1 Short title: Role of NTRC in developing tomato fruits...2019/09/16 · NTRC acts as central hub...
Transcript of 1 Short title: Role of NTRC in developing tomato fruits...2019/09/16 · NTRC acts as central hub...
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Short title: Role of NTRC in developing tomato fruits 1
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Corresponding author: 3
Liang-Yu Hou 4
Address: Ludwig-Maximilians-University Munich, Department Biology I, 82152 5
Planegg-Martinsried, Germany 6
Tel/Fax: +49 (089) 2180 74710 / +49 (089) 2180 74599 7
E-mail: [email protected] 8
Plant Physiology Preview. Published on September 16, 2019, as DOI:10.1104/pp.19.00911
Copyright 2019 by the American Society of Plant Biologists
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Article title: NTRC plays a crucial role in starch metabolism, redox balance and tomato fruit 9
growth 10
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Liang-Yu Hou*, Matthias Ehrlich, Ina Thormählen, Martin Lehmann, Ina Krahnert, Toshihiro 12
Obata, Francisco J. Cejudo, Alisdair R. Fernie, Peter Geigenberger 13
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Ludwig-Maximilians-University Munich, Department Biology I, 82152 Planegg-Martinsried, 15
Germany (L.-Y. H., M.E., I.T., M.L., P.G.); Max-Planck-Institute of Molecular Plant 16
Physiology, 14476 Potsdam-Golm, Germany (I.K., T.O., A.R.F.); Instituto de Bioquímica 17
Vegetal y Fotosíntesis, Universidad de Sevilla and Consejo Superior de Investigaciones 18
Científicas, 41092 Seville, Spain (F.J.C.) 19
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One sentence summary: 21
An established regulator of photosynthetic processes in leaves influences tomato fruit quality 22
by regulating starch accumulation, redox balance, and fruit growth. 23
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Footnotes: 24
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P.G. conceived the project; L.-Y.H., M.E., I.T., M.L., I.K., T.O., F.J.C. A.R.F., P.G. designed 26
the research; L.-Y.H. performed most of the experiments; M.E. performed the cloning; M.L. 27
performed GC-MS measurements; I.K. performed tomato transformation; L.-Y.H., I.T., M.L., 28
T.O., P.G. analyzed the data; I.T., A.R.F., P.G. supervised the experiments; L.-Y.H., P.G. 29
wrote the article. 30
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This work was supported by the Deutscher Akademischer Austauschdienst (grant to L.-Y.H. 32
and P.G.) and the Deutsche Forschungsgemeinschaft (TRR 175, grants to P.G, A.R.F and 33
M.L.). 34
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The author responsible for distribution of materials integral to the findings presented in this 36
article in accordance with the policy described in the Instructions for Authors 37
(www.plantphysiol.org) is: Peter Geigenberger ([email protected]). 38
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* Address correspondence to [email protected] 40
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Abstract 41
NADPH-thioredoxin reductase C (NTRC) forms a separate thiol-reduction cascade in 42
plastids, combining both NADPH-thioredoxin reductase and thioredoxin activities on a single 43
polypeptide. While NTRC is an important regulator of photosynthetic processes in leaves, its 44
function in heterotrophic tissues remains unclear. Here, we focus on the role of NTRC in 45
developing tomato (Solanum lycopersicum) fruits representing heterotrophic storage organs 46
important for agriculture and human diet. We used a fruit-specific promoter to decrease 47
NTRC expression by RNA interference in developing tomato fruits by 60-80% compared to 48
the wild type. This led to a decrease in fruit growth, resulting in smaller and lighter fully ripe 49
fruits containing less dry matter and more water. In immature fruits, NTRC down-regulation 50
decreased transient starch accumulation, which led to a subsequent decrease in soluble sugars 51
in ripe fruits. The inhibition of starch synthesis associated with a decrease in the 52
redox-activation state of ADP-glucose pyrophosphorylase and soluble starch synthase, which 53
catalyze the first committed and final polymerizing steps of starch biosynthesis, respectively. 54
This was accompanied by a decrease in the level of ADP-glucose. NTRC down-regulation 55
also led to a strong increase in the reductive states of NAD(H) and NADP(H) redox systems. 56
Metabolite profiling of NTRC-RNAi lines revealed increased organic and amino acid levels, 57
but reduced sugar levels, implying that NTRC regulates the osmotic balance of developing 58
fruits. These results indicate that NTRC acts as central hub in regulating carbon metabolism 59
and redox balance in heterotrophic tomato fruits, affecting fruit development as well as final 60
fruit size and quality. 61
62
Introduction 63
Reduction-oxidation (Redox) regulation appears to be a fundamental integrator of 64
metabolic pathways in different subcellular compartments (Geigenberger and Fernie, 2014). 65
In plant chloroplasts there are two different thiol redox systems, the ferredoxin 66
(Fdx)-thioredoxin (Trx) system, which depends on the reduction of Fdx by photosynthetic 67
electron transport in response to light, and the NADPH-dependent Trx reductase C (NTRC) 68
system, which relies on NADPH and thus may be linked to Fdx-NADPH reductase (FNR) in 69
the light or sugar metabolism in the dark (Buchanan and Balmer, 2005; Zaffagnini et al., 70
2018). NTRC is an unusual protein since it harbors both NADPH-Trx reductase (NTR) and 71
Trx domains on the same polypeptide (Serrato et al., 2004). This feature allows NTRC to use 72
NADPH as a source of electrons to regulate different chloroplast target proteins via 73
thiol-disulfide modulation (Spínola et al., 2008; Geigenberger et al., 2017). 74
In Arabidopsis (Arabidopsis thaliana) T-DNA insertion mutants, NTRC is involved in 75
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various chloroplast functions. There is in-planta evidence that NTRC is important to reduce 76
2-Cys peroxiredoxins (2-Cys Prxs) for the removal of hydrogen peroxide (Pérez-Ruiz et al., 77
2006; Pérez-Ruiz and Cejudo, 2009; Kirchsteiger et al., 2009) and to facilitate starch 78
synthesis via redox regulation of ADP-glucose pyrophosphorylase (AGPase) (Michalska et 79
al., 2009; Lepistö et al., 2013). Furthermore, NTRC has overlapping functions with the 80
Fdx-dependent Trx systems to optimize photosynthetic processes, such as non-photochemical 81
quenching of light energy (Thormählen et al., 2015; Naranjo et al., 2016; Yoshida and 82
Hisabori, 2016; Thormählen et al., 2017), Calvin-Benson cycle activity (Thormählen et al., 83
2015; Nikkanen et al., 2016; Yoshida and Hisabori, 2016), chlorophyll biosynthesis (Richter 84
et al., 2013; Pérez-Ruiz et al., 2014; Da et al., 2017) and ATP synthesis (Carrillo et al., 2016; 85
Nikkanen et al., 2016). In this context, NTRC indirectly maintains the reducing capacity of 86
the pool of Fdx-TRXs by restricting their re-oxidation via an oxidation loop involving 87
hydrogen peroxide and oxidized 2-Cys Prxs (Pérez-Ruiz et al., 2017). NTRC is, therefore, of 88
crucial importance to balance oxidation and reduction pathways in the chloroplast to optimize 89
photosynthesis, overall plant growth and development in response to light (Thormählen et al., 90
2015; Carrillo et al., 2016; Yoshida and Hisabori, 2016; Thormählen et al., 2017). 91
In addition to its role in chloroplasts, NTRC is also present in non-photosynthetic 92
plastids together with Fdx-dependent Trxs (Kirchsteiger et al., 2012). However, as yet little is 93
known on the role of NTRC in heterotrophic plant tissues that lack photosynthetic functions. 94
There is evidence that NTRC affects starch synthesis in Arabidopsis roots (Michalska et al., 95
2009), but this does not contribute to the growth phenotype of the Arabidopsis ntrc mutant 96
(Kirchsteiger et al., 2012). Complementation of the ntrc mutant by NTRC overexpression 97
under the control of a leaf-specific promoter led to wild-type (WT) phenotypes, but the 98
mutant phenotype remained unaltered when a root-specific promoter was used (Kirchsteiger 99
et al., 2012). These data thus indicate a role of NTRC in photosynthetic leaves, rather than in 100
non-photosynthetic roots. 101
Tomato (Solanum lycopersicum) is an important horticultural crop and major component 102
of the human diet serving as the primary model system to study fruit physiology and 103
development (Kanayama, 2017). Throughout the fruit developing process, a myriad of 104
changes in color, firmness, taste and flavor occur that are associated with developmentally 105
regulated metabolic shifts (e.g. changes in the levels of carbohydrates, acids and volatile 106
compounds) (Carrari et al., 2006). Notably, the abundance of carbohydrates is a key 107
determinant for fruit yield and quality (Kanayama, 2017). During development, immature 108
tomato fruits act as strong sink tissues, into which massive amounts of carbon are imported as 109
sucrose from leaf tissues (Osorio and Fernie, 2014). Most of the sucrose is mobilized to 110
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glucose and fructose and subsequently converted to starch as an intermediate carbon store to 111
promote the sink activity of the immature fruits (Osorio and Fernie, 2014). In later ripening 112
stages, starch is degraded to soluble sugars, which are key determinants of fruit taste and 113
quality (Davies and Hobson, 1981; Gierson and Kader, 1986). 114
Alterations of the transient starch pool of immature fruits have been proposed to be 115
important for the accumulation of soluble sugar in ripening fruits, which determines the final 116
fruit size and quality (Schaffer et al., 2000; Petreikov et al., 2009). In this context, AGPase 117
serves as a rate-limiting enzyme for starch synthesis in developing tomato fruits (Petriekov et 118
al. 2009). AGPase is a heterotetrameric enzyme located in the plastid comprised of two large 119
and two small subunits, which are subject to allosteric activation by glycerate-3-phosphate 120
(3-PGA) and inhibition by inorganic phosphate (Pi) (Preiss, 1973). AGPase is also subject to 121
redox regulation involving reversible reduction of a regulatory disulfide linking the small 122
subunits, which affects the allosteric properties of the enzyme (Fu et al., 1998; Tiessen et al., 123
2002). Manipulation of the cellular redox poise during tomato fruit development has 124
previously been demonstrated to lead to changes in the NADPH/NADP+ ratio, which were 125
accompanied by changes in the redox-activation states of NADP-malate dehydrogenase 126
(MDH) and AGPase of the developing fruits (Centeno et al., 2011). The changes in AGPase 127
redox state were additionally associated with alterations in starch synthesis in immature fruits 128
and subsequent alterations in soluble sugar content in mature fruits (Centeno et al., 2011). 129
Although these studies provide evidence that redox-regulation via thiol-disulfide modulation 130
is involved in tomato fruit development, the underlying fruit-specific thiol-redox systems 131
remain unresolved. 132
In this study, we down-regulated NTRC gene expression specifically in fruit tissues by 133
generating a RNA interference construct under the control of the fruit-specific patatin B33 134
promoter. The NTRC-RNAi lines were characterized by a 60-80% decrease in NTRC 135
transcripts and protein levels in developing fruits. In immature fruits, NTRC down-regulation 136
decreased transient starch accumulation by decreasing the redox-activation state of AGPase 137
and the activity of soluble starch synthase, which subsequently led to a decreased 138
accumulation of soluble sugars during ripening and to decreased fruit yield and quality in 139
fully ripe fruits. This was accompanied by an increased reduction state of the NAD(H) and 140
NADP(H) redox couples. These results provide evidence for a previously unknown function 141
of NTRC as a central hub in regulating carbon metabolism and redox balance in developing 142
fruits. 143
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Results 144
Generation of transgenic tomato plants with decreased expression of NTRC under 145
control of a fruit-specific promoter 146
To silence NTRC gene expression specifically in fruit tissues, we generated a 147
NTRC-RNAi construct under the control of the patatin B33 promoter, which has previously 148
been identified to confer fruit specific expression in tomato plants (Rocha-Sosa et al., 1989; 149
Frommer et al., 1994; Obiadalla-Ali et al., 2004), with kanamycin serving as a marker for 150
selection (Fig. 1A). The resulting construct was transformed via an Agrobacterium 151
tumefaciens-mediated process into tomato plants. To identify functional NTRC-RNAi lines, 152
samples from 35 and 65 day-after-flowering (DAF) fruits were harvested for genotyping. As 153
shown in Figure 1B, the NTRC-RNAi lines 2, 26 and 33 all harbored the neomycin 154
phosphotransferase II (nptII) gene, indicating the presence of the NTRC-RNAi cassette 155
within their genome. The transcript level of NTRC gene (LOC101254347) decreased by 156
60-80% in comparison to WT in the three RNAi lines, both in 35 (Fig. 1C) and 65 DAF fruit 157
samples (Supplemental Fig. S1A). Furthermore, the NTRC protein level of RNAi-2, 26 and 158
33 decreased by 50-80% compared to WT, both in 35 (Fig. 1, D and E) and 65 DAF fruit 159
samples (Supplemental Fig. S1, B and C). The expression of the second NTRC gene 160
(LOC101266017) (Nájera et al., 2017) was also analyzed, but its expression was too low to 161
be detectable in fruit tissues. Thus, we concluded the NTRC-RNAi lines 2, 26 and 33 were 162
appropriate to study the function of NTRC in tomato fruit. 163
164
Fruit-specific silencing of NTRC has no substantial effects on the process of fruit 165
development 166
The different stages of tomato fruit development are characterized by changes in 167
pigmentation and ripening. To analyze whether decreased expression of NTRC affects the 168
process of tomato fruit development, we analyzed chlorophyll and light-harvesting complex 169
II (Lhcb2) protein levels as well as the expression of genes involved in ethylene synthesis and 170
ripening in 35 DAF and 65 DAF fruits. In 35 DAF fruits, chlorophyll content and protein 171
level of Lhcb2 were not significantly changed in NTRC-RNAi lines compared to WT, while 172
both parameters were not detectable in 65 DAF fruits in all genotypes (Supplemental Fig. S2, 173
A and B). There were no visible changes in fruit color at 35 DAF (Fig. 2A) and 65 DAF 174
(Supplemental Fig. S3A) when NTRC-RNAi lines were compared to WT. To further analyze 175
whether NTRC down-regulation affected the ripening processes, we assayed the expression 176
of genes involved in ethylene production (S-adenosylhomocysteine hydrolase, SAHH2) 177
(Yang et al., 2017) and ripening (NON-RIPENING, NOR) (Ma et al., 2019) in both 35 DAF 178
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and 65 DAF fruits. Both genes were up-regulated when fruits became mature, but their 179
expression levels were not significantly changed between WT and NTRC-RNAi lines 180
(Supplemental Fig. S2, C and D). Taken together, NTRC down-regulation did not affect the 181
process of fruit development. While 35 DAF fruits represent the mature green stage and 65 182
DAF fruits represent the fully ripened stage, there were no stage-dependent differences 183
between NTRC-RNAi lines and WT. 184
185
Fruit-specific silencing of NTRC leads to smaller and lighter fruits with decreased dry 186
weight-to-fresh weight ratios 187
To investigate whether silencing of NTRC affects overall fruit growth, fully mature 188
fruits (65 DAF) were harvested for the measurement of final fruit size and weight. As shown 189
in Figure 2A, although both the WT and RNAi lines were fully ripe, the final fruit size of 190
NTRC-RNAi lines was clearly smaller than WT. Furthermore, the NTRC-RNAi lines 191
exhibited a 20% reduction in fruit height and 15% reduction in fruit width compared to WT 192
(Fig. 2B). The final tomato fruit fresh weight in NTRC-RNAi lines was 40% lower than WT 193
(Fig. 2C), and the NTRC-RNAi tomato fruits also contained less dry matter and more water 194
than WT (Fig. 2D). The effects of NTRC silencing on fruit growth and dry-matter content 195
were also observed at 35 DAF, indicating that they were not caused by developmental effects 196
(Supplemental Fig. S3). Taken together, fruit-specific silencing of NTRC led to the 197
generation of smaller and lighter fruits consisting of less dry matter in tomato plants. 198
199
Fruit-specific silencing of NTRC leads to lower accumulation of starch and soluble 200
sugars during fruit development 201
Tomato fruit development is characterized by a transient accumulation of starch in 202
immature fruits, which is subsequently mobilized to contribute to the accumulation of soluble 203
sugars, specifically glucose, in mature fruits (Schaffer et al., 2000). To investigate whether 204
the inhibition of fruit growth in NTRC-RNAi lines is due to a decrease in the transient starch 205
pool, we analyzed the levels of starch and soluble sugars during fruit development. In 206
immature fruits (35 DAF), NTRC down-regulation led to a strong decrease in starch 207
accumulation down to 25-50% of WT level (Fig. 3A). In mature fruits (65 DAF), where 208
starch has been almost completely mobilized in all genotypes, the NTRC-RNAi lines still 209
exhibited a trend of decreased residual starch levels compared to WT (Fig. 3B). In contrast to 210
this, NTRC silencing did not affect the protein content of the fruits at both developmental 211
stages (Supplemental Fig. S4). 212
To investigate whether the inhibition of starch accumulation was accompanied by an 213
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increase in soluble sugars, we analyzed the levels of glucose, fructose, sucrose and maltose in 214
NTRC-RNAi lines compared to WT. At both developmental stages, glucose level, the major 215
soluble sugar in tomato fruits, was strongly decreased in NTRC-RNAi lines compared to WT, 216
while fructose, sucrose and maltose levels showed only slight decreases (Fig. 3, C and D; Fig. 217
4A; Supplemental Table S1). Thus, interfering with NTRC expression in tomato fruits greatly 218
compromised transient starch accumulation and final soluble sugar accumulation during their 219
development, while there were no effects on the protein content of the fruits. 220
221
Interfering with NTRC expression in tomato fruits has minor effects on the levels of 222
phosphorylated intermediates and adenine nucleotides 223
To investigate whether the decrease in starch and sugar accumulation is related to 224
changes in sugar and energy metabolism, we further assayed the level of phosphorylated 225
metabolites and adenine nucleotides in developing tomato fruits. In NTRC-RNAi lines, the 226
level of glucose-6-phosphate (G6P), fructose-6-phosphate (F6P), and glucose-1-phosphate 227
(G1P) showed no consistent changes at 35 and 65 DAF when compared to WT 228
(Supplemental Table S1 and Table S2). Also, no substantial changes were detected in the 229
level of the glycolytic intermediate 3-PGA between WT and NTRC-RNAi lines at both 230
developmental stages. Triose phosphates were also assayed, but the level was under detection 231
limit, which is characteristic for the glycolytic pathway operating in heterotrophic tissues 232
(Hatzfeld and Stitt, 1990). Furthermore, the levels of ATP and ADP and the ATP/ADP ratio 233
in NTRC-RNAi lines were not significantly changed during fruit development when 234
compared to WT (Supplemental Table S2). The data also show the levels of the substrates 235
(G1P, ATP) and the allosteric regulators (3-PGA, Pi) of AGPase were not substantially 236
changed in the transgenic fruits. Taken together, these results demonstrate that interfering 237
with NTRC expression does not perturb glycolytic metabolism or adenylate energy state in 238
developing fruits. 239
240
Interfering with NTRC expression in tomato fruits leads to global changes in their 241
metabolite profile 242
To obtain in-depth insight into more global metabolite changes in NTRC-RNAi tomato 243
lines, we next performed metabolite profiling via GC-TOF-MS. We firstly confirmed the 244
NTRC-RNAi lines showed generally lower sugar levels compared to WT (Fig. 4A, left panel), 245
which was the case for glucose (55-80% of WT level), fructose (65-85% of WT level), and 246
sucrose (40-55% of WT level) as well as other sugars. Moreover, upon NTRC silencing, the 247
levels of several sugars decreased more strongly at 65 DAF than 35 DAF (Fig. 4A, right 248
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panel), including gentiobiose (3.6-fold), maltose (2-fold) and ribose (2.3-fold). With 249
exception of myo-inositol, there was no great change in sugar alcohols in NTRC-RNAi lines 250
(Fig. 4A). 251
By contrast, levels of several organic acids increased in NTRC-RNAi lines compared to 252
WT plants, including 2-imindazolidone-4-carboxylic acid, trans-aconitic acid, trans-caffeic 253
acid and quinic acid, which are involved in the synthesis of phenolic compounds, while 254
others, such as 1-pyrroline-2-carboxylate and gulonic acid, decreased (Fig. 4B). The decrease 255
in gulonic acid level might be attributable to lower glucose content, since this acid is formed 256
by oxidation of glucose. The intermediates of the tricarboxylic acid (TCA) cycle also showed 257
an increasing pattern compared to WT. Levels of citric acid and 2-oxo-glutaric acid were 2 to 258
4 times increased in NTRC-RNAi lines at both stages of fruit development, while malic acid, 259
fumaric acid and succinic acid only showed slight changes compared to WT (Fig. 4C). 260
Interestingly, the TCA cycle precursor, pyruvic acid, increased only in immature fruits of 261
NTRC-RNAi lines compared to WT (Fig. 4C, left panel). 262
In addition to organic acids, levels of amino acids also showed an increasing pattern 263
while NTRC expression was reduced. Asparagine, 3-cyano-alanine and homoserine were 1.5- 264
to 2-fold higher in NTRC-RNAi lines in immature fruits (Fig. 4C, left panel), and this change 265
became more severe while fruits ripened (Fig. 4C, right panel). Other amino acids, such as 266
tryptophan, histidine, glutamine and methionine, showed mild increases in NTRC-RNAi lines, 267
compared to WT. Metabolites, such as urea, amines, nucleosides and their derivate, were also 268
analyzed, but the results revealed only minor differences between the NTRC-RNAi lines and 269
WT at both stages (Fig. 4D). Overall, these results indicate interfering with NTRC expression 270
affects the balance between sugars, organic acids and amino acids during fruit development. 271
272
Interfering with NTRC expression in tomato fruits leads to an increase in the cellular 273
redox state of pyridine nucleotides 274
NAD(H) and NADP(H) redox couples are involved in various metabolic processes in 275
different subcellular compartments (Berger et al., 2004; Geigenberger and Fernie, 2014) and 276
also serve as cofactors for NTRC to reduce and regulate downstream plastidial proteins 277
(Pérez-Ruiz and Cejudo, 2009). To investigate whether NTRC down-regulation affects the 278
levels of these pyridine nucleotides, we next analyzed the levels of NAD(P)H and NAD(P)+ 279
in developing tomato fruits. In immature fruits (35 DAF), perturbing NTRC expression led to 280
a strong increase in the levels of the reduced forms, NADPH and NADH, while the levels of 281
the oxidized forms, NADP+ and NAD
+, decreased (Fig. 5A). This resulted in a 2- to 5-fold 282
increase in NADPH/NADP+ ratios and 1.5- to 2-fold increase in NADH/NAD
+ ratios in fruits 283
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of NTRC-RNAi lines in comparison to those of WT (Fig. 5C). 284
In fully ripe fruits (65 DAF), NTRC down-regulation also led to an increase in NADPH 285
and NADH, but the levels of the oxidized forms NADP+ and NAD
+ were not substantially 286
altered (Fig. 5B), which led to a minor increase in the ratios of NADPH/NADP+ and 287
NADH/NAD+ in NTRC-RNAi lines (Fig. 5D). Together, these results indicate reducing 288
NTRC expression caused an increased reduction state of the NAD(H) and NADP(H) redox 289
couples in developing tomato fruits with the effect being mitigated when fruits matured. 290
291
Interfering with NTRC expression in tomato fruits leads to a decrease in the 292
redox-activation state of AGPase and decreased levels of ADP-Glc 293
To investigate possible mechanisms involved in the inhibition of fruit starch 294
accumulation in response to decreased NTRC expression, we measured activities of enzymes 295
involved in starch metabolism. We first analyzed AGPase, which catalyzes the first 296
committed step of starch biosynthesis, since its activity is regulated via thiol-disulfide 297
modulation involving Trx and NTRC systems in the plastid (Geigenberger and Fernie, 2014; 298
Geigenberger et al., 2017). Since AGPase is activated by reduction of an intermolecular 299
disulfide bond between the two small subunits (AGPase small-subunit 1, APS1) of the 300
heterotetrameric holoenzyme (Ballicora et al., 2000; Tiessen et al., 2002; Hendriks et al., 301
2003), we assayed the changes in APS1 redox state by analyzing its monomerisation pattern 302
when NTRC expression was reduced. In developing fruits (35 DAF), perturbing NTRC 303
expression slightly decreased the monomerization ratios of APS1 (Fig. 6A) by approximately 304
20-30%, indicating an increased oxidation of the protein. Samples from ripe fruits (65 DAF) 305
were also tested, but there were no consistent differences between WT and RNAi lines in the 306
monomerization of APS1 (Supplemental Fig. S5A). 307
AGPase is also subject to allosteric regulation, being activated by 3-PGA and inhibited 308
by Pi (Kleczkowski, 1999; Hwang et al., 2005). Redox-regulation of AGPase alters the 309
kinetic properties of the holoenzyme, with oxidation of APS1 leading to an enzyme less 310
sensitive for allosteric activation (Tiessen et al., 2002; Hendriks et al., 2003). To investigate 311
whether perturbing NTRC expression in tomato fruits leads to a change in the kinetic 312
properties of AGPase, we assayed AGPase activity in the absence or presence of its allosteric 313
activator, 3-PGA, in 35 DAF fruits. As shown in Figure 6B, the NTRC-RNAi lines showed 314
comparable AGPase activity to WT when 3-PGA was omitted from the enzyme assay, but 315
there was a 30% decrease in AGPase activity compared to WT in the presence of 10 mM 316
3-PGA. To investigate 3-PGA activation kinetics in more detail, we performed AGPase 317
enzyme assays in the presence of varied concentrations of 3-PGA. Figure 6C shows that, in 318
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WT, increasing concentrations of 3-PGA from 0.025-10 mM led to a progressive increase in 319
AGPase activity from 0.2-6.0 nmol min-1
mg protein-1
. AGPase from NTRC-RNAi lines was 320
less sensitive to 3-PGA activation leading to 30-50% lower activities when incubated in the 321
presence of 0.25-10 mM 3-PGA. This concentration range corresponds to the in vivo 322
concentration of 3-PGA in the plastid, estimated to be around 0.3-0.5 mM using non-aqueous 323
fraction of growing potato (Solanum tuberosum) tubers (Farré et al., 2001; Tiessen et al., 324
2002) and developing barley (Hordeum vulgare) seeds (Tiessen et al., 2012), and 0.7-1 mM 325
using the 3-PGA levels of Supplemental Table S1 (70-100 nmol g FW-1
), given that the 326
vacuole contains no 3-PGA and comprises 90% of the cell volume. The overall activities of 327
AGPase in fully ripe fruits (65 DAF), which are characterized by an overall suppression of 328
starch synthesis (Fig. 3B), were 10-times lower than in immature fruits (35 DAF) either in the 329
presence or absence of 3-PGA in the enzyme assay, and there was no significant difference 330
between WT and NTRC-RNAi lines (Supplemental Fig. S5B). 331
ADP-glucose (ADP-Glc) is the direct product of the reaction catalyzed by AGPase and 332
represents the ultimate precursor of the final polymerizing reactions of starch synthesis. In 35 333
DAF fruits, NTRC down-regulation decreased ADP-Glc concentration to 50-75% of WT 334
level (Fig. 6D). This provides independent confirmation that NTRC silencing leads to an 335
inhibition of AGPase activity in vivo. 336
337
NTRC down-regulation leads to deceased activity of soluble starch synthase 338
In addition to AGPase, we also assayed other enzymes involved in starch metabolism in 339
35 DAF fruits. Soluble and granule-bound starch synthases catalyze the final polymerizing 340
reactions of starch biosynthesis. As shown in Table 1, perturbing NTRC expression decreased 341
the activity of soluble starch synthase to 25-40% of WT level, while the activity of 342
granule-bound starch synthase only slightly decreased. Soluble starch synthase is subject to 343
post-translational redox regulation and a potential target of NTRC in vitro (Skryhan et al., 344
2015; Skryhan et al., 2018). In contrast, NTRC down-regulation had no significant effect on 345
the activities of starch phosphorylases and starch hydrolases, which are involved in starch 346
degradation (Table 1). Together these results imply down-regulation of NTRC expression 347
leads to decreased starch accumulation via a joint inhibition of both AGPase and soluble 348
starch synthase. 349
350
NTRC down-regulation has minor effects on the redox state of 2-Cys Prxs in tomato 351
fruits 352
2-Cys Prxs are well-known targets of NTRC in Arabidopsis leaves where they are 353
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13
involved in hydrogen peroxide removal during photosynthesis (Cejudo et al., 2012). 354
Activation of 2-Cys Prxs involves reduction of an intermolecular disulfide bond linking the 355
two subunits of the homodimer (Pérez-Ruiz et al., 2006; Pérez-Ruiz et al., 2017). To 356
investigate the role of NTRC in reduction of 2-Cys Prxs in tomato fruits, we assayed the 357
monomer/dimer ratio of 2-Cys Prxs in tomato fruit tissues of NTRC-RNAi lines compared to 358
WT using non-reducing SDS gels. NTRC down-regulation slightly decreased the 359
monomerization ratio of 2-Cys Prxs in both 35 and 65 DAF tomato fruits (Supplemental Fig. 360
S6), even though the decrease was not consistent in all transgenic lines. The role of NTRC in 361
regulating the redox state of 2-Cys Prxs seems to be less important in tomato fruit than in 362
Arabidopsis leaves, where NTRC deficiency leads to a strong degree of 2-Cys-Prx oxidation 363
(Cejudo et al., 2012). However, other Trxs might have compensated for the deficiency of 364
NTRC with respect to 2-Cys-Prx reduction in tomato fruits. 365
366
Discussion 367
NTRC is a NADPH-dependent thiol redox enzyme, which has been found in 368
chloroplasts as well as in non-photosynthetic plastids (Kirchsteiger et al., 2012). In 369
photosynthetic tissues, the role of NTRC in carbon metabolism and redox balance of 370
chloroplasts in response to fluctuating light environments has been well characterized 371
(Geigenberger et al., 2017; Zaffagnini et al., 2018). Previous studies led to the perspective 372
that NTRC might function differently in heterotrophic tissues; however, our understanding as 373
to how remains fragmentary (Kirchsteiger et al., 2012). In this work, we used genetic and 374
biochemical approaches to directly test the role of NTRC in developing tomato fruits, which 375
represent heterotrophic tissues dependent on sucrose import with no substantial 376
photosynthetic carbon assimilation (Lytovchenko et al., 2011). Our results show NTRC does 377
not affect developmental and ripening programs of the fruit but promotes transient starch 378
accumulation by activating AGPase and soluble starch synthase to increase overall sink 379
activity and hence final fruit size, dry matter and sugar content. In contrast to this, NTRC 380
leads to a decreasing pattern of organic and amino acids and a decrease in the NAD(P)(H) 381
reduction state. This indicates NTRC serves as an important hub to orchestrate carbon 382
metabolism and redox balance in heterotrophic fruit tissues to optimize fruit growth and 383
quality in response to sucrose supply. 384
385
NTRC is important to optimize fruit size and quality 386
Tomato is an important horticultural crop and serves as major component of our diet 387
(Carrari and Fernie, 2006). There has been considerable interest to optimize tomato fruit yield 388
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14
and quality by using genetic approaches to alter sink metabolism and ripening (Giovannoni, 389
2004; Carrari and Fernie, 2006), but there is a lack of studies investigating the role of redox 390
regulation in this context. In the present paper, NTRC expression was specifically decreased 391
by 50-75% in developing tomato fruits using RNA interference under the control of a 392
fruit-specific promoter (Fig. 1). Perturbing NTRC expression did not affect developmental 393
and ripening programs of the fruit (Supplemental Fig. S2) but led to a decrease in final fruit 394
size and weight with fully mature fruits containing less dry matter and more water than WT 395
(Fig. 2). The phenotype was accompanied by a decrease in the levels of soluble sugars (Fig. 396
3D; Fig. 4A; Supplemental Table S1) and an opposing increase in many organic acids and 397
amino acids as well as phenolic compounds (Fig. 4, B, C and D; Supplemental Table S1). 398
Domesticated tomatoes tend to accumulate high amounts of glucose and fructose. Thus, the 399
total abundance of glucose, fructose and other minor sugars determines the sweetness of 400
tomato fruits. The acid-to-sugar ratio determines the sourness of fruits, and it is also an 401
important quality parameter (Davies and Hobson, 1981; Gierson and Kader, 1986). Thus, it is 402
very likely the changes we observed in response to NTRC down-regulation would eventually 403
influence the taste and flavor of tomato fruits. 404
NTRC affects growth and development of Arabidopsis plants (Pérez-Ruiz et al., 2006; 405
Lepisto et al., 2009; Toivola et al., 2013; Thormählen et al., 2015), but this is mainly ascribed 406
to its role in photosynthesis and leaf metabolism, rather than in non-photosynthetic tissues 407
(Kirchsteiger et al., 2012). The present study provides direct evidence for a previously 408
unrecognized role of NTRC to regulate the development of heterotrophic storage organs to 409
optimize yield quantity and quality parameters. This shows NTRC has important functions 410
beyond photosynthesis that affect growth and development. Although immature fruits are 411
green and contain chlorophyll, it is unlikely the changes in fruit growth reported above are 412
due to a role of NTRC in regulating photosynthetic processes within the fruit (Supplemental 413
Fig. S2). Indeed, previous studies have shown knock-out of chlorophyll synthesis in pale 414
tomato fruits does not have significant effects on fruit physiology, growth and development 415
(Lytovchenko et al., 2011). 416
There are conflicting studies concerning the effect of NTRC overexpression on the 417
growth of Arabidopsis plants. While Toivola et al. (2013) provide evidence that mild 418
overexpression of NTRC results in bigger plants, there is evidence from Ojeda et al. (2018) 419
that strong NTRC overexpression may have opposite effects. It is tempting to speculate 420
whether increased expression of NTRC may lead to larger tomato fruits. However, given the 421
results in Arabidopsis, this may depend on the level of NTRC overexpression. More studies 422
will be necessary to clarify this question. 423
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15
424
NTRC regulates transient starch accumulation in developing fruits 425
Tomato fruit development is characterized by a transient accumulation of starch in 426
immature fruits which serves as a carbon reserve for soluble sugar accumulation during 427
ripening and finally affects fruit size (Schaffer et al., 2000). In the present study, interfering 428
with NTRC expression resulted in a 50-75% decrease in the level of the transient starch pool 429
in immature fruits (Fig. 3A), which was accompanied by a decrease in soluble sugars during 430
maturation and decreased fruit size (Fig. 3, C and D; Fig. 4A). Our results show the decrease 431
in transient starch accumulation associates with a decrease in the redox-activation of AGPase 432
(Fig. 6), which is the rate-limiting enzyme in the pathway of starch synthesis in developing 433
tomato fruits (Schaffer et al., 2000) and other plant tissues (Geigenberger et al., 2004; 434
Vigeolas et al., 2004; Neuhaus and Stitt, 2006; Faix et al., 2012) and a known target of 435
NTRC (Geigenberger et al., 2017). NTRC down-regulation led to a decrease in the reduction 436
state of the small subunit of AGPase (Fig. 6A), which was accompanied by a 30-50% 437
decrease in the sensitivity of the AGPase holoenzyme for its allosteric activator 3-PGA (Fig. 438
6, B and C). While the subcellular concentration of 3-PGA in the plastid has been estimated 439
to yield 0.3-0.5 mM (Farré et al., 2001; Tiessen et al., 2002; Tiessen et al., 2012), the results 440
of Figure 6B indicate NTRC down-regulation will lead to a 30-50% lower AGPase activity 441
under in vivo conditions in developing tomato fruits. In confirmation to this, NTRC silencing 442
led to a 40% decrease in the in vivo level of ADP-Glc (Fig. 6D), which is the immediate 443
product of the reaction catalyzed by AGPase, providing independent in vivo evidence that 444
NTRC down-regulation restricts AGPase activity in developing fruits. 445
This is consistent with previous studies documenting the primary role of AGPase in 446
regulating starch accumulation, soluble sugar content and growth of tomato fruits. First, 447
increased AGPase activity in near-isogenic tomato lines led to elevated starch accumulation 448
in immature fruits, followed by increased soluble sugar content and larger size of fully ripe 449
fruits (Petreikov et al., 2009). Secondly, increased AGPase redox-activation by manipulation 450
of NAD(P)(H) redox states led to elevated rates of starch synthesis in immature fruits and a 451
subsequent increase in the content of soluble sugars in fully ripe fruits (Centeno et al., 2011). 452
Interestingly, NTRC down-regulation also led to a strong decrease in the activity of 453
soluble starch synthase catalyzing the subsequent step in the pathway of starch synthesis, 454
while enzyme activities involved in starch degradation were not significantly affected in 455
developing fruits (Table 1). Soluble starch synthase has been identified as a redox-regulated 456
enzyme and potential target of NTRC in vitro, with reduction via NTRC or Trx f leading to 457
an increase in its activity in vitro (Skryhan et al., 2015; Skryhan et al., 2018). Our results 458
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16
provide evidence the relationship between NTRC and soluble starch synthase, which has 459
been shown in vitro, also extends to the in vivo situation of developing fruits. Intriguingly, 460
our data show a combined effect of NTRC on both AGPase and soluble starch synthase, 461
catalyzing the first committed and final polymerizing steps of starch biosynthesis, 462
respectively. This indicates a role of NTRC in stimulating starch accumulation and ultimately 463
growth and quality of tomato fruits by combined redox-activation of both enzymes. 464
NTRC is also involved in transient starch accumulation in leaves of Arabidopsis plants 465
during day-night cycles (Lepistö et al., 2013; Thormählen et al., 2015). NTRC deficiency in 466
Arabidopsis T-DNA insertion mutants leads to a 30-40% restriction in redox-activation of 467
AGPase and starch synthesis upon illumination, both parameters being further attenuated in 468
double mutants with combined deficiencies in NTRC and Trx f1 (Thormählen et al., 2015). In 469
leaves, NTRC is dependent on NADPH produced by the photosynthetic light reactions. 470
Furthermore, it is involved in the regulation of the Calvin-Benson cycle, and therefore in the 471
provision of the substrates and the allosteric activator of AGPase, which will indirectly 472
contribute to the regulation of starch synthesis (Thormählen et al., 2015). This phenomenon 473
contrasts to the situation in heterotrophic tissues where AGPase redox-state and starch 474
synthesis are both regulated by the momentary supply of sugars via the phloem (Tiessen et al. 475
2002; Michalska et al. 2009). Sugar translocation from leaves to heterotrophic organs has 476
been shown to vary in response to development (Walker and Ho, 1977) and diurnal 477
alterations (Bénard et al., 2015). Changes in sugar availability may affect the activity of the 478
oxidative pentose phosphate pathway (OPPP) (Geigenberger and Fernie, 2014), leading to 479
alterations in the NADP(H) redox state and, thus, in NTRC activity. Little is known about 480
changes in NADP(H) levels in plant tissues. There is evidence for diurnal alterations in 481
NADP(H) levels and redox states in Arabidopsis leaves (Thormählen et al., 2015). Moreover, 482
glucose - but not sucrose - feeding leads to an increase in the NADPH/NADP+ ratio in 483
heterotrophic potato tuber tissue (Kolbe et al., 2005). This implies that in developing fruits, 484
NTRC modulates starch synthesis via redox regulation of AGPase and soluble starch 485
synthase in response to changes in sugar import, which lead to alterations of NADPH by 486
affecting the OPPP. This is important to adjust the sink activity of the fruit to the supply of 487
sucrose from the leaves to optimize fruit growth. 488
489
NTRC regulates the redox-balance of NAD(P)H in developing fruits 490
In heterotrophic plastids lacking photochemical reactions, redox homeostasis depends 491
mainly on NADPH, so NTRC, which depends on NADPH, may play an essential role in 492
maintaining the plastidial redox poise. Our results show interfering with NTRC expression 493
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17
strongly affects the cellular redox states of NAD(H) and NADP(H) in developing tomato 494
fruits (Fig. 5), which provides evidence that NTRC is a major regulator of cellular redox 495
homeostasis in this heterotrophic tissue. While, at the moment, the underlying mechanisms 496
are unresolved, NTRC uses NADPH as a substrate and, thus, may influence the 497
NADPH/NADP+ ratio directly or via regulation of plastidial pathways consuming or 498
producing NADPH, such as the OPPP. However, direct effects of NTRC on enzymes of the 499
OPPP have not been studied yet. A further possibility is that NTRC affects the redox shuttles 500
between plastids and mitochondria leading to changes in interorganellar redox regulation. 501
Previous studies in tomato fruits provide evidence that plastidial and mitochondrial 502
NAD(P)H systems are interconnected via the malate valve (Centeno et al., 2011). An 503
increase in the NADPH redox state in response to NTRC down-regulation in the plastid may 504
therefore be transmitted to mitochondria leading to an increase in NADH redox state and vice 505
versa. Since adenylate energy states were not significantly changed in NTRC-RNAi lines 506
(Supplemental Table S2), it is quite unlikely that the changes in redox states are due to 507
changes in respiration rates. 508
NTRC deficiency also leads to an increase in NADP(H) and NAD(H) reduction states in 509
Arabidopsis plants (Thormählen et al., 2015; Thormählen et al., 2017). However, this is most 510
probably due to NTRC regulating the balance between photosynthetic light reactions and the 511
Calvin-Benson cycle as well as NADPH export via NADP-dependent MDH of the malate 512
valve. Taken together, these results identify NTRC as a central hub controlling the cellular 513
redox poise in non-photosynthetic as well as photosynthetic tissues. 514
Based on the work in Arabidopsis plants, NTRC has been found to play a major role in 515
detoxification of hydrogen peroxide by providing reductants to 2-Cys Prxs, with NTRC 516
deficiency leading to a strong decreased in the 2-Cys Prxs reduction state in 517
photosynthesizing leaves (Pérez-Ruiz et al., 2006; Pérez-Ruiz et al., 2017). Compared to this, 518
our results show NTRC down-regulation leads to a relative minor decrease in 2-Cys Prxs 519
reduction in developing and ripe tomato fruits (Supplemental Fig. S6). However, it must be 520
noted that photosynthetic activity leads to increased rates of hydrogen peroxide production, 521
which requires increased antioxidant activity via the NTRC-2-Cys Prxs system (Pérez-Ruiz et 522
al., 2006; Pérez-Ruiz et al., 2017). These results suggest the antioxidant function of NTRC is 523
less important in non-photosynthetic than in photosynthetic tissues. 524
525
NTRC affects the carbon-nitrogen balance in developing fruits 526
While our results show NTRC down-regulation leads to a decrease in soluble sugar 527
levels, there is an opposing increase in the levels of several amino acids in developing fruits 528
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18
(Fig. 4; Supplemental Table S1). Since total protein level remains unchanged (Supplemental 529
Fig. S4), it is more likely NTRC down-regulation leads to a stimulation of amino acid 530
synthesis, rather than protein degradation. Because amino acid biosynthesis requires reducing 531
equivalents as substrates, production of amino acids may have been promoted by the 532
increased NAD(P)H/NAD(P)+ ratios in developing fruits of the NTRC-RNAi lines (Fig. 5). 533
Increased levels of amino acids in the face of decreased sugar levels are also observed in 534
leaves of Arabidopsis mutants lacking NTRC (Lepisto et al., 2009; Thormählen et al., 2015), 535
indicating there are similar effects of NTRC on amino acid synthesis in photosynthetic and 536
non-photosynthetic tissues. Inverse relationships between sugars and amino acids have also 537
been found in other plant tissues, such as barley seeds (Faix et al., 2012) or potato tubers 538
(Trethewey et al., 1998; Fernie et al., 2003; Roessner-Tunali et al., 2003) across different 539
genetic and physiological manipulations. Overall, these results suggest NTRC is involved in 540
the regulation of the sugar-amino acid balance to prevent osmotic imbalances in the 541
developing fruit. 542
543
Conclusions 544
NTRC is a NADPH-dependent thiol redox system located in photosynthetic as well as in 545
non-photosynthetic plastids. While previous approaches were mainly performed in 546
Arabidopsis and photosynthetic leaves to date, there has been a lack of knowledge on the role 547
of NTRC in heterotrophic tissues and crop plants. By establishing NTRC-RNA interference 548
specifically in tomato fruit tissues, our results uncover the importance of NTRC in 549
developing tomato fruits. In immature fruits, NTRC promotes transient starch accumulation 550
by activating AGPase and soluble starch synthase to increase the carbohydrate stock for 551
soluble sugar accumulation during ripening to optimize final fruit yield and quality. NTRC 552
also plays a pivotal role in regulating the redox poise of NAD(P)(H) during fruit development, 553
which represents the primary redox system in non-photosynthetic tissues. In summary, NTRC 554
acts as a central hub in regulating carbon metabolism and redox balance in heterotrophic 555
tomato fruits, similar to its established role in photosynthetic tissues. Our results indicate the 556
activity of NTRC is indispensable to optimize fruit growth as well as tomato yield quantity 557
and quality. 558
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19
Materials and Methods 559
Plant cultivation and harvest 560
Tomato (Solanum lycopersicum cv Moneymaker) were grown as detailed in the 561
literature (Carrari et al., 2003; Nunes-Nesi et al., 2005). Fruits were harvested at the mature 562
green (35 DAF) and ripe stage (65 DAF) with the intact pericarp being immediately frozen in 563
liquid nitrogen for further analysis. 564
565
NTRC-RNAi construct and plant transformation 566
The NTRC gene fragment was amplified from potato (Solanum tuberosum) cDNA with 567
attB1-RNAi-NTRC (5’-ggggacaagtttgtacaaaaaagcaggctcagatgtcaggaattttgatca-3’) and 568
attB2-RNAi-NTRC (5’-ggggaccactttgtacaagaaagctgggtcgcttgtcagatatctctccactgata-3’), and 569
cloned into modified pBINAR-RNAi vector (pBIN19 backbone containing a patatin B33 570
promoter and a GATEWAY cassette) using a commercial GATEWAY cloning kit (Invitrogen). 571
Tomato plants were transformed as described before (Tauberger et al., 2000) and the 572
transformants were selected by kanamycin. The patatin B33 promoter acts as a fruit-specific 573
promoter when expressed in tomato plants (Rocha-Sosa et al., 1989; Frommer et al., 1994; 574
Obiadalla-Ali et al., 2004). 575
576
Molecular characterization of NTRC-RNAi lines 577
Genomic DNA extraction followed the CTAB based protocol (Doyle and Doyle, 1987). 578
The kanamycin resistance gene (nptII) was amplified with primer pairs, Kana-F 579
(5’-aactgttcgccaggctcaag-3’) and Kana-R (5’-tcagaagaactcgtcaagaagg-3’), and the internal 580
control (CAC, SGN-U314153) was amplified with CAC-F (5’-cctccgttgtgatgtaactgg-3’) and 581
CAC-R (5’-attggtggaaagtaacatcatcg-3’). The RNAzol (Sigma-Aldrich) was used for RNA 582
extraction, and 500 μg of total RNA was used for preparing cDNA with Maxima Reverse 583
Transcriptase (Thermo). The SYBG reagent (Biorad) was used for real-time PCR, and the 584
signal was recorded via iQ5 Multicolor Real-Time PCR Detection System (Biorad). The 585
primers, NTRC-F (5’-aggtgtatttgcagctggagatg-3’), NTRC-R (5’-ttcttcagtagggggctggtggaa-3’), 586
NTRC-2F (5’-ggaccaactaagggcatcagcaa-3’) and NTRC-2R (5’-cgagcacagactcttctcctg-3’), 587
were used to amplify NTRC genes (LOC101254347; LOC101266017), respectively. For 588
detecting the expression of ethylene production and ripening genes, the primer pairs, 589
SAHH2-F (5’-gacgctcacagtggaacaacga-3’), SAHH2-R (5’-tcaactttccagcaaaatatcccc-3’), 590
NOR-F (5’-agagaacgatgcatggaggtttgt-3’) and NOR-R (5’-actggctcaggaaattggcaatgg-3’), were 591
used for amplification of SAHH2 and NOR genes, respectively (Yang et al., 2017; Ma et al., 592
2019). The CAC gene described above served as an internal control (Expósito-Rodríguez et 593
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20
al., 2008). Total protein was extracted from 35 mg of pulverized sample with 150 μl of 594
double strength Laemmli buffer (Laemmli, 1970) containing 10 mM DTT, and 10 μl of 595
protein extract was used for SDS-PAGE and immunoblotting. The membrane was blocked 596
with 5% (w/v) non-fat milk and incubated with the first antibody of NTRC (1:1000 dilution), 597
Lhcb2 (Agrisera, 1:5000) or RbCL (Agrisera, 1:10000 dilution) at 4oC overnight, respectively. 598
The secondary antibody conjugating horseradish peroxidase (HRP) (Agrisera, 1:10000 599
dilution) was applied onto the membrane, and the membrane was incubated at 25oC for one 600
hour. The membrane reacted with Prime ECL solution (GE Healthcare), and the signal was 601
visualized with X-ray film (Fuji). 602
603
Spectrophotometric metabolite measurements 604
(a) Starch and soluble sugar quantification 605
The extraction and detection of starch and soluble sugar followed the method described 606
previously (Hendriks et al., 2003). In brief, 20 mg of pulverized sample was extracted with 607
250 μl of 80% ethanol twice followed by another 250 μl of 50% ethanol at 90oC for 30 min. 608
The supernatant was used for the detection of soluble sugar, and the pellet was dried out for 609
the detection of starch. For measuring starch level, the pellet was re-suspended with 400 μl of 610
0.1 M sodium hydroxide and incubated at 95oC for one hour. The solution was then 611
neutralized to pH 7 with HCl-acetate buffer (0.5 M HCl prepared in 0.1 M acetate-NaOH 612
buffer, pH 4.9), and 40 μl of aliquot was incubated with 110 μl of starch degradation buffer 613
(2.8 U ml-1
amyloglucosidase, 4 U ml-1
α-amylase, 50 mM acetate-NaOH buffer) at 37oC 614
overnight. Fifty microliters of digested sample was firstly mixed with 160 μl of glucose 615
determination buffer (3.4 U ml-1
G6PDH, 3 mM ATP, 1.3 mM NADP, 3 mM MgCl2, 100 mM 616
HEPES-KOH, pH 7.0). When the reaction saturated, and 0.45 U hexokinase was applied to 617
detect the formation of NADPH, which was proportional to starch amount, at 340 nm via a 618
spectrophotometer (FilterMax F5, Molecular Device). 619
For measuring soluble sugar level, 30 μl of supernatant was firstly mixed with 200 μl of 620
detection buffer (0.7 U ml-1
G6PDH, 1.7 mM ATP, 0.8 mM NADP, 5 mM MgCl2, 50 mM 621
HEPES-KOH, pH 7). When the reaction was stable, 0.5 U hexokinase, 1.2 U 622
phosphoglucoisomerase and 133 U invertase were applied to detect glucose, fructose and 623
sucrose, respectively, via the formation of NADPH at 340 nm. 624
For the detection of maltose, the method described previously was followed with several 625
modifications (Shirokane et al., 2000). Briefly, the above ethanol extract was dried out by 626
heating at 95oC and substituted with equal volume of double distilled water. Then, 30 μl of 627
extract was mixed with 200 μl of detection buffer described above with addition of 0.5 U 628
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21
hexokinase. When the reaction was stable, 0.2 U maltose phosphorylase was applied to detect 629
maltose via NADPH formation in the florescence mode of spectrometer (Ex: 340 nm, Em: 630
465 nm). Varied concentration of maltose (Sigma-Aldrich) served as the standard for 631
calculation. 632
(b) Hexose phosphates and 3-phosphoglycerate (3-PGA) 633
The assay of hexose-phosphates and 3-PGA was performed as described before (Häusler 634
et al., 2000). Briefly, 50 mg of pulverized sample was suspended in 500 μl of ice-cold 0.5 M 635
HClO4 and incubated at 4oC for 10 min. Then, leaf residue was removed by centrifuging for 636
10 min at 20,000 g. The pH of the supernatant was adjusted to the range from 5.0 to 6.0 using 637
2.5 M K2CO3 solution, and the supernatant was then centrifuged for 10 min at 20,000 g to 638
remove formed precipitant. Around 5 mg of active charcoal was applied into solution, which 639
was incubated on ice for 20 min. Charcoal was further removed via centrifuging for 10 min at 640
20,000 g in 4oC. For measuring hexose phosphates, 50 μl of supernatant was mixed with 50 641
μl of water and reacted with 100 μl of detection buffer (200 mM Tris-HCl, pH 8.1, 1.6 mM 642
NADP, 10 mM MgCl2, 2 μM glucose-1,6-bisphosphate). When the reaction was stable, 0.1 U 643
glucose-6-phosphate dehydrogenase, 0.35 U phosphoglucoisomerase and 0.04 U 644
phosphoglucomutase were applied to detect G6P, F6P and G1P, respectively, via NADPH 645
formation in the fluorescence mode of the spectrometer (Ex: 340 nm, Em: 465 nm). For 646
detecting 3-PGA, 50 μl of supernatant was mixed with 50 μl of water and reacted with 100 μl 647
of detection buffer (200 mM HEPES-NaOH, pH 7.0, 2 mM MgCl2, 2 mM ATP, 8 μM NADH, 648
0.8 U ml-1
Glyceraldehyde 3-phosphate dehydrogenase). When the reaction was stable, 0.45 649
U phosphoglycerate kinase was applied to detect 3-PGA via NADH consumption in the 650
florescence mode of spectrometer described above. Varied concentration of NADPH and 651
NADH (Carl Roth) served as the standard for calculation. 652
(c) Pyridine nucleotides 653
The assay of pyridine nucleotides (NAD+, NADP
+, NADH and NADPH) was performed 654
as described previously (Lintala et al., 2014). In short, 25 mg of pulverized sample was 655
suspended in 250 μl of 0.1 M HClO4 for detecting NAD+ and NADP
+. Another 25 mg of 656
sample was suspended in 250 μl of 0.1 M KOH for detecting NADH and NADPH. Samples 657
were further incubated on ice for 10 min and then centrifuged at 20,000 g for 10 min at 4oC 658
to remove the pellet. The supernatant was incubated at 95oC for 2 min, and the pH value was 659
adjusted to the range from 8.0 to 8.5 with 0.1 M KOH prepared in 0.2 M Tris-HCl (pH 8.4) 660
or HClO4 prepared in 0.2 M Tris-HCl (pH 8.4), respectively. For measuring NAD(H), 50 μl 661
of extract was mixed with 50 μl of water and reacted with 50 μl of detection buffer 662
containing 300 mM Tricine-KOH (pH 9.0), 12 mM EDTA, 1.5 M ethanol, 0.3 mM phenazine 663
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22
ethosulfate (PES), 1.8 mM methylthiazolydiphenyl-tetrazolium bromide (MTT), and 18 U 664
ml-1
alcohol dehydrogenase. For measuring NADP(H), another 50 μl of extract was mixed 665
with 50 μl of water and reacted with 50 μl of detection buffer containing 300 mM 666
Tricine-KOH (pH 9), 12 mM EDTA, 9 mM Glc-6-P, 0.3 mM PES, 1.8 mM MTT, and 9 U 667
ml-1
Glc-6-P dehydrogenase. The absorbance at wavelength 570 nm was recorded in a 668
spectrometer (FilterMax F5, Molecular Device). Pyridine nucleotides (Carl Roth) solutions 669
prepared in water were used to generate the standard curve for calculation. 670
(d) ADP-glucose (ADP-Glc) 671
The extraction of ADP-Glc was performed as described previously with minor 672
modification (Lunn et al., 2006). Briefly, 50 mg of pulverized sample was suspended in 375 673
μl of ice-cold chloroform/methanol mixture (3:7) and vortexed vigorously at 4oC for 10 min 674
followed by the addition of 300 μl of double distilled water for phase partitioning. After 675
centrifugation at 18000 g at 4oC for 10 min, the supernatant was transferred to a new tube and 676
dehydrated in a vacuum at ambient temperature. The dried pellet was re-suspended in 70 μl 677
of water, and 10 μl of the suspension was used for detecting ADP-Glc. 678
ADP-Glc was analyzed by LC-MS using a Dionex Ultimate 3000 UHPLC (Thermo 679
Fisher Scientific, Waltham, Massachusetts, USA) and a timsTOF QTOF (Bruker Daltonik, 680
Fahrenheitstr. 4, 28359 Bremen, Germany) in combination. The assay supernatant was 681
injected into a C18 column (Ultra AQ C18, 150 x 2.1 mm, 3 µm, Restek, Bellefonte, 682
Pennsylvania, USA) with 300 µl min-1
flow at 22°C. The solvents used were (A) water and 683
(B) acetonitrile, both including 0.1% (v/v) formic acid. The gradient started at 1% B for 1 684
min, followed by a ramp to 25% B within 10 min. After a 4 min washing step at 95% B, the 685
gradient turned back to 1% B and kept constant for 3 min equilibration. After the column 686
compartment, the analytes passed a diode array detector (DAD) at 254 nm before entering the 687
mass spectrometer. An electrospray ionization (ESI) source in negative mode was used at 688
5000 V capillary voltage and nitrogen as dry gas, 8 L min-1
at 200°C. The timsTOF mass 689
spectrometer was run in MS mode from 50-1300 m/z mass ranges. Compounds were 690
identified using the specific mass (m/z), isotope pattern, and retention time after standard 691
injection. Data were acquired by otofControl 4.0. The evaluation was performed by 692
DataAnalysis 5.1. All software tools were provided by Bruker (Bruker Daltonik, 693
Fahrenheitstr. 4, 28359 Bremen, Germany). All solvents were supplied in LC-MS-grade by 694
Biosolve. The ADP-Glc standard was bought from Sigma-Aldrich. 695
(e) Total chlorophyll 696
The quantification of total chlorophyll was performed according to a previous approach 697
(Porra et al., 1989). In short, 35 mg of pulverized sample was extracted with 1 ml of ice-cold 698
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23
80% acetone and vortexed at 4oC for 2 min followed by centrifugation to remove insoluble 699
residues. Then, 250 μl of supernatant was mixed with 750 μl of 80% acetone and applied into 700
a quartz cuvette. The optical density of extract was determined by measuring the absorbance 701
at wavelength 664 and 647 nm. 702
703
Measurement of adenylate nucleotide 704
The extraction of adenylate nucleotides followed a previously published method (Jelitto 705
et al., 1992). Briefly, 50 mg of pulverized sample was extracted with 600 μl of extraction 706
buffer (16% TCA, 5 mM EGTA), and rigorously vortexed at 4oC for one hour. The 707
supernatant was then washed twice with 4 ml of cold water-saturated diethyl ether to remove 708
TCA. The lower water phase was transferred to a new microtube, and the pH was adjusted to 709
between 6.0 and 7.0 with neutralization buffer (5 M KOH, 1 M triethanoamine). The sample 710
was placed under a hood for one hour to let the remaining diethylether evaporate. The 711
detection procedure was described before (Haink and Deussen, 2003) with several 712
modifications: 85 μl of extract reacted with 20 μl of citrate buffer (62 mM Citric acid, 76 mM 713
KH2PO4, pH 5.2) and 20 μl of 50% chloroacetaldehyde at 80oC for 15 min. Then, 20 μl of 714
sample was applied into a NUCLEODUR 100-5 C18 ec column (MACHEREY-NAGEL) and 715
eluted with the mixture of mobile buffer A (10 mM KH2PO4, 5.7 mM 716
tetrabutylammoniumhydrogensulfate, pH 5.4) and mobile phase B (90% acetonitrile) under a 717
flow rate of 0.8 ml min-1
. The gradient was as described below: 0 min (100% A / 0% B), 18 718
min (86% A / 14% B), 36 min (56% A / 44% B), 38.4 min (17.5% A / 82.5% B), 39.6 min 719
(100% A / 0% B), 42 min (100% A / 0% B). The signal was analyzed by a fluorescence 720
detector (FINNIGAN SURVEYOR PDA Plus Detector, Thermo), which was set to 280 nm 721
excitation wavelength and 410 nm emission wavelength. Standards were provided by 722
Sigma-Aldrich. 723
724
Metabolite profiling: GC-TOF-MS 725
Metabolic profiling was performed using a method previously described with 726
modification (Roessner et al., 2001; Lisec et al., 2006; Erban et al., 2007). In brief, for the 727
extraction, 50 mg of tissue (fresh weight) was ground in 360 µl of cold (-20°C) methanol 728
containing internal standards (10 μl of ribitol, 0.2 mg ml-1
in water and 10 μl of 13C-sorbitol, 729
0.2 mg ml-1
in water) for the relative quantification. First, the samples were incubated at 4°C 730
for 30 min, and then the extract was carefully mixed with 200 μl of chloroform and 400 µl of 731
water. To force the separation of the polar and non-polar phase, a 15 min centrifugation step 732
at 25000 g was performed. For derivatization, 50 μl of the upper (polar) phase was dried in 733
www.plantphysiol.orgon October 18, 2020 - Published by Downloaded from Copyright © 2019 American Society of Plant Biologists. All rights reserved.
24
vacuum. The pellet was resuspended in 10 μl of methoxyaminhydrochloride (20 mg ml-1
in 734
pyridine) and agitated for 90 min at 40°C. After the addition of 20 µl of BSTFA 735
(N,O-Bis[trimethylsilyl]trifluoroacetamide) containing 2.5 μl of retention time standard 736
mixture of linear alkanes (n-decane, n-dodecane, n-pentadecane, n-nonadecane, n-docosane, 737
n-octacosane, n-dotriacontane), the whole mixture was incubated at 40°C for 45 min. Finally, 738
1 μl of sample was injected into a GC–TOF–MS system (Pegasus HT, Leco, St Joseph, USA). 739
Samples were prepared by an auto-sampler system (Combi PAL, CTC Analytics AG, 740
Zwingen, Switzerland). Helium acted as carrier gas at a constant flow rate of 1 ml min-1
. Gas 741
chromatography was performed on an Agilent GC (7890A, Agilent, Santa Clara, USA) using 742
a 30 m VF-5ms column with 10 m EZ-Guard column. The injection temperature of the 743
split/splitless injector was set to 250°C, as well as the transfer line and the ion source. The 744
initial oven temperature (70°C) was permanently increased to a final temperature of 350°C 745
by 9°C min-1
. To avoid solvent contaminations, the solvent delay time was set to 340 s. 746
Compounds were ionized and fractionated at 70 eV. Mass spectra were recorded at 20 scans 747
s-1
with an m/z 35-800 scanning range. Chromatograms and mass spectra were evaluated 748
using ChromaTOF 4.5 and TagFinder 4.1 software (Luedemann et al., 2008). All solvents 749
were supplied in LC-MS-grade by Biosolve (Leenderweg 78, 5555 Valkenswaard, NL). 750
Standards were provided by Sigma-Aldrich. 751
752
Redox shift assay of APS1 and 2-Cys Prxs 753
To preserve the redox state, the total protein was extracted using a TCA-based method 754
(Hendriks et al., 2003). In short, 35 mg of frozen fruit tissue was extracted with 1 ml of 755
ice-cold 16% TCA prepared in diethylether and incubated at -20oC overnight. After 756
centrifugation, the supernatant was discarded, and the pellet was washed with ice-cold 757
acetone three times. The pellet was then suspended with double strength Laemmli buffer 758
(Laemmli, 1970) without reducing reagent and incubated at 90oC for 5 min with 1400 rpm 759
shaking speed. Then, 10 μl of protein extract was applied into 10 or 12% SDS-acrylamide gel 760
to perform immunoblot analyses detecting APS1 or 2-Cys Prxs, respectively. The 761
immunoblotting procedure was performed as described before (Hendriks et al., 2003; 762
Pérez-Ruiz et al., 2006), and Image-J was used for quantification of the protein signals. 763
764
Enzyme activity assay of starch metabolism 765
(a) ADP-glucose pyrophosphorylase (AGPase) 766
The AGPase activity assay followed a previously described method (Tiessen et al., 2002) 767
with several modifications. Briefly, 25 mg of pulverized frozen fruit tissue was extracted with 768
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25
150 μl of 50 mM HEPES-KOH buffer (pH 7.5), and incubated on ice for 10 min followed by 769
centrifugation to remove leaf residue. The crude protein extract was applied into a Zeba 770
desalting column (Thermo) as described in the manual. Then, 20 μl of protein extract was 771
incubated with 180 μl of AGPase reaction buffer (50 mM HEPES-KOH, pH 7.5, 7 mM 772
MgCl2, 1.5 mM ATP, 0.5 mM G1P, varied concentration of 3-PGA) at 25oC for 30 min with 773
900 rpm shaking speed, and the reaction was terminated by mixing rigorously with 100 μl of 774
chloroform. Two microliters of the supernatant was used for quantification of ADP-Glc 775
produced during the enzyme assay. ADP-Glc was detected as described in the section of 776
ADP-Glc assay with modifications that an electrospray ionization (ESI) source in negative 777
mode was used at 4000 V capillary voltage and nitrogen as dry gas, 8 L min-1
at 250°C. 778
(b) Soluble starch synthase and granule-bound starch synthase 779
The assay was performed as previously described with modification (Kulichikhin et al., 780
2016). Briefly, 25 mg of pulverized sample was suspended in 150 μl of ice-cold buffer (100 781
mM HEPES-NaOH, pH 7.5, 8 mM MgCl2, 2 mM EDTA, 12.5% glycerol, 5% PVP-10, 782
protease inhibitor cocktail), and incubated on ice for 10 min followed by centrifugation to 783
separate the supernatant and insoluble pellet. For detecting soluble starch synthase, 50 μl of 784
the supernatant was applied into a self-packed Sephadex G25 column equilibrated with 50 785
mM HEPES-NaOH (pH 7.5) for desalting and mixed with 50 μl of double strength reaction 786
buffer (100 mM HEPES-NaOH, pH 7.5, 2 mM ADP-Glc, 0.2% amylopectin) followed by 787
incubation at 30oC for 30 min. The reaction was terminated by the addition of one-third 788
volume chloroform with vigorous vortex, and the upper phase was used for detecting the 789
production of ADP during the reaction. For the granule-bound starch synthase, the insoluble 790
pellet was washed twice with 1 ml of 50 mM HEPES-NaOH (pH 7.5) and re-suspended in 791
100 μl of reaction buffer (50 mM HEPES-NaOH, pH 7.5, 1 mM ADP-Glc) followed by 792
incubation at 30oC for 30 min with mild shaking. The reaction was terminated by the addition 793
of chloroform, and 10 μl of the upper phase was used for detecting ADP. ADP was analyzed 794
as described in the section of AGPase assay. ADP standards were provided by Sigma-Aldrich. 795
(c) Starch phosphorylase 796
The activity of starch phosphorylase was assayed by detecting the number of G1P 797
produced in the reaction (Zeeman et al., 1998; Häusler et al., 2000). In brief, 25 mg of 798
pulverized sample was extracted with 150 μl of extraction buffer (40 mM HEPES-NaOH, pH 799
7.5, 1 mM EDTA) and incubated on ice for 10 min followed by centrifugation to remove fruit 800
residues. Then, 25 μl of protein extract was mixed with 75 μl of water and 100 μl of double 801
strength reaction buffer (200 mM Tris-HCl, pH 8.1, 1.6 mM NADP+, 10 mM MgCl2, 2 μM 802
glucose-1,6-bisphosphate, 20 mM sodium phosphate buffer, pH 6.9, 0.1% amylopectin, 0.11 803
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26
U glucose-6-phosphate dehydrogenase, 0.07 U phosphoglucomutase). The NADPH 804
formation was detected in the fluorescence mode of the spectrophotometer (Ex: 340 nm, Em: 805
465 nm). 806
(d) Starch hydrolase 807
The activity of starch hydrolase was assayed by detecting the number of reducing groups 808
released from starch with DNS reagent (Miller, 1959). In short, 25 mg of pulverized sample 809
was extracted with 150 μl of extraction buffer (20 mM sodium phosphate buffer, pH 6.9, 6 810
mM NaCl) and incubated on ice for 10 min followed by centrifugation to remove fruit 811
residues. The supernatant was applied to a self-packed Sephadex G-25 column for desalting. 812
Then, 50 μl of protein extract was mixed with equal volume of 1% soluble starch followed by 813
incubation at 60oC for 30 min, and protein extract without the input of starch served as the 814
blank reaction. The reaction was terminated by the addition of 100 μl of DNS reagent (1% 815
3,5-dinitrosalicylic acid, 30% sodium potassium tartrate tetrahydrate), and the whole mixture 816
was incubated at 95oC for 5 min. The amount of reducing sugar released from starch was 817
determined in the absorbance mode of the spectrophotometer (wavelength: 540 nm). Maltose 818
served as the standard for quantification. 819
The protein concentration was measured by Bradford reagent (Carl Roth) and Pierce 660 820
nm protein reagent (Thermo) according to the manual. 821
822
Statistical analysis 823
All figures and statistical analyses were made via Graphpad Prism 8. ANOVA and 824
Dunnett’s test were used to evaluate the difference between WT and NTRC-RNAi lines 825
unless otherwise stated. 826
827
Accession numbers 828
Sequence data from this article can be found in the GenBank/EMBL data libraries under 829
accession numbers SlNTRC (LOC101254347; LOC101266017), SlSAHH2 (Solyc09g092380), 830
SlNOR (Solyc10g006880), SlCAC (SGN-U314153). 831
832
Supplemental Data 833
Supplemental Figure S1. The molecular characterization in 65 DAF tomato fruits of 834
NTRC-RNAi lines compared to wild type (WT). 835
836
Supplemental Figure S2. Characterization of developmental stage in tomato fruits. 837
838
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27
Supplemental Figure S3. Phenotypic analysis of mature green tomato fruits (35 DAF) of 839
NTRC-RNAi lines (RNAi-2, 26 and 33) compared to wild type (WT). 840
841
Supplemental Figure S4. Total protein content of fruits of NTRC-RNAi lines compared to 842
wild type (WT). 843
844
Supplemental Figure S5. Redox and allosteric properties of AGPase in 65 DAF tomato 845
fruits of NTRC-RNAi lines (RNAi-2, 26 and 33) compared to wild type (WT). 846
847
Supplemental Figure S6. The redox state of and 2-Cys Prxs in 35 and 65 DAF tomato fruits 848
of wild type (WT) and NTRC-RNAi lines. 849
850
Supplemental Table S1. Fold changes in metabolite profiles in tomato fruits of wild type 851
(WT) and NTRC-RNAi lines. 852
853
Supplemental Table S2. Changes in the level of hexose phosphates, 3-PGA and adenine 854
nucleotides in wild-type (WT) and NTRC-RNAi tomato fruits. 855
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28
Acknowledgements 856
We thank Anne Bierling (LMU Munich) for excellent technical assistance. We are also 857
grateful to all greenhouse staff in the Max-Planck-Institute of Molecular Plant Physiology for 858
taking care of tomato plants. 859
860
Tables 861
Table 1. Changes in the activity of starch metabolism enzymes in 35 DAF tomato fruits of 862
wild type (WT) and NTRC-RNAi lines. Results are the mean with standard error from six 863
biological replicates. Statistical analysis used ANOVA with Dunnett’s test (*, different from 864
WT, P < 0.05). 865
866
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29
Figure legends 867
Figure 1. Molecular characterization of NTRC-RNAi lines (2, 26 and 33) compared to wild 868
type (WT) in 35 DAF tomato fruits. A, The RNA interference construct targeting the NTRC 869
gene. B, Genotyping by PCR analysis for detection of nptII gene in NTRC-RNAi lines. C, 870
Detection of NTRC transcript level using RT-qPCR. D, Detection of NTRC proteins by 871
immunoblot analysis. E, Quantification of NTRC protein abundance using Image-J software. 872
The level of clathrin adaptor complexes (CAC) serves as a reference gene for genotyping and 873
RT-qPCR. The protein level of ribulose 1,5-bisphosphate carboxylase/oxygenase large 874
subunit (RbcL) served as a loading control for immunoblotting. Results are the mean with 875
standard error from six biological replicates. Statistical analysis used ANOVA with Dunnett’s 876
test (*, different from WT, P < 0.05). 877
878
Figure 2. Phenotypic analysis of fully-ripened tomato fruits (65 DAF) of NTRC-RNAi lines 879
(RNAi-2, 26 and 33) compared to wild type (WT). A, Visible phenotype of ripe fruits. B, 880
Fruit length and width. C, Fruit fresh weight. D, Ratio of fruit dry weight to fresh weight. 881
Results are the mean with standard error from six biological replicates (each replicate 882
contains data from 1 to 3 fruits). Statistical analysis used ANOVA with Dunnett’s test (*, 883
different from WT, P < 0.05). The red scale indicates 2 cm in length. 884
885
Figure 3. The accumulation of starch and soluble sugars in tomato fruits of wild type (WT) 886
and NTRC-RNAi lines (RNAi-2, 26 and 33). The levels of starch (A and B) and soluble 887
sugars (C and D) were measured in fruits harvested at 35 (A and C) and 65 DAF (B and D). 888
Results are the mean with standard error from six biological replicates. Statistical analysis 889
used ANOVA with Dunnett’s test (*, different from WT, P < 0.05). 890
891
Figure 4. Metabolite profile of NTRC-RNAi tomato fruits (RNAi-2, 26 and 33) compared to 892
wild type (WT). The metabolite profile was measured in fruits harvested at 35 (left panel) and 893
65 DAF (right panel) by using GC-TOF-MS. A. Sugars and sugar alcohols. B. Organic acids. 894
C. Amino acids and TCA cycle intermediates. D. Miscellaneous. Results are normalized to 895
WT with log2 transformation and visualized as a heatmap with hierarchical clustering done 896
by R software. Data are taken from Supplemental Table S1. 897
898
Figure 5. Changes in pyridine nucleotide level in tomato fruits of NTRC-RNAi lines in 899
comparison to wild type (WT). The levels of NADP+, NADPH, NAD
+ and NADH (A and B) 900
and the ratios of NADPH/NADP+ and NADH/NAD
+ (C and D) were measured in fruits 901
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30
harvested at 35 (A and C) and 65 DAF (B and D). Results are the mean with standard error 902
from six biological replicates. Statistical analysis used ANOVA with Dunnett’s test (*, 903
different from WT, P < 0.05). 904
905
Figure 6. Redox and allosteric properties of AGPase and endogenous ADP-glucose content 906
in 35 DAF tomato fruits of NTRC-RNAi lines (RNAi-2, 26 and 33) compared to wild type 907
(WT). A, the redox state of intermolecular disulfide bond in AGPase small subunit 1 (APS1). 908
B, AGPase activity assayed with or without the activation of 10 mM 3-PGA. C, AGPase 909
activity kinetics in the presence of varied concentration of 3-PGA as indicated in the figure. 910
D, the content of endogenous ADP-glucose. The monomer/dimer ratios of the respective 911
proteins were quantified by Image-J software. Results are the mean with standard error 912
(n=5-6). Statistical analysis used ANOVA with Dunnett’s test (*, different from WT, P < 913
0.05). 914
915
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Figure 1. Molecular characterization of NTRC-RNAi lines (2, 26 and 33) compared to wild type (WT) in 35 DAF tomato fruits. A, The RNA interference construct targeting the NTRC gene. B, Genotyping by PCR analysis for detection of nptII gene in NTRC-RNAi lines. C, Detection of NTRC transcript level using RT-qPCR. D, Detection of NTRC proteins by immunoblot analysis. E, Quantification of NTRC protein abundance using Image-J software. The level of clathrin adaptor complexes (CAC) serves as a reference gene for genotyping and RT-qPCR. The protein level of ribulose 1,5-bisphosphate carboxylase/oxygenase large subunit (RbcL) served as a loading control for immunoblotting. Results are the mean with standard error from six biological replicates. Statistical analysis used ANOVA with Dunnett’s test (*, different from WT, P < 0.05).
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A B
Figure 2. Phenotypic analysis of fully-ripened tomato fruits (65 DAF) of NTRC-RNAi lines (RNAi-2, 26 and 33) compared to wild type (WT). A, Visible phenotype of ripe fruits. B, Fruit length and width. C, Fruit fresh weight. D, Ratio of fruit dry weight to fresh weight. Results are the mean with standard error from six biological replicates (each replicate contains data from 1 to 3 fruits). Statistical analysis used ANOVA with Dunnett’s test (*, different from WT, P < 0.05). The red scale indicates 2 cm in length.
Height Width0
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8
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ght (
%)
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Figure 2
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D
B
Figure 3. The accumulation of starch and soluble sugars in tomato fruits of wild type (WT) and NTRC-RNAi lines (RNAi-2, 26 and 33). The levels of starch (A and B) and soluble sugars (C and D) were measured in fruits harvested at 35 (A and C) and 65 DAF (B and D). Results are the mean with standard error from six biological replicates. Statistical analysis used ANOVA with Dunnett’s test (*, different from WT, P < 0.05).
A
C
Figure 3
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RNAi-26
RNAi-33
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rch
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50
60
70
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uble
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ar (µ
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60
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uble
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ar (µ
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* *
*
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WT
RNAi-2RNAi-26RNAi-33WT
RNAi-2RNAi-26RNAi-33
210-1-2
WT
RNAi-2RNAi-26RNAi-33WT
RNAi-2RNAi-26RNAi-33
210-1-2
A
Figure 4. Metabolite profile of NTRC-RNAi tomato fruits (RNAi-2, 26 and 33) compared to wild type (WT). The metabolite profile was measured in fruits harvested at 35 (left panel) and 65 DAF (right panel) by using GC-TOF-MS. A. Sugars and sugar alcohols. B. Organic acids. C. Amino acids and TCA cycle intermediates. D. Miscellaneous. Results are normalized to WT with log2 transformation and visualized as a heatmap with hierarchical clustering done by R software. Data are taken from Supplemental Table S1.
WT
RNAi-2RNAi-26RNAi-33WT
RNAi-2RNAi-26RNAi-33
210-1-2
B
D
WT
RNAi-2RNAi-26RNAi-33WT
RNAi-2RNAi-26RNAi-33
210-1-2
C
Figure 4
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Figure 5
Figure 5. Changes in pyridine nucleotide level in tomato fruits of NTRC-RNAi lines in comparison to wild type (WT). The levels of NADP+, NADPH, NAD+ and NADH (A and B) and the ratios of NADPH/NADP+ and NADH/NAD+ (C and D) were measured in fruits harvested at 35 (A and C) and 65 DAF (B and D). Results are the mean with standard error from six biological replicates. Statistical analysis used ANOVA with Dunnett’s test (*, different from WT, P < 0.05).
B
NADPH/NADP+ NADH/NAD+0.0
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1.0
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ativ
e ra
tio
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Figure 6. Redox and allosteric properties of AGPase and endogenous ADP-glucose content in 35 DAF tomato fruits of NTRC-RNAi lines (RNAi-2, 26 and 33) compared to wild type (WT). A, the redox state of intermolecular disulfide bond in AGPase small subunit 1 (APS1). B, AGPase activity assayed with or without the activation of 10 mM 3-PGA. C, AGPase activity kinetics in the presence of varied concentration of 3-PGA as indicated in the figure. D, the content of endogenous ADP-glucose. The monomer/dimer ratios of the respective proteins were quantified by Image-J software. Results are the mean with standard error (n=5-6). Statistical analysis used ANOVA with Dunnett’s test (*, different from WT, P < 0.05).
Figure 6
B
ox�
re�
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1
Enzymes WT RNAi-2 RNAi-26 RNAi-33
Soluble starch synthase
(nmol min-1
mg protein-1
) 44.8±4.2 16.8±2.6* 11.4±3.3* 18.6±6.2*
Granule-bound starch synthase
(pmol min-1
mg FW-1
) 2.9±0.2 2.0±0.1* 2.0±0.1* 2.4±0.2
Starch phosphorylase
(nmol min-1
mg protein-1
) 49.6±10.9 76.5±18.3 30.7±4.2 40.0±11.3
Starch hydrolases
(μmol min-1
mg protein-1
) 2.4±0.4 2.3±0.7 1.1±0.2 1.0±0.4
Table 1. Changes in the activity of starch metabolism enzymes in 35 DAF tomato fruits of wild type (WT)
and NTRC-RNAi lines. Results are the mean with standard error from six biological replicates. Statistical
analysis used ANOVA with Dunnett’s test (*, different from WT, P < 0.05).
Table 1
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