Boron transport in plants:Boron transport in plants:coordinated regulationcoordinated regulation

of transporters

Kyoko Miwa, Toru FujiwaraBiotechnology Research Center,

The University of Tokyo

Boron (B) is essential for plantsIntroduction1

B deficiency symptoms Cross-linking of RG-II in cell wall

Boron (B) is essential for plants

B deficiency symptoms Cross linking of RG II in cell wall

Watanabe (1985) Kobayashi et al.(1996),O’Neill et al.(20

Occurrence of B deficiency was reported in 80 countries and 132 crop species(Shorrocks, 1997)

High concentration of B is toxicIntroduction2

High concentration of B is toxic.B toxicity symptoms

(Watanabe，1985)

Plants need to control B concentration in optimum range.Presentation by Sakamoto, 30th, 3:00pm

Boron transporters?Introduction3

･B(OH)3 + H2O → B(OH)4- + H +

（pKa = 9.25, 25 °C ）（p , ）

Boron is transported passively along transpiration streamBoron is transported passively along transpiration stream.

B

(Marschner 1995)

B homeostasis by regulation of B transporters

Low B High B

(2)Preferential distribution of B into

ti b NIP6 1(3)Exclusion of

toxic B by BOR4

young portions by NIP6;1

(1)Efficient uptake and l l di f Bxylem loading of B

by NIP5;1 and BOR1

Arabidopsis thaliana bor1-1 is d f ti i l l di f bdefective in xylem loading of boron.

100 WTbor1-1

WTbor1-1

100

Root cell sap Xylem exudate

µM µM

60

80bor1-1bor1-1

60

80

20

40

20

40

Tracer B in media (µM)

00 20 40 60 80 100

00 20 40 60 80 100

Tracer B in media (µM)

(Noguchi et al., 1997, Takano et al., 2002 Nature)

BOR1 B ra

tion

DW

)

567 vector

concentrates B into xylem under B

con

cen

trol

B/

kg D

345

BOR1ylow B supply.

(Xylem loading) Cel

lula

r B

（m

mo

12

(Xylem loading)

X l

C

0

CortexPericycleXylem

parenchyma

Casparian stripEpidermisMedia

Endodermis

XylemBOR1

２

(Takano et al., 2002, Nature)

BOR1 accumulation is regulated at post transcriptional level

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at post-transcriptional level.Protein

mRNAroot

rootroot

BOR1PBOR1

-B +B

BOR1

GFPBOR1P35S BOR1-GFP

+B 0 min

+B 30 min

Subcellularlocalization

+B 120 min

-B 120 min

+B 60 min

(Takano et al., 2005, PNAS)

Stele

B transport model by BOR1

Cortex

Stele

Pericycle Xylemparenchyma

Casparian stripEpidermisMedia

EndodermisHigh B

Xylem

BOR1

Low B

XylemBOR1

BOR1Xylem-directed export of B by BOR1

NIP5;1 is upregulated by low B NIP5;1 mRNA level in roots

-B 24h--> +B 24h B 24h

(Takano et al. 2006Plant Cell）

NIP5;1 is important for efficient B uptake in roots under low BB uptake in roots under low B.

(Takano et al. Plant Cell 2006)

Stele

B transport model in Arabidopsis

Cortex

Stele

Pericycle Xylemparenchyma

Casparian stripEpidermisMedia

EndodermisHigh B

Xylem

XylemBOR1

Low BNIP5;1

NIP5;1

yNIP5;1

Transcriptional regulation

post-transcriptional regulation

Coordinated expression of transporters and channelstransporters and channels

From Soils Toward XylemFrom Soils Toward Xylem

NIP5;1 BOR1 BBB

Channels facilitate influx

Transporters export out f th ll i tfacilitate influx

into cellsof the cells against

concentration gradient

NIP6;1 preferentially distributes B into young leaves under limited Binto young leaves under limited B.

- BB

+B

(Tanaka et al. 2008 Plant Cell)

A Rice BOR1 ortholog also

OsBOR1 (XII)

functions in B transportosbor1-2osbor1-13830 bp (2137 bp)

100 bp

OsBOR1 (XII)LB

osbor1 2LB

osbor1 13830 bp (2137 bp)

; exon ; intron ; Tos17 ; primer

WT

osbor110 cm

- Bosbor1

(Nakagawa et al.,2007 Plant Cell)

Presence of efflux B transporter for B tolerance was predicted in barleyB tolerance was predicted in barley.

+B ++BRoot B concentration

Sensitive cultivar

Tolerant cultivar

B in media (mM)SensitiveSensitive

(Hayes et al., 2004)Tolerant

(Sutton et al., 2007)

Tolerant

Which is for B tolerance among six BORs in Arabidopsis?

Required for growth

among six BORs in Arabidopsis?

BOR1BOR2BOR3

q gunder B limitation

BOR4BOR7

S ifi ll

0.05

O

BOR5BOR6

Specificallyexpressed in pollen

BOR4-GFP was not decreased hi h B lupon high B supply.

GFPBOR4Pro35S

line4-B +B ++B120

(kDa)B +B ++B

Root120

BOR4-GFP100

8080

(Miwa et al., 2007 Science)

B concentrations were decreased i BOR4 i liin BOR4 expressing lines.

80

100WT

60

80at

ion

gDW

)++ B(3 mM)

40

once

ntra

mol

B/k

g

WTBOR4line

(3 mM)

0

20

B c

o（

m BOR4line

0Roots Shoots

(Miwa et al., 2007)

BOR4 is localized to distal side of l b f t id iplasma membrane of root epidermis.

GFPBOR4genomeProBOR4

(Miwa et al., 2007)

Overexpression of BOR1 improved l t th d B li it tiplant growth under B limitation

GFPBOR1P35S 0.5 µMB

L15L10Wild type BOR1 overexpressor

BOR1 over Wild type -expressor

L10(Miwa et al., 2006, Plant J)

Enhanced expression of NIP5;1improved root growthimproved root growth

Wild type NIP5;1AC/ NIP5;1AC/35S-BOR1P35S ProNIP5;1 NIP5;1P35S ProNIP5;1 NIP5;1

Wild type NIP5;1AC/35S-BOR1

NIP5;1AC/35S-BOR1

35S BOR1

Grown on -B media for 23 daysBar : 5 cm

(Kato et al., 2009)(Line 1) (Line 6)

Overexpression of BOR4 conferred high B toleranceconferred high B tolerance.

Wild type BOR4 overexpressing ArabidopsisGFPBOR4Pro35S

Wild type BOR4 overexpressing Arabidopsis

++BToxic level B

10 mM

(Miwa et al., 2007,Science) Line 5 Line 4 Line 12

B homeostasis by regulation of B transporters

Low B High B

(2)Preferential distribution of B into

ti b NIP6 1(3)Exclusion of

toxic B by BOR4

young portions by NIP6;1

(1)Efficient uptake and l l di f Bxylem loading of B

by NIP5;1 and BOR1

ConclusionConclusion

• Plants regulate B transportPlants regulate B transport.• Expressions of B transporters are regulated

at mRNA and protein levels in response toat mRNA and protein levels in response to B conditions.P l l li ti f th t t i lik l• Polar localization of the transporters is likely to be important for directional trans-cellular t ttransport.

• B transporters are useful for improvement of B-stress tolerance.