Root exudates

What are they, what are their functions and how can we collect them

Ellis Hoffland, Angela Straathof, Diederik Keuskamp

What are they?

2

Jones et al. 2009 Plant Soil 321: 5Dennis etl al. 2010 FEMS Microbiol Ecol 72: 313

Rhizodeposits vs root exudates

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� Root exudation

● Active process, specific (ABC) transporters, controlled

● Organic acids, amino acids

● Quantitatively of limited importance

� Basal root exudation

● Meristematic regions behind root cap

● Vast majority of exudates

Rhizodeposits vs root exudates

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� Rhizodeposits

● Mucilage produced by border cells

● Lysis of sloughed-off root cap and border cells

● Mucilage, glycoproteins, etc. produced by root hairs

● Rhizodermal lysis of older cells

� Both

● High turn-over rates

5

Dennis et al. 2010 FEMS Microbiol. Ecol. 72: 313

Types of compounds

� Soluble, insoluble, volatile

� Sugars, amino acids, organic acids, fatty acids, sterols, enzymes, etc.

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Dennis et al. 2010 FEMS Microbiol. Ecol. 72: 313

How much?

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� Kuzyakov & Domanski 2000 J Plant Nutr. Soil Sci. 163: 421

Rhizodeposits: 11% of total C fixed

500 kg C ha-1

� Jones et al. 2009 Plant Soil 321: 5

Why are root deposits relevant?

� They (probably) shape the rhizobiome

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Berendsen et al. 2012 Trends Plant Sci. 17: 478

Shaping the rhizobiome

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Dis

trib

ution (

%)

Proteobacteria: families

Soil

Rhiz

osphere

Soil

Rhiz

osphere

Family:

Phyllum:

Bacteria: all phylla

Lundberg et al. 2012 Nature 488: 86

Shaping the rhizobiome

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Philippot et al. 2013 Nature Rev. Microbiol. 11:789

Com

positio

n (

%)

Bacterial phylla

Plant

Disclaimer: the environment matters!

11

Cúcio et al. 2016 Front. Microbiol.7: 440

Why are root deposits relevant?

� They affect cycles of C, N, P, Zn, Fe, ........

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C cycle

� Rhizodeposits:500 kg C ha-1

Kuzyakov & Domanski 2000 J Plant Nutr. Soil Sci. 163: 421

� Half life 30 – 120 h

� But: Half life bacterial cells: 224 d

Miltner 2012 Biogeochemistry 111:

41

� Labile litter forms stable SOM

Cotrufo et al. 2013 Global Change Biol. 19: 988

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Bradford et al. 2013 Biogeochemistry 113: 271

C cycle

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Brolsma et al. 2017 In preparation

� Plant species’ rhizobiomesdifferentially support decomposition of labile compounds

Plant species Phenylalanine

µmol CO2 g-1

soil h-1

Agrostis capillaris 0.21 b

Anthoxantum odoratum 0.29 b

Festuca rubra 0.61 a

Holcus lanatus 0.08 b

Centaurea jacea 0.16 b

Leucanthemum vulgare 0.61 a

Plantago lanceolata 0.62 ab

Rumex acetosa 0.84 a

C cycle - Priming

15Keiluweit et al. 2015 Nature Climate Change 5: 588

Plant – Plant Interactions

16Mommer et al. 2016 Trends Plant Sci. 21: 3

Plant – Plant Interactions

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Disso

lved P

Ca

P

Org

anic

P

a b

Competition

Org

anic

P

Disso

lved P

Ca

P

a b

Complementarity

Disso

lved P

Org

anic

P

Ca

P

a b

Facilitation

Hinsinger et al. 2011 Plant Physiol. 156:1078

Neumann 2010

Neumann & Martinoia 2009

YoungpH 5-6Citrate: 350

OldpH 6-7Citrate: 80mmol/h/g fw

MaturepH 4-5Citrate:1400

Root exudate: Citric acidRoot exudate: Citric acid

Root exudates – Facilitation

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55 52 51

290 280 203

Content peanut shoot (mg kg-1)

Zn 44 44 44

Fe 206 206 206

Zou & Zhang 2009 Agron. Sustain. Dev. 29: 63

20Semchenko et al. 2014 New Phytol. 204: 631

Plant – Plant Interactions - Signalling

Root exudate collection methods

� Angela Straathof & Diederik Keuskamp

� Soil - Rhizoboxes

� (Sterile) hydroponic systems

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Sterile vs. Non-Sterile

22Kuijken et al. 2015 Plant Soil 387: 131

Net exudation of a range of organic acids in sterile (white bars) and non-sterile (black bars) rhizosphere conditions of cultivarMoneymaker

The Rhizobox

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40 cm

12.5 cm

12.5 cm

50o

Pros:• Full grown

plants• Exact location• Timing• Soil(types)

Cons:• Not Sterile

• Turnover• Producing

compounds• Small volumes• Adsorption

The Rhizobox - Mini Lysimeters

24Shen & Hoffland 2007 Plant Soil 292: 161

Rhizobox 2.0

25Oburger et al. 2013 Exp. Environm. Bot. 87: 235

Pros:• Soil!• Test different soils• More relevant?!• Different methods

in 1

Cons:• Too Complex?• Expensive?• Still artificial?

Sorption materials

26Neumann et al. 2014 Front. Microbiol. 5: 131

Pros:• Soil!• Test different soils• More relevant?!

Cons:• Effectiveness• Adsorption by

soil• Specificity?• More relevant?!

Decades ago......

27Hoffland et al. 1989 Plant Soil 113: 161

Hydroponic system

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Pros:• Relatively

simple• Plants of any

size/age

Cons:• Non sterile• Dilution effect• Root

morphology

Sterile Hydroponic systems

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Pros:• Relatively

Simple• Plants of any

size/age• Sterile

Cons:• Morphology• Anaerobic• Dilution

effect• Sterile?!

Kaiser et al. 2002 Plant Physiol. 130: 1263

Sterile Hydroponic systems

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Pros:• Relatively

Simple• Plants of any

size/age• Sterile

Cons:

• Anaerobic• Dilution

effect• Sterile!!!

Strehmel et al. 2014 Pytochemistry 108: 35

PCR tubes

3 weeks

50 ml bottles

Sterile Hydroponic systems

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Pros:• Relatively

Simple• Plants of any

size/age• Sterile

Cons:

Kuijken et al. 2015 Plant Soil 387: 131

Thank you

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