www.cilr.uq.edu.au

ARC Centre of Excellencefor Integrative Legume Research

How Legumes Make Nodules

Lisette Pregelj – Education and Outreach Manager

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Legume NodulesLegume Nodules

• Legume nodules are special plant organs that house nitrogen-fixing bacteria called Rhizobium

• Nodules form on Legume roots

• Nodules can also form on stems in some species

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Rhizobium Filled Cells>25,000 per cell Nodule Cross-Section

Sclerenchyma

Vascular Bundles

Nodule Cortex

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Nodule FormationNodule Formation

1. Rhizobia attracted to root

2. Rhizobia attach to root hairs

3. Root hair curling

4. Infection thread formation

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Nodule FormationNodule Formation

5. Root cortical cell division

6. Rhizobia invade cortical cells

7. The nodule grows

8. Fully functional nodule

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1: Rhizobia attracted to root1: Rhizobia attracted to root

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1. Rhizobia attracted to root1. Rhizobia attracted to root

• Legume roots exude flavonoids

(Soybean exudes the isoflavone genistein)

• Rhizobia are attracted to flavonoids

Petri dish contains a bacterial lawn

Bacteria turn blue when a reporter gene is switched on by plant exudates (flavonoids)

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10 m

2. Rhizobia attach to root hairs2. Rhizobia attach to root hairs

Rhizobia

Root Hair

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2. Rhizobia attach to root hairs2. Rhizobia attach to root hairs

Deformed root hairs

Curled root hair

Vascular bundle

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50 mm

3. Root hair curling3. Root hair curling

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4. Rhizobia infect root hair4. Rhizobia infect root hair

Root Hair Root Hair Curling

Rhizobia

Infection Thread

Legume Root

Infection Pocket

Vascular Bundle

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50 m

4. Rhizobia infect root hair4. Rhizobia infect root hair

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100m

5. Root cortical cells divide5. Root cortical cells divide

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100 m

5. Root cortical cells divide5. Root cortical cells divide

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6. 6. Rhizobia Rhizobia invade cortical cellsinvade cortical cells

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2-4 mm

7. The nodule grows7. The nodule grows

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root nodule

Rhizobia (green marker)

100 m

8. Fully functional nodule8. Fully functional nodule

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Nodulation TimelineNodulation Timeline

Inoculation (day 0)

Roots 1-2 days post inoculation (dpi)

4 dpi

4-6 dpi 10 dpi 3 weeks pi

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Chemical SignalsChemical Signals

• Nodulation involved chemical signals• Released by both the Legume plant and the

Rhizobium bacteria

Nod Factors

Flavones

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Flavones and IsoflavonesFlavones and Isoflavones

• Released by Legume roots• Signal Rhizobia in the soil that a Legume is

present and ready to nodulate• Soybean releases Genistein, an isoflavone

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Flavones and IsoflavonesFlavones and Isoflavones

Flavonoids in

Root tip and

Thickened root

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Nod FactorsNod Factors

• Rhizobia in response to flavones release Nod factors

• Nod factors are sugars with specific signalling functions

• Nod factors signal to a legume that Rhizobia are present in the soil and ready to live in nodules

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Nod FactorsNod Factors

• Each Rhizobum species releases unique Nod factors• All Nod factors have a concerved backbone

• Difference is in the decorations (R1 R2 R3 R4 R5) and number (n) of sugar repeats

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Nod Factor ReceptorsNod Factor Receptors

• Each legume species has receptors specific to a unique Nod factor

• Therefore each legume species nodulates with its own Rhizobium

• Soybean -Bradyrhizobium japonicum

• Lotus – Mesorhizobium loti

• General – Rhizobium NGR234

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• University of Queensland

• Australian National University

• University of Melbourne

• University of Newcastle

CILRCILR

www.cilr.uq.edu.au

CILRCILR

ARC Centre of Excellence for Integrative Legume Research

Contact UsHQ: University of Queensland

Tel: 3365 3550Email: director.CILR@uq.edu.au

lisette.pregelj@uq.edu.au

Visit Our Website!www.cilr.uq.edu.au

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Picture and Diagram CreditsPicture and Diagram Credits

• Uli Mathesius • Mark Kinkema• Peter Gresshoff• Dana Hoffmann• Michael Sheahan • Sureeporn (Ning) Nontachaiyapoom• Paul Scott• Brett Ferguson