ARC Centre of Excellence

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ARC Centre of Excellence for Integrative Legume
Research How Legumes Make Nodules Lisette
Pregelj Education and Outreach Manager
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Legume 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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Vascular Bundles
Nodule Cortex
Sclerenchyma
Rhizobium Filled Cells gt25,000 per cell
Nodule Cross-Section
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Nodule 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 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 root
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1. Rhizobia attracted to root

• Legume roots exude flavonoids
• (Soybean exudes the isoflavone genistein)
• Rhizobia are attracted to flavonoids

Bacteria turn blue when a reporter gene is
switched on by plant exudates (flavonoids)
Petri dish contains a bacterial lawn
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2. Rhizobia attach to root hairs
Rhizobia
Root Hair
10 mm
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2. Rhizobia attach to root hairs
Deformed root hairs
Curled root hair
Vascular bundle
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3. Root hair curling
50 mm
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4. Rhizobia infect root hair
Root Hair Curling
Infection Thread
Root Hair
Rhizobia
Infection Pocket
Legume Root
Vascular Bundle
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4. Rhizobia infect root hair
50 mm
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5. Root cortical cells divide
100 mm
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5. Root cortical cells divide
100 mm
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6. Rhizobia invade cortical cells
50 mm
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7. The nodule grows
2-4 mm
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8. Fully functional nodule
root
nodule
Rhizobia (green marker)
100 mm
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Nodulation Timeline
Roots
1-2 days post inoculation (dpi)
Inoculation (day 0)
4 dpi
4-6 dpi
10 dpi
3 weeks pi
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Chemical Signals

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

Flavones
Nod Factors
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Flavones 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 Isoflavones
Flavonoids in Root tip and Thickened root
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Nod 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 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 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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CILR

• University of Queensland
• Australian National University
• University of Melbourne
• University of Newcastle

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CILR

• ARC Centre of Excellence for Integrative Legume
  Research
• Contact Us
• HQ University of Queensland
• Tel [phone]
• Email director.CILR_at_uq.edu.au
• lisette.pregelj_at_uq.edu.au
• Visit Our Website!
• www.cilr.uq.edu.au

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Picture and Diagram Credits

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