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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
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CILRCILR
ARC Centre of Excellence for Integrative Legume Research
Contact UsHQ: University of Queensland
Tel: 3365 3550Email: [email protected]
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