This presentation describes about the nodule formation and nitrogen fixation

1. Plant – Microbe Interaction
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2. 2

3. Symbiosis
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 Two/ more organisms living together
 Temporarily/ Long time
 At least one organism benefits from the association

4. 4
Mutualistic Relationship
1. Legumes – Bacteria
2. Algae – Fungus (Lichen)
3. Mycorrhiza – Plant
4. Endophytes - Plants

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1. Legumes – Bacteria
 Importance of nitrogen fixation
N2
 Nitrogen fixation
NH3
NH3 Nitrites Nitrates

6. 6
Nitrogen fixing microorganisms
1. Bacteria - Rhizobium and Bradyrhizobium (Rhizobia)
2. Actinomycete (filamentous bacteria)
3. Cyanobacteria

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 Rhizobium
 Nodules
 Bacteriods
 Nature of nodule environment

8. 8
Nodule formation

9. Rhizobia naturally colonize the rhizosphere, metabolizing organic compounds secreted by
roots. Signals released from a compatible host trigger a chain of events that lead to the
invasion of plant root cells by rhizobia
Successful signaling leads to cortical cell divisions and the development of a new root
organ, the nodule
Penetration of the root tissue can occur by two different strategies
In root hair-independent (intercellular) entry (a), disruption of the epidermal cell layer
allows rhizobia to enter the root. Rhizobia multiply within these infection pockets and
eventually invade plant cells. Note that in some (but not all) cases, Nod factors are
required for this colonization strategy
This contrasts with the classic model of root hair invasion (b). In this model, flavonoids
released by legume roots trigger the synthesis of rhizobial Nod factors, which induce root
hair curling, bacterial penetration at the centre of the infection pocket and division of
cortical cells.
Infection threads extend through root hairs towards the cortical cells of the root.
Infection threads ramify in nodule primordia (which are formed by dividing cortical cells),
into which rhizobia are released
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Limiting factors of Nitrogen fixation
1. Acid soils
2. Nitrogen fertilizer
3. Phosphorus deficiency
4. Molybdenum deficiency
5. Water logged

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 A boundary layer - tightly packed cells with very few air spaces
surrounds the central tissue of the nodule
 To pass through this layer, O2 must diffuse through the cell
contents and not through intercellular spaces
 Oxygen diffusion coefficient of water lower than air
 Very little oxygen reaches the centre of the nodule

15. Many plants are infected by endophytic fungi that are not considered to be
pathogenic and do not produce any disease symptoms on their host during
at least part of their life cycle
The association between the endophyte and the plant is believed to be
mutualistic because the endophyte gains nutrients and protection from the
plant but at the same time it produces chemicals that protect the plant from
attack by animals and, perhaps, plant pathogens
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Endophytic fungi