Plant – Microbe Interaction

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2

Symbiosis

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Two/ more organisms living together

Temporarily/ Long time

At least one organism benefits from the association

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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

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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

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Nodule formation

Rhizobia naturally colonize the rhizosphere, metabolizing organic compounds secreted byroots. Signals released from a compatible host trigger a chain of events that lead to theinvasion of plant root cells by rhizobia

Successful signaling leads to cortical cell divisions and the development of a new rootorgan, the nodule

Penetration of the root tissue can occur by two different strategiesIn root hair-independent (intercellular) entry (a), disruption of the epidermal cell layerallows rhizobia to enter the root. Rhizobia multiply within these infection pockets andeventually invade plant cells. Note that in some (but not all) cases, Nod factors arerequired for this colonization strategy

This contrasts with the classic model of root hair invasion (b). In this model, flavonoidsreleased by legume roots trigger the synthesis of rhizobial Nod factors, which induce roothair curling, bacterial penetration at the centre of the infection pocket and division ofcortical 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 spacessurrounds the central tissue of the nodule

To pass through this layer, O2 must diffuse through the cellcontents and not through intercellular spaces

Oxygen diffusion coefficient of water lower than air

Very little oxygen reaches the centre of the nodule

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