Plant Microbe Symbiosis: Nitrogen Fixation

Nitrogen fixation is crucial for Australian agriculture, contributing significantly to the economy.
- Estimated at $2,000,000,000$ annually, but current value is likely much higher.
Australian farming systems rely heavily on nitrogen fixation compared to other countries.
Reliance on nitrogen fixation is decreasing due to increased use of nitrogen fertilizers.
Pasture legumes contribute more to nitrogen fixation than crops due to larger cultivated areas.
Area under legumes is declining while area under cereals is increasing due to market forces.
- Leads to greater dependence on fertilizer nitrogen.

All nitrogen used in agriculture originates from the fixation of molecular nitrogen from the air.
Nitrogen fixation can occur:
- Biologically: E.g., rhizobial symbiosis.
- Industrially: High temperatures and pressures, less efficient than biological processes.

Symbiosis: Both partners benefit, but there are also costs.
Plants provide bacteria with carbon, and bacteria provide plants with nitrogen in the form of ammonium ( $NH_4$ ).
- Molecular nitrogen ( $N_2$ ) is converted to ammonium by bacteria.
If benefits don't outweigh costs, it's parasitism, not symbiosis.
Example: Field pea inoculation with rhizobia shows significant growth difference compared to no inoculation.

Specific signaling between plants and rhizobia is required for colonization.
Both roots and bacteria undergo changes for symbiosis to occur.
Plants provide carbon, and bacteria provide fixed nitrogen as ammonium ( $NH_4$ ).
Recognition is key for nodulation to start.
- Mutual recognition of hosts and bacteria is essential.

Rhizobia occur naturally in soils, but numbers are small.
Match between legume and available rhizobia may not be optimal.
Commercial inoculants for specific crops are important (e.g., Lucerne and Medicago).
Inoculation is vital because relying on naturally occurring rhizobia is often insufficient.
Inoculation should occur annually to boost rhizobia numbers.
Crop rotation means the match between rhizobia and the previous legume crop may not apply.
Match between legume crop and specific rhizobia is important for effective nodulation.

Recognition involves complex signaling.
Flavonoids:
- Organic compounds exuded by host legumes.
- Specific flavonoids for specific legume species.
- Rhizobia recognize specific flavonoids.
Flavonoids induce node gene transcription and production of node factors.
Node factors are released into the soil and recognized by the host.
Morphological and biochemical changes enable nodulation.
Recognition occurs at two points:
- Specific flavonoid-rhizobia interaction.
- Specific node factors-legume interaction.

Specificity is crucial in symbiotic relationships.
Example: Casuarinas and Frankia bacteria.
Not all Frankia species can effectively infect and produce nodules in all Casuarina species.
A match is needed for effective inoculation and nitrogen fixation.
Rhizobia with Fabaceae (legume family) and Frankia with Casuarina demonstrate specificity.

Flavonoids start the process, and node genes/factors initiate nodulation.
Root hair curls around and encapsulates bacteria.
Plants enclose bacteria in a peribacteroid membrane.
- Plants recognize bacteria as foreign objects, triggering immune responses.
- The peribacteroid membrane has transporters for ammonium ( $NH_4$ ).
Nodules tend to concentrate at the crown (stem-root junction).

Rhizobia enter root hair through infection thread.
Plants form peribacteroid membranes.
- Isolate bacteria.
- Control ammonium ( $NH_4$ ) transport.
Each bacterium represents a rhizobium cell changed during colonization.
Bacteroids produce nitrogenase.
- Enzyme that fixes atmospheric molecular nitrogen into ammonium.
- Works under very low oxygen conditions.
Molybdenum and iron are structural components of nitrogenase.

Nitrogenase requires a very low oxygen environment to function.
Plant tissues are typically oxygenated.
The nodule has a cortex containing bacteroids.
Bacteroids need oxygen for metabolic processes.
The nodule has a barrier that limits oxygen diffusion to the central fixation zone.
Oxygen comes from plant tissues and soil.
Barriers minimize oxygen diffusion to the central part of the nodule.
Less than 50 nanometers of oxygen is needed for nitrogenase to work effectively.

Symbiotic relationships can turn parasitic.
Checkpoint:
- Plants recognize node factors from bacteria.
- If not recognized, colonization stops.
Plants can develop oxygen barriers.
- Starve bacteria of oxygen if they become parasitic.

Nitrogen fixation is vital, especially in Australia.
- Dependence is decreasing but still important.
- Nitrogen fertilizers are expensive.
Australian agriculture relies on nitrogen fixation more than other systems.
Symbiotic relationships involve costs:
- Plants give carbohydrates to microorganisms.
- Microorganisms give nitrogen to plants.
Flavonoids are chemical signals triggering node genes.
Very low oxygen is needed for nitrogenase.