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**Rhizobia Overview:**
– History:
– First known species identified in 1889.
– Initial grouping under Rhizobium genus.
– Focus on crop and forage legumes like clover, alfalfa, beans, peas, and soybeans.
– Taxonomy:
– Paraphyletic group within Pseudomonadota classes.
– Majority in order Hyphomicrobiales.
– Includes non-symbiotic bacteria.
– Importance in Agriculture:
Nitrogen fixation in legumes.
– Essential for soil fertility and crop yield.
– Beneficial agricultural practice.

**Symbiotic Relationship:**
– Unique nitrogen-fixing bacteria in symbiosis with legumes.
– Mutualism with common legumes like peas, beans, clover, and soy.
– Exchange of nutrients for organic compounds.
– Challenges with cheater strains and resource exploitation.

**Evolutionary Aspects:**
Symbiosis evolution over 66 million years.
– Comparison to ancient endomycorrhizal symbiosis.
– Genetic pathways shared with endomycorrhizae.
– Evolutionary benefits from pre-existing signaling processes.

**Other Diazotrophs:**
– Comparison to other nitrogen-fixing bacteria.
– Actinomycete Frankia and cyanobacteria associations.
– Diazotrophic bacterial endophytes’ role in nitrogen fixation.

**Research and Studies:**
– Molecular genetics of plant-microbe interactions.
– Partner choice mechanisms in legume-rhizobium interaction.
– Survival and reproduction in starving rhizobia.
– Intracellular accommodation of microbes by plants.
– Bacteria in agrobiology and plant growth responses.

Rhizobia (Wikipedia)

Rhizobia are diazotrophic bacteria that fix nitrogen after becoming established inside the root nodules of legumes (Fabaceae). To express genes for nitrogen fixation, rhizobia require a plant host; they cannot independently fix nitrogen. In general, they are gram negative, motile, non-sporulating rods.

Root nodules, each containing billions of Rhizobiaceae bacteria

Rhizobia are a "group of soil bacteria that infect the roots of legumes to form root nodules". Rhizobia are found in the soil and, after infection, produce nodules in the legume where they fix nitrogen gas (N2) from the atmosphere, turning it into a more readily useful form of nitrogen. From here, the nitrogen is exported from the nodules and used for growth in the legume. Once the legume dies, the nodule breaks down and releases the rhizobia back into the soil, where they can live individually or reinfect a new legume host.

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