Rhizobia
Rhizobia are diazotrophic bacteria that fix nitrogen after becoming established inside the root nodules of legumes (Fabaceae).
[7] Although much of the nitrogen is removed when protein-rich grain or hay is harvested, significant amounts can remain in the soil for future crops.
Cheater strains may hoard plant resources such as polyhydroxybutyrate for the benefit of their own reproduction without fixing an appreciable amount of nitrogen.
[17][18] These flavonoids then promote the DNA binding activity of NodD, which belongs to the LysR family of transcriptional regulators and triggers the secretion of nod factors after the bacteria have entered the root hair.
[20] Inside the nodule, the bacteria differentiate morphologically into bacteroids and fix atmospheric nitrogen into ammonium using the enzyme nitrogenase.
In response to underperforming rhizobia, legume hosts can respond by imposing sanctions of varying severity to their nodules.
There is evidence for sanctions in soybean plants, which reduce rhizobium reproduction (perhaps by limiting oxygen supply) in nodules that fix less nitrogen.
[26] Likewise, wild lupine plants allocate fewer resources to nodules containing less-beneficial rhizobia, limiting rhizobial reproduction inside.
[30][31] Although evolution tends to swing toward one species taking advantage of another in the form of noncooperation in the selfish-gene model, management of such symbiosis allows for the continuation of cooperation.
For example, the plant recognition gene SYMRK (symbiosis receptor-like kinase) is involved in the perception of both the rhizobial Nod factors as well as the endomycorrhizal Myc-LCOs.
[36] The shared similar processes would have greatly facilitated the evolution of rhizobial symbiosis because not all the symbiotic mechanisms would have needed to develop.
Instead, the rhizobia simply needed to evolve mechanisms to take advantage of the symbiotic signaling processes already in place from endomycorrhizal symbiosis.
[43] The legume-rhizobia mutualism is context dependent; the benefits provided by rhizobia are lessened or absent under unfavorable environmental conditions.
[44] Perturbations can alter the balance of symbiotic relationships between species as reduced benefits provided can lead to antagonistic behavior, such as parasitism.
[44] For example, nutrient deposition has led to the emergence of less productive strains of rhizobia[46] and increased ambient temperatures have legumes reducing investment in the resource mutualism.
This increases photosynthesis costs, further destabilizing the legume-rhizobia mutualism as the legume suffers fitness consequences and is unable to provide benefits to rhizobia.
[47] Many other species of bacteria are able to fix nitrogen (diazotrophs), but few are able to associate intimately with plants and colonize specific structures like legume nodules.
[48][49] Additionally, loosely associated plant bacteria, termed endophytes, have been reported to fix nitrogen in planta.
[50] These bacteria colonize the intercellular spaces of leaves, stems, and roots in plants [51] but do not form specialized structures like rhizobia and Frankia.
