Rhizobia - Wikipedia
Rhizobia are symbiotic diazotrophs (prokaryotic organisms that carry out N2 fixation in legumes by bacterial symbionts such as Rhizobium etli. is important for maintaining the symbiotic relationship with legumes. In a symbiotic relationship with the soil bacteria known as 'rhizobia', legumes form nodules on their roots (or stems, see figure below) to 'fix' nitrogen into a form . The Rhizobium-legume (herb or tree) symbiosis is suggested to be the ideal solution to The element nitrogen, or “azote,” meaning “without life,” as Antonie Lavoisier . management of the symbiotic relationship between plants and bacteria.
In response to underperforming rhizobia, legume hosts can respond by imposing sanctions of varying severity to their nodules. Within a nodule, some of the bacteria differentiate into nitrogen fixing bacteroids, which have been found to be unable to reproduce.
Legume-Rhizobium - microbewiki
This ability to reinforce a mutual relationship with host sanctions pushes the relationship toward a mutualism rather than a parasitism and is likely a contributing factor to why the symbiosis exists.
However, other studies have found no evidence of plant sanctions. There is evidence for sanctions in soybean plants, which reduce rhizobium reproduction perhaps by limiting oxygen supply in nodules that fix less nitrogen. Some studies support the partner choice hypothesis. The partner choice hypothesis is not exclusive from the host sanctions hypothesis, as it is apparent that both of them are prevalent in the symbiotic relationship.
To understand the evolutionary history of this symbiosis, it is helpful to compare the rhizobia-legume symbiosis to a more ancient symbiotic relationship, such as that between endomycorrhizae fungi and land plants, which dates back to almost million years ago. Instead the rhizobia simply needed to evolve mechanisms to take advantage of the symbiotic signaling processes already in place from endomycorrhizal symbiosis. Other diazotrophs[ edit ] Many other species of bacteria are able to fix nitrogen diazotrophsbut few are able to associate intimately with plants and colonize specific structures like legume nodules.
Bacteria that do associate with plants include the actinobacteria Frankiawhich form symbiotic root nodules in actinorhizal plantsalthough these bacteria have a much broader host range implying the association is less specific than in legumes. Diazotrophic bacterial endophytes have very broad host ranges, in some cases colonizing both monocots and dicots.
Microbiology and Molecular Biology Reviews: Because legumes form nodules with rhizobia, they have high levels of nitrogen available to them. Their abundance of nitrogen is beneficial not only to the legumes themselves, but also to the plants around them.
There are other sources of nitrogen in the soil, but are not always provided at the levels required by plants, making the symbiotic relationship between legumes and rhizobia highly beneficial.
In return for the fixed nitrogen that they provide, the rhizobia are provided shelter inside of the plant's nodules and some of the carbon substrates and micronutrients that they need to generate energy and key metabolites for the cellular processes that sustain life Sprent, Nodulation and nitrogen fixation by rhizobia is not exclusive to legumes; rhizobia form root nodules on Parasponis Miq. The picture on the right shows "stem" nodules on Sesbania rostrata - stem nodules are produced from lateral or adventitious roots and are typically found in those few water-tolerant legume groups Neptunia, Sesbania that prefer wet or water-logged soils Goormachtig et al.
Plants, bacteria, animals, and manmade and natural phenomena all play a role in the nitrogen cycle. The plant supplies the rhizobia with energy in the form of amino acids and the rhizobia fix nitrogen from the atmosphere for plant uptake. The reduction of atmospheric dinitrogen into ammonia is the second most important biological process on earth after photosynthesis Sylvia, The actual process of dinitrogen fixation can only be carried out by diazotrophs that contain the enzyme dinitrogenase.
Nitrogen is the most critical nutrient needed to support plant growth. These include electrical N2 fixation by lightning where oxides of N come to ground with rain, the Haber-Bosch process in industrial fertilizer production, and biological N2 fixation in legumes by bacterial symbionts such as Rhizobium etli. Biological fixation of nitrogen was the leading form of annual nitrogen input until the last decade of the 20th century Russelle, It is gaining attention once again as sustainability becomes a central focus to feed a world population of over 7 billion people.
Nitrogen Fixation and the Nitrogen Cycle
Generally, legumes gain extra nitrogen for plant growth to offset the loss of photosynthate in this mutualistic association. The rhizobia invade plant roots and induce a nodule in which the bacteria reduce atmospheric nitrogen to ammonia and supply the plant with nitrogenous compounds Young, The plant gains the ability to grow in nitrogen poor soils, and the bacteria gain a protected niche where they multiply and eventually escape back into the surrounding soil when the nodule senesces Young, Because biological N2 fixation requires such a large amount of energy, it is important to understand the energy transfer in the process.
The stepwise reaction of energy transfer is characterized by the following steps: Each yield requires 2 e- for a total of 6 electrons needed. Electrons come in via Fe protein and are donated by ferredoxin. Nodulation The actual process of nodulation is a very coordinated effort between the legume and the Rhizobium bacteria in the soil.
Infection typically occurs in root hairs of legumes. Many rhizobia and host plants are highly specific and legumes can either attract rhizobia to root hairs directly by excretory compounds or by induction of nod gene activity in the bacteria.
Molecular determinants of host specificity during nitrogen-fixing symbiosis. Communication between legume and Rhizobium 1. Flavonoids are released by the host root. The flavonoid is at the highest concentration at the root and interacts with the product of bacterial nodD gene.
The nodD gene produces the protein, nodD, which is the sensor that recognizes chemicals excreted by host plant roots Russelle, Rhizobia colonize the soil in the vicinity of the root hair in response to the flavonoids. This process is autoregulated where favonoids stimulate Nod factor production, which stimulates flavonoid secretion Russelle,