Rhizobia
Rhizobia are
Rhizobia are a "group of soil bacteria that infect the roots of legumes to form
History
The first known species of rhizobia, Rhizobium leguminosarum, was identified in 1889, and all further species were initially placed in the Rhizobium genus. Most research has been done on crop and forage legumes such as clover, alfalfa, beans, peas, and soybeans; more research is being done on North American legumes.[citation needed]
Taxonomy [a]
Rhizobia are a
These groups include a variety of non-symbiotic bacteria. For instance, the plant pathogen Agrobacterium is a closer relative of Rhizobium than the Bradyrhizobium that nodulate soybean.[7]
Importance in agriculture
Although much of the nitrogen is removed when
Specific strains of rhizobia are required to make functional nodules on the roots able to fix the N2.[9] Having this specific rhizobia present is beneficial to the legume, as the N2 fixation can increase crop yield.[10] Inoculation with rhizobia tends to increase yield.[11]
Legume inoculation has been an agricultural practice for many years and has continuously improved over time.[10] 12–20 million hectares of soybeans are inoculated annually. An ideal inoculant includes some of the following aspects; maximum efficacy, ease of use, compatibility, high rhizobial concentration, long shelf-life, usefulness under varying field conditions, and survivability.[10][12][13]
These inoculants may foster success in legume cultivation.[14] As a result of the nodulation process, after the harvest of the crop, there are higher levels of soil nitrate, which can then be used by the next crop.
Symbiotic relationship
Rhizobia are unique in that they are the only nitrogen-fixing bacteria living in a
Nature of the mutualism
The legume–rhizobium
Infection and signal exchange
The formation of the symbiotic relationship involves a signal exchange between both partners that leads to mutual recognition and the development of symbiotic structures. The most well understood mechanism for the establishment of this symbiosis is through intracellular infection. Rhizobia are free living in the soil until they are able to sense
Inside the nodule, the bacteria differentiate morphologically into bacteroids and fix atmospheric nitrogen into ammonium using the enzyme nitrogenase. Ammonium is then converted into amino acids like glutamine and asparagine before it is exported to the plant.[16] In return, the plant supplies the bacteria with carbohydrates in the form of organic acids.[16] The plant also provides the bacteroid oxygen for cellular respiration, tightly bound by leghaemoglobins, plant proteins similar to human hemoglobins. This process keeps the nodule oxygen poor in order to prevent the inhibition of nitrogenase activity.[16]
Recently, a Bradyrhizobium strain was discovered to form nodules in Aeschynomene without producing nod factors, suggesting the existence of alternative communication signals other than nod factors, possibly involving the secretion of the plant hormone cytokinin.[16][20]
It has been observed that root nodules can be formed spontaneously in Medicago without the presence of rhizobia.[21] This implies that the development of the nodule is controlled entirely by the plant and simply triggered by the secretion of nod factors.
Evolutionary hypotheses
The sanctions hypothesis
There are two main hypotheses for the mechanism that maintains legume-rhizobium symbiosis (though both may occur in nature). The sanctions hypothesis theorizes that legumes cannot recognize the more parasitic or less nitrogen fixing rhizobia and must counter the parasitism by post-infection legume sanctions. In response to underperforming rhizobia, legume hosts can respond by imposing sanctions of varying severity to their nodules.[22] These sanctions include, but are not limited to, reduction of nodule growth, early nodule death, decreased carbon supply to nodules, or reduced oxygen supply to nodules that fix less nitrogen. Within a nodule, some of the bacteria differentiate into nitrogen fixing bacteroids, which have been found to be unable to reproduce.[23] Therefore, with the development of a symbiotic relationship, if the host sanctions hypothesis is correct, the host sanctions must act toward whole nodules rather than individual bacteria because individual targeting sanctions would prevent any reproducing rhizobia from proliferating over time. This ability to reinforce a mutual relationship with host sanctions pushes the relationship toward mutualism rather than parasitism and is likely a contributing factor to why the symbiosis exists.
However, other studies have found no evidence of plant sanctions.[24]
The partner choice hypothesis
The partner choice hypothesis proposes that the plant uses prenodulation signals from the rhizobia to decide whether to allow nodulation, and chooses only noncheating rhizobia. There is evidence for sanctions in soybean plants, which reduce rhizobium reproduction (perhaps by limiting oxygen supply) in nodules that fix less nitrogen.[25] Likewise, wild lupine plants allocate fewer resources to nodules containing less-beneficial rhizobia, limiting rhizobial reproduction inside.[26] This is consistent with the definition of sanctions, although called "partner choice" by the authors. Some studies support the partner choice hypothesis.[27] While both mechanisms no doubt contribute significantly to maintaining rhizobial cooperation, they do not in themselves fully explain the persistence of mutualism. 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.[28]
Evolutionary history
The symbiosis between nitrogen fixing rhizobia and the legume family has emerged and evolved over the past 66 million years.[29][30] 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.[31] When the relative fitness of both species is increased, natural selection will favor symbiosis.
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
Endomycorrhizal symbiosis can provide many insights into rhizobia symbiosis because recent genetic studies have suggested that rhizobia co-opted the signaling pathways from the more ancient endomycorrhizal symbiosis.[33] Bacteria secrete Nod factors and endomycorrhizae secrete Myc-LCOs. Upon recognition of the Nod factor/Myc-LCO, the plant proceeds to induce a variety of intracellular responses to prepare for the symbiosis.[34]
It is likely that rhizobia co-opted the features already in place for endomycorrhizal symbiosis because there are many shared or similar genes involved in the two processes. 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.[35] 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.
Other diazotrophs
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. Bacteria that do associate with plants include the actinomycete, Frankia, which form symbiotic root nodules in actinorhizal plants, although these bacteria have a much broader host range, implying the association is less specific than in legumes.[16] Additionally, several cyanobacteria like Nostoc are associated with aquatic ferns, Cycas, and Gunneras, although they do not form nodules.[36][37]
Additionally, loosely associated plant bacteria, termed endophytes, have been reported to fix nitrogen in planta.[38] These bacteria colonize the intercellular spaces of leaves, stems, and roots in plants [39] but do not form specialized structures like rhizobia and Frankia. Diazotrophic bacterial endophytes have very broad host ranges, in some cases colonizing both monocots and dicots.[40]
Note
- ^ As with many bacterium classifications, taxonomy work is still in progress as new genetic data and discoveries re-shuffle the existing phylogenetic tree
References
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- ^ a b Herridge, David (2013). "Rhizobial Inoculants". GRDC.
- ^ "Current taxonomy of rhizobia". Archived from the original on 2013-06-04. Retrieved 2013-12-02.
- ^ Weir, Bevan (2016). "The current taxonomy of rhizobia". Retrieved 2023-11-18.
- ^ "Bacteria confused with rhizobia, including Agrobacterium taxonomy". Archived from the original on 2013-12-03. Retrieved 2013-12-02.
- ^ "Taxonomy of legume nodule bacteria (rhizobia) and agrobacteria". Archived from the original on 2018-10-17. Retrieved 2013-12-02.
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- ^ "What is Rhizobia". Archived from the original on 2012-07-20. Retrieved 2008-07-01.
- ^ Rachaputi, Rao; Halpin, Neil; Seymour, Nikki; Bell, Mike. "rhizobium inoculation" (PDF). GRDC. Archived (PDF) from the original on 2014-11-29. Retrieved 2015-04-23.
- ^ a b c Catroux, Gerard; Hartmann, Alain; Revillin, Cecile (2001). Trends in rhizobium inoculant production and use. Netherlands: Kluwer Academic Publishers. pp. 21–30.
- ^ Purcell, Larry C.; Salmeron, Montserrat; Ashlock, Lanny (2013). "Chapter 5" (PDF). Arkansas Soybean Production Handbook - MP197. Little Rock, AR: University of Arkansas Cooperative Extension Service. p. 5. Archived from the original on 4 March 2016. Retrieved 21 February 2016.
- ^ Shrestha, R; Neupane, RK; Adhikari, NP. "Status and Future Prospects of Pulses in Nepal" (PDF). Government of Nepal. Archived (PDF) from the original on 2015-07-06. Retrieved 2015-04-23.
- ^ Bennett, J. Michael; Hicks, Dale R.; Naeve, Seth L.; Bush Bennett, Nancy (2014). The Minnesota Soybean Field Book (PDF). St Paul, MN: University of Minnesota Extension. p. 79. Archived from the original (PDF) on 30 September 2013. Retrieved 21 February 2016.
- ^ Stephens, J.H.G; Rask, H.M (2000). Inoculant production and formulation. Saskatoon: MicroBio RhizoGen Corporation. pp. 249–258.
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Further reading
- Jones, KM; Kobayashi, H; Davies, BW; Taga, ME; Walker, GC; et al. (2007). "How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model". Nature Reviews Microbiology. 5 (8): 619–33. PMID 17632573.