Rhizobia

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.α-proteobacteriaβ-proteobacteria 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. 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. Rhizobia are a paraphyletic group that fall into two classes of proteobacteria—the alphaproteobacteria and betaproteobacteria. As shown below, most belong to the order Rhizobiales, but several rhizobia occur in distinct bacterial orders of the proteobacteria. 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 (and might not really be a separate genus). 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. This is especially important when nitrogen fertilizer is not used, as in organic rotation schemes or some less-industrialized countries. Nitrogen is the most commonly deficient nutrient in many soils around the world and it is the most commonly supplied plant nutrient. Supply of nitrogen through fertilizers has severe environmental concerns. Rhizobia is 'the 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. Specific strains of rhizobia are required to make functional nodules on the roots able to fix the N2. Having this specific rhizobia present is beneficial to the legume, as the N2 fixation can increase crop yield. Inoculation with rhizobia tends to increase yield. Legume inoculation has been an agriculture practice for many years and has continuously improved over time. 12–20 million hectares of soybeans are inoculated annually. The technology to produce these inoculants are microbial fermenters. 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. Many poor countries have trouble with the introduction of new crops and reaching attainable yield. As they introduce new crops into their soils, these inoculants may foster legume growth and success in the area, therefore giving farmers more options for planting. Using these inoculants provide many other benefits as well such as not having to use nitrogen fertilizers. 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, and little to no nitrogen fertilizer is needed. Many poor farmers do not have access to fertilizers, so the sustainability of rhizobial inoculum is an important aspect in saving money for the essentials.