Rhizobacteria

Part of a series on
Microbiomes
Plant microbiomes Endosphere Phyllosphere Rhizosphere laimosphere root microbiome soil microbiome spermosphere
Marine microbiomes Seagrass Coral Cetacean Marine microbial symbiosis
Human microbiomes Human milk Fecal transplant Gut–brain axis Placental Salivary Uterine Necrobiome
Other microbiomes Mycobiome Phycosphere Built environment Drosophila gut
Microbiota Plant endophyte epiphyte rhizobacteria Human gut lung oral skin vaginal Marine Microbial community Initial acquisition Accessible carbohydrates Flora (microbiology)
Holobionts Plant holobiont Marine holobiont coral crab seagrass sponge rhodolith Hologenomics hologenome evolution
Viromes Bat Human Mangrove Viriome Virosphere
Related Biomass partitioning Dysbiosis Gnotobiosis Phytobiome Quorum sensing Biological dark matter Microbial population biology Microbial cooperation Metagenomics viral Metatranscriptomics Metabolomics Pan-genome Superorganism Symbiogenesis
Projects Human Microbiome Project Earth Microbiome Project
Category

Nitrogen fixation is one of the most beneficial processes performed by rhizobacteria. Nitrogen is a vital nutrient to plants and gaseous nitrogen (N₂) is not available to them due to the high energy required to break the triple bonds between the two atoms.[4] Rhizobacteria, through nitrogen fixation, are able to convert gaseous nitrogen (N₂) to ammonia (NH₃) making it an available nutrient to the host plant which can support and enhance plant growth. The host plant provides the bacteria with amino acids so they do not need to assimilate ammonia.[5] The amino acids are then shuttled back to the plant with newly fixed nitrogen. Nitrogenase is an enzyme involved in nitrogen fixation and requires anaerobic conditions. Membranes within root nodules are able to provide these conditions. The rhizobacteria require oxygen to metabolize, so oxygen is provided by a hemoglobin protein called leghemoglobin which is produced within the nodules.[4] Legumes are well-known nitrogen-fixing crops and have been used for centuries in crop rotation to maintain the health of the soil.

The symbiotic relationship between rhizobacteria and their host plants is not without costs. For the plant to be able to benefit from the added available nutrients provided by the rhizobacteria, it needs to provide a place and the proper conditions for the rhizobacteria to live. Creating and maintaining root nodules for rhizobacteria can cost between 12–25% of the plant's total photosynthetic output. Legumes are often able to colonize early successional environments due to the unavailability of nutrients. Once colonized, though, the rhizobacteria make the soil surrounding the plant more nutrient rich, which in turn can lead to competition with other plants. The symbiotic relationship, in short, can lead to increased competition.[4]

PGPRs increase the availability of nutrients through the solubilization of unavailable forms of nutrients and by the production of siderophores which aids in the facilitating of nutrient transport. Phosphorus, a limiting nutrient for plant growth, can be plentiful in soil, but is most commonly found in insoluble forms. Organic acids and phosphotases released by rhizobacteria found in plant rhizospheres facilitate the conversion of insoluble forms of phosphorus to plant-available forms such as H₂PO₄⁻. PGPR bacteria include Pseudomonas putida, Azospirillum fluorescens, and Azospirillum lipoferum and notable nitrogen-fixing bacteria associated with legumes includes Allorhizobium, Azorhizobium, Bradyrhizobium, and Rhizobium.[5]

Though microbial inoculants can be beneficial for crops, they are not widely used in industrial agriculture, as large-scale application techniques have yet to become economically viable. A notable exception is the use of rhizobial inoculants for legumes such as peas. Inoculation with PGPRs ensures efficient nitrogen fixation, and they have been employed in North American agriculture for over 100 years.

Wikiversity has learning resources about Plant growth promoting bacteria

Plant growth-promoting rhizobacteria (PGPR) were first defined by Kloepper and Schroth[6] to be soil bacteria that colonize the roots of plants following inoculation onto seed and that enhance plant growth.[7] The following are implicit in the colonization process: ability to survive inoculation onto seed, to multiply in the spermosphere (region surrounding the seed) in response to seed exudates, to attach to the root surface, and to colonize the developing root system.[8] The ineffectiveness of PGPR in the field has often been attributed to their inability to colonize plant roots.[3][9] A variety of bacterial traits and specific genes contribute to this process, but only a few have been identified. These include motility, chemotaxis to seed and root exudates, production of pili or fimbriae, production of specific cell surface components, ability to use specific components of root exudates, protein secretion, and quorum sensing. The generation of mutants altered in expression of these traits is aiding our understanding of the precise role each one plays in the colonization process.[10][11]

Progress in the identification of new, previously uncharacterized genes is being made using nonbiased screening strategies that rely on gene fusion technologies. These strategies employ reporter transposons[12] and in vitro expression technology (IVET)[13] to detect genes expressed during colonization.

Using molecular markers such as green fluorescent protein or fluorescent antibodies, it is possible to monitor the location of individual rhizobacteria on the root using confocal laser scanning microscopy.[3][14][15] This approach has also been combined with an rRNA-targeting probe to monitor the metabolic activity of a rhizobacterial strain in the rhizosphere and showed that bacteria located at the root tip were most active.[16]

Studies conducted on sugar beet crops found that some root-colonizing bacteria were deleterious rhizobacteria (DRB). Sugar beet seeds inoculated with DRB had reduced germination rates, root lesions, reduced root elongation, root distortions, increased fungi infection, and decreased plant growth. In one trial the sugar beet yield was reduced by 48%.[23]

Six strains of rhizobacteria have been identified as being DRB. The strains are in the genera Enterobacter, Klebsiella, Citrobacter, Flavobacterium, Achromobacter, and Arthrobacter. Due to a large number of taxonomic species yet to be described, complete characterization has not been possible as DRB are highly variable.[23]

The presence of PGPRs has proven to reduce and inhibit the colonization of DRB on sugar beet roots. Plots inoculated with PGPRs and DRBs had an increase in production of 39% while plots only treated with DRBs had a reduction in production of 30%.[23]

Rhizobacteria are also able to control plant diseases that are caused by other bacteria and fungi. Disease is suppressed through induced systemic resistance and through the production of antifungal metabolites. Pseudomonas biocontrol strains have been genetically modified to improve plant growth and improve the disease resistance of agricultural crops. In agriculture, inoculant bacteria are often applied to the seed coat of seeds prior to being sown. Inoculated seeds are more likely to establish large enough rhizobacterial populations within the rhizosphere to produce notable beneficial effects on the crop.[1]

They can also combat pathogenic microbes in cattle. Different forage species regulate their own rhizosphere to varying degrees and favouring various microbes. Kyselková et al 2015 find planting forage species known to encourage native rhizobacteria retards the spread within the soil of antibiotic resistance genes of cow faeces bacteria.[24][25]