Rhizobacteria are root-colonizing bacteria that form symbiotic relationships with many plants. The name comes from the Greek rhiza, meaning root. Though parasitic varieties of rhizobacteria exist, the term usually refers to bacteria that form a relationship beneficial for both parties (mutualism). They are an important group of microorganisms used in Biofertilizer. Biofertilization accounts for approximately 65% of the nitrogen supply to crops worldwide. Rhizobacteria are often referred to as plant growth-promoting rhizobacteria, or PGPRs. The term PGPRs was first used by Joseph W. Kloepper in the late 1970s and has become commonly used in scientific literature. PGPRs have different relationships with different species of host plants. The two major classes of relationships are rhizospheric and Endophytic. Rhizospheric relationships consist of the PGPRs that colonize the surface of the root, or superficial intercellar spaces of the host plant, often forming Root nodules. The dominant species found in the rhizosphere is a microbe from the genus Azospirillum. Endophytic relationships involve the PGPRs residing and growing within the host plant in the apoplastic space.
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 molecules. 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. 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 rood nodules are able to provide these conditions. The rhizobacteria require oxygen to metabolize therefore oxygen is provided by a hemoglobin protein called Leghemoglobin which is produced within the nodules. Legumes are well known nitrogen fixing crops and have been utilized for centuries in crop rotation to maintain the health of the soil.
The symbiotic relationship between rhizobacteria and its host plant 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 plants 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.
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.
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 ensure efficient nitrogen fixation, and they have been employed in North American agriculture for over 100 years.
Studies conducted on sugar beet crops found that some root-colonizing bacteria were deleterious rhizobacteria (DRB). It was found that 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%.
Six strains of rhizobacteria have been identified as being DRB. The strains are in the genera Enterobacter, Klebsiella, Citrobacter, Flavobacterium, Achromobacter, Arthrobacter. Due to a large number of taxonomic species yet to be described complete characterization has not been possible as DRB are highly variable.
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%.
Rhizobacteria are also able to control plant diseases that are caused by other bacteria and fungi. Disease is suppressed through induced systematic resistance and through the production of anti-fungal 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 rhizobacteria populations within the rhizosphere to produce notable beneficial effects on the crop.
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