Growth Promotion of Legumes by Inoculation of Rhizosphere Bacteria

Most plants grown in fields are colonized by diverse groups of rhizosphere bacteria that form beneficial or pathogenic relationships with their hosts. The root exudates encourage the development of beneficial bacterial communities in the root zone capable of producing secondary metabolites that improve plant growth and crop yield. These beneficial associations facilitate plant growth either by enhancing crop nutrition, releasing plant growth stimulating hormones, reducing damages caused by pathogens/pests by producing antibiotics, bacteriocins, siderophores, hydrolytic enzymes and other secondary metabolites or by improving resistance to environmental pollutants. Rhizosphere bacteria also supply biologically fixed nitrogen, solubilize bound phosphorus and may provide other nutrients, such as, potassium, iron and sulfur to plants. These beneficial associations hence, reduce the requirement of chemical fertilizers used for crop productivity. Moreover, some rhizobacteria are used to relieve the toxicity of metals and organic toxicants, either through stimulation of microbial degradation of pollutants in the rhizosphere, or by uptake of pollutants/toxicants by the plant. The inoculation of the legumes with such rhizosphere bacteria has often been found to increase symbiotic properties, plant biomass and yields under green house or field conditions. Tremendous progress has been made recently in characterizing the process of rhizosphere colonization, identification and cloning of bacterial genes involved in nitrogen fixation, phosphorus solubilization, production of plant growth regulators and in suppression of plant diseases. The interactions/relationships of rhizosphere bacteria with their hosts and performance of wild-type and genetically manipulated beneficial bacterial populations are discussed for their efficient utilization in legume production under sustainable agriculture systems.