Bacteria living in vicinity of plant roots (rhizosphere) are called as Rhizo (root) bacteria. They live either in free living state or symbiotic association. Both beneficial and deleterious rhizobacteria are present in the plant rhizosphere. The microbial activity is highly intense and stimulated in rhizosphere. This is because rhizosphere is enriched with different types of root exudates (sugar, amino acids, fatty acids, esters etc) secreted by host plant. Rhizobacteria utilize these as a source of carbon and energy and in turn promote and protect growth and increase subsequent yield of the host plant. This interaction between rhizobacteria and plant is one of the well known examples of mutualistic associations in the soil.
Many different mechanisms are responsible for plant growth promotion like production of growth regulators, decomposition and nutrient cycling. These activities are more concentrated in the rhizosphere because plant roots provide structural support, water and nutrients to the plant and associated microbes. These rhizobacteria also maintain plant health and offers protection from pathogens; thus they have biocontrol activities like production of antagonistic metabolites and killing or suppression of phytopathogenic organisms. A single or more than one mechanism favors plant growth by rhizobacterium. The rhizobacteria promoting plant growth and with biocontrol property are termed as plant growth promoting rhizobacteria (PGPR). The increases in germination, seedling emergence, vigor, height, root system development and increased yield are some of the criteria that are the effects of PGPR. Various plant growth promoting properties of rhizobacteria are biological nitrogen fixation, solubilization of insoluble phosphate compounds, mineralization of organic phosphates, enhanced nutrient uptake and hence plant nutrition, Sequestering and provision of micronutrients like iron, calcium, zinc, copper and manganese in utilizable form, denitrification to reduce excess nitrate load in the soil, exopolysaccharide production to bind soil particle and to retain water potential and aeration, secretion of various plant growth regulating metabolites like auxins, cytokinins, gibberellins, abscisic acid, and ethylene.
Rhizobacterial antagonistic or biocontrol properties include production of production of toxic compounds, antibiotics, cyanides to suppress pathogen attack and prevent crop damage caused by major plant pathogens. Rhizobacteria also interact competitively with other soil micro fauna for nutrients and niche by production of secondary metabolites like antibiotics, chelators like siderophores, bacteriocins and eradicate them to establish and colonize in the plant rhizosphere. Rhizobacteria also decrease the usage of pesticides and fertilizers. Many rhizobacteria represents special traits to live under stress conditions exerted by nutrient limitation, drought, salinity, cold temperature or heavy metal pollution. Such rhizobacteria find immense environmental applications such as rhizoremediation. The important rhizobacterial genera showing potential for plant growth promotion and biocontrol are: Rhizobium, Bradyrhizobium, Acetobacter, Enterobacter, Azotobacter, Azospirillium, Bacillus, Proteus, Burkholderia, Serratia and Pseudomonas. These rhizobacteria have been essential components of the biofertilizers.
Another type of rhizobacteria is deleterious bacteria which do not promote plant growth. They do not colonize plant roots but opportunistic plant parasites as they infect plant at wounded sites. In addition to this they compete for nutrients present in the rhizosphere, reduce root growth and nutrient uptake and inhibit seed germination. But these deleterious properties have been utilized as a solution to weed problems. The agricultural losses due to weeds have been a great issue and application of Weedicides or herbicides was never sufficient to control weeds. Exploitation of deleterious rhizobacteria (Agrobacterium sp., Pseudomonas syringae and Flavobacterium sp.) and their incorporation into integrated weed management would always be ecofriendly and a perfect solution.
Rhizosphere soil environment is dynamic and very diverse. The search for novel rhizobacteria for their beneficial traits, their interactions with host plants, some unexplored properties and development of bioinocula formulations for sustainable agriculture would therefore be continued.
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