As plant grows, roots release large quantity of metabolites from living root hairs or fibrous root system. These metabolites act as chemical signals for Rhizobacteria to move to the root surface. And variety bacteria colonize in rhizosphere. Some of the bacteria that inhabit in this area that able to colonize very efficiently in roots or the rhizosphere soil as a bio-film and these bacteria promote the growth of plant. These bacteria are referred to as plant growth promoting rhizobacteria (PGPR). The phytohormones are very important in plant growth and development because they regulate numerous biological processes from cell division, elongation and differentiation to tropic responses, food development and senescence. Auxins induce cell division, cell elongation, rooting callus formation, flowering and so on. Naturally occurring substances with indole nucleus possessing growth-promoting activity are referred to as auxins; chemically it is Indole acetic acid. (IAA) Not only plants but also microorganisms can synthesize phytohormone like auxins and cytokinins. The ability to synthesize phytohormone is widely distributed among rhizobacteria. 80% of the bacteria isolated from plant rhizosphere produce IAA.
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