PLANT HORMONES SYNTHESIZED BY MICROORGANISMS AND THEIR ROLE IN BIOFERTILIZER-A REVIEW ARTICLE.

Plant growth promoting rhizobacteria (PGPR) are the soil bacteria inhabiting around/on the root surface and are directly or indirectly involved in promoting plant growth and development via production and secretion of various regulatory chemicals in the vicinity of rhizosphere Various species of bacteria like Pseudomonas, Azospirillum, Azotobacter, Klebsiella, Enterobacter, Alcaligenes, Arthrobacter, Burkholderia, Bacillus and Serratia have been reported to enhance the plant growth directly by either assisting in resource acquisition (nitrogen, phosphorus and essential minerals) or modulating plant hormone levels, or indirectly by decreasing the inhibitory effects of various pathogens in the forms of biocontrol agents. The ability to synthesize growth stimulating phytohormones by numerous prokaryotic and eukaryotic microorganisms including numerous soil bacteria and fungi is reviewed, with emphasis on their effect on plant physiology and development. A phytohormone is an organic substance synthesized in defined organs of the plant that can be translocated to other sites, where it triggers specific biochemical, physiological and morphological responses. The commonly recognized classes of phytohormones, regarded as the ?classical five?, are: the auxins, gibberellins, cytokinins, abscisic acid and ethylene. Several PGPR are reported to produce IAA, gibberellic acid and cytokinins in the rhizospheric soil and thereby play a significant role in increasing the root surface area and number of root tips in many plants. Plant hormones contribute to the coordination of diverse physiological process in plants, including the regulation of quiescence and seed germination, root formation, fluorescence, branching, tillering, and fruit ripening. They increase plant resistance to environmental factors and induce or suppress the expression of genes and the synthesis of enzymes, pigments and metabolites (Arsad and Frankenberger, 1991; Kulaeva and Kuznetsov, 2002).