The rhizobia-legume symbioses exhibit variation in symbiotic performance as measured by plant yield, nodulation and nitrogenase activity. The previous studies have demonstrated that variation in symbiotic performance is dependent on both rhizobial strains and plant species (or cultivars). It has also been found that symbiotic variation is connected to some characteristics of rhizobia including serological and morphological phenotypes, tolerance to stresses, host range, plasmid profile as well as some cryptic plasmids. Rhizobia are important members of plant-growth-promoting rhizobacteria (PGPR) that exert the positive effects on plant growth via direct and indirect mechanisms. Plant-growth-promoting (PGP) activities include the production of phytohormones, siderophores and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase as well as the solubilization of inorganic phosphate. Many rhizobial strains produce phytohormones such as indole-3-acetic acid (IAA) with auxin activity and gibberellins (GAs). The role of IAA produced by rhizobia on plant-growth promotion has been demonstrated. GAs are involved in the formation of infection pocket and infection thread. Siderophores are iron chelators that chemically bind and solubilize iron. The production of siderophores is connected to the decrease or prevention of deleterious effect of pathogenic microorganisms. Phosphate-solubilizing activity of rhizobia can increase phosphorus availability to plants. The plant-growth promotion by rhizobia with phosphate-solubilizing activity has been reported. The ACC-producing rhizobia promote nodulation and root elongation of their host plants. These PGP activities make rhizobia superior PGPR for legumes and non-legumes.
Keywords: 1-Aminocyclopropane-1-carboxylic acid (ACC) deaminase, Gibberellin (GA), Host range, Indole-3-acetic acid (IAA), Phosphate solubilization, Plantgrowth- promoting rhizobacteria (PGPR), Siderophore, Symbiotic effectiveness, Symbiotic variation, Rhizobia.