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.
