Different mechanisms were found to be involved in plant growth promotion, either by rhizosphere or endophytic bacteria. Plant growth regulators produced by some bacteria are signal molecules acting as chemical messengers and play a fundamental role as growth and development regulators in the plant. Five of the six pathways for auxin biosynthesis in bacteria rely on tryptophan as the main IAA precursor. On the other hand, phosphate solubilizing bacteria could convert insoluble phosphates into available forms for plant via the process of acidification, chelation, exchange reactions, and production of gluconic acid
PGPR has been known to improve plant root growth and nutrition