Plant growth promotion is a complex phenomenon rarely attributable to a single mechanism as most PGP microbes influence plant growth through multiple mechanisms, and in some cases their PGP effect may only occur through interactions with other microbes. Any microbial agent added to the rhizosphere has to interact not only with the plant but also with any other organism sharing the same ecological niche. To be successful the inoculant has to maintain a critical population mass in the soil and have the right conditions to exert its beneficial activity. Fig. 1 presents the
steps from PGP isolation to commercialization.