Abstract
Root-colonizing bacteria (rhizobacteria) that exert beneficial effects on plant development via direct or indirect mechanisms have been defined as plant growth-promoting rhizobacteria (PGPR). These natural bioresources provide essential nutrients to plants and improve growth, competitiveness, and responses to external stress factors by an array of mechanisms under different agro-ecosystems. The PGPR facilitate plant growth by synthesizing or altering: the concentration of phytohormones: asymbiotic and symbiotic N-2 fixation, antagonism against phytopathogenic microorganisms by producing siderophores, antibiotics and cyanide; solubilization of mineral phosphates and other nutrients; and by synthesizing 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which helps to reduce the inhibitory effects of ethylene on plants. And hence, use of such PGPR may be a viable alternative to chemical fertilizers for increasing the productivity of various crops. However, despite their proven ability of growth promotion, PGPR have yet to fulfil their promise and potential as commercial bioinoculants. Understanding functional diversity of PGPR is vital for low-input sustainable production. Recent progress focusing on the principles and mechanisms of action of PGPR is reviewed and discussed.