Plant growth promotion mediated by bacterial rhizosphere colonizers

Abstract

Plant growth-promoting rhizobacteria (PGPR) represents a diverse subgroup of rhizosphere-colonizing bacteria. The host list of PGPR now includes barley, bean, canola (rapeseed), cotton, maize, groundnut, rice, vegetables, wheat, and woody species. In addition to increasing crop yields, different strains of PGPR can exert various effects on plants including biological control of soil-borne pathogens, promotion of legume nodulation by nitrogen-fixing rhizobia, and enhancement of seedling emergence rates. Reported mechanisms of action for PGPR have focussed on the indirect mechanisms of siderophore, antibiotic, or hydrogen cyanide production. Such indirect mechanisms reduce the population densities of deleterious microorganisms and thereby result in increased plant growth. Direct growth promotion by PGPR in the absence of deleterious microorganisms has been recently described.