Plant growth-promoting rhizobacteria

Plant growth-promoting rhizobacteria (PGPR) are a group of free-living bacteria that colonize the rhizosphere and benefit the root growth. Bacteria of diverse genera were identified as PGPR of which Bacillus and Pseudomonas spp. are predominant. PGPR exert a direct effect on plant growth by production of phytohormones, solubilization of inorganic phosphates, increased iron nutrition through iron-chelating siderophores and volatile compounds that affect the plant signaling pathways. Additionally, by antibiosis, competition for space and nutrients, and induction of systemic resistance in plants against a broad-spectrum of root and foliar pathogens, PGPR reduce the populations of root pathogens and other deleterious microorganisms in the rhizosphere, thus benefiting the plant growth. Root colonization, influenced by a number of biotic and abiotic components, is a limiting factor for the success of PGPR. Diverse reporter genes and nucleic acid-based methods were developed to track the introduced PGPR in the rhizosphere, and also to determine their metabolic status, and their effect on the native rhizosphere microbial communities. Quality of the PGPR formulations, in terms of viability and efficacy, determines their large-scale adoption at the end-user level. We discuss the importance of PGPR in sustainable agriculture with special reference to the mechanisms involved in their action and factors affecting their efficacy, along with the possibilities for their improvement.