Free-Living PGPRs in Biotic Stress Management

Abstract

Plant growth-promoting rhizobacteria (PGPR) is a heterogeneous group of microorganisms found in the rhizosphere. They live in association with roots and stimulate the plant growth and/or reduce the incidence of plant disease. The term PGPR is used to describe soil bacteria that colonize the rhizosphere of plants, growing in, on, or around plant tissues that stimulate plant growth by several mechanisms. The PGPRs are involved in various biotic activities of the soil ecosystem to make it dynamic for nutrient turnover and sustainable crop production by affecting plant growth. Generally, PGPR promotes plant growth directly due to their ability for nutrient supply (nitrogen, phosphorus, potassium, and essential minerals) or modulating plant hormone levels or indirectly by decreasing the inhibitory effect of various pathogens on plant growth and development in the form of biocontrol agents, root colonizers, and environment protectors. PGPRs can protect plants from diseases by a wide variety of mechanisms like antibiosis, induction of systemic resistance, siderophore production, production of 1-aminocyclopropane-1-carboxylate deaminase (ACC), signal interference while quorum sensing (QS) and inhibition of biofilm formation, production of lytic enzymes, production of volatile organic compounds (VOCs), promoting beneficial plant–microbe symbioses by competition for nutrients and niches, interference with pathogen toxin production, etc. A particular PGPR may affect plant diseases by using any one, or more, of these mechanisms. Bacteria of diverse genera have been identified as PGPRs, of which Bacillus and Pseudomonas spp. are predominant and have been implied in biocontrol due to their effective competitive interactions with bacteria, fungi, oomycetes, protozoa, viruses, and nematodes attacking a variety of crops.