Plant Growth-Promoting Rhizobacteria as Biostimulants in Sustainable Crop Production

Plant growth-promoting rhizobacteria (PGPR) are regarded as the biostimulants for plant growth and development. These bioagents are also used as the bioprotectants of plants to combat the biotic and abiotic stresses in plants. The diverse mechanisms of the PGPR for regulating plant nutrition and defending the stressful conditions of plant growth are the major triggering indicators for their growth-promoting feature. Bacteria from diverse taxonomic genera such as Pseudomonas, Bacillus, Paraburkholderia, Serratia, Enterobacter, Azospirillum, Rhizobium, Delftia and Azotobacter belong to the group of PGPR that show high potential as biofertilizers and biopesticides. Therefore, the PGPR have been applied for plant growth enhancement and combating plant diseases and abiotic stresses, simultaneously. Cycling, solubilization and enhancement of use efficiency of essential plant nutrient elements, and production of phytohormones (indole acetic acid, gibberellins, cytokinins, etc.), siderophores, and ACC deaminase are predominant mechanisms of plant growth promotion by the applied PGPR. However, the key challenges for the universal application of the PGPR include storage incompatibility, poor shelf-life, inconsistent field performance and delicate application techniques required in field conditions. A consortium application of the multifunctional and compatible PGPR is the formulation that shows better performance than the formulation of a single bacterium. A variety of multi-omic approaches are applied to overcome these shortcomings for sustainable plant nutrition, moderation of phytohormone chemistry, induction of gene expression in host plants and screening of the target genes for overall genetic advancement of the PGPR in agriculture. A large body of literature is available on the beneficial effects of the PGPR and their mode of action on crop plants. This chapter focuses on the contemporary research and knowledge gap concerning the widespread application of the PGPR for promoting low-input eco-friendly agriculture. This systematic and meta-analysis of the PGPR should offer a new opportunity for further research and practical application of the PGPR-based formulations as sustainable biostimulants in crop production under the changing climate.