Biostimulant and Beyond: Bacillus spp., the Important Plant Growth-Promoting Rhizobacteria (PGPR)-Based Biostimulant for Sustainable Agriculture

Bacteria that colonize plant roots and increase plant growth are known as plant growth promoting rhizobacteria (PGPR). PGPRs can influence plant growth in two different ways, directly and indirectly, and this important interaction is important for sustainable agriculture. Different species of Bacillus spp. can increase growth, yield and biomass of plants by increasing the availability of nutrients, enhance solubilization of nutrients which may cause uptake of nutrients, synthesis of indole-3-acetic acid, nitrogen fixation and phosphorous solubilization, and promote the production of phytohormones, stimulate growth, production and improve fruit and crop quality via enhancing the production of carotenoids, flavonoids, phenols, and antioxidants, increase synthesis of indoleacetic acid (IAA), gibberellins, siderophores, carotenoids, nitric oxide and different cell surface components. The most noticeable mechanisms of biological control of various species of Bacillus spp. are through antibiotics, lytic enzymes, and siderophores. Two important mechanisms of plant growth promotion are through nutrient availability, and phytohormone production. The aim of this manuscript is to survey the effects of Azospirillum spp. and Bacillus spp. by presenting case studies and successful paradigms in several horticultural and agricultural plants.