Plant growth-promoting bacteria and their application in agriculture

The rhizosphere is a thin layer (1-2 mm) of soil around the plant roots that are affected by the root exudates. The number and bacterial diversity in the rhizosphere are affected by the plant roots. Rhizosphere bacteria (rhizobacteria) show promoting (PGPB), deleterious, or neutral effects on plants. PGPRs increase plant growth by direct or indirect mechanisms. Direct mechanisms include plant hormones production (such as auxin, and gibberellin ...), phosphorus solubility, nitrogen fixation, siderophore production, sulfur oxidation, and ACC-deaminase production and indirect mechanisms include antibiotics production, pathogen cell wall destroying enzymes (such as chitinase), increasing the Induced systemic resistance ( ISR), HCN production, creating competition with pathogens, volatile compounds production. In this paper, the direct and indirect mechanisms and the application of PGPB on plants and other important affecting factors are discussed. Finally, the considerations that should be noticed in the use of PGPB and their marketing issues are proposed.