Soil Bacteria and Phytohormones for Sustainable Crop Production

Abstract

Plant growth-promoting rhizobacteria (PGPRs) synthesizes and exports phytohormones which are called plant growth regulators (PGRs). These PGRs may play regulatory role in plant growth and development. PGRs are organic substances that influence physiological processes of plants at extremely low concentrations. Among five classes of well-known PGRs, namely auxins, gibberellins, cytokinins, ethylene and abscisic acid, the most common, best characterized and physiologically active auxin in plants is indole-3-acetic acid (IAA) that stimulate both rapid (e.g. increases in cell elongation) and long-term (e.g. cell division and differentiation) responses in plants. Some bacteria also release indole-3-butyric acid (IBA), Trptophan and tryptophol, or indole-3-ethanol (TOL) that can indirectly contribute to plant growth promotion. On the other hand, cytokinins are usually present in small amounts, but enhance cell division leading to root hair formation and root development. Microorganisms have been found to contain over 30 growth-promoting compounds of the cytokinin group and about 90 % of microorganisms found in the rhizosphere are capable of releasing cytokinins when cultured in vitro. Soil bacteria also produce gibberellins (GAs) and over 100 GAs are known. The most widely recognized gibberellin is GA3 (gibberellic acid), and the most active GA in plants is GA1, which is primarily responsible for stem elongation. In addition, abscisic acid (ABA) has been detected by radioimmunoassay in supernatants of bacterial cultures held responsible for stomatal closure. Its presence in the rhizosphere could be extremely important for crop survival under a water-stressed soil environment, such as is found in arid and semiarid climates. Ethylene is a potent plant growth regulator that affects many aspects of plant growth, development and senescence. In addition to its recognition as a ripening hormone, ethylene promotes formation of adventurous root and root hair, stimulates germination and breaks dormancy of seeds. Soil bacteria promote plant growth especially seed germination by lowering the levels of ethylene in plants/seed rhizosphere. The enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, hydrolyzes ACC, the immediate biosynthesis precursor of ethylene in plants. The products of this hydrolysis, ammonia and α-ketobutyrate, can be used by the bacterium as a source of nitrogen and carbon for growth. Soil bacterium acts as a sink for ACC and thus lowers ethylene level in plants, preventing some of the potentially deleterious consequences of high ethylene concentrations. Soil bacteria along PGPRs also play an important role in production of phosphatases, β-glucanase, dehydrogenase, antibiotic, solubilization of mineral nutrients, stabilization of soil aggregates, improving in soil organic matter and soil structure. PGRs producing soil bacteria help in reduction of/supplementing the need for chemical fertilizers N and P for sustainable crop productivity.