Plant beneficial rhizospheric microbes (PBRMs): Prospects for increasing productivity and sustaining the resilience of soil fertility

Abstract

The efficient microorganisms in the vicinity of plant roots that exert positive effects on plant growth are known as plant beneficial rhizospheric microbes (PBRMs). One of the salient features for the effectiveness of PBRMs is their ability to proliferate at hosts' rhizosphere, rhizoplane or the root interior. They are directly or indirectly involved in plant growth promotion and development by releasing particular compounds, increasing the uptake of certain nutrients from the soil and lessening or preventing the plants from deleterious pathogens. Indirect plant growth promotion is achieved by the inhibition of the detrimental effects of phytopathogenic organisms, which can be obtained by the production of siderophores, i.e. metal-chelating agent. Several bacterial species also have been reported in biocontrol of soil-borne phytopathogens and the synthesis of antibiotics. Production of hydrogen cyanide (HCN) and/or fungal cell wall degrading enzymes (e.g. chitinase and ß-1, 3-glucanase) help PBRMs to suppress the effect of phytopathogens. Direct plant growth promotion involves symbiotic and non-symbiotic PBRMs which function through production of plant hormones such as auxins, gibberellins, ethylene, cytokinins and abscisic acid. Some of the PBRMs are functioning as a sink for 1-aminocyclopropane-1-carboxylate (ACC), the immediate precursor of ethylene in higher plants, by hydrolyzing it into a-ketobutyrate and ammonia and in this way promoting root growth by lowering indigenous ethylene levels in the rhizospheric environment. PBRMs also help in solubilization of nutrient-bearing minerals (indigenous sources of phosphates, potash and other nutrients), enhance resistance to plant stress, stabilize soil aggregates and improve soil structure and organic matter content. PBRMs could able to retain more soil organic N and other nutrients in the plant-soil system, which can reduce the need for fertilizer N and P and help in nutrient acquisition. Evidently, PBRMs maintain enormous prospects in advanced and sustainable plant production, including, improved plant tolerance to stress, better uptake of plant nutrient from soils and reduced application of chemical inputs. Besides, PBRMs in the soil bear a peculiar relation to soil fertility. Various investigations have documented the increased health and productivity of different plant species by the application of plant beneficial rhizospheric microorganisms under both the normal and abiotic-stressed conditions. Attempts should be directed towards maximizing the identified benefits of PBRMs in all developing countries. If the benefits of PBRMs in crop production can be maximized, this will certainly help to fight against hunger. This chapter discusses the major functions of PBRMs in broad terms, but efforts were made to present specific usage of PBRMs to enhance plant nutrient uptake, for better plant response to environmental stress, and unexplored potentials in developing countries.