Microbes-mediated sulphur cycling in soil: Impact on soil fertility, crop production and environmental sustainability.

Reduction in soil fertility and depletion of natural resources due to current intensive agricultural practices along with climate changes are the major constraints for crop productivity and global food security. Diverse microbial populations' inhabiting the soil and rhizosphere participate in biogeochemical cycling of nutrients and thereby, improve soil fertility and plant health, and reduce the adverse impact of synthetic fertilizers on the environment. Sulphur is 4th most common crucial macronutrient required by all organisms including plants, animals, humans and microorganisms. Effective strategies are required to enhance sulphur content in crops for minimizing adverse effects of sulphur deficiency on plants and humans. Various microorganisms are involved in sulphur cycling in soil through oxidation, reduction, mineralization, and immobilization, and volatalization processes of diverse sulphur compounds. Some microorganisms possess the unique ability to oxidize sulphur compounds into plant utilizable sulphate (SO42-) form. Considering the importance of sulphur as a nutrient for crops, many bacteria and fungi involved in sulphur cycling have been characterized from soil and rhizosphere. Some of these microbes have been found to positively affect plant growth and crop yield through multiple mechanisms including the enhanced mobilization of nutrients in soils (i.e., sulphate, phosphorus and nitrogen), production of growth-promoting hormones, inhibition of phytopathogens, protection against oxidative damage and mitigation of abiotic stresses. Application of these beneficial microbes as biofertilizers may reduce the conventional fertilizer application in soils. However, large-scale, well-designed, and long-term field trials are necessary to recommend the use of these microbes for increasing nutrient availability for growth and yield of crop plants. This review discusses the current knowledge regarding sulphur deficiency symptoms in plants, biogeochemical cycling of sulphur and inoculation effects of sulphur oxidizing microbes in improving plant biomass and crop yield in different crops.

Interkingdom signaling in plant-rhizomicrobiome interactions for sustainable agriculture.

The rhizosphere is a complex ecosystem around plant roots that comprises an integrated network of plant roots, the microbiome and soil. Wide spread communication between prokaryotes and eukaryotes occurs within this integrated network via a complex set of signal molecules secreted during both beneficial and harmful interactions. Intra- and inter-species communication among microbes occurs through various signal molecules that coordinate and control the behaviours of microorganisms in mixed communities. In addition, interkingdom signal exchange between plants and microbes occurs through the release of root exudates from the host plants. The diverse chemical substances released in root exudates affect the structural and physical heterogeneity of the soil. Moreover, chemical compounds released in root exudates trigger various signaling pathways in microbial populations that influence rhizosphere biology. Therefore, deciphering the language of interkingdom communication and understanding the mechanisms involved is innovative and promising approach for improving crop production in sustainable agriculture. This chapter describes briefly the shaping of the rhizomicrobiome in response to released root exudates. Moreover, predicting and controlling the microbiome structure and its function in the rhizosphere by understanding of rhizomicrobiome communication through different secreted compounds will allow us to better harness beneficial plant-microbe interactions. The recent progress in understanding interkingdom communication and interactions between plants and microbes is discussed in relation to plant growth, gene expression, nutrient uptake and resistance to pests and diseases along with mitigation of abiotic stresses in plants to improve plant ecosystem productivity for sustainable agriculture.