Silicon and plant nutrition: Dynamics, mechanisms of transport, and role of silicon solubilizer microbiomes in sustainable agriculture : A review

Silicon (Si) is the second most abundant element in the Earth's crust and has numerous roles in both soils and plants, although it is inaccessible to plants in its native state (insoluble silicate minerals). This inaccessibility can lead to insufficiency, which induces anomalies in plant growth and development. Specifically, Si alleviates various biotic and abiotic stresses in plants by enhancing tolerance mechanisms at different stages of uptake/deposition as a monosilicic acid. Exclusive utilization of conventional silicon (Si) fertilizers to boost agricultural productivity in an attempt to meet the world's ever-increasing food demand may result in surface and ground water pollution, waterway eutrophication, soil fertility depletion, and the accumulation of hazardous elements such as arsenic (As) in the soil. By addressing the drawbacks of chemical fertilizers, this review emphasizes an environmentally friendly alternative, namely, silicon-solubilizing microbiomes (SSMs), as bio-inoculants to liberate soluble Si and thus making it available to plants. A deep understanding of recent advances in the functional diversity, colonization patterns, modes of action, role in biogeochemical cycling, and phyto-stress tolerance for SSMs, and their implementation in Si-based agro-waste management through the fabrication of nanoparticles, could help to establish a reliable tool for economically and environmentally sustainable agriculture. From this perspective, in this review, we comprehensively summarize the latest methodologies for the isolation, screening, and characterization of SSMs, and Si solubilization mechanisms, including ligands (divalent cations), acids (inorganic and organic), alkali (nucleophilic attack), extracellular polysaccharides, and factors affecting them, as well as Si-mediated regulation of gene expression involved in Si uptake, transportation, and mineralization. We have critically revised the role of silicon-solubilizing microbes according to the current literature. The contributions of SSMs to bio-fertilization are still being explored; hence, we also discuss trajectories for future research in relation to SSM-mediated increases in bio-available Si. This will create new strategies by which to reduce the use of agrochemicals, improve plant health, and help us to globally gravitate towards more sustainable agricultural practices, based on the concept of a circular economy.