Silicon: an essential element for plant nutrition and phytohormones signaling mechanism under stressful conditions

Silicon (Si) is one of the essential and important elements that plays a vital role in the growth and productivity of crop plants by improving their nutritional status. The exogenous application of Si activates plant defense and phytohormones signaling mechanisms under biotic and abiotic stresses. Different soil factors such as soil pH, texture, organic matter, and temperature significantly influence the bioavailability and solubility of Si in the soil system. However, the uptake, transport, and accumulation of Si within the plants depend upon Si-transporters including LSi1, LSi2, and LSi6 that are present in the roots of plants. From the past few decades, the role of Si in mineral nutrient deficiencies, toxicities, biotic and abiotic stresses is being explored in cereals crops. Si improves the plant resistance against pathogenic stress, salinity, drought, heat, and heavy metals by regulating the defense system. In addition, Si facilitates the uptake of essential nutrients and restricts metal ions by making conjugates, provides mechanical strength to plant cell wall, and enhances the resistance against unwanted environmental conditions. It potentially regulates phytohormones biosynthesis, improves photosynthetic attributes, and increases the activities of antioxidant enzymes to reduce the harmful impacts of reactive oxygen species (ROS) and other toxic ions. Furthermore, the actual mechanisms behind Si-mediated alterations in plants under mineral nutrient stress are still unclear; however, a little literature is available on other abiotic stresses. Therefore, this study summarizes the findings from various investigations for better understanding the mechanism, regulation, and crosstalk among different phytohormones, micro- and macro-nutrient disorders, other biotic and abiotic stresses and their possible solutions in response to exogenous applications of Si (soil or foliar). Overall, this study suggested that Si supplementation significantly enhances the physio-biochemical attributes, defensive mechanism, hormonal regulation, and activates the regulation of expression pattern of stress responsive genes under stressful conditions.