Deciphering the Role of Silicon in Mitigation of Arsenic Toxicity in Soil–Plant Interface – An Overview

Approximately 500 million people around the globe is affected by arsenic (As) contamination of ground water. The regions of India, Bangladesh, Nepal, Vietnam, and China that make up the South and Southeast Asian Belt are the most arsenic-polluted areas. Although the amounts are often lower in contrast to Asian countries, developed countries such as the USA, Mexico, and Canada also face extensive levels of groundwater contamination from As. A worldwide concern is the toxic concentration of As in soil and water and its short- and long-term impacts on human and animal health. In addition to being a health risk to humans and animals when consumed in tainted water, food produced on soil contaminated with As also poses a serious threat to human health. Plants may utilise As in soil in a variety of ways, mostly as arsenate [As(V)] and arsenite [As(III)]. The availability of As in soil depends on a number of soil physiochemical parameters, including pH and water content. Plants use the uptake pathways for phosphate (P) and silicon (Si) since they lack a unique mechanism for the uptake and transport of As. To lessen the harmful effects on plants and human health, it is essential to comprehend the chemistry of As in soil, how it is absorbed, and the physiological and metabolic changes it causes in plants. Effective management of As can be done by various agronomical interventions such as water management, application of inorganic fertilizers containing P, Si and molybdenum (Mo) and soil incorporation of biochar, identification and/or development of varieties which accumulate less As in the consumable part and where ratio of inorganic to organic forms of As is low, growing of As tolerant cultivars, adoption of phytoremediation technique and increased use of different organic manures and green manure crops. Among the various management strategies, application of Si fertilizer proves promising, as it reduces soil available As and further, being a structural analogue of As, limiting the As uptake by the plants thereby improves plant health. In addition to reducing As toxicity through competitive absorption, Si aids in the growth and development of plants under a variety of harsh environmental circumstances. Numerous studies have shown that Si may help plants cope with a variety of biotic and abiotic stressors. Besides sharing similar transporters with As, Si also have positive role on uptake of different macro and micronutrients and improve the physiological parameters of the crop thereby alleviate the negative impacts of As toxicity. According to studies, exogenous application of Si reduces As toxicity by improving secondary root development, imparting mechanical strength, and limiting As uptake through cell wall lignification. This review has emphasized that application of Si stimulated the activity of the enzyme arsenate reductase, improved pigment synthesis, scavenged reactive oxygen species (ROS) through both enzymatic and non-enzymatic routes, and promoted the synthesis of phytochelatins in plants exposed to arsenic toxicity. Moreover, the continuous application of biomass rich in Si has no negative effects on the Si status of soil as well as soil health. Therefore, Si fertilization may be considered as an effective and environmentally responsible way to reduce As toxicity at the field level for improving food safety and human health around the globe.