Beneficial effects of silicon on salt tolerance in plants

High salinity is one of the major abiotic stressful factors affecting crop growth and productivity in agriculture system of the world. Therefore, improving the salt-tolerance of crops has attracted worldwide attention. Silicon element (Si) is the second most prevalent element in the earth’s crust excepting for oxygen.When the pH of a solution is less than 9, silicon is usually absorbed in the form of silicic acid Si(OH)₄ in plants. Although silicon has not been recognized as essential element for higher plants, it is considered to be a ‘beneficial element’. Especially, silicon can increase plants resistance to multiple stresses and improve the growth and development of plants under stress conditions. Many studies suggested that silicon actively involves the physiological and biochemical processes in plants under salt stress, and silicon has a crosstalk with the signaling molecules that include ethylene (ET), salicylic acid (SA), and polyamines (PAs). In this paper, the silicon accumulation and transportation, the beneficial regulatory role of Si are reviewed when the plants are subjected to the salt stress. Major progresses: 1) Si could be absorbed by plants in active, passive and rejective routes. Silicon transporters play important roles in silicon uptake, but the silicon uptake systems and their functions in different plant species need more investigations. 2) Silicon could alleviate salt damage through mediating ion balance under high salinity. Application of silicon can specifically decrease the uptake and transport of Na from roots to shoots, and make evenly distribute Na⁺ crossing the whole root section. In addition, silicon affects the uptake of some essential nutrients (e.g. Ca, K, N) in plant to alleviate adsorption competition between salt ions and essential nutrients. Recent studies suggested that polyamines play a regulatory role in promoting uptake of silicon-mediated nutrients under salt stress. 3) Both the up-regulation of silicon-mediated aquaporin gene expression and osmotic adjustment play important roles in increasing water uptake. 4) Silicon application alleviates oxidative stress damage to the plants by regulating the antioxidant defense and decreasing the production of reactive oxygen species (ROS). Meanwhile, silicon could alleviate the salt stress damage to the photosynthetic apparatus and prevent salt stress from destroying pigment, and thus improving the photosynthetic process. 5) Omics-based technologies, transcriptomic and proteomic analyses, provide powerful tools for better understanding the responsive mechanisms of silicon-triggered in alleviating environmental stresses at the molecular level. Both the transcriptome and proteome studies reveal that silicon could regulate the plants responses to salt stress through modulating the expressions of transcription factors and hormone-related genes as well as the translation of associated proteins. 6) The utilization of silicon mutant will be helpful to better understand the regulatory role of silicon in the physiological-biochemical metabolic processes in plants.