Silicon and Abiotic Stress in Plants: Current Knowledge and Future Research Needs.

Silicon (Si) is the second most abundant element in soils. Although Si has not yet been listed among the essential elements for higher plants, its beneficial roles in improving the growth and development of plants, particularly under stressful environments, have been well documented. More recently, rapid progress has been made in cloning and functional analysis of genes responsible for silicon uptake, transport and distribution in plants. Silicon is well-known to effectively mitigate various forms of abiotic stress such as salinity, drought, freezing, high temperature and UV radiation stresses and metal toxicities including aluminum, iron, manganese, cadmium, arsenic and zinc toxicity etc. However, the mechanisms underlying remain poorly understood. In this paper, we reviewed the updated knowledge of the roles of silicon-enhanced resistance to abiotic stresses in plants. There are a number of external and internal strategies by which plants are adapted or tolerant to abiotic stresses. For examples, Si-enhanced metal tolerance can be attributed to 1) external mechanisms including reduced metal ion activity because of silicate-caused pH rise and Si co-deposition with metals in growth media, and 2) in planta mechanisms including metals and Si co-deposition within plants, inhibited root-to-shoot metal transport, more metals bound to root cell walls and Si-mediated antioxidant defense capacity. For salinity tolerance, Si-mediated osmotic stress and ionic rebalance with enhanced K but reduced Na uptake and transport, and reduced membrane lipid peroxidation and improved membrane structure and stability caused by enhanced antioxidant defense activities contribute mainly to Si-enhanced salinity tolerance. However, we think that Si-enhanced antioxidant defense capacity is the universal and major strategy for Si-mediated tolerance to such abiotic stress as salinity, freezing, drought and heavy metals stress in plants. Finally we discussed future research needs for Si-mediated alleviation of abiotic stresses, and for Si-improved eco-environmental quality and health.