Beneficial effects of silicon on salt and drought tolerance in plants

Soil salinity and drought are major abiotic factors that limit crop growth and productivity worldwide. Indeed, soil salinity and drought disrupt the cellular ionic and osmotic balance. Although silicon (Si) is generally considered nonessential for plant growth and developments, Si uptake by plants can alleviate both biotic and abiotic stresses. Silicon application could therefore improve crop production under adverse climate and soil conditions. Several reports have reviewed the benefits of silicon application on crop growth, but the mechanisms of silicon action have not been systematically discussed. Here, we review recent advances on silicon uptake, transport, and accumulation in plants and how silicon alleviates salinity toxicity and drought stress. The major points are the following: (1) both passive and active silicon uptake may coexist in plants; (2) although silicon transporters have been identified in some plants, more silicon transporters remain to be identified, and the process of silicon transport needs further clarification; (3) the mechanisms for silicon-mediated tolerance of salinity and drought have been extensively investigated at both physiological and biochemical levels. The physiological aspects include increasing water uptake by roots, maintaining nutrient balance, decreasing water loss from leaves, and promoting photosynthetic rate. At the biochemical level, silicon may improve antioxidant defense abilities by increasing the activities of antioxidant enzymes and the contents of non enzymatic antioxidants; silicon may also contribute to osmotic adjustment and increase photosynthetic enzymatic activities; and (4) silicon can regulate the levels of endogenous plant hormones under stress conditions, whereas silicon involvement in signaling and regulation of gene expression related to increasing stress tolerance remains to be explored.