Ionic Basis of Salt Tolerance in Plants: Nutrient Homeostasis and Oxidative Stress Tolerance

Abstract

Salinity, recognized as a major threat in agriculture, causes 4.0–6.3% yield loss annually across the world. The problem is aggravated due to increasing irrigation with suboptimal quality of irrigation water and more salinization of coastal area due to the rise in sea level because of climate change. In saline soil, excessive concentrations of Na⁺ and Cl⁻ impair absorption of other beneficial ions such as K⁺ and Ca²⁺ that in turn inhibit plant growth and productivity. Maintenance of cellular K⁺ level and K⁺/Na⁺ ratio is still considered the most important factor for salt tolerance. Under high-Na⁺ environment, excess Na⁺ competes with K⁺ thereby hindering its uptake. Tolerant plants by employing a number of strategies restrict Na⁺ movement to young meristematic tissues and allow greater movement and/or tissue retention of K⁺ to physiologically more active tissues. Under salt stress different K⁺- and Na⁺-specific transporters, viz. SOS, NHX, and HKT family transporters (regulate cellular Na⁺ movement) and HAK, AKT, KT, and KUP (regulate K⁺ movement), either by upregulation or downregulation, control the cellular ion homeostasis and salt tolerance in plants. SOS1, a plasma membrane-bound Na⁺/H⁺ antiporter, mostly active in root tissue, removes the excess salt from the plant body by pumping them back to the rhizosphere in an energy-dependent process. Tonoplast-bound vacuolar Na⁺/H⁺ antiporters (NHX family transporters) play crucial role in Na⁺ compartmentalization inside the vacuole in mature cell in both root and leaf tissues. Storing excess salts in vacuole imparts tolerance in multifaceted manner, viz. imparting tissue and osmo-tolerance. Biosynthesis of organic osmolytes, a more energy-expensive process, is sometimes substituted by the accumulation of excess Na⁺ in non-active tissues under salt stress. Improved Ca²⁺ status inside the plant tissue is another important factor associated with salt tolerance and acts as a key signalling molecule to initiate Na⁺ exclusion. Several QTLs and miRNAs were reported to impart salt tolerance in several crops. Managing salinity beyond crop improvement strategies was also deliberated, e.g. lowering salt effect through K⁺ supplementation and phytohormones, etc. In this compilation, emphasis has been given on how nutrient/ionic imbalance causes deleterious effects on plants under saline conditions and what are the possible adaptive strategies plants employ to maintain the ionic homeostasis in saline environment.