Adaptive traits in plants for salinity stress tolerance: A critical review

Abstract

Soil salinity negatively affects plants at multiple levels, inducing ionic and metabolic imbalances that directly affect growth and productivity. Plants counter salinity stress through a combination of adaptive traits that facilitate cellular ion homeostasis and prevent excessive sodium (Na⁺) accumulation or its detrimental effects. This review provides in-depth information focusing on plant salinity tolerance mechanisms. The Na⁺-exclusion, xylem loading, and Na⁺-vacuolar sequestration by compartmentalization are primarily achieved by the set of antiporters (HKT1, NHX1, NHX2, and SOS1) located at the root cell plasma membrane, xylem parenchyma, and the tonoplast. Plants regulate Na⁺ xylem loading to restrict Na⁺ translocation to aerial tissues, mediated by SOS1 and HKT1.  Cellular compartmentalization is regulated by both Na⁺/H⁺ and K⁺/H⁺ antiporters, which maintain ion homeostasis. Osmoregulation in cells is achieved with the help of both organic osmolytes (proline, glycine betaine, sugars, polyols, etc.) and inorganic ions (Na⁺, K⁺, Ca²⁺), which counter salt-induced osmotic stress. However, oxidative stress is mitigated by various enzymatic antioxidant proteins (SOD, CAT, POD, APX, GR, DHAR) and non-enzymatic antioxidant molecules (e.g., vitamin C, glutathione, etc.). Apart from that, the roles of various hormones, for instance, abscisic acid, ethylene, salicylic acid, jasmonates, brassinosteroids, auxins, gibberellins, and cytokinins are discussed at length in this review.