Salt stress in higher plants: Mechanisms of toxicity and defensive responses

Soil salinity is a major constraint to crop performance. The main contributors to salt toxicity at a global scale are Na and Cl ions which affect up to 50 % of irrigated soils. Effects of salt exposure occur at the organismic, cellular, and molecular levels and are pleiotropic, involving (1) osmotic and water deficit syndromes, (2) specific Na and Cl inhibitions, (3) nutritional imbalance, and (4) oxidative stress. We review herein the responses elicited by salt-stressed plants to face all these challenges. With the only exception of halobacteria, all other organisms are not halotolerant at the molecular level. Instead, they have developed strategies to keep salts out of the cell. Then, induction of systems for salt extrusion to the rhizosphere and salt compartmentation into the vacuole play key roles in salt tolerance, aided by the synthesis and accumulation of compatible osmolytes and of antioxidant enzymes and metabolites. Expression of these effector genes is modulated by a complex network of salt-responsive transcription factors and signaling molecules. We discuss the progress made towards increasing salt tolerance in crops by engineering genes whose products operate at all these stages, from sensing and regulation to effector proteins, and identify key open questions that remain to be addressed.