THE ROLE OF CALCIUM IN PLANTS' SALT TOLERANCE

High concentrations of sodium (Na) are toxic to most plant species, making soil salinity a major abiotic stress in plant productivity world wide. It has been shown that, calcium (Ca²⁺) is an important determinant for plant salt tolerance and confers protective effects on plants under growing in sodic soils. Calcium plays an essential role in processes that preserve the structural and functional integrity of plant cell membranes, stabilizes cell wall structures, regulates ion transport and selectivity, and controls ion-exchange behavior as well as cell wall enzyme activities. The nature of these responses will vary depending on the plant genotype. One of the essential functions of Ca²⁺ is acting as a second messenger in stress signaling. Genetic evidence suggests that perception of salt stress leads to a cytosolic calcium-signal that activates the calcium sensor protein SOS₃. SOS₃ binds to and activates a ser/thr protein kinase SOS₂. The activated SOS₂ kinase regulates the activities of SOS₁, a plasma membrane Na⁺/H⁺ antiporter, and NHX₁, a tonoplast Na⁺/H⁺ antiporter. This results in either Na⁺ efflux out of cytosol or its compartmentation in vacuole.