Figure 1.

A proposed model of salt-stress tolerance in plants at cellular level. The sensing of ionic stress induces a transient elevation of cytosolic Ca²⁺ and pH.⁶,³⁸,³⁹,⁴³ Cytosolic-Ca²⁺ elevation is attributed with the influx of Ca²⁺ from cell wall, as well as from the vacuole. ER and mitochondria may also contribute to cytosolic-Ca²⁺ elevation. The cytosolic-pH increase seems to be linked with vacuolar-pH decreases.³⁸ Cytosolic Ca²⁺ elevation and pH increase, either independently or in coordination, induces the SOS pathway for ionic homeostasis through inhibition of Na⁺ entry, enhancing K⁺ uptake into the cell, and sequestration of cytosolic Na⁺ either into the apoplast or vacuole.²⁴,³⁴,⁵⁵ Cell organelles, such as nucleus, mitochondria, ER etc., might also have their own Ca²⁺ elevation and subsequent signaling cascade for downstream responses.⁸⁵,⁸⁶ Osmotic stress, in contrast to ionic toxicity, may either decrease or increase cytosolic-Ca²⁺ level, which might be linked with the production of reactive oxygen species (ROS).⁵² ROS then activates the MAPK pathways for osmotic homeostasis and detoxification responses.