Fig. (2).

Simplified view of the complex mechanisms of salinity tolerance in plants. Salinity stress is composed of two components, an osmotic component which imposes osmotic stress similar to drought and an ionic component which involves accumulation of Na⁺ leading to cellular damage. The response to the osmotic component is still unclear but appears to involve the maintenance of the osmotic potential through the production of osmolytes. The response to the ionic component of salinity stress is much clearer and involves osmotic adjustment and protection through the synthesis of compatible solutes and osmoprotectant proteins through both ABA-dependent as well as ABA-independent signaling pathways. Ionic stress is also abated through either the exclusion of Na⁺ from the cells or through its sequestration in the vacuole. Ca²⁺ plays an integral role in both the osmotic and ionic stress responses. A probable cross-talk between the cytokinin, jasmonic acid, gibberellic acid and ABA signaling in the salinity stress response has been shown. The crosstalk shows the positive roles of jasmonate and ABA in the salinity stress response while cytokinins and gibberellic acid (GA) have a negative regulatory role. The involvement of a two component histidine-aspartate phosphorelay is also hypothesized based on the positive regulatory role of the Arabidopsis histidine kinase, AHK1, in the salinity and osmotic stress response. AHK1 activates Histidine phosphotransfer proteins (Hpts) which in turn activate type-B response regulators (RRs) which ultimately regulate gene expression.