Figure 8.

Hypothetical model for CIPK21 function in osmotic and salt stress signaling. Under normal (top left) and salt stress conditions (bottom left), CIPK21 is localized in the cytoplasm and the nucleus. Upon coexpression of CBL2 or CBL3, CIPK21-CBL2/CBL3 complex formation initiates in the vacuolar membrane (top right), which is enhanced under salt stress (bottom right). Possibly under salt stress, CBL2/CBL3 detects the rise in cytoplasmic Ca²⁺, activates CIPKs (CIPK21), and targets them to the tonoplast. Possible targets of CIPK21 may be vacuolar membrane Na⁺-channels/transporters facilitating Na⁺ sequestration from cytosol into the vacuole under salt stress, which also might be responsible for the osmoregulation in plant cell.