Fig. 9

Working model of HD-Zip during developmental and stress (salt) response. The unknown receptor present on the plasma membrane perceives the stress or growth signal. Phytohormone, stress, or a change in the concentration of cytosolic Ca²⁺ may serve as the signal for the expression of HD-Zip transcription factors, which in turn regulate the expression of target genes resulting in growth, development and stress tolerance. In the case of salt exposure, the Na⁺ enters inside non-selective cation channels (NSCC) and some other transporters. Change in the concentration of cytosolic Ca²⁺ activates the CBL-CIPK signaling pathway leading to the increased activity of Na⁺/H⁺ antiporter SOS1 (salt overly sensitive1), which transport the Na⁺ in the vacuole or outside the cell. Phytohormone ABA levels also regulate the Na⁺/H⁺ antiporter NHX1 and other ion transporters to overcome with stress