Figure 7.

Proposed model for the transcriptional regulation of transporter genes involved in whole-plant ion distribution by vacuolar ion uptake during early salinity. The expression of plasma membrane-localized transport proteins involved in long-distance ion transport is tuned by the efficiency of vacuolar Cl⁻ fluxes in the vasculature at the onset of salinity stress. The reduced Cl⁻ storage in the vacuoles of almt9 mutants might perturb the intracellular homeostasis of Na⁺ and Cl⁻ (black circles represent positively and negatively charged ions). This might signal an enhanced NaCl stress and initiate elevated expression levels of transporters such as the Na⁺ transporter HKT1;1 (HKT). In accordance, wild-type and E196A plants have more efficient vacuolar Cl⁻ uptake, and exhibit lower salt-induced HKT1;1 transcript increase. Similarly, transcript levels of other genes encoding for transporters (x) involved in the regulation of long-distance ion translocation in shoots and roots might be up- or down-regulated (dashed lines) in response to the capacity of storing Cl⁻ in the vacuoles of the vascular system.