The plasma membrane proton ATPase is an important determinant of salt tolerance in plants

Abstract

Maintaining ion homeostasis is critical for adaptation in a NaCl stress environment. Induced activities of secondary active ion transporters for extracellular evacuation and vacuolar compartmentation of Na\sp{+} are crucial for lowering cytosolic Na\sp{+} levels. The plasma membrane (PM) proton (H\sp{+})-ATPase generates the H\sp{+} electrochemical potential required by proton-coupled secondary active transporters. Thus, modulation of this pump can exert tight control over Na\sp{+} homeostasis regulation in cells. The transcript level of PM H\sp{+}-ATPase is up-regulated in response to NaCl in suspension cultures of glycophyte tobacco and halophyte Atriplex nummularia. Message accumulation was induced by NaCl in roots of both plants and in expanded leaves of A. nummularia. The gene expression was more responsive to NaCl in the halophyte than in the glycophyte. The high level root-specific induction of message is localized predominantly in the elongation zone. In response to NaCl, the mRNA accumulates more in the protoderm (pre-epidermis) of root apical meristem where vascular tissues are not fully differentiated. Serving as one important barrier of roots for ion uptake, the epidermal cells may function actively in regulation of Na\sp{+} uptake, as demonstrated by the increased PM H\sp{+}-ATPase message. When roots have developed vascular tissues, the induction of the mRNA accumulation is localized predominantly in the endodermis. The endodermis has the important role of restricting apoplastic ion movements in mature roots, and this restriction may be accomplished by secondary active transporters supported by the PM H\sp{+}-ATPase. In expanded A. nummularia leaves, NaCl induction of the mRNA level is localized primarily in the bundle sheath cells, where regulation of Na\sp{+} flux must occur as the xylem unloads Na\sp{+}. The fact that induction of the mRNA accumulation correlates with the enhanced pump activity (Braun et al., 1986) indicates an important role of its gene regulation in plant adaptation to salt stress. The PM H\sp{+}-ATPase is actively involved in cells where regulation of Na\sp{+} homeostais occurs, and the capacity to regulate this gene in response to NaCl may be a determinant of salt tolerance