Differential Expression of Vacuolar H+-ATPase Subunit c Genes in Tissues Active in Membrane Trafficking and Their Roles in Plant Growth as Revealed by RNAi

Acidification of intracellular compartments by the vacuolar-type H(+)-ATPases (VHA) is known to energize ion and metabolite transport, though cellular processes influenced by this activity are poorly understood. At least 26 VHA genes encode 12 subunits of the V(1)V(o)-ATPase complex in Arabidopsis, and how the expression, assembly, and activity of the pump are integrated into signaling networks that govern growth and adaptation are largely unknown. The role of multiple VHA-c genes encoding the 16-kD subunit of the membrane V(o) sector was investigated. Expression of VHA-c1, monitored by promoter-driven beta-glucuronidase (GUS) activity was responsive to light or dark in an organ-specific manner. VHA-c1 expression in expanding cotyledons, hypocotyls of etiolated seedlings, and elongation zone of roots supported a role for V-ATPase in cell enlargement. Mutants reduced in VHA-c1 transcript using dsRNA-mediated interference showed reduction in root growth relative to wild-type seedlings. In contrast, VHA-c3 promoter::GUS expression was undetectable in most organs of seedlings, but strong in the root cap. Interestingly, dsRNA-mediated mutants of vha-c3 also showed reduced root length and decreased tolerance to moderate salt stress. The results suggest that V-ATPase functions in the root cap influenced root growth. Expression of VHA-c1 and VHA-c3 in tissues with active membrane flow, including root cap, vascular strands, and floral style would support a model for participation of the V(o) sector and V(1)V(o)-ATPase in membrane trafficking and fusion. Two VHA-c genes are thus differentially expressed to support growth in expanding cells and to supply increased demand for V-ATPase in cells with active exocytosis.