Evidence for a role in growth and salt resistance of a plasma membrane H+-ATPase in the root endodermis

The plasma membrane of plant cells is energized by an electrochemical gradi ent produced by P-type H+-ATPases (proton pumps). These pumps are encoded b y at least 12 genes in Arabidopsis. Here we provide evidence that isoform A HA4 contributes to solute transport through the root endodermis. AHA4 is ex pressed most strongly in the root endodermis and flowers, as suggested by p romoter-GUS reporter assays. A disruption of this pump (aha4-1) was identif ied as a T-DNA insertion in the middle of the gene (after VFP574). Truncate d aha4-1 transcripts accumulate to approximately 50% of the level observed for AHA4 mRNA in wild-type plants. Plants homozygous for aha4-1 (-/-) show a subtle reduction in root and shoot growth compared with wild-type plants when grown under normal conditions. However, a mutant phenotype is very cle ar in plants grown under salt stress (e.g., 75 or 110 mm NaCl). In leaves o f mutant plants subjected to Na stress, the ratio of Na to K increased 4-5- fold. Interestingly, the aha4-1 mutation appears to be semidominant and was only partially complemented by the introduction of additional wild-type co pies of AHA4. These results are consistent with the hypothesis that aha4-1 may produce a dominant negative protein or RNA that partially disrupts the activity of other pumps or functions in the root endodermal tissue, thereby compromising the function of this cell layer in controlling ion homeostasi s and nutrient transport.