A grape berry (Vitis vinifera L.) cation/proton antiporter is associated with berry ripening

We have cloned and characterized VvNHX1, a gene encoding a vacuolar cation/H+ antiporter from Vitis vinifera cv. Cabernet Sauvignon. VvNHX1 belongs to the vacuolar NHX protein family and showed high similarity to other known vacuolar antiporters. The expression of VvNHX1 partially complements the salt- and hygromycin-sensitive phenotypes of an ena1-4 nhx1 yeast strain. Immunoblots of vacuoles of yeast expressing a VvNHX1, together with the expression of a VvNHX1GFP (green fluorescent protein) chimera demonstrated that VvNHX1 localized to the vacuoles. VvNHX1 displayed low affinity K+/H+ and Na+/H+ exchange activities (12.8 and 40.2mM, respectively). The high levels of expression of VvNHX1 during the veraison and post-veraison stages would indicate that the increase in vacuolar K+ accumulation, mediated by VvNHX1, is needed for vacuolar expansion. This process, together with the rapid accumulation of reducing sugars, would drive water uptake to the berry and the concomitant berry size increase, typical of the post-veraison stage of growth.