Functional characterization of VvCAX3: a grapevine cation/H + exchanger that transports Ca2+ and other cations

Abstract

Grapevines are economically one of the most important fruit species worldwide. Thus, it is essential for winegrowers to guarantee fruit quality upon adverse climate conditions, including heavy rains before harvest that cause severe skin cracking and fruit spoilage. A close relationship has been demonstrated between increased tomato fruit integrity, increased calcium levels and increased activity of CAX-type cation/H+ exchangers. Therefore, the identification and characterization of grapevine CAX transporters is a landmark towards understanding calcium dynamics in the grape berry. In this study we have identified and functionally characterized the Vitis vinifera calcium-H+ antiporter VvCAX3, by heterologous expression of a yeast strain deficient in Ca2+ detoxification. Results showed that the presence of VvCAX3 restored the ability of the strain to grow in high levels of Ca2+ (100-200 mM), validating the role of the protein in Ca2+ transport. In addition, evidence for its role as exchanger of other cations, namely Na+, Li+, Mn2+ and Cu2+ was obtained. The data was complemented by gene expression studies in grape cell suspensions (Cabernet Sauvignon Berry) that showed a regulation of transcript abundance by cation levels in the culture medium. Studies in planta demonstrated that the transporter is highly expressed in green berries (pea size), decreasing gradually as development progresses, in accordance to the pattern of calcium accumulation in the fruit. VvCAX3 transcripts were also detected in grapevine roots, stems and leaves. Altogether the present study provided powerful insights on the role of VvCAX3 in grapevine calcium homeostasis.