Expression of the Grapevine Anion Transporter ALMT2 in Arabidopsis Roots Decreases the Shoot Cl-/NO3- Ratio under Salt Stress

Grapevines (Vitis vinifera, Vvi) are economically important crop plants which, when challenged with salt (NaCl) in soil and/or irrigation water, tend to accumulate Na+ and Cl- in aerial tissues, impacting yield and berry acceptability for winemaking. Grapevine (Vitis spp.) rootstocks vary in their capacity for shoot Cl- exclusion. Here, we characterize two putative anion transporter genes-Aluminium-activated Malate Transporter VviALMT2 and VviALMT8-that were differentially expressed in the roots of efficient (140 Ruggeri) and inefficient (K51-40) Cl--excluding rootstocks, to explore their potential for impacting shoot Cl- exclusion. Using the Xenopus laevis oocyte expression system, VviALMT2 and VviALMT8 formed conductive channels that were highly permeable to NO3-, slightly to moderately permeable to other substrates including Cl- and malate, but impermeable to SO42-. Quantitative reverse transcription-PCR analyses revealed that VviALMT2 was more highly expressed in the root vasculature and up-regulated by high [NO3-] re-supply post-starvation, while the fluorescently tagged translational fusion of VviALMT2 localized to the plasma membrane. As VviALMT8 showed no such features, we selected VviALMT2 as our salt exclusion candidate and assessed its function in planta. Expression of VviALMT2 in Arabidopsis thaliana root vasculature reduced shoot [Cl-]/[NO3-] after NaCl treatment, which suggests that VviALMT2 can be beneficial to plants under salt stress.