Two tonoplast MATE proteins function as turgor-regulating chloride channels in Arabidopsis

The central vacuole in a plant cell occupies the majority of the cellular volume and plays a key role in turgor regulation. The vacuolar membrane (tonoplast) contains a large number of transporters that mediate fluxes of solutes and water, thereby adjusting cell turgor in response to developmental and environmental signals. We report that two tonoplast Detoxification efflux carrier (DTX)/Multidrug and Toxic Compound Extrusion (MATE) transporters, DTX33 and DTX35, function as chloride channels essential for turgor regulation in Arabidopsis Ectopic expression of each transporter in Nicotiana benthamiana mesophyll cells elicited a large voltage-dependent inward chloride current across the tonoplast, showing that DTX33 and DTX35 each constitute a functional channel. Both channels are highly expressed in Arabidopsis tissues, including root hairs and guard cells that experience rapid turgor changes during root-hair elongation and stomatal movements. Disruption of these two genes, either in single or double mutants, resulted in shorter root hairs and smaller stomatal aperture, with double mutants showing more severe defects, suggesting that these two channels function additively to facilitate anion influx into the vacuole during cell expansion. In addition, dtx35 single mutant showed lower fertility as a result of a defect in pollen-tube growth. Indeed, patch-clamp recording of isolated vacuoles indicated that the inward chloride channel activity across the tonoplast was impaired in the double mutant. Because MATE proteins are widely known transporters of organic compounds, finding MATE members as chloride channels expands the functional definition of this large family of transporters.