title: Malate transport by the vacuolar AtALMT6 channel in guard cells is subject to multiple regulation.
abstract: Gas exchange in plants is controlled by guard cells, specialized cells acting as turgor pressure-driven valves. Malate is one of the major anions accumulated inside the vacuole during stomatal opening counteracting the positive charge of potassium. AtALMT6, a member of the aluminum-activated malate transporter family, is expressed in guard cells of leaves and stems as well as in flower organs of Arabidopsis thaliana. An AtALMT6-GFP fusion protein was targeted to the vacuolar membrane both in transient and stable expression systems. Patch-clamp experiments on vacuoles isolated from AtALMT6-GFP over-expressing Arabidopsis plants revealed large inward-rectifying malate currents only in the presence of micromolar cytosolic calcium concentrations. Further analyses showed that vacuolar pH and cytosolic malate regulate the threshold of activation of AtALMT6-mediated currents. The interplay of these two factors determines the AtALMT6 function as a malate influx or efflux channel depending on the tonoplast potential. Guard cell vacuoles isolated from Atalmt6 knock-out plants displayed reduced malate currents compared with wild-type vacuoles. This reduction, however, was not accompanied by phenotypic differences in the stomatal movements in knock-out plants, probably because of functional redundancy of malate transporters in guard cell vacuoles.
authors: Institute of Plant Biology, University of Zurich, Zurich, Switzerland.
authors: Stefan
authors: S
authors: Meyer
authors: Joachim
authors: J
authors: Scholz-Starke
authors: Alexis
authors: A
authors: De Angeli
authors: Peter
authors: P
authors: Kovermann
authors: Bo
authors: B
authors: Burla
authors: Franco
authors: F
authors: Gambale
authors: Enrico
authors: E
authors: Martinoia
mesh_terms: D017360:Arabidopsis / Q000166:cytology / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism; D055786:Gene Knockout Techniques; D006863:Hydrogen-Ion Concentration; D008293:Malates / Q000378:metabolism*; D008564:Membrane Potentials; D016254:Mutagenesis, Insertional; D027361:Organic Anion Transporters / Q000235:genetics / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D054046:Plant Stomata / Q000378:metabolism; D014617:Vacuoles / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D008293:Malates; D027361:Organic Anion Transporters; C030298:malic acid; D002118:Calcium