Malate transport by the vacuolar AtALMT6 channel in guard cells is subject to multiple regulation

Plant J. 2011 Jul;67(2):247-57. doi: 10.1111/j.1365-313X.2011.04587.x. Epub 2011 Apr 26.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Arabidopsis / cytology
  • Arabidopsis / genetics
  • Arabidopsis / metabolism*
  • Arabidopsis Proteins / genetics
  • Arabidopsis Proteins / metabolism*
  • Biological Transport
  • Calcium / metabolism
  • Gene Knockout Techniques
  • Hydrogen-Ion Concentration
  • Malates / metabolism*
  • Membrane Potentials
  • Mutagenesis, Insertional
  • Organic Anion Transporters / genetics
  • Organic Anion Transporters / metabolism*
  • Patch-Clamp Techniques
  • Plant Stomata / metabolism
  • Vacuoles / metabolism*


  • Arabidopsis Proteins
  • Malates
  • Organic Anion Transporters
  • malic acid
  • Calcium