MALATE-SENSITIVE ANION CHANNELS ENABLE GUARD-CELLS TO SENSE CHANGES IN THE AMBIENT CO2 CONCENTRATION

Malate is a characteristic metabolite in the photosynthesis of C4 and CAM plants. Furthermore, changes in the intracellular concentration of this organic acid provide part of the osmotic motor for guard cells. Since alterations in the malate concentration influence the photosynth etic capacity on one side and stomatal action on the other, it was stu died whether the extracellular malate level represents an indicator of changes in the ambient CO2 concentration and a key regulator of ion t ransport in guard cells. Here it is demonstrated that alterations in t he ambient CO2 level modify the extracellular malate concentration of Vicia faba leaves. Elevated external malate caused stomatal closure in a concentration-dependent manner (K-m(mal) = 0.3 mM). Slight variatio ns in the external malate concentration strongly regulate the voltage- dependent properties of GCAC1, an anion-release channel in the plasma membrane of guard cells. Superfusion of guard cell protoplasts with ma late levels in the physiological range (K-m(mal) = 0.4 mM) caused the voltage gate to shift towards the resting potential of the cell-activa ting GCAC1. Single-channel conductance was dependent on the extracellu lar chloride concentration (K-m(Cl) = 3 mM). In the absence of extrace llular chloride the plasma membrane lacked anion conductance until the addition of malate induced channel opening. Isophthalate was a powerf ul agonist in both malate-induced processes, channel regulation and st omatal closure, indicating that modulation of GCAC1 is a key step in s tomatal action. It was thus concluded that feedback regulation of volu me and turgor with respect to the ambient CO2 concentration via malate -sensitive anion channels may provide a CO2 sensor to guard cells.