Malate‐sensitive anion channels enable guard cells to sense changes in the ambient CO₂ concentration

Abstract

SummaryMalate 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 photosynthetic capacity on one side and stometal action on the other, it was studied whether the extracellular malate level represents an indicator of changes in the ambient CO2 concentration and a key regulator of ion transport in guard cells.Here it is demonstrated that alterations in the ambient C02 level modify the extracellular malate concentration of Vicia faba leaves. Elevated external malete caused stometal closure in a Concentrationdependent manner (Kin real = 0.3 mM). Slight variations in the external malate concentration strongly regulate the voltsge-depandent properties of GCAC1, an anion-release channel in the plasma membrane of guard cells. Suparfusion of guard ceil protoplasts with malate levels in the physiological range (Kin ma~ = 0.4 mM) caused the voltage gate to shift towards the resting potential of the cell-activeting GCAC1. Single-channel conductance was dependent on the extracellular chloride concentration (Kin c= = 3 mM). In the absence of extraceliular chloride the plasma membrane lacked anion conductance until the addition of malate induced channel opening. Isophthalete was a powerful agonist in both malateinduced processes, channel regulation and stomatal closure, indicating that modulation of GCACl is a key step in stomatal action. It was thus concluded that feedback regulation of volume and turgor with respect to the ambient CO2 concentration via maletesensitive anion channels may provide a CO 2 sensor to guard cells.

Studying guard cells in the intact plant: modulation of stomatal movement by apoplastic factors

“…Indeed the apoplastic malate concentration increases at high CO 2 concentrations in the atmosphere (Hedrich et al , 1994). Furthermore, malate induces closure of stomata in epidermal strips of V. faba with a half maximal concentration of 0.3 mM.…”

“…Furthermore, the activation of anion channels will depolarize the plasma membrane, which leads to the activation outward rectifying K + channels and subsequently an efflux of K + (Blatt, 1991). Plasma membrane anion channels of guard cells are sensitive to externally applied anions (Hedrich et al , 1994;. In the absence of organic anions, fast anion channels are deactivated at potentials negative of -100 mV (Fig.…”

“…Whole cell anion currents were measured in the presence of 84 mM chloride and after a change to 3 mM chloride plus 1 mM malate. Redrawn after Hedrich & Marten (1993) and Hedrich et al (1994) The pH of the cell wall also affects guard cells via more indirect pathways. Lowering the external pH may cause a decrease of the intracellular pH, which inhibits outward K + channels (Blatt & Armstrong, 1993;, activates inward K + channels (Grabov & Blatt, 1997) and enhances the activity of fast anion channels (Schulz-Lessdorf et al, 1996).…”

The ABC transporter AtABCB14 is a malate importer and modulates stomatal response to CO2

“…It remains to be determined which cell types are the major source of apoplastic malate during the response to high [CO 2 ], since both guard cells and mesophyll cells have been reported to release malate into apoplast 4,5,30 . Our results support the hypothesis that CO 2 acts on two independent pathways that both modulate guard cell movement.…”

“…2a), and thus suggest that the function of AtABCB14 during stomatal closing under high [CO 2 ] depends on the presence of apoplastic malate. Apoplastic malate concentrations have been reported to increase from 1 to more than 3 mM in response to an increase in the CO 2 level from ambient to 672 ppm 5 . The opposite phenotype to that observed in our study was recently described for the SLAC1 mutants 25,26 .…”