Malate transport and vacuolar ion channels in CAM plants

Malate is a ubiquitous vacuolar anion in terrestrial plants that plays an important role in carbon metabolism and ionic homeostasis. In plants showing crassulacean acid metabolism (CAM), malate is accumulated as a central intermediary in the process of photosynthetic carbon assimilation, and it is also one of the major charge-balancing anions present in the vacuole. During the CAM cycle, malic acid produced as a result of dark CO3 fixation accumulates in the vacuole at night (2 H+ per malate), and is remobilized from the vacuole in the following light period. CAM plants thus provide a good model for studying both the mechanism and control of malate transport across the tonoplast. Thermodynamic considerations suggest that malate(2-) (the anionic species transported out of the cytosol) is passively distributed across the tonoplast. Malic acid accumulation could thus be explained by malate(2-) transport into the vacuole occurring electrophoretically in response to the transmembrane electrical potential difference established by the tonoplast H+-ATPase and/or H+-PPase. Recent studies using the patch-clamp technique have provided evidence for the existence of a vacuolar malate-selective anion channel (VMAL) in both CAM species and C-3 species. The VMAL current has a number of distinctive properties that include strong rectification (opening only at cytosolic-side negative membrane potentials that would favour malate uptake into the vacuole), lack of Ca2+ dependence, and slow activation kinetics. The kinetics of VMAL activation can be resolved into three components, consisting of an instantaneous current and two slower components with voltage-independent time constants of 0.76 s and 5.3 s in Kalanchoe daigremontiana. These characteristics Suggest that the VMAL channel represents the major pathway for malate transport into the vacuole, although the strong rectification of the channel means there may be a separate, still-to-be-identified, transport mechanism for malate efflux.