Malate transport and vacuolar ion channels in CAM plants
Por:
Cheffings C.M., Pantoja O., Ashcroft F.M., Smith J.A.C.
Publicada:
1 ene 1997
Resumen:
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 CO2 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 malate2- (the anionic species transported out of the cytosol) is passively distributed across the tonoplast. Malic acid accumulation could thus be explained by malate2- 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 realate-selective anion channel (VMAL) in both CAM species and C3 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 realate 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.
Filiaciones:
Cheffings C.M.:
Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, United Kingdom
Pantoja O.:
Instituto de Biotecnología, Univ. Nac. Autonoma de México, Apdo. 510-3, Cuernavaca, Morelos 62271, Mexico
Ashcroft F.M.:
University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, United Kingdom
Smith J.A.C.:
Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, United Kingdom
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