Two different effects of calcium on aquaporins in salinity-stressed pepper plants

Two different effects of calcium were studied, respectively, in plasma membrane vesicles and in protoplasts isolated from roots of control pepper plants (Capsicum annuum L cv. California) or of plants treated with 50 mM NaCl, 10 mM CaCl2 or 10 mM CaCl2 + 50 mM NaCl. Under saline conditions, osmotic water permeability (P-f ) values decreased in protoplasts and plasma membrane vesicles, and the same reduction was observed in the PIP1 aquaporin abundance, indicating inhibitory effects of NaCl on aquaporin functionality and protein abundance. The cytosolic Ca2+ concentration, [Ca2+](cyt), was reduced by salinity, as observed by confocal microscope analysis. Two different actions of Ca2+ were observed. On the one hand, increase in free cytosolic calcium concentrations associated with stress perception may lead to aquaporin closure. On the other hand, when critical requirements of Ca2+ were reduced (by salinity), and extra-calcium would lead to an upregulation of aquaporins, indicating that a positive role of calcium at whole plant level combined with an inhibitory mechanism at aquaporin level may work in the regulation of pepper root water transport under salt stress. However, a link between these observations and other cell signalling in relation to water channel gating remains to be established.