Callose biosynthesis as a Ca2+-regulated process and possible relations to the induction of other metabolic changes

Chitosan and poly-L-lysine induce electrolyte leakage in suspension-cultured soy bean cells due to their polycationic nature. Similar effects are caused by Polymyxin B at certain concentrations. After any of these treatments electrolyte leakage is followed by a rapid onset of callose synthesis, as studied quantitatively by its fluorescence with Aniline Blue. In addition to membrane perturbations resulting in membrane leakage callose synthesis requires the presence of external Ca2+ at microM concentrations. It is suggested that concomitant with electrolyte leakage a Ca2+ influx occurs, which leads to a local increase in Ca2+ at the cytoplasmic side of the membrane, resulting in activation of the (1----3)-beta-glucan synthase. The activity of this enzyme in microsomal preparations depends directly and reversibly on Ca2+, with half-saturation at about 5 microM. Inhibitor studies suggest that activation of the (1----3)-beta-glucan synthase by Ca2+ is not mediated by calmodulin but possibly by phospholipids. The enzyme can also be activated in vitro by limited proteolysis. However, this type of activation does not appear to be involved in chitosan-elicited callose formation. It appears of interest in regard to the function of callose in cell and tissue repair mechanisms that its formation is inhibited by unsaturated fatty acids and lysophosphatidylcholine, substances that presumably can result during membrane damage. Callose synthesis may be regarded as an indicator of chitosan-induced increase in cytoplasmic Ca2+ concentration. It is speculated that this may also be part of the signal chain for initiation of de novo synthesis of enzymes reported to regulate phytoalexin production.