Ca²⁺ as a Signal in the Induction of Callose Synthesis

Abstract

In suspension-cultured cells and protoplasts, callose synthesis can be triggered by various biochemically unrelated substances. Laser scanning microscopy shows that the polycation chitosan is bound to the surface of protoplasts. Induction of callose synthesis by chitosan increases with the degree of chitosan polymerization up to several thousand, whereas N-acetylation of chitosan — statistically at every fourth to fifth glucosamine residue — decreases its potency. These results suggest that interaction of chitosan might occur over a large surface area with the phospholipid head groups on the plasma membrane. In contrast, the primary interaction of other callose elicitors (e.g. saponins, polyene antibiotics, acylated cyclic peptides) may occur with various constituents in the lipid phase of the plasma membrane. Taken together, signal perception for callose synthesis appears not to involve complementary receptors in the classical sense.

Common to all types of callose elicitors is the induction of a rapid K⁺ efflux, which is correlated with an external alkalinization and followed temporally by net Ca²⁺ uptake. As the plasma membrane-located 1,3-ß-glucan synthase has an absolute requirement for Ca²⁺ in the μM range, it has been suggested that Ca²⁺ uptake may lead to an increase in cytoplasmic [Ca²⁺] and thereby trigger callose synthesis. This idea is supported by the observations that external Ca²⁺ is essential for callose induction, and that inhibition of Ca²⁺ uptake by putative Ca²⁺-channel blockers decreases callose synthesis. Increasing Ca²⁺-uptake alone, however, appears not to be sufficient for the induction of callose formation as shown with the Ca²⁺-ionophore A 23187 and by low doses of Amphotericin B. Plasma membrane depolarization by itself appears not to represent an additional signal since callose synthesis is increased by the hyperpolarizing toxin fusicoccin and is decreased by protonophores.

Some of the substances capable of rapidly inducing callose synthesis have also been shown to elicit the slower production of phytoalexins, suggesting that the signal transduction mechanism involved in callose synthesis may also contribute to the regulation of other metabolic pathways.