The phytotoxin syringomycin elicits Ca2+‐dependent callose synthesis in suspension‐cultured cells of Catharanthus roseus

Deposition of the 1,3‐β‐glucan callose onto the cell wall represents one of the defence reactions of plants against pathogens. This process can be induced in suspension cells of Catharanthus roseus by subtoxic concentrations of the bacterial phytotoxin syringomycin and is associated with a slight increase in Ca2+ uptake and some K+ release. Under these conditions callose formation can be prevented by complexing external Ca2+, indicating that some Ca2+ uptake is essential as a signal. However, higher syringomycin concentrations elicit increased Ca2+ uptake without increasing callose formation, although the potential for callose synthesis is not exhausted – as shown using digitonin as an additional elicitor. These results suggest that, superimposed on Ca2+, another, yet unknown signal is also involved in the regulation of callose synthesis.