Astonishingly remarkable parallelism in induction of different defence reactions in allium cepa cell culture

Plants respond to fungal attack with induction of various defences including synhtesis of antibiotic substances phytoalexins (PA) and deposition of the 1,3-β-glucan callose onto a cell wall. We are studying the mechanisms of signal perception and transduction in Allium cepa suspension cell culture in the process of induction of different defence reactions. PA accumulation in onion suspension culture can be induced by treatment with biotic elicitor derived from Botrytis cinerea culture filtrate. Callose deposition can be induced in suspension cells of Allium cepa by the component of a fungal cell wall chitosan. In our experiments, chitosan failed to induce PA accumulation in onion cell culture, and vice versa. At the same time, clearly pronounced parallelism between both early induction processes was observed. Ca²⁺ is likely to be involved in elicitation of both defence reactions. The accumulation of phytoalexins and callose was significantly reduced in the presence of inhibitors of the calcium cascade such as the Ca²⁺-channel blocker (verapamil) and the vacuolar Ca²⁺ blocker (TMB-8). However, PA and callose synthesis significantly increased after treatments of cell suspension both with elicitors and Ca²⁺ ionophore A23187. Increasing Ca²⁺-uptake alone is sufficient for the induction of both defences, as shown with the A23187. Similarly, the treatment with dibutyril-cAMP or cAMP agonist theophylline induced both defence reactions in the absence of elicitors. This cAMP-induced accumulation of PA and callose could be prevented by pretreatment with Ca²⁺ antagonist verapamil. In responce to treatment with each of elicitors, an increase in cytosolic Ca²⁺ concentration in onion cells was observed in experiments with fluorescent calcium indicator indo-1. How the same messenger system mediates distinct defence reactions remains to be studied.