Induction of H2O2 synthesis by β-glucan elicitors in soybean is independent of cytosolic calcium transients

Soybean cell suspension cultures have been used to investigate the role of the elevation of the cytosolic Ca2+ concentration in β-glucan elicitors-induced defence responses, such as H2O2 and phytoalexin production. The intracellular Ca2+ concentration was monitored in transgenic cells expressing the Ca2+-sensing aequorin. Two lines of evidence showed that a transient increase of the cytosolic Ca2+ concentration is not necessarily involved in the induction of H2O2 generation: (i) a Bradyrhizobium japonicum cyclic β-glucan induced the H2O2 burst without increasing the cytosolic Ca2+ concentration; (ii) two ion channel blockers (anthracene-9-carboxylate, A9C; 5-nitro-2-(3-phenylpropylamino)-benzoate, NPPB) could not prevent a Phytophthora soja β-glucan elicitor-induced H2O2 synthesis but did prevent a cytosolic Ca2+ concentration increase. Moreover, A9C and NPPB inhibited P. sojae β-glucan-elicited defence-related gene inductions as well as the inducible accumulation of phytoalexins, suggesting that the P. sojae β-glucan-induced transient cytosolic Ca2+ increase is not necessary for the elicitation of H2O2 production but is very likely required for phytoalexin synthesi