Involvement of oxidative processes in the signaling mechanisms leading to the activation of glyceollin synthesis in soybean (Glycine max)

The efficiency of hydroperoxides (tert-butyl hydroperoxide, hydrogen peroxide) and sulfhydryl reagents (iodoacetamide, p-chloromercuribenzene sulfonic acid) as glyceollin elicitors was examined in relation to sulfhydryl oxidation, or alteration, and to lipid peroxidation, in 3-d-old soybean hypocotyl/radicle, Glycine max. These oxidative events were investigated as possible early steps in the transduction mechanisms leading to phytoalexin synthesis. Free protein sulfhydryl groups were not modified after any of the eliciting treatments, thus indicating that immediate massive protein oxidation or modification cannot be considered a signal transduction step. Unlike sulfhydryl reagents, which led to a decrease of the free nonprotein sulfhydryl group (free np-SH) pool under all of the eliciting conditions, the results obtained with hydroperoxides indicated that immediate oxidation of the np-SH is not required for the signal transduction. Moreover, elicitation with 10 millimoles tert-butyl hydroperoxide did not lead to further oxidation or to changes in np-SH level during the critical phase of phenylalanine ammonia-lyase activation (the first 20 h), suggesting that np-SH modifications are probably not involved in hydroperoxide-induced elicitation. On the other hand, all treatments leading to significant glyceollin accumulation were able to trigger a rapid (within 2 h) lipid peroxidation process, whereas noneliciting treatments did not. In addition, transition metals, such as Fe2+ and C+, were shown to stimulate both hydrogen peroxide-induced lipid peroxidation and glyceollin accumulation, again emphasizing that the two processes are at least closely linked in soybean. Among the oxidative processes triggered by activated oxygen species, oxidation of sulfhydryl compounds, or lipid peroxidation, our results suggest that lipid peroxidation is sufficient to initiate glyceollin accumulation in soybean. This further supports the hypothesis that lipid peroxidation could be involved as a step in the signal cascade that leads to induction of plant defenses.