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 p eroxide) and sulfhydryl reagents (iodoacetamide, p-chloromercuribenzen e sulfonic acid) as glyceollin elicitors was examined in relation to s ulfhydryl oxidation, or alteration, and to lipid peroxidation, in 3-d- old soybean hypocotyl/radicle, Glycine max. These oxidative events wer e 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 th at immediate massive protein oxidation or modification cannot be consi dered a signal transduction step. Unlike sulfhydryl reagents, which le d to a decrease of the free nonprotein sulfhydryl group (free np-SH) p ool under all of the eliciting conditions, the results obtained with h ydroperoxides indicated that immediate oxidation of the np-SH is not r equired for the signal transduction. Moreover, elicitation with 10 mM tertbutyl hydroperoxide did not lead to further oxidation or to change s in np-SH level during the critical phase of phenylalanine ammonialya se activation (the first 20 h), suggesting that np-SH modifications ar e probably not involved in hydroperoxide-induced elicitation. On the o ther hand, all treatments leading to significant glyceollin accumulati on were able to trigger a rapid (within 2 h) lipid peroxidation proces s, whereas noneliciting treatments did not. In addition, transition me tals, such as Fe2+ and Cu+, were shown to stimulate both hydrogen pero xide-induced lipid peroxidation and glyceollin accumulation, again emp hasizing 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 lip id peroxidation could be involved as a step in the signal cascade that leads to induction of plant defenses.