Signaling in Soybean Phenylpropanoid Responses (Dissection of Primary, Secondary, and Conditioning Effects of Light, Wounding, and Elicitor Treatments)

The spatial and temporal deployment of plant defense responses involves a complex interplay of signal events, often resulting in superimposition of signaling processes. We have employed a minimal-wound protocol to clearly separate and characterize the specific contributions of light, wounding, and a wall glucan elicitor preparation (PWG) from Phytophthora sojae (Kauf. and Gerde.) to the regulation of phenylpropanoid defense responses in soybean (Glycine max L. [Merr.]) cotyledon tissues. The assay also allowed us to clearly reconstitute responses to combinations of these primary signals and to examine the effects of other pathogenesis-related molecules on the responses in a defined manner. Light specifically triggers accumulation of malonylglucosyl conjugates of the 5-hydroxy-isoflavone, genistein, which is normally found in epidermal cells. PWG selectively induces accumulation of conjugates of the 5-deoxy-isoflavone daidzein, the first committed precursor of the phytoalexin glyceollin. Wounding initiates phenolic polymer deposition, a process greatly potentiated by PWG and light. Whereas glutathione selectively enhances light induction of genistein conjugates, methyl jasmonate enhances both light and PWG-induced isoflavone conjugate accumulations. Wound exudate fully activates the cell's capacity (competency) for the phenolic polymer and glyceollin responses to PWG, whereas glutathione partially restores competency, favoring coumestrol and phenolic polymer responses to PWG. Abscisic acid inhibits all induced phenylpropanoid responses.