The role of octadecanoids and functional mimics in soybean defense responses

Oxylipins of the jasmonate pathway and synthetic functional analogs have been analyzed for their elicitor like activities in an assay based on the induced accumulation of glyceollins, the phytoalexins of soybean (Glycine max L.), in cell suspension cultures of this plant. Jasmonic acid (JA) and its methyl ester showed weak phytoalexininducing activity when compared to an early jasmonate biosynthetic precursor, 12-oxophytodienoic acid (OPDA), as well as to the bacterial phytotoxin coronatine and certain 6-substituted indanoylLisoleucine methyl esters, which all were highly active. Interestingly, different octadecanoids and indanoyl conjugates induced the accumulation of transcripts of various defenserelated genes to different degrees, indicating distinct induction competencies. Therefore, these signaling compounds and mimics were further analyzed for their effects on signal transduction elements, such as the transient enhancement of the cytosolic Ca2+ concentration and MAP kinase activation, which are known to be initiated by a soybean pathogenderived [beta]glucan elicitor. In contrast to the [beta]glucan elicitor, none of the other compounds tested triggered these early signaling elements. Moreover, endogenous levels of OPDA and JA in soybean cells were shown to be unaffected after treatment with [beta]glucans. Thus, OPDA and JA, which are functionally mimicked by coronatine and a variety of 6-substituted derivatives of indanoylLisoleucine methyl ester, represent highly efficient signaling compounds of a lipidbased pathway not deployed in the [beta]glucan elicitorinitiated signal transduction.