Elicitation of Glyceollin Synthesis in Cultured Soybean Cells Containing 9β,19-Cyclopropylsterols

Abstract

Among the vast array of responses of higher plants to pathogens, the synthesis of phytoalexins is believed to play in some cases an important role in the development of resistance. As an example, soybean (Glycine max L.) seedlings produce and accumulate a mixture of structurally related isoflavonoid derivatives (glyceollins) following either inoculation with the fungus Phytophthora megasperma f sp. glycinea (Pmg) or treatment with a ß-glucan elicitor isolated from the fungal cell walls. While the different steps leading to the glyceollin synthesis in soybean cells are now well documented (1), the sequence of reactions involved between the recognition and/or the binding of the elicitor and the cellular response remains to be investigated. The external surface of the plasma membrane (PM) of soybean cells has been shown to contain high affinity binding sites for ß-glucans (2). The PM also contains the bulk of cellular free sterols as a mixture of sitosterol, stigmasterol and 24-methylcholesterol (3). In order to shed some light on the possible involvement of sterols in some plant defense reactions, especially the elicitation of phytoalexin synthesis, cultured soybean (Glycine max L. cv Harosoy 63) cells were grown in the presence of fenpropimorph, a sterol biosynthesis inhibitor, which triggers the replacement of up to 95 % of the usual Δ5-sterols by 9ß,19-cyclopropylsterols, and then tested for their ability to synthesize glyceollins in response to the addition of a Pmg ß-glucan elicitor. In such cells, the elicitor was found to induce an earlier synthesis and a five-fold higher accumulation of glyceollins than in elicited control (i.e. with a non modified sterol profile) cells. These isoflavonoids are undetectable in unelicited cultures treated only with the sterol biosynthesis inhibitor, indicating that the overstimulation of their synthesis requires the presence of the elicitor.KeywordsPlasma MembranePlant PhysiologyPlant PathologyCellular ResponseDefense ResponseThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.