Oligosaccharide Elicitors: Structures and Recognition

Abstract

Oligosaccharide elicitors capable of inducing one or more plant defense responses have been prepared from plant (homogalacturonan) and fungal (s-glucan, chitin, chitosan) cell wall polysaccharides. An overview of the structures and activities of these elicitors is presented. In addition, recent biochemical investigations of the cellular signaling pathways triggered by these elicitors will reviewed. The signal pathway leading to the biosynthesis and accumulation of phytoalexins in soybean tissues will be highlighted. This pathway is triggered by nanomolar concentrations of a branched hepta-s-glucoside elicitor from fungal walls. Current research is focused on the first step in the signaling pathway, the recognition of the elicitor by a specific receptor. A radio-labeled derivative of the elicitor has been prepared and used to demonstrate the presence of specific, high-affinity binding protein(s) (EBPs) (putative receptors) for the elicitor in soybean root membranes. The EBPs co-migrate with a plasma membrane marker in isopycnic sucrose density gradients. As a first step toward their purification, the EBPs have been solubilized from the soybean membranes using non-ionic detergents, and the solubilized proteins retain their high affinity and specificity for the hepta-s-glucoside elicitor. The membrane-localized and solubilized forms of the EBPs recognize the same structural elements of the hepta-s-glucoside elicitor that are essential for its phytoalexininducing activity, suggesting that the EBPs are physiological elicitor receptors.