Cell Wall-Derived Elicitors—Are they Natural (Endogenous) Regulators?

Abstract

Development of the idea that carbohydrates digested from cell walls could carry the information necessary to “activate” host defense responses to pathogens began in the 1970s. Early work (Ayers et al., 1976) demonstrated that a glucan derived from cell walls of the soybean pathogen Phytophthora megasperma f. sp. glycinea could elicit the synthesis of phytoalexins by soybean tissues. Subsequent studies have identified a heptasaccharide subfragment of the hyphal wall glucan, presumably generated by the action of soybean-produced glucan hydrolases, as the most active component (Sharp et al., 1984). Studies in recent years have identified similar responses in several cell culture and tissue systems. The production of phytoalexins and other presumed “defense” compounds in response to elicitors derived from host, as well as pathogen, cell walls has been described (Darvill and Albersheim, 1984; Lamb et al., 1986). Most recently these studies have utilized the techniques of molecular genetics to follow responses at the level of gene transcription (e.g., Bell et al., 1986). Genome activation with a lag time as short as 5 to 10 minutes has been shown (Lawton and Lamb, 1987). Questions about the means by which cells perceive the presence of elicitors (Schmidt and Ebel, 1987) and then mobilize their responses remain. However, it is clear that the biochemical confrontation at the infection site often includes many enzymes that can generate elicitor-active wall fragments.