PHYTOALEXIN SYNTHESIS: THE BIOCHEMICAL ANALYSIS OF THE INDUCTION PROCESS

The biochemical basis of defense mechanisms conferring disease resistance on plants is now under active investigation. Plant resistance mechanisms are effective at different levels in host-parasite interactions and include preformed physical and chemical defense barriers as well as defenses triggered by the invader (69). One inducible defense response is the synthesis of phytoalexins at the site of attempted infection. Phytoalexins have been defined as anti­ microbial compounds of low-molecular weight that both are synthesized by and accumulate in plants after the exposure of the plant to microorganisms (113). They show the range and structural complexity typical of higher-plant natural products and are predominantly phenylpropanoids, isoprenoids, and acetylenes (5). The discovery of phytoalexins in interactions between parasites and host plants has encouraged many investigators to believe that these compounds are critical factors in regulating the interactions. Although this chapter presents considerable evidence that supports this view, it also discusses other results that may indicate a more limited role for phytoalexins. One particularly interesting aspect of these compounds originates from the fact that, in many systems, phytoalexin synthesis is strongly enhanced not only upon challenge of plant tissues by parasites but also following treatment with substances called elicitors. Because of their suspected role in disease processes, the nature and activity of several elicitors are described. Suitable experimental systems of reduced complexity, e.g. plant-cell cultures and elicitors, as well