Sesquiterpene syntheses: occidenol and phytuberin
The word phytoalexin was derived from the greek "phyton," meaning plant, and "alexin," to ward off. Phytoalexins were originally proposed to be anti-fungal substances produced by plants in response to fungal infection. Acumulation of phytoalexins within infected tissue was believed to restrict the growth of invading fungi. Early investigators were aware that many parasitic fungi were specialized for specific host plants. A fungus isolated from one type of plant was unable to parasitize closely related species. A common feature associated with the failure of a fungi to develop on non-hosts was the rapid death of the cells at the site of attempted infection. The infection hyphae stopped growing among the dead cells and the cause has been postulated as the release of poison from these very cells. The rapid death of plant cells at sites of attempted infection has been temied hypersensitivity. This has been shown to be an effective form of resistance and has subsequently been found to occur in many plants as they express resistance to fungal and bacterial parasites.
Although the earliest studies on phytoalexins involved microbial infection resistance, more recent smdies have shown that the pathways responsible for their production may be activated through other means.^ For example, infestation of plant tissue by weevils or nematodes leads to the production of phytoalexins. Furthermore, many chemicals (abiotic elicitors) cause phytoalexins to accumulate. Along these same lines, certain physical agents such as physical wounds, partial freezing, and irradiation with certain wavelength light have been shown to induce very rapid phytoalexin formation in many plants.