Isolation and Biological Activity of Plant Derived Phototoxins

While it has been known for some time that certain species of plants contain photosensitizers, it is only recently that the raison d’etre of light activated toxins has been demonstrated. All of these photosensitizers fall into a diverse group of secondary compounds with no known function in primary metabolism such as photosynthesis, nitrogen assimilation etc., but have been shown to act as defenses against phytophagous insects and fungal pests. Evolution of the biosynthetic pathways leading to these compounds has probably occurred through a co-evolutionary process where pressure by herbivores or other pests has led to the selection of chance mutations which have produced novel, increasingly complex and toxic secondary substances. For example, work by Berenbaum (1983) has shown that while the widely distributed phytochemical umbelliferone is relatively non-toxic to armyworms, its derivative, xanthotoxin of the Apiaceae, formed biosynthetically by prenylation and cycli-zation, is phototoxic to this insect when ingested. Expression of toxicity was clearly shown to be dependent on irradiation by photosensitizing wavelengths. Other studies have shown that ingested plant photooxidants such as alpha terthienyl or phenylheptatriyne (Champagne et al.,1986; Downum et al., 1984) can cause black necrotic lesions and pupal deformities to develop in phytophagous larvae at acute doses, or at lower doses lengthen their development time and reduce final weights.