Phytotoxins.

Abstract

Abstract Evidence of the existence of a wide range of secondary compounds and proteins in plants with pharmacological activity or with the potential to cause adverse effects in other living organisms is now firmly embedded in the scientific literature. The term 'phytotoxins' is conventionally used to describe these substances, but there are issues associated with nomenclature which are addressed below. In this chapter, the ecology of phytotoxins, including several glycosides, non-protein amino acids, furanocoumarins, condensed tannins, gossypol and specific anti-nutritional proteins, is considered. A number of these compounds occur exclusively in tropical plants while others are more universally distributed but with particular relevance in temperate environments. The aforementioned phytotoxins have been selected for diverse manifestations and implications in toxicology. Effects in mammals include digestive dysfunction caused by anti-nutritional proteins in legume seeds; irreversible spastic paralysis and cognition defects induced by cyanogenic glycosides in cassava; goitrogenic activity precipitated by Brassica glucosinolate breakdown products; favism associated with pyrimidine glycosides in faba beans; cardiotoxicity and reproductive abnormalities caused by gossypol in cottonseed; phytodermatitis following contact with furanocoumarins in celery and other plants; and cancer induced by ptaquiloside in bracken fern. Lower organisms possess variable mechanisms for metabolizing several of these phytotoxins. Nevertheless, it is consistently maintained that phytotoxins serve in a protective role in plants against invertebrate herbivores and fungal pathogens. It is concluded that a defence system based on protein phytotoxins may be relatively more robust than those involving secondary compounds. Furthermore, it is suggested that these heat-labile protein phytotoxins should form part of a plant breeding programme to enhance pest and pathogen resistance without compromising food safety. However, the challenge for the future is to exploit the wide array of the other phytotoxins as environmentally friendly protectants for food crops. Structure-activity and other functional relationships have been established for a limited number of phytotoxins, thus improving the prospects for the development of more effective bio-pesticides.