ABSTRACT
Phytoalexins are inducible compounds first observed by Müller and Borger in 1940 and defined by Müller (1958) as antibiotics produced as a result of biochemical interactions between a host plant and a parasite. This definition has been modified by several workers (Harborne, 1977; Deverall, 1982). Essentially, they are chemical compounds that accumulate in the living hypersensitive tissues around the infection sites (Bailey, 1973; Rahe, 1973; Mansfield et al., 1974). The cause of hypersensitive reaction may be infection due to microorganisms or to the toxic effects of their breakdown products and metabolites (biotic inducers), or chemical and physical factors (abiotic inducers) (Cruickshank and Perrin, 1968; Hadwiger and Schwochau, 1971; Bailey et al., 1980). There are indications of different modes of activity for these two groups of inducers. Biotic inducers appear to activate defense genes in the host, which leads to enzyme synthesis for the production of the compounds (Darvill and Albersheim, 1984; Dhawale et al., 1989; Ellis et al., 1989; Preisig et al., 1991); the mode of action of abiotic inducers is not clear since phytoalexins have been known to accumulate in tissues treated with agents blocking genetic transcription and translation processes (Yoshikawa, 1978). There are indications to suggest that on infection constitutive compounds in the host cause the parasite to release elicitors which activate the phytoalexin production genes (Kiraly et al., 1972; Keen et al., 1983; Ersek and Kiraly, 1986). The induced phytoalexins demonstrate toxicity against nonpathogens as well as pathogens; pathogens, however, usually have a way of avoiding their toxic effects by catabolism or the formation of conjugates with glycosides (Tani and Mayama, 1982; Weltring et al., 1982; Willeke and Barz, 1982; Denny and Van Etten, 1983; Tahara et al., 1987). 334Plants which rapidly accumulate phytoalexins are known to be more resistant to infection. There are, however, doubts about their primary role in disease resistance (Ersek and Kiraly, 1986).
