SECONDARY METABOLITES IN PLANT DEFENSE-MECHANISMS

Many secondary metabolites found in plants have a role in defence agai nst herbivores, pests and pathogens. In this review, a few examples ar e described and discussed, and some of the problems in determining the precise role(s) df such metabolites highlighted. The role of secondar y metabolites in defence may involve deterrence/anti-feedant activity, toxicity or acting as precursors to physical defence systems. Many sp ecialist herbivores and pathogens do not merely circumvent the deterre nt or toxic effects of secondary metabolites but actually utilize thes e compounds as either host recognition cues or nutrients (or both). Th is is true of both cyanogenic glucosides and glucosinolates, which are discussed in detail as examples of defensive compounds. Their biochem istry is compared and contrasted. An enormous variety of secondary met abolites are derived from shikimic acid or aromatic amino acids, many of which have important roles in defence mechanisms. Several classes o f secondary products are 'induced' by infection, wounding or herbivory , and examples of these are given. Genetic variation in the speed and extent of such induction may account, at least in part, for the differ ence between resistant and susceptible varieties. Both salicylates and jasmonates have been implicated as signals in such responses and in m any other physiological processes, though their prescise roles and int eractions in signalling and development are not fully understood.