Many secondary metabolites found in plants have a role in defence against herbivores, pests and pathogens. In this review, a few examples are described and discussed, and some of the problems in determining the precise role(s) of such metabolites highlighted. The role of secondary metabolites in defence may involve deterrence/anti-feedant activity, toxicity or acting as precursors to physical defence systems. Many specialist herbivores and pathogens do not merely circumvent the deterrent or toxic effects of secondary metabolites but actually utilize these compounds as either host recognition cues or nutrients (or both). This is true of both cyanogenic glucosides and glucosinolates, which art discussed in detail as examples of defensive compounds. Their biochemistry is compared and contrasted. An enormous variety of secondary metabolites are derived from shikimic acid or aromatic amino acids, many of which have important roles in defence mechanisms. Several classes of 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 difference between resistant and susceptible varieties. Both salicylates and jasmonates have been implicated as signals in such responses and in many other physiological processes, though their prescise roles and interactions in signalling and development are not fully understood. Contents Summary 617 I. Introduction 617 II. Cyanogenic glucosides 618 III. Glucosinolates 619 IV. Non-protein amino acids 621 V. Alkaloids 622 VI. Plant phenoiics 623 VII. Plant terpenes, sesquiterpenoids and sterols 626 VIII. Phytoalexins 626 IX. Salicylic acid and methyl jasnionate 627 X. Conclusions 628 References 629.
Keywords: Cyanogenic glucosides; alkaloids; glucosinolates; methyl jasmonate; phenolics; phytoalexins; salicylic acid.