SUMMARY
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.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.1994.tb02968.x

Secondary metabolites in plant defence mechanisms

Defence mechanisms against fungi in bark, xylem, roots, and foliage of woody angiosperms and gymnosperms are addressed in this newest addition to the "Springer Series in Wood Science". The topic is approached from an anatomical and physiological angle, with emphasis on the relationships between plant structure and function and their role in defensive reactions and wound repair. Contributions on the biochemistry of defensive reactions, in-depth treatises on specific diseases such as Dutch elm disease, and Fusiform rust of pines, as well as chapters on wound reactions in bark tissues, microenvironmental aspects of xylem decay, the role of mycorrhizal fungi in defence, physiological responses of trees to fungal symbionts of insects, and the application of tissue culture and immunocytochemistry to the study of defense mechanisms in trees are included.

Defense Mechanisms Of Woody Plants Against Fungi

Beech bark disease in North America: Over a century of research revisited

Phenols from Fagus sylvatica and their role in defence against Cryptococcus fagisuga.

Concentrations of amino acids, total amino nitrogen, and phenols, ratio of phenol to amino nitrogen, and pH in the bark of American beech, Fagus grandifolia, were determined to see if these chemical constituents were correlated with susceptibility of trees to infestation by the beech scale, Cryptococcus fagisuga. The relationship of these bark constituents with scale infestation levels (none, light, and moderate), tree size (large vs. small trees), and bark shading was determined. Levels of some individual amino acids and total amino acid content in moderately infested trees were significantly higher than in uninfested trees. Large uninfested beech trees, which tend to be more readily infested by scale, had significantly higher concentrations of aspartic acid. Concentrations of phenol in the outer bark of both infested and uninfested trees were significantly higher than in the inner bark. However in bark infected by Nectria, phenol levels in the inner bark were significantly higher than in the outer bark.

Amino nitrogen and phenolic constituents of bark of American beech, Fagus grandifolia, and infestation by beech scale, Cryptococcus fagisuga

The amount of total extractable phenols was determined for bark sections obtained from behind cankers naturally induced by Nectria coccinea var. faginata and from behind mechanically inflicted wounds on stems of American beech. Healthy bark from susceptible trees was found to contain a similar level of phenolics as bark from trees determined to be resistant to the beech bark disease. Six months after wounding, wound-altered bark from susceptible trees was found to be lower in phenols than woundaltered bark from resistant trees. Inoculation of wounds with N. coccinea var. faginata resulted in decreased phenolic levels in bark sections nearest the wound surface, and increased phenolic levels in sections nearest the vascular cambium, several mm distant. Phenolic levels in injured or infected bark appear to follow similar patterns as those resulting from injury or infection of xylem tissues.

W. D. Ostrofsky

Papers

Bark phenolics of American beech (Fagus grandifolia) in relation to the beech bark disease