Antiherbivore compounds

Denton, A., Chapman, A.R.O., and Markham, J., Size-specific concentrations of phlorotannins (antiherbivore compounds) in three species of Fucus, Mar Ecol. Prog. Ser., 65, 103, 1990. 

Although the biologieal activities of many alkaloids have not yet been studied and their ecological functions remain to be elucidated or proved, we can nevertheless safely say that alkaloids are neither waste nor functionless molecules, but rather they are important fitness factors, probably mostly antiherbivore compounds. Since Nature obviously favored multitasking, additional activities, such as allelopathic or antimicrobial activities, are plausible. For quinolizidine and pyrrolizidine alkaloids, these multiple functions are already well documented (Tables 1-X). 

Lawrey JD (1983) Vulpinic and pinastric acids as lichen antiherbivore compounds contrary evidence. Bryologist 86 365-369... 

Table 11.1 Classes of plant compound with antiherbivore defense functions...

Aromatic and phenolic compounds can mediate UV-protecting activities, which might be favorable for plants living in UV-rich environments, such as high altitudes [1[. Alkaloids (such as isoquinoline, quinoline, and indole alkaloids) that derive from aromatic amino acids, such as phenylalanine, tyrosine, and tryptophan, may have UV-absorbing properties, besides antiherbivoral and antimicrobial activities. 

Many alkaloids and other allelochemicals are known for their diuretic activity (312). For an herbivore, an increased diuresis would also mean an augmented elimination of water and essential ions. Since Na is already limited in plant food (an antiherbivore device ), long-term exposure to diuretic compounds would reduce the fitness of an herbivore substantially. 

Allergenic effects. A number of secondary metabolites influence the immune system of animals, such as coumarins, furanocoumarins, hypericin, and helenalin. Common to these compounds is a strong allergenic effect on those parts of the skin or mucosa that have come into contact with the compounds (4,17,312). Activation or repression of the immune response is certainly a target that was selected during evolution as an antiherbivore strategy. The function of alkaloids in this context is hardly known. 

We can distinguish between secondary metabolites that are already present prior to an attack or wounding, so-called constitutive compounds, and others that are induced by these processes and made de novo. Inducing agents, which have been termed elicitors by phytopathologists, can be cell wall fragments of microbes, the plant itself, or many other chemical constituents (4,17,22-24). The induced compounds are called phytoalexins, which is merely a functional term, since these compounds often do not differ in structure from constitutive natural products. In another way this term is misleading, since it implies that the induced compound is only active in plant-microbe interactions, whereas in reality it often has multiple functions that include antimicrobial and antiherbivoral properties (see below). 

Lawrey JD (1989) Lichen Secondary Compounds Evidence for a Correspondence between Antiherbivore and Antimicrobial Function. Bryologist 92 326... 

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