Resistance to herbivores and fungal pathogens: Variations on a common theme? A review comparing the effect of secondary metabolites, induced and constitutive, on herbivores and fungal pathogens

There is a great deal of similarity between the signaling pathways that operate following herbivory and pathogen attack. Among the constitutive and induced secondary plant metabolites that are active in plants against herbivores and against microorganisms, and, in particular, pathogenic fungi, are alkaloids, cyanogenic Glycosides, glucosinolates, phenolics, and accumulated ions such as selenate, terpenoids, saponins, non-protein amino acids, and proteinase inhibitors. The signaling pathways evoked by salicylate, jasmonate, and nitric oxide, which are activated by herbivore attack or fungal pathogens, are compared. Almost all secondary metabolites active against herbivores are also active against fungi. The same is also true for compounds active against fungi, such as the phytoalexins, which in almost every case have an effect on herbivores. The conclusion is that evolution of defense mechanisms follows the principles of parsimony and there is no clear dichotomy between defenses against herbivores and against pathogenic microorganisms.