This chapter explores the reasons why plants rarely succumb to pathogens or herbivores. Even though plants are constantly under attack from viruses, bacteria, fungi, oomycetes or nematodes, the occurrence of disease is the exception. Similarly, plant defences effectively limit the damage caused by chewing or sucking insects. After a discussion of pathogenicity determinants and preformed defences, this chapter describes a concept unifying the inducible local immune responses to non-adapted and adapted pathogens. Perception and signal transduction events are explored. Next, systemic resistance is explained—that is, the activation of defence mechanisms in tissues distant from the site of a pathogen attack. Also covered is gene silencing as a defence against phytopathogenic viruses. Like pathogen defences, herbivore defences are both constitutive and inducible. In addition, they can be not only direct but also indirect, meaning that the enemies of herbivores are attracted and recruited for defence by the plant. Recognition of herbivore attack and the ensuing chain of signalling events are described. Plant defences are often chemical—that is, based on complex mixtures of bioactive secondary metabolites. The synthesis of glucosinolates is elaborated on to illustrate how herbivory acts as a driver of genetic diversity. Finally, parasitic plants and allelopathy are discussed as two additional forms of biotic stress.
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