Adaptive Defense Responses to Pathogens and Insects

The relative resistance of a plant to pathogens is determined by preformed, constitutive defenses, and the quality and diversity of the induced defenses deployed upon attack. Pathogens have evolved strategies to breach structural barriers and avoid or counter preformed and induced chemical defenses of their host plants. Plants have evolved sensitive mechanisms to perceive bioagressors and innate immune responses are activated when plants encounter non-adapted microbes, virulent microbes and insects, or when plants engage in a gene-for-gene resistance response. The earliest events in innate immunity are triggered by the perception of the mechanical and chemical signals associated with injury and highly conserved molecules called pathogen-associated molecular patterns (PAMPs), which signal the presence of a nonself organism (i.e., a microbial or animal aggressor). Using mitogen-activated protein kinase (MAPK) cascades, PAMP-triggered immunity integrates with the salicylic acid (SA)-, jasmonic acid (JA)-, and ethylene (ET)-regulated defense pathways. This complex and dynamic network provides effective, inducible defenses to resist pathogens and herbivores. Consistent with the defense arms race of plants and pathogens, these intruders attempt to derail defenses by the strategic deployment of virulence factors or effectors. In this chapter, the similarities and distinctions of the adaptive defense strategies used by pathogens and insect herbivores to recover nutrients and promote their establishment and proliferation on host plants will be emphasized. The role of PAMPs and the distinctive chemical nature of herbivore elicitors and effectors in plant-herbivore interactions are discussed.