ABSTRACT
In a plant disease context, most of the reports concerning plant responses to pathogens are in reality reports of responses to damage or metabolic stress caused by pathogens. They are not specific responses to specific pathogens. A virus, bacterium, or fungus can cause the accumulation of the same phytoalexins, pathogenesis-related proteins (PR-proteins), and structural biopolymers. Recently, emphasis in the field of plant immunology has shifted from the structural elucidation of defense compounds and pathways for their biosynthesis to the factors governing gene expression, leading to the synthesis and accumulation of defense compounds and establishment of a resistant or susceptible state. Much of the recent literature has concerned salicylic acid as a plant signal for defense compounds and disease resistance, as well as the complex metabolic interactions related to signal transduction. This chapter focuses on the signals and their mode of action for the activation of the diverse responses of plants to pathogens and how these responses relate to disease resistance.
