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IN NATURAL HABITATS, plants are surrounded by an enormous number of potential enemies. Nearly all ecosystems contain a wide variety of bacteria, viruses, fungi, nematodes, mites, insects, mammals, and other herbivorous animals. By their nature, plants cannot avoid these herbivores and pathogens simply by moving away; they must protect themselves in other ways. The cuticle (a waxy outer layer) and the periderm (secondary protective tissue), besides retarding water loss, provide barriers to bacterial and fungal entry. In addition, a group of plant compounds known as secondary metabolites defend plants against a variety of herbivores and pathogenic microbes | Chapter 3 Secondary Metabolites and Plant Defense IN NATURAL HABITATS plants are surrounded by an enormous number of potential enemies. Nearly all ecosystems contain a wide variety of bacteria viruses fungi nematodes mites insects mammals and other herbivorous animals. By their nature plants cannot avoid these herbivores and pathogens simply by moving away they must protect themselves in other ways. The cuticle a waxy outer layer and the periderm secondary protective tissue besides retarding water loss provide barriers to bacterial and fungal entry. In addition a group of plant compounds known as secondary metabolites defend plants against a variety of herbivores and pathogenic microbes. Secondary compounds may serve other important functions as well such as structural support as in the case of lignin or pigments as in the case of the anthocyanins. In this chapter we will discuss some of the mechanisms by which plants protect themselves against both herbivory and pathogenic organisms. We will begin with a discussion of the three classes of compounds that provide surface protection to the plant cutin suberin and waxes. Next we will describe the structures and biosynthetic pathways for the three major classes of secondary metabolites terpenes phenolics and nitrogen-containing compounds. Finally we will examine specific plant responses to pathogen attack the genetic control of host-pathogen interactions and cell signaling processes associated with infection. CUTIN WAXES AND SUBERIN All plant parts exposed to the atmosphere are coated with layers of lipid material that reduce water loss and help block the entry of pathogenic fungi and bacteria. The principal types of coatings are cutin suberin and waxes. Cutin is found on most aboveground parts suberin is present on underground parts woody stems and healed wounds. Waxes are associated with both cutin and suberin. 284 Chapter 13 Cutin Waxes and Suberin Are Made Up of Hydrophobic Compounds Cutin is a macromolecule a .
