Defense mechanisms and structural responses of plants to diseases, pests, and mechanical injury

This chapter describes the various anatomical responses of higher plants to wounds or infections, and includes an analysis of how plants have evolved growth forms to cope with the changing stress patterns in natural environments. Multiple defenses occur in plants in response to stress and invasion of pathogens. Some of these defenses are structural and are often accompanied by chemical alterations in cells and tissues. Plant defenses can be pre-infectional or post-infectional responses. Defenses can be passive in the sense that they are preformed barriers to infection or active mechanisms of resistance that are formed in direct response to an injury or the activities of harmful invaders. The cuticle provides a very effective passive defense. The principal active anatomical response of plants to wounds and infections is to wall off the injured or diseased region. Lignification of cell walls and callose deposition are other common responses of plant tissues to injury. Several structural features that can potentially increase plant resistance include thickening and lignification of cell walls, and increasing the amount of sclerenchymatous tissue, in addition to foliar pubescence, surface waxes, and mineral accumulation. Laticifers can also serve a defensive function by clogging the mouthparts of insects attempting to feed on a latex-producing plant.