Recognition and Response between Host and Parasite as Determinants in Resistance and Disease Development

Abstract

The entire metabolic organization that characterizes life forms is dependent upon the capacity to detect ambient conditions, or changes in ambient conditions, and respond appropriately. This capacity is readily apparent in interactions between vascular parasites and potential hosts. Soil-borne, fungal, root parasites, for example produce thick-walled, resting structures that can survive in a dormant state for long periods of time. As plant roots ramify through the soil, root cap cells are sloughed off and root exudates leach out of these cells and from young roots in a gradient that may extend a millimeter or two from the root surface (Griffin 1969). The resting spores are able to detect the presence of these nutrients (and probably the gradient) and respond by rapid germination (Griffin 1981), growth toward the source, and colonization of the root rhizosphere, thizoplane, and to varying degrees, cells of the epidermis, cortex (Gerik 1984; Gerik and Huisman 1985) and, sometimes, vascular tissues. These precesses, and the invasion of the vascular elements is apparently enhanced by senescence (Hepple 1963) or nematode infestation of roots (Bergeson et al 1970; Conroy et al 1972). When tissues have been invaded for some time and nutrients, especially carbohydrates, become depleted, cells of the parasites are rapidly converted into new, thick-walled, resting structures (Meyers and Cook 1972) to await the arrival of a new nutrient source. Thus, these parasites possess a remarkable system of recognition and response that permits them to take advantage of sometimes fleeting nutrient sources.