Abstract

The coevolution of interacting plants and microbes has given rise to a diverse array of exchanged signals and responses. Microbes that elicit a host response can be met variously with hospitable acceptance (as is the case with symbionts such as nitrogen-fixing Rhizobium bacteria), with tardy recognition and moderately effective defenses (as for most interactions that result in disease), or with a strong and rapid defense response that blocks further infection (Dixon and Lamb, 1990; Keen, 1990; Long and Staskawicz, 1993). This latter form of disease resistance forms the subject of this review and is known variously as race-specific resistance, gene-for-gene resistance, or hypersensitive resistance. Activation of gene-for-gene resistance typically depends on specific recognition of the invading pathogen by the plant (Keen, 1990). Numerous individual plant genes have been identified that control gene-for-gene resistance, and these genes are known as resistance (R) genes. Study of gene-for-gene resistance might be justified solely by the intrigue of plant-pathogen coevolution or as a model for signal transduction research in which an organism perceives and responds to its environment. However, the topic takes on greater interest dueto its pivotal impact on crop health and food production. Plant diseases cause billions of dollars in lost harvest annually, and in some instances, these losses have severe consequences for humans (Agrios, 1988; Schumann, 1991). One of the most convenient, inexpensive, and environmentally sound ways to control plant disease is to utilize disease-resistam varieties, and plant breeders make extensive use of classically defined R genes (Agrios, 1988). Recent work has revealed the structure of a number of plant R genes, and a striking degree of similarity among these genes has been observed. After briefly introducing the subject of R genes and avirulence (Avo genes, this review provides an overview of the conserved structural components that are predicted in the proteins encoded by R genes. The cloning of R genes has stimulated additional research that is also discussed, including structure-function analysis of R gene-encoded proteins, isolation of additional R genes, identification of functionally related components of the defense signal transduction cascade, and engineering of improved disease resistance in plants. RESISTANCE GENES, AVIRULENCE GENES, AND PLANT DEFENSE