Approaches to cloning plant genes conferring resistance to fungal pathogens

Fungal diseases are known in virtually all natural populations of plants and are a major cause of reduction in yields of agricultural and horticultural plants. In most plant-improvement programs, breeding for resistance to fungal diseases is a primary target for the plant breeder. Some fungal species (necrotrophs) cause death of plant cells and then grow by extracting nutrients from the dead cells. Other pathogens (biotrophs), such as those that cause rust and mildew, depend on living cells of the host plant for their nutrient supply. In most cases, host-plant resistance to an avirulent pathogen is manifested through a hypersensitive reaction in which the plant tissues immediately surrounding the initial point of pathogen attack die and thus prevent the establishment of pathogen growth. Despite the many examples of this hypersensitive reaction, and despite the economic importance of fungal diseases in agricultural plants, little has been learned of the basic mechanisms of recognition that must exist between the host and pathogen. In this paper we consider how the techniques of molecular biology that have been developed in the past few years may now be used to investigate the molecular basis of gene-for-gene interactions, and how they are providing some hope that resistance genes will be cloned and studied in detail.