Plants are continually attacked by a diverse range of phytopathogenic organisms. These organisms cause substantial losses to crops each year. Traditional approaches for control of plant diseases have been the use of chemical treatment and the construction of interspecific hybrids between resistant crops and their wild-type relatives as sources of resistant germplasm. However, environmental and economic concerns make chemical pesticides undesirable, while the traditional interspecific breeding is inefficient and often cannot eliminate the undesired traits of the wild species. Thus, the discovery of pest and pathogen-resistant genes provides a new approach to control plant disease.
Several genes responsible for disease resistance have been identified and isolated from plants. See Staskawicz et al. (1995) Science 268:661-667. Recently, the sugar beet Hsl.sup.pro-1 gene that confers resistance to the beet cyst nematode was cloned. See Cai et al. (1997) Science 275:832-834; and Moffat (1997) Science 275:757. Transformation of plants or plant tissues with the resistance genes can confer disease resistance to susceptible strains. See, for example, PCT Publication WO93/19181; and Cai et al. (1997) Science 275:832-834.
Nematode infection is prevalent in many crops. For example, soybean cyst nematode (Heterodera glycines) is a widespread pest that causes substantial damage to soybeans every year. Such damage is the result of the stunting of the soybean plant caused by the cyst nematode. The stunted plants have smaller root systems, show symptoms of mineral deficiencies in their leaves, and wilt easily. The soybean cyst nematode is believed to be responsible for yield losses in soybeans that are estimated to be in excess of $500 million per year.
Nematicides such as Aldicarb and its breakdown products are known to be highly toxic to mammals. As a result, government restrictions have been imposed on the use of these chemicals. Thus, there is a great need for the isolation of genes that can provide an effective method of controlling nematodes without causing health and environmental problems.