Nematode pathogenesis and resistance in plants.

Nematodes comprise a large phylum of animals that includes plant and animal parasites as well as many free-living species (Maggenti, 1981). Plant parasitic nematodes are obligate parasites, obtaining nutrition only from the cytoplasm of living plant cells. These tiny roundworms (generally 4 mm long and barely visible to the human eye) damage food and fiber crops throughout the world and cause billions of dollars in losses annually (Sasser and Freckman, 1987). Some plant parasitic nematodes are ectoparasites, living outside their host. These species cause severe root damage and can be important virus vectors (Brown et al., 1995). Other species spend much of their lives inside roots as migratory or sedentary endoparasites. Migratory parasites move through the root, causing massive cellular necrosis. However, it is the sedentary endoparasites of the family Heteroderidae that cause the most economic damage worldwide. This group is the focus of the review. The Heteroderidae can be divided into two groups: the cyst nematodes, which include the genera Heterodera and Globodera; and the root-knot nematodes (genus Meloidogyne). The soybean cyst nematode (Heterodera glycines) is the most economically important pathogen of soybean in the United States. Potato cyst nematodes (Globodera pallida and G. rosfochiensis) cause losses in potato-growing areas worldwide (Ross, 1986). Root-knot nematodes, so-called for the characteristic root galls or root knots that they form on many hosts (Figure lA), infect thousands of plant species and cause sevele losses in yield of many crops throughout the world (Mai, 1985). Symptoms of diseased plants infected by these groups of nematodes include stunted growth, wilting, and susceptibility to other pathogens. Nematodes in these three genera have complex interactions with their host plants that generally last more than a month and result in major morphological and developmental changes in both organisms. During the infection, elaborate developmental and morphological changes occur in host root cells, especially in those that become the feeding cells that provide the sole source of nutrients for the nematode. There has been a recent burst of activity in the investigation of the molecular changes that mediate the host-parasite interaction. This ac-