EMBARGOED until 5 p.m. EDT Monday, Aug. 17 For the first time, researchers have identified a single plant gene that confers resistance to two distantly related pests belonging to completely different animal categories or phyla. Results of the UC Davis study, which showed that a gene known as "Mi" provides tomato plants with resistance both to aphids and to tiny plant-parasitic roundworms or nematodes, will appear in the Aug. 18 issue of the journal Proceedings of the National Academy of Sciences. Mi is the first example of a plant resistance-gene active against two such distantly related organisms. It is also the first isolate-specific insect resistance-gene to be cloned and shown to be a member of a family of plant genes that confer resistance to other plant pests, such as fungi. Other members of this gene family confer resistance to pathogens or disease-causing agents through recognition of the protein product of a corresponding gene in the pest. Although not yet demonstrated for Mi, it is possible that Mi also functions through this type of "gene-for-gene" interaction. Collaborating on the study were Valerie Williamson, a UC Davis professor of nematology and an authority on the molecular and genetic basis of pest-resistance in crop plants, and UC Davis entomology professor Diane Ullman. Williamson recently cloned the Mi gene, previously identified as the gene responsible for producing resistance to root-knot nematodes in tomato plants. These microscopic roundworms damage tomatoes by entering the roots and sucking out the plant fluids. Aphids, on the other hand, feed on sap drawn from the plant's leaf veins. Researchers in the labs of Williamson and Ullman showed that the Mi gene is present in the leaves of aphid-resistant tomato plants, but not in tomato plants susceptible to aphid attack. And they demonstrated that resistance could be conferred on tomato plants normally vulnerable to aphids by inserting the Mi gene into those plants. The researchers are hopeful that the findings will lead to a better understanding of the mechanisms governing host-plant resistance and ultimately, to the development of more effective and environmentally friendly methods of pest control in crops.