Root-knot nematodes and potato aphids are serious agricultural pests and cause great yield losses worldwide. Root-knot nematodes are microscopic round worms that parasitize over 2,000 plant species including tomato. In contrast to the wide host range of root-knot nematodes, potato aphids are able to feed on a few members of the Solanaceae, which include tomato and potato. Besides causing direct damage to plants, this aphid species is also a vector of plant viruses. The tomato Mi-1 gene mediates resistance to three species of root-knot nematodes and to potato aphids. This gene is the only cloned plant resistance gene that confers resistance to root-knot nematodes and aphids. Mi-1 encodes a protein with structural similarities to other genes conferring resistance to a number of plant pathogens such as viruses, fungi and bacteria. This proposal seeks to understand how Mi-1 mediates resistance to nematodes and aphids. The approaches entail both genetic and functional analyses. Both existing tomato mutants, impaired in resistance signaling, and novel mutants will be used to assess their role in Mi-1-mediated resistance. To accelerate the identification of additional members of the Mi-1 signaling network, functional screens will be performed in an analogous system. Identifying members of the Mi-1 signal network will lead to a better understanding of how this resistance works against these two distinct agronomic pests. The broader impact from this work will result in less reliance on pesticides to control nematodes and aphids. Results from this work will lead to identification of approaches that will improve resistance to these pests in other crop systems where no sources of resistance have been identified. This research will also provide training opportunities in genetics and functional approaches to a postdoctoral fellow and a number of undergraduate students.