Independent action and contrasting phenotypes of resistance genes against spotted alfalfa aphid and bluegreen aphid in Medicago truncatula

Host resistance to aphids is poorly understood. Medicago truncatula, a model legume and cultivated pasture species, was used to elucidate defense against two aphid species, Therioaphis trifolii f. maculata (spotted alfalfa aphid, SAA) and Acyrthosiphon kondoi (bluegreen aphid, BGA). Aphid performance and plant damage were compared between near-isogenic cultivars, Mogul and Borung, that differ in resistance to both aphids. Analyses of aphid resistance in Mogul x Borung F-2 plants and their progeny revealed modes of action and chromosome locations of resistance genes. Separate genes were identified for SAA resistance (TTR) and BGA resistance (AKR); both mapped to chromosome 3 but were found to act independently to reduce survival and growth of their target aphid species. The TTR locus controls distinct, and contrasting, local and systemic plant responses between the near-isogenic cultivars. TTR-mediated plant responses imply interaction between a resistance factor(s) in vascular tissue and a bioactive component(s) of SAA saliva. Features of both resistance traits suggest homology to aphid resistance in other legumes; elucidation of their molecular mechanisms will likely apply to other aphid-plant interactions.