Molecular and genetic study of plant resistance to Therioaphis trifolii in the model legume Medicago truncatula

Aphids are phloem feeding insects that utilize their slender stylets to consume copious amounts of photo-assimilates from plants. The spotted alfalfa aphid (SAA, Therioaphis trifolii) causes severe and widespread damage to important pasture legumes, such as subterranean clover (Trifolium subterraneum) and alfalfa (Medicago sativa) due to direct feeding damage and by vectoring plant viruses. Medicago truncatula, also known as barrel medic, is closely related to alfalfa/lucerne and a host of SAA. M. truncatula has recently been developed to be an important model plant to study plant resistance against various aphids at the physiological, biochemical, and molecular levels. The objectives of this project were to identify the genetic basis and to decipher the defence mechanisms of resistance to T. trifolii in M. truncatula. Three accessions of M. truncatula were identified through my studies and previous work as being useful materials: a highly resistant accession Jester, a moderately resistant accession A17 and a highly susceptible accession A20 to T. trifolii. Among these accessions, I characterised three forms of resistance modalities to T. trifolii, namely antibiosis, antixenosis and tolerance. Moreover, analysis of T. trifolii feeding behaviour using electrical penetration graph (EPG) procedures showed that the proportion of time that the aphids spent on ingesting phloem sap was significantly shorter on Jester and A17 than on A20. These results suggested resistance to T. trifolii is phloem-mediated. The treatment with exogenous salicylic acid, jasmonic acid and ethylene had negative impacts on the feeding of T. trifolii but more on the T. trifolii resistance accessions than the susceptible accession.