Bluegreen aphid resistance in breeding lines of the model legume Medicago truncatula

Medicago truncatula Gaertn is one of several legume species bred in Australia for use in pastures. Because of its small diploid genome size, short generation time, and ability to be genetically transformed, it has also recently become a popular model plant for molecular genetic studies. Aphids, including the bluegreen aphid (Acyrthosiphon kondoi Shinji), are important pests of Australian pasture legumes. Improved aphid resistance is one of the priorities of the breeding program for M truncatula and other forage legumes (eg. subterranean clover, lucerne). Aphid resistance has been introgressed into three popular M truncatula varieties (Jemalong, Cyprus, Borung) from resistant stock to create new aphid-resistant cultivars (Jester, Caliph, Mogul). Mass exposure to aphids during bulk screening of progeny from these recurrent backcrosses is effective for selecting resistant lines, but provides no information about the nature of the resistance. Additional information can be obtained by determining what aspects of aphid behaviour or biology are affected by the resistant phenotypes. In this study, mean relative growth rate (MRGR) and survival of bluegreen aphid were compared among aphid-resistant varieties, their susceptible parent cultivars, and the accession lines from which the aphid resistance was first bred. Both MRGR and survival were reduced on the resistant lines compared to the susceptible. Additional information from electrical penetration graph (EPG) recordings has indicated that the resistance is primarily phloem-based. Thus it appears the resistance is affecting both the biology and behaviour of the aphids. This information forms the basis for new studies investigating the molecular basis for aphid resistance in M truncatula, including the identification of plant defence genes induced by aphid feeding.