Genetic Dissection Of Plant Resistance To Aphids In Medicago Truncatula

Aphids are uniqiotous and serious pests that cause substantial losses to agriculture by draining plant nutrients, injecting plant elicitors and transmitting pathogenic viruses. While the biology of resistance to aphids has been studied in detail, the genetic basis underlying this resistance is still poorly understood.Medicago truncatula, an important pasture, was severiously damaged by aphids in Austalia in the 1970s. Since then, several cultivars with high level of resistance have been developed. One of these cultivars, Jester, showed strong resistance to Acyrthosiphon kondoi (bluegreen aphid; BGA) conditioned by the gene AKR. Jester also exhibits resistance to an Australian biotype of pea aphid (PA; A. pisum). Since BGA and PA belong to the same genus, Acyrthosiphon, and the loci for the the resistance to them were linked, we wonder whether the PA resistance of Jester is also conferred by the same resistant gene AKR. To answer this question, we first evaluated the four progenitors of Jester for resistance to BGA and PA, respectively. The results showed that SA1499 is the donor of BGA resistance, while SA10733 is the donor of PA resistance. Hence, the origins of resistant abilities for BGA and PA in Jester are derived from different progenitors, suggesting two distinct resistant genes.Then, a F2:3 population was created from a cross between Jester and the susceptible accession A20 and used for analyzing PA resistance in Jester. The segregation ratio indicated that the PA resistance was also conferred by a single dominant gene. Furthermore, this gene was mapped to the middle of chromosome 3, between the SSR markers h2₃9a22a and h2₁80m21a, which is independent of the locus of BGA resistance gene AKR. Here, we termed the locus for PA resistance as the Acyrthosiphon pisum resistance gene (APR). The PA resistance genes APR1 in SA10733 was also mapped to the the same region as APR in Jester corresponding to the fact that SA10733 is the origin of PA resistance in Jester.To identify novel sources of resistance to PA in M. truncatula, a total of 34 accessions from M. truncatula core collection were examined by a time-course experiment based on both the aphid performance and the plant reaction to infestation, with Jester as a resistant check. No accessions showed higher level of resistance than Jester while eight accessions including Caliph showed similar level of resistance to PA. Since Caliph has the same progenitors as Jester, SA10733 is also the donor of PA resistance in Caliph. For the rest of seven resistant accessions, two random accessions SA10481 and SA1516 were used in the following experiments. Both of them showed tolerance and antibiosis to PA and the resistance was phloem-based, which was not significantly different from Jester, Caliph and SA10733. The PA resistance gene APR2 in SA10481 was also found in that region on chromosome 3 as APR in Jester and APR1 in SA10733. In addition, allelism tests between SA10733, SA10481 and SA1516 indicated that the PA resistance genes from them were allelic genes or closely linked to each other. Taken the information together, we can conclude that the genetic bases of these PA resistant accessions are similar (if not the same).Besides charaterisation of qualitative resistance genes, quantitative resistance to PA was also important for M. truncatula breeding. Reference genotype A17 of M. truncatula shows moderate resistance to both BGA and PA in comparison with Jester and A20. Previous studies have reported that the feeding of both BGA and PA induced hypersensitive response (HR) in A17 conferred by the same gene AIN. For BGA,AIN-mediated resistance to BGA involves antibiosis but not plant tolerance. However, it remained unclear the role of the AIN-mediated response following PA infestation on A17. In this study, both choice and no choice tests revealed the antibiosis effect of A17 to PA. Analysis of feeding behavior found that the resistance to PA in A17 occurred at the level of the phloem. Host choice experiment suggested that there was no antixenosis involved in the resistance to PA in A17. To dissect the genetic basis of moderate resistance, the quantitative trait loci (QTL) analysis using recombinant inbred lines derived from a cross between A17 and A20 was carried out. For both PA and BGA, plant biomass loss and aphid colony biomass were conferred by independent loci. Moreover, aphid colony biomass co-located closely with AIN. However, plant biomass losses due to aphid infestation were mapped to different chromosomes for PA and BGA. The identification of QTLs for two closely-related aphid species will lead to a better understanding of the genetic basis of moderate resistance to aphid in plants and allow the improvement of aphid resistance in pasture crops through marker-assisted breeding.