Virus symptomatology in accessions of the Medicago truncatula core collection and identification of virus resistance phenotypes

Plants of 212 accessions from the core collection of model legume species Medicago truncatula were inoculated with infective sap containing Alfalfa mosaic virus (AMV, isolate EW), Bean yellow mosaic virus (BYMV, isolate MI) or Cucumber mosaic virus (CMV, isolate SN-1). A diverse range of systemic symptoms were obtained that varied widely in severity depending on the combination of virus isolate and accession, or, especially with AMV, some accessions became infected but did not display symptoms. The delay between virus inoculation and symptom appearance normally varied from 1 to 4 weeks, but with CMV it took up to 8 weeks in two accessions. Five (AMV), 59 (BYMV) and 22 (CMV) core accessions remained uninfected systemically. Plants of most of these accessions, and some that died or gave susceptible phenotypes, were then inoculated with two additional isolates of AMV (eight accessions), or two distinct strains of BYMV (58 accessions) and CMV (21 accessions). Plants of accession 11715 remained uninfected by CMV isolates CP (CMV subgroup 1) and LW (CMV subgroup 2), but those of all other previously uninfected accessions became infected systemically by all three viruses. All accessions inoculated with AMV isolates Aq and Hu, and most inoculated with BYMV isolate LKoj1-NN (generalist strain), BYMV isolate LP-1 (lupin strain), and CMV isolates CP and LW developed typical susceptible phenotypes. However, systemic hypersensitive phenotypes developed with BYMV LKoj1-NN and LP-1 in plants of 4456, or with LKoj1-NN only in 774, 1526, 4327, 14829, 15268, 22922 and 25654; 15268 and 25654 had developed this phenotype previously with MI (generalist strain). Similarly, plants of 21362 developed this phenotype with CMV CP and LW, while plants of 1526, 2748 and 31443 developed it with CP; 2748, 21632 and 31443 had developed it previously with SN-1 (mixture of subgroups 1 and 2). Once the genetic bases of the BYMV and CMV resistances found in M. truncatula are understood, they may prove useful in future virus resistance breeding among crop and pasture legumes.