The use of green fluorescent protein-tagged recombinant viruses to test Lettuce mosaic virus resistance in lettuce

Abstract

Seed certification and the use of cultivars containing one of two, probably allelic, recessive genes, mol1 and mol2, are the principal control methods for Lettuce mosaic virus (LMV) in lettuce. Although for a few LMV isolates, mol2 confers resistance with most isolates, the genes mol1 or mol2 confer a tolerance, and virus accumulation is readily detected in mol-carrying plants. This phenotype complicates evaluation of the resistance status, in particular for mol1, for which there are no viral strains against which a true resistance is expressed. Two green fluorescent protein (GFP)-tagged viruses were constructed, derived from a non-resistance breaking isolate (LMV-0) and from a resistance-breaking isolate (LMV-E). An evaluation of 101 cultivars of known status was carried out with these recombinant viruses. Using the LMV-0-derived recombinant, identification of mol-carrying cultivars was simple because, contrary to its wild-type parent, systemic movement of LMV-0-GFP was abolished in resistant plants. This assay detected four cases of misidentification of resistance status. In all these cases, further tests confirmed that the prior resistance status information was incorrect, so that a 100% correlation was observed between LMV-0-GFP behavior and the mol resistance status. Similarly, the LMV-E-derived recombinant allowed the identification of mol2 lettuce lines because its systemic movement was restricted in mol2 lines but not in susceptible or in mol1 lines. The tagged viruses were able to systemically invade another host, pea, irrespective of its resistance status against another member of the genus Potyvirus, Pea seed-borne mosaic virus. The use of these recombinant viruses could therefore greatly facilitate LMV resistance evaluation and speed up lettuce breeding programs.