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The Use of Green Fluorescent Protein-Tagged Recombinant Viruses to Test Lettuce mosaic virus Resistance in Lettuce

First, second, fourth, and fifth authors: Equipe de Virologie, UMR GD2P, IBVM, Centre INRA de Bordeaux, BP81, 33883 Villenave d'Ornon cedex, France; and third author: Unité de Génétique et d'Amélioration des Fruits et Légumes, INRA, Domaine Saint Maurice, BP94, 84143 Montfavet cedex, France

Seed certification and the use of cultivars containing one of two, probably allelic, recessive genes, mo1¹ and mo1², are the principal control methods for Lettuce mosaic virus (LMV) in lettuce. Although for a few LMV isolates, mo1² confers resistance with most isolates, the genes mo1¹ or mo1² confer a tolerance, and virus accumulation is readily detected in mo1-carrying plants. This phenotype complicates evaluation of the resistance status, in particular for mo1¹, 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 mo1-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 mo1 resistance status. Similarly, the LMV-E-derived recombinant allowed the identification of mo1² lettuce lines because its systemic movement was restricted in mo1² lines but not in susceptible or in mo1¹ 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.