Olivier Navaud,3 and
1INRA, UR407 Pathologie Végétale, F-84140 Montfavet, France; 2INRA, UR1052 Génétique et Amélioration des Fruits et Légumes, F-84140 Montfavet, France; 3INRA, UMR441 Laboratoire des Interactions Plantes Microorganismes, F-31326, Castanet-Tolosan, France
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Accepted 17 February 2010.
To understand why the Pvr4 resistance of pepper against Potyvirus spp. remained durable in field conditions while virulent Potato virus Y (PVY) variants could be selected in the laboratory, we studied the molecular mechanisms which generated these variants and the consequences on viral fitness. Using a reverse genetics approach with an infectious cDNA clone of PVY, we found that the region coding for the NIb protein (RNA-dependent RNA polymerase) of PVY was the avirulence factor corresponding to Pvr4 and that a single nonsynonymous nucleotide substitution in that region, an adenosine to guanosine substitution at position 8,424 of the PVY genome (A8424G), was sufficient for virulence. This substitution imposed a high competitiveness cost to the virus against an avirulent PVY variant in plants devoid of Pvr4. In addition, during serial passages in susceptible pepper plants, the only observed possibility of the virulent mutant to increase its fitness was through the G8424A reversion, strengthening the high durability potential of the Pvr4 resistance. This is in accordance with the fact that the NIb protein is one of the most constrained proteins expressed by the PVY genome and, more generally, by Potyvirus spp., and with a previously developed model predicting the durability of virus resistances as a function of the evolutionary constraint applied on corresponding avirulence factors.
The American Phytopathological Society, 2010