Previous resistance analyses of Arabidopsis thaliana mutants knocked out for eukaryotic translation initiation factors showed that disruption of the At-eIF(iso)4E or both the AteIF(iso)4G1 and At-eIF(iso)4G2 genes resulted in resistance against turnip mosaic virus (TuMV). This study selected TuMV virulent variants that overcame this resistance and showed that two independent mutations in the region coding for the viral genome-linked protein (VPg) were sufficient to restore TuMV virulence in At-eIF(iso)4E and At-eIF(iso)4G1¾At-eIF(iso)4G2 knockout plants. As a VPg-eIF(iso)4E interaction has been shown previously to be critical for TuMV infection, a systematic analysis of the interactions between A. thaliana eIF4Es and VPgs of virulent and avirulent TuMVs was performed. The results suggest that virulent TuMV variants may use an eIF4F-independent pathway.The eukaryotic translation initiation factors eIF4E and eIF4G play a key role during virus infection in plants ( Robaglia & Caranta, 2006). During mRNA translation, eIF4E provides the cap-binding function and is associated with eIF4G, the scaffold for the other components of the translation initiation complex, to form the eIF4F complex (Marcotrigiano et al., 1999). In plants, a second eIF4F complex called eIF(iso)4F is present, and results from the assembly of eIF(iso)4E and eIF(iso)4G isoforms (Browning, 2004). These two complexes perform essentially the same task in translation, but have different affinities for certain classes of mRNA substrates and are probably involved in different cellular events (Gallie & Browning, 2001). Plant genes encoding proteins from the eIF4F and eIF(iso)4F complexes belong to small gene families. In Arabidopsis thaliana, three genes encode the eIF4E subfamily proteins (eIF4E1, eIF4E2 and eIF4E3), one encodes eIF(iso)4E and one a non-canonical eIF4E-like protein, known as novel cap-binding protein (nCBP). A single gene encodes eIF4G, whilst two genes encode the eIF(iso)4G subfamily proteins [eIF(iso)4G1 and eIF(iso)4G2] ( Robaglia & Caranta, 2006). Viruses that depend on these factors to perform their infectious cycle include members of the genus Potyvirus, the largest and the most diverse genus of plant viruses (Adams et al., 2005;Shukla et al., 1994). Potyviruses are characterized by a positive-sense, single-stranded RNA genome of about 10 kb that encodes at least ten multifunctional proteins. The genome is 39-polyadenylated and covalently linked at its 59 end to a virus-encoded protein (VPg) (Murphy et al., 1991;Revers et al., 1999).Potyviruses may selectively use either the eIF4E or eIF(iso)4E isoform in A. thaliana to achieve a successful infection (Sato et al., 2005). We have demonstrated that potyviruses, including turnip mosaic virus (TuMV), selectively use members of the eIF4G family in a fashion that parallels the selective recruitment of eIF4E isoforms (Nicaise et al., 2007;Sato et al., 2005 ; data not shown).In order to confirm that each single amino acid change in the VPg cistron of TuMV determined its virulence on the At...