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...

SUMMARYArabidopsis thaliana represents a valuable and efficient model to understand mechanisms underlying plant susceptibility to viral diseases. Here, we describe the identification and molecular cloning of a new gene responsible for recessive resistance to several isolates of Watermelon mosaic virus (WMV, genus Potyvirus) in the Arabidopsis Cvi-0 accession. rwm1 acts at an early stage of infection by impairing viral accumulation in initially infected leaf tissues. Map-based cloning delimited rwm1 on chromosome 1 in a 114-kb region containing 30 annotated genes. Positional and functional candidate gene analysis suggested that rwm1 encodes cPGK2 (At1g56190), an evolutionary conserved nucleus-encoded chloroplast phosphoglycerate kinase with a key role in cell metabolism. Comparative sequence analysis indicates that a single amino acid substitution (S78G) in the N-terminal domain of cPGK2 is involved in rwm1-mediated resistance. This mutation may have functional consequences because it targets a highly conserved residue, affects a putative phosphorylation site and occurs within a predicted nuclear localization signal. Transgenic complementation in Arabidopsis together with virus-induced gene silencing in Nicotiana benthamiana confirmed that cPGK2 corresponds to rwm1 and that the protein is required for efficient WMV infection. This work uncovers new insight into natural plant resistance mechanisms that may provide interesting opportunities for the genetic control of plant virus diseases.

SummaryMedicago truncatula has been widely adopted as a model plant for crop legume species of the Vicieae. Despite the availability of transformation and regeneration protocols, there are currently limited tools available in this species for the systematic investigation of gene function. Within the framework of the European Grain Legumes Integrated Project (http:// www.eugrainlegumes.org), chemical mutagenesis was applied to M. truncatula to create two mutant populations that were used to establish a TILLING (targeting induced local lesions in genomes) platform and a phenotypic database, allowing both reverse and forward genetics screens. Both populations had the same M2 line number, but differed in their M1 population size: population 1 was derived from a small M1 population (one-tenth the size of the M2 generation), whereas population 2 was generated by single seed descent and therefore has M1 and M2 generations of equal size. Fifty-six targets were screened, 10 on both populations, and 546 point mutations were identified. Population 2 had a mutation frequency of 1/485 kb, twice that of population 1. The strategy used to generate population 2 is more efficient than that used to generate population 1, with regard to mutagenesis density and mutation recovery. However, the design of population 1 allowed us to estimate the genetically effective cell number to be three in M. truncatula . Phenotyping data to help forward screenings are publicly available, as well as a web tool for ordering seeds at

SummarySharka is a devastating viral disease caused by the Plum pox virus (PPV) in stone fruit trees and few sources of resistance are known in its natural hosts. Since any knowledge gained from Arabidopsis on plant virus susceptibility factors is likely to be transferable to crop species, Arabidopsis's natural variation was searched for host factors essential for PPV infection.To locate regions of the genome associated with susceptibility to PPV, linkage analysis was performed on six biparental populations as well as on multiparental lines. To refine quantitative trait locus (QTL) mapping, a genome-wide association analysis was carried out using 147 Arabidopsis accessions.Evidence was found for linkage on chromosomes 1, 3 and 5 with restriction of PPV longdistance movement. The most relevant signals occurred within a region at the bottom of chromosome 3, which comprises seven RTM3-like TRAF domain-containing genes. Since the resistance mechanism analyzed here is recessive and the rtm3 knockout mutant is susceptible to PPV infection, it suggests that other gene(s) present in the small identified region encompassing RTM3 are necessary for PPV long-distance movement.In consequence, we report here the occurrence of host factor(s) that are indispensable for virus long-distance movement.