The eIF4E and eIF(iso)4E cDNAs from several genotypes of lettuce (Lactuca sativa) that are susceptible, tolerant, or resistant to infection by Lettuce mosaic virus (LMV; genus Potyvirus) were cloned and sequenced. Although Ls-eIF(iso)4E was monomorphic in sequence, three types of Ls-eIF4E differed by point sequence variations, and a short in-frame deletion in one of them. The amino acid variations specific to Ls-eIF4E(1) and Ls-eIF4E(2) were predicted to be located near the cap recognition pocket in a homology-based tridimensional protein model. In 19 lettuce genotypes, including two near-isogenic pairs, there was a strict correlation between these three allelic types and the presence or absence of the recessive LMV resistance genes mo1(1) and mo1(2). Ls-eIF4E(1) and mo1(1) cosegregated in the progeny of two separate crosses between susceptible genotypes and an mo1(1) genotype. Finally, transient ectopic expression of Ls-eIF4E restored systemic accumulation of a green fluorescent protein-tagged LMV in LMV-resistant mo1(2) plants and a recombinant LMV expressing Ls-eIF4E degrees from its genome, but not Ls-eIF4E(1) or Ls-eIF(iso)4E, accumulated and produced symptoms in mo1(1) or mo1(2) genotypes. Therefore, sequence correlation, tight genetic linkage, and functional complementation strongly suggest that eIF4E plays a role in the LMV cycle in lettuce and that mo1(1) and mo1(2) are alleles coding for forms of eIF4E unable or less effective to fulfill this role. More generally, the isoforms of eIF4E appear to be host factors involved in the cycle of potyviruses in plants, probably through a general mechanism yet to be clarified.

Lettuce downy mildew caused by Bremia lactucae is the most important disease of lettuce worldwide. Breeding for resistance to this disease is a major priority for most lettuce breeding programs. Many genes and factors for resistance to B. lactucae have been reported by multiple researchers over the past *50 years. Their nomenclature has not been coordinated, resulting in duplications and gaps in nominations. We have reviewed the available information and rationalized it into 51 resistance genes and factors and 15 quantitative trait loci along with supporting documentation as well as genetic and molecular information. This involved multiple rounds of consultation with many of the original authors. This paper provides the foundation for naming additional genes for resistance to B. lactucae in the future as well as for deploying genes to provide more durable resistance. Keywords

The tobacco (Nicotiana tabacum) element Tnt1 is one of the few identified active retrotransposons in plants. These elements possess unique properties that make them ideal genetic tools for gene tagging. Here, we demonstrate the feasibility of gene tagging using the retrotransposon Tnt1 in lettuce (Lactuca sativa), which is the largest genome tested for retrotransposon mutagenesis so far. Of 10 different transgenic bushes carrying a complete Tnt1 containing T-DNA, eight contained multiple transposed copies of Tnt1. The number of transposed copies of the element per plant was particularly high, the smallest number being 28. Tnt1 transposition in lettuce can be induced by a very simple in vitro culture protocol. Tnt1 insertions were stable in the progeny of the primary transformants and could be segregated genetically. Characterization of the sequences flanking some insertion sites revealed that Tnt1 often inserted into genes. The progeny of some primary transformants showed phenotypic alterations due to recessive mutations. One of these mutations was due to Tnt1 insertion in the gibberellin 3b-hydroxylase gene. Taken together, these results indicate that Tnt1 is a powerful tool for insertion mutagenesis especially in plants with a large genome.

Two resistances to downy mildew derived from Lactuca serriola were characterized genetically and mapped using molecular markers. Classical genetic analysis suggested monogenic inheritance; however, the presence of multiple, tightly-linked genes in each case could not be eliminated. Therefore, they were designated resistance factors R17 and R18. Analysis with molecular markers known to be linked to clusters of resistance genes quickly revealed linkage of R18 to the major cluster of resistance genes and provided six linked markers, three RAPD (Random Amplified Polymorphic DNA) markers and three codominant SCAR (Sequence Characterized Amplified Region) markers. The mapping of R17 required the screening of arbitrary RAPD markers using bulked segregant analysis; this provided five linked markers, three of which segregated in the basic mapping population. This demonstrated loose linkage to a second cluster of resistance genes and provided additional linked markers. Two RAPD markers linked to R17 were converted into SCARs. The identification of reliable PCR-based markers flanking each gene will aid in selection and in combining these resistance genes with others.