Involvement of the cylindrical inclusion (CI) protein in the overcoming of an eIF4E-mediated resistance against Lettuce mosaic potyvirus

Abstract

The capacity of Lettuce mosaic virus to overcome the lettuce resistance conferred by the mo1(1) and mo1(2) alleles of the gene for eukaryotic translation initiation factor 4E (eIF4E) was analysed using reverse genetics. Mutations in the virus genome-linked protein (VPg) allowed mo1(1) only to be overcome, but mutations in the C-terminal portion of the cylindrical inclusion (CI) protein allowed both alleles to be overcome. Site-directed mutagenesis pinpointed a key role of the amino acid at position 621 in the virulence. This is the first example of the involvement of a potyviral CI protein in the breaking of an eIF4E-mediated resistance.

Figure 1

Structure and biological properties of susceptible and resistant lettuce plants of the Lettuce mosaic virus (LMV) recombinants used in this study. Left: schematic representations of the genomes of the recombinants constructed between LMV‐0 and LMV‐E. Coding sequences of LMV‐0 and LMV‐E are represented by white and grey boxes, respectively. Restriction sites used to construct the recombinants and their position along the LMV genome are indicated. Right: behaviour of the parental LMV isolates and of the recombinants in susceptible (mo1⁰, cultivar Salinas) or resistant (mo1¹, cultivar Mantilia; mo1 ², cultivar Salinas 88) lettuce plants. Virus, viral accumulation in systemically infected leaves; Sympt, symptoms; –, no viral accumulation or no symptoms; +, viral accumulation or symptoms. *(+) denotes sporadic detection of low‐level (80% less than in susceptible plants), symptomless, systemic LMV‐0 accumulation in some Salinas 88 plants.

Figure 2

Effects of mutations of amino acid 621 on the accumulation of the various viruses in susceptible and resistant lettuce. Plants from the varieties Salinas (mo1⁰, susceptible), Salinas 88 (mo1²) and Mantilia (mo1¹) were assayed. The accumulation of each virus was determined by enzyme‐linked immunosorbent assay (ELISA) and expressed as a percentage of the average value measured for LMV‐0 on the susceptible lettuce cultivar Salinas. The samples (at least three plants for each virus) were collected from non‐inoculated leaves (level 3 from the top of the plant) at 21 days post‐inoculation (dpi) in at least four independent experiments. Average values are shown, together with standard deviations.