Recessive resistance: VPg-eIF4E interactions

Genetic resistance is the major means of protecting crops against virus diseases and globally both natural and transgenic strategies have been effectively deployed. The scope of the European interest in virus resistance is the topic of a EU Co-ordinated action programme (FOOD-CT-2005-006961) called “Resistvir” (www.resistvir-db.org/ ). This project aims to co-ordinated European activities in this area and develop resources that will assist researchers and plant breeders in identifying existing and new sources of virus resistance.

Pea seed-borne mosaic virus (PSbMV) is a member of the potyvirus group. PSbMV's primary host is pea, but also causes disease in lentils and broadbeans. Some lines of pea carry recessive resistances to PSbMV. Resistance gene sbm-1 belongs to one of two clusters of recessive genes in pea on linkage groups II and IV which confer resistance to pathotypes of PSbMV and other viruses of the potyvirus group.

Through a combination of positional cloning and candidate gene analysis, we have characterized sbm-1 as a mutant of the eukaryotic translation initiation factor 4E (eIF4E) (Gao et al, 2004a).

Factor eIF4E normally mediates one of the first steps in mRNA translation by binding to the 5’ m7G-cap. Mutant eIF4Es mediating resistance have altered amino acids adjacent to the cap-binding pocket although a block to cap-binding appears not to be essential to achieve resistance.

Associated with the resistance phenotype we showed that eIF4E also had the capacity to move from cell-to-cell (Gao et al, 2004b). The role of eIF4E as a dominant susceptibility factor is not yet clear but we are exploring the possibility that it might in some way potentiate virus movement.

In addition, we are attempting to crystallise pea eIF4E alone and in a complex with VPg that will allow us to understand the nature of the interaction and make predictions about novel potential resistance alleles.

VPg as a regulator of virus movement

As part of our earlier investigations as to the relationship between PSbMV VPg and the sbm-1 gene in pea, we utilised VPg as a bait in a yeast two-hybrid screen for protein interactors.  We identified a class of specific cysteine-rich proteins from several different hosts that interact with the VPg protein from several different potyviruses. We have called this protein potyvirus VPg-interacting protein 1 (PVIP1). This protein has no direct connection with recessive resistance genes, but Arabidopsis plants down-regulated for PVIP expression using RNAi have delayed symptoms indicative of an effect on virus long distance movement (Dunoyer et al, 2004).

From the amino acid sequence of PVIP, it is not obvious what its role is in virus movement might be.

We are using genetic, cell biological and biochemical techniques to determine the function of PVIP and its role in potyvirus infection.