The coat protein of tobamovirus acts as elicitor of both L2 and L4 gene-mediated resistance in Capsicum

In Capsicum, the resistance conferred by the L-2 gene is effective against all of the pepper-infecting tobamoviruses except Pepper mild mottle virus (PMMoV), whereas that conferred by the L-4 gene is effective against them all. These resistances are expressed by a hypersensitive response, manifested through the formation of necrotic local lesions (NLLs) at the primary site of infection. The Capsicum L-2 gene confers resistance to Paprika mild mottle virus (PaMMV), while the L-4 gene is effective against both PaMMV and PMMoV. The PaMMV and PMMoV coat proteins (CPs) were expressed in Capsicum frutescens ((LL2)-L-2) and Capsicum chacoense((LL4)-L-4) plants using the heterologous Potato virus X (PVX)-based expression system. In C. frutescens ((LL2)-L-2) plants, the chimeric PVX virus containing the PaMMV CP was localized in the inoculated leaves and produced NLLs, whereas the chimeric PVX containing the PMMoV CP infected the plants systemically. Thus, the data indicated that the PaMMV CP is the only tobamovirus factor required for the induction of the host response mediated by the Capsicum L-2 resistance gene. In C. chacoense ((LL4)-L-4) plants, both chimeric viruses were localized to the inoculated leaves and produced NLLs, indicating that either PaMMV or PMMoV CPs are required to elicit the L-4 gene-mediated host response. In addition, transient expression of PaMMV CP into C. frutescens ((LL2)-L-2) leaves and PMMoV CP into C. chacoense ((LL4)-L-4) leaves by biolistic co-bombardment with a beta-glucuronidase reporter gene led to the induction of cell death and the expression of host defence genes in both hosts. Thus, the tobamovirus CP is the elicitor of the Capsicum L-2 and L-4 gene-mediated hypersensitive response.