Non-structural Functions of Hordeivirus Capsid Protein Identified in Plants Infected by a Chimeric Tobamovirus

Capsid proteins (CPs) of (+)RNA–containing plant viruses are multifunctional proteins involved in many stages of viral infection cycle, in addition to their main function of virus capsid formation. For example, the tobamoviral CP ensures virus systemic transport in plants and defines the virus–host interactions, thereby influencing the virus host range, virus infectivity, pathogenicity, and manifestation of infection symptoms. Hordeiviruses and tobamoviruses belong to the Virgaviridae family and have rod–shaped virions with a helical symmetry; their CPs are similar in structure. However, no non–structural functions of hordeiviral CPs have been described so far. In this study, we assayed possible non–structural functions of CP from the barley stripe mosaic virus (BSMV) (hordeivirus). To do this, the genome of turnip vein clearing virus (TVCV) (tobamovirus) was modified by substituting the TVCV CP gene with the BSMV CP gene or its mutants. We found that BSMV CP efficiently replaced TVCV CP at all stages of viral infection. In particular, BSMV CP performed the role of tobamoviral CP in the long–distance transport of the chimeric virus, acted as a hypersensitive response elicitor, and served as a pathogenicity determinant that influenced the symptoms of the viral infection. The chimeric tobamovirus coding for the C –terminally truncated BSMV CP displayed an increased infectivity and was transported in plants in a form of atypical virions (ribonucleoprotein complexes).