Properties of a previously undescribed grapevine nepovirus from Tunisia

A virus with isometric particles c. 30 nm in diameter and angular contour was isolated by inoculation of sap from a Tunisian grapevine with mild mottling and leaf deformation. The virus sedimented in sucrose density gradients as three components: T (empty shells), M (particles containing a molecule of ssRNA with an apparent size of 5,800 nucleotides, constituting 35% of the particle weight) and B (particles containing a molecule of ssRNA with apparent size of 6,800 nucleotides, constituting 41% of the particle weight). Virus particles had buoyant densities of 1.31 (T), 1.45 (M), and 1.49 g/cm3 (B) in cesium chloride equilibrium gradients. The coat protein subunits consisted of a single polypeptide with mol. wt. of c. 59,000 daltons. An antiserum was produced with a titer of 1:256, which did not react with healthy plant antigens. Cells of artificially infected herbaceous hosts showed cytoplasmic vesiculate-vacuolate inclusion bodies, virus-containing tubules, mostly associated with plasmodesmata and/or cell wall protrusions, and crystalline aggregates of virus particles and empty capsids. The physicochemical and ultrastructural properties of this virus resemble very much those of nepoviruses. However, it was serologically unrelated to 19 different members of the group, including all those reported to infect grapevines. Therefore, the virus is possibly a hitherto unreported nepovirus for which the name of grapevine Tunisian ringspot virus (GTRV) is proposed.