Structure of Citrus Ringspot-Psorosis-Associated Virus Particles: Implications for Diagnosis and Taxonomy

Abstract

Negative staining and electron microscopy has revealed particles of similar morphology in preparations of citrus and Chenopodium quinoa experimentally infected with several virus isolates associated with citrus ringspot and psorosis symptoms. The morphology is complex; the filamentous particles come in a t least two sizes (contour lengths) and occur in an open circular fonn and a linear form, plus intermediates. The linear (or L) form may appear as a sinuous structure about 10 nm in diameter and up to 2,500 nm long; it is in reality a two-stranded structure with loops a t the ends, formed from a collapsed circle. The open circular form (0 form) is a highly kinked filament 3 to 4 nm in diameter. The morphology suggests that this type of virus represents a new genus of viruses similar to the Tenuiviruses and the Bunyaviridae; these possess an ambisense divided ssRNA genome, each RNA being circularized by means of a panhandle structure of complementary base sequences a t the 5' and 3' termini. We propose the name 'Ophiovirus' (from the Greek ophis, meaning 'serpent') for this new genus, because the particles have a snaky appearance. The finding of morphologically indistinguishable particles in isolates associated with ringspot symptoms or with psorosis symptoms is further evidence that both types of symptom are associated with the same kind of virus.