Stability of rice yellow mottle virus and cellular compartmentalization during the infection process in Oryza sativa (L.).

Rice yellow mottle virus (RYMV) is icosahedral in morphology and known to swell in vitro, but the biological function of swollen particles remains unknown. Anion-exchange chromatography was used to identify three markedly stable forms of RYMV particles from infected plants: (1) an unstable swollen form lacking Ca2+ and dependent upon basic pH; (2) a more stable transitional form lacking Ca2+ but dependent upon acidic pH; and (3) a pH-independent, stable, compact form containing Ca2+. Particle stability increased over the time course of infection in rice plants: transitional and swollen forms were abundant during early infection (2 weeks postinfection), whereas compact forms increased during later stages of infection. Electron microscopy of infected tissue revealed virus particles in vacuoles of xylem parenchyma and mesophyll cells early in the time course of infection and suggested that vacuoles and other vesicles were the major storage compartments for virus particles. We propose a model in which virus maturation is associated with the virus accumulation in vacuoles. In this acidic compartment, virus particles may bind Ca2+ to produce a highly stable, compact form of the virus. The localization of subcellular RYMV isoforms in infected cells and the corresponding biological properties of the virus are discussed.