Movement and Intracellular Location of Sonchus Yellow Net Virus within Infected Nicotiana edwardsonii

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Abstract
Summary Systemic movement of Sonchus yellow net virus to leaves and roots was first detected by ELISA 24 h after mechanical inoculation. Thereafter, virus levels rose to a maximum 10 days after inoculation; the highest levels were between 4.0 and 7.3 µg/g tissue, in leaves which were not yet fully expanded. Electron microscopy of tissue sections revealed that when the virus content of tissues was greatest, virtually all leaf and root cells were infected. Most of the virions were in the perinuclear space; only a few were scattered in the cytoplasm. Nuclei contained large viroplasms associated with viral nucleocapsids. Between 10 and 20 days after inoculation, levels of virus antigen and viral RNA fell to about 20% of their maximum. By 20 days after inoculation, no more than 10% of cells contained virus particles and almost all the virions were in the cytoplasm. These results suggest that this virus spreads systemically until most or all cells are infected. The plants then undergo a recovery phase during which virus disappears from the nuclei of infected cells and vesiculates into the cytoplasm.