Morphogenesis of Sonchus yellow net virus in cowpea protoplasts

The development of Sonchus yellow net virus was studied in ultrathin sections of protoplasts sampled at various intervals after inoculation. The sequence of major ultrastructural changes consisted of appearance in the nucleus, 10 hr after inoculation, of a granular matrix with coiled nucleocapsid strands at its edge, budding of virus particles at the inner nuclear membrane followed by the release of virus particles into the perinuclear space and their spread in the endoplasmic reticulum. Fusion of viral envelopes with endoplasmic reticulum membranes occurred from 24 hr after inoculation and was followed by release of coiled nucleocapsid strands in the cytoplasm. Incubation of infected protoplasts in the presence of tunicamycin prevented budding of virus particles and cores accumulated in the nucleus. Neither virus particles nor cores were detected in the cytoplasm.