The structure of beet yellows virus filaments

Abstract

Distribution and conformation of the beet yellow stunt virus (BYSV) was investigated in the phloem tissue of Sonchus oleraceus L. (Asteraceae). Virus was found in companion cells, phloem parenchyma cells, and some sieve elements. None was encountered outside the phloem. Some parenchymatic cells contained only vesicles with fibrillar networks, others, vesicles and a small number of virus particles. In most of the infected cells virus particles, scattered or in aggregates, were not accompanied by vesicles. The vesicles had a single delimiting membrane in an apparently early stage of infection of a cell. Later, individual vesicles or groups of them became enclosed in a second membrane, which probably originated from the endoplasmic reticulum. Individual parenchyma cells with virus were partly or completely degenerated. In such cells, ribosomes were the last cell component to disintegrate. Finally, virus also appeared to degenerate. Some nuclei contained virus without displaying degenerative changes in the plane of the sections. Others showed rupture of the nuclear envelope. In such nuclei, virus particles were continuous between cytoplasm and nucleus through the gap in the envelope. The effect of BYSV on Sonchus is compared with that of the same virus on the sugarbeet and with the effect of the similar beet yellows virus (BYV) on its hosts.