The mechanism of infection of plant protoplasts by viruses.

The process of virus infection of protoplasts isolated from tobacco leaves has been examined by means of electron microscopy. Immediately after inoculation, virus particles appear at two types of site: trapped in complex surface lesions of the plasmalemma, or in peripheral cytoplasmic vesicles. The complex lesions are only visible after treatment of the protoplasts with inocula containing poly-L-ornithine. With infection by tobacco mosaic virus and cowpea chlorotic mottle virus, which require poly-L-ornithine, the majority of virus particles occur at lesion sites. Pea enation mosaic virus, which does not require poly-L-ornithine for infection to become established, is found predominantly and in high numbers in peripheral vesicles. The behaviour of these three viruses is discussed in terms of a probable mechanism for infection of the protoplasts.