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.
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