Abstract
Ten new thermosensitive (ts) mutants of alfalfa mosaic virus (AMV), a plant virus with a tripartite genome, are described. Stable ts mutants were induced by u.v. irradiation of purified middle (M) component. Mutants were isolated by subculturing in bean plants (instead of in the usual plant host, tobacco), in which unstable ts mutants apparently arise spontaneously. As well as ts mutants, we found a mutant which in contrast to the parent strain (AMV 425) was able to infect bean plants systemically. As expected, all stable mutations mapped on M component. Complementation analysis confirmed the presence of two complementation groups on M component. This complementation is probably intracistronic. Furthermore, we found that mutants belonging to the same complementation group often interfered with the multiplication of each other, even at the permissive temperature. Taken together, these results could suggest that the product directed by AMV RNA 2 (the RNA inside M component) has two functional domains and is active in a multimeric form.