An alfalfa mosaic virus RNA 2 mutant, which does not induce a hypersensitive reaction in cowpea plants, is multiplied to a high concentration in cowpea protoplasts.

A mutant of alfalfa mosaic virus (AMV), which in contrast to wild type (wt) can invade cowpea plants systemically, is replicated more efficiently in cowpea protoplasts than the wt. Mutant preparations isolated from infected cowpea protoplasts contained a higher amount of middle component (M, containing RNA 2) than wt preparations. Both in cowpea plants and in cowpea protoplasts a wt phenotype is obtained upon addition of wt M to this mutant, suggesting a correlation between the type of plant reaction evoked by the virus infection and the regulation of viral RNA synthesis.