SUMMARY B component RNA (B-RNA) of red clover mottle comovirus (RCMV) was not transported between cells of inoculated leaf tissue unless it was co-inoculated with the M component. However when host plants were infected with sunn-hemp mosaic tobamovirus (SHMV) or mutant Nill8 of tobacco mosaic virus (TMV) before superinoculation, RCMV B-RNA was transported readily between cells. Plants were infected with SHMV or Nill8 and, 5 days after superinoculation with RCMV B component, protoplasts were isolated from the infected leaves and inoculated with RCMV M component. About 30% of such protoplasts multiplied RCMV (identified by immunofluorescence microscopy) whereas only 3 to 5 % of protoplasts from similarly treated plants not initially infected with SHMV or Nil 18 multiplied RCMV. Thus B-RNA spread from mixedly infected (SHMV + B-RNA or Nil 18 + B-RNA) cells in the absence of M-RNA to the neighbouring cells. RCMV B-RNA spread in leaves infected with the temperature-sensitive coat protein TMV mutant Nil 18 and grown at nonpermissive temperature. Thus TMV coat protein is not involved in enabling RCMV RNA to be transported.

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