Two plant species, Nicotiana clevelandii and Nicotiana glutinosa, were chosen for inoculation of the plasmid cDNA clones. Unless specified otherwise, all inocula used were a mixture of the three genomic cDNA clones (corresponding to RNAs 1, 2 and 3), each at an equal concentration of 10 mg/10 ml per plant. For each test, at least five, four-leaf plants of each of the above two species were inoculated mechanically after a 24-h dark treatment. Each test was repeated at least once.