Plasmid vector for cloning infectious cDNAs from plant RNA viruses: high infectivity of cDNA clones of tomato aspermy cucumovirus.

An improved version of the previously obtained cloning vector pCass was constructed by partially duplicating the 35S promoter used to drive the transient transcription of cloned viral cDNAs. Full-length cDNAs of the three genomic RNAs of tomato aspermy cucumovirus (TAV) cloned in this improved pCass (designated pCass2) gave a 3-fold higher infectivity in two plant species tested than the same cDNAs cloned in pCass1 with only a single 35S promoter. Host range, symptoms, morphology of viral particles and viral progeny RNAs induced by these sets of infectious cDNA clones analysed were identical to those induced by the wild-type virus. A mutant of genomic TAV RNA 3 containing a 163 nt deletion in the 3' untranslated region was stably maintained in the progeny RNAs, indicating that these cDNA clones may facilitate a study of virus function. This is the first report of infectious cDNA clones of TAV as well as of infectious cDNA clones with a duplicated 35S promoter of CaMV.