RECOMBINATION BETWEEN DEFECTIVE TOMBUSVIRUS RNAS GENERATES FUNCTIONALHYBRID GENOMES

The tombusviruses represent a group of small icosahedral plant viruses that contain monopartite positive-sense RNA genomes. Tombusviruses ar e able to generate small replicating deletion mutants of their genomes (i.e., defective interfering RNAs) during infections via RNA recombin ation and/or rearrangement. To further study the process of RNA recomb ination and to determine whether tombusviruses were capable of trans-r ecombination, protoplasts were coinoculated with in vitro-generated tr anscripts of a nonreplicating 3'-truncated genomic RNA of cucumber nec rosis tombusvirus and either replicative or replication-defective DI R NAs of tomato bushy stunt tombusvirus. After a 48-hr incubation, two d ominant replicative chimeric recombinant viral RNA populations were de tected that contained various large contiguous 5' segments of the cucu mber necrosis tombusvirus genomic RNA fused to 3'-terminal regions of the tomato bushy stunt tombusvirus defective interfering RNA. Some of the larger chimeric recombinants formed in protoplasts were able to sy stemically infect plants and induce wild-type symptoms. In addition, a functional chimeric genome was generated in planta after direct coino culation of whale plants with the defective RNA components. These resu lts indicate that (i) RNA recombination can occur relatively efficient ly in single-cell infections, (ii) trans-recombination can occur with nonreplicating viral RNA components, and (iii) functional chimeric gen omes can be generated via recombination. Possible mechanisms for the f ormation of the recombinants are proposed, and evolutionary implicatio ns are discussed.