Conundrum of the lack of defective RNAs (dRNAs) associated with tobamovirus infections: dRNAs that can move are not replicated by the wild-type virus; dRNAs that are replicated by the wild-type virus do not move

Two classes of artificially constructed defective RNAs (dRNAs) of Tobacco m osaic virus (TMV) were examined in planta with helper viruses that expresse d one (183 kDa) or both (126 and 183 kDa) of the replicase-associated prote ins. The first class of artificially constructed dRNAs had the helicase and polymerase (POL) domains deleted; the second had an intact 126-kDa protein open reading frame (ORF). Despite extremely high levels of replication in protoplasts, the first class of dRNAs did not accumulate in plants. The dRN As with an intact 126-kDa protein ORF were replicated at moderate levels in protoplasts and in planta when supported by a TMV mutant that expressed th e 183-kDa protein but not the 126-kDa protein (183F), These dRNAs were not supported by helper viruses expressing both replicase-associated proteins. De novo dRNAs were generated in plants infected by 183F but not in plants i nfected with virus with the wild-type replicase, These novel dRNAs each con tained a new stop codon near the location of the wild-type stop codon for t he 126-kDa protein and had most of the POL domain deleted. The fact that on ly dRNAs that contained a complete 126-kDa protein ORF moved systemically s uggests that expression of a functional 126-kDa protein or the presence of certain sequences and/or structures within this ORF is required for movemen t of dRNAs. At least two factors may contribute to the lack of naturally oc curring dRNAs in association with wild-type TMV infections: an inability of TMV to support dRNAs that can move in plants and the inability of dRNAs th at can be replicated by TMV to move in plants.