THE CAPSID PROTEIN GENE OF TOMATO BUSHY STUNT VIRUS IS DISPENSABLE FOR SYSTEMIC MOVEMENT AND CAN BE REPLACED FOR LOCALIZED EXPRESSION OF FOREIGN GENES

The requirement of the capsid protein for viability of tomato bushy stunt virus (TBSV), a small, spherical plant virus, was analyzed by inactivating the corresponding gene in full-length cDNA clones followed by bioassays of in vitro-generated transcripts. The results demonstrated that the coat protein of TBSV is not involved in replication or gene expression in protoplasts. In addition, cell-to-cell spread of viral RNA as well as long-distance movement in plants occurred in the absence of coat protein expression. Symptoms on systemic tobacco hosts in the absence of an intact coat protein gene were quite typical of a TBSV infection, except that most infected plants survived, whereas infection with the wild-type virus resulted in a lethal necrosis. Viral RNA in which the coat protein gene was replaced with a reporter gene expressed this foreign gene at high levels in protoplasts as well as in the inoculated leaves of plants. Expression of the reporter gene was greatly reduced in upper leaves of systemically infected hosts because the nonviral sequences were rapidly deleted from the genome during the process of systemic infection. The capsid protein of TBSV was not responsible for the elicitation of necrotic lesions on Chenopodium amaranticolor. Analyses of beta-glucuronidase expression by TBSV on inoculated leaves of this local lesion host suggested that replication and cell-to-cell spread were not affected prior to 1 day postinoculation. The hypersensitive response is probably initiated at some time after establishment of a localized infection and limited cell-to-cell movement.