On the stability of sequences inserted into viral genomes

Willemsen, Anouk; Zwart, Mark P

It has been shown that many different plant viruses can express foreign genes, and they have the advantage being able to express these directly in vivo. As an initial strategy to express foreign genes in plants, on many occasions viral genes were replaced with the gene of interest (gene replacement instead of gene insertion). This strategy appeared to be (partially) successful in plant ssDNA viruses (Hayes et al. 1988; Ward, Etessami, and Stanley 1988; Hayes, Coutts, and Buck 1989) , as the replaced coat protein did not appear to play an essential role in virus spread throughout the plant host (Ward, Etessami, and Stanley 1988) . However, viral ssRNA(þ) genomes seem to be less plastic as the replacement strategy was mostly unsuccessful in plant RNA viruses. Although the RNA viral vectors permitted the expression of replaced genes, either they were only viable in protoplasts and not in whole plants (French, Janda, and Ahlquist 1986; Joshi, Joshi, and Ow 1990) , or they were unable to establish systemic infections (Takamatsu et al. 1987; Dawson, Bubrick, and Grantham 1988) .