CYTOPLASMIC INHIBITION OF CAROTENOID BIOSYNTHESIS WITH VIRUS-DERIVED RNA

The carotenoid biosynthetic pathway in higher plants was manipulated b y using an RNA viral vector. A cDNA encoding phytoene synthase and a p artial cDNA encoding phytoene desaturase (PDS) were placed under the t ranscriptional control of a tobamovirus subgenomic promoter. One to tw o weeks after inoculation, systemically infected Nicotiana benthamiana plants were analyzed for phytoene. Leaves from transfected plants exp ressing phytoene synthase developed a bright orange phenotype and accu mulated high levels of phytoene. Cytoplasmic inhibition of plant gene expression by viral RNA was demonstrated with an antisense RNA transcr ipt to a partial PDS cDNA derived from tomato. The leaves of the plant s transfected with the antisense PDS sequence developed a white phenot ype and also accumulated high levels of phytoene. A partial cDNA to th e corresponding N. benthamiana PDS gene was isolated and found to shar e significant homology with the tomato antisense PDS transcript. This work demonstrates that an episomal RNA viral vector can be used to del iberately manipulate a major, eukaryotic biosynthetic pathway. In addi tion, our results indicate that an antisense transcript generated in t he cytoplasm of a plant cell can turn off endogenous gene expression.