TRANSGENIC PLANT-VIRUS RESISTANCE MEDIATED BY UNTRANSLATABLE SENSE RNAS - EXPRESSION, REGULATION, AND FATE OF NONESSENTIAL RNAS

Haploid leaf tissue of tobacco cultivars K326 and K149 was transformed with several transgenes containing cDNA of the potato virus Y (PVY) c oat protein (CP) open reading frame (ORF). The various transgenes cont aining the PVY CP ORF sequence produced (1) the expected mRNA and CP p roduct, (2) an mRNA rendered untranslatable by introduction of a stop codon immediately after the initiation codon, or (3) an antisense RNA that was untranslatable as a result of the incorrect orientation of th e PVY CP ORF behind the transcriptional promoter. Homozygous doubled h aploid (DH) (diploid) plants were generated, and selfed progeny from t hese plants were examined. Resistance was virus specific, functioning only against PVY. An inverse correlation between transgene-derived PVY transcript steady state levels and resistance was generally noted wit h lines expressing the untranslatable sense version of the PVY CP ORF. A collection of DH lines, derived from a single transformation event of a common haploid plant and isogenic for the PVY transgenes expressi ng untranslatable sense RNA, displayed different levels of PVY resista nce. Lines with actively transcribed, methylated transgene sequences h ad low steady state levels of transgene transcript and a virus-resista nt phenotype. These results are discussed within the context of sense suppression in plants.