Genetic and biochemical dissection of transgenic RNA-mediated virus resistance

RNA-mediated virus resistance has been observed in transgenic plants at varying frequencies, suggesting that a nuclear requirement or other pre-condition must be met. This study was undertaken to characterize genetically transgenes that confer a highly resistant state to infection by tobacco etch virus (TEV). Transgenic tobacco line 2RC-6.13, expressing an untranslatable mRNA containing the TEV coat protein open reading frame, had three distinct transgene integration events that segregated as two linkage groups. A genetic series of plants that contained zero, one, two, or all three transgene inserts in both homozygous and heterozygous conditions was produced and examined. Genetic and biochemical data suggested that RNA-mediated virus resistance is a multigenic trait in line 2RC-6.13; three or more transgenes were necessary to establish the highly resistant state, One or two transgene copies resulted in an inducible form of resistance (i.e., recovery). Transcription rates and steady state RNA levels of the transgene-derived transcript present in different members of the genetic series supported a post-transcriptional RNA degradation process as the underlying mechanism for transgene transcript reduction and virus resistance, This degradation process appeared to initiate via cleavage of specific sites within the target RNA sequence, as determined by RNA gel blot and primer extension analyses of transgene-derived mRNA from various transgenic plant lines.