NONTARGET DNA-SEQUENCES REDUCE THE TRANSGENE LENGTH NECESSARY FOR RNA-MEDIATED TOSPOVIRUS RESISTANCE IN TRANSGENIC PLANTS

RNA-mediated virus resistance has recently been shown to be the result of post-transcriptional transgene silencing in transgenic plants. Thi s study was undertaken to characterize the effect of transgene length and nontarget DNA sequences on RNA-mediated tospovirus resistance in t ransgenic plants. Transgenic Nicotiana benthamiana plants were generat ed to express different regions of the nucleocapsid (N) protein of tom ato spotted wilt (TSWV) tospovirus, Transgenic plants expressing half- gene segments (387-453 bp) of the N gene displayed resistance through post-transcriptional gene silencing. Although smaller N gene segments (92-235 bp) were ineffective in conferring resistance when expressed a lone in transgenic plants, these segments conferred resistance when fu sed to the nontarget green fluorescent protein gene DNA. These results demonstrate that (i) a critical length of N transgene (236-387 bp) is required for a high level of transgene expression and consequent gene silencing, and (ii) the post-transcriptional gene silencing mechanism can trans inactivate the incoming tospovirus genome with homologous t ransgene segments that are as short as 110 bp. Therefore, the activati on of post-transcriptional transgene silencing requires a significantl y larger transgene than is required for the trans inactivation of the incoming viral genome. These results raise the possibility of developi ng a simple new strategy for engineering multiple virus resistance in transgenic plants.