An RNA-Dependent RNA polymerase gene in Arabidopsis is required for posttranscriptional gene silencing mediated by a transgene but not by a virus

Posttranscriptional gene silencing is a defense mechanism in plants that is similar to quelling in fungi and RNA interference in animals. Here, we des cribe four genetic loci that are required for posttranscriptional gene sile ncing in Arabidopsis. One of these, SDE1, is a plant homolog of QDE-1 in Ne urospora crassa that encodes an RNA-dependent RNA polymerase. The sde1 muta tion was specific for posttranscriptional gene silencing induced by transge nes rather than by viruses. We propose that the role of SDE1 is to synthesi ze a double-stranded RNA initiator of posttranscriptional gene silencing. A ccording to this idea, when a virus induces posttranscriptional gene silenc ing, the virus-encoded RNA polymerase would produce the double-stranded RNA and SDE1 would be redundant.