More than a decade ago, a surprising observation was made in petunias. While trying to deepen the purple color of these flowers, Rich Jorgensen and colleagues introduced a pigment-producing gene under the control of a powerful promoter. Instead of the expected deep purple color, many of the flowers appeared variegated or even white. Jorgensen named the observed phenomenon "cosuppression", since the expression of both the introduced gene and the homologous endogenous gene was suppressed (1-5).

First thought to be a quirk of petunias, cosuppression has since been found to occur in many species of plants. It has also been observed in fungi, and has been particularly well characterized in Neurospora crassa, where it is known as "quelling" (1-3).

But what causes this gene silencing effect? Although transgene-induced silencing in some plants appears to involve gene-specific methylation (transcriptional gene silencing, or TGS), in others silencing occurs at the post-transcriptional level (post-transcriptional gene silencing, or PTGS). Nuclear run-on experiments in the latter case show that the homologous transcript is made, but that it is rapidly degraded in the cytoplasm and does not accumulate (1, 3, 6).

Introduction of transgenes can trigger PTGS, however silencing can also be induced by the introduction of certain viruses (2, 3). Once triggered, PTGS is mediated by a diffusible, trans-acting molecule. This was first demonstrated in Neurospora, when Cogoni and colleagues showed that gene silencing could be transferred between nuclei in heterokaryotic strains (1, 7). It was later confirmed in plants when Palauqui and colleagues induced PTGS in a host plant by grafting a silenced, transgene-containing source plant to an unsilenced host (8). From work done in nematodes and flies, we now know that the trans-acting factor responsible for PTGS in plants is dsRNA (1-3).