A Responsive Regulatory System is Revealed by Sense Suppression of Pigment Genes in Petunia Flowers

Abstract

Plant transgenes can suppress the expression of homologous genes in at least two ways. One is a phenomenon known as epigene conversion,whereby one copy of a gene causes the transcriptional silencing of another copy, and the altered expression state of the silenced gene is heritable after loss of the gene causing the silencing (as in paramutation). The other mode of homology-dependent gene silencing is best referred to as sense suppression, a posttranscriptional phenomenon usually resulting in the cosuppression of homologous transgenes and endogenes. The homology required for sense suppression can be limited to the transcribed regions of the genes. Sense suppression is observed frequently in transgenic plants, generally as a paradoxical outcome of attempts to increase expression of an endogenous gene by the introduction of a chimeric transgene comprised of a strong promoter fused to a copy of the endogene’s coding sequence in the sense orientation. The varied observations and possible mechanisms for homology-dependent transgene silencing phenomena in plants have been much discussed in the literature, citations to which may be found in Matzke and Matzke (1995), de Carvahlo Niebel et al. (1995), and Jorgensen (1995).