Introduction of a Chimeric Chalcone Synthase Gene into Petunia Results in Reversible Co-Suppression of Homologous Genes in trans (original) (raw)
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Phenotype of the Transgene in Plants: Expression and Silencing
Floriculture, Ornamental and Plant Biotechnology: Advances and Topical Issues Vol. II, 2006
When we overexpress the transgene in plants, we encounter two completely different phenomena, namely, successful expression and silencing of the transgene. In the first half of this paper, we introduce several strategies for a high-level expression of the transgene. In the latter half we discuss another aspect of the transgene's fate, i.e., cosuppression. The RNA interference (RNAi) pathway seems to account for the molecular basis of cosuppression. The presence of siRNA, a key molecule of the RNAi pathway, is an indication of plants displaying cosuppression. However, its presence does not always corresponded to the incidence of RNA silencing, suggesting the presence of regulatory mechanisms involved in the action of siRNA, especially in cosuppression between transgenes and the corresponding plant endogenous genes. Elucidation of the molecular bases of cosuppression facilitates the development of feasible strategies for boosting the accumulation of the recombinant proteins of interest by evading cosuppression.
Altered gene expression in plants due to trans interactions between homologous genes
Trends in biotechnology, 1990
We have attempted to explain co-suppression in terms of a hypothesis whereby homologous sequences are able to interact somatically in trans, in a manner influenced by sequence context or location. We have speculated that there might be mechanistic similarities between co-suppression and some other trans interaction and epigenetic phenomena in plants, fungi and animals; and that it might be the sequence context in which a gene lies, or its location in the genome that influences the likelihood that it will participate in these phenomena. We suspect that elucidation of the mechanisms behind these phenomena will play a role in developing a better understanding of the relationship between nuclear architecture and gene expression. This, in turn, will be helpful in understanding the developmental regulatory mechanisms that exert control over somatic trans interactions in plants, and perhaps in understanding some aspects of the basis of cellular differentiation in development.
Homologyâbased control of gene expression patterns in transgenic petunia flowers
Developmental Genetics, 1998
Plant transgenes may participate in two types of homology-based gene silencing. One requires transcript homology, is post-transcriptional, and is referred to as cosuppression; the other requires promoter homology, is transcriptional, and is similar to paramutation. This paper uses flower color transgenes to address the hierarchical operation of both mechanisms in plants carrying two transgene copies. It is shown that cosuppression of homologous, endogenous flower color genes by single-copy transgenes requires that the transgene be driven by a strong promoter and that the degree of cosuppression is highly sensitive to increasing transgene dosage. Together, these observations suggest that cosuppression should be a sensitive reporter of epigenetic changes in transgene transcription, such as might be caused by paramutation-like interactions between transgene loci. Intercrosses bringing together two homologous transgene loci, one a known epimutable reporter and the other a transgene inverted repeat, result in complete loss of cosuppression in some outcross progeny and a qualitative change in morphology-based patterns of cosuppression in other outcross progeny. This paramutation-like behavior suggests that the transgenes may be altered at the transcriptional level, eliminating cosuppression altogether or changing the spatial pattern of transgene transcription to produce a new pattern of cosuppression. Dev. Genet. 22:100-109, 1998. 1998
Plant Journal, 1998
A single-copy sense Chalcone synthase (Chs) transgene driven by a strong promoter and producing a fully translatable transcript was converted to an allelic antisense Chs transgene by Cre-lox-mediated DNA recombination in petunia. The sense Chs allele suppressed flower pigmentation in a simple pattern determined by cells at the junctions between adjacent petals, as is typical of single-copy sense Chs transgenes of this type, whereas the antisense Chs allele produced a different pattern of Chs suppression with white petal edges and reduced pigmentation throughout the petal limbs, as is typical of antisense Chs transgenes. In plants carrying a lox-flanked Chs transgene, the presence of Cre protein can cause both sense-specific and antisense-specific patterns to be superimposed in the same flower, suggesting that sense and antisense suppression by single-copy transgenes are mediated by different mechanisms or occur in different cellular or developmental compartments. The presence of Cre also causes the production of numerous, non-clonal white spots, suggesting that the turnover state is not cell-autonomous.
Review Article: The Silence of Genes in Transgenic Plants
Annals of Botany
In genetically modified plants, the introduced transgenes are sometimes not expressed. They can be silenced. Transgenes can also cause the silencing of endogenous plant genes if they are sufficiently homologous, a phenomenon known as co-suppression. Silencing occurs transcriptionally and post-transcriptionally but silencing of endogenous genes seems predominantly post-transcriptional. If viral transgenes are introduced and silenced, the posttranscriptional process also prevents homologous RNA viruses from accumulating ; this is a means of generating virus-resistant plants. A major goal of current research is to dissect the mechanism(s) of these sequence-homologydependent gene silencing phenomena. Various factors seem to play a role, including DNA methylation, transgene copy number and the repetitiveness of the transgene insert, transgene expression level, possible production of aberrant RNAs, and ectopic DNA-DNA interactions. The causal relationship between these factors and the link between transcriptional and post-transcriptional silencing is not always clear. In this review we discuss various observations associated with gene silencing and attempt to relate them.
Molecular and cellular biology, 1998
Posttranscriptional silencing of chalcone synthase (Chs) genes in petunia transformants occurs by introducing T-DNAs that contain a promoter-driven or promoterless Chs transgene. With the constructs we used, silencing occurs only by T-DNA loci which are composed of two or more T-DNA copies that are arranged as inverted repeats (IRs). Since we are interested in the mechanism by which these IR loci induce silencing, we have analyzed different IR loci and nonsilencing single-copy (S) T-DNA loci with respect to the expression and methylation of the transgenes residing in these loci. We show that in an IR locus, the transgenes located proximal to the IR center are much more highly methylated than are the distal genes. A strong silencing locus composed of three inverted T-DNAs bearing promoterless Chs transgenes was methylated across the entire locus. The host Chs genes in untransformed plants were moderately methylated, and no change in methylation was detected when the genes were silenc...
Susceptibility of transgene loci to homology-dependent gene silencing
MGG Molecular & General Genetics, 1994
Previous work has shown that two unlinked, partially homologous transgene loci can interact in plant nuclei, leading to reversible methylation and inactivation of one transgene locus in the presence of the second. To study whether the chromosomal location of a transgene influences its susceptibility to trans-inactivation, we retransformed four transgenic lines, which contained the same construct (H) integrated in different chromosomal locations, with a second, partially homologous construct (K). At least 50 double transformants (DTs) were regenerated from each single transformant (ST) and screened for inactivation of markers [chloramphenicol acetyltransferase (CAT); hygromycin resistance (HYGR)] at the resident H locus. For two STs, H locus markers were inactivated in less than 1% of the DTs, suggesting that, at these integration sites, H was relatively resistant to trans-inactivation. In contrast, the other two STs appeared to be more sensitive to trans-inactivation: 4-10% of the DTs were CAT-and/or Hyg s. Inactivation of H locus markers could be attributed to two distinct phenomena:
A Responsive Regulatory System is Revealed by Sense Suppression of Pigment Genes in Petunia Flowers
Stadler Genetics Symposia Series, 1996
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 ...