The Frequency and Degree of Cosuppression by Sense Chalcone Synthase Transgenes Are Dependent on Transgene Promoter Strength and Are Reduced by Premature Nonsense Codons in the Transgene Coding Sequence

Post-transcriptional silencing of chalcone synthase in petunia using a geminivirus-based episomal vector

A vector that produces DNA replicons (multicopy plant episomes) was constructed using elements of the geminivirus tobacco yellow dwarf virus (TYDV). All plant cells contain an integrated chromosomal T-DNA copy of the TYDV elements that provides a template for the production of episomes in the cell nucleus. Transgenic Petunia hybrida plants containing a CaMV 35S promoter-driven chalcone synthase A (ChsA) gene cloned into the episomal vector produced flowers with a white-spotted phenotype at high frequency. The spots were found at random locations in the petals and occurred in corresponding positions in both the upper and lower epidermis, indicating that the spots were non-clonal. The spotted phenotype was somatically stable and was inherited through meiosis. In white-spotted flower tissue, steady-state ChsA mRNA levels were downregulated but rates of RNA transcription were unaffected, suggesting that the phenotype resulted from post-transcriptional gene silencing of the endogenous and episomal ChsA genes. Increases in both the frequency and extent of gene silencing in flowers correlated with increases in episome copy number in mature flowers, flower buds and young and fully expanded leaves. Relatively small increases in episome copy number (less than threefold) appeared sufficient to trigger the gene-silenced phenotype.

A transgene with repeated DNA causes high frequency, post-transcriptional suppression of ACC-oxidase gene expression in tomato

Gene silencing with sense genes is an important method for down-regulating the expression of endogenous plant genes, but the frequency of silencing is unpredictable. Fifteen per cent of tomato plants transformed with a 35S-ACC-oxidase ( ACO 1) sense gene had reduced ACC-oxidase activity. However, 96% of plants transformed with an ACC-oxidase sense gene, containing two additional upstream inverted copies of its 5' untranslated region, exhibited reduced ACC-oxidase activity compared to wild-type plants. In the three plants chosen for analysis, there were substantially reduced amounts of both endogenous and transgenic ACO RNA, indicating that this was an example of co-suppression. Ribonuclease protection assays using probes spanning intron-exon borders showed that the reduced accumulation of endogenous ACO mRNA occurred post-transcriptionally since the abundance of unprocessed transcripts was not affected. The ACO1 transgene with the repeated 5'UTR also strongly inhibited the accumulation of RNA from the related ACO 2 gene in flowers, although there is little homology between the 5'UTRs of ACO 1 and ACO 2. These results indicate that although repeated DNA in a transgene greatly enhances the probability of gene silencing of an endogenous gene, it also involves generation of a trans -acting silencing signal produced, at least partly, from sequences external to the repeat.

Transgenes are dispensable for the RNA degradation step of cosuppression

Cosuppression results in the degradation of RNA from host genes and homologous transgenes after transcription in the nucleus. By using grafting experiments, we have shown previously that a systemic signal mediates the propagation of cosuppression of Nia host genes and 35S-Nia2 transgenes from silenced 35S-Nia2 transgenic stocks to nonsilenced 35S-Nia2 transgenic scions but not to wild-type scions. Here, we examined the requirements for triggering and maintenance of cosuppression in various types of scions. Grafting-induced silencing occurred in 35S-Nia2 transgenic lines over-accumulating Nia mRNA whether they are able to spontaneously trigger cosuppression or not and in 35S-Nia2 transgene-free plants over-accumulating host Nia mRNA caused by metabolic derepression. When grafting-induced silenced scions were removed from the silenced stocks and regrafted onto wild-type plants, silencing was not maintained in the 35S-Nia2 transgene-free plants and in the 35S-Nia2 transgenic lines that are not able to trigger cosuppression spontaneously. Conversely, silencing was maintained in the 35S-Nia2 transgenic lines that are able to trigger cosuppression spontaneously. Our results indicate that the presence of a 35S-Nia2 transgene is dispensable for the RNA degradation step of posttranscriptional silencing when host Nia mRNA over-accumulate above the level of wild-type plants. They also suggest that grafting-induced RNA degradation does not result in the production of the systemic silencing signal required for spontaneous triggering and maintenance.

Overexpression of a homeobox gene, LeT6, reveals indeterminate features in the tomato compound leaf

The cultivated tomato (Lycopersicon esculentum) has a unipinnate compound leaf. In the developing leaf primordium, major leaflet initiation is basipetal, and lobe formation and early vascular differentiation are acropetal. We show that engineered alterations in the expression of a tomato homeobox gene, LeT6, can cause dramatic changes in leaf morphology. The morphological states are variable and unstable and the phenotypes produced indicate that the tomato leaf has an inherent level of indeterminacy. This is manifested by the production of multiple orders of compounding in the leaf, by numerous shoot, inflorescence, and floral meristems on leaves, and by the conversion of rachis-petiolule junctions into "axillary" positions where floral buds can arise. Overexpression of a heterologous homeobox transgene, kn1, does not produce such phenotypic variability. This indicates that LeT6 may differ from the heterologous kn1 gene in the effects manifested on overexpression, and that 35S-LeT6 plants may be subject to alterations in expression of both the introduced and endogenous LeT6 genes. The expression patterns of LeT6 argue in favor of a fundamental role for LeT6 in morphogenesis of leaves in tomato and also suggest that variability in homeobox gene expression may account for some of the diversity in leaf form seen in nature.

Phenylpropanoid biosynthesis and its regulation

In the past year progress has been made in the manipulation of phenylpropanoid metabolism but several studies highlight gaps in our understanding of the biochemistry of these pathways. New components involved in transcriptional regulation of phenylpropanoid genes have been identified, including transcription factors and novel proteins that function upstream of DNA-binding proteins.

A model for RNA-mediated gene silencing in higher plants

Homology-dependent gene silencing (HdGS) which is the generic term for transcriptional gene silencing (TGS), post-transcriptional gene silencing (PTGS) and RNA-mediated virus-resistance (RmVR) has been shown to frequently occur in transgenic plants. The role of RNA as a target and initiator of PTGS and RmVR is more and more manifested. Because TGS is assumed to be induced by a DNA-DNA interaction-mediated promoter methylation, a possible involvement of RNA in TGS was not really considered up to now. In this review we attempt to demonstrate that all three types of HdGS could be triggered by one RNA-based mechanism. A model proposing TGS as a consequence of RNA-directed DNA methylation (RdDM) and a refined mRNA threshold mechanism are presented. In contrast to the view that high amounts of mRNA are required we assume that the concentration of RNAs that can serve as efficient templates for a plant-encoded RNA-directed RNA polymerase (RdRP) plays a key role in HdGS and possibly also in natural gene regulation of non-transformed cells. According to this idea a particular information must be encoded to render mRNA turn-over products a suitable RdRP substrate. It will be discussed that such a mechanism could account for the silencing phenomena of poorly transcribed transgenes. Finally, an explanation for the coherency between PTGS and DNA methylation is documented.

Controlling gene expression in transgenics

The repertoire of cis-regulatory elements has increased to a level of sophistication that offers considerable spatial and temporal control over transgene expression. Recent advances made with transgenes have revealed that the control of their expression is also influenced by factors that range from transgene copy number and arrangement to nuclear architecture and chromosomal location. These factors must now be included with the standard considerations of transcriptional and translational enhancers of gene expression during transgene design.