Transgene inactivation in Petunia hybrida is influenced by the properties of the foreign gene

An arginine-to-histidine mutation in flavanone-3-hydroxylase results in pink strawberry fruits

Fruit color is a very important external commodity factor for consumers. Compared to the most typical red octoploid strawberry (Fragaria × ananassa), the pink strawberry often sells for a more expensive price and has a higher economic benefit due to its outstanding color. However, few studies have examined the molecular basis of pink-colored strawberry fruit. Through an EMS mutagenesis of woodland strawberry (Fragaria vesca), we identified a mutant with pink fruits and green petioles. Bulked-segregant analysis sequencing analysis and gene function verification confirmed that the responsible mutation resides in a gene encoding flavanone-3-hydroxylase (F3H) in the anthocyanin synthesis pathway. This nonsynonymous mutation results in an arginine-to-histidine change at position 130 of F3H. Molecular docking experiments showed that the arginine-to-histidine mutation results in a reduction of intermolecular force-hydrogen bonding between the F3H protein and its substrates. Enzymatic experiments showed a greatly reduced ability of the mutated F3H protein to catalyze the conversion of the substrates and hence a blockage of the anthocyanin synthesis pathway. The discovery of a key residue in the F3H gene controlling anthocyanin synthesis provides a clear target of modification for the molecular breeding of strawberry varieties with pink-colored fruits, which may be of great commercial value.

Transgene Was Silenced in Hybrids between Transgenic Herbicide-Resistant Crops and Their Wild Relatives Utilizing Alien Chromosomes

The commercialization of transgenic herbicide-resistant (HR) crops may cause gene flow risk. If a transgene in progenies of transgenic crops and wild relatives is silencing, these progenies should be killed by the target herbicide, thus, the gene flow risk could be decreased. We obtained the progenies of backcross generations between wild Brassca juncea (AABB, 2n = 36) and glufosinate-resistant transgenic Brassica napus (AACC, 2n = 38, PAT gene located on the C-chromosome). They carried the HR gene but did not express it normally, i.e., gene silencing occurred. Meanwhile, six to nine methylation sites were found on the promoter of PAT in transgene-silencing progenies, while no methylation sites occurred on that in transgene-expressing progenies. In addition, transgene expressing and silencing backcross progenies showed similar fitness with wild Brassica juncea. In conclusion, we elaborate on the occurrence of transgene-silencing event in backcross progenies between transgenic crop utilizing alien chromosomes and their wild relatives, and the DNA methylation of the transgene promoter was an important factor leading to gene silencing. The insertion site of the transgene could be considered a strategy to reduce the ecological risk of transgenic crops, and applied to cultivate lower gene flow HR crops in the future.

Molecular Characterization and Event-Specific Real-Time PCR Detection of Two Dissimilar Groups of Genetically Modified Petunia (Petunia x hybrida) Sold on the Market

Petunia plants with unusual orange flowers were noticed on the European market and confirmed to be genetically modified (GM) by the Finnish authorities in spring 2017. Later in 2017, inspections and controls performed by several official laboratories of national competent authorities in the European Union detected several GM petunia varieties with orange flowers, but also another group of unusually colored flowers. In the latter group, a so far undetected gene coding for a flavonoid 3'5' hydroxylase (F3'5'H) responsible for the purple color was identified by German and Dutch authorities, suggesting that the petunias found on the markets contain different genetic constructs. Here, a strategy is described for the identification of GM petunia varieties. It is based on an initial GMO screening for known elements using (real-time) PCR and subsequent identification of the insertion sites by a gene walking-like approach called ALF (amplification of linearly-enriched fragments) in combination with Sanger and MinION sequencing. The results indicate that the positively identified GM petunias can be traced back to two dissimilar GM events used for breeding of the different varieties. The test results also confirm that the transgenic petunia event RL01-17 used in the first German field trial in 1991 is not the origin of the GM petunias sold on the market. On basis of the obtained sequence data, event-specific real-time PCR confirmatory methods were developed and validated. These methods are applicable for the rapid detection and identification of GM petunias in routine analysis. In addition, a decision support system was developed for revealing the most likely origin of the GM petunia.

MinION sequencing technology to characterize unauthorized GM petunia plants circulating on the European Union market

In order to characterize unauthorized genetically modified petunia, an integrated strategy has been applied here on several suspected petunia samples from the European market. More precisely, DNA fragments of interest were produced by DNA walking anchored on key targets, earlier detected by real-time PCR screening analysis, to be subsequently sequenced using the MinION platform from Oxford Nanopore Technologies. This way, the presence of genetically modified petunia was demonstrated via the characterization of their transgene flanking regions as well as unnatural associations of elements from their transgenic cassette.

Repression of anthocyanin biosynthesis by R3-MYB transcription factors in lily (Lilium spp.)

Lily R3-MYB transcription factors are involved in negative regulation to limit anthocyanin accumulation in lily flowers and leaves and create notable color patterns on ectopically expressed petunia flowers. In eudicots, both positive and negative regulators act to precisely regulate the level of anthocyanin accumulation. The R3-MYB transcription factor is among the main factors repressing anthocyanin biosynthesis. Although, in monocots, the positive regulators have been well characterized, the negative regulators have not been examined. Two R3-MYBs, LhR3MYB1 and LhR3MYB2, which were identified in lily transcriptomes, were characterized in this study to understand the regulatory mechanisms of anthocyanin biosynthesis. LhR3MYB1 and LhR3MYB2 had a C2 suppressor motif downstream of a single MYB repeat; the similar amino acid motif appears only in AtMYBL2 among the eudicot R3-MYB proteins. Stable and transient overexpression of LhR3MYB1 and LhR3MYB2 in tobacco plants showed suppression of anthocyanin biosynthesis by both; however, suppression by LhR3MYB2 was stronger than that by LhR3MYB1. In the lily plant, the LhR3MYB2 transcript was detected in leaves with light stimulus-induced anthocyanin accumulation and in pink tepals. Although LhR3MYB1 was expressed in some, but not all tepals, its expression was not linked to anthocyanin accumulation. In addition, LhR3MYB1 expression levels in the leaves remained unchanged by the light stimulus, and LhR3MYB1 transcripts predominantly accumulated in the ovaries, which did not accumulate anthocyanins. Thus, although LhR3MYB1 and LhR3MYB2 have an ability to repress anthocyanin accumulation, LhR3MYB2 is more strongly involved in the negative regulation to limit the accumulation than that by LhR3MYB1. In addition, the overexpression of LhR3MYB2 generated notable color patterns in petunia flowers; thus, the usefulness of the LhR3MYB genes for creating unique color patterns by genetic engineering is discussed.

Great Cause-Small Effect: Undeclared Genetically Engineered Orange Petunias Harbor an Inefficient Dihydroflavonol 4-Reductase

A recall campaign for commercial, orange flowering petunia varieties in spring 2017 caused economic losses worldwide. The orange varieties were identified as undeclared genetically engineered (GE)-plants, harboring a maize dihydroflavonol 4-reductase (DFR, A₁), which was used in former scientific transgenic breeding attempts to enable formation of orange pelargonidin derivatives from the precursor dihydrokaempferol (DHK) in petunia. How and when the A₁ cDNA entered the commercial breeding process is unclear. We provide an in-depth analysis of three orange petunia varieties, released by breeders from three countries, with respect to their transgenic construct, transcriptomes, anthocyanin composition, and flavonoid metabolism at the level of selected enzymes and genes. The two possible sources of the A₁ cDNA in the undeclared GE-petunia can be discriminated by PCR. A special version of the A₁ gene, the A₁ type 2 allele, is present, which includes, at the 3'-end, an additional 144 bp segment from the non-viral transposable Cin4-1 sequence, which does not add any functional advantage with respect to DFR activity. This unequivocally points at the first scientific GE-petunia from the 1980s as the A₁ source, which is further underpinned e.g., by the presence of specific restriction sites, parts of the untranslated sequences, and the same arrangement of the building blocks of the transformation plasmid used. Surprisingly, however, the GE-petunia cannot be distinguished from native red and blue varieties by their ability to convert DHK in common in vitro enzyme assays, as DHK is an inadequate substrate for both the petunia and maize DFR. Recombinant maize DFR underpins the low DHK acceptance, and, thus, the strikingly limited suitability of the A₁ protein for a transgenic approach for breeding pelargonidin-based flower color. The effect of single amino acid mutations on the substrate specificity of DFRs is demonstrated. Expression of the A₁ gene is generally lower than the petunia DFR expression despite being under the control of the strong, constitutive p35S promoter. We show that a rare constellation in flavonoid metabolism-absence or strongly reduced activity of both flavonol synthase and B-ring hydroxylating enzymes-allows pelargonidin formation in the presence of DFRs with poor DHK acceptance.

Genetically engineered orange petunias on the market

Unauthorized genetically engineered orange petunias were found on the market. Genetic engineering of petunia was shown to lead to novel flower color some 20 years ago. Here we show that petunia lines with orange flowers, generated for scientific purposes, apparently found their way to petunia breeding programmes, intentionally or unintentionally. Today they are widely available, but have not been registered for commerce.

Epigenetic silencing in transgenic plants

Epigenetic silencing is a natural phenomenon in which the expression of genes is regulated through modifications of DNA, RNA, or histone proteins. It is a mechanism for defending host genomes against the effects of transposable elements and viral infection, and acts as a modulator of expression of duplicated gene family members and as a silencer of transgenes. A major breakthrough in understanding the mechanism of epigenetic silencing was the discovery of silencing in transgenic tobacco plants due to the interaction between two homologous promoters. The molecular mechanism of epigenetic mechanism is highly complicated and it is not completely understood yet. Two different molecular routes have been proposed for this, that is, transcriptional gene silencing, which is associated with heavy methylation of promoter regions and blocks the transcription of transgenes, and post-transcriptional gene silencing (PTGS), the basic mechanism is degradation of the cytosolic mRNA of transgenes or endogenous genes. Undesired transgene silencing is of major concern in the transgenic technologies used in crop improvement. A complete understanding of this phenomenon will be very useful for transgenic applications, where silencing of specific genes is required. The current status of epigenetic silencing in transgenic technology is discussed and summarized in this mini-review.

Characterization of the mannan synthase promoter from guar (Cyamopsis tetragonoloba)

Guar seed gum, consisting primarily of a high molecular weight galactomannan, is the most cost effective natural thickener, having broad applications in the food, cosmetics, paper, pharmaceutical and petroleum industries. The properties of the polymer can potentially be enhanced by genetic modification. Development of suitable endosperm-specific promoters for use in guar is desirable for metabolic engineering of the seed gum. A ~1.6 kb guar mannan synthase (MS) promoter region has been isolated. The MS promoter sequence was fused with the GUS reporter gene and overexpressed in the heterologous species alfalfa (Medicago sativa). The potential strength and specificity of the MS promoter was compared with those of the constitutive 35S promoter and the seed specific β-phaseolin promoter. Quantitative GUS assays revealed that the MS promoter directs GUS expression specifically in endosperm in transgenic alfalfa. Thus, the guar MS promoter could prove generally useful for directing endosperm-specific expression of transgenes in legume species.

Greenhouse and field cultivations of antigen-expressing potatoes focusing on the variability in plant constituents and antigen expression

The production of plant-derived pharmaceuticals essentially requires stable concentrations of plant constituents, especially recombinant proteins; nonetheless, soil and seasonal variations might drastically interfere with this stability. In addition, variability might depend on the plant organ used for production. Therefore, we investigated the variability in plant constituents and antigen expression in potato plants under greenhouse and field growth conditions and in leaves compared to tubers. Using potatoes expressing VP60, the only structural capsid protein of the rabbit haemorrhagic disease virus (RHDV), CTB, the non-toxic B subunit (CTB) of the cholera toxin (CTA-CTB(5)) and the marker protein NPTII (neomycinphosphotransferase) as a model, we compare greenhouse and field production of potato-derived antigens. The influence of the production organ turned out to be transgene specific. In general, yield, plant quality and transgene expression levels in the field were higher than or similar to those observed in the greenhouse. The variation (CV) of major plant constituents and the amount of transgene-encoded protein was not influenced by the higher variation of soil properties observed in the field. Amazingly, for specific events, the variability in the model protein concentrations was often lower under field than under greenhouse conditions. The changes in gene expression under environmental stress conditions in the field observed in another event do not reduce the positive influence on variability since events like these should excluded from production. Hence, it can be concluded that for specific applications, field production of transgenic plants producing pharmaceuticals is superior to greenhouse production, even concerning the stability of transgene expression over different years. On the basis of our results, we expect equal or even higher expression levels with lower variability of recombinant pharmaceuticals in the field compared to greenhouse production combined with approximately 10 times higher tuber yield in the field.

Stable integration and expression of wasabi defensin gene in "Egusi" melon (Colocynthis citrullus L.) confers resistance to Fusarium wilt and Alternaria leaf spot

Production of "Egusi" melon (Colocynthis citrullus L.) in West Africa is limited by fungal diseases, such as Alternaria leaf spot and Fusarium wilt. In order to engineer "Egusi" resistant to these diseases, cotyledonary explants of two "Egusi" genotypes, 'Ejagham' and NHC1-130, were transformed with Agrobacterium tumefaciens strain EHA101 harbouring wasabi defensin gene (isolated from Wasabia japonica L.) in a binary vector pEKH1. After co-cultivation for 3 days, infected explants were transferred to MS medium containing 100 mg l(-l) kanamycin to select transformed tissues. After 3 weeks of culture, adventitious shoots appeared directly along the edges of the explants. As much as 19 out of 52 (36.5%) and 25 out of 71 (35.2%) of the explants in genotype NHC1-130 and 'Ejagham', respectively, formed shoots after 6 weeks of culture. As much as 74% (14 out of 19) of the shoots regenerated in genotype NHC1-130 and 72% (18 out of 25) of those produced in genotype 'Ejagham' were transgenic. A DNA fragment corresponding to the wasabi defensin gene or the selection marker nptII was amplified by PCR from the genomic DNA of all regenerated plant clones rooted on hormone-free MS medium under the same selection pressure, suggesting their transgenic nature. Southern blot analysis confirmed successful integration of 1-5 copies of the transgene. RT-PCR, northern and western blot analyses revealed that wasabi defensin gene was expressed in transgenic lines. Transgenic lines showed increased levels of resistance to Alternaria solani, which causes Alternaria leaf spot and Fusarium oxysporum, which causes Fusarium wilt, as compared to that of untransformed plants.

Production of red-flowered plants by genetic engineering of multiple flavonoid biosynthetic genes

Orange- to red-colored flowers are difficult to produce by conventional breeding techniques in some floricultural plants. This is due to the deficiency in the formation of pelargonidin, which confers orange to red colors, in their flowers. Previous researchers have reported that brick-red colored flowers can be produced by introducing a foreign dihydroflavonol 4-reductase (DFR) with different substrate specificity in Petunia hybrida, which does not accumulate pelargonidin pigments naturally. However, because these experiments used dihydrokaempferol (DHK)-accumulated mutants as transformation hosts, this strategy cannot be applied directly to other floricultural plants. Thus in this study, we attempted to produce red-flowered plants by suppressing two endogenous genes and expressing one foreign gene using tobacco as a model plant. We used a chimeric RNAi construct for suppression of two genes (flavonol synthase [FLS] and flavonoid 3'-hydroxylase [F3'H]) and expression of the gerbera DFR gene in order to accumulate pelargonidin pigments in tobacco flowers. We successfully produced red-flowered tobacco plants containing high amounts of additional pelargonidin as confirmed by HPLC analysis. The flavonol content was reduced in the transgenic plants as expected, although complete inhibition was not achieved. Expression analysis also showed that reduction of the two-targeted genes and expression of the foreign gene occurred simultaneously. These results demonstrate that flower color modification can be achieved by multiple gene regulation without use of mutants if the vector constructs are designed resourcefully.

Pathogen-induced expression of a cecropin A-melittin antimicrobial peptide gene confers antifungal resistance in transgenic tobacco

Expression of defensive genes from a promoter that is specifically activated in response to pathogen invasion is highly desirable for engineering disease-resistant plants. A plant transformation vector was constructed with transcriptional fusion between the pathogen-responsive win3.12T promoter from poplar and the gene encoding the novel cecropin A-melittin hybrid peptide (CEMA) with strong antimicrobial activity. This promoter-transgene combination was evaluated in transgenic tobacco (Nicotiana tabacum L. cv. Xanthi) for enhanced plant resistance against a highly virulent pathogenic fungus Fusarium solani. Transgene expression in leaves was strongly increased after fungal infection or mechanical wounding, and the accumulation of CEMA transcripts was found to be systemic and positively correlated with the number of transgene insertions. A simple and efficient in vitro regeneration bioassay for preliminary screening of transgenic lines against pathogenic fungi was developed. CEMA had strong antifungal activity in vitro, inhibiting conidia germination at concentrations that were non-toxic to tobacco protoplasts. Most importantly, the expression level of the CEMA peptide in vivo, regulated by the win3.12T promoter, was sufficient to confer resistance against F. solani in transgenic tobacco. The antifungal resistance of plants with high CEMA expression was strong and reproducible. In addition, leaf tissue extracts from transgenic plants significantly reduced the number of fungal colonies arising from germinated conidia. Accumulation of CEMA peptide in transgenic tobacco had no deleterious effect on plant growth and development. This is the first report showing the application of a heterologous pathogen-inducible promoter to direct the expression of an antimicrobial peptide in plants, and the feasibility of this approach to provide disease resistance in tobacco and, possibly, other crops.

Cre-mediated site-specific gene integration for consistent transgene expression in rice

To minimize expression variability amongst transgenic lines, we have utilized the strategy of Cre/lox-mediated site-specific gene integration. This method allows the precise integration of a transgene in a lox site previously placed in the genome. Using the biolistic method for DNA delivery, we have generated several site-specific integrant lines, derived from three different target lines. About 80% of the selected lines contain precise integration of the gusA reporter gene and fall into two categories: single-copy (SC) lines that contain site-specific integration without additional random integrations, and multicopy (MC) lines that contain random integrations in addition to the site-specific integration. The expression of the gusA gene was studied in callus cells and regenerated plants. The isogenic SC lines displayed significantly lower expression variation, whereas much higher expression variation was observed in MC lines. Furthermore, stable inheritance of the gusA gene was observed in T1 plants derived from a subset of SC lines. This demonstrates that consistent gene expression can be obtained in rice by Cre-mediated site-specific integration.

Neither inverted repeat T-DNA configurations nor arrangements of tandemly repeated transgenes are sufficient to trigger transgene silencing

Transgene expression was analysed in Arabidopsis T-DNA transformants carrying defined numbers and arrangement of different reporter genes. All transgenes were placed under the control of the strong constitutive CaMV 35S promoter. High, stable transgene expression was observed in plants containing two copies of the beta-glucuronidase (GUS) gene, two or four copies of the green fluorescent protein (GFP) gene and two, four or six copies of the streptomycin phosphotransferase (SPT) gene. Thus, the mere presence of multiple promoter and/or transgene sequences did not result in gene silencing. In none of the cases analysed were tandem repeat arrangements of transgenes and/or inverted repeat (IR) T-DNA structures sufficient to trigger silencing of the different reporter genes. Instead, post-transcriptional gene silencing (PTGS) correlated with the copy number of the highly expressed transgenes. Twelve copies of the SPT and four copies of the GUS gene triggered silencing. Silencing is frequently associated with repetitive T-DNA structures. We favour the idea that in many cases this may be attributed to the high transgene doses rather than the repeat arrangements themselves.

Heterologous expression of dihydroflavonol 4-reductases from various plants

Dihydroflavonol 4-reductases (DFR) catalyze the stereospecific reduction of dihydroflavonols to the respective flavan 3,4-diols (leucoanthocyanidins) and might also be involved in the reduction of flavanones to flavan-4-ols, which are important intermediates in the 3-deoxyflavonoid pathway. Several cDNA clones encoding DFR have been isolated from different plant species. Despite the important function of these enzymes in the flavonoid pathway, attempts at heterologous expression of cDNA clones in Escherichia coli have failed so far. Here, three well known heterologous expression systems for plant-derived genes were tested to obtain the functional protein of DFR from Gerbera hybrids. Successful synthesis of an active DFR enzyme was achieved in eukaryotic cells, using either baker's yeast (Saccharomyces cerevisiae) or tobacco protoplasts (Nicotiana tabacum), transformed with expression vectors containing the open reading frame of Gerbera DFR. These expression systems provide useful and powerful tools for rapid biochemical characterization, in particular the substrate specificity, of the increasing number of cloned DFR sequences. Furthermore, this tool allows the stereospecific synthesis of (14)C-labeled leucoanthocyanidins in high quality and quantity, which is a prerequisite for detailed biochemical investigation of the less understood enzymatic reactions located downstream of DFR in anthocyanin, catechin and proanthocyanidin biosynthesis.

Sequence elements within an HSP70 promoter counteract transcriptional transgene silencing in Chlamydomonas

We have shown previously that the HSP70A (A) promoter, when fused upstream of other promoters, significantly improves their performance in driving transgene expression in Chlamydomonas. Here, we employed the bacterial resistance gene ble, driven by the RBCS2 (R) promoter or an AR promoter fusion, to determine, by which mechanism(s) the A promoter may exert its enhancing effect. We observed that transformation rates of AR-ble constructs were significantly higher than those of R-ble constructs. However, ble mRNA levels in pools of transformants generated with either construct type were the same. Co-transformation experiments revealed that the R-ble transgene was silenced in 80% of the transformants, whereas this fraction was reduced to 36% in transformants harbouring the AR-ble transgene. We conclude that the A promoter acts by decreasing the probability that a transgene becomes transcriptionally silenced. We mapped two elements within the A promoter that are responsible for this effect. The core of the first element appears to be located between nucleotides - 7 and + 67 relative to the HSP70A transcriptional start site. Its activity is strongly dependent on its spatial setting with respect to the R promoter and is increased by upstream sequences (- 196 to - 8). The second element is independent of the first and is located to the region from - 754 to - 197. Its activity is spacing-independent and additive to the first element.

A human CpG island randomly inserted into a plant genome is protected from methylation

In vertebrate genomes the dinucleotide CpG is heavily methylated, except in CpG islands, which are normally unmethylated. It is not clear why the CpG islands are such poor substrates for DNA methyltransferase. Plant genomes display methylation, but otherwise the genomes of plants and animals represent two very divergent evolutionary lines. To gain a further understanding of the resistance of CpG islands to methylation, we introduced a human CpG island from the proteasome-like subunit I gene into the genome of the plant Arabidopsis thaliana. Our results show that prevention of methylation is an intrinsic property of CpG islands, recognized even if a human CpG island is transferred to a plant genome. Two different parts of the human CpG island - the promoter region/ first exon and exon 2-4 - both displayed resistance against methylation, but the promoter/ exon1 construct seemed to be most resistant. In contrast, certain sites in a plant CpG-rich region used as a control transgene were always methylated. The frequency of silencing of the adjacent nptII (KmR) gene in the human CpG constructs was lower than observed for the plant CpG-rich region. These results have implications for understanding DNA methylation, and for construction of vectors that will reduce transgene silencing.

The frequency of silencing in Arabidopsis thaliana varies highly between progeny of siblings and can be influenced by environmental factors

In a collection of 111 transgenic Arabidopsis thaliana lines, silencing of the nptII gene was observed in 62 (56%) of the lines and three distinct nptII-silencing phenotypes were identified. Two T-DNA constructs were used, which differed in distance and orientation of the marker gene relative to the border sequences. Comparison of the sets of lines generated with each vector, indicate that the T-DNA construct configuration influence the incidence of lines displaying silencing, as well as the distribution of silencing phenotypes. Twenty lines were investigated more thoroughly. The frequency of silencing varied between siblings in 19 lines, including three lines containing a single T-DNA copy. The last line showed 100% silencing. The gus gene present in both constructs could be expressed in the presence of a silenced nptII gene. Investigation of methylation at a single site in the pnos promoter revealed partial methylation in multi-copy lines, but no methylation in single-copy lines. For 16 lines, the overall frequencies of silencing differed significantly between control plants and plants exposed to temperature stress; in 11 of these lines at the 0.1% level. In several cases, the frequency of silencing in progeny of stress-treated plants was higher than for the control group, while other lines showed higher frequencies of kanamycin-resistant progeny for the stress-treated sibling plants.

Alteration of a single amino acid changes the substrate specificity of dihydroflavonol 4-reductase

Many plant species exhibit a reduced range of flower colors due to the lack of an essential gene or to the substrate specificity of a biosynthetic enzyme. Petunia does not produce orange flowers because dihydroflavonol 4-reductase (DFR) from this species, an enzyme involved in anthocyanin biosynthesis, inefficiently reduces dihydrokaempferol, the precursor to orange pelargonidin-type anthocyanins. The substrate specificity of DFR, however, has not been investigated at the molecular level. By analyzing chimeric DFRs of Petunia and Gerbera, we identified a region that determines the substrate specificity of DFR. Furthermore, by changing a single amino acid in this presumed substrate-binding region, we developed a DFR enzyme that preferentially reduces dihydrokaempferol. Our results imply that the substrate specificity of DFR can be altered by minor changes in DFR.

Transcriptional gene silencing in plants: targets, inducers and regulators

Gene silencing can occur either through repression of transcription, termed transcriptional gene silencing (TGS), or through mRNA degradation, termed post-transcriptional gene silencing (PTGS). Initially, TGS was associated with the regulation of transposons through DNA methylation in the nucleus, whereas PTGS was shown to regulate virus infection through double-stranded RNA in the cytoplasm. However, several breakthroughs in the field have been reported recently that blur this neat distinction. First, in plants TGS and DNA methylation can be induced by either dsRNA or viral infection. Second, a mutation in the plant MOM gene reverses TGS without affecting DNA methylation. Third, in Caenorhabditis elegans mutation of several genes that control RNA interference, a form of PTGS, also affect the regulation of transposons. TGS and PTGS, therefore, appear to form two alternative pathways to control incoming, redundant and/or mobile nucleic acids.

Hierarchical patterns of transgene expression indicate involvement of developmental mechanisms in the regulation of the maize P1-rr promoter

The maize P1-rr gene encodes a Myb-homologous transcription factor that regulates the synthesis of red flavonoid pigments. Maize plants transformed with segments of the P1-rr promoter driving a GUS reporter gene exhibit significant variation in transgene expression, both between independent transformation events and among sibling plants derived from a single event. Interestingly, variability in spatial expression is not random; rather, transgene activity occurs predominantly in five patterns that fit a hierarchy: expression is most common in kernel pericarp, with sequential addition of expression in cob glumes, husk, silk, and tassel. The hierarchical expression pattern of P-rr::GUS transgenes suggests a possible model for developmental regulation of the P1-rr gene. Our results demonstrate that variability in transgene expression, a common occurrence in transgenic plant studies, can be informative if adequately analyzed to uncover underlying patterns of gene expression.

Plants as bioreactors for protein production: avoiding the problem of transgene silencing

Plants are particularly attractive as large-scale production systems for proteins intended for therapeutical or industrial applications: they can be grown easily and inexpensively in large quantities that can be harvested and processed with the available agronomic infrastructures. The effective use of plants as bioreactors depends on the possibility of obtaining high protein accumulation levels that are stable during the life cycle of the transgenic plant and in subsequent generations. Silencing of the introduced transgenes has frequently been observed in plants, constituting a major commercial risk and hampering the general economic exploitation of plants as protein factories. Until now, the most efficient strategy to avoid transgene silencing involves careful design of the transgene construct and thorough analysis of transformants at the molecular level. Here, we focus on different aspects of the generation of transgenic plants intended for protein production and on their influence on the stability of heterologous gene expression.

(TRANS)GENE SILENCING IN PLANTS: How Many Mechanisms?

Epigenetic silencing of transgenes and endogenous genes can occur at the transcriptional level (TGS) or at the posttranscriptional level (PTGS). Because they can be induced by transgenes and viruses, TGS and PTGS probably reflect alternative (although not exclusive) responses to two important stress factors that the plant's genome has to face: the stable integration of additional DNA into chromosomes and the extrachromosomal replication of a viral genome. TGS, which results from the impairment of transcription initiation through methylation and/or chromatin condensation, could derive from the mechanisms by which transposed copies of mobile elements and T-DNA insertions are tamed. PTGS, which results from the degradation of mRNA when aberrant sense, antisense, or double-stranded forms of RNA are produced, could derive from the process of recovery by which cells eliminate pathogens (RNA viruses) or their undesirable products (RNA encoded by DNA viruses). Mechanisms involving DNA-DNA, DNA-RNA, or RNA-RNA interactions are discussed to explain the various pathways for triggering (trans)gene silencing in plants.

Cymbidium hybrida dihydroflavonol 4-reductase does not efficiently reduce dihydrokaempferol to produce orange pelargonidin-type anthocyanins

Some angiosperms are limited to a range of possible flower colors. This limitation can be due to the lack of an anthocyanin biosynthetic gene or to the substrate specificity of a key anthocyanin biosynthetic enzyme, dihydroflavonol 4-reductase (DFR). Cymbidium hybrida orchid flowers primarily produce cyanidin-type (pink to red) anthocyanins and lack the pelargonidin-type (orange to brick-red) anthocyanins. To investigate the underlying molecular mechanism of this flower color range, we cloned a Cymbidium DFR gene and transformed it into a DFR- petunia line. We found that the Cymbidium DFR did not efficiently reduce dihydrokaempferol (DHK), which is an essential step for pelargonidin production. Phylogenetic analysis of a number of DFR sequences indicate that the inability to catalyze DHK reduction has occurred at least twice during angiosperm evolution. Our results indicate that developing a pelargonidin-type orange flower color in Cymbidium may require the transformation of a DFR gene that can efficiently catalyze DHK reduction.

Novel coloured flowers

The floricultural industry has focused its attention primarily on the development of novel coloured and longer living cut flowers. The basis for this was laid down some years ago through the isolation of 'blue' genes and ethylene biosynthesis genes. Recently, a novel 'blue' gene has been discovered and yellow pigments were produced in petunias by addition of a new branch to the phenylpropanoid pathway. More insight was obtained into the sequestration of anthocyanin pigments into storage vacuoles. Significant progress has been achieved in the commercialisation of genetically modified flower varieties.

DNA METHYLATION IN PLANTS

Methylation of cytosine residues in DNA provides a mechanism of gene control. There are two classes of methyltransferase in Arabidopsis; one has a carboxy-terminal methyltransferase domain fused to an amino-terminal regulatory domain and is similar to mammalian methyltransferases. The second class apparently lacks an amino-terminal domain and is less well conserved. Methylcytosine can occur at any cytosine residue, but it is likely that clonal transmission of methylation patterns only occurs for cytosines in strand-symmetrical sequences CpG and CpNpG. In plants, as in mammals, DNA methylation has dual roles in defense against invading DNA and transposable elements and in gene regulation. Although originally reported as having no phenotypic consequence, reduced DNA methylation disrupts normal plant development.

RNA-mediated virus resistance in transgenic plants

In recent years the concept of pathogen-derived resistance (PDR) has been successfully exploited for conferring resistance against viruses in many crop plants. Starting with coat protein-mediated resistance, the range has been broadened to the use of other viral genes as a source of PDR. However, in the course of the efforts, often no clear correlation could be made between expression levels of the transgenes and observed virus resistance levels. Several reports mentioned high resistance levels using genes incapable of producing protein, but in these cases, even plants accumulating high amounts of transgene RNA were not most resistant. To accommodate these unexplained observations, a resistance mechanism involving specific breakdown of viral RNAs has been proposed. Recent progress towards understanding the RNA-mediated resistance mechanism and similarities with the co-suppression phenomenon will be discussed.

Position effects and epigenetic silencing of plant transgenes

Nuclear processes that silence plant transgenes are being revealed by analyses of natural triggers of epigenetic modifications, particularly cytosine methylation, and by comparisons of the genomic environments of differentially expressed transgene loci. It is increasingly apparent that plant genomes can sense and respond to the presence of foreign DNA in certain sequence contexts and at multiple dispersed sites. Determining the basis of this sensitivity and how nuclear defense systems are activated poses major challenges for the future.

Phenotypic variation in transgenic tobacco expressing mutated geminivirus movement/pathogenicity (BC1) proteins

Tobacco plants were transformed with the movement protein (pathogenicity) gene (BC1) from tomato mottle geminivirus (TMoV), using Agrobacterium-mediated transformation. Different transgenic tobacco lines that expressed high levels of the BC1 protein had phenotypes ranging from plants with severe stunting and leaf mottling (resembling geminivirus symptoms) to plants with no visible symptoms. The sequence data for the BC1 transgene from the transgenic plants with the different phenotypes indicated an association of spontaneously mutated forms of the BC1 gene in the transformed tobacco with phenotype variations. One mutated transgene associated with an asymptomatic phenotype had a major deletion at the C terminus of 119 amino acid residues with a recombination resulting in the addition of 26 amino acid residues of unidentified origin. This asymptomatic, mutated BC1 attenuated the phenotypic expression of the symptomatic BC1 in a tobacco line containing both copies of the BC1 gene. Another mutated form of the BC1 gene amplified from an asymptomatic, multicopy transgenic tobacco plant did not induce symptoms when transiently expressed in tobacco via a virus vector. The symptom attenuation in the transgenic tobacco by the asymptomatic BC1 may involve trans-dominant negative interference.