Expression of randomly integrated single complete copy transgenes does not vary in Arabidopsis thaliana

Generation of the salicylic acid deficient Arabidopsis via a synthetic salicylic acid hydroxylase expression cassette

BACKGROUND

Salicylic acid (SA) is one of the plant hormones, which plays crucial roles in signaling transduction in plant growth, disease resistance, and leaf senescence. Arabidopsis (Arabidopsis thaliana) SA 3-hydroxylase (S3H) and 5-hydroxylase (S5H) are key enzymes which maintain SA homeostasis by catalyzing SA to 2,3-dihydroxybenzoic acid (DHBA) and 2,5-DHBA, respectively.

RESULTS

SA deficient transgenic Arabidopsis lines were generated by introducing two binary vectors S5Hpro::EGFP-S3H and 35Spro::EGFP-S3H respectively, in which the expression of S3H is under the control of the S5H promoter or CaMV 35S promoter. Compared with the constitutive expression of S3H gene under the control of 35S promoter, the S3H gene under the native S5H promoter is activated by endogenous SA and results in a dynamic control of SA catabolism in a feedback mode. The SA accumulation, growth, leaf senescence, and pathogen resistance of the S5Hpro::GFP-S3H transgenic plants were investigated in parallel with NahG transgenic plants. The SA levels in the S5Hpro::EGFP-S3H transgenic plants were similar to or slightly lower than those of NahG transgenic Arabidopsis and resulted in SA deficient phenotypes. The low-SA trait of the S5Hpro::EGFP-S3H transgenic lines was inherited stably in the later generations.

CONCLUSIONS

Compared with NahG transgenic lines producing by-product catechol, S5Hpro::EGFP-S3H transgenic lines reduce SA levels by converting SA to a native product 2,3-DHBA for catabolism. Together, we provide new SA-deficient germplasms for the investigations of SA signaling in plant development, leaf senescence, and disease resistance.

Codon optimization of chicken β Gallinacin-3 gene results in constitutive expression and enhanced antimicrobial activity in transgenic Medicago sativa L

Gallinacin-3 (Gal-3) is a newly discovered epithelial beta-defensin that acts as cationic antimicrobial peptides, and plays an important role in chicken innate immunity. However, the gallinacin-3 precursor containeda lengthy C-terminal region, which often hindered itsexpression. After codon optimization of Gal-3 and construction of an expression vector, the transgenic plants of Medicago sativa were obtained. Transgenic plants were validated and expression of proteins was detected. The antimicrobial activity of chicken β Gal-3 was analyzed and effects of chicken β Gal-3 on the body weight and intestinal microflora of mice were described. Our results demonstrated that the codon optimized chicken Gal-3 was stably expressed in transgenic Medicago sativa using the pCAMBIA3301 expression vector under the control of protein phosphatase (Ppha) promoter. Five transgenic plants with the highest expression of chicken β Gal-3 were selected, and were evaluated for the in vitro antimicrobial activity against Escherichia coli, Staphylococcus aureus and Salmonella typhi. Our findings confirmed that the Minimum Inhibitory Concentration (MIC) of the three bacterial strains were 32, 16 and 128 μg/mL, respectively. In addition, the effect of chicken Gal-3 on the body weight of mice fed with transgenic plants showed no significant deviation compared with that of the control group. Similarly, no loss of intestinal microflora was evident in the experimental group compared with the control group. Together, our findings demonstrate an alternative method for the stable expression of chicken Gal-3 withsignificant antibacterial effects and potential probiotics uses. In addition, this study may also be useful in the development of resistant M. sativa plants against pathogenic bacteria in future studies.

Development of the binary vector pTACAtg1 for stable gene expression in plant: Reduction of gene silencing in transgenic plants carrying the target gene with long flanking sequences

Genetic modification in plants helps us to understand molecular mechanisms underlying on plant fitness and to improve profitable crops. However, in transgenic plants, the value of gene expression often varies among plant populations of distinct lines and among generations of identical individuals. This variation is caused by several reasons, such as differences in the chromosome position, repeated sequences, and copy number of the inserted transgene. Developing a state-of-art technology to avoid the variation of gene expression levels including gene silencing has been awaited. Here, we developed a novel binary plasmid (pTACAtg1) that is based on a transformation-competent artificial chromosome (TAC) vector, harboring long genomic DNA fragments on both sides of the cloning sites. As a case study, we cloned the cauliflower mosaic virus 35S promoter:β-glucuronidase (35S:GUS) gene cassettes into the pTACAtg1, and introduced it with long flanking sequences on the pTACAtg1 into the plants. In isolated transgenic plants, the copy number was reduced and the GUS expressions were detected more stably than those in the control plants carrying the insert without flanking regions. In our result, the reduced copy number of a transgene suppressed variation and silencing of its gene expression. The pTACAtg1 vector will be suitable for the production of stable transformants and for expression analyses of a transgene.

Uniform Expression and Relatively Small Position Effects Characterize Sister Transformants in Maize and Soybean

Development of transgenic cell lines or organisms for industrial, agricultural, or medicinal applications involves inserting DNA into the target genome in a way that achieves efficacious transgene expression without a deleterious impact on fitness. The genomic insertion site is widely recognized as an important determinant of success. However, the effect of chromosomal location on transgene expression and fitness has not been systematically investigated in plants. Here we evaluate the importance of transgene insertion site in maize and soybean using both random and site-specific transgene integration. We have compared the relative contribution of genomic location on transgene expression levels with other factors, including cis-regulatory elements, neighboring transgenes, genetic background, and zygosity. As expected, cis-regulatory elements and the presence/absence of nearby transgene neighbors can impact transgene expression. Surprisingly, we determined not only that genomic location had the least impact on transgene expression compared to the other factors that were investigated but that the majority of insertion sites recovered supported transgene expression levels that were statistically not distinguishable. All 68 genomic sites evaluated were capable of supporting high-level transgene expression, which was also consistent across generations. Furthermore, multilocation field evaluation detected no to little decrease in agronomic performance as a result of transgene insertion at the vast majority of sites we evaluated with a single construct in five maize hybrid backgrounds.

Impact of differential DNA methylation on transgene expression in cotton (Gossypium hirsutum L.) events generated by targeted sequence insertion

Targeted Genome Optimization (TGO) using site-specific nucleases to introduce a DNA double-strand break (DSB) at a specific target locus has broadened the options available to breeders for generation and combination of multiple traits. The use of targeted DNA cleavage in combination with homologous recombination (HR)-mediated repair, enabled the precise targeted insertion of additional trait genes (2mepsps, hppd, axmi115) at a pre-existing transgenic locus in cotton. Here we describe the expression and epigenome analyses of cotton Targeted Sequence Insertion (TSI) events over generations. In a subset of events, we observed variability in the level of transgene (hppd, axmi115) expression between independent but genetically identical TSI events. Transgene expression could also be differential within single events and variable over generations. This expression variability and silencing occurred independently of the transgene sequence and could be attributed to DNA methylation that was further linked to different DNA methylation mechanisms. The trigger(s) of transgene DNA methylation remains elusive but we hypothesize that targeted DSB induction and repair could be a potential trigger for DNA methylation.

Overexpression and cosuppression of xylem-related genes in an early xylem differentiation stage-specific manner by the AtTED4 promoter

Tissue-specific overexpression of useful genes, which we can design according to their cause-and-effect relationships, often gives valuable gain-of-function phenotypes. To develop genetic tools in woody biomass engineering, we produced a collection of Arabidopsis lines that possess chimeric genes of a promoter of an early xylem differentiation stage-specific gene, Arabidopsis Tracheary Element Differentiation-related 4 (AtTED4) and late xylem development-associated genes, many of which are uncharacterized. The AtTED4 promoter directed the expected expression of transgenes in developing vascular tissues from young to mature stage. Of T2 lines examined, 42%, 49% and 9% were judged as lines with the nonrepeat type insertion, the simple repeat type insertion and the other repeat type insertion of transgenes. In 174 T3 lines, overexpression lines were confirmed for 37 genes, whereas only cosuppression lines were produced for eight genes. The AtTED4 promoter activity was high enough to overexpress a wide range of genes over wild-type expression levels, even though the wild-type expression is much higher than AtTED4 expression for several genes. As a typical example, we investigated phenotypes of pAtTED4::At5g60490 plants, in which both overexpression and cosuppression lines were included. Overexpression but not cosuppression lines showed accelerated xylem development, suggesting the positive role of At5g60490 in xylem development. Taken together, this study provides valuable results about behaviours of various genes expressed under an early xylem-specific promoter and about usefulness of their lines as genetic tools in woody biomass engineering.

Intron sequences that stimulate gene expression in Arabidopsis

Related motifs strongly increase gene expression when added to an intron located in coding sequences. Many introns greatly increase gene expression through a mechanism that remains elusive. An obstacle to understanding intron-mediated enhancement (IME) has been the difficulty of locating the specific intron sequences responsible for boosting expression because they are redundant, dispersed, and degenerate. Previously we used the IMEter algorithm in two independent ways to identify two motifs (CGATT and TTNGATYTG) that are candidates for involvement in IME in Arabidopsis. Here we show that both motifs are sufficient to increase expression. An intron that has little influence on expression was converted into one that increased mRNA accumulation 24-fold and reporter enzyme activity 40-fold relative to the intronless control by introducing 11 copies of the more active TTNGATYTG motif. This degree of stimulation is twice as large as that of the strongest of 15 natural introns previously tested in the same reporter gene. Even though the CGATT and TTNGATYTG motifs each increased expression, and CGATT matches the NGATY core of the longer motif, combining the motifs to make TTCGATTTG reduced the stimulating ability of the TTNGATYTG motif. Additional substitutions were used to test the contribution to IME of other residues in the TTNGATYTG motif. The verification that these motifs are active in IME will improve our ability to predict the stimulating ability of introns, to engineer any intron to increase expression to a desired level, and to explore the mechanism of IME by seeking factors that might interact with these sequences.

Development of disease-resistant rice by pathogen-responsive expression of WRKY45

WRKY45 is an important transcription factor in the salicylic acid signalling pathway in rice that mediates chemical-induced resistance against multiple pathogens. Its constitutive overexpression confers extremely strong resistance against Magnaporthe oryzae and Xanthomonas oryzae pv. oryzae to rice, but has adverse effects on agronomic traits. Here, a new strategy to confer rice with strong disease resistance without any negative effects on agronomic traits was established by expressing WRKY45 under the control of pathogen-responsive promoters in combination with a translational enhancer derived from a 5'-untranslated region (UTR) of rice alcohol dehydrogenase (ADH). Rice promoters that responded to M. oryzae and X. oryzae pv. oryzae infections within 24 h were identified, and 2-kb upstream sequences from nine of them were isolated, fused to WRKY45 cDNA with or without the ADH 5'-UTR, and introduced into rice. Although pathogen-responsive promoters alone failed to confer effective disease resistance, the use of the ADH 5'-UTR in combination with them, in particular the PR1b and GST promoters, enhanced disease resistance. Field trials showed that overall, PR1b promoter-driven (with ADH 5'-UTR) lines performed the best and one had agronomic traits comparable to control untransformed rice. Thus, expressing WRKY45 under the control of the PR1b promoter with the ADH 5'-UTR is an excellent strategy to develop disease-resistant rice, and the line established could serve as a mother line for breeding disease-resistant rice.

Molecular Analyses of Transgenic Plants

One of the major challenges in plant molecular biology is to generate transgenic plants that express transgenes stably over generations. Here, we describe some routine methods to study transgene locus structure and to analyze transgene expression in plants: Southern hybridization using DIG chemiluminescent technology for characterization of transgenic locus, SYBR Green-based real-time RT-PCR to measure transgene transcript level, and protein immunoblot analysis to evaluate accumulation and stability of transgenic protein product in the target tissue.

CaMV-35S promoter sequence-specific DNA methylation in lettuce

KEY MESSAGE

We found 35S promoter sequence-specific DNA methylation in lettuce. Additionally, transgenic lettuce plants having a modified 35S promoter lost methylation, suggesting the modified sequence is subjected to the methylation machinery. We previously reported that cauliflower mosaic virus 35S promoter-specific DNA methylation in transgenic gentian (Gentiana triflora × G. scabra) plants occurs irrespective of the copy number and the genomic location of T-DNA, and causes strong gene silencing. To confirm whether 35S-specific methylation can occur in other plant species, transgenic lettuce (Lactuca sativa L.) plants with a single copy of the 35S promoter-driven sGFP gene were produced and analyzed. Among 10 lines of transgenic plants, 3, 4, and 3 lines showed strong, weak, and no expression of sGFP mRNA, respectively. Bisulfite genomic sequencing of the 35S promoter region showed hypermethylation at CpG and CpWpG (where W is A or T) sites in 9 of 10 lines. Gentian-type de novo methylation pattern, consisting of methylated cytosines at CpHpH (where H is A, C, or T) sites, was also observed in the transgenic lettuce lines, suggesting that lettuce and gentian share similar methylation machinery. Four of five transgenic lettuce lines having a single copy of a modified 35S promoter, which was modified in the proposed core target of de novo methylation in gentian, exhibited 35S hypomethylation, indicating that the modified sequence may be the target of the 35S-specific methylation machinery.

Development of disease-resistant rice by optimized expression of WRKY45

The rice transcription factor WRKY45 plays a central role in the salicylic acid signalling pathway and mediates chemical-induced resistance to multiple pathogens, including Magnaporthe oryzae and Xanthomonas oryzae pv. oryzae. Previously, we reported that rice transformants overexpressing WRKY45 driven by the maize ubiquitin promoter were strongly resistant to both pathogens; however, their growth and yield were negatively affected because of the trade-off between the two conflicting traits. Also, some unknown environmental factor(s) exacerbated this problem. Here, we report the development of transgenic rice lines resistant to both pathogens and with agronomic traits almost comparable to those of wild-type rice. This was achieved by optimizing the promoter driving WRKY45 expression. We isolated 16 constitutive promoters from rice genomic DNA and tested their ability to drive WRKY45 expression. Comparisons among different transformant lines showed that, overall, the strength of WRKY45 expression was positively correlated with disease resistance and negatively correlated with agronomic traits. We conducted field trials to evaluate the growth of transgenic and control lines. The agronomic traits of two lines expressing WRKY45 driven by the OsUbi7 promoter (PO sUbi7 lines) were nearly comparable to those of untransformed rice, and both lines were pathogen resistant. Interestingly, excessive WRKY45 expression rendered rice plants sensitive to low temperature and salinity, and stress sensitivity was correlated with the induction of defence genes by these stresses. These negative effects were barely observed in the PO sUbi7 lines. Moreover, their patterns of defence gene expression were similar to those in plants primed by chemical defence inducers.

Simplifying transgene locus structure through Cre-lox recombination

Transgene silencing is often associated with multicopy integrations, which occur frequently during plant transformation. Transgene expression can be restored in a number of multicopy loci by converting them to single copy. This chapter describes a plant transformation protocol based on use of the Cre-lox system, which allows conversion of a multicopy transgene locus into single copy. The strategy is based on designing a transformation vector with lox sites, developing transgenic lines, and introducing Cre activity to initiate Cre-lox recombination, which leads to the simplification of a multicopy locus to a single- or low-copy state. This method is compatible with both gene gun and Agrobacterium-mediated gene delivery and should be particularly useful for crops that are difficult to transform.

Progressive 35S promoter methylation increases rapidly during vegetative development in transgenic Nicotiana attenuata plants

BACKGROUND

Genetically modified plants are widely used in agriculture and increasingly in ecological research to enable the selective manipulation of plant traits in the field. Despite their broad usage, many aspects of unwanted transgene silencing throughout plant development are still poorly understood. A transgene can be epigenetically silenced by a process called RNA directed DNA methylation (RdDM), which can be seen as a heritable loss of gene expression. The spontaneous nature of transgene silencing has been widely reported, but patterns of acquirement remain still unclear.

RESULTS

Transgenic wild tobacco plants (Nicotiana attenuata) expressing heterologous genes coding for antimicrobial peptides displayed an erratic and variable occurrence of transgene silencing. We focused on three independently transformed lines (PNA 1.2, PNA 10.1 and ICE 4.4) as they rapidly lost the expression of the resistance marker and down-regulated transgene expression by more than 200 fold after only one plant generation. Bisulfite sequencing indicated hypermethylation within the 35S and NOS promoters of these lines. To shed light on the progress of methylation establishment, we successively sampled leaf tissues from different stages during plant development and found a rapid increase in 35S promoter methylation during vegetative growth (up to 77% absolute increase within 45 days of growth). The levels of de novo methylation were inherited by the offspring without any visible discontinuation. A secondary callus regeneration step could interfere with the establishment of gene silencing and we found successfully restored transgene expression in the offspring of several regenerants.

CONCLUSIONS

The unpredictability of the gene silencing process requires a thorough selection and early detection of unstable plant lines. De novo methylation of the transgenes was acquired solely during vegetative development and did not require a generational change for its establishment or enhancement. A secondary callus regeneration step provides a convenient way to rescue transgene expression without causing undesirable morphological effects, which is essential for experiments that use transformed plants in the analysis of ecologically important traits.

Evaluation of four phloem-specific promoters in vegetative tissues of transgenic citrus plants

'Mexican' lime (Citrus aurantifolia Swingle) was transformed with constructs that contained chimeric promoter-gus gene fusions of phloem-specific rolC promoter of Agrobacterium rhizogenes, Arabidopsis thaliana sucrose-H(+) symporter (AtSUC2) gene promoter of Arabidopsis thaliana, rice tungro bacilliform virus (RTBV) promoter and sucrose synthase l (RSs1) gene promoter of Oryza sativa (rice). Histochemical β-glucuronidase (GUS) analysis revealed vascular-specific expression of the GUS protein in citrus. The RTBV promoter was the most efficient promoter in this study while the RSs1 promoter could drive low levels of gus gene expression in citrus. These results were further validated by reverse transcription real-time polymerase chain reaction and northern blotting. Southern blot analysis confirmed stable transgene integration, which ranged from a single insertion to four copies per genome. The use of phloem-specific promoters in citrus will allow targeted transgene expression of antibacterial constructs designed to battle huanglongbing disease (HLB or citrus greening disease), associated with a phloem-limited Gram-negative bacterium.

The gypsy insulator of Drosophila melanogaster, together with its binding protein suppressor of Hairy-wing, facilitate high and precise expression of transgenes in Arabidopsis thaliana

The variation of expression pattern exhibited by a transgene as a result of random integration, known as position effect, is, among other mechanisms, a particular challenge to reverse genetics. We present a strategy to counteract position effect in Arabidopsis thaliana by flanking the transgenes with the gypsy insulator from Drosophila melanogaster. In addition, Suppressor of Hairy-wing [Su(Hw)], the binding protein of the gypsy insulator, was coexpressed. Results indicated that the gypsy insulators could efficiently improve the expression levels of reporter genes driven by various kinds of promoters by 8- to 13-fold. Coexpression of the Su(Hw) protein led to a more uniform expression level of transgenes, as the coefficient of variation of expression levels was reduced further. The gypsy-Su(Hw) system enhanced expression levels, but did not alter the specificity of promoter activities, as experimentally evidenced by the promoters of the PIN and the AFB gene families. Interestingly, the gypsy insulator was also able to improve the expression of a selectable marker gene outside the insulated region, which facilitated the screen of transformants. Our system will likely decrease the number of lines that experimenters need to create and examine for a given transgene by contributing to relatively high and precise expression of transgenes in plants. Certain features of the gypsy insulator in Arabidopsis also provide new perspectives on the insulator field.

Strict de novo methylation of the 35S enhancer sequence in gentian

A novel transgene silencing phenomenon was found in the ornamental plant, gentian (Gentiana triflora x G. scabra), in which the introduced Cauliflower mosaic virus (CaMV) 35S promoter region was strictly methylated, irrespective of the transgene copy number and integrated loci. Transgenic tobacco having the same vector did not show the silencing behavior. Not only unmodified, but also modified 35S promoters containing a 35S enhancer sequence were found to be highly methylated in the single copy transgenic gentian lines. The 35S core promoter (-90)-introduced transgenic lines showed a small degree of methylation, implying that the 35S enhancer sequence was involved in the methylation machinery. The rigorous silencing phenomenon enabled us to analyze methylation in a number of the transgenic lines in parallel, which led to the discovery of a consensus target region for de novo methylation, which comprised an asymmetric cytosine (CpHpH; H is A, C or T) sequence. Consequently, distinct footprints of de novo methylation were detected in each (modified) 35S promoter sequence, and the enhancer region (-148 to -85) was identified as a crucial target for de novo methylation. Electrophoretic mobility shift assay (EMSA) showed that complexes formed in gentian nuclear extract with the -149 to -124 and -107 to -83 region probes were distinct from those of tobacco nuclear extracts, suggesting that the complexes might contribute to de novo methylation. Our results provide insights into the phenomenon of sequence- and species- specific gene silencing in higher plants.

The HSP terminator of Arabidopsis thaliana increases gene expression in plant cells

To express a foreign gene in plants effectively, a good expression system is required. Here we describe the identification of a transcriptional terminator that supports increased levels of expression. The terminators of several Arabidopsis genes were examined in transfected Arabidopsis T87 protoplasts. The heat shock protein 18.2 (HSP) terminator was the most effective in supporting increased levels of expression. The HSP terminator increases mRNA levels of both transiently and stably expressed transgenes approximately 2-fold more than the NOS (nopaline synthase) terminator. When combined with the HSP terminator, a translational enhancer increased gene expression levels approximately 60- to 100-fold in transgenic plants.

A position effect on the heritability of epigenetic silencing

In animals and yeast, position effects have been well documented. In animals, the best example of this process is Position Effect Variegation (PEV) in Drosophila melanogaster. In PEV, when genes are moved into close proximity to constitutive heterochromatin, their expression can become unstable, resulting in variegated patches of gene expression. This process is regulated by a variety of proteins implicated in both chromatin remodeling and RNAi-based silencing. A similar phenomenon is observed when transgenes are inserted into heterochromatic regions in fission yeast. In contrast, there are few examples of position effects in plants, and there are no documented examples in either plants or animals for positions that are associated with the reversal of previously established silenced states. MuDR transposons in maize can be heritably silenced by a naturally occurring rearranged version of MuDR. This element, Muk, produces a long hairpin RNA molecule that can trigger DNA methylation and heritable silencing of one or many MuDR elements. In most cases, MuDR elements remain inactive even after Muk segregates away. Thus, Muk-induced silencing involves a directed and heritable change in gene activity in the absence of changes in DNA sequence. Using classical genetic analysis, we have identified an exceptional position at which MuDR element silencing is unstable. Muk effectively silences the MuDR element at this position. However, after Muk is segregated away, element activity is restored. This restoration is accompanied by a reversal of DNA methylation. To our knowledge, this is the first documented example of a position effect that is associated with the reversal of epigenetic silencing. This observation suggests that there are cis-acting sequences that alter the propensity of an epigenetically silenced gene to remain inactive. This raises the interesting possibility that an important feature of local chromatin environments may be the capacity to erase previously established epigenetic marks.

Epigenetics involves the heritable alteration of gene activity without changes in DNA sequence. Although clearly a repository for heritable information, what makes epigenetic states distinct is that they are far more labile than those associated with DNA sequence. The epigenetic landscape of eukaryotic genomes is far from uniform. Vast stretches of them are effectively epigenetically silenced, while other regions are largely active. The experiments described here suggest that the propensity to maintain heritable epigenetic states can vary depending on position within the genome. Because transposable elements, or transposons, move from place to place within the genome, they make an ideal probe for differences in epigenetic states at various positions. Our model system uses a single transposon, MuDR in maize, and a variant of MuDR, Mu killer (Muk). When MuDR and Muk are combined genetically, MuDR elements become epigenetically silenced, and they generally remain so even after Muk is lost in subsequent generations. However, we have identified a particular position at which the MuDR element reactivates after Muk is lost. These data show that there are some parts of the maize genome that are either competent to erase epigenetic silencing or are incapable of maintaining it. These results suggest that erasure of heritable information may be an important component of epigenetic regulation.

Genome-wide transposon tagging reveals location-dependent effects on transcription and chromatin organization in Arabidopsis

The interphase nucleus exists as a highly dynamic system, the physical properties of which have functional importance in gene regulation. Not only can gene expression be influenced by the local sequence context, but also by the architecture of the nucleus in three-dimensions (3D), and by the interactions between these levels via chromatin modifications. A challenging task is to resolve the complex interplay between sequence- and genome structure-based control mechanisms. Here, we created a collection of 277 Arabidopsis lines that allow the visual tracking of individual loci in living plants while comparing gene expression potential at these locations, via an identical reporter cassette. Our studies revealed regional gene silencing near a heterochromatin island, via DNA methylation, that is correlated with mobility constraint and nucleolar association. We also found an example of nucleolar association that does not correlate with gene suppression, suggesting that distinct mechanisms exist that can mediate interactions between chromatin and the nucleolus. These studies demonstrate the utility of this novel resource in unifying structural and functional studies towards a more comprehensive model of how global chromatin organization may coordinate gene expression over large scales.

Local DNA features affect RNA-directed transcriptional gene silencing and DNA methylation

Transcription of a nopaline synthase promoter (pNOS) inverted repeat provides an RNA signal that can trigger transcriptional gene silencing and methylation of pNOS promoters in trans. The degree of silencing is influenced by the local DNA features close to the target promoter integration sites. Among 26 transgenic Arabidopsis thaliana lines harbouring single copies of a T-DNA including a pNOS-NPTII reporter gene at different chromosomal loci, NPTII RNA levels showed limited variation. When challenged by the silencer transgene providing the pNOS RNA signal, reduction of the NPTII RNA levels in the F(1) generation varied by more than 100-fold, ranging from no reduction to reduction to <1% of the non-silenced level. Silencing was generally correlated with proportional DNA methylation in the pNOS region, except for one target transgene showing substantial DNA methylation without adequate silencing. Silencing was progressive through generations. Differences in the degree of silencing among the target transgenes were transmitted at least to the F(3) generation, and seemed to be influenced by transgene-flanking sequences. Apparently, close-by repeats promoted, whereas close-by functional genes diminished, the response to the silencing signal.

The Quest to Understand the Basis and Mechanisms that Control Expression of Introduced Transgenes in Crop Plants

We discuss mechanisms and factors that influence levels and stability of expressed heterologous proteins in crop plants. We have seen substantial progress in this field over the past two decades in model experimental organisms such as Arabidopsis and tobacco. There is no question such studies have resulted in furthering our understanding of key processes in the plant cell and the elaboration of sophisticated models to explain underlying mechanisms that might influence the fate, levels and stability of expression of recombinant heterologous proteins in plants. However, very often, such information is not applicable outside these laboratory experimental models. In order to generate a knowledge basis that can be used to achieve high levels and stability of heterologous proteins in relevant crop plants it is imperative to perform such studies on the target crops. With this in mind, we discuss key elements of the process at the DNA, RNA and protein levels. We believe it is essential to discuss recombinant protein production in crops in a holistic manner in order to develop a comprehensive knowledge base that will in turn serve plant biotechnology applications well.

Soybean mosaic virus helper component-protease enhances somatic embryo production and stabilizes transgene expression in soybean

Soybean mosaic virus (SMV) helper component protease (HC-Pro), a suppressor of post-transcriptional gene silencing, was evaluated for its ability to enhance production of soybean hygromycin-resistant somatic embryos (HR-SEs), and stabilize transgene expression. Immature soybean cotyledonary explants were co-cultured with Agrobacterium tumefaciens strain KYRT1 harboring either pCAMBIA1302, carrying a hygromycin phosphotransferase gene (hpt) and a gene encoding green fluorescent protein; pCAMBIA1305.1, carrying hpt and beta-glucuronidase (uidA) genes; pG2-HC-Pro, a derivative of pCAMBIA1305.1 containing SMV G2 HC-Pro; or pG5-HC-Pro, a derivative of pCAMBIA1305.1 containing SMV G5 HC-Pro, but lacking uidA. Significantly (rho<0.02) higher numbers of HR-SEs were obtained from explants transformed with Agrobacterium harboring either pG2-HC-Pro or pG5-HC-Pro than with either of the vector controls (pCAMBIA1302 or pCAMBIA1305.1). Beta-glucuronidase (GUS) expression was significantly (rho<0.003) higher in 50-day-old transgenic plants expressing GUS along with SMV-HC-Pro and in SMV-infected GUS transgenic plants than in transgenic plants expressing GUS alone. Together, these data suggest that SMV-HC-Pro enhanced recovery of HR-SEs by suppressing silencing of the hygromycin phosphotransferase gene.