'GM-gene-deletor': fused loxP-FRT recognition sequences dramatically improve the efficiency of FLP or CRE recombinase on transgene excision from pollen and seed of tobacco plants

Development of a simple and efficient system for excising selectable markers in Arabidopsis using a minimal promoter::Cre fusion construct

The development of rapid and efficient strategies to generate selectable marker-free transgenic plants could help increase the consumer acceptance of genetically modified (GM) plants. To produce marker-free transgenic plants without conditional treatment or the genetic crossing of offspring, we have developed a rapid and convenient DNA excision method mediated by the Cre/loxP recombination system under the control of a -46 minimal CaMV 35S promoter. The results of a transient expression assay showed that -46 minimal promoter::Cre recombinase (-46::Cre) can cause the loxP-specific excision of a selectable marker, thereby connecting the 35S promoter and β-glucuronidase (GUS) reporter gene. Analysis of stable transgenic Arabidopsis plants indicated a positive correlation between loxP-specific DNA excision and GUS expression. PCR and DNA gel-blot analysis further revealed that nine of the 10 tested T(1) transgenic lines carried both excised and nonexcised constructs in their genomes. In the subsequent T(2) generation plants, over 30% of the individuals for each line were marker-free plants harboring the excised construct only. These results demonstrate that the -46::Cre fusion construct can be efficiently and easily utilized for producing marker-free transgenic plants.

Recombinase-mediated gene stacking as a transformation operating system

The current method for combining transgenes into a genome is through the assortment of independent loci, a classical operating system compatible with transgenic traits created by different developers, at different times and/or through different transformation techniques. However, as the number of transgenic loci increases over time, increasingly larger populations are needed to find the rare individual with the desired assortment of transgenic loci along with the non-transgenic elite traits. Introducing a transgene directly into a field cultivar would bypass the need to introgress the engineered trait. However, this necessitates separate transformations into numerous field cultivars, along with the characterization and regulatory approval of each independent transformation event. Reducing the number of segregating transgenic loci could be achieved if multiple traits are introduced at the same time, a preferred option if each of the many traits is new or requires re-engineering. If re-engineering of previously introduced traits is not needed, then appending a new trait to an existing locus would be a rational strategy. The insertion of new DNA at a known locus can be accomplished by site-specific integration, through a host-dependent homology-based process, or a heterologous site-specific recombination system. Here, we discuss gene stacking through the use of site-specific recombinases.

[Advances on transgene containment technologies]

The biosecurity of transgenic organism has been widely concerned and extremely restricted its application. Recently, many technological strategies have been developed to ensure its biosecurity. Thus, transgene containment technologies have become one of the hotspots in current transgenic research. In this paper, several transgene containment technologies, such as marker-free transgenic technology, safety marker transgenic technology, chloroplast transgenic technologies, terminator technology, male sterility technology, and 'GM-gene-deletor'technology were reviewed and evaluated. 'GM-gene-deletor' technology, as one of these technologies, demonstrated a prosperous future for safe application of transgenic organisms. Finally, the strategies for developing new transgene containment technologies have been suggested.

[Effect of transgenic insect-resistant rice on biodiversity]

Rice is the most important food crops in maintaining food security in China. The loss of China's annual rice production caused by pests is over ten million tons. Present studies showed that the transgenic insect-resistant rice can substantially reduce the application amount of chemical pesticides. In the case of no pesticide use, the pest density in transgenic rice field is significantly lower than that in non-transgenic field, and the neutral insects and natural enemies of pests increased significantly, indicating that the ecological environment and biodiversity toward the positive direction. The gene flow frequency from transgenic rice is dramatically reduced with the distance increases, reaching less than 0.01% at the distance of 6.2 m. Application of transgenic insect-resistant rice in China has an important significance for ensuring food security, maintaining sustainable agricultural development, and protecting the ecological environment and biodiversity. This review summarized the research progress in transgenic insect-resistant rice and its effect on biodiversity. The research directions and development trends of crop pest controlling in future are discussed. These help to promote better use of transgenic insect-resistant rice.

[Transgenic technology and soybean quality improvement]

Soybean is an important source of edible oil, protein and protein diet. The breeding process of high quality soybean can be accelerated via employment of transgenic technology, by which the key genes for soybean quality traits could be directly manipulated. Thus, various soybean varieties could be bred to fulfill different needs for specific consumers. Here, we reviewed the contribution of transgenic technology to improvement of soybean qualities in recent years. We also introduce some newly developed safe transgenic technologies and hope this information could relieve some concerns on the GM food.

An efficient and rapid transgenic pollen screening and detection method using flow cytometry

Assaying for transgenic pollen, a major vector of transgene flow, provides valuable information and essential data for the study of gene flow and assessing the effectiveness of transgene containment. Most studies have employed microscopic screening methods or progeny analyses to estimate the frequency of transgenic pollen. However, these methods are time-consuming and laborious when large numbers of pollen grains must be analyzed to look for rare transgenic pollen grains. Thus, there is an urgent need for the development of a simple, rapid, and high throughput analysis method for transgenic pollen analysis. In this study, our objective was to determine the accuracy of using flow cytometry technology for transgenic pollen quantification in practical application where transgenic pollen is not frequent. A suspension of non-transgenic tobacco pollen was spiked with a known amount of verified transgenic tobacco pollen synthesizing low or high amounts of green fluorescent protein (GFP). The flow cytometric method detected approximately 75% and 100% of pollen grains synthesizing low and high amounts of GFP, respectively. The method is rapid, as it is able to count 5000 pollen grains per minute-long run. Our data indicate that this flow cytometric method is useful to study gene flow and assessment of transgene containment.

Transgene excision in pollen using a codon optimized serine resolvase CinH-RS2 site-specific recombination system

Transgene escape, a major environmental and regulatory concern in transgenic crop cultivation, could be alleviated by removing transgenes from pollen, the most frequent vector for transgene flow. A transgene excision vector containing a codon optimized serine resolvase CinH recombinase (CinH) and its recognition sites RS2 were constructed and transformed into tobacco (Nicotiana tabacum cv. Xanthi). CinH recombinase recognized 119 bp of nucleic acid sequences, RS2, in pollen and excised the transgene flanked by the RS2 sites. In this system, the pollen-specific LAT52 promoter from tomato was employed to control the expression of CinH recombinase. Loss of expression of a green fluorescent protein (GFP) gene under the control of the LAT59 promoter from tomato was used as an indicator of transgene excision. Efficiency of transgene excision from pollen was determined by flow cytometry (FCM)-based pollen screening. While a transgenic event in the absence of CinH recombinase contained about 70% of GFP-synthesizing pollen, three single-copy transgene events contained less than 1% of GFP-synthesizing pollen based on 30,000 pollen grains analyzed per event. This suggests that CinH-RS2 recombination system could be effectively utilized for transgene biocontainment.

Transgene introgression in crop relatives: molecular evidence and mitigation strategies

Incorporation of crop genes into wild and weedy relative populations (i.e. introgression) has long been of interest to ecologists and weed scientists. Potential negative outcomes that result from crop transgene introgression (e.g. extinction of native wild relative populations; invasive spread by wild or weedy hosts) have not been documented, and few examples of transgene introgression exist. However, molecular evidence of introgression from non-transgenic crops to their relatives continues to emerge, even for crops deemed low-risk candidates for transgene introgression. We posit that transgene introgression monitoring and mitigation strategies are warranted in cases in which transgenes are predicted to confer selective advantages and disadvantages to recipient hosts. The utility and consequences of such strategies are examined, and future directions provided.

Recombinase technology: applications and possibilities

The use of recombinases for genomic engineering is no longer a new technology. In fact, this technology has entered its third decade since the initial discovery that recombinases function in heterologous systems (Sauer in Mol Cell Biol 7(6):2087-2096, 1987). The random insertion of a transgene into a plant genome by traditional methods generates unpredictable expression patterns. This feature of transgenesis makes screening for functional lines with predictable expression labor intensive and time consuming. Furthermore, an antibiotic resistance gene is often left in the final product and the potential escape of such resistance markers into the environment and their potential consumption raises consumer concern. The use of site-specific recombination technology in plant genome manipulation has been demonstrated to effectively resolve complex transgene insertions to single copy, remove unwanted DNA, and precisely insert DNA into known genomic target sites. Recombinases have also been demonstrated capable of site-specific recombination within non-nuclear targets, such as the plastid genome of tobacco. Here, we review multiple uses of site-specific recombination and their application toward plant genomic engineering. We also provide alternative strategies for the combined use of multiple site-specific recombinase systems for genome engineering to precisely insert transgenes into a pre-determined locus, and removal of unwanted selectable marker genes.

Evaluating biological containment strategies for pollen-mediated gene flow

Several biological containment methods have been developed to reduce pollen dispersal; many of them only have a proof of concept in a model plant species. This review focuses on biological containment measures which were tested for their long-term efficiency at the greenhouse or field scale level, i.e. plastid transformation, transgene excission, cleistogamy and cytoplasmic male sterility (CMS). Pollen-mediated gene transfer in transplastomic tobacco could occur at very low frequencies if the predominant mode of inheritance is maternal. Transgene excision from tobacco pollen can be made highly efficient by coexpression of two recombinases. For cleistogamous oilseed rape it was shown that some flowers were partially open depending on genotypes, environment and recording dates. Reports on the stability of CMS in maize and sunflower indicated that there is a high variability for different genotypes under different environmental conditions and over successive years. But for both crop types some stable lines could be selected. These data demonstrate that the biological containment methods discussed are very promising for reducing gene flow but that no single containment strategy provides 100% reduction. However, the necessary efficiency of biological containment methods depends on the level of containment required. The containment level may need to be higher for safety purposes (e.g. production of special plant-made pharmaceuticals), while much lower containment levels may already be sufficient to reach coexistence goals. It is concluded that where pollen-mediated gene flow must be prevented altogether, combinations of complementary containment systems will be required.

Sustainable use of biotechnology for bioenergy feedstocks

Done correctly, cellulosic bioenergy should be both environmentally and economically beneficial. Carbon sequestration and decreased fossil fuel use are both worthy goals in developing next-generation biofuels. We believe that biotechnology will be needed to significantly improve yield and digestibility of dedicated perennial herbaceous biomass feedstocks, such as switchgrass and Miscanthus, which are native to the US and China, respectively. This Forum discusses the sustainability of herbaceous feedstocks relative to the regulation of biotechnology with regards to likely genetically engineered traits. The Forum focuses on two prominent countries wishing to develop their bioeconomies: the US and China. These two countries also share a political desire and regulatory frameworks to enable the commercialization and wide release of transgenic feedstocks with appropriate and safe new genetics. In recent years, regulators in both countries perform regular inspections of transgenic field releases and seriously consider compliance issues, even though the US framework is considered to be more mature and stringent. Transgene flow continues to be a pertinent environmental and regulatory issue with regards to transgenic plants. This concern is largely driven by consumer issues and ecological uncertainties. Regulators are concerned about large-scale releases of transgenic crops that have sexually compatible crops or wild relatives that can stably harbor transgenes via hybridization and introgression. Therefore, prior to the commercialization or extensive field testing of transgenic bioenergy feedstocks, we recommend that mechanisms that ensure biocontainment of transgenes be instituted, especially for perennial grasses. A cautionary case study will be presented in which a plant's biology and ecology conspired against regulatory constraints in a non-biomass crop perennial grass (creeping bentgrass, Agrostis stolonifera), in which biocontainment was not attained. Appropriate technologies that could be applied to perennial grass feedstocks for biocontainment are discussed.

Zinc finger nuclease-mediated transgene deletion

A transgene, flanked by zinc finger nuclease (ZFN) cleavage sites, was deleted from a stably transformed plant by crossing it with a second plant expressing a corresponding ZFN gene. A target construct, containing a GUS reporter gene flanked by ZFN cleavage sites, a GFP reporter gene and a PAT selectable marker gene, was transformed into tobacco. Basta-resistant plants were regenerated and screened for GUS and GFP expression. A second construct, containing a ZFN gene driven by the constitutive CsVMV promoter and an HPT selectable marker gene, was also transformed into tobacco. Selected T(0) plants were grown to maturity and allowed to self-pollinate. Homozygous target plants, which expressed GUS and GFP, were crossed with homozygous ZFN plants, which expressed the ZFN gene. Numerous GUS-negative plants were observed among the hybrids with one particular cross displaying approximately 35% GUS-negative plants. Evidence for complete deletion of a 4.3 kb sequence comprising the GUS gene was obtained and sequence confirmed. Co-segregation in F(2) progenies of 'truncated' and 'intact' target sequences with expected reporter gene phenotypes were observed. Since ZFNs can be designed to bind and cleave a wide range of DNA sequences, these results constitute a general strategy for creating targeted gene deletions.

Transgenic hybrid aspen overexpressing the Atwbc19 gene encoding an ATP-binding cassette transporter confers resistance to four aminoglycoside antibiotics

Antibiotic-resistance genes of bacterial origin are invaluable markers for plant genetic engineering. However, these genes are feared to pose possible risk to human health by horizontal gene transfer from transgenic plants to bacteria, potentially resulting in antibiotic-resistant pathogenic bacteria; this is a considerable regulatory concern in some countries. The Atwbc19 gene, encoding an Arabidopsis thaliana ATP-binding cassette transporter, has been reported to confer resistance to kanamycin specifically as an alternative to bacterial antibiotic-resistance genes. In this report, we transformed hybrid aspen (Populus canescens x P. grandidentata) with the Atwbc19 gene. Unlike Atwbc19-transgenic tobacco that was only resistant to kanamycin, the transgenic Populus plants also showed resistance to three other aminoglycoside antibiotics (neomycin, geneticin, and paromomycin) at comparable levels to plants containing a CaMV35S-nptII cassette. Although it is unknown why the transgenic Populus with the Atwbc19 gene is resistant to all aminoglycoside antibiotics tested, the broad utility of the Atwbc19 gene as a reporter gene is confirmed here in a second dicot species. Because the Atwbc19 gene is plant-ubiquitous, it might serve as an alternative selectable marker to current bacterial antibiotic-resistance marker genes and alleviate the potential risk for horizontal transfer of bacterial-resistance genes in transgenic plants.

Cre-mediated seed-specific transgene excision in tobacco

Here we report the production of marker-free transgenic plants expressing phenolic compounds with high pharmacological value. Our strategy consisted in simultaneous delivery of lox-target and cre-containing constructs into the plant genome by cotransformation. In the Cre-vector, the cre recombinase gene was controlled by a seed-specific napin promoter. In the lox-target construct the selectable bar gene was placed between two lox sites in direct orientation, while a napin promoter driven vstI gene was inserted outside of the lox sites. Upon seed-specific cre induction the bar expression cassette was excised from the tobacco genome. Genetic and molecular analysis of T1 progeny plants indicated DNA excision in all 10 transgenic lines tested. RP-HPLC analysis demonstrated that the expression of the vstI gene resulted in accumulation of trans-resveratrol and its glycosylated derivative piceid in seeds of all marker free lines. These findings indicate that the seed-specific marker gene excision did not interfere with the expression of the gene of interest. Our data demonstrated the feasi of a developmentally controlled cre gene to mediate site-specific excision in tobacco very efficiently.

A novel mannose-based selection system for plant transformation using celery mannose-6-phosphate reductase gene

To investigate its potential application as a selectable marker for plant transformation, the mannitol producing, celery mannose-6-phosphate reductase gene (M6PR) was transformed into Arabidopsis and tobacco using Agrobacterium tumefaciens-mediated transformation. Mannose-tolerance assays in transgenic materials revealed that the M6PR can act as a selectable marker gene in either a positive or a negative selection mode depending on the plant species. For mannose sensitive species, such as Arabidopsis, expression of M6PR enhanced mannose tolerance and provided a positive selection for transgenic seeds. On medium containing 2 g/L mannose, transgenic seeds germinated, whereas wild type (WT) seeds did not. For mannose-tolerant species, expression of M6PR increased mannose sensitivity in tobacco and enabled a negative selection for transgenic leaves and seeds. Mannose at 30 g/L blanched leaf explants from all 29 transgenic tobacco events with M6PR. In contrast, 30 g/L mannose did not inhibit shoot regeneration from leaf explants of WT or transgenic plants with either an antisense M6PR or a plasmid control. Similarly, mannose at 30 g/L inhibited seed germination of transgenic tobacco seeds with M6PR but not that of WT or transgenic tobacco with either the antisense M6PR or the plasmid control. Northern blot confirmed transcripts of the M6PR in transgenic tobacco, and accumulation of mannitol verified activity of the M6PR in tobacco leaves. Either positive or negative selection using the celery M6PR is versatile for plant transformation. Additionally, the celery M6PR is a potential target gene for improving salt-tolerance in plants due to mannitol accumulation.

Genetically modified plants for non-food or non-feed purposes: straightforward screening for their appearance in food and feed

Genetically modified (GM) plants aimed at producing food/feed are part of regular agriculture in many areas of the World. Commodity plants have also found application as bioreactors, designated non-food/non-feed GM (NFGM) plants, thereby making raw material for further refinement to industrial, diagnostic or pharmaceutical preparations. Many among them may pose health challenge to consumers or livestock animals, if occurring in food/feed. NFGM plants are typically released into the environment, but are grown under special oversight and any among several containment practices, none of which provide full protection against accidental dispersal. Adventitious admixture with food or feed can occur either through distributional mismanagement or as a consequence of gene flow to plant relatives. To facilitate NFGM surveillance we propose a new mandatory tagging of essentially all such plants, prior to cultivation or marketing in the European Union. The suggested tag--Plant-Made Industrial or Pharmaceutical Products Tag (PMIP-T)--is envisaged to occur as a transgenic silent DNA identifier in host plants and designed to enable technically simple identification and characterisation of any NFGM. Implementation of PMIP-T would permit inexpensive, reliable and high-throughput screening for NFGM specifically. The paper outlines key NFGM prospects and challenges as well as the PMIP-T concept.

Genetic load and transgenic mitigating genes in transgenic Brassica rapa (field mustard) x Brassica napus (oilseed rape) hybrid populations

Background

One theoretical explanation for the relatively poor performance of Brassica rapa (weed) × Brassica napus (crop) transgenic hybrids suggests that hybridization imparts a negative genetic load. Consequently, in hybrids genetic load could overshadow any benefits of fitness enhancing transgenes and become the limiting factor in transgenic hybrid persistence. Two types of genetic load were analyzed in this study: random/linkage-derived genetic load, and directly incorporated genetic load using a transgenic mitigation (TM) strategy. In order to measure the effects of random genetic load, hybrid productivity (seed yield and biomass) was correlated with crop- and weed-specific AFLP genomic markers. This portion of the study was designed to answer whether or not weed × transgenic crop hybrids possessing more crop genes were less competitive than hybrids containing fewer crop genes. The effects of directly incorporated genetic load (TM) were analyzed through transgene persistence data. TM strategies are proposed to decrease transgene persistence if gene flow and subsequent transgene introgression to a wild host were to occur.

Results

In the absence of interspecific competition, transgenic weed × crop hybrids benefited from having more crop-specific alleles. There was a positive correlation between performance and number of B. napus crop-specific AFLP markers [seed yield vs. marker number (r = 0.54, P = 0.0003) and vegetative dry biomass vs. marker number (r = 0.44, P = 0.005)]. However under interspecific competition with wheat or more weed-like conditions (i.e. representing a situation where hybrid plants emerge as volunteer weeds in subsequent cropping systems), there was a positive correlation between the number of B. rapa weed-specific AFLP markers and seed yield (r = 0.70, P = 0.0001), although no such correlation was detected for vegetative biomass. When genetic load was directly incorporated into the hybrid genome, by inserting a fitness-mitigating dwarfing gene that that is beneficial for crops but deleterious for weeds (a transgene mitigation measure), there was a dramatic decrease in the number of transgenic hybrid progeny persisting in the population.

Conclusion

The effects of genetic load of crop and in some situations, weed alleles might be beneficial under certain environmental conditions. However, when genetic load was directly incorporated into transgenic events, e.g., using a TM construct, the number of transgenic hybrids and persistence in weedy genomic backgrounds was significantly decreased.

Evaluation of seven promoters to achieve germline directed Cre-lox recombination in Arabidopsis thaliana

Site-specific recombination systems, such as Cre-lox from bacteriophage P1, have become very important tools for plant genome engineering. In many cases a constitutive promoter is used to express the recombinase gene. However, for certain research and commercial applications constitutive Cre-mediated recombination may not be desirable. We have evaluated the potential of seven different germline promoter:cre fusions to remove a stably integrated lox cassette through Cre-mediated recombination in Arabidopsis thaliana. We monitored the functionality of each promoter in the germline of primary transformants by analyzing the presence of the recombined lox cassette in T(2) progeny. The selected germline promoters are involved in different developmental cues, including early stem cell identity (CLAVATA3), flower meristem identity (LEAFY, APETALA1), floral organ identity (AGAMOUS), and meiosis (SOLO DANCERS, DMC1, SWITCH1). For five out of these seven promoters we were able to show that efficient Cre-mediated recombination does, indeed, occur and that the recombination takes place at some point during germline development. Furthermore, a recombination efficiency of 100% is obtained when Cre-expression is regulated by the CLAVATA3 promoter. In addition, with these promoters, we observe much less variation in recombination frequency than previously reported for the 35S promoter. For these reasons, we believe that germline-specific Cre-lox recombination provides an additional tool to the site-specific recombination technology in plants.

Developmentally regulated site-specific marker gene excision in transgenic B. napus plants

We have developed a self-excision Cre-vector to remove marker genes from Brassica napus. In this vector cre recombinase gene and bar expression cassette were inserted between two lox sites in direct orientation. These lox-flanked sequences were placed between the seed-specific napin promoter and the gene of interest (vstI). Tissue-specific cre activation resulted in simultaneous excision of the recombinase and marker genes. The vector was introduced into B. napus by Agrobacterium-mediated transformation. F1 progeny of seven lines with single and multiple transgene insertions was subjected to segregation and molecular analysis. Marker-free plants could be detected and confirmed by PCR and Southern blot in all transgenic lines tested. The recombination efficiency expressed as a ratio of plants with complete gene excision to the total number of investigated plants varied from 13 to 81% dependent on the transgene copy number. Potential application of this system would be the establishment of marker-free transgenic plants in generatively propagated species.

Auto-excision of selectable marker genes from transgenic tobacco via a stress inducible FLP/FRT site-specific recombination system

Antibiotic resistance marker genes are powerful selection tools for use in plant transformation processes. However, once transformation is accomplished, the presence of these resistance genes is no longer necessary and can even be undesirable. We herein describe the successful excision of antibiotic resistance genes from transgenic plants via the use of an oxidative stress-inducible FLP gene. FLP encodes a recombinase that can eliminate FLP and hpt selection genes flanked by two FRT sites. During a transformation procedure in tobacco, transformants were obtained by selection on hygromycin media. Regenerants of the initial transformants were screened for selective marker excision in hydrogen peroxide supplemented media and both the FLP and hpt genes were found to have been eliminated. About 13-41% of regenerated shoots on hydrogen peroxide media were marker-free. This auto-excision system, mediated by the oxidative stress-inducible FLP/FRT system to eliminate a selectable marker gene can be very readily adopted and used to efficiently generate marker-free transgenic plants.

Research progress in techniques for detecting genetically modified organisms

The techniques used to detect genetically modified organisms (GMO), including qualitative polymerase chain reaction (PCR), quantitative PCR, enzyme-linked immunosorbent assay (ELISA) and many others, are systematically described and discussed. The application progress of GMO in species-specific detection, endogenous genes, standard substances and restraining factors influencing detection are reviewed. The ongoing problems and development prospects of detection techniques of GMO are also pointed out.

Intein-mediated protein assembly in transgenic wheat: production of active barnase and acetolactate synthase from split genes

Engineering traits by the assembly of non-functional gene products is a promising tool for modern plant biotechnology. In this article, we describe the establishment of male sterility and herbicide resistance in wheat (Triticum aestivum) by complementing inactive precursor protein fragments through a split intein system. N- and C-terminal fragments of a barnase gene from Bacillus amyloliquifaciens were fused to intein sequences from the Synechocystis sp. gene DnaB and delivered into the wheat genome via biolistic particle bombardment. Both barnase fragments were expressed under the control of a tapetum-specific promoter. High efficiency of the split barnase system was achieved by introducing GGGGS linkers between the fusion domains of the assembled protein. Depending on the vector version that was transformed, up to 51% of primary transformed plants produced sterile pollen. In the F(1) progeny, the male-sterile phenotype segregated with both barnase gene fragments. Expression of the cytotoxic barnase in the tapetum did not apparently affect the vegetative phenotype and remained stable under increased temperatures. In addition, the reconstitution of sulphonylurea resistance was achieved by DnaE intein-mediated assembly of a mutated acetolactate synthase (ALS) protein from rice. The impacts of the technical advances revealed in this study on the concepts for trait control, transgene containment and hybrid breeding are discussed.

Expression of active Streptomyces phage phiC31 integrase in transgenic wheat plants

Site-specific recombination systems are becoming an important tool for the genetic modification of crop plants. Here we report the functional expression of the Streptomyces phage-derived phiC31 recombinase (integrase) in wheat. T-DNA constructs containing a phiC31 integrase transgene were stably transformed into wheat plants via particle gun bombardment. A plant-virus-based assay system was used to monitor the site-specific recombination activity of the recombinant integrase protein in vivo. We established several independent doubled haploid (DH) inbred lines that constitutively express an active integrase enzyme without any apparent detrimental effects on plant growth and development. The potential of phiC31 integrase expression in crop plants related to transgene control technologies or hybrid breeding systems is discussed.

Feasibility of the seed specific cruciferin C promoter in the self excision Cre/loxP strategy focused on generation of marker-free transgenic plants

This work is focused on the generation of selectable marker-free transgenic tobacco plants using the self excision Cre/loxP system that is controlled by a strong seed specific Arabidopsis cruciferin C (CRUC) promoter. It involves Agrobacterium-mediated transformation using a binary vector containing the gus reporter gene and one pair of the loxP sites flanking the cre recombinase and selectable nptII marker genes (floxed DNA). Surprisingly, an ectopic activation of CRUC resulting in partial excision of floxed DNA was observed during regeneration of transformed cells already in calli. The regenerated T(0) plants were chimeric, but no ongoing ectopic expression was observed in these one-year-long invitro maintained plants. The process of the nptII removal was expected in the seeds; however, none of the analysed T(0) transgenic lines generated whole progeny sensitive to kanamycin. Detailed analyses of progeny of selected T(0)-30 line showed that 10.2% GUS positive plants had completely removed nptII gene while the remaining 86.4% were still chimeras. Repeated activation of the cre gene in T(2) seeds resulted in increased rate of marker-free plants, whereas four out of ten analysed chimeric T(1) plants generated completely marker-free progenies. This work points out the feasibility as well as limits of the CRUC promoter in the Cre/loxP strategy.

Plants to power: bioenergy to fuel the future

Bioenergy should play an essential part in reaching targets to replace petroleum-based transportation fuels with a viable alternative, and in reducing long-term carbon dioxide emissions, if environmental and economic sustainability are considered carefully. Here, we review different platforms, crops, and biotechnology-based improvements for sustainable bioenergy. Among the different platforms, there are two obvious advantages to using lignocellulosic biomass for ethanol production: higher net energy gain and lower production costs. However, the use of lignocellulosic ethanol as a viable alternative to petroleum-based transportation fuels largely depends on plant biotechnology breakthroughs. We examine how biotechnology, such as lignin modification, abiotic stress resistance, nutrition usage, in planta expression of cell wall digestion enzymes, biomass production, feedstock establishment, biocontainment of transgenes, metabolic engineering, and basic research, can be used to address the challenges faced by bioenergy crop production.

Excision of selectable marker gene from transgenic tobacco using the GM-gene-deletor system regulated by a heat-inducible promoter

To excise a selectable marker gene from transgenic plants, a new binary expression vector based on the 'genetically modified (GM)-gene-deletor' system was constructed. In this vector, the gene coding for FLP site-specific recombinase under the control of a heat shock-inducible promoter HSP18.2 from Arabidopsis thaliana and isopentenyltransferase gene (ipt), as a selectable marker gene under the control of the cauliflower mosaic virus 35S (CaMV 35S) promoter, were flanked by two loxP/FRT fusion sequences as recombination sites in direct orientation. Histochemical staining for GUS activity showed that, upon induction by heat shock, all exogenous DNA, including the selectable marker gene ipt, beta-glucuronidase (gusA) gene and the FLP recombinase gene, between two loxP/FRT sites was eliminated efficiently from primary transgenic tobacco plants. Molecular analysis further confirmed that excision of the marker gene (ipt) was heritable and stable. Our approach provides a reliable strategy for auto-excising a selectable marker gene from calli, shoots or other tissues of transgenic plants after transformation and producing marker-free transgenic plants.

Creation and analysis of a novel chimeric promoter for the complete containment of pollen- and seed-mediated gene flow

Effective containment of gene flow in transgenic plants requires a promoter that is highly specific for male and female gametes or tissues. Here, we report the creation of a novel pollen-, stigma- and carpel-specific (PSC) promoter through the fusion of the pollen-specific LAT52 and carpel-specific AGL5 enhancers to a stigma-specific SLG promoter. Gene expression analysis showed that fusion of the LAT52 enhancer to the SLG promoter enables the latter to gain pollen-specific activity while the acquirement of carpel-specific activity requires the correct orientation of the inserted AGL5 enhancer in the PSC promoter, and only a forward- but not a reverse-oriented one is functional. The resulting fPSC promoter, when fused to DT-A, generated at least three aberrant gynoecium phenotypes. Type I plants exhibited shortened stigmatic tissues, resembling plants containing the DT-A gene controlled by the SLG promoter. However, type II and III plants displayed partial or complete ablation of gynoecia, and were unable to support the reproductive process. Type II and III plants also produced severely perturbed anthers and pollen in comparison to type I or SLG::DT-A plants, and transgenic pollen grains were unable, when out-crossed with control plants, to pass the transgene to the next generation in all plants examined, indicating that they are selectively eliminated. This tissue-specific ablation or perturbation is highly specific, and does not compromise vegetative growth. Evidently, the fPSC promoter faithfully acquires tissue specificity from the incorporated enhancers and promoter, and should have a practical application for transgene containment in non-fruit and -grain producing plant crops.

The second intron of AGAMOUS drives carpel- and stamen-specific expression sufficient to induce complete sterility in Arabidopsis

Gene containment technologies that prevent transgene dispersal through pollen, fruit and seed are in immediate demand to address concerns of gene flow from transgenic crops into wild species or close relatives. In this study, we isolated the enhancer element of Arabidopsis AGAMOUS that drives gene expression specifically in stamens and carpels. By fusing this AG enhancer to a minimal 35S promoter fragment, two tissue-specific promoters, fAGIP and rAGIP in forward and reverse orientations, respectively, were created and fused to the GUS reporter. Transgenic Arabidopsis plants harboring either fAGIP::GUS or rAGIP::GUS displayed similar GUS expression specifically in carpel and stamen tissues and their primordial cells. To test their utility for engineering sterility, the promoters were fused to the Diphtheria toxin A (DT-A) gene coding for a ribosome inactivating protein as well as the Barnase gene coding for an extracellular ribonuclease, and tested for tissue-specific ablation. Over 89% of AGIP::DT-A and 68% of AGIP::Barnase transgenic plants displayed specific and precise ablation of stamens and carpels and are completely sterile. These transgenic plants showed normal vegetative development with prolonged vegetative growth. To evaluate the stability of the sterile phenotype, 16 AGIP::DT-A lines underwent two consecutive cutback generations and showed no reversion of the floral phenotype. This study demonstrates a simple, precise and efficient approach to achieve absolute sterility through irreversible ablation of both male and female floral organs. This approach should have a practical application for transgene containment in ornamental, landscaping, and woody species, whose seeds and fruits are of no economic value.

Pathogen Phytosensing: Plants to Report Plant Pathogens

Real-time systems that provide evidence of pathogen contamination in crops can be an important new line of early defense in agricultural centers. Plants possess defense mechanisms to protect against pathogen attack. Inducible plant defense is controlled by signal transduction pathways, inducible promoters and cis-regulatory elements corresponding to key genes involved in defense, and pathogen-specific responses. Identified inducible promoters and cis-acting elements could be utilized in plant sentinels, or ‘phytosensors’, by fusing these to reporter genes to produce plants with altered phenotypes in response to the presence of pathogens. Here, we have employed cis-acting elements from promoter regions of pathogen inducible genes as well as those responsive to the plant defense signal molecules salicylic acid, jasmonic acid, and ethylene. Synthetic promoters were constructed by combining various regulatory elements supplemented with the enhancer elements from the Cauliflower mosaic virus (CaMV) 35S promoter to increase basal level of the GUS expression. The inducibility of each synthetic promoter was first assessed in transient expression assays using Arabidopsis thaliana protoplasts and then examined for efficacy in stably transgenic Arabidopsis and tobacco plants. Histochemical and fluorometric GUS expression analyses showed that both transgenic Arabidopsis and tobacco plants responded to elicitor and phytohormone treatments with increased GUS expression when compared to untreated plants. Pathogen-inducible phytosensor studies were initiated by analyzing the sensitivity of the synthetic promoters against virus infection. Transgenic tobacco plants infected with Alfalfa mosaic virus showed an increase in GUS expression when compared to mock-inoculated control plants, whereas Tobacco mosaic virus infection caused no changes in GUS expression. Further research, using these transgenic plants against a range of different pathogens with the regulation of detectable reporter gene could provide biological evidence to define the functional differences between pathogens, and provide new technology and applications for transgenic plants as phytosensors.

Safety assessment of food products from r-DNA animals

Recombinant-DNA (transgenic) animals intended for food production are approaching the market. Among them, recombinant-DNA fishes constitute the most advanced case. As a result, intergovernmental organizations are working on guidelines which would eventually become international standards for national food safety assessments of these products. This article reviews the emerging elements for the food safety assessment of products derived from recombinant-DNA animals. These elements will become highly relevant both for researchers and regulators interested in developing or analyzing recombinant-DNA animals intended to be used in the commercial elaboration of food products. It also provides references to science-based tools that can be used to support food safety assessments. Finally, it proposes recommendations for the further development of biosafety assessment methodologies in this area.

FLP recombinase-mediated site-specific recombination in rice

The feasibility of using the FLP/FRT site-specific recombination system in rice for genome engineering was evaluated. Transgenic rice plants expressing the FLP recombinase were crossed with plants harbouring the kanamycin resistance gene (neomycin phosphotransferase II, nptII) flanked by FRT sites, which also served to separate the corn ubiquitin promoter from a promoterless gusA. Hybrid progeny were tested for excision of the nptII gene and the positioning of the ubiquitin promoter proximal to gusA. While the hybrid progeny from various crosses exhibited beta-glucuronidase (GUS) expression, the progeny of selfed parental rice plants did not show detectable GUS activity. Despite the variable GUS expression and incomplete recombination displayed in hybrids from some crosses, uniform GUS staining and complete recombination were observed in hybrids from other crosses. The recombined locus was shown to be stably inherited by the progeny. These data demonstrate the operation of FLP recombinase in catalysing excisional DNA recombination in rice, and confirm that the FLP/FRT recombination system functions effectively in the cereal crop rice. Transgenic rice lines expressing active FLP recombinase generated in this study provide foundational stock material, thus facilitating the future application and development of the FLP/FRT system in rice genetic improvement.

Generation of single-copy T-DNA transformants in Arabidopsis by the CRE/loxP recombination-mediated resolution system

We investigated whether complex T-DNA loci, often resulting in low transgene expression, can be resolved efficiently into single copies by CRE/loxP-mediated recombination. An SB-loxP T-DNA, containing two invertedly oriented loxP sequences located inside and immediately adjacent to the T-DNA border ends, was constructed. Regardless of the orientation and number of SB-loxP-derived T-DNAs integrated at one locus, recombination between the outermost loxP sequences in direct orientation should resolve multiple copies into a single T-DNA copy. Seven transformants with a complex SB-loxP locus were crossed with a CRE-expressing plant. In three hybrids, the complex T-DNA locus was reduced efficiently to a single-copy locus. Upon segregation of the CRE recombinase gene, only the simplified T-DNA locus was found in the progeny, demonstrating DNA had been excised efficiently in the progenitor cells of the gametes. In the two transformants with an inverted T-DNA repeat, the T-DNA resolution was accompanied by at least a 10-fold enhanced transgene expression. Therefore, the resolution of complex loci to a single-copy T-DNA insert by the CRE/loxP recombination system can become a valuable method for the production of elite transgenic Arabidopsis thaliana plants that are less prone to gene silencing.

Marker-free transgenic plants through genetically programmed auto-excision

We present here a vector system to obtain homozygous marker-free transgenic plants without the need of extra handling and within the same time frame as compared to transformation methods in which the marker is not removed. By introducing a germline-specific auto-excision vector containing a cre recombinase gene under the control of a germline-specific promoter, transgenic plants become genetically programmed to lose the marker when its presence is no longer required (i.e. after the initial selection of primary transformants). Using promoters with different germline functionality, two modules of this genetic program were developed. In the first module, the promoter, placed upstream of the cre gene, confers CRE functionality in both the male and the female germline or in the common germline (e.g. floral meristem cells). In the second module, a promoter conferring single germline-specific CRE functionality was introduced upstream of the cre gene. Promoter sequences used in this work are derived from the APETALA1 and SOLO DANCERS genes from Arabidopsis (Arabidopsis thaliana) Columbia-0 conferring common germline and single germline functionality, respectively. Introduction of the genetic program did not reduce transformation efficiency. Marker-free homozygous progeny plants were efficiently obtained, regardless of which promoter was used. In addition, simplification of complex transgene loci was observed.

Molecular genetic improvement of cereals: transgenic wheat (Triticum aestivum L.)

Only modest progress has been made in the molecular genetic improvement of wheat following the production of the first transgenic plants in 1992, made possible by the development of efficient, long-term regenerable embryogenic cultures derived from immature embryos and use of the biolistics method for the direct delivery of DNA into regenerable cells. Transgenic lines expressing genes that confer resistance to environmentally friendly non-selective herbicides, and pests and pathogens have been produced, in addition to lines with improved bread-making and nutritional qualities; some of these are ready for commercial production. Reduction of losses caused by weeds, pests and pathogens in such plants not only indirectly increases available arable land and fresh water supplies, but also conserves energy and natural resources. Nevertheless, the work carried out thus far can be considered only the beginning, as many difficult tasks lie ahead and much remains to be done. The challenge now is to produce higher-yielding varieties that are more nutritious, and are resistant or tolerant to a wide variety of biotic as well as abiotic stresses (especially drought, salinity, heavy metal toxicity) that currently cause substantial losses in productivity. How well we will meet this challenge for wheat, and indeed for other cereal and non-cereal crops, will depend largely on establishing collaborative partnerships between breeders, molecular biologists, biotechnologists and industry, and on how effectively they make use of the knowledge and insights gained from basic studies in plant biology and genetics, the sequencing of plant/cereal genomes, the discovery of synteny in cereals, and the availability of DNA-based markers and increasingly detailed chromosomal maps.

GM maize from site-specific recombination technology, what next?

The term plant genetic engineering has long conveyed a highly efficient and precise process for the manipulation of plant genomes. For nearly two decades, research on recombinase-based applications has steadily advanced the surgical capabilities of plant genome rearrangements. Once considered interesting laboratory exercises, a first crop plant derived from this type of DNA acrobatics is heading to market. Originally configured for a specific application, to remove a selectable marker, it could be the first of more to come - and not just market-free plants.