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

Cre-mediated autoexcision of selectable marker genes in soybean, cotton, canola and maize transgenic plants

Efficient selectable marker gene autoexcision in transgenic plants of soybean, cotton, canola, and maize is achieved by effective Cre recombinase expression. Selectable marker genes are often required for efficient generation of transgenic plants in plant transformation but are not desired once the transgenic events are obtained. We have developed Cre/loxP autoexcision systems to remove selectable marker genes in soybean, cotton, canola and maize. We tested a set of vectors with diverse promoters and identified promising promoters to drive cre expression for each of the four crops. We evaluated both the efficiency of generating primary transgenic events with low transgene copy numbers, and the frequency of marker-free progeny in the next generation. The best performing vectors gave no obvious decrease in the transformation frequency in each crop and generated homozygous marker-free progeny in the next generation. We found that effective expression of Cre recombinase for marker gene autoexcision can be species dependent. Among the vectors tested, the best autoexcision frequency (41%) in soybean transformation came from using the soybean RSP1 promoter for cre expression. The cre gene expressed by soybean RSP1 promoter with an Arabidopsis AtpE intron delivered the best autoexcision frequency (69%) in cotton transformation. The cre gene expressed by the embryo-specific eUSP88 promoter from Vicia faba conferred the best marker excision frequency (32%) in canola transformation. Finally, the cre gene expressed by the rice CDC45-1 promoter resulted in 44% autoexcision in maize transformation. The Cre/loxP recombinase system enables the generation of selectable marker-free transgenic plants for commercial product development in four agriculturally important crops and provides further improvement opportunities for more specific and better marker excision efficiency.

Comparison of three Agrobacterium-mediated co-transformation methods for generating marker-free transgenic Brassica napus plants

BACKGROUND

Generation of marker-free transgenic plants is very important to the regulatory permission and commercial release of transgenic crops. Co-transformation methods that enable the removal of selectable marker genes have been extensively used because they are simple and clean. Few comparisons are currently available between different strain/plasmid co-transformation systems, and also data are related to variation in co-transformation frequencies caused by other details of the vector design.

RESULTS

In this study, we constructed three vector systems for the co-transformation of allotetraploid Brassica napus (B. napus) mediated by Agrobacterium tumefaciens and compared these co-transformation methods. We tested a mixed-strain system, in which a single T-DNA is harbored in two plasmids, as well as two "double T-DNA" vector systems, in which two independent T-DNAs are harbored in one plasmid in a tandem orientation or in an inverted orientation. As confirmed by the use of PCR analysis, test strips, and Southern blot, the average co-transformation frequencies from these systems ranged from 24 to 81% in T₀ plants, with the highest frequency of 81% for 1:1 treatment of the mixed-strain system. These vector systems are valuable for generating marker-free transgenic B. napus plants, and marker-free plants were successfully obtained in the T₁ generation from 50 to 77% of T₀ transgenic lines using these systems, with the highest frequency of 77% for "double T-DNA" vector systems of pBID RT Enhanced. We further found that marker-free B. napus plants were more frequently encountered in the progeny of transgenic lines which has only one or two marker gene copies in the T₀ generation. Two types of herbicide resistant transgenic B. napus plants, Bar ⁺ with phosphinothricin resistance and Bar ⁺ EPSPS ⁺ GOX ⁺ with phosphinothricin and glyphosate resistance, were obtained.

CONCLUSION

We were successful in removing selectable marker genes in transgenic B. napus plants using all three co-transformation systems developed in this study. It was proved that if a appropriate mole ratio was designed for the specific length ratio of the twin T-DNAs for the mixed-strain method, high unlinked co-insertion frequency and overall success frequency could be achieved. Our study provides useful information for the construction of efficient co-transformation system for marker-free transgenic crop production and developed transgenic B. napus with various types of herbicide resistance.

Cold inducible promoter driven Cre-lox system proved to be highly efficient for marker gene excision in transgenic barley

A Cre-lox based auto-excision strategy has been adapted for barley, capable of cre and selectable marker gene (SMG) removal. The cold inducible wheat promoter called wcs120 was utilised for driving Cre expression. The binary vector was carrying the transgene (uidA) and a so called 'recombination cassette' flanked by the lox sequences. This part included both the recombinase gene and the SMG (bar) under the control of a constitutive promoter. T₀, T₁ and T₂ transgenic plants were subjected to low temperature (at 4°C, 10°C and 12°C) at different developmental stages to induce recombination. The presence of uidA, cre, and bar genes and recombination footprints were studied by PCR and DNA sequencing, while cre transcription was followed by qRT-PCR. These analyses indicated that, cold treatment of the germinating seeds (4°C for 3days) followed by plant growing at higher temperature (24°C) has been the most efficient (90-100%), and this treatment lead to heritable changes in the genome. Thermal separation of Cre accumulation (at low temperature) from Cre enzyme activity (at higher temperature) could have prevented the premature excision of its own encoding gene, and lead to high expression level thereby increasing recombination frequency.

Novel recombinant binary vectors harbouring Basta (bar) gene as a plant selectable marker for genetic transformation of plants

Genetic transformation is one of the most widely used technique in crop improvement. However, most of the binary vectors used in this technique, especially cloning based, contain antibiotic genes as selection marker that raise serious consumer and environmental concerns; moreover, they could be transferred to non-target hosts with deleterious effects. Therefore, the goal of this study was reconstruction of the widely used pBI121 binary vector by substituting the harmful antibiotic selection marker gene with a less-harmful selection marker, Basta (herbicide resistance gene). The generated vectors were designated as pBI121NB and pBI121CB, in which Basta gene was expressed under the control of Nos or CaMV 35S promoter, respectively. The successful integration of the new inserts into both the vectors was confirmed by PCR, restriction digestion and sequencing. Both these vectors were used in transforming Arabidopsis, Egyptian wheat and barley varieties using LBA4404 and GV3101 Agrobacterium strains. The surfactant Tween-20 resulted in an efficient transformation and the number of Arabidopsis transformants was about 6-9 %. Soaked seeds of wheat and barley were transformed with Agrobacterium to introduce the bacteria to the growing shoot apices. The percentage of transgenic lines was around 16-17 and 14-15 % for wheat and barley, respectively. The quantitative studies presented in this work showed that both LBA4404 and GV3101 strains were suitable for transforming Egyptian wheat and barley.

Selectable Marker Gene Removal and Expression of Transgene by Inducible Promoter Containing FFDD Cis-Acting elements in Transgenic Plants

BACKGROUND

Selectable marker gene (SMG) systems are critical for generation of transgenic crops. Transgenic crop production without using SMG is not economically feasible. However, SMGs are non-essential once an intact transgenic plant has been established. Elimination of SMGs from transgenic crops both increases public acceptance of GM crops and prepares gene stacking possibility for improvement of complex traits. Synthetic inducible promoters provide an efficient and flexible strategy to regulate transgene expression.

OBJECTIVES

This study aimed to construct a transformation vector based on Cre/loxP recombination system to enhance efficiency of SMG-free transgenic plant production followed by post-excision expression of gene of interest in transgenic plants by a pathogen inducible promoter.

MATERIALS AND METHODS

In pG-IPFFDD-creint-gusint construct, cre recombinase and selectable marker gene (nptII) cassettes were placed between the two loxP recognition sites in direct orientation. Seed-specific Napin promoter was used for regulation of Cre expression in transgenic seeds. In the construct, loxP flanked sequence containing nptII and recombinase cassettes, located between a pathogen inducible promoter containing FFDD cis-acting elements and β-glucuronidase coding region. The cunstuct was transformed into Nicotiana tabaccum via Agrobacterium-mediated transformation.

RESULTS

The results showed that both cre and nptII excision occurs in T1 progeny tobacco plants through seed-specific cre expression. The excisions were confirmed by methods activation of the gus gene, germination test on kanamycin-containing medium and molecular analysis. Inducibility of gus expression by FFDD-containing promoter in T1 leaf tissues was confirmed by histochemical Gus staining assay.

CONCLUSIONS

The established system is not only an efficient tool for marker gene elimination but also provides possibility for inducible expression of the transgene.

High-throughput transformation pipeline for a Brazilian japonica rice with bar gene selection

The goal of this work was to establish a transformation pipeline for upland Curinga rice (Oryza sativa L. ssp. japonica) with bar gene selection employing bialaphos and phosphinothricin as selection agents. The following genes of interest: AtNCED3, Lsi1, GLU2, LEW2, PLD-alpha, DA1, TOR, AVP1, and Rubisco were cloned into the binary vector p7i2x-Ubi and were transferred into Agrobacterium strain EHA 105. Embryogenic calli derived from the mature embryos were transformed, and transgenic cells and shoots were selected on the medium supplemented with bialaphos or phosphinothricin (PPT) using a stepwise selection scheme. Molecular analyses were established using polymerase chain reaction and Southern blot for the bar gene and the NOS terminator. Overall, 273 putative transgenic plants were analyzed by Southern blot with 134 events identified. In total, 77 events had a single copy of the transgene integrated in the plant genome while 29 events had two copies. We tested backbone integration in 101 transgenic plants from all constructs and found 60 transgenic plants having no additional sequence integrated in the plant genome. The bar gene activity was evaluated by the chlorophenol red test and the leaf painting test using phosphinothricin with several transgenic plants. The majority of T0 plants carrying the single copy of transgene produced T1 seeds in the screen house.

The pollen- and embryo-specific Arabidopsis DLL promoter bears good potential for application in marker-free Cre/loxP self-excision strategy

KEY MESSAGE

Marker-free transgenic plants can be generated with high efficiency by using the Cre/ lox P self-excision system controlled by the pollen- and embryo-specific Arabidopsis DLL promoter. In this work, we aimed to study the feasibility of using the pollen- and embryo-specific DLL promoter of the At4g16160 gene from Arabidopsis thaliana in a Cre/loxP self-excision strategy. A Cre/loxP self-excision cassette controlled by the DLL promoter was introduced into the tobacco genome via Agrobacterium-mediated transformation. No evidence for premature activation of the Cre/loxP system was observed in primary transformants. The efficiency of nptII removal during pollen and embryo development was investigated in transgenic T1 progenies derived from eight self- and four cross-pollinated T0 lines, respectively. Segregation and rooting assays were performed to select recombined T1 plants. Molecular analyses of these plants confirmed the excision event in all analysed T0 lines and marker-free transgenic T1 plants were obtained with efficiency of up to 96.2%. The Arabidopsis DLL promoter appears to be a strong candidate to drive Cre-mediated recombination not only in tobacco as a model plant, but also in other plant species.

Development of selectable marker free, insect resistant, transgenic mustard (Brassica juncea) plants using Cre/lox mediated recombination

Background

Antibiotic/ herbicide resistant marker genes have been proven to be very useful in plant transformation for the initial selection of desired transgenic events. However, presence of these genes in the genetically modified crops may render the crop less acceptable to the consumers. Among several different approaches, the effectiveness of Cre/lox mediated recombination strategy for selectable marker gene (SMG) elimination has previously been demonstrated by different groups in several plants including Brassica. In the present study exploiting Cre/lox mediated recombination strategy, attempt has been made for selectable marker gene elimination from Allium sativum leaf agglutinin (ASAL) expressing Brassica plants with hemipteran insect resistant phenotype.

Results

Allium sativum leaf agglutinin (ASAL) linked with lox flanked hygromycin resistant (hpt) gene was introduced in mustard. Cre recombinase gene cassette was also integrated in separate event. A Cre/lox mediated recombination using crossing strategy was adopted to remove the hpt gene from the subsequent generation of selected hybrid events. Reciprocal crosses were made between T₁ASAL-lox-hpt-lox and cre-bar plants. Marker gene elimination was confirmed in the resulting F₁ hybrid progenies by PCR analysis, using hpt, cre and ASAL specific primers followed by Southern hybridization. In marker free plants, expression of ASAL was also confirmed by western blotting and ELISA analysis. Retention of functionality of expressed ASAL was investigated by agglutination assay using rabbit erythrocytes. Expressed ASAL was also found to be thermo-sensitive. In planta insect bioassay on F₁ hybrid progenies exhibited detrimental effect on the performance of devastating target pest, Lipaphis erysimi. The F₁ hybrid hpt negative, ASAL positive plants were allowed to self- fertilize to obtain F₂ progeny plants. In some of these plants cre gene was found to be segregated out of the ASAL gene by genetic segregation yielding completely marker free plants.

Conclusions

The present study establishes the efficient expression of the newly introduced insect resistant ASAL gene even after Cre/lox mediated recombination resulting in elimination of selectable marker gene.

Excision of a selectable marker gene in transgenic banana using a Cre/lox system controlled by an embryo specific promoter

Antibiotic and herbicide resistance genes have been used in transgene technology as powerful selection tools. Nonetheless, once transgenic events have been obtained their presence is no longer needed and can even be undesirable. In this work, we have developed a system to excise the selectable marker and the cre recombinase genes from transgenic banana cv. 'Grande Naine' (Musa AAA). To achieve this, the embryo specific REG-2 promoter was isolated from rice and its expression pattern in banana cell clumps, somatic embryos and regenerated plantlets was characterized by using a pREG2::uidA fusion construct. Subsequently, the REG-2 promoter was placed upstream of the cre gene, conferring Cre functionality in somatic embryos and recombination of lox sites resulting in excision of the selectable marker and cre genes. PCR analysis revealed that 41.7 % of the analysed transgenic plants were completely marker free, results that were thereafter confirmed by Southern blot hybridization. These results demonstrate the feasibility of using developmentally controlled promoters to mediate marker excision in banana. This system does not require any extra handling compared to the conventional transformation procedure and might be useful in other species regenerating through somatic embryogenesis.

Tissue-specifically regulated site-specific excision of selectable marker genes in bivalent insecticidal, genetically-modified rice

Marker-free, genetically-modified rice was created by the tissue-specifically regulated Cre/loxP system, in which the Cre recombinase gene and hygromycin phosphotransferase gene (hpt) were flanked by two directly oriented loxP sites. Cre expression was activated by the tissue-specific promoter OsMADS45 in flower or napin in seed, resulting in simultaneous excision of the recombinase and marker genes. Segregation of T1 progeny was performed to select recombined plants. The excision was confirmed by PCR, Southern blot and sequence analyses indicating that efficiency varied from 10 to 53 % for OsMADS45 and from 12 to 36 % for napin. The expression of cry1Ac and vip3A was detected by RT-PCR analysis in marker-free transgenic rice. These results suggested that our tissue-specifically regulated Cre/loxP system could auto-excise marker genes from transgenic rice and alleviate public concerns about the security of GM crops.

A developmentally regulated Cre-lox system to generate marker-free transgenic Brassica napus plants

In this chapter, a strategy for engineering marker-free Brassica napus plants is described. It is based on the Cre-lox site-specific recombination system and includes three essential steps. At first, the binary vector pLH-nap-lx-cre-35S-bar-lx-vst has been designed. In this vector, the cre gene and the bar expression cassette are flanked by two lox sites in direct orientation. The lox-flanked sequence is placed between a seed-specific napin promoter and a coding region for the vstI gene. At the second step, the cre-bar vector was transferred into B. napus hypocotyl explants by Agrobacterium tumefaciens-mediated transformation. Finally, T1 progeny was tested for excision of the marker gene at phenotypic and molecular levels. PCR, sequencing, and Southern blot analysis confirmed complete and precise deletion of the lox-flanked DNA region. This developmentally regulated Cre-lox system can be applied to remove undesirable DNA in transgenic plants propagated by seeds.

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.

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.