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

Level of tissue differentiation influences the activation of a heat-inducible flower-specific system for genetic containment in poplar (Populus tremula L.)

Differentiation level but not transgene copy number influenced activation of a gene containment system in poplar. Heat treatments promoted CRE gene body methylation. The flower-specific transgene deletion was confirmed. Gene flow between genetic modified trees and their wild relatives is still motive of concern. Therefore, approaches for gene containment are required. In this study, we designed a novel strategy for achieving an inducible and flower-specific transgene removal from poplar trees but still expressing the transgene in the plant body. Hence, pollen carrying transgenes could be used for breeding purposes under controlled conditions in a first phase, and in the second phase genetic modified poplars developing transgene-free pollen grains could be released. This approach is based on the recombination systems CRE/loxP and FLP/frt. Both gene constructs contained a heat-inducible CRE/loxP-based spacer sequence for in vivo assembling of the flower-specific FLP/frt system. This allowed inducible activation of gene containment. The FLP/frt system was under the regulation of a flower-specific promoter, either CGPDHC or PTD. Our results confirmed complete CRE/loxP-based in vivo assembling of the flower-specific transgene excision system after heat treatment in all cells for up to 30 % of regenerants derived from undifferentiated tissue cultures. Degradation of HSP::CRE/loxP spacer after recombination but also persistence as extrachromosomal DNA circles were detected in sub-lines obtained after heat treatments. Furthermore, heat treatment promoted methylation of the CRE gene body. A lower methylation level was detected at CpG sites in transgenic sub-lines showing complete CRE/loxP recombination and persistence of CRE/loxP spacer, compared to sub-lines with incomplete recombination. However, our results suggest that low methylation might be necessary but not sufficient for recombination. The flower-specific FLP/frt-based transgene deletion was confirmed in 6.3 % of flowers.

In vitro regeneration and Agrobacterium tumefaciens-mediated genetic transformation in asakura-sanshoo (Zanthoxylum piperitum (L.) DC. F. inerme Makino) an important medicinal plant

Context:

Asakura-sanshoo (Zanthoxylum piperitum [L.] DC. f. inerme Makino) is an important medicinal plant in East Asia. Transgenic technique could be applied to improve plant traits and analyze gene function. However, there is no report on regeneration and genetic transformation in Asakura-sanshoo.

Aims:

To establish a regeneration and Agrobacterium tumefaciens-mediated genetic transformation system in Asakura-sanshoo, which could be used for cultivar improvement and gene function analysis.

Settings and Design:

The various combinations of indole-3-butyric acid (IBA), 6-benzylaminopurine (BA) and naphthalene acetic acid (NAA) were explored for the optimal plant regeneration from petiole and stem of Asakura-sanshoo. The half-strength woody plant medium (WPM) with different concentrations of NAA and IBA was used to induce root. For genetic transformation, A. tumefaciens strain EHA-105 harboring the plasmid pBin-Ex-H-ipt which carries the isopentenyl transferase (ipt) gene, β-glucuronidase (GUS) gene and kanamycin resistance gene neomycin phosphotransferase II (NPTII) were used. The transformation efficiency was detected by the kanamycin resistant frequency.

Materials and Methods:

Petioles and stems were obtained from the in vitro cultured Asakura-sanshoo. The petiole and stem segments were precultured for 3 days, and then inflected using the bacterium at the concentration of OD₆₀₀ 0.5–0.8 for 10 min, followed by 3 days co-cultivation. Selection of the transgenic plants was carried out after 7 days the regeneration using gradient kanamycin at 30 mg/L and 50 mg/L, respectively. Successful transformed plants were confirmed by GUS histochemical assays, polymerase chain reaction (PCR), reverse transcription-PCR (RT-PCR), and Southern blotting analysis.

Results:

The highest shoots regeneration was obtained on WPM supplement with 0.5 mg/L BA and 0.2 mg/L NAA. The optimal rooting medium was half strength macro-element WPM. The kanamycin resistant frequency of petiole and stem was 24.66% and 25.93%, respectively. Thirty-five shoots in thousands adventitious buds were confirmed through GUS histochemical assays, PCR, RT-PCR, and Southern blotting. The regeneration shoot per explants elevated 5.85 fold compared with the wild-type plants.

Conclusions:

Individual transgenic Asakura-sanshoo lines were obtained. In this paper, it first revealed the expression of ipt gene significantly promoted the adventitious buds induction in Asakura-sanshoo as the same action as in other plants.

Efficient auto-excision of a selectable marker gene from transgenic citrus by combining the Cre/loxP system and ipt selection

KEY MESSAGE

A highly efficient Cre-mediated deletion system, offering a good alternative for producing marker-free transgenic plants that will relieve public concerns regarding GMOs, was first developed in citrus. The presence of marker genes in genetically modified crops raises public concerns regarding their safety. The removal of marker genes can prevent the risk of their flow into the environment and hasten the public's acceptance of transgenic products. In this study, a new construct based on the Cre/loxP site-recombination system was designed to delete marker genes from transgenic citrus. In the construct, the selectable marker gene isopentenyltransferase gene (ipt) from Agrobacterium tumefaciens and the Cre recombinase gene were flanked by two loxP recognition sites in the direct orientation. The green fluorescent protein (gfp) reporter gene for monitoring the transformation of foreign genes was located outside of the loxP sequences. Transformation and deletion efficiencies of the vector were investigated using nopaline synthase gene (NosP) and CaMV 35S promoters to drive expression of Cre. Analysis of GFP activity showed that 28.1 and 13.6 % transformation efficiencies could be obtained by NosP- and CaMV 35S-driven deletions, respectively. Molecular analysis demonstrated that 100 % deletion efficiency was observed in the transgenic plants. The complete excision of the marker gene was found in all deletion events driven by NosP and in 81.8 % of deletion events driven by CaMV 35S. The results showed that Cre/loxP-mediated excision was highly efficient and precise in citrus. This approach provides a reliable strategy for auto-deletion of selectable marker genes from transgenic citrus to produce marker-free transgenic plants.

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.

Approaches for gene targeting and targeted gene expression in plants

Transgenic science and technology are fundamental to state-of-the-art plant molecular genetics and crop improvement. The new generation of technology endeavors to introduce genes 'stably' into 'site-specific' locations and in 'single copy' without the integration of extraneous vector 'backbone' sequences or selectable markers and with a 'predictable and consistent' expression. Several similar strategies and technologies, which can push the development of 'smart' genetically modified plants with desirable attributes, as well as enhance their consumer acceptability, are discussed in this review.

Heat-inducible RNAi for gene functional analysis in plants

Controlling gene expression during plant development is an efficient method to explore gene function and RNA interference (RNAi) is now considered as a powerful technology for gene functional analysis. However, constitutive gene silencing cannot be used with genes involved in fundamental processes such as embryo viability or plant growth and alternative silencing strategies avoiding these limitations should be preferred. Tissue-specific and inducible promoters, able to control gene expression at spatial and/or temporal level, can be used to circumvent viability problems. In this chapter, after a rapid overview of the inducible promoters currently used for transgenic approaches in plants, we describe a method we have developed to study gene function by heat-inducible RNAi. This system is easy to use and complementary to those based on chemical gene inducer treatments and might be useful for both research and biotechnological applications.

FLP/FRT recombination from yeast: application of a two gene cassette scheme as an inducible system in plants

Phytosensors are plants that are genetically engineered for sensing and reporting the presence of a specific contaminant, including agriculturally important biological agents. Phytosensors are constructed by transforming plants to contain specific biotic- or abiotic-inducible promoters fused to a reporter gene. When such transgenic plants encounter the target biotic or abiotic agent, the specific inducible promoter is triggered and subsequently drives the expression of the reporter gene, which produces a signal for detection. However, several systems lack robustness, rapid induction and promoter strength. Here, we tested the FLP/FRT recombination system in a construct containing a two gene cassette organization and examined its potential in transgenic Arabidopsis and tobacco plants using a β-glucuronidase (GUS) reporter. In this model system, a heat-shock inducible promoter was employed to control the expression of the FLP recombinase gene. Upon heat induction and subsequent active FLP-mediated excision event, the GUS gene was placed in close proximity to the 35S promoter resulting in an active GUS reporter expression. Our results demonstrate that the two gene cassette scheme of inducible FLP/FRT recombination system is functional in tobacco and Arabidopsis, providing additional insights into its possible application in phytosensing such as creating strong readout capabilities.