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

Split-Cre mediated deletion of DNA no longer needed after site-specific integration in rice

N-cre and C-cre added in separate lines reassemble functional Cre in F1 progeny to excise unnecessary DNA, including cre DNA, thereby eliminating generations needed to cross in and out cre. Crop improvement via transgenesis can benefit through efficient DNA integration strategies. As new traits are developed, new transgenes can be stacked by in planta site-specific integration near previous transgenes, thereby facilitating their introgression to field cultivars as a single segregation locus. However, as each round of integration often requires use of selectable markers, it is more convenient to reuse the selection scheme. The Cre recombinase can be used to delete away previously used selection genes, and other DNA no longer needed after transformation, but the constitutive production of this DNA scanning protein can also affect plant growth. We had previously described in Arabidopsis a split Cre protein fragment complement scheme to reassemble a functional Cre recombinase. As our goal for developing this system was to deploy its use in major crop plants, here we show that Cre protein fragment complementation works in rice with precise recombination structures confirmed by DNA sequencing. As each N-terminal and C-terminal fragment is also flanked by lox recombination sites, they can also self-excise to avoid the need to segregate away the cre DNA. Options to form F1 hybrids homozygous for one transgene, or hemizygous for two different transgenes at the same chromosome location, are discussed.

Clean gene technology to develop selectable marker-free pod borer-resistant transgenic pigeon pea events involving the constitutive expression of Cry1Ac

The most crucial yield constraint of pigeon pea is susceptibility to the pod borer Helicoverpa armigera, which causes extensive damage and severe economic losses every year. The Agrobacterium-mediated plumular meristem transformation technique was applied for the development of cry1Ac transgenic pigeon pea. Bioactivity of the cry1Ac gene was compared based on integration and expression driven by two promoters, the constitutive CaMV35S promoter and the green-tissue-specific ats1A promoter, in those transgenic events. The transgenic events also contained the selectable marker gene nptII flanked by loxP sites. Independent transgenic events expressing the Cre recombinase gene along with a linked bar selection marker were also developed. Integration and expression patterns of both cry1Ac and cre were confirmed through Southern and western blot analysis of T₁ events. The constitutive expression of the Cry1Ac protein was found to be more effective for conferring resistant activity against H. armigera larvae in comparison to green-tissue-specific expression. Constitutively expressing Cry1Ac T₁ events were crossed with Cre recombinase expressing T₁ events. The crossing-based Cre/lox-mediated marker gene elimination strategy was demonstrated to generate nptII-free Cry1Ac-expressing T₂ events. These events were subsequently analyzed in the T₃ generation for the segregation of cre and bar genes. Five Cry1Ac-expressing T₃ transgenic pigeon pea events were devoid of the nptII marker as well as cre-bar genes. H. armigera larval mortality in those marker-free T₃ events was found to be 80-100%. The development of such nptII selectable marker-free Cry1Ac-expressing pigeon pea transgenics for the first time would greatly support the sustainable biotechnological breeding program for pod borer resistance in pigeon pea. KEY POINTS: • Constitutive expression of Cry1Ac conferred complete resistance against Helicoverpa armigera • Green-tissue-specific expression of Cry1Ac conferred partial pest resistance • Cre/lox-mediated nptII elimination was successful in constitutively expressing Cry1Ac transgenic pigeon pea events.

Development of marker-free transgenic pigeon pea (Cajanus cajan) expressing a pod borer insecticidal protein

Pigeon pea, a grain legume of the semiarid tropics, is a rich source of high-quality protein. The productivity of this pulse is seriously affected by lepidopteron insect pests. To generate a sustainable insect-resistant plant, synthetically prepared bioactive key constituents of a crystal protein (Syn Cry1Ab) of Bacillus thuringiensis were expressed in pigeon pea under the guidance of a tissue-specific promoter of the RuBP carboxylase/oxygenase small subunit (rbcS) gene. Regenerated transgenic plants with the cry1Ab expression cassette (cry1Ab-lox-bar-lox) showed the optimum insect motility rate (90%) in an in vitro insect bioassay with second instar larvae, signifying the insecticidal potency of Syn Cry1Ab. In parallel, another plant line was also generated with a chimaeric vector harbouring a cre recombinase gene under the control of the CaMV 2 × 35S promoter. Crossing between T₁ plants with a single insertion of cry1Ab-lox-bar-lox T-DNA and T₁ plants with moderate expression of a cre gene with a linked hygromycin resistance (hptII) gene was performed to exclude the bialaphos resistance (bar) marker gene. Excision of the bar gene was achieved in T₁F₁ hybrids, with up to 35.71% recombination frequency. Insect-resistant pigeon pea plants devoid of selectable marker genes (syn Cry1Ab- bar and cre-hptII) were established in a consecutive generation (T₁F₂) through genetic segregation.

Current achievements and future prospects of genetic engineering in Indian mustard (Brassica juncea L. Czern & Coss.)

Transgenic technology in Indian mustard has expedited crop improvement programs. Further, there is a need to optimize gene editing protocols and find out the suitable target genes to harvest the benefits of gene editing technology in this important edible oilseed crop. Brassica juncea is an economically and industrially important oilseed crop being grown mainly in India and in some parts of Canada, Russia, China and Australia. Besides being consumed as edible oil, it also has numerous applications in food and paint industry. However, its overall production and productivity are being hampered by a number of biotic and abiotic stress factors. Further, its oil and seedmeal quality needs to be improved for increasing food as well as feed value. However, the lack of resistant crossable germplasm or varieties necessitated the use of genetic engineering interventions in Indian mustard crop improvement. A number of genes conferring resistance to biotic stresses including lectins for aphids' control, chitinase, glucanase and osmotin for disease control and for abiotic stresses, CODA, LEA and ion antiporter genes have been transferred to Indian mustard. Both antisense and RNAi technologies have been employed for improving oil and seedmeal quality. Efforts have been made to improve the phytoremediation potential of this crop through genetic engineering approach. The deployment of barnase/barstar gene system for developing male sterile and restorer lines has really expedited hybrid development programs in Indian mustard. Further, there is a need to optimize gene editing protocols and to find out suitable target genes for gene editing in this crop. In this review paper, authors have attempted to review various genetic transformation efforts carried out in Indian mustard for its improvement to combat biotic and abiotic stress challenges, quality improvement and hybrid development.

Marker-free transgenic rice expressing the vegetative insecticidal protein (Vip) of Bacillus thuringiensis shows broad insecticidal properties

Genetically engineered rice lines with broad insecticidal properties against major lepidopteran pests were generated using a synthetic, truncated form of vegetative insecticidal protein (Syn vip3BR) from Bacillus thuringiensis. The selectable marker gene and the redundant transgene(s) were eliminated through Cre/ lox mediated recombination and genetic segregation to make consumer friendly Bt -rice. For sustainable resistance against lepidopteran insect pests, chloroplast targeted synthetic version of bioactive core component of a vegetative insecticidal protein (Syn vip3BR) of Bacillus thuringiensis was expressed in rice under the control of green-tissue specific ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit gene promoter. The transgenic plants (in Oryza sativa indica Swarna cultivar) showed high insect mortality rate in vitro against major rice pests, yellow stem borer (Scirpophaga incertulas), rice leaf folder (Cnaphalocrocis medinalis) and rice horn caterpillar (Melanitis leda ismene) in T1 generation, indicating insecticidal potency of Syn vip3BR. Under field conditions, the T1 plants showed considerable resistance against leaf folders and stem borers. The expression cassette (vip-lox-hpt-lox) as well as another vector with chimeric cre recombinase gene under constitutive rice ubiquitin1 gene promoter was designed for the elimination of selectable marker hygromycin phosphotransferase (hptII) gene. Crossing experiments were performed between T1 plants with single insertion site of vip-lox-hpt-lox T-DNA and one T1 plant with moderate expression of cre recombinase with linked bialaphos resistance (syn bar) gene. Marker gene excision was achieved in hybrids with up to 41.18 % recombination efficiency. Insect resistant transgenic lines, devoid of selectable marker and redundant transgene(s) (hptII + cre-syn bar), were established in subsequent generation through genetic segregation.

Development of Marker-Free Transgenic Potato Tubers Enriched in Caffeoylquinic Acids and Flavonols

Potato (Solanum tuberosum L.) is a major crop worldwide that meets human economic and nutritional requirements. Potato has several advantages over other crops: easy to cultivate and store, cheap to consume, and rich in a variety of secondary metabolites. In this study, we generated three marker-free transgenic potato lines that expressed the Arabidopsis thaliana flavonol-specific transcriptional activator AtMYB12 driven by the tuber-specific promoter Patatin. Marker-free potato tubers displayed increased amounts of caffeoylquinic acids (CQAs) (3.35-fold increases on average) and flavonols (4.50-fold increase on average). Concentrations of these metabolites were associated with the enhanced expression of genes in the CQA and flavonol biosynthesis pathways. Accumulation of CQAs and flavonols resulted in 2-fold higher antioxidant capacity compared to wild-type potatoes. Tubers from these marker-free transgenic potatoes have therefore improved antioxidant properties.

Development of selectable marker-free transgenic potato plants expressing cry3A against the Colorado potato beetle (Leptinotarsa decemlineata Say)

BACKGROUND

Elimination of selectable marker genes (SMGs) is important for the safe assessment and commercial use of transgenic plants. The destructive and invasive Colorado potato beetle (CPB) poses a serious threat to potato production. In response to this need, selectable marker-free transgenic potato lines expressing cry3A were developed to control the damage and spread of CPB.

RESULTS

We simultaneously introduced cry3A and npt II genes harboured in different plasmids into the potato genome using the Agrobacterium-mediated cotransformation method. Four selectable marker-free transgenic potato (CT) lines expressing cry3A were developed by self-crossing segregation and molecular analyses, including Southern blot, western blot and enzyme-linked immunosorbent assay (ELISA) assays. CT lines were used in a resistance bioassay against CPB in the laboratory and field. In the laboratory, CT lines exhibited high resistance to CPB, and 100% mortality of first-instar larvae occurred 6 days after infestation. In the field, untransformed plant leaves were almost entirely consumed, with an average of 155 larvae present per plant 25 days after inoculation. However, CT lines showed no damage symptoms, with approximately 2.5 larvae surviving per plant.

CONCLUSION

We successfully eliminated SMGs from the transgenic potato lines expressing cry3A in order to decrease CPB damage, control the spread of this pest eastwards and alleviate the concern regarding the safe assessment of regulatory requirements. © 2015 Society of Chemical Industry.

Gene stacking in plant cell using recombinases for gene integration and nucleases for marker gene deletion

BACKGROUND

Practical approaches for multigene transformation and gene stacking are extremely important for engineering complex traits and adding new traits in transgenic crops. Trait deployment by gene stacking would greatly simplify downstream plant breeding and trait introgression into cultivars. Gene stacking into pre-determined genomic sites depends on mechanisms of targeted DNA integration and recycling of selectable marker genes. Targeted integrations into chromosomal breaks, created by nucleases, require large transformation efforts. Recombinases such as Cre-lox, on the other hand, efficiently drive site-specific integrations in plants. However, the reversibility of Cre-lox recombination, due to the incorporation of two cis-positioned lox sites, presents a major bottleneck in its application in gene stacking. Here, we describe a strategy of resolving this bottleneck through excision of one of the cis-positioned lox, embedded in the marker gene, by nuclease activity.

METHODS

All transgenic lines were developed by particle bombardment of rice callus with plasmid constructs. Standard molecular approach was used for building the constructs. Transgene loci were analyzed by PCR, Southern hybridization, and DNA sequencing.

RESULTS

We developed a highly efficient gene stacking method by utilizing powerful recombinases such as Cre-lox and FLP-FRT, for site-specific gene integrations, and nucleases for marker gene excisions. We generated Cre-mediated site-specific integration locus in rice and showed excision of marker gene by I-SceI at ~20 % efficiency, seamlessly connecting genes in the locus. Next, we showed ZFN could be used for marker excision, and the locus can be targeted again by recombinases. Hence, we extended the power of recombinases to gene stacking application in plants. Finally, we show that heat-inducible I-SceI is also suitable for marker excision, and therefore could serve as an important tool in streamlining this gene stacking platform.

CONCLUSIONS

A practical approach for gene stacking in plant cell was developed that allows targeted gene insertions through rounds of transformation, a method needed for introducing new traits into transgenic lines for their rapid deployment in the field. By using Cre-lox, a powerful site-specific recombination system, this method greatly improves gene stacking efficiency, and through the application of nucleases develops marker-free, seamless stack of genes at pre-determined chromosomal sites.

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.

Aphid-repellent pheromone E-β-farnesene is generated in transgenic Arabidopsis thaliana over-expressing farnesyl diphosphate synthase2

BACKGROUND AND AIMS

Plant-synthesized sesquiterpenes play a pivotal role in chemotactic interactions with insects. Biosynthesis of functionally diverse sesquiterpenes is dependent on the availability of a pool of the precursor farnesyldiphosphate (FDP). In Arabidopsis thaliana, FPS2, encoding cytosolic farnesyldiphosphate synthase, is implicated in the synthesis of cytosolic FDP, but it is not known whether enhanced levels of FDP have a commensurate effect on sesquiterpene-mediated defence responses. This study examined transgenic arabidopsis plants generated to over-express FPS2 in order to determine if any effects could be observed in the response of aphids, Myzus persicae.

METHODS

Transgenic arabidopsis plants were generated to over-express FPS2 to produce FPS2 in either the cytosol or the chloroplasts. Morphochemical analyses of the transgenic plants were carried out to detremine growth responses of roots and shoots, and for GC-MS profiling of sesquiterpenes. Aphid response to hyrdo-distillate extracts and head-space volatiles from transgenic plants was assessed using a bioassay.

KEY RESULTS

Either over-expression of FPS2 in the cytosol or targetting of its translated product to chlorplasts resulted in stimulatory growth responses of transgenic arabidopsis at early and late developmental stages. GC-MS analysis of hydro-distillate extracts from aerial parts of the plants revealed biosynthesis of several novel sesquiterpenes, including E-β-farnesene, an alarm pheromone of aphids. Both entrapped volatiles and hydro-distillate extracts of the transgenic leaves triggered agitation in aphids, which was related to both time and dose of exposure.

CONCLUSIONS

Over-expression of FPS2 in the cytosol and targeting of its translated product to chloroplasts in arabidopsis led to synthesis of several novel sesquiterpenes, including E-β-farnesene, and induced alarm responses in M. persicae. The results suggest a potential for engineering aphid-resistant strains of arabidopsis.