Cre/lox-mediated marker gene excision in transgenic maize (Zea mays L.) plants

Development of an inducible excision system of a visual marker Ipomoea batatas Myb gene from the genome of transgenic cells

In the plant genetic transformation process, single selection by a chemical-resistant marker gene occasionally allows the proliferation of non-transgenic cells, escaping selection pressure. The additional use of a visual marker gene is effective for accurate selection. For instance, R2R3-MYB genes are used for regulating anthocyanin biosynthesis; however, constitutive Myb expression in transgenic plants is not always desirable and may cause developmental abnormalities due to excess anthocyanin accumulation. To overcome the remaining problems in the use of Myb as a visible marker, we developed T-DNA. Ipomoea batatas Myb (IbMyb) and Cre expression cassettes were inserted between two loxP sequences, and the hygromycin phosphotransferase (HPT) and green fluorescent protein (GFP) expression cassettes were located outside of the loxP-IbMyb-Cre-loxP region. In the developed system, IbMyb and Cre were excised from the genomes of transgenic cells using heat-inducible Cre-loxP recombination. Upon heat treatment in a general incubator, green shoots emerged from purple tobacco transgenic calli that were pigmented with IbMyb expression. The excision of IbMyb from the genome of green shoots was confirmed using polymerase chain reaction (PCR) and sequencing. GFP expression was observed in the roots of the obtained green transgenic plants. We report that the system developed here operated successfully in tobacco, showing the potential to provide an easier and cheaper visual selection of transgenic cells in the genetic transformation process.

Unclasping potentials of genomics and gene editing in chickpea to fight climate change and global hunger threat

Genomics and genome editing promise enormous opportunities for crop improvement and elementary research. Precise modification in the specific targeted location of a genome has profited over the unplanned insertional events which are generally accomplished employing unadventurous means of genetic modifications. The advent of new genome editing procedures viz; zinc finger nucleases (ZFNs), homing endonucleases, transcription activator like effector nucleases (TALENs), Base Editors (BEs), and Primer Editors (PEs) enable molecular scientists to modulate gene expressions or create novel genes with high precision and efficiency. However, all these techniques are exorbitant and tedious since their prerequisites are difficult processes that necessitate protein engineering. Contrary to first generation genome modifying methods, CRISPR/Cas9 is simple to construct, and clones can hypothetically target several locations in the genome with different guide RNAs. Following the model of the application in crop with the help of the CRISPR/Cas9 module, various customized Cas9 cassettes have been cast off to advance mark discrimination and diminish random cuts. The present study discusses the progression in genome editing apparatuses, and their applications in chickpea crop development, scientific limitations, and future perspectives for biofortifying cytokinin dehydrogenase, nitrate reductase, superoxide dismutase to induce drought resistance, heat tolerance and higher yield in chickpea to encounter global climate change, hunger and nutritional threats.

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.

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.

Gene activation via Cre/lox-mediated excision in cowpea (Vigna unguiculata)

Expression of Cre recombinase by AtRps5a_pro or AtDD45_pro enabled Cre/lox-mediated recombination at an early embryonic developmental stage upon crossing, activating transgenes in the hybrid cowpea and tobacco. Genetic engineering ideally results in precise spatiotemporal control of transgene expression. To activate transgenes exclusively in a hybrid upon fertilization, we evaluated a Cre/lox-mediated gene activation system with the Cre recombinase expressed by either AtRps5a or AtDD45 promoters that showed activity in egg cells and young embryos. In crosses between Cre recombinase lines and transgenic lines harboring a lox-excision reporter cassette with ZsGreen driven by the AtUbq3 promoter after Cre/lox-mediated recombination, we observed complete excision of the lox-flanked intervening DNA sequence between the AtUbq3_pro and the ZsGreen coding sequence in F₁ progeny upon genotyping but no ZsGreen expression in F₁ seeds or seedlings. The incapability to observe ZsGreen fluorescence was attributed to the activity of the AtUbq3_pro. Strong ZsGreen expression in F₁ seeds was observed after recombination when ZsGreen was driven by the AtUbq10 promoter. Using the AtDD45_pro to express Cre resulted in more variation in recombination frequencies between transgenic lines and crosses. Regardless of the promoter used to regulate Cre, mosaic F₁ progeny were rare, suggesting gene activation at an early embryo-developmental stage. Observation of ZsGreen-expressing tobacco embryos at the globular stage from crosses with the AtRps5a_proCre lines pollinated by the AtUbq3_prolox line supported the early activation mode.

Genetic Transformation of Recalcitrant Upland Switchgrass Using Morphogenic Genes

Switchgrass (Panicum virgatum) is an excellent feedstock for biofuel production. While genetic transformation is routinely done in lowland switchgrass, upland cultivars remain recalcitrant to genetic transformation. Here we report the establishment of an efficient and reproducible transformation protocol for two upland cultivars, 'Summer' and 'Blackwell', by ectopic overexpression of morphogenic genes, Baby boom (Bbm) and Wuschel2 (Wus2). Two auxotrophic Agrobacterium strains, LBA4404Thy- and EHA105Thy-, each harboring the same construct containing ZmBbm, ZmWus2, and a green fluorescence protein (GFP) gene, ZsGreen1, were used to infect immature leaf segments derived from in vitro grown seedlings. The Agrobacterium strains also contain a transformation helper plasmid that carry additional copies of Agrobacterium virulence genes. GFP-expressing calli were identified and selected for regeneration. The highest transformation efficiency of 6% was obtained for the tetraploid cultivar Summer when LBA4404Thy- was used for infection, which is twice of that for the octoploid cultivar Blackwell. LBA4404Thy- consistently outperformed EHA105Thy- on transformation frequency across the two cultivars. Fifteen randomly selected putative transgenic plants of Summer and Blackwell, representing independent callus events, were confirmed as transgenic by the presence of the transgene, ZmAls, and the absence of AtuFtsZ, a chromosomal gene specific to the Agrobacterium strain LBA4404 using polymerase chain reaction. Transgene integration and expression was further confirmed by the detection of GFP in roots, and the resistance to herbicide injury to leaves of selected putative transgenic plants. The ZmBbm and ZmWus2 genes were successfully removed from 40 to 33.3% of the transgenic plants of Summer and Blackwell, respectively, via the Cre-Lox recombination system upon heat treatment of GFP-expressing embryogenic calli. Our successful transformation of recalcitrant upland switchgrass provides a method for gene function analysis and germplasm enhancement via biotechnology.

Sequential Genome Editing and Induced Excision of the Transgene in N. tabacum BY2 Cells

While plant cells in suspension are becoming a popular platform for expressing biotherapeutic proteins, the need to pre-engineer these cells to better comply with their role as host cell lines is emerging. Heterologous DNA and selectable markers are used for transformation and genome editing designated to produce improved host cell lines for overexpression of recombinant proteins. The removal of these heterologous DNA and selectable markers, no longer needed, can be beneficial since they limit additional gene stacking in subsequent transformations and may pose excessive metabolic burden on the cell machinery. In this study we developed an innovative stepwise methodology in which the CRISPR-Cas9 is used sequentially to target genome editing, followed by its own excision. The first step included a stable insertion of a CRISPR-Cas9 cassette, targeted to knockout the β(1,2)-xylosyltranferase (XylT) and the α(1,3)-fucosyltransferase (FucT) genes in Nicotiana tabacum L. cv Bright Yellow 2 (BY2) cell suspension. The second step included the excision of the inserted cassette of 14.3 kbp by induction of specific sgRNA designed to target the T-DNA boundaries. The genome editing step and the transgene removal step are achieved in one transformation run. This mechanism enables CRISPR genome editing and subsequently eliminating the introduced transgenes thus freeing the cells from foreign DNA no longer needed.

Sequential Genome Editing and Induced Excision of the Transgene in N. tabacum BY2 Cells

While plant cells in suspension are becoming a popular platform for expressing biotherapeutic proteins, the need to pre-engineer these cells to better comply with their role as host cell lines is emerging. Heterologous DNA and selectable markers are used for transformation and genome editing designated to produce improved host cell lines for overexpression of recombinant proteins. The removal of these heterologous DNA and selectable markers, no longer needed, can be beneficial since they limit additional gene stacking in subsequent transformations and may pose excessive metabolic burden on the cell machinery. In this study we developed an innovative stepwise methodology in which the CRISPR-Cas9 is used sequentially to target genome editing, followed by its own excision. The first step included a stable insertion of a CRISPR-Cas9 cassette, targeted to knockout the β(1,2)-xylosyltranferase (XylT) and the α(1,3)-fucosyltransferase (FucT) genes in Nicotiana tabacum L. cv Bright Yellow 2 (BY2) cell suspension. The second step included the excision of the inserted cassette of 14.3 kbp by induction of specific sgRNA designed to target the T-DNA boundaries. The genome editing step and the transgene removal step are achieved in one transformation run. This mechanism enables CRISPR genome editing and subsequently eliminating the introduced transgenes thus freeing the cells from foreign DNA no longer needed.

Site-specific recombinase genome engineering toolkit in maize

Site-specific recombinase enzymes function in heterologous cellular environments to initiate strand-switching reactions between unique DNA sequences termed recombinase binding sites. Depending on binding site position and orientation, reactions result in integrations, excisions, or inversions of targeted DNA sequences in a precise and predictable manner. Here, we established five different stable recombinase expression lines in maize through Agrobacterium-mediated transformation of T-DNA molecules that contain coding sequences for Cre, R, FLPe, phiC31 Integrase, and phiC31 excisionase. Through the bombardment of recombinase activated DsRed transient expression constructs, we have determined that all five recombinases are functional in maize plants. These recombinase expression lines could be utilized for a variety of genetic engineering applications, including selectable marker removal, targeted transgene integration into predetermined locations, and gene stacking.

Construction of Marker-Free Genetically Modified Maize Using a Heat-Inducible Auto-Excision Vector

Gene modification is a promising tool for plant breeding, and gradual application from the laboratory to the field. Selectable marker genes (SMG) are required in the transformation process to simplify the identification of transgenic plants; however, it is more desirable to obtain transgenic plants without selection markers. Transgene integration mediated by site-specific recombination (SSR) systems into the dedicated genomic sites has been demonstrated in a few different plant species. Here, we present an auto-elimination vector system that uses a heat-inducible Cre to eliminate the selectable marker from transgenic maize, without the need for repeated transformation or sexual crossing. The vector combines an inducible site-specific recombinase (hsp70::Cre) that allows for the precise elimination of the selectable marker gene egfp upon heating. This marker gene is used for the initial positive selection of transgenic tissue. The egfp also functions as a visual marker to demonstrate the effectiveness of the heat-inducible Cre. A second marker gene for anthocyanin pigmentation (Rsc) is located outside of the region eliminated by Cre and is used for the identification of transgenic offspring in future generations. Using the heat-inducible auto-excision vector, marker-free transgenic maize plants were obtained in a precisely controlled genetic modification process. Genetic and molecular analyses indicated that the inducible auto-excision system was tightly controlled, with highly efficient DNA excision, and provided a highly reliable method to generate marker-free transgenic maize.

Heat-shock-inducible CRISPR/Cas9 system generates heritable mutations in rice

Transient expression of CRISPR/Cas9 is an effective approach for limiting its activities and improving its precision in genome editing. Here, we describe the heat-shock-inducible CRISPR/Cas9 for controlled genome editing, and demonstrate its efficiency in the model crop, rice. Using the soybean heat-shock protein gene promoter and the rice U3 promoter to express Cas9 and sgRNA, respectively, we developed the heat-shock (HS)-inducible CRISPR/Cas9 system, and tested its efficacy in targeted mutagenesis. Two loci were targeted in rice, and the presence of targeted mutations was determined before and after the HS treatment. Only a low rate of targeted mutagenesis was detected before HS (~16%), but an increased rate of mutagenesis was observed after the HS treatment among the transgenic lines (50-63%). Analysis of regenerated plants harboring HS-CRISPR/Cas9 revealed that targeted mutagenesis was suppressed in the plants but induced by HS, which was detectable by Sanger sequencing after a few weeks of HS treatments. Most importantly, the HS-induced mutations were transmitted to the progeny at a high rate, generating monoallelic and biallelic mutations that independently segregated from the Cas9 gene. Additionally, off-target mutations were either undetectable or found at a lower rate in HS-CRISPR/Cas9 lines as compared to the constitutive-overexpression CRISPR/Cas9 lines. Taken together, this work shows that HS-CRISPR/Cas9 is a controlled and reasonably efficient platform for genome editing, and therefore, a promising tool for limiting genome-wide off-target effects and improving the precision of genome editing.

Heat-Shock-Induced Removal of Transgenes Using the Gene-Deletor System in Hybrid Aspen (Populus tremula × P. tremuloides)

To evaluate the efficacy of the gene-deletor system in aspen, we evaluated the system for foreign gene removal in a hybrid aspen clone, INRA 353-53 (Populus tremula × P. tremuloides). The recombinase flipping DNA (FLP) gene was under the control of the heat-inducible promoter of Gmhsp17.6-L, and the β-glucuronidase (gusA) gene which was under the control of the 35S promoter and were constructed using the gene-deletor system in the pCaLFGmFNLFG vector. Six transgenic plants and their sublines were heated at 42 °C for 8 h and gene deletion was verified by polymerase chain reaction (PCR). Three lines exhibited partial transgene deletion while the remaining three lines did not delete. Transgenic lines were evaluated by Southern-blot analyses, verifying that the six transgenic plant lines all had a single copy of transfer DNA (t-DNA). Two partial-deletion lines and two non-deletion lines were analysed for methylation and expression of promoter and recombinase. Hardly any methylation was detected in the Gmhsp17.6-L promoter or recombinase FLP gene sequences, however, the expression of the promoter and recombinase was increased significantly in the partial-deletion compared with the non-deletion line after heat-shock treatment. These results suggest that the excision efficiency had no direct relationship with methylation status of the Gmhsp17.6-L promoter and FLP recombinase, yet was affected by the expression of the Gmhsp17.6-L and FLP after heat-shock treatment.

Elimination of macro elements from inoculation and co-cultivation media enhances the efficiency of Agrobacterium-mediated transformation in Petunia

In order to evaluate the effect of inoculation and co-cultivation media elements on transformation frequency in Petunia hybrida, modified MS media with different elements were tested on Alvan and Large Flower Alvan (LF Alvan), two local cultivars. Leaf explants of both cultivars were inoculated with Agrobacterium tumefaciens strain LBA4404 (pBI121) containing neomycin phosphotransferase (nptII) and an intron-containing β-glucuronidase (gus) genes. When medium lacking KH2PO4, NH4NO3, KNO3, and CaCl2 was used as inoculation and co-cultivation medium, a higher frequency of transformation for Alvan (22%) and LF Alvan (16%) was obtained. Kanamycin resistant plantlets were stained blue by GUS assay. Furthermore, polymerase chain reaction (PCR) analysis revealed the presence of both gus and nptII genes in all putative transformants. Finally, southern blot hybridization confirmed insertion of 1-4 copies of gus gene in transgenic plants.

A new location to split Cre recombinase for protein fragment complementation

We have previously described a recombinase-mediated gene stacking system in which the Cre recombinase is used to remove lox-site flanked DNA no longer needed after each round of Bxb1 integrase-mediated site-specific integration. The Cre recombinase can be conveniently introduced by hybridization with a cre-expressing plant. However, maintaining an efficient cre-expressing line over many generations can be a problem, as high production of this DNA-binding protein might interfere with normal chromosome activities. To counter this selection against high Cre activity, we considered a split-cre approach, in which Cre activity is reconstituted after separate parts of Cre are brought into the same genome by hybridization. To insure that the recombinase-mediated gene stacking system retains its freedom to operate, we tested for new locations to split Cre into complementing fragments. In this study, we describe testing four new locations for splitting the Cre recombinase for protein fragment complementation and show that the two fragments of Cre split between Lys244 and Asn245 can reconstitute activity that is comparable to that of wild-type Cre.

A Chemical-Induced, Seed-Soaking Activation Procedure for Regulated Gene Expression in Rice

Inducible gene expression has emerged as a powerful tool for plant functional genomics. The estrogen receptor-based, chemical-inducible system XVE has been used in many plant species, but the limited systemic movement of inducer β-estradiol in transgenic rice plants has prohibited a wide use of the XVE system in this important food crop. Here, we constructed an improved chemical-regulated, site-specific recombination system by employing the XVE transactivator in combination with a Cre/loxP-FRT system, and optimized a seed-soaking procedure for XVE induction in rice. By using a gus gene and an hpRNAi cassette targeted for OsPDS as reporters, we demonstrated that soaking transgenic seeds with estradiol solution could induce highly efficient site-specific recombination in germinating embryos, resulting in constitutive and high-level expression of target gene or RNAi cassette in intact rice plants from induced seeds. The strategy reported here thereby provides a useful gene activation approach for effectively regulating gene expression in rice.

Microprojectile Bombardment Transformation of Date Palm Using the Insecticidal Cholesterol Oxidase (ChoA) Gene

The overall objective of this work is to optimize the transformation system for date palm as a first step toward production of date palm clones resistant to noxious pests. A construct harboring the cholesterol oxidase (ChoA) gene, which renders plant resistance against insect attack, is introduced into embryogenic date palm callus using the PDS-1000/He particle bombardment system. The process involves the establishment of embryogenic callus cultures as well as immature embryo-derived microcalli that are used as target tissues for shooting and optimization of transformation conditions. This chapter in addition explains molecular and histochemical assays conducted to confirm gene integration and expression.

Use of Zinc-Finger Nucleases for Crop Improvement

Over the past two decades, new technologies enabling targeted modification of plant genomes have been developed. Among these are zinc-finger nucleases (ZFNs) which are composed of engineered zinc-finger DNA-binding domains fused with a nuclease, generally the FokI nuclease. The zinc-finger domains are composed of a series of four to six 30 amino acid domains that can bind to trinucleotide sequences giving the entire DNA-binding domain specificity to 12-18 nucleotides. Since the FokI nuclease functions as a dimer, pairs of zinc-finger domains are designed to bind upstream and downstream of the cut site which increases the specificity of the complete ZFN to 24-36 nucleotides. The ability of these engineered nucleases to create targeted double-stranded breaks at designated locations throughout the genome has enabled precise deletion, addition, and editing of genes. These techniques are being used to create new genetic variation by deleting or editing endogenous gene sequences and enhancing the efficiency of transgenic product development through targeted insertion of transgenes to specific genomic locations and to sequentially add and/or delete transgenes from existing transgenic events.

Marker Removal in Transgenic Plants Using Cre Recombinase Delivered with Potato Virus X

In this chapter we present an alternative method to develop marker-free transgenic plants. It makes use of the Cre/loxP recombination system from bacteriophage P1 and consists of two essential components. The first component is the transgenic plant containing a loxP-flanked marker gene. The second component is a cre transient expression vector based on potato virus X. The great benefit of this transient delivery method consists in the avoidance of stable integration of the cre recombinase gene into the plant genome. Upon infection of the loxP-target plant with PVX-Cre, the virus spreads systemically through the plant and causes the recombinase-mediated excision of the marker gene. Marker-free transgenic loci can be transmitted to the progeny by plant regeneration from PVX-Cre systemically infected leaves or self-pollination of virus-infected plants. The protocol covers generation of loxP-target transgenic plants, PVX-mediated delivery of Cre recombinase protein, phenotypic and molecular analysis of recombination events, and transmission of marker-free transgenic loci to the next generation. The transient expression system described in this chapter can be adapted for marker gene removal in other plant species that are amenable for virus infection.

Marker-free PLRV resistant potato mediated by Cre-loxP excision and RNAi

An inverted repeat construct corresponding to a segment of the potato leaf roll virus coat protein gene was created under control of a constitutive promoter and transferred into a transformation vector with a heat inducible Cre-loxP system to excise the nptII antibiotic resistance marker gene. Fifty-eight transgenic events were evaluated for resistance to PLRV by greenhouse inoculations, which lead to the identification of 7 highly resistant events, of which 4 were extremely resistant. This resistance was also highly effective against accumulation in subsequent tuber generations from inoculated plants, which has not been reported before. Northern blot analysis showed correlation of PLRV specific siRNA accumulation with the level of PLRV resistance. Heat mediated excision of the nptII antibiotic resistance gene in PLRV resistant events was highly efficient in one event with full excision in 71 % of treated explants. On the other hand 8 out of 10 analyzed events showed truncated T-DNA insertions lacking one of the two loxP sites as determined by PCR and confirmed by sequencing flanking regions in 2 events, suggesting cryptic LB sites in the non-coding region between the nptII gene and the flanking loxP site. Accordingly, it is proposed to modify the Cre-loxP vector by reducing the 1 kb size of the region between nptII, loxP, and the LB.

Genetically modified (GM) crops: milestones and new advances in crop improvement

New advances in crop genetic engineering can significantly pace up the development of genetically improved varieties with enhanced yield, nutrition and tolerance to biotic and abiotic stresses. Genetically modified (GM) crops can act as powerful complement to the crops produced by laborious and time consuming conventional breeding methods to meet the worldwide demand for quality foods. GM crops can help fight malnutrition due to enhanced yield, nutritional quality and increased resistance to various biotic and abiotic stresses. However, several biosafety issues and public concerns are associated with cultivation of GM crops developed by transgenesis, i.e., introduction of genes from distantly related organism. To meet these concerns, researchers have developed alternative concepts of cisgenesis and intragenesis which involve transformation of plants with genetic material derived from the species itself or from closely related species capable of sexual hybridization, respectively. Recombinase technology aimed at site-specific integration of transgene can help to overcome limitations of traditional genetic engineering methods based on random integration of multiple copy of transgene into plant genome leading to gene silencing and unpredictable expression pattern. Besides, recently developed technology of genome editing using engineered nucleases, permit the modification or mutation of genes of interest without involving foreign DNA, and as a result, plants developed with this technology might be considered as non-transgenic genetically altered plants. This would open the doors for the development and commercialization of transgenic plants with superior phenotypes even in countries where GM crops are poorly accepted. This review is an attempt to summarize various past achievements of GM technology in crop improvement, recent progress and new advances in the field to develop improved varieties aimed for better consumer acceptance.

Maize (Zea mays) Hi-II Transformation via Agrobacterium-Mediated T-DNA Transfer

Genetic transformation of maize via Agrobacterium tumefaciens is still more art than science, with different researchers achieving substantially different transformation results. This article describes our advanced Agrobacterium-mediated transformation system in Hi-II maize. The system utilizes simple binary vectors and immature embryos for the transformation, employing the bar gene as a plant selectable marker in combination with bialaphos for subsequent culture selection. The transformation process is efficient and highly reproducible. Certain inbred maize lines can also be transformed with some modification of the system. © 2016 by John Wiley & Sons, Inc.

Expression of an engineered synthetic cry2Aa (D42/K63F/K64P) gene of Bacillus thuringiensis in marker free transgenic tobacco facilitated full-protection from cotton leaf worm (S. littoralis) at very low concentration

Emergence of resistant insects limits the sustainability of Bacillus thuringiensis (Bt) transgenic crop plants for insect management. Beside this, the presence of unwanted marker gene(s) in the transgenic crops is also a major environmental and health concern. Thus, development of marker free transgenic crop plants expressing a new class of toxin having a different mortality mechanism is necessary for resistance management. In a previous study, we generated an engineered Cry2Aa (D42/K63F/K64P) toxin which has a different mortality mechanism as compared to first generation Bt toxin Cry1A, and this engineered toxin was found to enhance 4.1-6.6-fold toxicity against major lepidopteran insect pests of crop plants. In the present study, we have tested the potency of this engineered synthetic Cry2Aa (D42/K63F/K64P) toxin as a candidate in the development of insect resistant transgenic tobacco plants. Simultaneously, we have eliminated the selectable marker gene from the Cry2Aa (D42/K63F/K64P) expressing tobacco plants by exploiting the Cre/lox mediated recombination methodology, and successfully developed marker free T2 transgenic tobacco plants expressing the engineered Cry2Aa toxin. Realtime and western blot analysis demonstrated the expression of engineered toxin gene in transgenic plants. Insect feeding assays revealed that the marker free T2 progeny of transgenic plants expressing Cry2Aa (D42/K63F/K64P) toxin showed 82-92 and 52-61 % mortality to cotton leaf worm (CLW) and cotton bollworm (CBW) respectively. Thus, this engineered Cry2Aa toxin could be useful for the generation of insect resistant transgenic Bt lines which will protect the crop damages caused by different insect pests such as CLW and CBW.

Gene stacking by recombinases

Efficient methods of stacking genes into plant genomes are needed to expedite transfer of multigenic traits to crop varieties of diverse ecosystems. Over two decades of research has identified several DNA recombinases that carryout efficient cis and trans recombination between the recombination sites artificially introduced into the plant chromosome. The specificity and efficiency of recombinases make them extremely attractive for genome engineering. In plant biotechnology, recombinases have mostly been used for removing selectable marker genes and have rarely been extended to more complex applications. The reversibility of recombination, a property of the tyrosine family of recombinases, does not lend itself to gene stacking approaches that involve rounds of transformation for integrating genes into the engineered sites. However, recent developments in the field of recombinases have overcome these challenges and paved the way for gene stacking. Some of the key advancements include the application of unidirectional recombination systems, modification of recombination sites and transgene site modifications to allow repeated site-specific integrations into the selected site. Gene stacking is relevant to agriculturally important crops, many of which are difficult to transform; therefore, development of high-efficiency gene stacking systems will be important for its application on agronomically important crops, and their elite varieties. Recombinases, by virtue of their specificity and efficiency in plant cells, emerge as powerful tools for a variety of applications including gene stacking.

Transgenic trait deployment using designed nucleases

The demand for crops requiring increasingly complex combinations of transgenes poses unique challenges for transgenic trait deployment. Future value-adding traits such as those associated with crop performance are expected to involve multiple transgenes. Random integration of transgenes not only results in unpredictable expression and potential unwanted side effects but stacking multiple, randomly integrated, independently segregating transgenes creates breeding challenges during introgression and product development. Designed nucleases enable the creation of targeted DNA double-strand breaks at specified genomic locations whereby repair can result in targeted transgene integration leading to precise alterations in DNA sequences for plant genome editing, including the targeting of a transgene to a genomic locus that supports high-level and stable transgene expression without interfering with resident gene function. In addition, targeted DNA integration via designed nucleases allows for the addition of transgenes into previously integrated transgenic loci to create stacked products. The currently reported frequencies of independently generated transgenic events obtained with site-specific transgene integration without the aid of selection for targeting are very low. A modular, positive selection-based gene targeting strategy has been developed involving cassette exchange of selectable marker genes which allows for targeted events to be preferentially selected, over multiple cycles of sequential transformation. This, combined with the demonstration of intragenomic recombination following crossing of transgenic events that contain stably integrated donor and target DNA constructs with nuclease-expressing plants, points towards the future of trait stacking that is less dependent on high-efficiency transformation.

Conditional Gene Expression/Deletion Systems for Marchantia polymorpha Using its Own Heat-Shock Promoter and Cre/loxP-Mediated Site-Specific Recombination

The liverwort Marchantia polymorpha is an emerging model plant suitable for addressing, using genetic approaches, various evolutionary questions in the land plant lineage. Haploid dominancy in its life cycle facilitates genetic analyses, but conversely limits the ability to isolate mutants of essential genes. To overcome this issue and to be employed in cell lineage, mosaic and cell autonomy analyses, we developed a system that allows conditional gene expression and deletion using a promoter of a heat-shock protein (HSP) gene and the Cre/loxP site-specific recombination system. Because the widely used promoter of the Arabidopsis HSP18.2 gene did not operate in M. polymorpha, we identified a promoter of an endogenous HSP gene, MpHSP17.8A1, which exhibited a highly inducible transient expression level upon heat shock with a low basal activity level. Reporter genes fused to this promoter were induced globally in thalli under whole-plant heat treatment and also locally using a laser-assisted targeted heating technique. By expressing Cre fused to the glucocorticoid receptor under the control of the MpHSP17.8A1 promoter, a low background, sufficiently inducible control for loxP-mediated recombination could be achieved in M. polymorpha. Based on these findings, we developed a Gateway technology-based binary vector for the conditional induction of gene deletions.

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.

Development of Selectable Marker-Free Transgenic Rice Plants with Enhanced Seed Tocopherol Content through FLP/FRT-Mediated Spontaneous Auto-Excision

Development of marker-free transgenic plants is a technical alternative for avoiding concerns about the safety of selectable marker genes used in genetically modified (GM) crops. Here, we describe the construction of a spontaneous self-excision binary vector using an oxidative stress-inducible modified FLP/FRT system and its successful application to produce marker-free transgenic rice plants with enhanced seed tocopherol content. To generate selectable marker-free transgenic rice plants, we constructed a binary vector using the hpt selectable marker gene and the rice codon-optimized FLP (mFLP) gene under the control of an oxidative stress-inducible promoter between two FRT sites, along with multiple cloning sites for convenient cloning of genes of interest. Using this pCMF binary vector with the NtTC gene, marker-free T₁ transgenic rice plants expressing NtTC were produced by Agrobacterium-mediated stable transformation using hygromycin as a selective agent, followed by segregation of selectable marker genes. Furthermore, α-, γ-, and total tocopherol levels were significantly increased in seeds of the marker-free transgenic TC line compared with those of wild-type plants. Thus, this spontaneous auto-excision system, incorporating an oxidative stress-inducible mFLP/FRT system to eliminate the selectable marker gene, can be easily adopted and used to efficiently generate marker-free transgenic rice plants. Moreover, nutritional enhancement of rice seeds through elevation of tocopherol content coupled with this marker-free strategy may improve human health and public acceptance of GM rice.

Purple chromoprotein gene serves as a new selection marker for transgenesis of the microalga Nannochloropsis oculata

Among the methods used to screen transgenic microalgae, antibiotics selection has raised environmental and food safety concerns, while the observation of fluorescence proteins could be influenced by the endogenous fluorescence of host chloroplasts. As an alternative, this study isolated the purple chromoprotein (CP) from Stichodacyla haddoni (shCP). A plasmid in which shCP cDNA is driven by a heat-inducible promoter was linearized and electroporated into 2.5×10⁸ protoplasts of Nannochloropsis oculata. Following regeneration and cultivation on an f/2 medium plate for two weeks, we observed 26 colonies that displayed a slightly dark green coloration. After individually subculturing and performing five hours of heat shock at 42°C, a dark brown color was mosaically displayed in five of these colonies, indicating that both untransformed and transformed cells were mixed together in each colony. To obtain a uniform expression of shCP throughout the whole colony, we continuously isolated each transformed cell that exhibited brown coloration and subcultured it on a fresh plate, resulting in the generation of five transgenic lines of N. oculata which stably harbored the shCP gene for at least 22 months, as confirmed by PCR detection and observation by the naked eye. As shown by Western blot, exogenous shCP protein was expressed in these transgenic microalgae. Since shCP protein is biodegradable and originates from a marine organism, both environmental and food safety concerns have been eliminated, making this novel shCP reporter gene a simple, but effective and ecologically safe, marker for screening and isolating transgenic microalgae.

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.

Simplifying transgene locus structure through Cre-lox recombination

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

Xcc-facilitated agroinfiltration of citrus leaves: a tool for rapid functional analysis of transgenes in citrus leaves

Xanthomonas citri subsp. citri pretreatment before agroinfiltration could significantly promote transient expression in citrus leaves which were previously recalcitrant to agroinfiltration. Transient expression via agroinfiltration is widely used in biotechnology but remains problematic in many economically important plants. Xanthomonas citri subsp. citri (Xcc)-facilitated agroinfiltration was employed to promote transient protein expression in Valencia sweet orange leaves, which are recalcitrant to agroinfiltration. However, it is unclear whether Xcc-facilitated agroinfiltration has broad application, i.e., whether Xcc-facilitated agroinfiltration could be used on other citrus varieties. In addition, we intended to investigate whether Xcc-facilitated agroinfiltration could be used to hasten transgene function assays, e.g., Cre/lox system and Cas9/sgRNA system. In this report, Xcc-facilitated agroinfiltration was further exploited to enhance β-glucuronidase (GUS) expression in five citrus varieties. Xcc-facilitated agroinfiltration also significantly increased GFP expression in six citrus varieties tested. Both GUS and GFP assays indicated that Xcc-facilitated agroinfiltration had the best performance in grapefruit. After Xcc-facilitated agroinfiltration was carried out in grapefruit, protoplast analysis of the transformed cells indicated that there were more than 20 % leaf cells expressing GFP. In grapefruit, usefulness of Xcc-facilitated agroinfiltration was assayed in three case studies: (1) fast functional analysis of Cre/lox system, (2) the heat shock regulation of HSP70B promoter derived from Arabidopsis, and (3) Cas9/sgRNA-mediated genome modification.

Genetic use restriction technologies: a review

Genetic use restriction technologies (GURTs), developed to secure return on investments through protection of plant varieties, are among the most controversial and opposed genetic engineering biotechnologies as they are perceived as a tool to force farmers to depend on multinational corporations' seed monopolies. In this work, the currently proposed strategies are described and compared with some of the principal techniques implemented for preventing transgene flow and/or seed saving, with a simultaneous analysis of the future perspectives of GURTs taking into account potential benefits, possible impacts on farmers and local plant genetic resources (PGR), hypothetical negative environmental issues and ethical concerns related to intellectual property that have led to the ban of this technology.

Maize transformation technology development for commercial event generation

Maize is an important food and feed crop in many countries. It is also one of the most important target crops for the application of biotechnology. Currently, there are more biotech traits available on the market in maize than in any other crop. Generation of transgenic events is a crucial step in the development of biotech traits. For commercial applications, a high throughput transformation system producing a large number of high quality events in an elite genetic background is highly desirable. There has been tremendous progress in Agrobacterium-mediated maize transformation since the publication of the Ishida et al. (1996) paper and the technology has been widely adopted for transgenic event production by many labs around the world. We will review general efforts in establishing efficient maize transformation technologies useful for transgenic event production in trait research and development. The review will also discuss transformation systems used for generating commercial maize trait events currently on the market. As the number of traits is increasing steadily and two or more modes of action are used to control key pests, new tools are needed to efficiently transform vectors containing multiple trait genes. We will review general guidelines for assembling binary vectors for commercial transformation. Approaches to increase transformation efficiency and gene expression of large gene stack vectors will be discussed. Finally, recent studies of targeted genome modification and transgene insertion using different site-directed nuclease technologies will be reviewed.

Engineered minichromosomes in plants

Platforms for the development of synthetic chromosomes in plants have been produced in several species using telomere mediated chromosomal truncation with the simultaneous inclusion of sites that facilitate further additions to the newly generated minichromosome. By utilizing truncated supernumerary or B chromosomes, the output of the genes on the minichromosome can be amplified. Proof of concept experiments have been successful illustrating that minichromosome platforms can be modified in vivo. Engineered minichromosomes can likely be combined with haploid breeding if they are incorporated into inducer lines given that the observations that basically inert chromosomes from haploid inducer lines can be recovered at workable frequencies in otherwise haploid plants. Future needs of synthetic chromosome development are discussed.

Identification and utilization of cleistogamy gene cl7(t) in rice (Oryza sativa L.)

Gene transformation is an important method for improvement of plants into elite varieties. However, the possibility of gene flow between genetically modified (GM) crops and similar species is a serious public issue that may potentially endanger ecological stability. Cleistogamy is expected to be an ideal genetic tool for preventing transgene propagation from GM crops. A rice mutant, cl7(t), was created by ethyl methanesulfonate mutagenesis. The mutant exhibited cleistogamy, and had closed spikelets, reduced plant height, and altered morphology of the leaves, panicle, and seeds. Anatomical investigations revealed that the cl7(t) mutant contained more vascular bundles and thicker stems than the wild type, which increased the mechanical strength of its internodes, and anti-lodging ability. Further studies demonstrated that the force required to open the lemma and palea was higher in the cl7(t) mutant, and there was weak swelling ability in the lodicules, which leads to cleistogamy. Allelic analyses and complementation tests indicated that cl7(t) was a novel allele of dep2, a mutant that was previously reported to have similar panicle morphology. Sequence analysis showed that cl7(t) had a single nucleotide substitution (C to A) in the third exon that leads to a Ser substitution with a stop codon, giving a truncated DEP2 protein. Quantitative RT-PCR and in situ hybridization tests demonstrated that there was lower CL7(t) expression level in the spikelets and weaker CL7(t) signals in the lodicules of the cl7(t) mutant compared with wild type, which implies that CL7(t) might participate in the development of lodicules. To improve the agronomic traits of cl7(t) to fit the needs of field production, the cl7(t) mutant was crossed with an intermediate-type rice variety named Guanghui102, which bears some important agronomic traits, including increased grain numbers and high rate of seed setting. Through multi-generational pedigree selection, cleistogamy lines with improved economic traits were obtained, which can be used for the selection of ecologically safe GM rice varieties.

Agrobacterium-mediated transformation of maize (Zea mays) immature embryos

Agrobacterium tumefaciens-mediated transformation is one of the most efficient and simple gene delivery systems for genetic improvement and biology studies in maize. This system has become more widely used by both public and private laboratories. However, transformation efficiencies vary greatly from laboratory to laboratory for the same genotype. Here, we illustrate our advanced Agrobacterium-mediated transformation method in Hi-II maize using simple binary vectors. The protocol utilizes immature embryos as starting explants and the bar gene as a selectable marker coupled with bialaphos as a selective agent. The protocol offers efficient transformation results with high reproducibility, provided that some experimental conditions are well controlled. This transformation method, with minor modifications, can be also employed to transform certain maize inbreds.

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.

The use of rice seeds to produce human pharmaceuticals for oral therapy

Rice (Oryza sativa L.) is the major staple food consumed by half of the world's population. Rice seeds have gained recent attention as bioreactors for the production of human pharmaceuticals such as therapeutic proteins or peptides. Rice seed production platforms have many advantages over animal cell or microbe systems in terms of cost-effectiveness, scalability, safety, product stability and productivity. Rice seed-based human pharmaceuticals are expected to become innovative therapies as edible drugs. Therapeutic proteins can be sequestered within natural cellular compartments in rice seeds and protected from harsh gastrointestinal environments. This review presents the state-of-the-art on the construction of gene cassettes for accumulation of pharmaceutical proteins or peptides in rice seeds, the generation of transgenic rice plants, and challenges involved in the use of rice seeds to produce human pharmaceuticals.

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.

Less is more: strategies to remove marker genes from transgenic plants

Selectable marker genes (SMGs) and selection agents are useful tools in the production of transgenic plants by selecting transformed cells from a matrix consisting of mostly untransformed cells. Most SMGs express protein products that confer antibiotic- or herbicide resistance traits, and typically reside in the end product of genetically-modified (GM) plants. The presence of these genes in GM plants, and subsequently in food, feed and the environment, are of concern and subject to special government regulation in many countries. The presence of SMGs in GM plants might also, in some cases, result in a metabolic burden for the host plants. Their use also prevents the re-use of the same SMG when a second transformation scheme is needed to be performed on the transgenic host. In recent years, several strategies have been developed to remove SMGs from GM products while retaining the transgenes of interest. This review describes the existing strategies for SMG removal, including the implementation of site specific recombination systems, TALENs and ZFNs. This review discusses the advantages and disadvantages of existing SMG-removal strategies and explores possible future research directions for SMG removal including emerging technologies for increased precision for genome modification.

PHA bioplastics, biochemicals, and energy from crops

Large scale production of polyhydroxyalkanoates (PHAs) in plants can provide a sustainable supply of bioplastics, biochemicals, and energy from sunlight and atmospheric CO(2). PHAs are a class of polymers with various chain lengths that are naturally produced by some microorganisms as storage materials. The properties of these polyesters make them functionally equivalent to many of the petroleum-based plastics that are currently in the market place. However, unlike most petroleum-derived plastics, PHAs can be produced from renewable feedstocks and easily degrade in most biologically active environments. This review highlights research efforts over the last 20 years to engineer the production of PHAs in plants with a focus on polyhydroxybutryrate (PHB) production in bioenergy crops with C(4) photosynthesis. PHB has the potential to be a high volume commercial product with uses not only in the plastics and materials markets, but also in renewable chemicals and feed. The major challenges of improving product yield and plant fitness in high biomass yielding C(4) crops are discussed in detail.

Agrobacterium-mediated co-transformation of rice using two selectable marker genes derived from rice genome components

A method for Agrobacterium-mediated co-transformation of rice (Oryza sativa L.) was developed using rice-derived selection markers. Two T-DNAs were efficiently introduced into separate loci using selectable marker gene cassettes consisting of the mutated acetolactate synthase gene (mALS) under the control of the callus-specific promoter (CSP) (CSP:mALS) and the ferredoxin nitrite reductase gene (NiR) under the control of its own promoter (NiR P:NiR). The CSP:mALS gene cassette confers sulfonylurea herbicide resistance to transgenic rice callus. The NiR P:NiR construct complements NiR-deficient mutant cultivars such as 'Koshihikari', which are defective in the regulation of nitrogen metabolism. In the present study, the CaMV35S:GUS and CaMV35S:GFP gene cassettes were co-introduced into the 'Koshihikari' genome using our system. Approximately 5-10 independent transgenic lines expressing both the GUS and GFP reporters were obtained from 100 Agrobacterium co-inoculated calli. Furthermore, transgenic 'Koshihikari' rice lines with reduced content of two major seed allergen proteins, the 33 and 14-16 kDa allergens, were generated by this co-transformation system. The present results indicate that the generation of selectable antibiotic resistance marker gene-free transgenic rice is possible using our rice-derived selection marker co-transformation system. Key message An improved rice transformation method was developed based on Agrobacterium-mediated co-transformation using two rice genome-derived selectable marker gene cassettes.