PVX-Cre-mediated marker gene elimination from transgenic plants

Viral delivery of recombinases activates heritable genetic switches in plants

Viral vectors provide an increasingly versatile platform for transformation-free reagent delivery to plants. RNA viral vectors can be used to induce gene silencing, overexpress proteins, or introduce gene editing reagents; however, they are often constrained by carrying capacity or restricted tropism in germline cells. Site-specific recombinases that catalyze precise genetic rearrangements are powerful tools for genome engineering that vary in size and, potentially, efficacy in plants. In this work, we show that viral vectors based on tobacco rattle virus (TRV) deliver and stably express four recombinases ranging in size from ∼0.6 to ∼1.5 kb and achieve simultaneous marker removal and reporter activation through targeted excision in transgenic Nicotiana benthamiana lines. TRV vectors with Cre, FLP, CinH, and Integrase13 efficiently mediated recombination in infected somatic tissue and led to heritable modifications at high frequency. An excision-activated Ruby reporter enabled simple and high-resolution tracing of infected cell lineages without the need for molecular genotyping. Together, our experiments broaden the scope of viral recombinase delivery and offer insights into infection dynamics that may be useful in developing future viral vectors.

Improving transient expression in N. benthamiana by suppression of the Nb-SABP2 and Nb-COI1 plant defence response related genes

Currently transient expression is one of the preferred plant-based technologies for recombinant protein manufacturing, particularly in respect to pharmaceutically relevant products. Modern hybrid transient expression systems combine the features of Agrobacterium tumefaciens and viral vectors. However, host plant reaction to Agrobacterium-mediated delivery of gene of interest can negatively affect foreign protein accumulation. In this study, we investigated whether the modulation of plant immune response through knockdown of the Nb-SABP2 and Nb-COI1 N. benthamiana genes could improve recombinant protein yield. In plants, the SABP2 and COI1 proteins are involved in the salicylic acid and jasmonic acid metabolism, respectively. We exemplified the utility of this approach with the green fluorescence (GFP) and β nerve growth factor (βNGF) proteins: compared to the tobacco mosaic virus (TMV)-based vector the Nb-SABP2 and Nb-COI1-suppressed plants provided an increased recombinant protein accumulation. We also show that this strategy is extendable to the expression systems utilizing potato virus X (PVX) as the vector backbone: the enhanced amounts of βNGF were detected in the Nb-SABP2 and Nb-COI1-depleted leaves co-infiltrated with the PVX-βNGF. These findings suggest that modulating host plant reaction to agrodelivery of expression vectors could be useful for improving transient foreign protein production in N. benthamiana.

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.

At-CycD2 Enhances Accumulation of Above-Ground Biomass and Recombinant Proteins in Transgenic Nicotiana benthamiana Plants

Transient expression in Nicotiana benthamiana holds great potential for recombinant protein manufacturing due to its advantages in terms of speed and yield compared to stably transformed plants. To continue improving the quantity of recombinant proteins the plant host will need to be modified at both plant and cellular levels. In attempt to increase leaf mass fraction, we transformed N. benthamiana with the At-CycD2 gene, a positive regulator of the cell cycle. Phenotypic characterization of the T₁ progeny plants revealed their accelerated above-ground biomass accumulation and enhanced rate of leaf initiation. In comparison to non-transgenic control the best performing line At-CycD2-15 provided 143 and 140% higher leaf and stem biomass fractions, respectively. The leaf area enlargement of the At-CycD2-15 genotype was associated with the increase of epidermal cell number compensated by slightly reduced cell size. The production capacity of the At-CycD2-15 transgenic line was superior to that of the non-transgenic N. benthamiana. The accumulation of transiently expressed GFP and scFv-TM43-E10 proteins per unit biomass was increased by 138.5 and 156.7%, respectively, compared to the wild type. With these results we demonstrate the potential of cell cycle regulator gene At-CycD2 to modulate both plant phenotype and intracellular environment of N. benthamiana for enhanced recombinant protein yield.

An Efficient Gene Excision System in Maize

Use of the morphogenic genes Baby Boom (Bbm) and Wuschel2 (Wus2), along with new ternary constructs, has increased the genotype range and the type of explants that can be used for maize transformation. Further optimizing the expression pattern for Bbm/Wus2 has resulted in rapid maize transformation methods that are faster and applicable to a broader range of inbreds. However, expression of Bbm/Wus2 can compromise the quality of regenerated plants, leading to sterility. We reasoned excising morphogenic genes after transformation but before regeneration would increase production of fertile T0 plants. We developed a method that uses an inducible site-specific recombinase (Cre) to excise morphogenic genes. The use of developmentally regulated promoters, such as Ole, Glb1, End2, and Ltp2, to drive Cre enabled excision of morphogenic genes in early embryo development and produced excised events at a rate of 25-100%. A different strategy utilizing an excision-activated selectable marker produced excised events at a rate of 53-68%; however, the transformation frequency was lower (13-50%). The use of inducible heat shock promoters (e.g. Hsp17.7, Hsp26) to express Cre, along with improvements in tissue culture conditions and construct design, resulted in high frequencies of T0 transformation (29-69%), excision (50-97%), usable quality events (4-15%), and few escapes (non-transgenic; 14-17%) in three elite maize inbreds. Transgenic events produced by this method are free of morphogenic and marker genes.

Enhanced foreign protein accumulation in Nicotiana benthamiana leaves co-infiltrated with a TMV vector and plant cell cycle regulator genes

In this short communication, we report that the cell cycle checkpoint genes At-CycD2 and At-CDC27a from Arabidopsis thaliana enhance the transient heterologous protein expression in Nicotiana benthamiana. We selected a well-studied and widely used virus expression vector based on TMV for the delivery of recombinant proteins into the host plant. Co-infiltration of TMV-gfp and binary expression vectors carrying the At-CycD2 and At-CDC27a genes, respectively, resulted in enhanced GFP fluorescence in agroinoculated leaves. These findings corresponded with the observation of (1) higher mRNA levels for TMV and gfp and (2) increased GFP protein accumulation. Furthermore, by co-delivery of the TMV-scFv-TM43-E10 and At-CycD2/At-CDC27a expressing constructs we observed an enhanced amount of the scFv-TM43-E10 antibody fragment compared to the delivery of the TMV-scFv-TM43-E10 alone. We anticipate that this finding might be adapted for enhancing foreign protein production in N. benthamiana as the host plant.

Transient plant production of Salmonella Typhimurium diagnostic antibodies

Salmonella Typhimurium is one of the most important zoonotic pathogens worldwide and a major cause of economic losses in the pig production chain. The emergence of multi-drug resistant strains over the past years has led to considerations about an enhanced surveillance of bacterial food contamination. Currently, ELISA is the method of choice for high throughput identification of S. Typhimurium. The sensitivity and specificity of this assay might be improved by application of new diagnostic antibodies. We focused on plant-based expression of candidate diagnostic TM43-E10 antibodies discovered using as antigen the S. Typhimurium OmpD protein. The scFv-TM43-E10 and scFv-Fc-TM43-E10 antibody derivatives have been successfully produced in N. benthamiana using a deconstructed movement-deficient PVX vector supplemented with the γb silencing suppressor from Poa semilatent virus. The plant-made antibodies showed the same antigen-binding specificity as that of the microbial/mammalian cell-produced counterparts and could recognize the OmpD antigen in S. Typhimurium infected plant samples.

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.

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.

Advances in genetic engineering of marine algae

Algae are a component of bait sources for animal aquaculture, and they produce abundant valuable compounds for the chemical industry and human health. With today's fast growing demand for algae biofuels and the profitable market for cosmetics and pharmaceuticals made from algal natural products, the genetic engineering of marine algae has been attracting increasing attention as a crucial systemic technology to address the challenge of the biomass feedstock supply for sustainable industrial applications and to modify the metabolic pathway for the more efficient production of high-value products. Nevertheless, to date, only a few marine algae species can be genetically manipulated. In this article, an updated account of the research progress in marine algal genomics is presented along with methods for transformation. In addition, vector construction and gene selection strategies are reviewed. Meanwhile, a review on the progress of bioreactor technologies for marine algae culture is also revisited.

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

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

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

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

Recombinase technology: applications and possibilities

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

Zinc finger nuclease-mediated transgene deletion

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

Cre-mediated seed-specific transgene excision in tobacco

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

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

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

Cre/lox system to develop selectable marker free transgenic tobacco plants conferring resistance against sap sucking homopteran insect

A binary expression vector was constructed containing the insecticidal gene Allium sativum leaf agglutinin (ASAL), and a selectable nptII marker gene cassette, flanked by lox sites. Similarly, another binary vector was developed with the chimeric cre gene construct. Transformed tobacco plants were generated with these two independent vectors. Each of the T(0) lox plants was crossed with T(0) Cre plants. PCR analyses followed by the sequencing of the target T-DNA part of the hybrid T(1) plants demonstrated the excision of the nptII gene in highly precised manner in certain percentage of the T(1) hybrid lines. The frequency of such marker gene excision was calculated to be 19.2% in the hybrids. Marker free plants were able to express ASAL efficiently and reduce the survivability of Myzus persiceae, the deadly pest of tobacco significantly, compared to the control tobacco plants. Results of PCR and Southern blot analyses of some of the T(2) plants detected the absence of cre as well as nptII genes. Thus, the crossing strategy involving Cre/lox system for the excision of marker genes appears to be very effective and easy to execute. Documentation of such marker excision phenomenon in the transgenic plants expressing the important insecticidal protein for the first time has a great significance from agricultural and biotechnological points of view.

Stable recombinase-mediated cassette exchange in Arabidopsis using Agrobacterium tumefaciens

Site-specific integration is an attractive method for the improvement of current transformation technologies aimed at the production of stable transgenic plants. Here, we present a Cre-based targeting strategy in Arabidopsis (Arabidopsis thaliana) using recombinase-mediated cassette exchange (RMCE) of transferred DNA (T-DNA) delivered by Agrobacterium tumefaciens. The rationale for effective RMCE is the precise exchange of a genomic and a replacement cassette both flanked by two heterospecific lox sites that are incompatible with each other to prevent unwanted cassette deletion. We designed a strategy in which the coding region of a loxP/lox5171-flanked bialaphos resistance (bar) gene is exchanged for a loxP/lox5171-flanked T-DNA replacement cassette containing the neomycin phosphotransferase (nptII) coding region via loxP/loxP and lox5171/lox5171 directed recombination. The bar gene is driven by the strong 35S promoter, which is located outside the target cassette. This placement ensures preferential selection of RMCE events and not random integration events by expression of nptII from this same promoter. Using root transformation, during which Cre was provided on a cotransformed T-DNA, 50 kanamycin-resistant calli were selected. Forty-four percent contained a correctly exchanged cassette based on PCR analysis, indicating the stringency of the selection system. This was confirmed for the offspring of five analyzed events by Southern-blot analysis. In four of the five analyzed RMCE events, there were no additional T-DNA insertions or they easily segregated, resulting in high-efficiency single-copy RMCE events. Our approach enables simple and efficient selection of targeting events using the advantages of Agrobacterium-mediated transformation.

Marker-free transgenic plants through genetically programmed auto-excision

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

Evaluation of CRE-mediated excision approaches in Arabidopsis thaliana

The ability of the CRE recombinase to catalyze excision of a DNA fragment flanked by directly repeated lox sites has been exploited to modify gene expression and proved to function well in particular case studies. However, very often variability in CRE expression and differences in efficiency of CRE-mediated recombination are observed. Here, various approaches were investigated to reproducibly obtain optimal CRE activity. CRE recombination was analyzed either by transforming the CRE T-DNA into plants containing a lox-flanked fragment or by transforming a T-DNA harboring a lox-flanked fragment into plants producing the CRE recombinase. Although somatic CRE-mediated excision of a lox-flanked fragment was obtained in all transformants, a variable amount of germline-transmitted deletions was found among different independent transformants, irrespective of the orientation of transformation. Also, the efficiency of CRE-mediated excision correlated well with the CRE mRNA level. In addition, CRE-mediated fragment excision was compared after floral dip and after root tissue transformation when transforming in a CRE-expressing background. Importantly, less CRE activity was needed to excise the lox-flanked fragment from the transferred T-DNA after root tissue transformation than after floral dip transformation. We hypothesize that this is correlated with the lower T-DNA copy number inserted during root transformation as compared to floral dip transformation.

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

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

Functionality of the beta/six site-specific recombination system in tobacco and Arabidopsis: a novel tool for genetic engineering of plant genomes

The beta recombinase is a member of the prokaryotic site-specific serine recombinases (invertase/resolvase family), which in the presence of a DNA bending cofactor can catalyse DNA deletions between two directly oriented 90-bp six recombination sites. We have examined here whether the beta recombinase can be expressed in plants and whether it displays in planta its specific catalytic activity excising DNA sequences that are flanked by six sites. In plant protoplasts, the enzyme could be expressed as a GFP-beta recombinase fusion which can localise to the cell nucleus. Beta recombinase stably expressed in tobacco plants can catalyse deletion of a spacer region that is flanked by directly oriented six sites and has been placed between promoter and a GUS reporter gene (preventing GUS expression). In transient transformation experiments, beta recombinase-mediated elimination of the spacer results in transcriptional induction of the GUS gene. Similarly, beta recombinase in stably double-transformed Arabidopsis plants deletes specifically the spacer region of a reporter construct that has been incorporated into the genome. In the segregating T1 generation, plants were identified that contain exclusively the recombined reporter construct. In summary, our results demonstrate that functional / recombinase can be expressed in plants and that the enzyme is suitable to precisely eliminate undesired sequences from plant genomes. Therefore, the beta/six recombination system (and presumably related recombinases) may become an attractive tool for plant genetic engineering.

Reactivation of an integrated disabled viral vector using a Cre-loxP recombination system in Arabidopsis thaliana

We developed an inactivated DNA replicon of Turnip Mosaic Virus (TuMV), which was reactivated by a recombination event based on the Cre-loxP system. Viral replication was prevented by the insertion of a translation terminator sequence flanked by two loxP sites at the junction of the P1-HCPro-coding genes. In vitro recombination was tested with purified Cre, which excised the floxed sequence from the TuMV DNA, leaving a single loxP site in the reactivated viral genome, and restored the open reading frame of the replicon. Arabidopsis thaliana plants were made transgenic for the inactivated TuMV replicon. Removal of the translation terminator sequence was achieved by the controlled expression of Cre. Delivery of the Cre recombinase to the transgenic plants was obtained by three methods: agroinfiltration, PVX-based production, or transgenic chemical-inducible expression. In each case, reactivation of TuMV replication was observed.

Self-excision of the antibiotic resistance gene nptII using a heat inducible Cre-loxP system from transgenic potato

Resistance to antibiotics mediated by selectable marker genes remains a powerful selection tool for transgenic event production. However, regulatory agencies and consumer concerns favor these to be eliminated from food crops. Several excision systems exist but none have been optimized or shown to be functional for clonally propagated crops. The excision of the nptII gene conferring resistance to kanamycin has been achieved here using a gene construct based on a heat-inducible cre gene producing a recombinase that eliminates cre and nptII genes flanked by two loxP sites. First-generation regenerants with the Cre-loxP system were obtained by selection on kanamycin media. Following a heat treatment, second generation regenerants were screened for excision by PCR using nptII, cre, and T-DNA borders primers. Excision efficiency appeared to be at 4.7% depending on the heat treatment. The footprint of the excision was shown by sequencing between T-DNA borders to correspond to a perfect recombination event. Selectable marker-free sprouts were also obtained from tubers of transgenic events when submitted to similar heat treatment at 4% frequency. Spontaneous excision was not observed out of 196 regenerants from untreated transgenic explants. Biosafety concerns are minimized because the expression of cre gene driven by the hsp70 promoter of Drosophila melanogaster was remarkably low even under heat activation and no functional loxP site were found in published Solanum sequence database. A new plant transformation vector pCIP54/55 was developed including a multiple cloning site and the self-excision system which should be a useful tool not only for marker genes in potato but for any gene or sequence removal in any plant.

Removal of the selectable marker gene from transgenic tobacco plants by expression of Cre recombinase from a tobacco mosaic virus vector through agroinfection

Selection markers are often indispensable during the process of plant transformation, but dispensable once transgenic plants have been established. The Cre/lox site-specific recombination system has been employed to eliminate selectable marker genes from transgenic plants. Here we describe the use of a movement function-improved Tobacco Mosaic Virus (TMV) vector, m30B, to express Cre recombinase for elimination of the selectable marker gene nptII from transgenic tobacco plants. The transgenic tobacco plants were produced by Agrobacterium-mediated transformation with a specially designed binary vector pGNG which contained in its T-DNA region a sequence complex of 35S promoter-lox-the gfp coding sequence-rbcS terminator-Nos promoter-nptII-Nos terminator-lox-the gus coding region-Nos terminator. The expression of the recombinant viral vector m30B:Cre in plant cells was achieved by placing the viral vector under the control of the 35S promoter and through agroinoculation. After co-cultivating the pGNG-leaf discs with agro35S-m30B:Cre followed by shoot regeneration without any selection, plants devoid of the lox-flanked sequences including nptII were obtained with an efficiency of about 34% as revealed by histochemical GUS assay of the regenerants. Three of 11 GUS expressing regenerants, derived from two independent transgenic lines containing single copy of the pGNG T-DNA, proved to be free of the lox-flanked sequences by Southern blot analysis. Excision of the lox-flanked sequences in the three plants could be attributed to transient expression of Cre from the viral vector at the early stage of co-cultivation, since the cre sequence could not be detected in the viral RNA molecules accumulated in the plants, nor in their genomic DNA. The parental marker-free genotype was inherited in their selfed progeny, and all of the progeny were virus-free, apparently because TMV is not seed-transmissible. Therefore, expression of Cre from a TMV-based vector could be used to eliminate selectable marker genes from transgenic tobacco plants without sexual crossing and segregation, and this strategy could be extended to other TMV-infected plant species and applicable to other compatible virus-host plant systems.

Biosafety considerations for selectable and scorable markers used in cassava (Manihot esculentaCrantz) biotechnology

Cassava is an important subsistence crop grown only in the tropics, and represents a major source of calories for many people in developing countries. Improvements in the areas of resistance to insects and viral diseases, enhanced nutritional qualities, reduced cyanogenic content and modified starch characteristics are urgently needed. Traditional breeding is hampered by the nature of the crop, which has a high degree of heterozygosity, irregular flowering, and poor seed set. Biotechnology has the potential to enhance crop improvement efforts, and genetic engineering techniques for cassava have thus been developed over the past decade. Selectable and scorable markers are critical to efficient transformation technology, and must be evaluated for biosafety, as well as efficiency and cost-effectiveness. In order to facilitate research planning and regulatory submission, the literature on biosafety aspects of the selectable and scorable markers currently used in cassava biotechnology is surveyed. The source, mode of action and current use of each marker gene is described. The potential for toxicity, allergenicity, pleiotropic effects, horizontal gene transfer, and the impact of these on food or feed safety and environmental safety is evaluated. Based on extensive information, the selectable marker genes nptII, hpt, bar/pat, and manA, and the scorable marker gene uidA, all have little risk in terms of biosafety. These appear to represent the safest options for use in cassava biotechnology available at this time.

Agroinfiltration as a tool for transient expression of cre recombinase in vivo

Agroinfiltration was used to express transiently cre recombinase from bacteriophage P1 in planta. Activation of gfp expression after cre-mediated excision of a bar intervening sequence served as a marker to monitor site-specific recombination events in lox-target N. benthamiana plants. Gfp expressing regenerants from A. tumefaciens infiltrated leaves were obtained with an efficiency of about 34%. In 20% of the regenerants bar gene excision was due to the expression of stably integrated cre gene, whereas in 14% of plants site-specific recombination was a consequence of transient cre expression. Phenotypic and molecular data indicated that the recombined state has been transferred to the T(1 )generation. These results demonstrate the suitability of agroinfiltration for the expression of cre recombinase in vivo.

Development and assessment of a potato virus X-based expression system with improved biosafety

Over the last decade, plant virus-based vectors have been developed and successfully exploited for high-yield production of heterologous proteins in plants. However, widespread application of recombinant viruses raises concerns about possible risks to the environment. One of the primary safety issues that must be considered is the uncontrolled spread of the genetically engineered virus from experimental plants to susceptible weeds or crops. Using a movement-deficient Potato virus X (PVX)-based transient gene expression vector which harbors the beta-glucuronidase (gus) gene, we established a plant viral expression system that provides containment of the recombinant virus and allows for safe and efficient protein production. By deletion of the viral 25k movement protein gene, systemic spread of the modified virus in non-transgenic Nicotiana benthamiana plants was successfully inhibited. In transgenic N. benthamiana plants expressing the 25K viral movement protein, this deficiency was complemented, thus resulting in systemic infection with the movement-deficient virus. While no differences in virus spread and accumulation were observed compared to infection caused by wild-type PVX in non-transgenic plants, the movement protein transgenic plants exhibited none of the normal symptoms of viral infection. Several biosafety aspects were investigated including the potential for recombination between the defective virus and the movement protein transgene, as well as complementation effects in non-transgenic plants doubly infected with the defective and the wild-type virus. Furthermore, the applicability of the safety system for the production of heterologous proteins was evaluated with gus as a model gene. With respect to the stability of the gus insert and the expression level of the GUS protein, there were no differences between the novel system developed and the conventional PVX-based expression system.