FLP-mediated recombination for use in hybrid plant production

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.

Glycosylphosphatidylinositol-Anchored Proteins in Arabidopsis and One of Their Common Roles in Signaling Transduction

Diverse proteins are found modified with glycosylphosphatidylinositol (GPI) at their carboxyl terminus in eukaryotes, which allows them to associate with membrane lipid bilayers and anchor on the external surface of the plasma membrane. GPI-anchored proteins (GPI-APs) play crucial roles in various processes, and more and more GPI-APs have been identified and studied. In this review, previous genomic and proteomic predictions of GPI-APs in Arabidopsis have been updated, which reveal their high abundance and complexity. From studies of individual GPI-APs in Arabidopsis, certain GPI-APs have been found associated with partner receptor-like kinases (RLKs), targeting RLKs to their subcellular localization and helping to recognize extracellular signaling polypeptide ligands. Interestingly, the association might also be involved in ligand selection. The analyses suggest that GPI-APs are essential and widely involved in signal transduction through association with RLKs.

Engineered selective plant male sterility through pollen-specific expression of the EcoRI restriction endonuclease

Unintended gene flow from transgenic plants via pollen, seed and vegetative propagation is a regulatory concern because of potential admixture in food and crop systems, as well as hybridization and introgression to wild and weedy relatives. Bioconfinement of transgenic pollen would help address some of these concerns and enable transgenic plant production for several crops where gene flow is an issue. Here, we demonstrate the expression of the restriction endonuclease EcoRI under the control of the tomato pollen-specific LAT52 promoter is an effective method for generating selective male sterility in Nicotiana tabacum (tobacco). Of nine transgenic events recovered, four events had very high bioconfinement with tightly controlled EcoRI expression in pollen and negligible-to-no expression other plant tissues. Transgenic plants had normal morphology wherein vegetative growth and reproductivity were similar to nontransgenic controls. In glasshouse experiments, transgenic lines were hand-crossed to both male-sterile and emasculated nontransgenic tobacco varieties. Progeny analysis of 16 000-40 000 seeds per transgenic line demonstrated five lines approached (>99.7%) or attained 100% bioconfinement for one or more generations. Bioconfinement was again demonstrated at or near 100% under field conditions where four transgenic lines were grown in close proximity to male-sterile tobacco, and 900-2100 seeds per male-sterile line were analysed for transgenes. Based upon these results, we conclude EcoRI-driven selective male sterility holds practical potential as a safe and reliable transgene bioconfinement strategy. Given the mechanism of male sterility, this method could be applicable to any plant species.

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.

Bioenergy grass feedstock: current options and prospects for trait improvement using emerging genetic, genomic, and systems biology toolkits

For lignocellulosic bioenergy to become a viable alternative to traditional energy production methods, rapid increases in conversion efficiency and biomass yield must be achieved. Increased productivity in bioenergy production can be achieved through concomitant gains in processing efficiency as well as genetic improvement of feedstock that have the potential for bioenergy production at an industrial scale. The purpose of this review is to explore the genetic and genomic resource landscape for the improvement of a specific bioenergy feedstock group, the C4 bioenergy grasses. First, bioenergy grass feedstock traits relevant to biochemical conversion are examined. Then we outline genetic resources available bioenergy grasses for mapping bioenergy traits to DNA markers and genes. This is followed by a discussion of genomic tools and how they can be applied to understanding bioenergy grass feedstock trait genetic mechanisms leading to further improvement opportunities.

phiC31 integrase-mediated site-specific recombination in barley

The Streptomyces phage phiC31 integrase was tested for its feasibility in excising transgenes from the barley genome through site-specific recombination. We produced transgenic barley plants expressing an active phiC31 integrase and crossed them with transgenic barley plants carrying a target locus for recombination. The target sequence involves a reporter gene encoding green fluorescent protein (GFP), which is flanked by the attB and attP recognition sites for the phiC31 integrase. This sequence disruptively separates a gusA coding sequence from an upstream rice actin promoter. We succeeded in producing site-specific recombination events in the hybrid progeny of 11 independent barley plants carrying the above target sequence after crossing with plants carrying a phiC31 expression cassette. Some of the hybrids displayed fully executed recombination. Excision of the GFP gene fostered activation of the gusA gene, as visualized in tissue of hybrid plants by histochemical staining. The recombinant loci were detected in progeny of selfed F(1), even in individuals lacking the phiC31 transgene, which provides evidence of stability and generative transmission of the recombination events. In several plants that displayed incomplete recombination, extrachromosomal excision circles were identified. Besides the technical advance achieved in this study, the generated phiC31 integrase-expressing barley plants provide foundational stock material for use in future approaches to barley genetic improvement, such as the production of marker-free transgenic plants or switching transgene activity.

DNA cleavage is independent of synapsis during Streptomyces phage phiBT1 integrase-mediated site-specific recombination

Bacteriophage-encoded serine recombinases have great potential in genetic engineering but their catalytic mechanisms have not been adequately studied. Integration of ϕBT1 and ϕC31 via their attachment (att) sites is catalyzed by integrases of the large serine recombinase subtype. Both ϕBT1 and ϕC31 integrases were found to cleave single-substrate att sites without synaptic complex formation, and ϕBT1 integrase relaxed supercoiled DNA containing a single integration site. Systematic mutation of the central att site dinucleotide revealed that cleavage was independent of nucleotide sequence, but rejoining was crucially dependent upon complementarity of the cleavage products. Recombination between att sites containing dinucleotides with antiparallel complementarity led to antiparallel recombination. Integrase-substrate pre-incubation experiments revealed that the enzyme can form an attP-integrase tetramer complex that then captures naked attB DNA, and suggested that two alternative assembly pathways can lead to synaptic complex formation.

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

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

Restoring pollen fertility in transgenic male-sterile eggplant by Cre/loxp-mediated site-specific recombination system

This study was designed to control plant fertility by cell lethal gene Barnase expressing at specific developmental stage and in specific tissue of male organ under the control of Cre/loxP system, for heterosis breeding, producing hybrid seed of eggplant. The Barnase-coding region was flanked by loxP recognition sites for Cre-recombinase. The eggplant inbred/pure line (‘E-38') was transformed with Cre gene and the inbred/pure line (‘E-8') was transformed with the Barnase gene situated between loxp. The experiments were done separately, by means of Agrobacterium co-culture. Four T₀ -plants with the Barnase gene were obtained, all proved to be male-sterile and incapable of producing viable pollen. Flowers stamens were shorter, but the vegetative phenotype was similar to wild-type. Five T ₀ -plants with the Cre gene developed well, blossomed out and set fruit normally. The crossing of male-sterile Barnase-plants with Cre expression transgenic eggplants resulted in site-specific excision with the male-sterile plants producing normal fruits. With the Barnase was excised, pollen fertility was fully restored in the hybrids. The phenotype of these restored plants was the same as that of the wild-type. Thus, the Barnase and Cre genes were capable of stable inheritance and expression in progenies of transgenic plants.

Transgene excision from wheat chromosomes by phage phiC31 integrase

The Streptomyces phage phiC31 integrase was tested for its ability to excise transgenic DNA from the wheat genome by site-specific recombination. Plants that stably express phiC31 integrase were crossed to plants carrying a target construct bearing the phiC31 recognition sites, attP and attB. In the progeny, phiC31 recombinase mediates recombination between the att sites of the target locus, which results in excision of the intervening DNA. Recombination events could be identified in 34 independent wheat lines by PCR and Southern blot analysis and by sequencing of the excision footprints. Recombinant loci were inherited to the subsequent generation. The results presented here establish the integrase-att system as a tool for catalysing the precise elimination of DNA sequences from wheat chromosomes.

Generation of marker-free transgenic maize with improved salt tolerance using the FLP/FRT recombination system

The possible release of selectable marker genes from genetically modified transgenic plants, or of gut microbes, to the environment, has raised worldwide public concerns. In this study, we showed the generation of marker-free transgenic maize plants constitutively expressing AtNHX1, a Na(+)/H(+) antiporter gene from Arabidopsis that conferred salt tolerance on plants, using the FLP/FRT site-specific recombination system. Transgenic plant expressing a modified FLP recombinase gene was crossed with transgenic plant harboring AtNHX1 and mutant als, a selectable marker gene flanked by two directed FRT sites. The sexual crossing led to precise and complete excision of the FRT-surrounded als marker gene in the F1 progenies. Further salt tolerance examinations indicated that marker-free AtNHX1 transgenic plants accumulated more Na(+) and K(+), and produced greater biomass and yields than did the wild-type plants when grown in high saline fields. These results demonstrate the feasibility of using this FLP/FRT-based marker elimination system to generate marker-free transgenic important cereal crops with improved salt tolerance.

Auto-excision of selectable marker genes from transgenic tobacco via a stress inducible FLP/FRT site-specific recombination system

Antibiotic resistance marker genes are powerful selection tools for use in plant transformation processes. However, once transformation is accomplished, the presence of these resistance genes is no longer necessary and can even be undesirable. We herein describe the successful excision of antibiotic resistance genes from transgenic plants via the use of an oxidative stress-inducible FLP gene. FLP encodes a recombinase that can eliminate FLP and hpt selection genes flanked by two FRT sites. During a transformation procedure in tobacco, transformants were obtained by selection on hygromycin media. Regenerants of the initial transformants were screened for selective marker excision in hydrogen peroxide supplemented media and both the FLP and hpt genes were found to have been eliminated. About 13-41% of regenerated shoots on hydrogen peroxide media were marker-free. This auto-excision system, mediated by the oxidative stress-inducible FLP/FRT system to eliminate a selectable marker gene can be very readily adopted and used to efficiently generate marker-free transgenic plants.

Expression of active Streptomyces phage phiC31 integrase in transgenic wheat plants

Site-specific recombination systems are becoming an important tool for the genetic modification of crop plants. Here we report the functional expression of the Streptomyces phage-derived phiC31 recombinase (integrase) in wheat. T-DNA constructs containing a phiC31 integrase transgene were stably transformed into wheat plants via particle gun bombardment. A plant-virus-based assay system was used to monitor the site-specific recombination activity of the recombinant integrase protein in vivo. We established several independent doubled haploid (DH) inbred lines that constitutively express an active integrase enzyme without any apparent detrimental effects on plant growth and development. The potential of phiC31 integrase expression in crop plants related to transgene control technologies or hybrid breeding systems is discussed.

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.

Highly efficient in vitro site-specific recombination system based on streptomyces phage phiBT1 integrase

The Streptomyces phage phiBT1 encodes a site-specific integrase of the large serine recombinase subfamily. In this report, the enzymatic activity of the phiBT1 integrase was characterized in vitro. We showed that this integrase has efficient integration activity with substrate DNAs containing attB and attP sites, independent of DNA supercoiling or cofactors. Both intra- and intermolecular recombinations proceed with rapid kinetics. The recombination is highly specific, and no reactions are observed between pairs of sites including attB and attL, attB and attR, attP and attL, or attP and attR or between two identical att sequences; however, a low but significant frequency of excision recombination between attL and attR is observed in the presence of the phiBT1 integrase alone. In addition, for efficient integration, the minimal sizes of attB and attP are 36 bp and 48 bp, respectively. This site-specific recombination system is efficient and simple to use; thus, it could have applications for the manipulation of DNA in vitro.

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

To excise a selectable marker gene from transgenic plants, a new binary expression vector based on the 'genetically modified (GM)-gene-deletor' system was constructed. In this vector, the gene coding for FLP site-specific recombinase under the control of a heat shock-inducible promoter HSP18.2 from Arabidopsis thaliana and isopentenyltransferase gene (ipt), as a selectable marker gene under the control of the cauliflower mosaic virus 35S (CaMV 35S) promoter, were flanked by two loxP/FRT fusion sequences as recombination sites in direct orientation. Histochemical staining for GUS activity showed that, upon induction by heat shock, all exogenous DNA, including the selectable marker gene ipt, beta-glucuronidase (gusA) gene and the FLP recombinase gene, between two loxP/FRT sites was eliminated efficiently from primary transgenic tobacco plants. Molecular analysis further confirmed that excision of the marker gene (ipt) was heritable and stable. Our approach provides a reliable strategy for auto-excising a selectable marker gene from calli, shoots or other tissues of transgenic plants after transformation and producing marker-free transgenic plants.

FLP recombinase-mediated site-specific recombination in rice

The feasibility of using the FLP/FRT site-specific recombination system in rice for genome engineering was evaluated. Transgenic rice plants expressing the FLP recombinase were crossed with plants harbouring the kanamycin resistance gene (neomycin phosphotransferase II, nptII) flanked by FRT sites, which also served to separate the corn ubiquitin promoter from a promoterless gusA. Hybrid progeny were tested for excision of the nptII gene and the positioning of the ubiquitin promoter proximal to gusA. While the hybrid progeny from various crosses exhibited beta-glucuronidase (GUS) expression, the progeny of selfed parental rice plants did not show detectable GUS activity. Despite the variable GUS expression and incomplete recombination displayed in hybrids from some crosses, uniform GUS staining and complete recombination were observed in hybrids from other crosses. The recombined locus was shown to be stably inherited by the progeny. These data demonstrate the operation of FLP recombinase in catalysing excisional DNA recombination in rice, and confirm that the FLP/FRT recombination system functions effectively in the cereal crop rice. Transgenic rice lines expressing active FLP recombinase generated in this study provide foundational stock material, thus facilitating the future application and development of the FLP/FRT system in rice genetic improvement.

pORE: a modular binary vector series suited for both monocot and dicot plant transformation

We present a series of 14 binary vectors suitable for Agrobacterium-mediated transformation of dicotyledonous plants and adaptable for biolistic transformation of monocotyledonous plants. The vector size has been minimized by eliminating all non-essential elements from the vector backbone and T-DNA regions while maintaining the ability to replicate independently. The smallest of the vector series is 6.3 kb and possesses an extensive multiple cloning site with 21 unique restriction endonuclease sites that are compatible with common cloning, protein expression, yeast two-hybrid and other binary vectors. The T-DNA region was engineered using a synthetic designer oligonucleotide resulting in an entirely modular system whereby any vector element can be independently exchanged. The high copy number ColE1 origin of replication has been included to enhance plasmid yield in Escherichia coli. FRT recombination sites flank the selectable marker cassette regions and allow for in planta excision by FLP recombinase. The pORE series consists of three basic types; an 'open' set for general plant transformation, a 'reporter' set for promoter analysis and an 'expression' set for constitutive expression of transgenes. The sets comprise various combinations of promoters (P (HPL), P (ENTCUP2) and P (TAPADH)), selectable markers (nptII and pat) and reporter genes (gusA and smgfp).

A Cre/loxP-mediated self-activating gene excision system to produce marker gene free transgenic soybean plants

Marker-gene-free transgenic soybean plants were produced by isolating a developmentally regulated embryo-specific gene promoter, app1, from Arabidopsis and developing a self-activating gene excision system using the P1 bacteriophage Cre/loxP recombination system. To accomplish this, the Cre recombinase gene was placed under control of the app1 promoter and, together with a selectable marker gene (hygromycin phosphotransferase), were cloned between two loxP recombination sites. This entire sequence was then placed between a constitutive promoter and a coding region for either beta-glucuronidase (Gus) or glyphosate acetyltransferase (Gat). Gene excision would remove the entire sequence between the two loxP sites and bring the coding region to the constitutive promoter for expression. Using this system marker gene excision occurred in over 30% of the stable transgenic events as indicated by the activation of the gus reporter gene or the gat gene in separate experiments. Transgenic plants with 1 or 2 copies of a functional excision-activated gat transgene and without any marker gene were obtained in T0 or T1 generation. This demonstrates the feasibility of using developmentally controlled promoters to mediate marker excision in soybean.

'GM-gene-deletor': fused loxP-FRT recognition sequences dramatically improve the efficiency of FLP or CRE recombinase on transgene excision from pollen and seed of tobacco plants

Pollen- and seed-mediated transgene flow is a concern in plant biotechnology. We report here a highly efficient 'genetically modified (GM)-gene-deletor' system to remove all functional transgenes from pollen, seed or both. With the three pollen- and/or seed-specific gene promoters tested, the phage CRE/loxP or yeast FLP/FRT system alone was inefficient in excising transgenes from tobacco pollen and/or seed, with no transgenic event having 100% efficiency. When loxP-FRT fusion sequences were used as recognition sites, simultaneous expression of both FLP and CRE reduced the average excision efficiency, but the expression of FLP or CRE alone increased the average excision efficiency, with many transgenic events being 100% efficient based on more than 25,000 T(1) progeny examined per event. The 'GM-gene-deletor' reported here may be used to produce 'non-transgenic' pollen and/or seed from transgenic plants and to provide a bioconfinement tool for transgenic crops and perennials, with special applicability towards vegetatively propagated plants and trees.

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.

RTS, a rice anther-specific gene is required for male fertility and its promoter sequence directs tissue-specific gene expression in different plant species

A tapetum-specific gene, RTS, has been isolated by differential screening of a cDNA library from rice panicles. RTS is a unique gene in the rice genome. RNA blot analysis and in situ hybridization indicates that this gene is predominantly expressed in the anther's tapetum during meiosis and disappears before anthesis. RTS has no introns and encodes a putative polypeptide of 94 amino acids with a hydrophobic N-terminal region. The nucleotide and deduced amino acid sequence of the gene do not show significant homology to any known sequences. However, a sequence in the promoter region, GAATTTGTTA, differs only by one or two nucleotides from one of the conserved motifs in the promoter region of two pollen-specific genes of tomato. Several other sequence motifs found in other anther-specific promoters were also identified in the promoter of the RTS gene. Transgenic and antisense RNA approaches revealed that RTS gene is required for male fertility in rice. The promoter region of RTS, when fused to the Bacillus amyloliquefaciens ribonuclease gene, barnase, or the antisense of the RTS gene, is able to drive tissue-specific expression of both genes in rice, creeping bentgrass (Agrostis stolonifera L.) and Arabidopsis, conferring male sterility to the transgenic plants. Light and near-infrared confocal microscopy of cross-sections through developing flowers of male-sterile transgenics shows that tissue-specific expression of barnase or the antisense RTS genes interrupts tapetal development, resulting in deformed non-viable pollen. These results demonstrate a critical role of the RTS gene in pollen development in rice and the versatile application of the RTS gene promoter in directing anther-specific gene expression in both monocotyledonous and dicotyledonous plants, pointing to a potential for exploiting this gene and its promoter for engineering male sterility for hybrid production of various plant species.

FLP-mediated site-specific recombination for genome modification in turfgrass

To develop molecular strategies for gene containment in genetically modified (GM) turfgrass, we have studied the feasibility of using the FLP/FRT site-specific DNA recombination system from yeast for controlled genome modification in turfgrass. Suspension cell cultures of creeping bentgrass (Agrostis stolonifera L.) and Kentucky bluegrass (Poa pratensis) were co-transformed with a FLP recombinase expression vector and a recombination-reporter test plasmid containing beta-glucuronidase (gusA) gene which was separated from the maize ubiquitin (ubi) promoter by an FRT-flanked blocking DNA sequence to prevent its transcription. GUS activity was observed in co-transformed cells, in which molecular analyses indicated that FLP-mediated excision of the blocking sequence had brought into proximity the upstream promoter and the downstream reporter gene, resulting in GUS expression. Functional evaluation of the FLP/FRT system using transgenic creeping bentgrass stably expressing FLP recombinase confirmed the observation in suspension cell culture. Our results indicate that FLP/FRT system is a useful tool for genetic manipulation of turfgrass, pointing to the great potential of exploiting the system to develop molecular strategies for transgene containment in perennials.

Site-specific recombination in Zea mays

The elimination of marker genes after selection is recommended for the commercial use of genetically modified plants. We compared the applicability of the two site-specific recombination systems Cre/lox and Flp/FRT for marker gene elimination in maize plants. The selection marker gene pat surrounded by two identically directed lox or FRT sites was introduced into maize. Sexual crossing with plants harboring the corresponding constitutively expressed recombinase led to the precise and complete excision of the lox-flanked marker gene in the F1 progeny, whereas Flp-mediated recombination of FRT sequences occurred rarely. Further examination of site-specific integration was done by biolistic bombardment of immature embryos harboring only one lox site with a lox.uidA sequence with results indicating directed integration.

Utility of the FLP-FRT recombination system for genetic manipulation of rice

To develop an FLP-FRT recombination system- (derived from 2 mu plasmid of Saccharomyces cerevisiae) based marker gene removal application for rice, we introduced the gene for FLP recombinase, under the control of the maize ubiquitin-1 promoter, into the rice genome. FLP activity was monitored in callus and regenerated plants by an assay based on the deletion of the FRT-flanked DNA fragment, leading to the activation of the beta-glucuronidase gene. FLP activity was detected both in the callus and leaves of some of the transgenic lines. Based on our comparison of the recombination efficiency of the FLP-FRT system expressed in the transgenic lines with that of the widely used Cre-lox system (derived from bacteriophage P1), we suggest that the FLP-FRT system is a useful tool for the genetic manipulation of rice.

Site-specific recombination in Arabidopsis plants promoted by the Integrase protein of coliphage HK022

The gene encoding the wild type Integrase protein of coliphage HK022 was integrated chromosomally and expressed in Arabidopsis thaliana plants. Double-transgenic plants cloned with the int gene as well as with a T-DNA fragment carrying the proper att sites in a tandem orientation showed that Int catalyzed a site-specific integration reaction (attP x attB) as well as a site-specific excision reaction (attL x attR). The reactions took place without the need to provide any of the accessory proteins that are required by Int in the bacterial host. When expressed in tobacco plants a GFP-Int fusion exhibits a predominant nuclear localization.

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

After the initial transformation and tissue culture process is complete, selectable marker genes, which are used in virtually all transformation approaches, are not required for the expression of the gene of interest in the transgenic plants. There are several advantages to removing the selectable marker gene after it is no longer needed, such as enabling the reuse of selectable markers and simplifying transgene arrays. We have tested the Cre/ lox system from bacteriophage P1 for its ability to precisely excise stably integrated marker genes from chromosomes in transgenic maize plants. Two strategies, crossing and autoexcision, have been tested and demonstrated. In the crossing strategy, plants expressing the Cre recombinase are crossed with plants bearing a transgene construct in which the selectable marker gene is flanked by directly repeated lox sites. Unlike previous reports in which incomplete somatic and germline excision were common, in our experiments complete somatic and germline marker gene excision occurred in the F(1) plants from most crosses with multiple independent Cre and lox lines. In the autoexcision strategy, the cre gene, under the control of a heat shock-inducible promoter, is excised along with the nptII marker gene. Our results show that a transient heat shock treatment of primary transgenic callus is sufficient for inducing cre and excising the cre and nptII genes. Genetic segregation and molecular analysis confirmed that marker gene removal is precise, complete and stable. The autoexcision strategy provides a way of removing the selectable marker gene from callus or other tissues such as embryos and kernels.

Expression of CCAAT-binding factor antisense transcripts in reproductive tissues affects plant fertility

We have previously isolated a CCAAT-binding factor B subunit gene ( BnCBF-B) from Brassica napus that is widely expressed in different plant tissues and whose role is still unknown. To investigate the importance of this transcription factor subunit in plant reproductive tissues, we targeted antisense BnCBF-B transcripts to the tapetum of transgenic B. napus plants. Of the 24 independent transformants, 13 yielded reduced quantities of viable pollen, of which five were unable to produce the elongated siliques indicative of normal seed set. The decrease in pollen viability probably resulted from the precocious degeneration of the tapetal cell layer observed in these plants. Surprisingly, the male-sterile phenotype was also accompanied by a decrease in female fertility, which could be due to the expression of the antisense BnCBF-B transcripts in the female reproductive structures of the transgenic plants. These results suggest that the BnCBF-B gene plays a critical non-redundant role in plant reproductive tissues.

A novel, two-component system for cell lethality and its use in engineering nuclear male-sterility in plants

Ablation of cells by the controlled expression of a lethal gene can be used to engineer plant traits such as male sterility and disease resistance. However, it may not be possible to achieve sufficient specificity of expression to prevent secondary effects in non-targeted tissues. In this paper we demonstrate that the extracellular ribonuclease, barnase, can be engineered into two complementary fragments, allowing overlapping promoter specificity to be used to enhance targeting specificity. Using a transient system, we first show that barnase can be split into two inactive peptide fragments, that when co-expressed can complement each other to reconstitute barnase activity. When a luciferase reporter gene was introduced into plant cells along with genes encoding both partial barnase peptides, a substantial reduction in luciferase activity was seen. Cytotoxicity of the reconstituted barnase was demonstrated by crossing together parents constitutively expressing each of the barnase fragments, then assaying their progeny for the presence of both partial barnase genes. None of over 300 tomato seeds planted resulted in a viable progeny that inherited both transgenes. When expression of the partial barnase genes was instead targeted to the tapetum, male sterility resulted. All 13 tomato progeny that inherited both transgenes were male sterile, whereas the three progeny inheriting only the N-terminal barnase gene were male fertile. Finally, we describe how male sterility generated by this type of two-component system can be used in hybrid seed production.

Excision of selectable marker genes from transgenic plants

Selectable marker genes are required to ensure the efficient genetic modification of crops. Economic incentives and safety concerns have prompted the development of several strategies (site-specific recombination, homologous recombination, transposition, and co-transformation) to eliminate these genes from the genome after they have fulfilled their purpose. Recently, chemically inducible site-specific recombinase systems have emerged as valuable tools for efficiently regulating the excision of transgenes when their expression is no longer required. The implementation of these strategies in crops and their further improvement will help to expedite widespread public acceptance of agricultural biotechnology

FLP/FRT-mediated restoration of normal phenotypes and clonal sectors formation in rolC transgenic tobacco

Site-specific recombination systems have been shown to excise transgene DNA sequences positioned between their cognate target sites, and thus be used to generate clonal sectors in transgenic plants. Here we characterized clonal sectors derived from genetic reversion of rolC (A. rhizogenes)--induced vegetative and reproductive phenotypes, mediated by FLP recombinase from S. cerevisiae, in tobacco. The constitutive expression of rolC induces pleiotropic effects including reduced apical dominance and plant height, lanceolate and pale green leaves and small, male-sterile flowers. Two transgenic male-sterile tobacco lines (N. tabacum, Samsun NN) expressing a 35sP-rolC gene construct flanked by two FRT (FLP recombinase target) sites, were cross-pollinated with pollen from a constitutive 35sP-FLP expressing line. Three main phenotypes were generated in result of recombinase-mediated excision of the 35sP-rolC locus in the F1 (FLP x FRT-35sP-rolC-FRT) hybrid progenies: (a) restoration of male fertility, associated with reversion to normal leaf phenotypes prior to flower bud formation, (b) development of normal and fertile lateral shoot sectors on the background of rolC-type plants, (c) restoration of partially fertile flowers, associated with display of peripheral normal leaf sectors surrounding rolC-type inner-leaf tissues, consistent with periclinal chimeras. These results, supported by DNA molecular analysis, indicate that site-specific recombination might be used as a relatively efficient tool for generation of transgenic periclinal chimeric plants.

Efficient gene activation in cultured mammalian cells mediated by FLP recombinase-expressing recombinant adenovirus

A recombinant adenovirus (rAd) expressing Cre recombinase derived from bacteriophage P1 has already been extensively used for the conditional gene activation and inactivation strategies in mammalian systems. In this study, we generated AxCAFLP, a rAd expressing FLP recombinase derived from Saccharomyces cerevisiae and carried out quantitative comparisons with Cre-expressing rAd in both in vitro and in cultured cells to provide another efficient gene regulation system in mammalian cells. In the in vitro experiments, the relative recombination efficiency of FLP expressed in 293 cells infected with FLP-expressing rAd was approximately one-thirtieth that of Cre even at 30 degrees C, the optimum temperature for FLP activity, and was approximately one-ninetieth at 37 degrees C. Co-infection experiments in HeLa cells using a target rAd conditionally expressing LacZ under the control of FLP showed that an FLP-expressing rAd, infected at a multiplicity of infection (MOI) of 5, was able to activate the transgene in almost 100% of HeLa cells whereas the Cre-expressing rAd was sufficient at an MOI of 0.2. Since an MOI of 5 is ordinarily used in rAd experiments, these results showed that the FLP-expressing rAd is useful for gene activation strategies and is probably applicable to a sequential gene regulation system in combination with Cre-expressing rAd in mammalian cells.