Excision of a selective marker in transgenic rice using a novel Cre/loxP system controlled by a floral specific promoter

Engineered Expression of Vip3A in Green Tissues as a Feasible Approach for the Control of Insect Pests in Maize

Genetic engineering technology offers opportunities to improve many important agronomic traits in crops, including insect-resistance. However, genetically modified (GM) exogenous proteins in edible tissues of transgenic crops has become an issue of intense public concern. To advance the application of GM techniques in maize, a Cre/loxP-based strategy was developed for manipulating the transgenes in green tissues while locking them in non-green tissues. In the strategy, the site-specific excision can be used to switch on or off the expression of transgenes at specific tissues. In this work, two basic transgenic maize, named KEY, carrying the Cre gene, and LOCK, containing the Vip3A gene with a blocked element, were obtained based on their separate fusion gene cassettes. The expression level and concentration of Vip3A were observed with a high specific accumulation in the green tissues (leaf and stem), and only a small amount was observed in the root and kernel tissues in the KEY × LOCK hybrids. The insect resistance of transgenic maize against two common lepidopteran pests, Ostrinia furnacalis and Spodoptera frugiperda, was assessed in the laboratory and field. The results indicate that the hybrids possessed high resistance levels against the two pests, with mortality rates above 73.6% and damage scales below 2.4 compared with the control group. Our results suggest that the Cre/loxP-mediated genetic engineering approach has a competitive advantage in GM maize. Overall, the findings from this study are significant for providing a feasible strategy for transgenes avoiding expression in edible parts and exploring novel techniques toward the biosafety of GM plants.

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.

A Prospective Review on Selectable Marker-Free Genome Engineered Rice: Past, Present and Future Scientific Realm

As a staple food crop, rice has gained mainstream attention in genome engineering for its genetic improvement. Genome engineering technologies such as transgenic and genome editing have enabled the significant improvement of target traits in relation to various biotic and abiotic aspects as well as nutrition, for which genetic diversity is lacking. In comparison to conventional breeding, genome engineering techniques are more precise and less time-consuming. However, one of the major issues with biotech rice commercialization is the utilization of selectable marker genes (SMGs) in the vector construct, which when incorporated into the genome are considered to pose risks to human health, the environment, and biodiversity, and thus become a matter of regulation. Various conventional strategies (co-transformation, transposon, recombinase systems, and MAT-vector) have been used in rice to avoid or remove the SMG from the developed events. However, the major limitations of these methods are; time-consuming, leftover cryptic sequences in the genome, and there is variable frequency. In contrast to these methods, CRISPR/Cas9-based marker excision, marker-free targeted gene insertion, programmed self-elimination, and RNP-based delivery enable us to generate marker-free engineered rice plants precisely and in less time. Although the CRISPR/Cas9-based SMG-free approaches are in their early stages, further research and their utilization in rice could help to break the regulatory barrier in its commercialization. In the current review, we have discussed the limitations of traditional methods followed by advanced techniques. We have also proposed a hypothesis, “DNA-free marker-less transformation” to overcome the regulatory barriers posed by SMGs.

An Efficient Marker Gene Excision Strategy Based on CRISPR/Cas9-Mediated Homology-Directed Repair in Rice

In order to separate transformed cells from non-transformed cells, antibiotic selectable marker genes are usually utilized in genetic transformation. After obtaining transgenic plants, it is often necessary to remove the marker gene from the plant genome in order to avoid regulatory issues. However, many marker-free systems are time-consuming and labor-intensive. Homology-directed repair (HDR) is a process of homologous recombination using homologous arms for efficient and precise repair of DNA double-strand breaks (DSBs). The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein-9 (Cas9) system is a powerful genome editing tool that can efficiently cause DSBs. Here, we isolated a rice promoter (Pssi) of a gene that highly expressed in stem, shoot tip and inflorescence, and established a high-efficiency sequence-excision strategy by using this Pssi to drive CRISPR/Cas9-mediated HDR for marker free (PssiCHMF). In our study, PssiCHMF-induced marker gene deletion was detected in 73.3% of T0 plants and 83.2% of T1 plants. A high proportion (55.6%) of homozygous marker-excised plants were obtained in T1 progeny. The recombinant GUS reporter-aided analysis and its sequencing of the recombinant products showed precise deletion and repair mediated by the PssiCHMF method. In conclusion, our CRISPR/Cas9-mediated HDR auto-excision method provides a time-saving and efficient strategy for removing the marker genes from transgenic plants.

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.

Generation of Marker-Free Transgenic Rice Resistant to Rice Blast Disease Using Ac/Ds Transposon-Mediated Transgene Reintegration System

Rice blast is one of the most serious diseases of rice and a major threat to rice production. Breeding disease-resistant rice is one of the most economical, safe, and effective measures for the control of rice blast. As a complement to traditional crop breeding, the transgenic method can avoid the time-consuming process of crosses and multi-generation selection. In this study, maize (Zea mays) Activator (Ac)/Dissociation (Ds) transposon vectors carrying green fluorescent protein (GFP) and red fluorescent protein (mCherry) genetic markers were used for generating marker-free transgenic rice. Double fluorescent protein-aided counterselection against the presence of T-DNA was performed together with polymerase chain reaction (PCR)-based positive selection for the gene of interest (GOI) to screen marker-free progeny. We cloned an RNAi expression cassette of the rice Pi21 gene that negatively regulates resistance to rice blast as a GOI into the Ds element in the Ac/Ds vector and obtained marker-free T1 rice plants from 13 independent transgenic lines. Marker-free and Ds/GOI-homozygous rice lines were verified by PCR and Southern hybridization analysis to be completely free of transgenic markers and T-DNA sequences. qRT-PCR analysis and rice blast disease inoculation confirmed that the marker-free transgenic rice lines exhibited decreased Pi21 expression levels and increased resistance to rice blast. TAIL-PCR results showed that the Ds (Pi21-RNAi) transgenes in two rice lines were reintegrated in intergenic regions in the rice genome. The Ac/Ds vector with dual fluorescent protein markers offers more reliable screening of marker-free transgenic progeny and can be utilized in the transgenic breeding of rice disease resistance and other agronomic traits.

Simultaneous resistance against the two viruses causing rice tungro disease using RNA interference

Rice tungro is the most important viral disease affecting rice in South and Southeast Asia, caused by two viruses rice tungro bacilliform virus (RTBV) and rice tungro spherical virus (RTSV). Transgenic resistance using RNA-interference (RNAi) has been reported individually against RTBV and RTSV earlier. Here we report the development of transgenic rice plants expressing RNAi against both RTBV and RTSV simultaneously. A DNA construct carrying 300 bp of RTBV DNA and 300 bp of RTSV cDNA were cloned as the two arms in hairpin orientation in a binary plasmid background to generate RNAi against both viruses simultaneously. Transgenic rice plants were raised using the above construct and their resistance against RTBV and RTSV was quantified at the T₁ plants. Levels of both the viral nucleic acids showed a fall of 100- to 500-fold in the above plants, compared with the non-transgenic controls, coupled with the amelioration of stunting. The transgenic plants also retained higher chlorophyll levels than the control non-transgenic plants after infection with RTBV and RTSV. Small RNA analysis of virus inoculated transgenic plants indicated the presence of 21 nt and 22 nt siRNAs specific to RTBV and RTSV. The evidence points towards an active RNAi mechanism leading to resistance against the tungro viruses in the plants analysed.

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.

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.

Split-Cre complementation restores combination activity on transgene excision in hair roots of transgenic tobacco

The Cre/loxP system is increasingly exploited for genetic manipulation of DNA in vitro and in vivo. It was previously reported that inactive ''split-Cre'' fragments could restore Cre activity in transgenic mice when overlapping co-expression was controlled by two different promoters. In this study, we analyzed recombination activities of split-Cre proteins, and found that no recombinase activity was detected in the in vitro recombination reaction in which only the N-terminal domain (NCre) of split-Cre protein was expressed, whereas recombination activity was obtained when the C-terminal (CCre) or both NCre and CCre fragments were supplied. We have also determined the recombination efficiency of split-Cre proteins which were co-expressed in hair roots of transgenic tobacco. No Cre recombination event was observed in hair roots of transgenic tobacco when the NCre or CCre genes were expressed alone. In contrast, an efficient recombination event was found in transgenic hairy roots co-expressing both inactive split-Cre genes. Moreover, the restored recombination efficiency of split-Cre proteins fused with the nuclear localization sequence (NLS) was higher than that of intact Cre in transgenic lines. Thus, DNA recombination mediated by split-Cre proteins provides an alternative method for spatial and temporal regulation of gene expression in transgenic plants.

Ectopic expression of OsMADS45 activates the upstream genes Hd3a and RFT1 at an early development stage causing early flowering in rice

BACKGROUND

The rice gene, OsMADS45, which belongs to the MADS-box E class gene, participates in the regulation of floral development. Previous studies have revealed that ectopic expression of OsMADS45 induces early flowering and influences reduced plant height under short-day (SD) conditions. However, the regulation mechanism of OsMADS45 overexpression remains unknown. We introduce an OsMADS45 overexpression construct Ubi:OsMADS45 into TNG67 plants (an Hd1 (Heading date 1) and Ehd1 (Early heading date 1) defective rice cultivar grown in Taiwan), and we analyzed the expression patterns of various floral regulators to understand the regulation pathways affected by OsMADS45 expression.

RESULTS

The transgenic rice exhibit a heading date approximately 40 days earlier than that observed in TNG67 plants, and transgenic rice display small plant size and low grain yield. OsMADS45 overexpression did not alter the oscillating rhythm of the examined floral regulatory genes but advanced (by approximately 20 days) the up-regulate of two florigens, Hd3a (Heading Date 3a) and RFT1 (RICE FLOWERING LOCUS T1) and suppressed the expression of Hd1 at the juvenile stage. The expression levels of OsMADS14 and OsMADS18, which are two well-known reproductive phase transition markers, were also increased at early developmental stages and are believed to be the major regulators responsible for early flowering in OsMADS45-overexpressing transgenic rice. OsMADS45 overexpression did not influence other floral regulator genes upstream of Hd1 and Ehd1, such as OsGI (OsGIGANTEA), Ehd2/Osld1/RID1 and OsMADS50.

CONCLUSION

These results indicate that in transgenic rice, OsMADS45 overexpressing ectopically activates the upstream genes Hd3a and RFT1 at early development stage and up-regulates the expression of OsMADS14 and OsMADS18, which induces early flowering.

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.

Tissue-specifically regulated site-specific excision of selectable marker genes in bivalent insecticidal, genetically-modified rice

Marker-free, genetically-modified rice was created by the tissue-specifically regulated Cre/loxP system, in which the Cre recombinase gene and hygromycin phosphotransferase gene (hpt) were flanked by two directly oriented loxP sites. Cre expression was activated by the tissue-specific promoter OsMADS45 in flower or napin in seed, resulting in simultaneous excision of the recombinase and marker genes. Segregation of T1 progeny was performed to select recombined plants. The excision was confirmed by PCR, Southern blot and sequence analyses indicating that efficiency varied from 10 to 53 % for OsMADS45 and from 12 to 36 % for napin. The expression of cry1Ac and vip3A was detected by RT-PCR analysis in marker-free transgenic rice. These results suggested that our tissue-specifically regulated Cre/loxP system could auto-excise marker genes from transgenic rice and alleviate public concerns about the security of GM crops.

Heat-shock-mediated elimination of the nptII marker gene in transgenic apple (Malus×domestica Borkh.)

Production of marker-free genetically modified (GM) plants is one of the major challenges of molecular fruit breeding. Employing clean vector technologies, allowing the removal of undesired DNA sequences from GM plants, this goal can be achieved. The present study describes the establishment of a clean vector system in apple Malus×domestica Borkh., which is based on the use of the neomycin phosphotransferase II gene (nptII) as selectable marker gene and kanamycin/paramomycin as selective agent. The nptII gene can be removed after selection of GM shoots via site-specific excision mediated by heat-shock-inducible expression of the budding yeast FLP recombinase driven by the soybean Gmhsp17.5-E promoter. We created a monitoring vector containing the nptII and the flp gene as a box flanked by two direct repeats of the flp recognition target (FRT) sites. The FRT-flanked box separates the gusA reporter gene from the Cauliflower Mosaic Virus 35S (CaMV 35S) promoter. Consequently, GUS expression does only occur after elimination of the FRT-flanked box. Transformation experiments using the monitoring vector resulted in a total of nine transgenic lines. These lines were investigated for transgenicity by PCR, RT-PCR and Southern hybridization. Among different temperature regimes tested, exposure to 42 °C for 3.5 to 4h led to efficient induction of FLP-mediated recombination and removal of the nptII marker gene. A second round of shoot regeneration from leaf explants led to GM apple plants completely free of the nptII gene.

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.

High-resolution genetic mapping and candidate gene identification of the SLP1 locus that controls glume development in rice

Stunted lemma palea 1 (slp1) is a rice mutant that displays dwarfism, shortened inflorescence lengths, severely degenerated lemmas/paleas, and sterility. The SLP1 locus was mapped between markers RM447 and D275 in the distal region of the long arm of chromosome 8, using the F2 progeny derived from the cross between the Slp1/slp1 mutant (Oryza sativa subsp. japonica) and the variety Taichung Native 1 (TN1, O. sativa subsp. indica). The SLP1 locus was further delimited to a 46.4-kb genomic region containing three putative genes: OsSPL16, OsMADS45, and OsMADS37. Comparisons of the sequence variations and expression levels of the three candidate genes between wild-type plants and homozygous slp1 mutants suggested that a missense mutation in the sixth amino acid of the OsSPL16 protein was likely responsible for the slp1 mutant phenotypes.

Recent advances in plant transformation

Plant genetic engineering has become one of the most important molecular tools in the modern molecular breeding of crops. Over the last decade, significant progress has been made in the development of new and efficient transformation methods in plants. Despite a variety of available DNA delivery methods, Agrobacterium- and biolistic-mediated transformation remain the two predominantly employed approaches. In particular, progress in Agrobacterium-mediated transformation of cereals and other recalcitrant dicot species has been quite remarkable. In the meantime, other transgenic-enabling technologies have emerged, including generation of marker-free transgenics, gene targeting, and chromosomal engineering. Although transformation of some plant species or elite germplasm remains a challenge, further advancement in transformation technology is expected because the mechanisms of governing the regeneration and transformation processes are now better understood and are being creatively applied to designing improved transformation methods or to developing new enabling technologies.

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.

Selectable antibiotic resistance marker gene-free transgenic rice harbouring the garlic leaf lectin gene exhibits resistance to sap-sucking planthoppers

Rice, the major food crop of world is severely affected by homopteran sucking pests. We introduced coding sequence of Allium sativum leaf agglutinin, ASAL, in rice cultivar IR64 to develop sustainable resistance against sap-sucking planthoppers as well as eliminated the selectable antibiotic-resistant marker gene hygromycin phosphotransferase (hpt) exploiting cre/lox site-specific recombination system. An expression vector was constructed containing the coding sequence of ASAL, a potent controlling agent against green leafhoppers (GLH, Nephotettix virescens) and brown planthopper (BPH, Nilaparvata lugens). The selectable marker (hpt) gene cassette was cloned within two lox sites of the same vector. Alongside, another vector was developed with chimeric cre recombinase gene cassette. Reciprocal crosses were performed between three single-copy T(0) plants with ASAL- lox-hpt-lox T-DNA and three single-copy T(0) plants with cre-bar T-DNA. Marker gene excisions were detected in T(1) hybrids through hygromycin sensitivity assay. Molecular analysis of T(1) plants exhibited 27.4% recombination efficiency. T(2) progenies of L03C04(1) hybrid parent showed 25% cre negative ASAL-expressing plants. Northern blot, western blot and ELISA showed significant level of ASAL expression in five marker-free T(2) progeny plants. In planta bioassay of GLH and BPH performed on these T(2) progenies exhibited radical reduction in survivability and fecundity compared with the untransformed control plants.

A novel mannose-based selection system for plant transformation using celery mannose-6-phosphate reductase gene

To investigate its potential application as a selectable marker for plant transformation, the mannitol producing, celery mannose-6-phosphate reductase gene (M6PR) was transformed into Arabidopsis and tobacco using Agrobacterium tumefaciens-mediated transformation. Mannose-tolerance assays in transgenic materials revealed that the M6PR can act as a selectable marker gene in either a positive or a negative selection mode depending on the plant species. For mannose sensitive species, such as Arabidopsis, expression of M6PR enhanced mannose tolerance and provided a positive selection for transgenic seeds. On medium containing 2 g/L mannose, transgenic seeds germinated, whereas wild type (WT) seeds did not. For mannose-tolerant species, expression of M6PR increased mannose sensitivity in tobacco and enabled a negative selection for transgenic leaves and seeds. Mannose at 30 g/L blanched leaf explants from all 29 transgenic tobacco events with M6PR. In contrast, 30 g/L mannose did not inhibit shoot regeneration from leaf explants of WT or transgenic plants with either an antisense M6PR or a plasmid control. Similarly, mannose at 30 g/L inhibited seed germination of transgenic tobacco seeds with M6PR but not that of WT or transgenic tobacco with either the antisense M6PR or the plasmid control. Northern blot confirmed transcripts of the M6PR in transgenic tobacco, and accumulation of mannitol verified activity of the M6PR in tobacco leaves. Either positive or negative selection using the celery M6PR is versatile for plant transformation. Additionally, the celery M6PR is a potential target gene for improving salt-tolerance in plants due to mannitol accumulation.

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.