High efficiency transgene segregation in co-transformed maize plants using an Agrobacterium tumefaciens 2 T-DNA binary system

Genomic consequences associated with Agrobacterium-mediated transformation of plants

Attenuated strains of the naturally occurring plant pathogen Agrobacterium tumefaciens can transfer virtually any DNA sequence of interest to model plants and crops. This has made Agrobacterium-mediated transformation (AMT) one of the most commonly used tools in agricultural biotechnology. Understanding AMT, and its functional consequences, is of fundamental importance given that it sits at the intersection of many fundamental fields of study, including plant-microbe interactions, DNA repair/genome stability, and epigenetic regulation of gene expression. Despite extensive research and use of AMT over the last 40 years, the extent of genomic disruption associated with integrating exogenous DNA into plant genomes using this method remains underappreciated. However, new technologies like long-read sequencing make this disruption more apparent, complementing previous findings from multiple research groups that have tackled this question in the past. In this review, we cover progress on the molecular mechanisms involved in Agrobacterium-mediated DNA integration into plant genomes. We also discuss localized mutations at the site of insertion and describe the structure of these DNA insertions, which can range from single copy insertions to large concatemers, consisting of complex DNA originating from different sources. Finally, we discuss the prevalence of large-scale genomic rearrangements associated with the integration of DNA during AMT with examples. Understanding the intended and unintended effects of AMT on genome stability is critical to all plant researchers who use this methodology to generate new genetic variants.

Effect of marker-free transgenic Chlamydomonas on the control of Aedes mosquito population and on plankton

BACKGROUND

More than half of the world's population suffers from epidemic diseases that are spread by mosquitoes. The primary strategy used to stop the spread of mosquito-borne diseases is vector control. Interference RNA (RNAi) is a powerful tool for controlling insect populations and may be less susceptible to insect resistance than other strategies. However, public concerns have been raised because of the transfer of antibiotic resistance marker genes to environmental microorganisms after integration into the recipient genome, thus allowing the pathogen to acquire resistance. Therefore, in the present study, we modified the 3-hydroxykynurenine transaminase (3hkt) and hormone receptor 3 (hr3) RNAi vectors to remove antibiotic resistance marker genes and retain the expression cassette of the inverse repeat sequence of the 3hkt/hr3 target gene. This recombinant microalgal marker-free RNAi insecticide was subsequently added to the suburban water in a simulated-field trial to test its ability to control mosquito population.

METHODS

The expression cassette of the 3hkt/hr3 inverted repeat sequence and a DNA fragment of the argininosuccinate lyase gene without the ampicillin resistance gene were obtained using restriction enzyme digestion and recovery. After the cotransformation of Chlamydomonas, the recombinant algae was then employed to feed Aedes albopictus larvae. Ten and 300 larvae were used in small- and large-scale laboratory Ae.albopictus feeding trials, respectively. Simulated field trials were conducted using Meishe River water that was complemented with recombinant Chlamydomonas. Moreover, the impact of recombinant microalgae on phytoplankton and zooplankton in the released water was explored via high-throughput sequencing.

RESULTS

The marker-free RNAi-recombinant Chlamydomonas effectively silenced the 3hkt/hr3 target gene, resulting in the inhibition of Ae. albopictus development and also in the high rate of Ae. albopictus larvae mortality in the laboratory and simulated field trials. In addition, the results confirmed that the effect of recombinant Chlamydomonas on plankton in the released water was similar to that of the nontransgenic Chlamydomonas, which could reduce the abundance and species of plankton.

CONCLUSIONS

The marker-free RNAi-recombinant Chlamydomonas are highly lethal to the Ae. albopictus mosquito, and their effect on plankton in released water is similar to that of the nontransgenic algal strains, which reduces the abundance and species of plankton. Thus, marker-free recombinant Chlamydomonas can be used for mosquito biorational control and mosquito-borne disease prevention.

Maize Interveinal Chlorosis 1 links the Yang Cycle and Fe homeostasis through Nicotianamine biosynthesis

The Yang cycle is involved in many essential metabolic pathways in plant growth and development. As extended products of the Yang cycle, the function and regulation network of ethylene and polyamines are well characterized. Nicotianamine (NA) is also a product of this cycle and works as a key metal chelator for iron (Fe) homeostasis in plants. However, interactions between the Yang cycle and NA biosynthesis remain unclear. Here, we cloned maize interveinal chlorosis 1 (mic1), encoding a 5'-methylthioadenosine nucleosidase (MTN), that is essential for 5'-methylthioadenosine (MTA) salvage and NA biosynthesis in maize (Zea mays). A single base G-A transition in the fourth exon of mic1 causes a Gly to Asp change, resulting in increased MTA, reduced Fe distribution, and growth retardation of seedlings. Knockout of ZmMIC1 but not its paralog ZmMTN2 by CRISPR/Cas9 causes interveinal chlorosis, indicating ZmMIC1 is mainly responsible for MTN activity in maize. Transcriptome analysis showed a typical response of Fe deficiency. However, metabolic analysis revealed dramatically reduced NA content in mic1, suggesting NA biosynthesis was impaired in the mutant. Exogenous application of NA transiently reversed the interveinal chlorosis phenotype of mic1 seedlings. Moreover, the mic1 mutant overexpressing a NA synthase gene not only recovered from interveinal chlorosis and growth retardation but was also fertile. These findings provide a link between the Yang cycle and NA biosynthesis, which highlights an aspect of Fe homeostasis regulation in maize.

ptxD/Phi as alternative selectable marker system for genetic transformation for bio-safety concerns: a review

Antibiotic and herbicide resistance genes are the most common marker genes for plant transformation to improve crop yield and food quality. However, there is public concern about the use of resistance marker genes in food crops due to the risk of potential gene flow from transgenic plants to compatible weedy relatives, leading to the possible development of “superweeds” and antibiotic resistance. Several selectable marker genes such as aph, nptII, aaC3, aadA, pat, bar, epsp and gat, which have been synthesized to generate transgenic plants by genetic transformation, have shown some limitations. These marker genes, which confer antibiotic or herbicide resistance and are introduced into crops along with economically valuable genes, have three main problems: selective agents have negative effects on plant cell proliferation and differentiation, uncertainty about the environmental effects of many selectable marker genes, and difficulty in performing recurrent transformations with the same selectable marker to pyramid desired genes. Recently, a simple, novel, and affordable method was presented for plant cells to convert non-metabolizable phosphite (Phi) to an important phosphate (Pi) for developing cells by gene expression encoding a phosphite oxidoreductase (PTXD) enzyme. The ptxD gene, in combination with a selection medium containing Phi as the sole phosphorus (P) source, can serve as an effective and efficient system for selecting transformed cells. The selection system adds nutrients to transgenic plants without potential risks to the environment. The ptxD/Phi system has been shown to be a promising transgenic selection system with several advantages in cost and safety compared to other antibiotic-based selection systems. In this review, we have summarized the development of selection markers for genetic transformation and the potential use of the ptxD/Phi scheme as an alternative selection marker system to minimize the future use of antibiotic and herbicide marker genes.

Current progress and challenges in crop genetic transformation

Plant transformation remains the most sought-after technology for functional genomics and crop genetic improvement, especially for introducing specific new traits and to modify or recombine already existing traits. Along with many other agricultural technologies, the global production of genetically engineered crops has steadily grown since they were first introduced 25 years ago. Since the first transfer of DNA into plant cells using Agrobacterium tumefaciens, different transformation methods have enabled rapid advances in molecular breeding approaches to bring crop varieties with novel traits to the market that would be difficult or not possible to achieve with conventional breeding methods. Today, transformation to produce genetically engineered crops is the fastest and most widely adopted technology in agriculture. The rapidly increasing number of sequenced plant genomes and information from functional genomics data to understand gene function, together with novel gene cloning and tissue culture methods, is further accelerating crop improvement and trait development. These advances are welcome and needed to make crops more resilient to climate change and to secure their yield for feeding the increasing human population. Despite the success, transformation remains a bottleneck because many plant species and crop genotypes are recalcitrant to established tissue culture and regeneration conditions, or they show poor transformability. Improvements are possible using morphogenetic transcriptional regulators, but their broader applicability remains to be tested. Advances in genome editing techniques and direct, non-tissue culture-based transformation methods offer alternative approaches to enhance varietal development in other recalcitrant crops. Here, we review recent developments in plant transformation and regeneration, and discuss opportunities for new breeding technologies in agriculture.

Comparison of three Agrobacterium-mediated co-transformation methods for generating marker-free transgenic Brassica napus plants

BACKGROUND

Generation of marker-free transgenic plants is very important to the regulatory permission and commercial release of transgenic crops. Co-transformation methods that enable the removal of selectable marker genes have been extensively used because they are simple and clean. Few comparisons are currently available between different strain/plasmid co-transformation systems, and also data are related to variation in co-transformation frequencies caused by other details of the vector design.

RESULTS

In this study, we constructed three vector systems for the co-transformation of allotetraploid Brassica napus (B. napus) mediated by Agrobacterium tumefaciens and compared these co-transformation methods. We tested a mixed-strain system, in which a single T-DNA is harbored in two plasmids, as well as two "double T-DNA" vector systems, in which two independent T-DNAs are harbored in one plasmid in a tandem orientation or in an inverted orientation. As confirmed by the use of PCR analysis, test strips, and Southern blot, the average co-transformation frequencies from these systems ranged from 24 to 81% in T₀ plants, with the highest frequency of 81% for 1:1 treatment of the mixed-strain system. These vector systems are valuable for generating marker-free transgenic B. napus plants, and marker-free plants were successfully obtained in the T₁ generation from 50 to 77% of T₀ transgenic lines using these systems, with the highest frequency of 77% for "double T-DNA" vector systems of pBID RT Enhanced. We further found that marker-free B. napus plants were more frequently encountered in the progeny of transgenic lines which has only one or two marker gene copies in the T₀ generation. Two types of herbicide resistant transgenic B. napus plants, Bar ⁺ with phosphinothricin resistance and Bar ⁺ EPSPS ⁺ GOX ⁺ with phosphinothricin and glyphosate resistance, were obtained.

CONCLUSION

We were successful in removing selectable marker genes in transgenic B. napus plants using all three co-transformation systems developed in this study. It was proved that if a appropriate mole ratio was designed for the specific length ratio of the twin T-DNAs for the mixed-strain method, high unlinked co-insertion frequency and overall success frequency could be achieved. Our study provides useful information for the construction of efficient co-transformation system for marker-free transgenic crop production and developed transgenic B. napus with various types of herbicide resistance.

Recent developments and applications of genetic transformation and genome editing technologies in wheat

Wheat (Triticum aestivum) is a staple crop across the world and plays a remarkable role in food supplying security. Over the past few decades, basic and applied research on wheat has lagged behind other cereal crops due to the complex and polyploid genome and difficulties in genetic transformation. A breakthrough called as PureWheat was made in the genetic transformation of wheat in 2014 in Asia, leading to a noticeable progress of wheat genome editing. Due to this great achievement, it is predicated that wheat biotechnology revolution is arriving. Genome editing technologies using zinc finger nucleases, transcription activator-like effector nuclease, and clustered regularly interspaced short palindromic repeats-associated endonucleases (CRISR/Cas) are becoming powerful tools for crop modification which can help biologists and biotechnologists better understand the processes of mutagenesis and genomic alteration. Among the three genome editing systems, CRISR/Cas has high specificity and activity, and therefore it is widely used in genetic engineering. Generally, the genome editing technologies depend on an efficient genetic transformation system. In this paper, we summarize recent progresses and applications on genetic transformation and genome editing in wheat. We also examine the future aspects of genetic transformation and genome editing. We believe that the technologies for wheat efficient genetic engineering and functional studies will become routine with the emergence of high-quality genomic sequences.

An optimized double T-DNA binary vector system for improved production of marker-free transgenic tobacco plants

OBJECTIVES

In the plant transformation process, marker genes play a vital role in identifying transformed cells from non-transformed cells. However, once transgenic plants have been obtained, the presence of marker genes may provoke public concern about environmental or biosafety issues. In our previous study, a double T-DNA vector system has been developed to obtain marker-free transgenic plants, but the T-DNA left border (LB) and right border (RB) of the vector showed an RB-LB-RB-LB pattern and led to high linkage integration between the selectable marker gene (SMG) and the gene of interest (GOI). To improve this double T-DNA vector system, we inverted the first T-DNA direction such that a LB-RB-RB-LB pattern resulted to avoid transcriptional read-through at the LB and the subsequent linkage transfer of the SMG and GOI.

RESULTS

We separately inserted the green fluorescent protein (GFP) gene as the GOI and the neomycin phosphotransferase II (NPTII) gene as the SMG in both optimized and original vectors and carried out Agrobacterium-mediated tobacco transformation. Statistical analysis revealed that the linkage frequency was 25.6% in T0 plants transformed with the optimized vector, which is a 42.1% decrease compared with that of the original vector (44.2%). The frequency of obtaining marker-free transgenic plants was 66.7% in T1 plants transformed with the optimized vector, showing a 33.4% increase compared with that of the original vector (50.0%).

CONCLUSION

Our results demonstrate that the optimized double T-DNA binary vector system is a more effective, economical and time-saving approach for obtaining marker-free transgenic plants.

Genome-scale mining of root-preferential genes from maize and characterization of their promoter activity

BACKGROUND

Modification of root architecture and improvement of root resistance to stresses can increase crop productivity. Functional analyses of root-specific genes are necessary for root system improvement, and root-specific promoters enable research into the regulation of root development and genetic manipulation of root traits. Maize is an important crop species; however, little systematic mining of root-specific genes and promoters has been performed to date.

RESULTS

Genomic-scale mining based on microarray data sets followed by transcript detection resulted in the identification of 222 root-specific genes. Gene Ontology enrichment analyses revealed that these 222 root-specific genes were mainly involved in responses to chemical, biotic, and abiotic stresses. Of the 222 genes, 33 were verified by quantitative reverse transcription polymerase chain reaction, and 31 showed root-preferential activity. About 2 kb upstream 5 of the 31 identified root-preferential genes were cloned from the maize genome as putative promoters and named p8463, p5023, p1534, p8531 and p6629. GUS staining of transgenic maize-derived promoter-GUS constructs revealed that the five promoters drove GUS expression in a root-preferential manner.

CONCLUSIONS

We mined root-preferential genes and their promoters in maize and verified p8463, p5023, p1534, p8531 and p6629 as root-preferential promoters. Our research enables the identification of other tissue-specific genes and promoters in maize and other species. In addition, the five promoters may enable enhancement of target gene(s) of maize in a root-preferential manner to generate novel maize cultivars with resistance to water, fertilizer constraints, or biotic stresses.

High efficiency Agrobacterium-mediated site-specific gene integration in maize utilizing the FLP-FRT recombination system

An efficient Agrobacterium-mediated site-specific integration (SSI) technology using the flipase/flipase recognition target (FLP/FRT) system in elite maize inbred lines is described. The system allows precise integration of a single copy of a donor DNA flanked by heterologous FRT sites into a predefined recombinant target line (RTL) containing the corresponding heterologous FRT sites. A promoter-trap system consisting of a pre-integrated promoter followed by an FRT site enables efficient selection of events. The efficiency of this system is dependent on several factors including Agrobacterium tumefaciens strain, expression of morphogenic genes Babyboom (Bbm) and Wuschel2 (Wus2) and choice of heterologous FRT pairs. Of the Agrobacterium strains tested, strain AGL1 resulted in higher transformation frequency than strain LBA4404 THY- (0.27% vs. 0.05%; per cent of infected embryos producing events). The addition of morphogenic genes increased transformation frequency (2.65% in AGL1; 0.65% in LBA4404 THY-). Following further optimization, including the choice of FRT pairs, a method was developed that achieved 19%-22.5% transformation frequency. Importantly, >50% of T0 transformants contain the desired full-length site-specific insertion. The frequencies reported here establish a new benchmark for generating targeted quality events compatible with commercial product development.

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

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

Improved folate accumulation in genetically modified maize and wheat

Folates are indispensable co-factors for one-carbon metabolism in all organisms. In humans, suboptimal folate intake results in serious disorders. One promising strategy for improving human folate status is to enhance folate levels in food crops by metabolic engineering. In this study, we cloned two GmGCHI (GTP cyclohydrolase I) genes (Gm8gGCHI and Gm3gGCHI) and one GmADCS (aminodeoxychorismate synthase) gene from soybean, which are responsible for synthesizing the folate precursors pterin and p-aminobenzoate, respectively. We initially confirmed their functions in transgenic Arabidopsis plants and found that Gm8gGCHI increased pterin and folate production more than Gm3gGCHI did. We then co-expressed Gm8gGCHI and GmADCS driven by endosperm-specific promoters in maize and wheat, two major staple crops, to boost their folate metabolic flux. A 4.2-fold and 2.3-fold increase in folate levels were observed in transgenic maize and wheat grains, respectively. To optimize wheat folate enhancement, codon-optimized Gm8gGCHI and tomato LeADCS genes under the control of a wheat endosperm-specific glutenin promoter (1Dx5) were co-transformed. This yielded a 5.6-fold increase in folate in transgenic wheat grains (Gm8gGCHI+/LeADCS+). This two-gene co-expression strategy therefore has the potential to greatly enhance folate levels in maize and wheat, thus improving their nutritional value.

Using Morphogenic Genes to Improve Recovery and Regeneration of Transgenic Plants

Efficient transformation of numerous important crops remains a challenge, due predominantly to our inability to stimulate growth of transgenic cells capable of producing plants. For years, this difficulty has been partially addressed by tissue culture strategies that improve regeneration either through somatic embryogenesis or meristem formation. Identification of genes involved in these developmental processes, designated here as morphogenic genes, provides useful tools in transformation research. In species from eudicots and cereals to gymnosperms, ectopic overexpression of genes involved in either embryo or meristem development has been used to stimulate growth of transgenic plants. However, many of these genes produce pleiotropic deleterious phenotypes. To mitigate this, research has been focusing on ways to take advantage of growth-stimulating morphogenic genes while later restricting or eliminating their expression in the plant. Methods of controlling ectopic overexpression include the use of transient expression, inducible promoters, tissue-specific promoters, and excision of the morphogenic genes. These methods of controlling morphogenic gene expression have been demonstrated in a variety of important crops. Here, we provide a review that highlights how ectopic overexpression of genes involved in morphogenesis has been used to improve transformation efficiencies, which is facilitating transformation of numerous recalcitrant crops. The use of morphogenic genes may help to alleviate one of the bottlenecks currently slowing progress in plant genome modification.

Generation of marker-free transgenic hexaploid wheat via an Agrobacterium-mediated co-transformation strategy in commercial Chinese wheat varieties

Genotype specificity is a big problem lagging the development of efficient hexaploid wheat transformation system. Increasingly, the biosecurity of genetically modified organisms is garnering public attention, so the generation of marker-free transgenic plants is very important to the eventual potential commercial release of transgenic wheat. In this study, 15 commercial Chinese hexaploid wheat varieties were successfully transformed via an Agrobacterium-mediated method, with efficiency of up to 37.7%, as confirmed by the use of Quickstix strips, histochemical staining, PCR analysis and Southern blotting. Of particular interest, marker-free transgenic wheat plants from various commercial Chinese varieties and their F₁ hybrids were successfully obtained for the first time, with a frequency of 4.3%, using a plasmid harbouring two independent T-DNA regions. The average co-integration frequency of the gus and the bar genes located on the two independent T-DNA regions was 49.0% in T₀ plants. We further found that the efficiency of generating marker-free plants was related to the number of bar gene copies integrated in the genome. Marker-free transgenic wheat plants were identified in the progeny of three transgenic lines that had only one or two bar gene copies. Moreover, silencing of the bar gene was detected in 30.7% of T₁ positive plants, but the gus gene was never found to be silenced in T₁ plants. Bisulphite genomic sequencing suggested that DNA methylation in the 35S promoter of the bar gene regulatory region might be the main reason for bar gene silencing in the transgenic plants.

Advances in Maize Transformation Technologies and Development of Transgenic Maize

Maize is the principal grain crop of the world. It is also the crop where genetic engineering has been employed to a great extent to improve its various traits. The ability to transform maize is a crucial step for application of gene technology in maize improvement. There have been constant improvements in the maize transformation technologies over past several years. The choice of genotype and the explant material to initiate transformation and the different types of media to be used in various stages of tissue culture can have significant impact on the outcomes of the transformation efforts. Various methods of gene transfer, like the particle bombardment, protoplast transformation, Agrobacterium-mediated, in planta transformation, etc., have been tried and improved over years. Similarly, various selection systems for retrieval of the transformants have been attempted. The commercial success of maize transformation and transgenic development is unmatched by any other crop so far. Maize transformation with newer gene editing technologies is opening up a fresh dimension in transformation protocols and work-flows. This review captures the various past and recent facets in improvement in maize transformation technologies and attempts to present a comprehensive updated picture of the current state of the art in this area.

Generation of Marker- and/or Backbone-Free Transgenic Wheat Plants via Agrobacterium-Mediated Transformation

Horizontal transfer of antibiotic resistance genes to animals and vertical transfer of herbicide resistance genes to the weedy relatives are perceived as major biosafety concerns in genetically modified (GM) crops. In this study, five novel vectors which used gusA and bar as a reporter gene and a selection marker gene, respectively, were constructed based on the pCLEAN dual binary vector system. Among these vectors, 1G7B and 5G7B carried two T-DNAs located on two respective plasmids with 5G7B possessing an additional virGwt gene. 5LBTG154 and 5TGTB154 carried two T-DNAs in the target plasmid with either one or double right borders, and 5BTG154 carried the selectable marker gene on the backbone outside of the T-DNA left border in the target plasmid. In addition, 5BTG154, 5LBTG154, and 5TGTB154 used pAL154 as a helper plasmid which contains Komari fragment to facilitate transformation. These five dual binary vector combinations were transformed into Agrobacterium strain AGL1 and used to transform durum wheat cv Stewart 63. Evaluation of the co-transformation efficiencies, the frequencies of marker-free transgenic plants, and integration of backbone sequences in the obtained transgenic lines indicated that two vectors (5G7B and 5TGTB154) were more efficient in generating marker-free transgenic wheat plants with no or minimal integration of backbone sequences in the wheat genome. The vector series developed in this study for generation of marker- and/or backbone-free transgenic wheat plants via Agrobacterium-mediated transformation will be useful to facilitate the creation of "clean" GM wheat containing only the foreign genes of agronomic importance.

A super twin T-DNA vector that allows independent gene expression during Agrobacterium-mediated transformation

In this study, we designed and constructed a super twin T-DNA vector (pTRIDT313-g) containing two independent T-DNA cassettes-one for the selection gene Hyg and the other for the target gene Gus-to produce marker-free transgenic lines. The resulting vector was transformed into tobacco, and polymerase chain reaction (PCR) analysis showed four types of gene combinations in the T1 and T2 generations: Gus only, Hyg only, Gus+Hyg, and untransformed lines. The intermediate region from the T-DNA of the right border of Hyg to the left border of Gus in the Hyg and Gus lines was not amplified. Genome walking confirmed that the Hyg and Gus T-DNA cassettes were independently inserted in different regions of the tobacco genome. Thus, the two T-DNA cassettes were integrated randomly as independent loci into the tobacco genome. The results of reverse transcription-PCR indicated that Hyg could normally be expressed in the roots, stems, and leaves of transgenic lines, and the resistance test showed that all Hyg transgenic lines could grow in the presence of 50mg/L hygromycin. All Gus transgenic lines showed obvious blue coloration in enzyme activity tests, indicating that the Gus gene could be normally expressed in all the lines. Therefore, the super twin T-DNA vector (pTRIDT313-g) exhibits independent integration, heredity, and normal gene function from two T-DNA cassettes. This vector could be a useful and valuable tool in the production of marker-free transgenic lines.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

TALEN-Based Mutagenesis of Lipoxygenase LOX3 Enhances the Storage Tolerance of Rice (Oryza sativa) Seeds

The deterioration of rice grain reduces the quality of rice, resulting in serious economic losses for farmers. Lipoxygenases (LOXs) catalyze the dioxygenation of polyunsaturated fatty acids with at least one cis,cis-1,4-pentadiene to form hydroperoxide, which is a major factor influencing seed longevity and viability. Recently, genome editing, an essential tool employed in reverse genetics, has been used experimentally to investigate basic plant biology or to modify crop plants for the improvement of important agricultural traits. In this study, we performed targeted mutagenesis in rice using transcription activator-like effector nucleases (TALENs) to improve seed storability. A modified ligation-independent cloning method (LIC) was employed to allow for the quick and efficient directional insertion of TALEN monomer modules into destination vectors used in plants. We demonstrated the feasibility and flexibility of the technology by developing a set of modular vectors for genome editing. After construction and validation, the TALEN pairs were used to create stable transgenic rice lines via Agrobacterium-mediated transformation. One heterozygous mutant (4%) was recovered from 25 transgenic NPTII-resistant lines, and the mutation was transmitted to the next generation. Further molecular and protein level experiments verified LOX3 deficiency and demonstrated the improvement of seed storability. Our work provides a flexible genome editing tool for improving important agronomic traits, as well as direct evidence that Lox3 has only a limited impact on seed longevity.

Maize transformation technology development for commercial event generation

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

Current issues in cereal crop biodiversity

The exploration, conservation, and use of agricultural biodiversity are essential components of efficient transdisciplinary research for a sustainable agriculture and food sector. Most recent advances on plant biotechnology and crop genomics must be complemented with a holistic management of plant genetic resources. Plant breeding programs aimed at improving agricultural productivity and food security can benefit from the systematic exploitation and conservation of genetic diversity to meet the demands of a growing population facing climate change. The genetic diversity of staple small grains, including rice, maize, wheat, millets, and more recently quinoa, have been surveyed to encourage utilization and prioritization of areas for germplasm conservation. Geographic information system technologies and spatial analysis are now being used as powerful tools to elucidate genetic and ecological patterns in the distribution of cultivated and wild species to establish coherent programs for the management of plant genetic resources for food and agriculture.

A new double right border binary vector for producing marker-free transgenic plants

BACKGROUND

Once a transgenic plant is developed, the selectable marker gene (SMG) becomes unnecessary in the plant. In fact, the continued presence of the SMG in the transgenic plant may cause unexpected pleiotropic effects as well as environmental or biosafety issues. Several methods for removal of SMGs that have been reported remain inaccessible due to protection by patents, while development of new ones is expensive and cost prohibitive. Here, we describe the development of a new vector for producing marker-free plants by simply adapting an ordinary binary vector to the double right border (DRB) vector design using conventional cloning procedures.

FINDINGS

We developed the DRB vector pMarkfree5.0 by placing the bar gene (representing genes of interest) between two copies of T-DNA right border sequences. The β-glucuronidase (gus) and nptII genes (representing the selectable marker gene) were cloned next followed by one copy of the left border sequence. When tested in a model species (tobacco), this vector system enabled the generation of 55.6% kanamycin-resistant plants by Agrobacterium-mediated transformation. The frequency of cotransformation of the nptII and bar transgenes using the vector was 66.7%. Using the leaf bleach and Basta assays, we confirmed that the nptII and bar transgenes were coexpressed and segregated independently in the transgenic plants. This enable separation of the transgenes in plants cotransformed using pMarkfree5.0.

CONCLUSIONS

The results suggest that the DRB system developed here is a practical and effective approach for separation of gene(s) of interest from a SMG and production of SMG-free plants. Therefore this system could be instrumental in production of "clean" plants containing genes of agronomic importance.

A comparison of strategies for multiple-gene co-transformation via hairy root induction

Hairy root is a transformed root tissue in which transfer DNA (T-DNA) is inserted in the genome by Agrobacterium rhizogenes. To establish a system for multiple-gene co-transformation in hairy roots, we evaluated four different strategies using A. rhizogenes. The genes gusA and mgfp5 were located in separate plasmids, which were transformed into two different batches of A. rhizogenes (strategy 2AR) or a single batch (strategy 2BV). The two reporter genes were also inserted in one T-DNA (strategy 1TD) or two different T-DNAs (strategy 2TD) in a binary vector. Over 90 % of infected Nicotiana tabacum leaf discs formed hairy roots in all four groups, which was not significantly different from the infection efficiency of wild-type A. rhizogenes. Proportions of co-transformed hairy roots with strategies 2AR, 2BV, 1TD, and 2TD were 65.4, 40.0, 78.6, and 82.1 %, respectively, which indicated that all of the strategies were suitable for co-transformation of multiple genes. High variation in growth rate and heterologous protein expression indicated that further screening is required to identify the clone with the highest productivity. Our results indicated that strategies 1TD and 2TD achieved the highest co-transformation efficiency. Combination with strategy 2AR or 2BV provides additional options for co-transformation of multiple transgenes.

Improved nutritive quality and salt resistance in transgenic maize by simultaneously overexpression of a natural lysine-rich protein gene, SBgLR, and an ERF transcription factor gene, TSRF1

Maize (Zea mays L.), as one of the most important crops in the world, is deficient in lysine and tryptophan. Environmental conditions greatly impact plant growth, development and productivity. In this study, we used particle bombardment mediated co-transformation to obtain marker-free transgenic maize inbred X178 lines harboring a lysine-rich protein gene SBgLR from potato and an ethylene responsive factor (ERF) transcription factor gene, TSRF1, from tomato. Both of the target genes were successfully expressed and showed various expression levels in different transgenic lines. Analysis showed that the protein and lysine content in T1 transgenic maize seeds increased significantly. Compared to non-transformed maize, the protein and lysine content increased by 7.7% to 24.38% and 8.70% to 30.43%, respectively. Moreover, transgenic maize exhibited more tolerance to salt stress. When treated with 200 mM NaCl for 48 h, both non-transformed and transgenic plant leaves displayed wilting and losing green symptoms and dramatic increase of the free proline contents. However, the degree of control seedlings was much more serious than that of transgenic lines and much more increases of the free proline contents in the transgenic lines than that in the control seedlings were observed. Meanwhile, lower extent decreases of the chlorophyll contents were detected in the transgenic seedlings. Quantitative RT-PCR was performed to analyze the expression of ten stress-related genes, including stress responsive transcription factor genes, ZmMYB59 and ZmMYC1, proline synthesis related genes, ZmP5CS1 and ZmP5CS2, photosynthesis-related genes, ZmELIP, ZmPSI-N, ZmOEE, Zmrbcs and ZmPLAS, and one ABA biosynthesis related gene, ZmSDR. The results showed that with the exception of ZmP5CS1 and ZmP5CS2 in line 9-10 and 19-11, ZmMYC1 in line 19-11 and ZmSDR in line 19-11, the expression of other stress-related genes were inhibited in transgenic lines under normal conditions. After salt treatment, the expressions of the ten stress-related genes were significantly induced in both wild-type (WT) and transgenic lines. However, compared to WT, the increases of ZmP5CS1 in all these three transgenic lines and ZmP5CS2 in line 9-10 were less than WT plants. This study provides an effective approach of maize genetic engineering for improved nutritive quality and salt tolerance.

Combinational transformation of three wheat genes encoding fructan biosynthesis enzymes confers increased fructan content and tolerance to abiotic stresses in tobacco

KEY MESSAGE

Seven kinds of transgenic tobacco plants transformed with combinations of three FBE genes were obtained. The transgenic plants transformed with Ta1-SST + Ta6-SFT genes appeared to have the highest fructan or soluble sugar content and the strongest salt tolerance. Fructan is thought to be one of the important regulators involved in plant tolerance to various abiotic stresses. In this study, wheat-derived genes, Ta1-SST, Ta6-SFT, and Ta1-FFT, encoding fructan biosynthesis enzymes (FBE) were isolated and cloned into vectors modified pBI121 or pZP211. Seven different combinations of the three target genes were transformed into tobacco plants through an Agrobacterium-mediated approach, and transgenic tobacco plants were identified by PCR, ELISA, and Southern blotting. Compared with tobacco plants transformed with other six combinations of the three target genes and with wild-type plants, the transgenic plants transformed with Ta1-SST + Ta6-SFT genes contained the highest fructan and soluble sugar content. All seven types of transgenic tobacco plants displayed a much higher level of tolerance to drought, low temperature, and high salinity compared with the wild type. Differences of drought and low temperature tolerance between the transgenic plants containing a single FBE gene and those harboring two or three FBE genes were not significant, but the salt tolerance level of the transgenic plants with different FBE gene combinations from high to low was: Ta1-SST + Ta6-SFT > Ta1-SST + Ta6-SFT + Ta1-FFT > Ta1-SST + Ta1-FFT > Ta1-SFT + Ta1-FFT > single FBE gene. These results indicated that the tolerances of the transgenic tobacco plants to various abiotic stresses were associated with the transformed target gene combinations and the contents of fructan and soluble sugar contained in the transgenic plants.

Agrobacterium-mediated gene transfer in plants and biosafety considerations

Agrobacterium, the natures' genetic engineer, has been used as a vector to create transgenic plants. Agrobacterium-mediated gene transfer in plants is a highly efficient transformation process which is governed by various factors including genotype of the host plant, explant, vector, plasmid, bacterial strain, composition of culture medium, tissue damage, and temperature of co-cultivation. Agrobacterium has been successfully used to transform various economically and horticulturally important monocot and dicot species by standard tissue culture and in planta transformation techniques like floral or seedling infilteration, apical meristem transformation, and the pistil drip methods. Monocots have been comparatively difficult to transform by Agrobacterium. However, successful transformations have been reported in the last few years based on the adjustment of the parameters that govern the responses of monocots to Agrobacterium. A novel Agrobacterium transferred DNA-derived nanocomplex method has been developed which will be highly valuable for plant biology and biotechnology. Agrobacterium-mediated genetic transformation is known to be the preferred method of creating transgenic plants from a commercial and biosafety perspective. Agrobacterium-mediated gene transfer predominantly results in the integration of foreign genes at a single locus in the host plant, without associated vector backbone and is also known to produce marker free plants, which are the prerequisites for commercialization of transgenic crops. Research in Agrobacterium-mediated transformation can provide new and novel insights into the understanding of the regulatory process controlling molecular, cellular, biochemical, physiological, and developmental processes occurring during Agrobacterium-mediated transformation and also into a wide range of aspects on biological safety of transgenic crops to improve crop production to meet the demands of ever-growing world's population.

Assessment of simple marker-free genetic transformation techniques in alfalfa

Methods to avoid the presence of selectable marker genes (SMG) in transgenic plants are available but not implemented in many crop species. We assessed the efficiency of simple marker-free Agrobacterium-mediated transformation techniques in alfalfa: regeneration without selection, or marker-less, and co-transformation with two vectors, one containing the SMG and one containing a non-selected gene. To easily estimate the efficiency of marker-less transformation, the nptII and the GUS markers were used as non-selected genes. After Agrobacterium treatment, somatic embryos were regenerated without selection. The percentage of transgenic embryos was determined by a second cycle of regeneration using the embryos as starting material, in the presence of kanamycin, by PCR screening of T1 progenies, and by the GUS test. In two experiments, from 0 to 1.7% of the somatic embryos were transgenic. Co-transformation was performed with two vectors, one with the hemL SMG and one with the unselected nptII gene, each carried by a different culture of Agrobacterium. Only 15 putative co-transformed plants were regenerated from two experiments, with an average co-transformation percentage of 3.7. Southern blot hybridizations and/or T(1) progeny segregation were used to confirm transgene integration, and qPCR was also used to estimate the T-DNA copy number. In the T(1) progenies obtained by crossing with a non-transgenic pollinator, marker-free segregants were obtained. Both marker-free approaches showed very low efficiency.

Transgene stacking and marker elimination in transgenic rice by sequential Agrobacterium-mediated co-transformation with the same selectable marker gene

Rice chitinase (chi11) and tobacco osmotin (ap24) genes, which cause disruption of fungal cell wall and cell membrane, respectively, were stacked in transgenic rice to develop resistance against the sheath blight disease. The homozygous marker-free transgenic rice line CoT23 which harboured the rice chi11 transgene was sequentially re-transformed with a second transgene ap24 by co-transformation using an Agrobacterium tumefaciens strain harbouring a single-copy cointegrate vector pGV2260::pSSJ1 and a multi-copy binary vector pBin19∆nptII-ap24 in the same cell. pGV2260::pSSJ1 T-DNA carried the hygromycin phosphotransferase (hph) and β-glucuronidase (gus) genes. pBin19∆nptII-ap24 T-DNA harboured the tobacco osmotin (ap24) gene. Co-transformation of the gene of interest (ap24) with the selectable marker gene (SMG, hph) occurred in 12 out of 18 T(0) plants (67%). Segregation of hph from ap24 was accomplished in the T(1) generation in one (line 11) of the four analysed co-transformed plants. The presence of ap24 and chi11 transgenes and the absence of the hph gene in the SMG-eliminated T(1) plants of the line 11 were confirmed by DNA blot analyses. The SMG-free transgenic plants of the line 11 harboured a single copy of the ap24 gene. Homozygous, SMG-free T(2) plants of the transgenic line 11 harboured stacked transgenes, chi11 and ap24. Northern blot analysis of the SMG-free plants revealed constitutive expression of chi11 and ap24. The transgenic plants with stacked transgenes displayed high levels of resistance against Rhizoctonia solani. Thus, we demonstrate the development of transgene-stacked and marker-free transgenic rice by sequential Agrobacterium-mediated co-transformation with the same SMG.

Improved Agrobacterium-mediated co-transformation and selectable marker elimination in transgenic rice by using a high copy number pBin19-derived binary vector

A high copy number, selectable marker gene (SMG)-free Agrobacterium binary vector pBin19ΔnptII was constructed by deleting the nptII gene from pBin19. The binary vectors with the RK2 and pVS replication origins exist in 12 and 3 copies, respectively, in Agrobacterium. The tobacco osmotin gene (ap24) was cloned in pBin19ΔnptII and the resultant plasmid pBin19ΔnptII-ap24 was mobilized into the Agrobacterium tumefaciens strain C58C1 Rif(r) harbouring the single-copy cointegrate vector pGV2260::pSSJ1. The T-DNA of the cointegrate vector harboured the hph (SMG) and gus genes. Transformation of Oryza sativa L. var. Pusa Basmati1 with Agrobacterium tumefaciens (pGV2260::pSSJ1, pBin19ΔnptII-ap24) yielded 14 independent hyg+/GUS+ transgenic plants. Southern blot analysis with hph and ap24 probes revealed that 12 out of the 14 transgenic plants were co-transformed and harboured hph, gus and ap24 genes. The new multi-copy binary vector yielded 86% co-transformation efficiency. SMG elimination by genetic separation of the cointegrate T-DNA with the hph/gus genes and binary vector T-DNA with the ap24 gene was accomplished in four out of ten primary co-transformants that were forwarded to the T₁ generation.

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.

Agrobacterium-mediated transformation of durum wheat (Triticum turgidum L. var. durum cv Stewart) with improved efficiency

An efficient Agrobacterium-mediated durum wheat transformation system has been developed for the production of 121 independent transgenic lines. This improved system used Agrobacterium strain AGL1 containing the superbinary pGreen/pSoup vector system and durum wheat cv Stewart as the recipient plant. Acetosyringone at 400 microM was added to both the inoculation and cultivation medium, and picloram at 10 mg l(-1) and 2 mg l(-1) was used in the cultivation and induction medium, respectively. Compared with 200 microM in the inoculation and cultivation media, the increased acetosyringone concentration led to significantly higher GUS (beta-glucuronidase) transient expression and T-DNA delivery efficiency. However, no evident effects of acetosyringone concentration on regeneration frequency were observed. The higher acetosyringone concentration led to an improvement in average final transformation efficiency from 4.7% to 6.3%. Furthermore, the concentration of picloram in the co-cultivation medium had significant effects on callus induction and regeneration. Compared with 2 mg l(-1) picloram in the co-cultivation medium, increasing the concentration to 10 mg l(-1) picloram resulted in improved final transformation frequency from 2.8% to 6.3%, with the highest frequency of 12.3% reached in one particular experiment, although statistical analysis showed that this difference in final transformation efficiency had a low level of significance. Stable integration of foreign genes, their expression, and inheritance were confirmed by Southern blot analyses, GUS assay, and genetic analysis. Analysis of T(1) progeny showed that, of the 31 transgenic lines randomly selected, nearly one-third had a segregation ratio of 3:1, while the remainder had ratios typical of two or three independently segregating loci.

Marker-free transgenic corn plant production through co-bombardment

The use of particle gun for the production of marker-free plants is scant in published literature. Perhaps this is a reflection of the widely held notion that the events generated through bombardment tend to have multiple copies of transgenes, usually integrated at a single locus, features which precludes segregating away the selectable marker gene. However, our previous studies have shown that single-copy integrants are obtained at a high frequency if limited quantity of DNA is used for bombardment. Also, the concatemerized insertion of transgenes has been demonstrated to be greatly reduced if "cassette DNA" is employed in place of whole plasmid DNA for bombardment. Based on the above findings, in the present study the feasibility of co-bombardment was evaluated for the production of marker-free plants in corn, employing a combination of limited quantity DNA and cassette DNA approaches for bombardment. Transgenic events were generated after co-bombardment of a selectable marker cassette containing the nptII gene (2.5 ng per shot) and a GUS gene cassette (15 ng per shot). Among these events single-copy integrants for nptII gene occurred at an average frequency of 68% within which the co-expression frequency of GUS and nptII genes ranged from 41% to 80%. Marker-free corn plants could be identified from the progeny of 28 out of the 103 R0 co-expressing events screened. The results demonstrate that by using cassette DNA and low quantities of DNA for bombardment, marker-free plants are produced at efficiencies comparable to that of Agrobacterium-based co-transformation methods.

Production of selectable marker-free transgenic tobacco plants using a non-selection approach: chimerism or escape, transgene inheritance, and efficiency

Public concern and metabolic drain were the main driving forces for the development of a selectable marker-free transformation system. We demonstrated here the production of transgenic tobacco plants using a non-selection approach by Agrobacterium tumefaciens-mediated transformation. A. tumefaciens-infected leaf explants were allowed to produce shoots on a shoot induction medium (SIM) containing no selective compounds. Up to 35.1% of the A. tumefaciens-infected leaf explants produced histochemically GUS(+) shoots, 3.1% of regenerated shoots were GUS(+), and 72% of the GUS(+) shoots were stably transformed by producing GUS(+) T1 seedlings. When polymerase chain reaction (PCR) was used to screen the regenerated shoots, 4% of the shoots were found to be PCR(+) for the transgene and 65% of the PCR(+) shoots were stable transformants. Also, generation of PCR(+) escapes decreased linearly as the number of subculture increased from one to three on SIM containing the antibiotic that kills the Agrobacterium. Twenty-five to 75% of the transformants were able to transmit transgene activity to the T1 generation in a Mendelian 3:1 ratio, and a transformation efficiency of 2.2-2.8% was achieved for the most effective binary vector. These results indicated that majority of the GUS(+) or PCR(+) shoots recovered under no selection were stable transformants, and only one-third of them were chimeric or escapes. Transgenes in these transgenic plants were able to transmit the transgene into progeny in a similar fashion as those recovered under selection.

Generation of selectable marker-free sheath blight resistant transgenic rice plants by efficient co-transformation of a cointegrate vector T-DNA and a binary vector T-DNA in one Agrobacterium tumefaciens strain

Co-transformation of Oryza sativa L. var. Pusa Basmati1 was done using an Agrobacterium tumefaciens strain harbouring a single-copy cointegrate vector and a multi-copy binary vector in the same cell. The T-DNA of the cointegrate vector pGV2260::pSSJ1 carried the hygromycin phosphotransferase (hph) and beta-glucuronidase (gus) genes. The binary vector pCam-chi11, without a plant selectable marker gene, harboured the rice chitinase (chi11) gene under maize ubiquitin promoter. Co-transformation of the gene of interest (chi11) with the selectable marker gene (hph) occurred in 4 out of 20 T(0) plants (20%). Segregation of hph from chi11 was accomplished in two (CoT6 and CoT23) of the four co-transformed plants in the T(1) generation. The selectable marker-free (SMF) lines CoT6 and CoT23 harboured single copies of chi11. Homozygous SMF T(2) plants were established in the lines CoT6 and CoT23. Northern and Western blot analysis of the homozygous SMF lines showed high level of transgene expression. In comparison to untransformed controls, chitinase specific activity was 66- and 22-fold higher in the homozygous SMF T(2) plants of lines CoT6 and CoT23, respectively. The lines CoT6 and CoT23 exhibited 38 and 40% reduction in sheath blight disease, respectively.

The pCLEAN dual binary vector system for Agrobacterium-mediated plant transformation

The development of novel transformation vectors is essential to the improvement of plant transformation technologies. Here, we report the construction and testing of a new multifunctional dual binary vector system, pCLEAN, for Agrobacterium-mediated plant transformation. The pCLEAN vectors are based on the widely used pGreen/pSoup system and the pCLEAN-G/pCLEAN-S plasmids are fully compatible with the existing pGreen/pSoup vectors. A single Agrobacterium can harbor (1) pCLEAN-G and pSoup, (2) pGreen and pCLEAN-S, or (3) pCLEAN-G and pCLEAN-S vector combination. pCLEAN vectors have been designed to enable the delivery of multiple transgenes from distinct T-DNAs and/or vector backbone sequences while minimizing the insertion of superfluous DNA sequences into the plant nuclear genome as well as facilitating the production of marker-free plants. pCLEAN vectors contain a minimal T-DNA (102 nucleotides) consisting of direct border repeats surrounding a 52-nucleotide-long multiple cloning site, an optimized left-border sequence, a double left-border sequence, restriction sites outside the borders, and two independent T-DNAs. In addition, selectable and/or reporter genes have been inserted into the vector backbone sequence to allow either the counter-screening of backbone transfer or its exploitation for the production of marker-free plants. The efficiency of the different pCLEAN vectors has been assessed using transient and stable transformation assays in Nicotiana benthamiana and/or Oryza sativa.

Development of an in planta method for transformation of alfalfa (Medicago sativa)

Conventional methods in transforming alfalfa (Medicago sativa) require multiple tissue culture manipulations that are time-consuming and expensive, while applicable only to a few highly regenerable genotypes. Here, we describe a simple in planta method that makes it possible to transform a commercial variety without employing selectable marker genes. Basically, young seedlings are cut at the apical node, cold-treated, and vigorously vortexed in an Agrobacterium suspension also containing sand. About 7% of treated seedlings produced progenies segregating for the T-DNA. The vortex-mediated seedling transformation method was applied to transform alfalfa with an all-native transfer DNA comprising a silencing construct for the caffeic acid o-methyltransferase (Comt) gene. Resulting intragenic plants accumulated reduced levels of the indigestible fiber component lignin that lowers forage quality. The absence of both selectable marker genes and other foreign genetic elements may expedite the governmental approval process for quality-enhanced alfalfa.

Agrobacterium-mediated transformation of maize

Maize may be transformed very efficiently using Agrobacterium tumefaciens-mediated methods. The most critical factor in the transformation protocol is the co-cultivation of healthy immature embryos of the correct developmental stage with A. tumefaciens; the embryos should be collected only from vigorous plants grown in well-conditioned glasshouses. With the protocol described here, approximately 50% of immature embryos from the inbred line A188 and 15% from inbred lines A634, H99 and W117 will produce transformants. About half of the transformed plants are expected to carry one or two copies of the transgenes, which are inherited by the progeny in a mendelian fashion. More than 90% of transformants are expected to be normal in morphology. The protocol takes about 3 months from the start of co-cultivation to the planting of transformants into pots.

Okadaic acid and trifluoperazine enhance Agrobacterium-mediated transformation in eastern white pine

Mature zygotic embryos of recalcitrant Christmas tree species eastern white pine (Pinus strobus L.) were used as explants for Agrobacterium tumefaciens strain GV3101-mediated transformation using the uidA (beta-Glucuronidase) gene as a reporter. Influence of the time of sonication and the concentrations of protein phosphatase inhibitor (okadaic acid) and kinase inhibitor (trifluoperazine) on Agrobacterium-mediated transformation have been evaluated. A high transformation frequency was obtained after embryos were sonicated for 45-50 s, or treated with 1.5-2.0 microM okadaic acid or treated with 100-200 microM trifluoperazine, respectively. Protein phosphatase and kinase inhibitors enhance Agrobacterium-mediated transformation in eastern white pine. A 2-3.5-fold higher rate of hygromycin-resistant callus was obtained with an addition of 2 microM okadaic acid or 150 microM trifluoperazine or sonicated embryos for 45 s. Stable integration of the uidA gene in the plant genome of eastern white pine was confirmed by polymerase chain reaction (PCR), Southern and northern blot analyses. These results demonstrated that a stable and enhanced transformation system has been established in eastern white pine and this system would provide an opportunity to transfer economically important genes into this Christmas tree species.

Engineering resistance to PVY in different potato cultivars in a marker-free transformation system using a 'shooter mutant' A. tumefaciens

In this work, Potato virus Y (PVY) resistant potatoes were generated using an environmentally safe construct. For this purpose, a 'shooter' mutant Agrobacterium-based transformation system was used. The isopentenyl transferase gene (ipt) present on the Ti plasmid of 'shooter' strains enhances shoot regeneration and can be used as a phenotypic selection marker. The introduced marker-free binary vector carried a hairpin construct derived from the coat protein gene of PVY-NTN strain in order to induce gene silencing. Transformation resulted in high regeneration rates (1.4-5.7 shoots per explant). With pre-selection for the ipt (+) phenotype the transformation frequency was 24-53%, while without selection 12-28% of the shoots were PCR positive. The presence of the transgene was verified by Southern hybridization. In 16 of 31 challenged transformant lines PVY could be detected neither by RT-PCR nor by back inoculation. A 62.5% of these resistant lines proved to be also ipt-free. This transformation system was reproducible in four potato cultivars, suggesting that it could easily be adapted for other species.

Thirty years of plant transformation technology development

Technology development is seminal to many aspects of basic and applied plant transgenic science. Through the development and commercialization of genetically modified crops, the evolution of plant transgenic technologies is also relevant to society as a whole. In this study, literature statistics were used to uncover trends in the development of these technologies. Publication volume and impact (citation) over the past 30 years were analysed with respect to economic zones, countries, species and DNA delivery method. This revealed that, following a dramatic expansion in the 1980s, publications focusing on the development of transgenic technology have been slowing down worldwide since the early mid-1990s, except in a few leading Asian countries. The implications of these trends on the future of plant transgenic science as a whole are discussed.

Agrobacterium-mediated transformation of cereals: a promising approach crossing barriers

Cereal crops have been the primary targets for improvement by genetic transformation because of their worldwide importance for human consumption. For a long time, many of these important cereals were difficult to genetically engineer, mainly as a result of their inherent limitations associated with the resistance to Agrobacterium infection and their recalcitrance to in vitro regeneration. The delivery of foreign genes to rice plants via Agrobacterium tumefaciens has now become a routine technique. However, there are still serious handicaps with Agrobacterium-mediated transformation of other major cereals. In this paper, we review the pioneering efforts, existing problems and future prospects of Agrobacterium-mediated genetic transformation of major cereal crops, such as rice, maize, wheat, barley, sorghum and sugarcane.

Molecular characterization of marker-free transgenic lines of indica rice that accumulate carotenoids in seed endosperm

A single Agrobacterium strain harbouring two binary plasmids was successfully used for the first time to develop a marker-free transgenic rice of improved nutritional value. Sixty-eight T0 co-transformants were obtained in three indica rice cultivars--two popular high-yielding Bangladeshi varieties (BR28 and BR29), and one high-iron rice cultivar (IR68144). Marker-free lines were obtained from 14 out of 24 selected co-transformants screened in the T1 generation. The accumulation of total carotenoids in polished T2 rice seeds of the primary transgenic VPBR29-17-37 reached levels of up to 3.0 microg/g, with the level of beta-carotene reaching 1.8 microg/g. In the cultivars BR28 and IR68144, total carotenoid levels in the transformants reached 2.0 microg/g of polished rice seeds. The levels of lutein and other carotenoids in the seeds were also significantly enhanced. T1 plants obtained from primary transgenics with simple gene-integration patterns tended to have a lower carotenoid content than the original parental lines. This study describes the development of marker-free transgenic rice lines containing high levels of carotenoids, and addresses the relationship between the rearrangement of transgenes and the presence of metabolic end products in transgenic rice.

Generation of marker-free transgenic maize by regular two-border Agrobacterium transformation vectors

By introducing additional T-DNA borders into a binary plasmid used in Agrobacterium-mediated plant transformation, previous studies have demonstrated that the marker gene and the gene of interest (GOI) can be carried by independent T-strands, which sometimes integrate in unlinked loci in the plant genome. This allows the recovery of marker-free transgenic plants through genetic segregation in the next generation. In this study, we have found that by repositioning the selectable marker gene in the backbone and leaving only the GOI in the T-DNA region, a regular two-border binary plasmid was able to generate marker-free transgenic maize plants more efficiently than a conventional single binary plasmid with multiple T-DNA borders. These results also provide evidence that both the right and left borders can initiate and terminate T-strands. Such non-canonical initiation and termination of T-strands may be the basis for the elevated frequencies of cotransformation and unlinked insertions.

A novel two T-DNA binary vector allows efficient generation of marker-free transgenic plants in three elite cultivars of rice (Oryza sativa L.)

A pilot binary vector was constructed to assess the potential of the 2 T-DNA system for generating selectable marker-free progeny plants in three elite rice cultivars (ZhongZuo321, Ariete and Khao Dawk Mali 105) known to exhibit contrasting amenabilities to transformation. The first T-DNA of the vector, delimited by Agrobacterium tumefaciens borders, contains the hygromycin phosphotransferase (hpt) selectable gene and the green fluorescent protein (gfp) reporter gene while the second T-DNA, delimited by Agrobacterium rhizogenes borders, bears the phosphinothricin acetyl transferase (bar) gene, featuring the gene of interest. 82-90% of the hygromycin-resistant primary transformants exhibited tolerance to ammonium glufosinate mediated by the bar gene suggesting very high co-transformation frequency in the three cultivars. All of the regenerated plants were analyzed by Southern blot which confirmed co-integration of the T-DNAs at frequencies consistent with those of co-expression and allowed determination of copy number for each gene as well as detection of two different vector backbone fragments extending between the two T-DNAs. Hygromycin susceptible, ammonium glufosinate tolerant phenotypes represented 14.4, 17.4 and 14.3% of the plants in T1 progenies of ZZ321, Ariete and KDML105 primary transformants, respectively. We developed a statistical model for deducing from the observed copy number of each T-DNA in T0 plants and phenotypic segregations in T1 progenies the most likely constitution and linkage of the T-DNA integration locus. Statistical analysis identified in 40 out of 42 lines a most likely linkage configuration theoretically allowing genetic separation of the two T-DNA types and out segregation of the T-DNA bearing the bar gene. Overall, though improvements of the technology would be beneficial, the 2 T-DNA system appeared to be a useful approach to generate selectable marker-free rice plants with a consistent frequency among cultivars.