Recent advances in development of marker-free transgenic plants: Regulation and biosafety concern

An industrial-grade Nicotiana benthamiana line for the production of glycoproteins carrying fucose-free galactosylated N-glycans

β1,4-galactosylation is a typical human N-glycan formation with functional impact on proteins, particularly known for IgGs. Therefore, the expression of recombinant proteins with controlled galactosylation is an important quality parameter in the biotech industry. Here we describe the establishment of a plant-based expression platform for the manufacturing of recombinant proteins carrying β1,4-galactosylated N-glycans. A genome-edited Nicotiana benthamiana glycosylation mutant (NbXF-KO) that synthesizes conserved eukaryotic GnGn structures served as a template for further elongation toward β1,4-galactosylated N-glycans. A hybrid β1,4-galactosyltransferase gene that targets the enzyme to a post-Golgi compartment was introduced into the NbXF-KO genome without any additional foreign DNA sequence. The efficient generation of "marker-free" transgenic lines (NbXF-KOGal) was achieved by using a dual-vector strategy and visual screening procedures. Of note, a monoclonal antibody expressed in NbXF-KOGal exhibited up to 70 % galactosylated, fucose/xylose-free N-glycans, in a batch-to-batch consistent manner. Given recent findings attributing anti-inflammatory activities to nonfucosylated, galactosylated IgG, our results gain new significance.

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.

OsRuvBL1a DNA helicase boost salinity and drought tolerance in transgenic indica rice raised by in planta transformation

RuvBL, is a member of SF6 superfamily of helicases and is conserved among the various model systems. Recently, rice (Oryza sativa L.) homolog of RuvBL has been biochemically characterized for its ATPase and DNA helicase activities; however its involvement in stress has not been studied so far. Present investigation reports the detailed functional characterization of OsRuvBL under abiotic stresses through genetic engineering. An efficient Agrobacterium-mediated in planta transformation protocol was developed in indica rice to generate the transgenic lines and study was focused on optimization of factors to achieve maximum transformation efficiency. Overexpressing OsRuvBL1a transgenic lines showed enhanced tolerance under in vivo salinity stress as compared to WT plants. The physiological and biochemical analysis of the OsRuvBL1a transgenic lines showed better performance under salinity and drought stresses. Several stress responsive interacting partners of OsRuvBL1a were identified using Y2H system revealed to its role in stress tolerance. Functional mechanism for boosting stress tolerance by OsRuvBL1a has been proposed in this study. This integration of OsRuvBL1a gene in rice genome using in planta transformation method helped to achieve the abiotic stress resilient smart crop. This study is the first direct evidence to show the novel function of RuvBL in boosting abiotic stress tolerance in plants.

A DEAD box helicase Psp68 positively regulates salt stress responses in marker-free transgenic rice plants

Helicases are the motor proteins not only involved in transcriptional and post-transcription process but also provide abiotic stress tolerance in many crops. The p68, belong to the SF2 (DEAD-box helicase) family proteins and overexpression of Psp68 providing enhanced tolerance to transgenic rice plants. In this study, salinity tolerant marker-free transgenic rice has been developed by overexpressing Psp68 gene and phenotypically characterized. The Psp68 overexpressing marker-free transgenic rice plants were initially screened in the rooting medium containing salt stress and 20% polyethylene glycol (PEG). Stable integration and overexpression of Psp68 in marker-free transgenic lines were confirmed by molecular analyses including PCR, southern, western blot, and qRT-PCR analyses. The marker-free transgenic lines showed enhanced tolerance to salinity stress as displayed by early seed germination, higher chlorophyll content, reduced necrosis, more survival rate, improved seedling growth and more grain yield per plant. Furthermore, Psp68 overexpressing marker-free transgenics also accumulated less Na+ and higher K+ ions in the presence of salinity stress. Phenotypic analyses also revealed that marker-free transgenic rice lines efficiently scavenge ROS-mediated damages as displayed by lower H2O2 and malondialdehyde content, delayed electrolyte leakage, higher photosynthetic efficiency, membrane stability, proline content and enhanced activities of antioxidants enzymes. Overall, our results confirmed that Psp68 overexpression confers salinity stress tolerance in marker-free transgenics, hence the technique could be utilized to develop genetically modified crops without any biosafety issues.

Methods of crop improvement and applications towards fortifying food security

Agriculture has supported human life from the beginning of civilization, despite a plethora of biotic (pests, pathogens) and abiotic (drought, cold) stressors being exerted on the global food demand. In the past 50 years, the enhanced understanding of cellular and molecular mechanisms in plants has led to novel innovations in biotechnology, resulting in the introduction of desired genes/traits through plant genetic engineering. Targeted genome editing technologies such as Zinc-Finger Nucleases (ZFNs), Transcription Activator-Like Effector Nucleases (TALENs), and Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) have emerged as powerful tools for crop improvement. This new CRISPR technology is proving to be an efficient and straightforward process with low cost. It possesses applicability across most plant species, targets multiple genes, and is being used to engineer plant metabolic pathways to create resistance to pathogens and abiotic stressors. These novel genome editing (GE) technologies are poised to meet the UN's sustainable development goals of "zero hunger" and "good human health and wellbeing." These technologies could be more efficient in developing transgenic crops and aid in speeding up the regulatory approvals and risk assessments conducted by the US Departments of Agriculture (USDA), Food and Drug Administration (FDA), and Environmental Protection Agency (EPA).

CRISPR/Cas genome editing in plants: Dawn of Agrobacterium transformation for recalcitrant and transgene-free plants for future crop breeding

Genome editing tools based on CRISPR/Cas system have been posed to solve many issues in agriculture and improve food production. Genetic engineering by Agrobacterium-mediated transformation has helped to impart specific traits straightaway in many crops. Many GM crops have also reached the field for commercial cultivation. Genetic engineering requires mostly a transformation protocol often mediated by Agrobacterium to insert a specific gene at a random locus. Genome editing with CRISPR/Cas system is a more precise technique for the targeted modification of genes/bases in the host plant genome. Unlike the conventional transformation system, wherein elimination of marker/foreign gene was possible only post-transformation, CRISPR/Cas system could generate transgene-free plants by delivering CRISPR/Cas reagents such as the Cas protein and guide RNAs gRNA(s) preassembled to form ribonucleoproteins (RNPs) into plant cells. CRISPR reagent delivery might be helpful to overcome issues with plants that are recalcitrant to Agrobacterium transformation and the legal hurdles due to the presence of the foreign gene. More recently, the grafting of wild-type shoots to transgenic donor rootstocks developed by the CRISPR/Cas system has reported transgene-free genome editing. CRISPR/Cas system also requires only a small piece of gRNA besides Cas9 or other effectors to target a specific region in the genome. So this system has been projected to be a key contributor to future crop breeding. In this article, we recap the main events of plant transformation, compare the difference between genetic transformation and CRISPR/Cas-mediated genome editing, and draw insights into the future application of the CRISPR/Cas system.

Plant Virus-Derived Vectors for Plant Genome Engineering

Advances in genome engineering (GE) tools based on sequence-specific programmable nucleases have revolutionized precise genome editing in plants. However, only the traditional approaches are used to deliver these GE reagents, which mostly rely on Agrobacterium-mediated transformation or particle bombardment. These techniques have been successfully used for the past decades for the genetic engineering of plants with some limitations relating to lengthy time-taking protocols and transgenes integration-related regulatory concerns. Nevertheless, in the era of climate change, we require certain faster protocols for developing climate-smart resilient crops through GE to deal with global food security. Therefore, some alternative approaches are needed to robustly deliver the GE reagents. In this case, the plant viral vectors could be an excellent option for the delivery of GE reagents because they are efficient, effective, and precise. Additionally, these are autonomously replicating and considered as natural specialists for transient delivery. In the present review, we have discussed the potential use of these plant viral vectors for the efficient delivery of GE reagents. We have further described the different plant viral vectors, such as DNA and RNA viruses, which have been used as efficient gene targeting systems in model plants, and in other important crops including potato, tomato, wheat, and rice. The achievements gained so far in the use of viral vectors as a carrier for GE reagent delivery are depicted along with the benefits and limitations of each viral vector. Moreover, recent advances have been explored in employing viral vectors for GE and adapting this technology for future research.

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.

Genetically modified foods: bibliometric analysis on consumer perception and preference

In this study, we present the bibliometric trends emerging from research outputs on consumer perception and preference for genetically modified (GM) foods and policy prescriptions for enabling the consumption using VOSviewer visualization software. Consumers' positive response is largely influenced by the decision of the governments to ban or approve the GM crops cultivation. Similarly, the public support increases when the potential benefits of the technology are well articulated, consumption increases with a price discount, people's trust on the government and belief in science increases with a positive influence by the media. Europe and the USA are the first region and country, respectively, in terms of the number of active institutions per research output, per-capita GDP publication and citations. We suggest research-, agri-food industries-, and society-oriented policies to be implemented by the stakeholders to ensure the safety of GM foods, encourage consumer-based studies, and increase public awareness toward these food products.

Construction of gene modification system with highly efficient and markerless for Monascus ruber M7

Monascus spp. are traditional medicinal and edible filamentous fungi in China, and can produce various secondary metabolites, such as Monascus pigments (MPs) and citrinin (CIT). Genetic modification methods, such as gene knock-out, complementation, and overexpression, have been used extensively to investigate the function of related genes in Monascus spp.. However, the resistance selection genes that can have been used for genetic modification in Monascus spp. are limited, and the gene replacement frequency (GRF) is usually <5%. Therefore, we are committed to construct a highly efficient gene editing system without resistance selection marker gene. In this study, using M. ruber M7 as the starting strain, we successfully constructed a so-called markerlessly and highly genetic modification system including the mutants ΔmrpyrGΔmrlig4 and ΔmrpyrGΔmrlig4::mrpyrG, in which we used the endogenous gene mrpyrG from M. ruber M7 instead of the resistance marker gene as the screening marker, and simultaneously deleted mrlig4 related to non-homologous end joining in M. ruber M7. Then, the morphology, the growth rate, the production of MPs and CIT of the mutants were analyzed. And the results show that the mutant strains have normal mycelia, cleistothecia and conidia on PDA+Uridine(U) plate, the biomass of each mutant is also no different from M. ruber M7. However, the U addition also has a certain effect on the orange and red pigments yield of M. ruber M7, which needs our further study. Finally, we applied the system to delete multiple genes from M. ruber M7 separately or continuously without any resistance marker gene, and found that the average GRF of ΔmrpyrGΔmrlig4 was about 18 times of that of M. ruber M7. The markerlessly and highly genetic modification system constructed in current study not only will be used for multi-gene simultaneous modification in Monascus spp., and also lays a foundation for investigating the effects of multi-genes modification on Monascus spp..

Engineering an oilseed crop for hyper-accumulation of carotenoids in the seeds without using a traditional marker gene

Ketocarotenoids were synthesized successfully in Camelina sativa seeds by genetic modification without using a traditional selection marker genes. This method provided an interesting tool for metabolic engineering of seed crops. Camelina sativa (L.) Crantz is an important oil crop with many excellent agronomic traits. This model oil plant has been exploited to accumulate value-added bioproducts using genetic manipulation that depends on antibiotic- or herbicide-based selection marker genes (SMG), one of the major concerns for genetically modified foods. Here we reported metabolic engineering of C. sativa to synthesize red ketocarotenoids that could serve as a reporter to visualize transgenic events without using a traditional SMG. Overexpression of a non-native β-carotene ketolase gene coupled with three other carotenogenous genes (phytoene synthase, β-carotene hydroxylase, and Orange) in C. sativa resulted in production of red seeds that were visibly distinguishable from the normal yellow ones. Constitutive expression of the transgenes led to delayed plant development and seed germination. In contrast, seed-specific transformants demonstrated normal growth and seed germination despite the accumulation of up to 70-fold the level of carotenoids in the seeds compared to the controls, including significant amounts of astaxanthin and keto-lutein. As a result, the transgenic seed oils exhibited much higher antioxidant activity. No significant changes were found in the profiles of fatty acids between transgenic and control seeds. This study provided an interesting tool for metabolic engineering of seed crops without using a disputed SMG.

Comparison of bacterial diversity, root exudates and soil enzymatic activities in the rhizosphere of AVP1-transgenic and non-transgenic wheat (Triticum aestivum L.)

AIMS

Soil microbial communities are among the most diverse communities that might be affected due to transgenic crops. Therefore, risk assessment studies on transgenes are essentially required as any adverse effects may depend not only on the specific gene and crop involved but also on soil conditions.

METHODS AND RESULTS

The present study deals with the comparison of bacterial populations, root exudates, and activities of soil enzymes in non-transgenic and AVP1-transgenic wheat rhizosphere, overexpressing vacuolar H+pyrophosphatase for salinity and drought stress tolerance. Amounts of organic acids and sugars produced as root exudates and activities of dehydrogenase, phosphatase, and protease enzymes in soil solution showed no significant differences in AVP1-transgenic and non-transgenic wheat rhizosphere, except for urease and phenol oxidase activities. The higher copy number of nifH gene showed the abundance of nitrogen-fixing bacteria in the rhizosphere of AVP1-transgenic wheat compared with non-transgenic wheat. nifH gene sequence analysis indicated the common diazotrophic genera Azospirillum, Bradyrhizobium, Rhizobium, and Pseudomonas in AVP1-transgenic and non-transgenic wheat except for Zoogloea detected only in non-transgenic wheat. Using 454-pyrosequencing of 16S rRNA gene from soil DNA, a total of 156, 282 sequences of 18 phyla were obtained, which represented bacterial (128,006), Archeal (7,928), and unclassified (21,568) sequences. Proteobacteria, Crenarchaeota, and Firmicutes were the most abundant phyla in transgenic and non-transgenic wheat rhizosphere. Further comparison of different taxonomic units at the genus level showed similar distribution in transgenic and non-transgenic wheat rhizosphere.

CONCLUSION

We conclude that AVP1 gene in transgenic wheat has no apparent adverse effects on the soil environment and different bacterial communities. However, bacterial community depends on several other factors not only genetic composition of the host plants.

SIGNIFICANCE OF THE STUDY

The present research supports introduction and cultivation of transgenic plants in agricultural systems without any adverse effects on indigenous bacterial communities and soil ecosystem.

Marker-Free Rice (Oryza sativa L. cv. IR 64) Overexpressing PDH45 Gene Confers Salinity Tolerance by Maintaining Photosynthesis and Antioxidant Machinery

Helicases function as key enzymes in salinity stress tolerance, and the role and function of PDH45 (pea DNA helicase 45) in stress tolerance have been reported in different crops with selectable markers, raising public and regulatory concerns. In the present study, we developed five lines of marker-free PDH45-overexpressing transgenic lines of rice (Oryza sativa L. cv. IR64). The overexpression of PDH45 driven by CaMV35S promoter in transgenic rice conferred high salinity (200 mM NaCl) tolerance in the T1 generation. Molecular attributes such as PCR, RT-PCR, and Southern and Western blot analyses confirmed stable integration and expression of the PDH45 gene in the PDH45-overexpressing lines. We observed higher endogenous levels of sugars (glucose and fructose) and hormones (GA, zeatin, and IAA) in the transgenic lines in comparison to control plants (empty vector (VC) and wild type (WT)) under salt treatments. Furthermore, photosynthetic characteristics such as net photosynthetic rate (Pn), stomatal conductance (gs), intercellular CO2 (Ci), and chlorophyll (Chl) content were significantly higher in transgenic lines under salinity stress as compared to control plants. However, the maximum primary photochemical efficiency of PSII, as an estimated from variable to maximum chlorophyll a fluorescence (Fv/Fm), was identical in the transgenics to that in the control plants. The activities of antioxidant enzymes, such as catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), and guaiacol peroxidase (GPX), were significantly higher in transgenic lines in comparison to control plants, which helped in keeping the oxidative stress burden (MDA and H2O2) lesser on transgenic lines, thus protecting the growth and photosynthetic efficiency of the plants. Overall, the present research reports the development of marker-free PDH45-overexpressing transgenic lines for salt tolerance that can potentially avoid public and biosafety concerns and facilitate the commercialization of genetically engineered crop plants.

A Strategy for the Production and Molecular Validation of Agrobacterium-Mediated Intragenic Octoploid Strawberry

Intragenesis is an all-native engineering technology for crop improvement. Using an intragenic strategy to bring genes from wild species to cultivated strawberry could expand the genetic variability. A robust regeneration protocol was developed for the strawberry cv. ‘Shanghai Angel’ by optimizing the dose of Thidiazuron and identifying the most suitable explants. The expression cassette was assembled with all DNA fragments from F. vesca, harboring a sugar transporter gene FvSTP8 driven by a fruit-specific FvKnox promoter. Transformed strawberry was developed through an Agrobacterium-mediated strategy without any selectable markers. Other than PCR selection, probe-based duplex droplet digital PCR (ddPCR) was performed to determine the T-DNA insert. Four independent transformed shoots were obtained with a maximum of 5.3% efficiency. Two lines were confirmed to be chimeras, while the other two were complete transformants with six and 11 copies of the intragene, respectively. The presence of a vector backbone beyond the T-DNA in these transformants indicated that intragenic strawberries were not obtained. The current work optimized the procedures for producing transformed strawberry without antibiotic selection, and accurately determined the insertion copies by ddPCR in the strawberry genome for the first time. These strategies might be promising for the engineering of ‘Shanghai Angel’ and other cultivars to improve agronomic traits.

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.

Converting endogenous genes of the malaria mosquito into simple non-autonomous gene drives for population replacement

Gene drives for mosquito population replacement are promising tools for malaria control. However, there is currently no clear pathway for safely testing such tools in endemic countries. The lack of well-characterized promoters for infection-relevant tissues and regulatory hurdles are further obstacles for their design and use. Here we explore how minimal genetic modifications of endogenous mosquito genes can convert them directly into non-autonomous gene drives without disrupting their expression. We co-opted the native regulatory sequences of three midgut-specific loci of the malaria vector Anopheles gambiae to host a prototypical antimalarial molecule and guide-RNAs encoded within artificial introns that support efficient gene drive. We assess the propensity of these modifications to interfere with the development of Plasmodium falciparum and their effect on fitness. Because of their inherent simplicity and passive mode of drive such traits could form part of an acceptable testing pathway of gene drives for malaria eradication.

Characterization of an algal phosphomannose isomerase gene and its application as a selectable marker for genetic manipulation of tomato

Establishing a transgenic plant largely relies on a selectable marker gene that can confer antibiotic or herbicide resistance to plant cells. The existence of such selectable marker genes in genetically modified foods has long been criticized. Plant cells generally exhibit too low an activity of phosphomannose isomerase (PMI) to grow with mannose as a sole carbon source. In this study, we characterized PMI from the green microalga Chlorococcum sp. and assessed its feasibility as a selectable marker for plant biotechnology. Chlorococcum sp. PMI (ChlPMI) was shown to be closely related to higher plants but more distant to bacterial counterparts. Overexpression of ChlPMI in tomato induced callus and shoot formation in media containing mannose (6 g/L) and had an average transformation rate of 3.9%. Based on this transformation system, a polycistronic gene cluster containing crtB, HpBHY, CrBKT and SlLCYB (BBBB) was co-expressed in a different tomato cultivar. Six putative transformants were achieved with a transformation rate of 1.4%, which produced significant amounts of astaxanthin due to the expression of the BBBB genes. Taken together, these findings indicate that we have established an additional tool for plant biotechnology that may be suitable for genetically modifying foods safely.

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.

Detection of genetically modified maize in Jordan

This study aimed to detect genetically modified maize (GMM) in seeds of eleven imported maize hybrids grown in Jordan. We used promoter 35 S and T-nos terminator for general screening of transgenic materials. Conventional PCR detected the specific events for the screening of Bt 11, MON810, and Bt176 events. Seeds of eleven maize hybrids samples showed a positive response to the 35 S promoter; nine out of eleven showed a positive response for T-nos terminator. Bt11 event was the most used in GMM seeds, where seven out of eleven samples showed positive results. Two out of eleven hybrids showed the presence of the Bt176 event; however, MON810 not detected in any of the tested hybrids. We studied the Bt11 event in imported GMM seeds in Jordan for the first time, reinforcing the need for a mandatory labeling system and a valid simple qualitative method in routine analysis of GMCs.

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.

Genetically modified crops: current status and future prospects

While transgenic technology has heralded a new era in crop improvement, several concerns have precluded their widespread acceptance. Alternative technologies, such as cisgenesis and genome-editing may address many of such issues and facilitate the development of genetically engineered crop varieties with multiple favourable traits. Genetic engineering and plant transformation have played a pivotal role in crop improvement via introducing beneficial foreign gene(s) or silencing the expression of endogenous gene(s) in crop plants. Genetically modified crops possess one or more useful traits, such as, herbicide tolerance, insect resistance, abiotic stress tolerance, disease resistance, and nutritional improvement. To date, nearly 525 different transgenic events in 32 crops have been approved for cultivation in different parts of the world. The adoption of transgenic technology has been shown to increase crop yields, reduce pesticide and insecticide use, reduce CO₂ emissions, and decrease the cost of crop production. However, widespread adoption of transgenic crops carrying foreign genes faces roadblocks due to concerns of potential toxicity and allergenicity to human beings, potential environmental risks, such as chances of gene flow, adverse effects on non-target organisms, evolution of resistance in weeds and insects etc. These concerns have prompted the adoption of alternative technologies like cisgenesis, intragenesis, and most recently, genome editing. Some of these alternative technologies can be utilized to develop crop plants that are free from any foreign gene hence, it is expected that such crops might achieve higher consumer acceptance as compared to the transgenic crops and would get faster regulatory approvals. In this review, we present a comprehensive update on the current status of the genetically modified (GM) crops under cultivation. We also discuss the issues affecting widespread adoption of transgenic GM crops and comment upon the recent tools and techniques developed to address some of these concerns.

Principles, Applications, and Biosafety of Plant Genome Editing Using CRISPR-Cas9

The terms genome engineering, genome editing, and gene editing, refer to modifications (insertions, deletions, substitutions) in the genome of a living organism. The most widely used approach to genome editing nowadays is based on Clustered Regularly Interspaced Short Palindromic Repeats and associated protein 9 (CRISPR-Cas9). In prokaryotes, CRISPR-Cas9 is an adaptive immune system that naturally protects cells from DNA virus infections. CRISPR-Cas9 has been modified to create a versatile genome editing technology that has a wide diversity of applications in medicine, agriculture, and basic studies of gene functions. CRISPR-Cas9 has been used in a growing number of monocot and dicot plant species to enhance yield, quality, and nutritional value, to introduce or enhance tolerance to biotic and abiotic stresses, among other applications. Although biosafety concerns remain, genome editing is a promising technology with potential to contribute to food production for the benefit of the growing human population. Here, we review the principles, current advances and applications of CRISPR-Cas9-based gene editing in crop improvement. We also address biosafety concerns and show that humans have been exposed to Cas9 protein homologues long before the use of CRISPR-Cas9 in genome editing.

Production of selectable marker gene-free Cavendish banana (Musa spp.) using a steroid-inducible recombinase platform

Genetic improvement of commercially accepted banana cultivars is strongly reliant on the ability to introduce genes that encode important agro-traits such as disease resistance. In most cases this can only be achieved using a transgenic approach. Public and regulatory acceptance of these events would greatly increase with “clean” single copy integration events free of the selectable marker gene and extraneous vector backbone. This would also allow for the successive addition of new genes and traits as they become available. In this study, we used the pMarker Free 1 (pMF1) vector containing the green fluorescent protein (gfp) reporter gene to assess the effectiveness of steroid-inducible recombination and positive/negative dual selection to regenerate transgenic Cavendish banana plants that were potentially free of the selectable marker gene. By examining the interaction of two different Agrobacterium strains with two different cultivars of Cavendish banana, namely Williams and Grand Naine, we describe a transformation and regeneration strategy that successfully produced marker-free, single transgene copy, gfp-expressing events. The system will provide a useful means of serially improving banana into the future.

A set of plasmids carrying antibiotic resistance markers and Cre recombinase for genetic engineering of nonconventional yeast Zygosaccharomyces rouxii

The so-called nonconventional yeasts are becoming increasingly attractive in food and industrial biotechnology. Among them, Zygosaccharomyces rouxii is known to be halotolerant, osmotolerant, petite negative, and poorly Crabtree positive. These traits and the high fermentative vigour make this species very appealing for industrial and food applications. Nevertheless, the biotechnological exploitation of Z. rouxii has been biased by the low availability of genetic engineering tools and the recalcitrance of this yeast towards the most conventional transformation procedures. Centromeric and episomal Z. rouxii plasmids have been successfully constructed with prototrophic markers, which limited their usage to auxotrophic strains, mainly derived from the Z. rouxii haploid type strain Centraalbureau voor Schimmelcultures (CBS) 732^T . By contrast, the majority of industrially promising Z. rouxii yeasts are prototrophic and allodiploid/aneuploid strains. In order to expand the genetic tools for manipulating these strains, we developed two centromeric and two episomal vectors harbouring KanMX^R and ClonNAT^R as dominant drug resistance markers, respectively. We also constructed the plasmid pGRCRE that allows the Cre recombinase-mediated marker recycling during multiple gene deletions. As proof of concept, pGRCRE was successfully used to rescue the kanMX-loxP module in Z. rouxii ATCC 42981 G418-resistant mutants previously constructed by replacing the MATα^P expression locus with the loxP-kanMX-loxP cassette.

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.

The CRISPR/Cas9 system and its applications in crop genome editing

The CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/CRISPR associated protein9) system is an RNA-guided genome editing tool that consists of a Cas9 nuclease and a single-guide RNA (sgRNA). By base-pairing with a DNA target sequence, the sgRNA enables Cas9 to recognize and cut a specific target DNA sequence, generating double strand breaks (DSBs) that trigger cell repair mechanisms and mutations at or near the DSBs sites. Since its discovery, the CRISPR/Cas9 system has revolutionized genome editing and is now becoming widely utilized to edit the genomes of a diverse range of crop plants. In this review, we present an overview of the CRISPR/Cas9 system itself, including its mechanism of action, system construction strategies, and the screening methods used to identify mutants containing edited genes. We evaluate recent examples of the use of CRISPR/Cas9 for crop plant improvement, and research into the function(s) of genes involved in determining crop yields, quality, environmental stress tolerance/resistance, regulation of gene transcription and translation, and the construction of mutant libraries and production of transgene-free genome-edited crops. In addition, challenges and future opportunities for the use of the CRISPR/Cas9 system in crop breeding are discussed.

Agrobacterium-mediated horizontal gene transfer: Mechanism, biotechnological application, potential risk and forestalling strategy

The extraordinary capacity of Agrobacterium to transfer its genetic material to host cell makes it evolve from phytopathogen to a powerful transgenic vector. Agrobacterium-mediated stable transformation is widely used as the preferred method to create transgenic plants for molecular plant biology research and crop breeding. Recent years, both mechanism and application of Agrobacterium-mediated horizontal gene transfer have made significant progresses, especially Agrobacterium-mediated transient transformation was developed for plant biotechnology industry to produce recombinant proteins. Agrobacterium strains are almost used and saved not only by each of microbiology and molecular plant labs, but also by many of plant biotechnology manufacturers. Agrobacterium is able to transfer its genetic material to a broad range of hosts, including plant and non-plant hosts. As a consequence, the concern of environmental risk associated with the accidental release of genetically modified Agrobacterium arises. In this article, we outline the recent progress in the molecular mechanism of Agrobacterium-meditated gene transfer, focus on the application of Agrobacterium-mediated horizontal gene transfer, and review the potential risk associated with Agrobacterium-meditated gene transfer. Based on the comparison between the infecting process of Agrobacterium as a pathogen and the transgenic process of Agrobacterium as a transgenic vector, we realize that chemotaxis is the distinct difference between these two biological processes and thus discuss the possible role of chemotaxis in forestalling the potential risk of Agrobacterium-meditated horizontal gene transfer to non-target plant species.

From plant metabolic engineering to plant synthetic biology: The evolution of the design/build/test/learn cycle

Genetic improvement of crops started since the dawn of agriculture and has continuously evolved in parallel with emerging technological innovations. The use of genome engineering in crop improvement has already revolutionised modern agriculture in less than thirty years. Plant metabolic engineering is still at a development stage and faces several challenges, in particular with the time necessary to develop plant based solutions to bio-industrial demands. However the recent success of several metabolic engineering approaches applied to major crops are encouraging and the emerging field of plant synthetic biology offers new opportunities. Some pioneering studies have demonstrated that synthetic genetic circuits or orthogonal metabolic pathways can be introduced into plants to achieve a desired function. The combination of metabolic engineering and synthetic biology is expected to significantly accelerate crop improvement. A defining aspect of both fields is the design/build/test/learn cycle, or the use of iterative rounds of testing modifications to refine hypotheses and develop best solutions. Several technological and technical improvements are now available to make a better use of each design, build, test, and learn components of the cycle. All these advances should facilitate the rapid development of a wide variety of bio-products for a world in need of sustainable solutions.

The potential of genetic engineering of plants for the remediation of soils contaminated with heavy metals

The genetic engineering of plants to facilitate the reclamation of soils and waters contaminated with inorganic pollutants is a relatively new and evolving field, benefiting from the heterologous expression of genes that increase the capacity of plants to mobilize, stabilize and/or accumulate metals. The efficiency of phytoremediation relies on the mechanisms underlying metal accumulation and tolerance, such as metal uptake, translocation and detoxification. The transfer of genes involved in any of these processes into fast-growing, high-biomass crops may improve their reclamation potential. The successful phytoextraction of metals/metalloids and their accumulation in aerial organs have been achieved by expressing metal ligands or transporters, enzymes involved in sulfur metabolism, enzymes that alter the chemical form or redox state of metals/metalloids and even the components of primary metabolism. This review article considers the potential of genetic engineering as a strategy to improve the phytoremediation capacity of plants in the context of heavy metals and metalloids, using recent case studies to demonstrate the practical application of this approach in the field.

A method for the production and expedient screening of CRISPR/Cas9-mediated non-transgenic mutant plants

Developing CRISPR/Cas9-mediated non-transgenic mutants in asexually propagated perennial crop plants is challenging but highly desirable. Here, we report a highly useful method using an Agrobacterium-mediated transient CRISPR/Cas9 gene expression system to create non-transgenic mutant plants without the need for sexual segregation. We have also developed a rapid, cost-effective, and high-throughput mutant screening protocol based on Illumina sequencing followed by high-resolution melting (HRM) analysis. Using tetraploid tobacco as a model species and the phytoene desaturase (PDS) gene as a target, we successfully created and expediently identified mutant plants, which were verified as tetra-allelic mutants. We produced pds mutant shoots at a rate of 47.5% from tobacco leaf explants, without the use of antibiotic selection. Among these pds plants, 17.2% were confirmed to be non-transgenic, for an overall non-transgenic mutation rate of 8.2%. Our method is reliable and effective in creating non-transgenic mutant plants without the need to segregate out transgenes through sexual reproduction. This method should be applicable to many economically important, heterozygous, perennial crop species that are more difficult to regenerate.

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.

Enabling Molecular Technologies for Trait Improvement in Wheat

Wheat is the major staple food crop and a source of calories for humans worldwide. A steady increase in the wheat production is essential to meet the demands of an ever-increasing global population and to achieve food security. The large size and structurally intricate genome of polyploid wheat had hindered the genomic analysis. However, with the advent of new genomic technologies such as next generation sequencing has led to genome drafts for bread wheat and its progenitors and has paved the way to design new strategies for crop improvement. Here we provide an overview of the advancements made in wheat genomics together with the available "omics approaches" and bioinformatics resources developed for wheat research. Advances in genomic, transcriptomic, and metabolomic technologies are highlighted as options to circumvent existing bottlenecks in the phenotypic and genomic selection and gene transfer. The contemporary reverse genetics approaches, including the novel genome editing techniques to inform targeted manipulation of a single/multiple genes and strategies for generating marker-free transgenic wheat plants, emphasize potential to revolutionize wheat improvement shortly.

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

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

Marker-free transgenic rice expressing the vegetative insecticidal protein (Vip) of Bacillus thuringiensis shows broad insecticidal properties

Genetically engineered rice lines with broad insecticidal properties against major lepidopteran pests were generated using a synthetic, truncated form of vegetative insecticidal protein (Syn vip3BR) from Bacillus thuringiensis. The selectable marker gene and the redundant transgene(s) were eliminated through Cre/ lox mediated recombination and genetic segregation to make consumer friendly Bt -rice. For sustainable resistance against lepidopteran insect pests, chloroplast targeted synthetic version of bioactive core component of a vegetative insecticidal protein (Syn vip3BR) of Bacillus thuringiensis was expressed in rice under the control of green-tissue specific ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit gene promoter. The transgenic plants (in Oryza sativa indica Swarna cultivar) showed high insect mortality rate in vitro against major rice pests, yellow stem borer (Scirpophaga incertulas), rice leaf folder (Cnaphalocrocis medinalis) and rice horn caterpillar (Melanitis leda ismene) in T1 generation, indicating insecticidal potency of Syn vip3BR. Under field conditions, the T1 plants showed considerable resistance against leaf folders and stem borers. The expression cassette (vip-lox-hpt-lox) as well as another vector with chimeric cre recombinase gene under constitutive rice ubiquitin1 gene promoter was designed for the elimination of selectable marker hygromycin phosphotransferase (hptII) gene. Crossing experiments were performed between T1 plants with single insertion site of vip-lox-hpt-lox T-DNA and one T1 plant with moderate expression of cre recombinase with linked bialaphos resistance (syn bar) gene. Marker gene excision was achieved in hybrids with up to 41.18 % recombination efficiency. Insect resistant transgenic lines, devoid of selectable marker and redundant transgene(s) (hptII + cre-syn bar), were established in subsequent generation through genetic segregation.

Minimizing the time and cost of production of transgenic alfalfa libraries using the highly efficient completely sequenced vector pPZP200BAR

Alfalfa is the most important forage legume worldwide. However, similar to other minor forage crops, it is usually harvested along with weeds, which decrease its nutrient quality and thus reduce its high value in the market. In addition, weeds reduce alfalfa yield by about 50 %. Although weeds are the limiting factor for alfalfa production, little progress has been made in the incorporation of herbicide-tolerant traits into commercial alfalfa. This is partially due to the high times and costs needed for the production of vast numbers of transgenic alfalfa events as an empirical approach to bypass the random transgenic silencing and for the identification of an event with optimal transgene expression. In this focus article, we report the complete sequence of pPZP200BAR and the extremely high efficiency of this binary vector in alfalfa transformation, opening the way for rapid and inexpensive production of transgenic events for alfalfa improvement public programs.

Assessing zygosity in progeny of transgenic plants: current methods and perspectives

Homozygosity is highly desirable in transgenic plants research to ensure the stable integration and inheritance of transgene(s). Simple, reliable and high-throughput techniques to detect the zygosity of transgenic events in plants are invaluable tools for biotechnology and plant breeding companies. Currently, a number of basic techniques are being used to determine the zygosity of transgenic plants in T1 generation. For successful application of any technique, precision and simplicity of approach combined with the power of resolution are important parameters. On the basis of simplicity, resolution and cost involved, the available techniques have been classified into three major classes which are conventional methods, current methods and next generation methods. Conventional methods include antibiotic marker-based selection and the highly labor intensive Southern blot analysis. In contrast, methods such as real time PCR, TAIL PCR and competitive PCR are not only cost effective but rapid as well. Moreover, methods such as NGS, digital PCR and loop-mediated isothermal amplification also provide a cost effective, fast and not so labor intensive substitute of current methods. In this review, we have attempted to compare and contrast all the available efficient methods to distinguish homozygous plants in progeny of transgenics. This review also provides information of various techniques available for determining zygosity in plants so as to permit researchers to make informed choices of techniques that best suit their analyses. More importantly, detection and subsequent selection of homozygous individuals is central for facilitating the movement of transgenic plants from the laboratory to the field.

Jasmonates: Multifunctional Roles in Stress Tolerance

Jasmonates (JAs) [Jasmonic acid (JA) and methyl jasmonates (MeJAs)] are known to take part in various physiological processes. Exogenous application of JAs so far tested on different plants under abiotic stresses particularly salinity, drought, and temperature (low/high) conditions have proved effective in improving plant stress tolerance. However, its extent of effectiveness entirely depends on the type of plant species tested or its concentration. The effects of introgression or silencing of different JA- and Me-JA-related genes have been summarized in this review, which have shown a substantial role in improving crop yield and quality in different plants under stress or non-stress conditions. Regulation of JAs synthesis is impaired in stressed as well as unstressed plant cells/tissues, which is believed to be associated with a variety of metabolic events including signal transduction. Although, mitogen activated protein kinases (MAPKs) are important components of JA signaling and biosynthesis pathways, nitric oxide, ROS, calcium, ABA, ethylene, and salicylic acid are also important mediators of plant growth and development during JA signal transduction and synthesis. The exploration of other signaling molecules can be beneficial to examine the details of underlying molecular mechanisms of JA signal transduction. Much work is to be done in near future to find the proper answers of the questions like action of JA related metabolites, and identification of universal JA receptors etc. Complete signaling pathways involving MAPKs, CDPK, TGA, SIPK, WIPK, and WRKY transcription factors are yet to be investigated to understand the complete mechanism of action of JAs.

Plant phosphomannose isomerase as a selectable marker for rice transformation

The E. coli phosphomannose isomerase (EcPMI) gene is widely used as a selectable marker gene (SMG) in mannose (Man) selection-based plant transformation. Although some plant species exhibit significant PMI activity and active PMIs were even identified in Man-sensitive plants, whether plant PMIs can be used as SMGs remains unclear. In this study, we isolated four novel PMI genes from Chlorella variabilis and Oryza sativa. Their isoenzymatic activities were examined in vitro and compared with that of EcPMI. The active plant PMIs were separately constructed into binary vectors as SMGs and then transformed into rice via Agrobacterium. In both Indica and Japonica subspecies, our results indicated that the plant PMIs could select and produce transgenic plants in a pattern similar to that of EcPMI. The transgenic plants exhibited an accumulation of plant PMI transcripts and enhancement of the in vivo PMI activity. Furthermore, a gene of interest was successfully transformed into rice using the plant PMIs as SMGs. Thus, novel SMGs for Man selection were isolated from plants, and our analysis suggested that PMIs encoding active enzymes might be common in plants and could potentially be used as appropriate genetic elements in cisgenesis engineering.

Development of Marker-Free Transgenic Potato Tubers Enriched in Caffeoylquinic Acids and Flavonols

Potato (Solanum tuberosum L.) is a major crop worldwide that meets human economic and nutritional requirements. Potato has several advantages over other crops: easy to cultivate and store, cheap to consume, and rich in a variety of secondary metabolites. In this study, we generated three marker-free transgenic potato lines that expressed the Arabidopsis thaliana flavonol-specific transcriptional activator AtMYB12 driven by the tuber-specific promoter Patatin. Marker-free potato tubers displayed increased amounts of caffeoylquinic acids (CQAs) (3.35-fold increases on average) and flavonols (4.50-fold increase on average). Concentrations of these metabolites were associated with the enhanced expression of genes in the CQA and flavonol biosynthesis pathways. Accumulation of CQAs and flavonols resulted in 2-fold higher antioxidant capacity compared to wild-type potatoes. Tubers from these marker-free transgenic potatoes have therefore improved antioxidant properties.

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.