Stable Transformation Frequency Research Articles

In vitro</em></strong><strong> direct regeneration and <em>Agrobacterium tumefaciens</em> mediated <em>in planta</em> transformation of <em>Ocimum sanctum L.

An in vitro regeneration system for propagation has been successfully developed for a valuable medicinal and aromatic plant ‘ Ocimum sanctum L’. In the present study, petiole explants, from in-vitro grown cultures of O. sanctum, was used for direct regeneration. The developed protocol employed 98% of regeneration frequency in addition to 9.6 shoots per explant when cultured on Murashige and Skoog (MS) medium fortified with 3 mg/L benzylamino purine (BAP) and 1 mg/L Naphthalene acetic acid (NAA). Furthermore, Agrobacterium tumefaciens Mediated genetic Transformation (ATMT) protocol (transient and stable) was also developed using LBA4404 strain harboring pBI121 with uid-A and neomycin phosphotransferase genes. The regenerated transformants were shifted on MS with kanamycin (50 mg/L) and afterwards placed on the half-strength MS medium. The validation was done through polymerase chain reaction (PCR) with neomycin phosphotransferase-II ( npt-II) & β-glucoronidase ( uid-A) gene primers. The maximum stable transformation frequency of 70% ± 0.35 was achieved. Hence, it is apparent that the established protocols i.e. in vitro direct regeneration and ATMT are appropriate for integrating novel enzymes/genes through high throughput techniques such as gene tagging, and targeted gene replacement to modulate the primary as well as secondary metabolic flux towards desired agronomic product or trait in planta.

Improvement of Gene Delivery and Mutation Efficiency in the CRISPR-Cas9 Wheat (Triticum aestivum L.) Genomics System via Biolistics.

Discovery of the CRISPR-Cas9 gene editing system revolutionized the field of plant genomics. Despite advantages in the ease of designing gRNA and the low cost of the CRISPR-Cas9 system, there are still hurdles to overcome in low mutation efficiencies, specifically in hexaploid wheat. In conjunction with gene delivery and transformation frequency, the mutation efficiency bottleneck has the potential to slow down advancements in genomic editing of wheat. In this study, nine bombardment parameter combinations using three gold particle sizes and three rupture disk pressures were tested to establish optimal stable transformation frequencies in wheat. Utilizing the best transformation protocol and a knockout cassette of the phytoene desaturase gene, we subjected transformed embryos to four temperature treatments and compared mutation efficiencies. The use of 0.6 μm gold particles for bombardment increased transformation frequencies across all delivery pressures. A heat treatment of 34 °C for 24 h resulted in the highest mutation efficiency with no or minimal reduction in transformation frequency. The 34 °C treatment produced two M0 mutant events with albino phenotypes, requiring biallelic mutations in all three genomes of hexaploid wheat. Utilizing optimal transformation and heat treatment parameters greatly increases mutation efficiency and can help advance research efforts in wheat genomics.

Co-culturing on dry filter paper significantly increased the efficiency of Agrobacterium-mediated transformations of maize immature embryos.

In the Agrobacterium tumefaciens-mediated transformations of maize immature embryos (IEs), the common co-culturing media used are MS or N6-based (MC). Here, we used a novel co-culturing method in which maize 'Qi319' IEs inoculated with Agrobacterium-harboring target vector were placed on dry filter paper (DC) in a petri dish. To compare the effects of the DC and MC co-culturing methods on transformation efficiency, we designed three experiments: (1) A. tumefaciens strain AGL1 independently carrying two plasmids, pXQD12 and pXQD70; (2) two A. tumefaciens strains, AGL1 and EHA105, carrying pXQD12; and (3) strains AGL1 and EHA105 each independently inoculated with pXQD12 and pXQD70 for different infiltration periods, 5, 10, 15, 20 and 25min. We used A. tumefaciens to inoculate IEs derived from maize ears 9-15d after pollination, and then IEs were placed in petri dishes for co-culturing. The DC treatment significantly increased the percentage of IEs expressing green fluorescence protein (%GFP), indicating positive transformants. DC-treated IEs had ~ 3 to 4 times the %GFP compared with MC-treated IEs at 8d after inoculation (3d co-culture and 5d restoration). The average regeneration frequency (%GFP positive regenerated calli of infected IEs) and stable transformation frequency (%GFP positive T0 plants of infected IEs) significantly increased with the DC treatment. Thus, the DC method may be used to develop a more efficient Agrobacterium-mediated transformation method for maize IEs.

Transfection by Electroporation.

Electroporation-the use of high-voltage electric shocks to introduce DNA into cells-can be used with most cell types, yields a high frequency of both stable transformation and transient gene expression, and, because it requires fewer steps, can be easier than alternate techniques. This unit describes electroporation of mammalian cells, including ES cells for the preparation of knock-out, knock-in, and transgenic mice. Protocols are described for the use of electroporation in vivo to perform gene therapy for cancer therapy and DNA vaccination. Also described are modifications for preparation and transfection of plant protoplasts. © 2018 by John Wiley & Sons, Inc.

Improved Agrobacterium-mediated transformation and direct plant regeneration in four cultivars of finger millet (Eleusine coracana (L.) Gaertn.)

We have developed an improved Agrobacterium-mediated transformation and rapid regeneration system for four cultivars (‘CO(Ra)-14’, ‘PR-202’, ‘Try-1’ and ‘Paiyur-2’) of finger millet using optimized transformation and direct plant regeneration conditions. The shoot apical meristems (SAMs) were used as explants in this study. Agrobacterium strain EHA105 carrying binary vector pCAMBIA1301 was used to optimize the transformation conditions. Concentration of hygromycin, the optical density of the culture, infection time, age of the explants, co-cultivation period, the concentrations of acetosyringone and antibiotics were optimized to improve the transformation frequency. The highest frequency of mean transient gus expression (85.1%) was achieved in cultivar ‘CO(Ra)-14’. The entire transformation procedure, from initiating SAMs to planting putative transgenic plantlets in the greenhouse, was completed within 45 days with the highest stable transformation frequency of 11.8% for ‘CO(Ra)-14’. PCR, gus staining and Southern blot analyses were performed in T0 and T1 generations to confirm the gene integration. Six events from T0 had a single copy of the transgene and showed a normal Mendelian pattern of segregation. To our knowledge, this is the first report on the high frequency transformation of finger millet by Agrobacterium and subsequent recovery of transgenic plants via direct plant regeneration without a callus phase, in short duration (45 days). The proposed protocol could be supportive in breaking through the bottleneck in transformation and regeneration of finger millet cultivars.

Transfection by Electroporation

Electroporation-the use of high-voltage electric shocks to introduce DNA into cells-can be used with most cell types, yields a high frequency of both stable transformation and transient gene expression, and, because it requires fewer steps, can be easier than alternative techniques. This unit describes electroporation of mammalian cells, including ES cells, for the preparation of knock-out, knock-in, and transgenic mice. Protocols are described for the use of electroporation in vivo to perform gene therapy for cancer, as well as for DNA vaccination. © 2017 by John Wiley & Sons, Inc.

An Agrobacterium tumefaciens Strain with Gamma-Aminobutyric Acid Transaminase Activity Shows an Enhanced Genetic Transformation Ability in Plants

Agrobacterium tumefaciens has the unique ability to mediate inter-kingdom DNA transfer, and for this reason, it has been utilized for plant genetic engineering. To increase the transformation frequency in plant genetic engineering, we focused on gamma-aminobutyric acid (GABA), which is a negative factor in the Agrobacterium-plant interaction. Recent studies have shown contradictory results regarding the effects of GABA on vir gene expression, leading to the speculation that GABA inhibits T-DNA transfer. In this study, we examined the effect of GABA on T-DNA transfer using a tomato line with a low GABA content. Compared with the control, the T-DNA transfer frequency was increased in the low-GABA tomato line, indicating that GABA inhibits T-DNA transfer. Therefore, we bred a new A. tumefaciens strain with GABA transaminase activity and the ability to degrade GABA. The A. tumefaciens strain exhibited increased T-DNA transfer in two tomato cultivars and Erianthus arundinacues and an increased frequency of stable transformation in tomato.

Agrobacterium T-DNA integration into the plant genome can occur without the activity of key non-homologous end-joining proteins.

Non-homologous end joining (NHEJ) is the major model proposed for Agrobacterium T-DNA integration into the plant genome. In animal cells, several proteins, including KU70, KU80, ARTEMIS, DNA-PKcs, DNA ligase IV (LIG4), Ataxia telangiectasia mutated (ATM), and ATM- and Rad3-related (ATR), play an important role in 'classical' (c)NHEJ. Other proteins, including histone H1 (HON1), XRCC1, and PARP1, participate in a 'backup' (b)NHEJ process. We examined transient and stable transformation frequencies of Arabidopsis thaliana roots mutant for numerous NHEJ and other related genes. Mutants of KU70, KU80, and the plant-specific DNA Ligase VI (LIG6) showed increased stable transformation susceptibility. However, these mutants showed transient transformation susceptibility similar to that of wild-type plants, suggesting enhanced T-DNA integration in these mutants. These results were confirmed using a promoter-trap transformation vector that requires T-DNA integration into the plant genome to activate a promoterless gusA (uidA) gene, by virus-induced gene silencing (VIGS) of Nicotiana benthamiana NHEJ genes, and by biochemical assays for T-DNA integration. No alteration in transient or stable transformation frequencies was detected with atm, atr, lig4, xrcc1, or parp1 mutants. However, mutation of parp1 caused high levels of T-DNA integration and transgene methylation. A double mutant (ku80/parp1), knocking out components of both NHEJ pathways, did not show any decrease in stable transformation or T-DNA integration. Thus, T-DNA integration does not require known NHEJ proteins, suggesting an alternative route for integration.

<i>Agrobacterium</i>-Mediated Transformation of Mexican Lime (<i>Citrus aurantifolia</i> Swingle) Using Optimized Systems for Epicotyls and Cotyledons

Transgenic Mexican lime (Citrus aurantifolia Swingle) was produced through two explant sources, each using systems previously optimized for each source. One used epicotyls segments, which was the predominant explant for transgenic Citrus production following co-cultivation with Agrobacterium, and has a well-established protocol. The other procedure used embryo cotyledons from mature seeds, which was developed in our lab as an alternative for stable Citrus transformation. Cotyledon transformation and regeneration protocols were optimized by comparing variables in culture medium composition on shoot regeneration and four parameters in transient transformation. The optimized protocols were compared, and frequency of regeneration, frequency of transgenic plant-recovery and stable transformation efficiency indicated the superiority of the cotyledon protocol for Agrobacterium-mediated genetic transformation in Mexican lime. The tissue choice resulted in marked improvement in shoot regeneration (14.1% of explants producing shoots in epicotyls; 55.8% in cotyledons), stable transformation frequency (11.4% of epicotyls explants; 40.2% in cotyledons), and frequency of transgenic plant-recovery (37.9% in epicotyl explants; 92.6% in cotyledons). Thus, easy availability of explants using embryo cotyledons from mature seeds, technical simplicity, shortening of transformation time-course, and higher transformation and regeneration frequencies makes this new system an attractive alternative over the previously published Citrus transformation protocols. In the course of this project, we generated Mexican lime with a Recombinase Mediated Exchange Cassette landing pad, which was designed for stacking transgenes.

Novel compounds that enhance Agrobacterium-mediated plant transformation by mitigating oxidative stress.

Agrobacterium tumefaciens caused tissue browning leading to subsequent cell death in plant transformation and novel anti-oxidative compounds enhanced Agrobacterium -mediated plant transformation by mitigating oxidative stress. Browning and death of cells transformed with Agrobacterium tumefaciens is a long-standing and high impact problem in plant transformation and the agricultural biotechnology industry, severely limiting the production of transgenic plants. Using our tomato cv. MicroTom transformation system, we demonstrated that Agrobacterium caused tissue browning (TB) leading to subsequent cell death by our correlation study. Without an antioxidant (lipoic acid, LA) TB was severe and associated with high levels of GUS transient expression and low stable transformation frequency (STF). LA addition shifted the curve in that most TB was intermediate and associated with the highest levels of GUS transient expression and STF. We evaluated 18 novel anti-oxidative compounds for their potential to enhance Agrobacterium-mediated transformation, by screening for TB reduction and monitoring GUS transient expression. Promising compounds were further evaluated for their effect on MicroTom and soybean STF. Among twelve non-antioxidant compounds, seven and five significantly (P < 0.05) reduced TB and increased STF, respectively. Among six antioxidants four of them significantly reduced TB and five of them significantly increased STF. The most efficient compound found to increase STF was melatonin (MEL, an antioxidant). Optimal concentrations and stages to use MEL in transformation were determined, and Southern blot analysis showed that T-DNA integration was not affected by MEL. The ability of diverse compounds with different anti-oxidative mechanisms can reduce Agrobacterium-mediated TB and increase STF, strongly supporting that oxidative stress is an important limiting factor in Agrobacterium-mediated transformation and the limiting factor can be controlled by these compounds at different levels.

Protocol for Direct Transformation through Gene Gun in Capsicum annuum L.Cv. Mathania

protocol for direct transformation through biolistic gun has been established for chilli (Capsicum annuum L. cv. Mathania). A disarmed strain of Agrobacterium tumefaciens EHA 105 carrying a binary vector plasmid p35SGUSINT has been used for transformation. This vector contains neomycin phosphotransferase gene (npt II), whose expression confirms Kanamycin resistance in transformants. In addition to npt II, plasmid encodes -glucuronidase, reporter enzyme used for studying the expression of foreign genes in plants. We report for the first time, the use of Nitrogen gas for the biolistic transformation of chilli. We found physical parameter like 6 cm target distance with 900 psi rupture disk for partical gun experiment the most efficient for transient GUS expression. In case of nitrogen gas the frequency of transient GUS expression was better both in leaves (67.74%) and hypocotyl (69.18%) and frequency of conversion of transient to stable transformation was 3.4% in leaves and 4.0% in hypotocyls as against 63.91 % transient GUS expression showed by leaves and 63.77% showed by hypocotyls when helium gas was used in biolistic transformation and stable transformation frequency was 3.0% in leaves and 4.0% in hypocotyls. The transgenic nature of the regenerated plants was confirmed by the histochemical staining of GUS, and polymerase chain reaction (PCR) analysis of npt II gene.

Increased 1‐aminocyclopropane‐1‐carboxylate deaminase activity enhances Agrobacterium tumefaciens‐mediated gene delivery into plant cells

Agrobacterium-mediated transformation is a useful tool for the genetic modification in plants, although its efficiency is low for several plant species. Agrobacterium-mediated transformation has three major steps in laboratory-controlled experiments: the delivery of T-DNA into plant cells, the selection of transformed plant cells, and the regeneration of whole plants from the selected cells. Each of these steps must be optimized to improve the efficiency of Agrobacterium-mediated plant transformation. It has been reported that increasing the number of cells transformed by T-DNA delivery can improve the frequency of stable transformation. Previously, we demonstrated that a reduction in ethylene production by plant cells during cocultivation with A. tumefaciens-expressing 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase resulted in increased T-DNA delivery into the plant cells. In this study, to further improve T-DNA delivery by A. tumefaciens, we modified the expression cassette of the ACC deaminase gene using vir gene promoter sequences. The ACC deaminase gene driven by the virD1 promoter was expressed at a higher level, resulting in a higher ACC deaminase activity in this A. tumefaciens strain than in the strain with the lac promoter used in a previous study. The newly developed A. tumefaciens strain improves the delivery of T-DNA into Solanum lycopersicum (tomato) and Erianthus ravennae plants and thus may be a powerful tool for the Agrobacterium-mediated genetic engineering of plants.

Optimisation of tomato Micro-tom regeneration and selection on glufosinate/Basta and dependency of gene silencing on transgene copy number

An efficient protocol of transformation and selection of transgenic lines of Micro-tom, a widespread model cultivar for tomato, is reported. RNA interference silencing efficiency and stability have been investigated and correlated with the number of insertions. Given its small size and ease of cultivation, the tomato (Solanum lycopersicon) cultivar Micro-tom is of widespread use as a model tomato plant. To create and screen transgenic plants, different selectable markers are commonly used. The bar marker carrying the resistance to the herbicide glufosinate/Basta, has many advantages, but it has been little utilised and with low efficiency for identification of tomato transgenic plants. Here we describe a procedure for accurate selection of transgenic Micro-tom both in vitro and in soil. Immunoblot, Southern blot and phenotypic analyses showed that 100% of herbicide-resistant plants were transgenic. In addition, regeneration improvement has been obtained by using 2mg/l Gibberellic acid in the shoot elongation medium; rooting optimisation on medium containing 1mg/l IAA allowed up to 97% of shoots developing strong and very healthy roots after only 10days. Stable transformation frequency by infection of leaf explants with Agrobacterium reached 12%. Shoots have been induced by combination of 1mg/l zeatin-trans and 0.1mg/l IAA. Somatic embryogenesis of cotyledon on medium containing 1mg/l zeatin+2mg/l IAA is described in Micro-tom. The photosynthetic psbS gene has been used as reporter gene for RNA silencing studies. The efficiency of gene silencing has been found equivalent using three different target gene fragments of 519, 398 and 328bp. Interestingly, silencing efficiency decreased from T0 to the T3 generation in plants containing multiple copies of the inserted T-DNA, while it was stable in plants containing a single insertion.

Transfection by Electroporation

Electroporation--the use of high-voltage electric shocks to introduce DNA into cells--can be used with most cell types, yields a high frequency of both stable transformation and transient gene expression, and, because it requires fewer steps, can be easier than alternate techniques. This unit describes electroporation of mammalian cells, including ES cells for the preparation of knock-out, knock-in, and transgenic mice. Protocols are described for the use of electroporation in vivo to perform gene therapy for cancer therapy and DNA vaccination. Also described are modifications for preparation and transfection of plant protoplasts.

Transfection by Electroporation

Electroporation--the use of high-voltage electric shocks to introduce DNA into cells--can be used with most cell types, yields a high frequency of both stable transformation and transient gene expression, and, because it requires fewer steps, can be easier than alternate techniques. This unit describes electroporation of mammalian cells, including ES cells for the preparation of knock-out, knock-in, and transgenic mice. Protocols are described for the use of electroporation in vivo to perform gene therapy for cancer therapy and DNA vaccination. Also described are modifications for preparation and transfection of plant protoplasts.

Transfection by Electroporation

Electroporation--the use of high-voltage electric shocks to introduce DNA into cells--can be used with most cell types, yields a high frequency of both stable transformation and transient gene expression, and, because it requires fewer steps, can be easier than alternate techniques. This unit describes electroporation of mammalian cells, including ES cells for the preparation of knock-out, knock-in, and transgenic mice. Protocols are described for the use of electroporation in vivo to perform gene therapy for cancer therapy and DNA vaccination. Also described are modifications for preparation and transfection of plant protoplasts.

Direct shoot organogenesis on hypocotyl explants with collar region from in vitro seedlings of Coffea canephora Pierre ex. Frohner cv. C × R and Agrobacterium tumefaciens-mediated transformation

Direct differentiation of shoot buds from the collar region of hypocotyl segments of Coffea canephora was obtained on Murashige and Skoog (MS) medium supplemented with 40 μM silver nitrate (AgNO3) and growth regulators indole-3-acetic acid (IAA) and N6 benzyladenine (BA). The highest response to shoot differentiation of 60% frequency and the maximum number of multiple shoots (2–3) per explant were obtained on MS medium containing 8.87 μM BA and 2.85 μM IAA. Apart from this, 70% of hypocotyl explants produced yellow friable embryogenic callus and also globular primary somatic embryos. Subsequent transfer onto the same medium induced secondary somatic embryogenesis. The micro-shoots, upon transfer to the same medium, in the following 6 weeks developed into 4-cm-long shoots with a single root. Further subculturing onto the same medium induced 4–5 roots in a 4-week period. The resulting plantlets were hardened and transferred to micro-pots containing sand:compost mixture (1:2), where 65% of them survived and resumed growth. By using optimal levels of AgNO3, it was possible to obtain effective direct organogenesis and embryogenesis. This system was used for genetic transformation using Agrobacterium tumefaciens. A stable transformation frequency of 2–5% was obtained when both types of explants, i.e., hypocotyl explants with collar region or hypocotyl explants without collar region, were co-cultivated with A. tumefaciens GV 3101 harboring pCAMBIA 1305.2 binary vector. This is the first report of a hypocotyl collar region-based Agrobacterium-mediated transformation protocol for the economically important tropical plant C. canephora.

Transformation of the Moss Physcomitrella patens Using T-DNA Mutagenesis

INTRODUCTIONIn this protocol, the transformation of moss (Physcomitrella patens) protoplasts is performed via Agrobacterium-mediated transfer of T-DNA. Protoplasts are incubated with Agrobacterium and acetoseringone in regeneration medium. They are then washed and plated on antibiotic-containing medium to select for T-DNA insertion in stable transformants. The transformation rate for this protocol is typically 10−4 (expressed as the frequency of stable transformants among regenerants surviving the transformation procedure).

Efficient, reproducible Agrobacterium-mediated transformation of sorghum using heat treatment of immature embryos

A number of parameters related to Agrobacterium-mediated infection were tested to optimize transformation frequencies of sorghum (Sorghum bicolor L.). A plasmid with a selectable marker, phosphomannose isomerase, and an sgfp reporter gene was used. First, storing immature spikes at 4 degrees C before use decreased frequency of GFP-expressing calli, for example, in sorghum variety P898012 from 22.5% at 0 day to 6.4% at 5 days. Next, heating immature embryos (IEs) at various temperatures for 3 min prior to Agrobacterium infection increased frequencies of GFP-expressing calli, of mannose-selected calli and of transformed calli. The optimal 43 degrees C heat treatment increased transformation frequencies from 2.6% with no heat to 7.6%. Using different heating times at 43 degrees C prior to infection showed 3 min was optimal. Centrifuging IEs with no heat or heating at various temperatures decreased frequencies of all tissue responses; however, both heat and centrifugation increased de-differentiation of tissue. If IEs were cooled at 25 degrees C versus on ice after heating and prior to infection, numbers with GFP-expressing cells increased from 34.2 to 49.1%. The most optimal treatment, 43 degrees C for 3 min, cooling at 25 degrees C and no centrifugation, yielded 49.1% GFP-expressing calli and 8.3% stable transformation frequency. Transformation frequencies greater than 7% were routinely observed using similar treatments over 5 months of testing. This reproducible frequency, calculated as numbers of independent IEs producing regenerable transgenic tissues, confirmed by PCR, western and DNA hybridization analysis, divided by total numbers of IEs infected, is several-fold higher than published frequencies.

Production of transgenic barrel medic (Medicago truncatula Gaernt.) using the ipt-type MAT vector system and impairment of Recombinase-mediated excision events

Expression of the uidA reporter gene was tested in transformation experiments of barrel medic (Medicago truncatula Gaertn.) with the ipt-type control vectors pIPT5, pIPT10 and pIPT20 and distinct in vitro culture conditions. The highest GUS expression levels were obtained with the pIPT10 construct carrying the ipt gene under the control of the native ipt promoter and using kanamycin as selective agent. The ipt-shooty transformants, characterized by the absence of both rooting ability and apical dominance associated with vitrification, were easily identified by visual selection. Using only the ipt gene as selectable marker, we obtained a stable transformation frequency of 9.8% with pIPT10 construct. The ipt-type MAT vector pEXM2 was then used to monitor the excision events mediated by the yeast Recombinase and the consequent production of ipt marker-free transgenic plants. Transgenic ipt-shooty lines were recovered at a frequency of 7.9% in the absence of kanamycin-based selection. The ipt-shooty phenotype was maintained in all the transgenic lines and no reversion to the normal phenotype occurred. PCR analysis revealed the presence of the 'hit and run' cassette in the genome of all the regenerated ipt-shooty lines while RT-PCR experiments confirmed the expression of the R gene, encoding the yeast Recombinase. A detailed molecular investigation, carried out to verify the integrity of the RS sites, revealed that these regions were intact in most cases. Our results with barrel medic suggest that the MAT system must be carefully evaluated and discussed on a case by case basis.