Development of an airgun device for particle bombardment

Construction and operation of an airgun device for transient gene-expression studies in monocots is described. Compressed air in a cylinder of an airgun was used as the source of propulsion for DNA-coated gold or tungsten particles. Under a partial vacuum of 700 mm Hg, velocity of the macrocarrier was measured at 520 m sec^?1 and 432 m sec^?1 at atmospheric pressure. Optimum distance from the stopping plate to different target cells during bombardment ranged from 4 to 7 cm. Mean transformation efficiency of the GUS-gene marker was estimated at 350 transformants per 65 mg fresh weight of the maize cultured cells. Up to 200 GUS transformed cells were detected per 100 mg of embryogenic rice callus. Use of gold flakes or tungsten powder as microcarriers resulted in similar transformation rates. No transformation was observed in any cells when DNA constructs contained prokaryotic translation initiation sequences for the GUS gene. Based on transient GUS assays, further modification of the airgun device will likely be necessary to obtain high stable transformation rates.     

Stable transformation of tomato cell cultures after bombardment with plasmid and YAC DNA

Summary Stable transformants were obtained after microprojectile particle bombardment of tomato cell suspensions (Lycopersicon esculentum cv VFNT Cherry and L. pennellii). The suspensions were bombarded with tungsten particles coated with either plasmid (6.3 kb) or yeast artificial chromosome (YAC) (80 kb) DNA containing the ß-glucuronidase (GUS) and neomycin phosphotransferase II (nptII) genes. The YAC DNA contained an insert of approximately 50 kb of DNA from VFNT Cherry. L. pennellii suspensions were more amenable to transformation than VFNT Cherry; more kanamycin-resistant calli were recovered from L. pennelli after bombardment with plasmid DNA, and only L. pennellii cells produced transformants after bombardment with YAC DNA. DNA gel blot analysis confirmed the presence of the nptll and GUS genes. This analysis also confirmed the integration of YAC DNA into the genome of the kanamycin-resistant calli and suggested that the level of intactness of the integrated YAC DNA was fairly high in four of the five transformants examined. Microprojectile bombardment of regenerable cultures with YACs may ultimately aid in map-based cloning of agriculturally-important genes.Abbreviations YAC yeast artificial chromosome - MS Murashige and Skoog - 2,4-D 2,4-dichlorophenoxy-acetic acid - IAA indole-3-acetic acid - GUS ß-glucuronidase - nptII neomycin phosphotransferase II     

Hydrolysis of glucuronide-based substrates mediated by tungsten, Cu2+, Fe2+, and Zn2+

A variety of metal microprojectiles are currently used for carrying foreign DNA into living cells via particle-acceleration techniques. While developing a microprojectile-mediated protocol for transforming cells of sugarbeet ( Beta vulgaris L.), formation of a blue precipitate was observed with the indigoqenic substrate 5-bromo-4-chloro-3-indolyl-β-D-glucuronic acid (X-gluc) in the absence of gusA DNA encoding β-D-glucuronidase (GUS). Tungsten microcarriers, but not gold or silicon carbide, proved capable of catalyzing the cleavage of the glucuronide residue from three histochemical substrates evaluated: X-gluc, salmon X-gluc and magenta X-gluc. Indigo-stained sugarbeet cells were observed following bombardment with tungsten in the absence of DNA. Addition of oxidative catalysts to tungsten microcarriers during substrate incubation had no apparent effect on the metal-mediated catalysis. Treatment of microcarriers with Proteinase K and heat ruled out the presence of enzymes. Microbiological evaluation indicated the absence of contaminating microbes. Similarly, metal-catalyzed hydrolysis of the fluorogenic substrate 4-methylumbelliferyl-β-D-glucuronic acid (4-MUG) was observed in the presence of tungsten spheres but not with gold or silicon carbide particles. With this substrate, hydrolysis also occurred with millimolar concentrations of Cu²⁺, Fe²⁺ and Zn²⁺ ions. Consequently, careful monitoring of DNA-minus controls and avoidance of millimolar concentrations of Cu²⁺, Fe²⁺ and Zn²⁺ ions are recommended in microprojectile bombardment experiments where transient assays for gusA expression are performed.     

Optimization of biolistic bombardment parameters for <Emphasis Type="Italic">Dendrobium</Emphasis> Sonia 17 calluses using GFP and GUS as the reporter system

Genetic transformation system of Dendrobium Sonia 17 was optimized using green fluorescent protein (GFP) and -glucuronidase (GUS) gene as the reporter systems. The 35S-sgfp-TYG-nos (p35S) and pSMDFR, carrying sgfp and gusA gene, respectively, were co-bombarded into the calluses. Parameters optimized were acceleration pressure, target distance, gold particle size, pre-bombardment cultured time, plasmid DNA precipitation, total plasmid DNA and the ratio of the plasmids co-bombarded. Both reporter systems responded similarly to the bombardment parameters investigated. Based on the GUS/GFP spot counts, the GFP expression rate was higher than that for GUS under the control of the same promoter, CaMV 35S. GFP could be used as the reporter system for the co-bombardment as it was rapid and non-destructive system to monitor the transformed tissues. A combination of GFP and antibiotic resistance gene was used to select stable putative transformants.     

Early events in microprojectile bombardment: cell viability and particle location

Tungsten and gold particles, coated with plasmid DNA harboring the β-glucuronidase (GUS) and neomycin phosphotransferase II (npt-II) genes, were delivered into tobacco primary leaves and suspension-cultured cells of maize using the helium particle inflow gun. Cell viability and particle localization were determined 1 and 2 days after bombardment. Of the counted particles, 7–10% penetrated into or through the epidermis. Blue spots on tobacco leaves appeared as a blue area around a single, densely stained particle-containing central cell. DNA-coated gold particles provoked smaller spots with less diffusion and gave rise to more individual events than tungsten particles. In more than 90% of the GUS-positive epidermal and mesophyll cells, a particle was detectable within their nucleus. Two days after bombardment, viability had decreased to 1–2% in particle-containing cells. Penetration of a cell by a particle was accompanied by callose formation in the wound area. Dead suspension culture cells of maize without callose formation but containing particles were detected just 1 h post-bombardment. Living cells with callose spots appeared more frequently after bombardment with tungsten than gold. As in tobacco, GUS expression was limited to those cells containing a particle in their nucleus, and the number of particle-containing, viable cells was low after 48 h. The frequency of stable expression events was compared to the number of surviving tobacco leaf cells. On average, four kanamycin-resistant calli or plantlets were recovered per bombarded dish, of which approximately 50% were also GUS-positive. This corresponds to a stable-to-transient ratio of approximately 0.8%, and is similar to the number of particle-containing cells surviving after 48 h.     

微藻高效基因枪转化方法的建立

基因枪法是外源基因导入微藻细胞的重要手段。然而,发展至今,微藻细胞基因枪转化效率一直偏低（10~50个转化子/μg DNA）,高价低效的转化方法阻碍了基于高通量转化子的基因功能分析。为了提高基因枪的转化效率,本研究以三角褐指藻为材料,从抗生素选择培养基的改良,微载体的选择、制备、包埋、点膜和轰击参数的优化,以及受体细胞的处理等方面进行了系统研究。结果显示,采用50%海水盐度f/2培养基可以提高博来霉素的效价,f/2固体培养基中2216E营养物质的加入能缩短1/3的平板筛选时间。微载体制备应选择对金（钨）粉没有吸附作用的离心管,制备量/管应少于3.5 mg。微载体轰击量每次大约为0.75 mg,过量将会造成一个轰击死亡圈,过少将导致轰击成本上升。当轰击间距A为6.35 mm,间距B为11 mm,间距C为6 cm时,可以获得最多的转化细胞。109个受体细胞铺成较厚的多细胞层能显著提高转化效率。经过上述优化与改进,本研究将现有文献报道的转化效率提高了4.7~30倍,达到295 ± 60个转化子/μg DNA。该方法除适用于三角褐指藻外,也可广泛应用于其他微藻（杜氏盐藻、小球藻）的基因枪转化研究,可以为微藻基因工程研究提供快速,高效和可靠的操作技术。     

Evaluation of parameters for high efficiency gene transfer via particle bombardment in Indian mulberry

Particle bombardment is a popular method of direct gene delivery into cell, tissue and organs since it requires minimum pre- and post-bombardment manipulation. In addition, this technique is much easier and fast to perform with intact tissue/organ and reduces the period of in vitro culture. Genetic transformation of mulberry, Morus indica cv. K2 was attempted by particle bombardment using hypocotyl, cotyledon, leaf and leaf callus explants. The effect of various physical and biological parameters during bombardment were studied by the histochemical localization of GUS reporter gene following two days of bombardment and by assessing the number of blue spots per explant. p35SGUSINT was used for optimization of different parameters. The percentage of GUS positive explants was very low with tungsten (20%) as compared to gold particles (36%) indicating tungsten toxicity to the tissue. Maximum GUS activity was observed at 1100 psi helium pressure and 9 cm target distance for hypocotyl, cotyledon and leaf. Double bombardment of explants with 10 microg of DNA loaded on macrocarriers clearly yielded a better (up to 56%) result as compared to a single bombardment (30%). Amongst the various plasmids tested, pBI221 gave the highest (100%) GUS positive explants in the leaf callus.     

Factors affecting gene delivery by particle bombardment ofDendrobium orchids

Summary Five parameters were examined for their effect on transformation ofDendrobium tissues by microprojectile bombardment. The superpromoter in pBI426 produced at least 1.5 times as many transient transformants as the single cauliflower mosaic virus 35S promoter in pBI121 (37 to 69% vs. 0 to 44%) with dark and frequent GUS (β-glucuronidase) staining. Tissue, genotype, and type of microparticle significantly affected transient GUS activity. Higher expression was seen in protocormlike bodies and in hybrid UH44 compared to etiolated shoots and protocorms and to hybrids M61 and K1329-39. Microparticles of 1.6-μm Bio-Rad gold were more effective than 1.0-μm ASI gold. Transient GUS activity did not differ among protocormlike bodies bombarded using helium propellant pressures of 650, 900, or 1100 psi. Transgenic plants were recovered fromDendrobium UH800 protocormlike bodies bombarded with pBI426-coated, 1.1-μm tungsten particles using an early-model gunpowder-driven apparatus with an estimated stable transformation rate of 11.7%. One transgenic plant ofDendrobium UH44 was recovered from etiolated shoot explants bombarded with pBI121-coated, 1.1-μm tungsten particles using the Dupont PDS-1000 with a stable transformation rate of 0.17%. Positive selection results showed 100 to 200 mg·liter⁻¹ kanamycin to be appropriate for regeneration of transgenic plants from protocormlike bodies, protocorms, and etiolated shoot explants over a 3- to 9.5-mo. period.     

Effects of microprojectile bombardment on embryogenic suspension cell cultures of maize (Zea mays L.) used for genetic transformation

We have investigated the interaction between tungsten and gold microprojectiles with suspension-culture cells of maize used for genetic transformation. Particle size measurements were evaluated before and after DNA precipitation to determine mean particle size and the effect of DNA precipitation on particle aggregation. Following particle bombardment, metal foils were examined by scanning electron microscopy to visualize dispersion of individual particles and aggregates. Particle penetration into suspension-culture cell clusters was examined in paraffin-embedded bombarded cells serially sectioned and viewed with light microscopy and by energy dispersive X-ray microanalysis. Acridine-orange-stained bombarded cells were examined to observe cellular response to particle penetration. Transient expression of reporter genes C1 and B and GUS, (-glucuronidase) were used to assess effects of particle bombardment on embryogenic cell types. Autoradiographic analysis of the transformable suspension cell culture SC82 (see Gordon-Kamm et al. 1990, Plant Cell 2, 603–618) was conducted to evaluate the S-phase and mitotic indices in embryogenic and nonembryogenic cells throughout a subculture passage and in response to DNA/particle delivery. The results of these investigations are discussed relative to cytodifferentiation of suspension cell clusters and recovery of transformed clonal sectors.Abbreviations GUS -glucuronidase - FAA formaldehyde-acetic acid-alcohol - SEM scanning electron microscopy     

Evidence That More than 90% of beta-Glucuronidase-Expressing Cells after Particle Bombardment Directly Receive the Foreign Gene in their Nucleus

Plasmid DNA harboring the β-glucuronidase (GUS) gene, coated on gold particles, was delivered into cultured tobacco (Nicotiana tabacum L. cv Bright Yellow-2) cells using a pneumatic particle gun. Cytological analyses of intracellular location of the introduced gold particles before and after GUS expression assay indicated that more than 90% of GUS-expressing cells after bombardment received a DNA-coated particle in their nucleus.     

Optimization of transient gene expression in pollen of Norway spruce (Picea abies) by particle acceleration

In order to optimize transient gene expression in Norway spruce pollen after DNA delivery with particle bombardment, effects of different conditions during homhardmenl were analysed using β-glucuroniduse (GUS) driven by the rice Act I promoter and Inciferase (LUS) driven by the tomato !at 52 promoter as reporter genes. Transient gene expression was significantly increased hy using two bombardments. Also the distance from the stopping plate to the sample was critical to gam maximum gene expression. There was no significant difference between gold and tungsten particles, and the number of positively stained pollen increased with increasing DNA concentration, from 5 to 40 pg DNA added in the DNA/tungsten solution The DNA delivery to Norway spruce pollen was most efficient at a chamber pressure above 70 kPa.     

Analysis of particle bombardment parameters to optimise DNA delivery into wheat tissues

 The objective of this study was to identify the major parameters controlling DNA delivery by particle bombardment to wheat (Triticum aestivum L.) scutellum and inflorescence tissue. The main factors studied were the DNA/gold precipitation process, bombardment parameters and tissue culture variables. Efficiency of DNA (uidA gene) delivery was assessed by scoring transient GUS expression in bombarded tissues. Of the parameters analysed, amount of plasmid DNA, spermidine concentration, presence of Ca⁺⁺ ions, calcium chloride concentration, amount of gold particles, gold particle size, acceleration pressure, chamber vacuum pressure, bombardment distance, osmotic conditioning of tissues and type of auxin had a clear influence on transient gene expression. A bombardment procedure suitable for elite wheat varieties was developed which allowed high-efficiency DNA delivery combined with reduced damage to target tissues. Received: 6 May 1998 / Revision received: 10 August 1998 / Accepted: 2 February 1999     

Improvement of DNA/Metal Particle Adsorption in Tungsten-Based Biolistic Bombardment; Alkaline pH is Necessary for DNA Adsorption and Suppression of DNA Degradation

Tungsten particles have long been used as microcarriers in biolistic bombardment because of their cost-effectiveness compared to alternative gold particles—even if the former have several drawbacks, including their DNA-degrading activity. We characterized tungsten-induced DNA degradation to assess the value of this metal particle and to improve tungsten-based biolistic bombardment. Alkaline pH, low temperature, and high salt concentration were found to diminish tungsten-induced DNA breakdown. The pH was the most influential factor in this phenomenon, both in aqueous solutions and on the particles. Furthermore, alkaline pH greater than 9.4 of an adsorption mixture was found to be essential for DNA binding to metal particles. Based on these findings, we propose a new formula of DNA/tungsten adsorption by using TE buffers that keep alkaline pH (>9.4) of the mixture, in which tungsten-bound plasmid DNA cleavage was suppressed to half the level of that in the conventional DNA-binding condition.     

Biological projectiles (phage, yeast, bacteria) for genetic transformation of plants

Summary Bacteriophage lambda particles, yeast cells, and bacterial cells were tested as projectiles to deliver marker/reporter genes into plant cells via the biolistic process. When phage particles were complexed to tungsten or gold particles and used to bombard tobacco cells, fewer than 15 cell clusters per plate transiently expressed β-glucuronidase (GUS). Cells of wildtype Saccharomyces cerevisiae were too large to be effective projectiles, but use of a reduced-size mutant resulted in a small number of transformants. Escherichia coli cells complexed with tungsten were the most effective projectile for plant transformation. Various methods to prepare E. coli were tested to reduce particle size, improve binding of bacteria to metal particles, and/or minimize particle clumping. In maize, the number of transformants was highest when bacteria/tungsten particles were air-dried onto macrocarriers from an aqueous solution. When maize cells were bombarded with bacteria/tungsten projectiles, rates of transient gene expression (2000 per plate) and stable transformation (50 per plate) were only two- to threefold lower than when purified DNA was used. Transformation of tobacco with E. coli projectiles was improved when the bacteria were treated with a series of ethanol and ether washes, then dried into a powder. Nevertheless, tobacco transformation was still 24- (transient) and 200-fold (stable) less than when purified DNA was used. Biological projectiles can be effective for plant transformation and are advantageous because once a DNA construct is made and put into the appropriate microorganism, the need to isolate and purify DNA for the biolistic process is eliminated, which saves time and lessens DNA shear. Such projectiles may be especially well suited where high molecular weight DNA constructs are needed.     

An efficient protocol for particle bombardment-mediated transformation of <Emphasis Type="Italic">Centella asiatica</Emphasis> callus

In this study, we report an optimization of particle bombardment transformation system for Centella asiatica callus. A total of eight parameters affecting the genetic transformation system were optimized using the synthetic green fluorescent protein (sGFP) as a reporter driven by the CaMV 35S promoter. The results indicated that DNA delivery conditions of 9-cm target distance, 1,100 psi helium pressure, 1.0 μm gold particles size, 27 mmHg chamber vacuum pressure, 2 times number of bombardment, spermidine as precipitation agent, 60 h post-bombardment incubation time and 2 μg plasmid DNA concentration were optimal for C. asiatica callus transformation. The expression of sGFP was monitored using fluorescence microscope and further confirmed using RT-PCR. This optimized genetic transformation system is applicable for rapid transient gene analysis and transgenic C. asiatica production.     

Particle-inflow-gun-mediated genetic transformation of buffel grass (Cenchrus ciliaris L.): optimizing biological and physical parameters

The present study was conducted to optimize various biological and physical parameters for developing an efficient and reproducible gene transfer method for genetic transformation of buffel grass. Transformation was carried out using a helium-driven particle inflow gun (PIG). Embryogenic calli produced from mature seeds of buffel grass cv. CC-119 were separately bombarded with four plasmids, containing Actin (pAct1DX), Ubiquitin (pAHC-25; pAHC-27) and CaMV-35S (pCaMVGUS) promoters, coated on tungsten and gold particles. The efficiency of transformation was monitored through transient GUS expression. Different parameters, viz., the type of promoter, type and size of microcarrier, helium gas pressure, distance and time of bombardment, were standardized for delivering DNA into embryogenic calli. Bombardment with plasmid DNA carrying the actin promoter coated on 1.6 micro gold particles, at a helium pressure of 4 bars, a distance of 10 cm for 10 micro sec and 28 mm Hg vacuum in the chamber, produced the best result in transient GUS expression. The Actin promoter was found to be more efficient in driving expression of the GUS gene in buffel grass, followed by Ubiquitin and CaMV-35S promoters. Lower helium pressure was found to be sub-optimal, while higher pressure produced a smaller number of blue spots, probably due to excessive damage to the cells. Maximum of 385 blue spots was observed with gold particles of 1.6 micro size, whereas only 213 blue spots were recorded for tungsten particles of 1.0 micro size. The optimized parameters can be employed for genetic transformation of buffel grass with genes of agronomic importance.     

The Biolistic® PDS-1000/He device

This paper describes the design, operation, and performance of the Biolistic® PDS-1000/He device, which is used to transform living organisms with foreign DNA. DNA is delivered to cells in association with microscopic metal particles, called microcarriers, that are propelled at high velocity towards target tissues. The microcarriers are accelerated on a plastic cylinder, called a macrocarrier, which is driven by a shock wave of helium gas. The effectiveness of the PDS-1000/He device was tested by bombarding tobacco cell suspension cultures with microcarriers that were coated with plasmid DNA containing the B-glucuronidase (GUS) and neomycin phosphotransferase II (NPTII) genes. Two days after bombardment, there were 6835 ± 594 cell clusters per petri plate that expressed the GUS gene. Kanamycin resistant colonies were observed 6 to 8 weeks after bombardment, at a rate of 838 ± 134 colonies per bombarded plate.     

Secondary embryogenesis and transient expression of the β-glucuronidase gene in hypocotyls of rapeseed microspore-derived embryos

Secondary embryogenesis from rapeseed microspore-derived embryos (MDEs) was studied in three Brassica napus L. cultivars Global, PF₇₀₄ and Option. The best results in terms of secondary embryogenesis percentage obtained in cultures of Global and PF₇₀₄ MDEs (75.88 and 65.97 %, respectively) and PF₇₀₄ produced the highest number of secondary embryos per each primary embryo (14.91 ± 2.18). After optimization of physical parameters, rapeseed hypocotyls of MDEs were bombarded with microcarriers coated with a plasmid containing GUS reporter gene. The highest levels of transient GUS expression were obtained using bombardment with gold particles of 1.6 μm, at helium pressure of 9.3 MPa, a bombardment distance of 9 cm, chamber vacuum pressure of 7.1 × 10⁻⁶ kPa and single bombardment in bombardment medium containing 0.4 M mannitol.     

Optimization of factors influencing microprojectile bombardment-mediated genetic transformation of seed-derived callus and regeneration of transgenic plants in <Emphasis Type="Italic">Eleusine coracana</Emphasis> (L.) Gaertn

Microprojectile bombardment mediated genetic transformation parameters have been standardized for seed derived callus of Eleusine coracana. Plasmid pCAMBIA 1381 harboring hygromycin phosphotransferase (hptII) as selectable marker gene and β-glucuronidase (gus A) as reporter gene, was used for the optimization of gene transfer conditions. The transient GUS expression and survival of putative transformants were taken into consideration for the assessment of parameters. Optimum conditions for the microprojectile bombardment mediated genetic transformation of finger millet were 1,100 psi rupture disk pressure with 3 cm distance from rupture disk to macrocarrier and 12 cm microprojectile travel distance. Double bombardment with gold particles of 1.0 μm size provided maximum transient GUS expression and transformation efficiency. Osmotic treatment of callus with 0.4 M sorbitol enhanced efficiency of particle bombardment mediated genetic transformation. Regenerative calli were bombarded at optimum conditions of bombardment and placed on regeneration medium with hygromycin to obtain transformed plants. The integration of hptII and gus A genes was confirmed with PCR amplification of 684 and 634 bp sizes of the bands respectively from putative transformants and Southern blot hybridization using PCR amplified DIG labeled hptII gene as probe. PCR analysis with hptII gene specific primers indicated the presence of transgene in T₁ generation plants. Thus a successful genetic transformation system was developed using particle bombardment in E. coracana with 45.3% transformation efficiency. The protocol will be helpful for the introgression of desired genes into E. coracana.     

Physical,chemical and physiological parameters for electroporation-mediated gene delivery into rice protoplasts

Transformation of cereal protoplasts has been reported using several methods; however, the efficiencies of transformations are still very low. We have evaluated a number of parameters that influence electroporation-mediated DNA uptake and have also compared the efficiency of transient GUS activity and stable transformation obtained using an optimized electroporation method with that of the PEG method. The electroporation conditions tested were ionic composition of buffer, ionic strength, resistivity of buffer, type of anions, voltage, and capacitance.Protoplasts isolated from suspension cultures derived from immature embryos of rice (cvs Radon and IR-54) were used for this study. Stable transformation or transient GUS expression experiments were carried out using a plasmid construct containing the CaMV 35S promoter driving thebar gene and a rice actin promoter driving thegus A (uid A) gene (pAG35bar). Electroporation under optimized conditions resulted in about 13-fold higher GUS activities compared to the PEG method. Protoplast survival following optimized electroporation conditions was 55–60%, compared to 35–40% with the PEG treatment. Protoplasts isolated from a suspension culture at different ages gave substantially different levels of transient GUS expression following electroporation-mediated DNA uptake. In contrast, the age of the suspension culture did not influence PEG-mediated DNA uptake and transient GUS activities, which remained low throughout the culture period examined (21 months). Putatively transformed calluses were selected after three to four weeks on medium containing phosphinothricin as the selection agent. The transformation frequencies ranged from 6.2×10^–5 to 5.4×10^–4 with the electroporation method compared to 1.3×10^–5 to 5.3×10^–5 with the PEG method. Southern blot analysis of PPT-resistant calluses obtained by the electroporation-mediated transformation showed simple intergration patterns of integrated DNA in most of the transformants.