Fusion characteristics of plant protoplasts in electric fields

Real-time detection of somatic hybrid cells during electrofusion of carrot protoplasts with stably labelled mitochondria

Somatic hybridisation in the carrot, as in other plant species, enables the development of novel plants with unique characteristics. This process can be induced by the application of electric current to isolated protoplasts, but such electrofusion requires an effective hybrid cell identification method. This paper describes the non-toxic fluorescent protein (FP) tagging of protoplasts which allows discrimination of fusion components and identification of hybrids in real-time during electrofusion. One of four FPs: cyan (eCFP), green (sGFP), yellow (eYFP) or the mCherry variant of red FP (RFP), with a fused mitochondrial targeting sequence, was introduced to carrot cell lines of three varieties using Agrobacterium-mediated transformation. After selection, a set of carrot callus lines with either GFP, YFP or RFP-labelled mitochondria that showed stable fluorescence served as protoplast sources. Various combinations of direct current (DC) parameters on protoplast integrity and their ability to form hybrid cells were assessed during electrofusion. The protoplast response and hybrid cell formation depended on DC voltage and pulse time, and varied among protoplast sources. Heterofusants (GFP + RFP or YFP + RFP) were identified by detection of a dual-colour fluorescence. This approach enabled, for the first time, a comprehensive assessment of the carrot protoplast response to the applied electric field conditions as well as identification of the DC parameters suitable for hybrid formation, and an estimation of the electrofusion success rate by performing real-time observations of protoplast fluorescence.

Electroporation of rapeseed protoplasts - transient and stable transformation

Protoplasts of Brassica napus hypocotyls were transfected using electroporation. Parameters such as discharge potential, protoplast density and buffer constituents were tested to determine the most suitable conditions for gene transfer. To monitor the introduction of DNA into protoplasts a plasmid containing the β-glucuronidase (EC 3.2.1.31), and the neomycin phospotransferase (EC 2.7.1.95) genes was used. By using this construct, expression of a screenable marker gene for transient expression analysis as well as an antibiotic resistance marker gene for selection of stable transformants were obtained. Refined electroporation conditions resulted in a frequency of 0.1% transiently transformed protoplasts. Microcalluses were cultured under selective conditions in a bead-type culture system. Resistant callus, with an absolute transformation frequency of 4.9 × 10^-5 and a relative transformation frequency of 0.3% could be achieved. X-ray irradiation of newly electroporated protoplasts did not enhance absolute transformation frequencies. From some of the resistant calluses, transgenic plants could be regenerated which were characterized by molecular analysis.

Selection of hybrid plants obtained by electrofusion of vacuolated x evacuolated plant protoplasts in hypo-osmolar solution

Vacuolated and evacuolated tobacco mesophyll protoplasts were electrically fused in hypo-osmolar media by using an alternating field of modulated amplitude for alignment. The vacuolated fusion partner was isolated from Nicotiana tabaccum L. cv Xanthi and the evacuolated one from the streptomycin-resistant strain Nicotiana tabaccum L. cv Petit Havana SR1. The field and osmolarity conditions used ensured relatively high yields of heterologous fusion products despite the differences in density and size of the parental cells. After removal of the evacuolated, streptomycin-resistant fused and unfused protoplasts by flotation of vacuole-containing cells on iso-osmolar sucrose medium, the cybrids and hybrids were cultured in 25 microliters drops of agarose. During the first 5 weeks the non-fused Xanthi-protoplasts were used as a nurse culture. After addition of streptomycin to the growth media, cybrids and hybrids were successfully selected whereas fused and unfused vacuole-containing protoplasts died within 6 days. Only the streptomycin-resistant cybrids and hybrids developed into whole plants. On average a yield of 0.025% of streptomycin-resistant plants (referred to the total number of parental cells) was obtained. Polyacrylamide gel electrophoresis of leaf extracts of these plants showed that at least 50% of the streptomycin-resistant plants had a hybrid-esterase isoenzyme pattern. The protocol can be generalised by fusion of iodoacetamide-inactivated vacuolated protoplasts with meristematic (or evacuolized) protoplasts carrying no genetic marker. Use of evacolated protoplasts for electrofusion with vacuole-containing protoplasts therefore offers a way of overcoming the lack of suitable genetic markers for hybrid selection.

Metabolic complementation for a single gene function associated with partial and total loss of donor DNA in interspecific somatic hybrids

We report here on the obtainment of interspecific somatic, asymmetric, and highly asymmetric nuclear hybrids via protoplast fusion. Asymmetric nuclear hybrids were obtained after fusion of mesophyll protoplasts from a nitrate reductase-deficient cofactor mutant of N. plumbaginifolia with irradiated (100 krad) kanamycin resistant leaf protoplasts of a haploid N. tabacum. Selection for nitrate reductase (NR) and/or kanamycin (Km) resistance resulted in the production of three groups of plants (NR(+), NR(+), Km(R), and NR(-)Km(R)). Cytological analysis of some hybrid regenerants showed the presence of numerous tobacco chromosomes and chromosome fragments, besides a polyploid N. plumbaginifolia genome (tetra or hexaploid). All the regenerants tested were male sterile but some of them could be backcrossed to the recipient partner. In a second experiment, somatic and highly asymmetric nuclear hybrids were obtained after fusion of mesophyll protoplasts from the universal hybridizer of N. plumbaginifolia with suspension protoplasts of a tumor line of N. tabacum. Selection resulted in two types of colonies: nonregenerating hybrid calli turned out to be true somatic hybrids, while cytological analysis of regenerants obtained on morphogenic calli did not show any presence of donor-specific chromosomes. Forty percent of the hybrid regenerants were completely fertile, while the others could only be backcrossed to the recipient N. plumbaginifolia. Since the gene we selected for is not yet cloned, we were not able to demonstrate the transfer of genetic material at the molecular level. However, since no reversion frequency for the nitrate reductase mutant is known, and due to a detailed cytological knowledge of both fusion partners, we feel confident in speculating that intergenomic recombination between N. plumbaginifolia and N. tabacum has occurred.

Facilitated electrofusion of vacuolated x evacuolated oat mesophyll protoplasts in hypo-osmolar media after alignment with an alternating field of modulated strength

Electrofusion of evacuolated and vacuolated oat leaf protoplasts is difficult because of the different size and density of these cells which results in separation of the two fusion partners during dielectrophoresis. The fusion yield of this cell system was considerably enhanced by electrofusion in hypo-osmolar media containing 0.4 M mannitol, 0.1 mM calcium acetate and 0.1% bovine serum albumin. This increase in yield was only achieved if the dielectrophoretically induced membrane contact between the two fusion partners was enhanced by an initial short 'burst' of higher field strength (500 V/cm, peak to peak, for 5 s followed by a reduction of to 90 V/cm, peak to peak, for 20 s, frequency 1 MHz). Due to the high field strength of the alternating field at the beginning of cell chain formation separation of fusion partners of different size and density was mainly avoided. Simultaneously, the short duration of this high field 'burst' avoided the generation of lethal effects in the cell membranes. The subsequent low field strength of the alternating field was sufficient to keep the aligned cells in position. Optimum fusion was induced by a single square pulse of 750 V/cm and 30 musec duration. The time required for rounding up of the heterologous fusion products decreased with decreasing osmolarity. Fusion resulted in a 5.7 +/- 1.2% yield of heterologous fusion products (compared to 0.7% using the conventional electrofusion protocol) as determined by flow cytometric assay. About 50% of the vacuolated oat protoplasts and 20-50% of the heterologous fusion products regenerated their cell walls within 5 days after hypo-osmolar treatment, but no cell divisions could be observed. Evacuolated oat protoplasts died after 2-3 days in culture without any detectable cell wall regeneration.

Determination of physical membrane properties of plant cell protoplasts via the electrofusion technique: prediction of optimal fusion yields and protoplast viability

By variation of physical parameters (field strength, pulse duration) which result in electrofusion and electroporation, properties of the plasma membrane of different types of plant cell protoplasts were analyzed. The lower threshold for that field pulse intensity at which membrane breakdown occurred (recorded as fusion event) depended on pulse duration, protoplast size, and protoplast type (tobacco, oat; vacuolated, evacuolated). This fusion characteristic of plant protoplasts can also be taken as a measure of the charging process of the membrane and allows thus a non-invasive determination of the time constant and the specific membrane capacitance. Although the fusion yield was comparable at pulse duration/field strength couples of, e.g., 10 μs/1.5 kV*cm(-1) and 200 μs/0.5 kV*cm(-1), hybrid viability was not. Rates of cell wall regeneration and cell division of tobacco mesophyll protoplasts were not affected but may have been increased at short pulse duration/high field strength. Plating efficiency, in contrast, was significantly decreased with longer pulse duration at low field strengths.

Molecular, cytogenetic and morphological characterization of somatic hybrids of dihaploid Solanum tuberosum and diploid S. brevidens

Fifty-eight somatic hybrid plants, produced both by chemical (11) and electrical fusion (47) of protoplasts of dihaploid Solanum tuberosum and S. brevidens, have been analysed by molecular, cytological and morphological methods. The potentially useful euploid plants constituted 34% of the total, of which 20% were tetraploid and 14% hexaploid; the remainder were aneuploid at the tetraploid, hexaploid and octoploid levels. Analysis of chloroplast DNA showed that 55% of hybrids contained chloroplasts from S. brevidens and 45% from S. tuberosum. Hexaploids, the products of three protoplasts fusing together, were analyzed with specific DNA probes, and this revealed that nuclear genome dosages could be either 2∶1 S. tuberosum∶S. brevidens, or vice-versa. Chloroplast types of hexaploids were not influenced by nuclear genome dosage, and all six possible combinations of genome dosage and chloroplast types were found amongst tetraploids and hexaploids. To examine the morphology of the hybrid population and its possible relation to the chromosome number and chloroplast DNA type, 18 morphological characteristics were measured on greenhouse-grown plants and analyzed by principal component and canonical variate analyses. Both analyses showed that nuclear ploidy has the most prominent influence on the overall morphology of the hybrids. Differential parental genome expression in the morphology of the hybrids is discussed. These results provide useful data on the range of genetic combinations that can be expected to occur amongst somatic hybrid plants.

Production of somatic hybrids by electrofusion in Solanum

Conditions are described for large scale electrofusion of mesophyll protoplasts of dihaploid S. tuberosum with those of diploid S. brevidens. Overall fusion frequencies of 20%-30% were achieved, and following fusion, large numbers of protoplast-derived calli were obtained. Putative somatic hybrid plants were selected from the regenerated shoots by examining their morphological characteristics. Twenty-one somatic hybrids were confirmed by isoenzyme analysis and six somatic hybrids were further confirmed by Southern hybridization. Tetraploid hybrids were obtained, but cytogenetic studies indicated that more of the regenerated hybrids were hexaploid than had previously been found following chemical fusion of the same partners. Some advantages of electrofusion over chemical fusion are discussed.

Resistance to potato leaf roll virus and potato virus Y in somatic hybrids between dihaploid Solanum tuberosum and S. brevidens

Many somatic fusion hybrids have been produced between a dihaploid potato Solanum tuberosum and the sexually-incompatible wild species S. brevidens using both chemical and electrical fusion techniques. S. brevidens was resistant to both potato leaf roll virus (PLRV) and potato virus Y (PVY), the viruses being either at low (PLRV) or undetectable (PVY) concentrations as determined by enzyme-linked immunosorbent assay (ELISA). The S. tuberosum parent was susceptible to both viruses. A wide range of resistance, expressed as a decrease in virus concentration to both viruses was found amongst fusion hybrids, four of which were especially resistant. The practicality of introducing virus resistance from S. brevidens into cultivated potatoes by somatic hybridisation is discussed.

High-speed electro-fusion and electro-transfection of plant protoplasts by a continuous flow electro-manipulator

A continuous flow electro-manipulator available both for mass production of fused and of transfected plant protoplasts was devised using a flow chamber with gold-coated glass panel electrodes. Up to 100 ml of protoplasts suspension were treated within 20 min at the rate of approximately 1×10(6) protoplasts / min. The yield of diheterokaryons between tobacco mesophyll and carrot root protoplasts reached approximately 10 % of total protoplasts by flow electro-fusion. More than 95 % of tobacco and cowpea mesophyll protoplasts became infected with tobacco mosaic virus RNA by flow electro-transfection.

The permeability of electroporated cells and protoplasts of sugar beet

A simple method has been developed to determine the changes in permeability of protoplasts and intact cells when electroporated. Cells and protoplasts of sugar beet, Beta vulgaris L., were subjected to electric pulse treatments of different field strengths, pulse number and pulse duration, and the ability to accumulate and retain the hydrophilic dye phenosafranine was determined spectrophotometrically. Results of timecourse studies of phenosafranine accumulation and retention indicated that pores are formed or enlarged rapidly in the plasmamembrane and remain permeable to phenosafranine for relatively long periods; the half-life of the 'pores" was temperaturedependent. Both cells and protoplasts retained the highest levels of phenosafranine when supplied with a series of five rectangular pulses of 50 μs duration and of field strength 2500 V·cm(-1). If these parameters were exceeded, The phenosafranine content was reduced, concomitant with a decline in viability as indicated by fluorescein-diacetate staining, indicating the loss of the integrity of the plasmamembrane. The pattern of accumulation and retention by protoplasts of radioactivity from [(3)H]pABD1, a modified bacterial plasmid, was similar to that of phenosafranine, but uptake of the plasmid by cells was not demonstrated. The mothod can be used to determine conditions for the optimum permeabilization of protoplasts for direct gene transfer.

Factors affecting protoplast electrofusion efficiency

The electrofusion efficiency of protoplasts isolated from a carrot (Daucus carota) suspension culture was increased by treatment with 0.1 mg/ml lysolecithin, 2.5% dimethylsulfoxide (DMSO), or 0.5 mM Ca(2+). The lysolecithin and DMSO treatments substantially increased protoplast lysis, whereas calcium treatment did not. The enzymes used for protoplast isolation were also found to have a dramatic effect on the efficiency of fusion. A mixture of Cellulysin and Driselase led to a two-fold enhancement of fusion as compared with Driselase alone. The stimulation by Cellulysin appears to be due to enzymatic modification of the cell surface. However, comparison of the time course for wall digestion with the development of susceptibility to electrofusion suggests that the effect of Cellulysin is not simply due to removal of the cell wall. Brief treatment of the cells with pronase or proteinase K also doubled the efficiency of fusion. Taken together, these results indicate that electrofusion efficiency can be enhanced by the method used for protoplast isolation; they also suggest that modification of membrane/cell-surface proteins during protoplast isolation may be particularly important in determining electrofusion efficiencies.

A versatile low-cost apparatus for cell electrofusion and other electrophysiological treatments

Instrumentation is described which will generate AC fields and DC square pulses for cell electrofusion, electroporation and other electrophysiological experiments. The equipment can generate a constant or intermittent AC field and a single square pulse, either alone or in combination. It can also generate a train of intermittent square pulses. The components for the instrumentation are commercially available and can be assembled by persons with little or no training in electronics. A design is given for a mass treatment chamber that can be easily constructed and viewed with a microscope.