Individual selection, culture and manipulation of higher plant cells

Isolation and cultivation of embryogenic microspores from barley (Hordeum vulgare L.)

Two different techniques for mechanical isolation of microspores from the barley cultivar 'Igri' have been evaluated. The anthers were subjected to mannitol pretreatment prior to microspore isolation, which was performed either by maceration with a pestle or by blending of the excised anthers. The microspores were purified by centrifugation and washing and cultured in liquid medium on a membrane support. In the following four weeks the microspores developed into embryoids, which were subsequently regenerated to plants on solid medium. Microblending of the anthers was found to be more reproducible than pestle maceration, and the yield of large microspores was 100% higher using this method. With the microblending technique a mean of 9.4 green plants and 0.4 albino plants were regenerated per plated anther while a mean of only 2.8 green and 0.17 albino plants per anther were regenerated from microspores isolated after pestle maceration of the anthers. Microspores isolated from mass cultures were also cultured as single cells in microdroplets, and it was shown that microspores isolated from 3-5 days old mass cultures could develop into plants although at a low frequency (0.3%). Finally, the potential of using microinjection for transforming embryogenic microspores has been evaluated.

Manipulating the genetic identity and biochemical surface properties of individual cells with electric-field-induced fusion

A method for cell-cell and cell-liposome fusion at the single-cell level is described. Individual cells or liposomes were first selected and manipulated either by optical trapping or by adhesion to a micromanipulator-controlled ultramicroelectrode. Spatially selective fusion of the cell-cell or cell-liposome pair was achieved by the application of a highly focused electric field through a pair of 5-micrometer o.d. carbon-fiber ultramicroelectrodes. The ability to fuse together single cells opens new possibilities in the manipulation of the genetic and cellular makeup of individual cells in a controlled manner. In the study of cellular networks, for example, the alteration of the biochemical identity of a selected cell can have a profound effect on the behavior of the entire network. Fusion of a single liposome with a target cell allows the introduction of the liposomal content into the cell interior as well as the addition of lipids and membrane proteins onto the cell surface. This cell-liposome fusion represents an approach to the manipulation of the cytoplasmic contents and surface properties of single cells. As an example, we have introduced a membrane protein (gamma-glutamyltransferase) reconstituted in liposomes into the cell plasma membrane.

Isolation of a full-length cDNA encoding calreticulin from a PCR library of in vitro zygotes of maize

A full-size cDNA clone (1614 bp) encoding calreticulin was isolated from a PCR-based cDNA library of maize in vitro zygotes. Calreticulin is a major Ca2+ storage protein located mainly in the lumen of the endoplasmic reticulum but also in the nucleus and/or cytoplasm of some cells. A differential screening between cDNA libraries originating from 104 in vitro zygotes (18 h after in vitro fertilization) and 128 unfertilized egg cells was performed to isolate newly expressed genes or genes expressed more abundantly after fertilization. The expression of the isolated cDNA clone is enhanced after fertilization and strongly correlated to cell division. Sequence comparison to a shorter maize calreticulin cDNA isolated from a PCR based cDNA library construction from a few plant cells [12]. It is further shown that calreticulins in maize are probably transcribed from a small gene family differentially expressed in abundance in diverse tissues. The deduced amino acid sequence encodes an acidic protein (pI 4.17) of 48 kDa sharing 77-92% and 50-54% homology to other plant and animal calreticulins, respectively. The described calreticulin gene represents to our knowledge the first cDNA clone isolated from a RT/PCR cDNA library originating from only a few plant cells and is the first gene isolated from zygotes of higher plants.

An in Vitro System for Adhesion and Fusion of Maize Gametes

The development of in vitro fertilization systems in flowering plants is important for understanding and controlling the mechanisms of fertilization. Here a method is described in which isolated maize gametes fuse. In a medium containing 5 mM calcium chloride, sperm and egg cells adhere for several minutes and then fuse within 10 seconds. The method is specific to male-female gamete pairs and results in 80 percent fusion, whereas fusions with other combinations of gametic and mesophyllic cells are less frequent. Eggs fertilized in vitro do not fuse with additional male gametes, which suggests that a block to polyspermy exists.

Analysis of single protoplasts and regenerated plants by PCR and RAPD technology

We investigated the use of the polymerase chain reaction (PCR) and the associated random amplification of polymorphic DNA (RAPD) technique in the analysis of DNA and specific genes in plant cells at different stages of regeneration in in vitro cultures. We demonstrate that both procedures can be used to differentiate reproducibly between closely related species as well as to reveal levels of DNA polymorphism in regenerated plants. We also demonstrate that both procedures, using protocols that we have developed, are applicable at all tissue culture stages, from single isolated protoplasts to regenerated plants. Possible explanations for the variation levels detected in regenerated wheat plants are advanced.

Traveling-wave dielectrophoresis of microparticles

The traveling-wave-induced linear transfer of dielectric particles like living cells and artificial objects of microscopic dimensions is analyzed. It is shown that the electrode geometries must correspond to particle sizes to allow an effective manipulation of particles immersed in weakly electrolytic solutions by high frequency traveling waves. The theoretical model elaborated in this paper is in good agreement with experimental results obtained in microfabricated chambers of linearly arranged electrodes. It explains the behavior of homogeneous cellulose spheres as well as that of membrane-covered pine polls. The traveling-wave-driven electrodes are described by a superposition of time-dependent point charges. Subsequently, each of these point charges has to be considered as polarizing the dielectric particle and interacting with the polarized particle. This results in forces which effectively translocate the particle.

An improved approach for transformation of plant cells by microinjection: molecular and genetic analysis

A new culture method for the injection of tobacco mesophyll protoplasts has been established. The protoplasts are embedded in a thin layer of alginate and are nourished from the medium in the underlying basis layer. In the alginate layer the protoplasts regenerate to calli at a frequency of up to 80%. Embedded protoplasts can be selected either with 50 mg l-1 kanamycin or 5 mg l-1 paromomycin. Single resistant cells can be recovered from about 10,000 sensitive cells in one alginate layer. Injection of the neo gene (coding for neomycin phosphotransferase II) into protoplast derived single cells in the alginate layer results in kanamycin resistant colonies that can be regenerated to mature plants. These plants express the neomycin phosphotransferase as shown by enzyme activity assay. The integration of the transgene into the plant genome could be proved by Southern hybridization to high molecular weight DNA. With this culture method 100 cells can be injected per hour. Transformation frequencies range from 2 to 20%. In crossing experiments, it was shown that the foreign gene is transmitted to the next generation in a Mendelian fashion.

Transfer of defined numbers of chloroplasts into albino protoplasts using an improved subprotoplast/protoplast microfusion procedure: transfer of only two chloroplasts leads to variegated progeny

A procedure is described by which it is possible to perform controlled microfusion of microscopically selected protoplast fusion partners with high efficiencies. The procedure is applied to fusion of Nicotiana tabacum (line 92V37. N. undulata cytoplasm) plastid albino protoplasts as a recipient and spontaneously formed subprotoplasts of green N. tabacum (line SR1) as donor. Products of individual electrofusion events are cloned via single cell nurse culture and the derived cell lines are analysed for the occurrence of variegated or green regenerating shoots, which are indicative of the establishment of the transferred organelles in the cell progeny. The plastid population in green regenerants recovered after the transfer of only two chloroplasts was demonstrated to have originated from the donor subprotoplast organelles by restriction analysis of total DNA using a plastome-specific probe.

Organelle transfer by microfusion of defined protoplast-cytoplast pairs

Defined cybridization was performed by one-to-one electrofusion (microfusion) of preselected protoplast-cytoplast pairs of male-fertile, streptomycin-resistant Nicotiana tabacum and cytoplasmic male-sterile, streptomycin-sensitive N. tabacum cms (N. bigelovii), followed by microculture of the fusion products until plant regeneration. Dominant selectable markers, namely, kanamycin resistance (nptII) and hygromycin B resistance (hpt) genes had been previously integrated in the nuclear genomes of the otherwise almost fully isogenic parental strains using direct gene transfer to protoplasts. In addition to chromosome counts indicating the expected allotetraploid tobacco count of 48, the absence of the nucleus from the cytoplast donor line was confirmed by Southern blot hybridization using nptII and hpt probes, as well as by an in vitro selection test with leaf expiants and the corresponding enzyme assays for 30 cybrids. The cytoplasmic composition of the cybrids obtained was analyzed for chloroplast type using the streptomycin resistance/sensitivity locus. The fate of mitochondria in cybrids was checked by species-specific patterns in Southern analysis of restriction endonuclease digests of total DNA with N. sylvestris mitochondrial DNA probes.

Somatic hybridization by microfusion of defined protoplast pairs in Nicotiana: morphological, genetic, and molecular characterization

Somatic hybrid/cybrid plants were obtained by microfusion of defined protoplast pairs from malefertile, streptomycin-resistant Nicotiana tabacum and cytoplasmic male-sterile (cms), streptomycin-sensitive N. tabacum cms (N. bigelovii) after microculture of recovered fusants. Genetic and molecular characterization of the organelle composition of 30 somatic hybrid/cybrid plants was performed. The fate of chloroplasts was assessed by an in vivo assay for streptomycin resistance/ sensitivity using leaf explants (R0 generation and R1 seedlings). For the analysis of the mitochondrial (mt) DNA, species-specific patterns were generated by Southern hybridization of restriction endonuclease digests of total DNA and mtDNA, with three DNA probes of N. sylvestris mitochondrial origin. In addition, detailed histological and scanning electron microscopy studies on flower ontogeny were performed for representative somatic hybrids/cybrids showing interesting flower morphology. The present study demonstrates that electrofusion of individually selected pairs of protoplasts (microfusion) can be used for the controlled somatic hybridization of higher plants.

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.

Vital DNA staining of agarose-embedded protoplasts and cell suspensions of Nicotiana plumbaginifolia

The DNA of agarose-embedded protoplasts of Nicotiana plumbaginifolia was stained with Hoechst 33342 by immersing microscope slides, coated with immobilized protoplasts, into Erlenmeyer flasks containing consecutively dye solution, pH-correcting washing solutions and culture medium. After staining, protoplasts regenerated cell walls, started to divide and proliferated to calli. The culture system with immobilized protoplasts permits rapid change of culture media and accurate control of experimental conditions. The staining technique offers the opportunity for continuous observation of chromosomal behaviour and cell dynamics in individual plant cells.The same staining procedure was successfully applied to DNA of plant cells in suspension. Flow cytometric analysis revealed a retarding effect of the dye on the cell cycle, but within hours the cells recovered and showed their normal growth characteristics as compared to the controls.