Somatic hybrid plants of potato and tomato regenerated from fused protoplasts

Somatic hybridization in potato: use of γ-irradiated protoplasts of Solanum pinnatisectum in genetic reconstruction

Leaf mesophyll protoplasts of Solanum pinnatisectum (2n=24) γ-irradiated at doses of 200 Gy and consequently unable to divide were fused with untreated protoplasts of genomic chlorophyll deficient mutant IvP 841-1 (2n=24) containing the germplasms of S. tuberosum and S. phureja. Two types of plants differing in their pigmentation characteristics were selected. The regenerants of one group were identified as true somatic hybrids by using isozyme analyses of esterase and aspartate aminotransferase. The anthocyanin marker of S. pinnatisectum was phenotypically expressed in these regenerants and could be used as an additional selection trait for hybrid screening in this species combination. The regenerants of the second group were corrected for the gene controlling chlorophyll deficiency but contained species-specific isozymes of the potato cultivar only. Restriction analysis of chloroplast DNA revealed chloroplasts of the S. pinnatisectum type in all but one of the plants tested. The fusion experiments involving γ-irradiated protoplasts show that this approach in potato reconstruction has the advantage of producing a wide range of genetically novel plants.

Individual selection, culture and manipulation of higher plant cells

Due to the heterogeneity in morphology, physiological and morphogenetical capabilities of higher plant cells in mass culture, the development of methods for individually culturing defined cells seemed to be useful and necessary. Individual cell culture represents a powerful tool for studies on the physiology of different cell types, the analysis of differentiation programs, the genetic manipulation of plant cells and cell-cell interactions. An improved microculture system based on a computer-controlled set-up for the efficient selection, transfer and individual culture of defined higher plant cells until regeneration of whole plants is described. Related experimental approaches for individually manipulating higher plant cells under controlled conditions, such as electrofusion of defined pairs of protoplasts and subprotoplasts, cell reconstruction and intranuclear microinjection of protoplasts and karyoplasts - mainly performed with cells of the crop plant Brassica napus L. - are presented.

Selection of a universal hybridizer in Sinapis turgida Del. and regeneration of plantlets from somatic hybrids with Brassica species

A double-mutant cell line, which was unable to grow in a medium with NO 3 (-) as the sole nitrogen source and was resistant to 5-methyl-tryptophan (5MT), was selected from cell suspensions of Sinapis turgida Del. (Brassicaceae) by culturing the cells in AA medium (Toriyama and Hinata, 1985, Plant Sci. 41, 179-183) supplemented with 50 mM chlorate and 229 μM 5MT. Protoplasts of this cell line were fused with mesophyll protoplasts of Brassica oleracea L. with dextran, and six somatic hybrids were selected initially by culture in the NO 3 (-) medium and then by transfer to the NO 3 (-) medium supplemented with 229 μM 5MT. The somatic hybrids produced embryoids, leaves and plantlets on a regeneration medium. The hybrid characters were confirmed by investigations of acid phosphatase (EC 3.1.3.2) and peroxidase (EC 1.11.1.7) isoenzymes, chromosome number, growth on NO 3 (-) medium, 5MT resistance, and capacity to regenerate plants. Somatic hybrids between S. turgida Del. and B. nigra (L.) Koch were also obtained using this method. These results indicate that the double-mutant cell line established here will be able to serve as a universal hybridizer to select somatic hybrids after protoplast fusion with any other wild-type partner.

Fertility of somatic hybrids from protoplast fusions of Solanum brevidens and S. tuberosum

Two sets of somatic hybrids between Solanum brevidens (2x) and S. tuberosum (2x and 4x) were evaluated for male fertility, meiotic regularity and female fertility. The somatic hybrids were tetraploids from 2x + 2x fusions and hexaploids from 2x + 4x fusions. Pollen stainability ranged from 0 to 83% in tetraploids and from 0 to 23% in hexaploids. The tetraploids had more regular meiosis, lower levels of micropollen and fewer unassociated chromosomes than hexaploids. However, except for a low level of selfing, the pollen of both sets of hybrids was ineffective in pollinations. The tetraploids, as females, crossed poorly with 2x and 4x tester species and selfed only at low levels. The hexaploid fusion hybrids also crossed poorly with the 2x tester species and selfed only to a limited degree; however, they crossed well with 4x testers. Seed set in crosses with S. tuberosum Group Andigena, and S. tuberosum Group Tuberosum cultivars 'Kathadin' and 'Norland' averaged 16.7, 15.6 and 28.6 seeds per fruit, respectively. Progeny from these crosses had 5x or nearly 5x ploidy levels. The results indicate that reasonable levels of female fertility can be obtained in somatic fusion hybrids of S. brevidens and S. tuberosum.

Restriction endonuclease studies on the chloroplast and mitochondrial DNAs of alfalfa (Medicago sativa L.) protoclones

Alfalfa protoclones were regenerated from the mesophyll protoplasts of two cloned source plants (parents), RS-K1 and RS-K2, initiated from Regen S seed. Because of the high frequency of karyotypic upset previously observed in these plants, chloroplast DNAs (cpDNA) from 23 protoclones and mitochondrial DNAs (mtDNA) from 20 protoclones were examined by restriction endonuclease analysis in order to assess recombination in their cytoplasmic genomes. Seven and four endonucleases were separately used for cpDNA and mtDNA analysis, respectively. Data were consistent with no, or a low frequency of, major sequence rearrangements in either the chloroplast or the mitochondrial genomes as a result of protocloning. However, two types of cpDNA were detected in the 23 protoclones, with only one protoclone possessing the cpDNA type of the cloned parental populations sampled. Possible explanations include a preferential selection during protocloning for one of two parental cpDNA types, an in planta sorting out of cpDNA types in the parental material or both.

Somatic hybrids between Solanum brevidens and Solanum tuberosum: Expression of a late blight resistance gene and potato leaf roll resistance

Hexaploid somatic hybrids resulting from mesophyll protoplast fusions between Solanum brevidens Phil., PI 218228, and Solanum tuberosum L., PI 203900 were tested for late blight resistance using two races of Phytophthora infestans Monte., de Bary. The S. tuberosum parent was a "late blight differential" possessing the R4 gene which confers resistance to race 0. The S. brevidens parent is resistant to potato leaf roll virus. Inoculations with both compatible (race 1.3.4.5) and incompatible (race 0) races of P. infestans clearly demonstrated the expression of the late blight resistance gene in all of the hybrid progeny tested. Most of the hybrids tested were also resistant to potato leaf roll virus (PLRV), indicating that the S. brevidens genes for PLRV resistance were present and expressed.

Regeneration of somatic hybrid plants formed between Lycopersicon esculentum and Solatium rickii

A single somatic hybrid callus clone was identified following the fusion of Lycopersicon esculentum protoplasts and Solanum rickii suspension culture protoplasts. The hybrid nature of the callus and the plants regenerating from it was determined by assaying phosphoglucomutase-2 isozyme expression. The chloroplast genome present in four somatic hybrid plants was characterized by probing digests of total DNA with nick translated L. esculentum chloroplast DNA(cpDNA). All four somatic hybrid plants had inherited S. rickii cpDNA. Two clones of plant mitochondrial DNA (mtDNA), soybean 18S and 5S rDNA and maize cytochrome oxidase subunit II were used to characterize the mtDNA present in total DNA digests of four somatic hybrid plants. In both cases, the somatic hybrid plants had inherited most but not all of the S. rickii specific fragments, but none of the L. esculentum specific fragments.

Production and characterization of somatic hybrids between Solanum melongena L. and S. sisymbriifolium Lam

Protoplasts of 6-azauracil (AU) resistant cell lines of Solanum melongena L. were fused with protoplasts of S. sisymbriifolium Lam. to create somatic hybrids between these sexually-incompatible species. Following fusion, colonies were selected which were capable of growth in medium containing 1mM AU. These colonies were placed on medium containing zeatin which had been shown to stimulate anthocyanin production during shoot organogenesis in tissue explants of S. sisymbriifolium but not in S. melongena. A total of 37 anthocyanin-producing colonies were identified from which 26 hybrid plants were regenerated. The morphological traits intermediate to those of the parents included: flower colour, leaf shape, and trichome density. Cytogenetic analysis revealed that all hybrids were aneuploids but their chromosome numbers were close to the expected number of 48. Isozyme analysis revealed that nuclear genes of both parents were expressed in the hybrids. In addition, isoelectric focussing of the large subunit of ribulose 1,5-bisphosphate carboxylase (Rubisco) provided evidence that each hybrid expressed only the S. sisymbriifolium chloroplast genome. All hybrids regenerated thus far have been sterile.

Intra-specific fusions in Solanum tuberosum

Plants were regenerated from callus arising from protoplast fusion of two S. tuberosum diploids. Tetraploid progeny from the fusion of the two diploid partners had increased vigor. Isozyme analysis confirmed the presence of proteins from both partners in the fusion progeny. Pigmentation of tubers and anthers was heightened substantially in the fusion products. This fusion, the first intra-specific fusion within S. tuberosum, indicates that somatic fusion may be useful for transferring traits within this group.

Steroidal glycoalkaloid content of potato, tomato and their somatic hybrids

Analyses of leaves and 'tubers' from somatic hybrids of potato and tomato ('pomato' with plastids of potato, 'topato' with plastids of tomato) produced by fusion of protoplasts from liquid cultures of dihaploid potato and mesophyll of tomato revealed the presence of the two major potato glycoalkaloids (α-solanine and α-chaconine) as well as the tomato glycoalkaloid (αtomatine). The total alkaloid content of leaves was greater than that of 'tubers' and similar to levels in the foliage of parent plants. However, glycoalkaloids were more abundant in hybrid 'tubers' than in normal potato tubers by a factor of 5-15. In hybrid foliage, approximately 98% of the alkaloid present was of potato origin whereas in 'tubers' the reverse was the case, with tomatine comprising 60-70% of the total alkaloid. The similarities in alkaloid content and ratios between the pomato and the topato lines indicate that plastomes do not influence the biosynthesis and distribution of these alkaloids. The results indicate that major secondary metabolites may prove useful for assessing the hybrid nature of such plants.

Somatic hybridization between Lycopersicon esculentum and Lycopersicon pennellii

Selection and screening methods were devised which resulted in the identification of a number of somatic hybrid callus clones following fusion of Lycopersicon esculentum protoplasts and L. pennellii suspension culture protoplasts. Visual selection for callus morphology combined with a high fusion frequency and irradiation of one parental protoplast type ((137)Cs source, 1.5 Krads) resulted in selection of a callus clone population containing a high proportion of somatic hybrids. Analysis of a dimeric isozyme for the presence of a heterodimeric form was found to be satisfactory for distinguishing parental-type calli, somatic hybrid calli, and mixed calli derived from both types of unfused parental cells. No somatic hybrid calli produced shoots, although the sexual hybrid between L. esculentum and L. pennellii regenerated well under the culture conditions employed. This result suggests that the non-regenerable growth habit of the L. pennellii suspension culture was dominant in the somatic hybrid. The culture conditions described here are suitable for obtaining regenerated plants from L. esculentum mesophyll protoplasts. L. esculentum protoplast calli from fusion cultures gave rise to shoots with L. esculentum phenotype at higher frequency than calli from control unfused L. esculentum mesophyll protoplast cultures. The use of probes for species-specific organelle DNA fragments allowed identification of organelle DNA restriction fragments in digests of total DNA from small samples of individual callus clones. The callus clones analyzed either carried predominantly one parental plastid DNA type or mixtures of both types. Use of a mitochondrial DNA (mtDNA) probe which distinguishes two parental mtDNA fragments revealed that the L. pennellii-specific fragment was present in all clones examined, but the L. esculentum fragment was absent or in low proportion.

Increasing the variability of Lycopersicon peruvianum Mill. by protoplast fusion with Petunia hybrida L

Somatic hybridization of Lycopersicon peruvianum and Petunia hybrida was carried out to transfer cytoplasmic male sterility from Petunia to Lycopersicon. Cytological, morphological and biochemical analyses were performed to characterize the regenerated plants. Two regenerated plants, R3 and R6, were male sterile. R3 possessed chromosomes morphologically similar to those of both parental types. Leaf morphologies of these two plants and a third plant, R7, were intermediate between the two parents. The stability of RUBPCase was verified during parental plant development and after in vitro culture. Plant R7 presented a new form of the large subunit of RUBPCase.

Generation of heteroplastidic Nicotiana cybrids by protoplast fusion: analysis for plastid recombinant types

Protoplasts of a mutant line of Nicotiana tabacum having a maternally-transmitted chlorophyll deficiency were fused with protoplasts of two alloplasmic-male-sterile Nicotiana lines by the "donor-recipient" technique. In both fusion experiments variegated plantlets were regenerated which were shown to contain cytoplasms of mixed chloroplast nature. This confirms that with the "donor-recipient" method one can obtain mixed cytoplasms of genetically different chloroplasts. We present a convenient system to assay for genetic recombination between chloroplasts by combining use of several cytoplasmic markers: vis. chlorophyll pigmentation, chloroplast DNA restriction patterns, tentoxin resistance and male sterility. Within the limits of the experiment no recombinant types were recovered.

Genetic manipulation in plant breeding: somatic versus generative

A comparison is made between molecular/ in vitro/somatic and plant-level/generative approaches in the reconstruction of genotypes and reproductive systems. Although classical methods will remain the basis of plant breeding, a number of new somatic as well as generative genetic manipulation techniques are definitely applicable in several special situations. The first are technically more demanding, the latter are often conceptually more difficult, and both are laborious. Choice of approach is determined by the plant species, the stage of development of the techniques, the amount of background genetic information and the genetic diversity available, and the capacity of the institution involved. In the final stages of the program traditional selection and testing procedures remain indispensable. Whether any particular breeding program will profit from the incorporation of sophisticated genetic manipulation techniques must be carefully analysed. This discussion is intended to provide a basis for this analysis.

Cybrids containing mixed and sorted-out chloroplasts following interspecific somatic fusions in Nicotiana

Streptomycin resistance was transferred by "donor-recipient" protoplast fusion from Nicotiana tabacum (SR-1) protoplasts into Nicotiana tabacum (cytoplasmic male sterile - Line 92) protoplasts in one case and into Nicotiana sylvestris protoplasts in another. It is demonstrated that streptomycin resistance (SR-1) is a chloroplast marker which segregates independently from a mitochondrial marker.In the fusion experiment where Nicotiana tabacum (Line 92) was the recipient, microcalli were plated in the presence of streptomycin. In this case, chloroplast sorting out occurred at a stage preceeding plant regeneration, producing stable streptomycin resistant cybrids. In the fusion whre Nicotiana sylvestris was the recipient, no direct selection for streptomycin resistance was performed. In this case chloroplast sorting out was incomplete, thus producing cybrid plants with a mixed chloroplast population. In some plants, sorting out of streptomycin resistant and sensitive chloroplasts was still apparent in the second generation progeny.

Two dimensional analysis of chloroplast proteins from normal and cytoplasmic male sterile Brassica napus

Stromal and thylakoid proteins isolated from normal (N) and cytoplasmic male sterile (cms) lines of Brassica napus have been compared using a two dimensional gel separation. It has been shown that: 1) stromal compartments of the two lines were very similar; 2) although there was extensive homology between protein maps of thylakoids isolated from the two lines, these could be distinguished by the spots corresponding to the β subunits of the coupling factor CF1 from the ATPase complex.

Chromosomal and isozyme studies of Nicotiana tabacum - Glycine max hybrid cell lines

The chromosomal stability of a number of somatic hybrids derived from soybean (Glycine max (L.) Merr.) and Nicotiana tabacum var. 'Xanthi' were investigated. Several of the hybrid cell lines retained more than half the complement of N. tabacum chromosomes after 7 months of culturing. A number of chromosomal abnormalities were observed. The hybrids were positively identified by employing isozyme analysis of several dehydrogenases and aspartate aminotransferase.

Restriction endonuclease analysis of chloroplast DNA in interspecies somatic Hybrids of Petunia

Restriction endonuclease cleavage pattern analysis of chloroplast DNA (cpDNA) of three different interspecific somatic hybrid plants revealed that the cytoplasms of the hybrids contained only cpDNA of P. parodii. The somatic hybrid plants analysed were those between P. parodii (wild type) + P. hybrida (wild type); P. parodii (wild type)+P. inflata (cytoplasmic albino mutant); P. parodii (wild type) + P. parviflora (nuclear albino mutant). The presence of only P. parodii chloroplasts in the somatic hybrid of P. parodii + P. inflata is possibly due to the stringent selection used for somatic hybrid production. However, in the case of the two other somatic hybrids P. parodii + P. hybrida and P. parodii + P. parviflora it was not possible to determine whether the presence of only P. parodii chloroplasts in these somatic hybrid plants was due to the nature of the selection schemes used or simply occurred by chance. The relevance of such somatic hybrid material for the study of genomic-cytoplasmic interaction is discussed, as well as the use of restriction endonuclease fragment patterns for the analysis of taxonomic and evolutionary inter-relationships in the genus Petunia.

Somatic hybridization of an atrazine resistant biotype of Solanum nigrum with Solanum tuberosum : Part 1: Clonal variation in morphology and in atrazine sensitivity

Plants were regenerated from protoplast fusion experiments with haploid Solanum tuberosum L. and an atrazine resistant biotype of the normally susceptible S. nigrum L. Sixty clones which were unlike the parents were selected by types of hairs and leaf pigmentation of young shoots and characterized by: chromosome numbers, response to atrazine, branching, hairs of the calyx, shapes and pigmentation of leaves, and morphology of flowers. Twenty five clones showed vegetatively stable differences from the parental clones; enough combinations of mixed characters suggested the clear origin from fusants of at least nine clones. Observed diversities within and between protoplastderived clones are interpreted as expressions of variation during the development of the regenerants.

Comparison of Nicotiana tabacum and Nicotiana nesophila hybrids produced by ovule culture and protoplast fusion

Somatic hybrids were produced between Nicotiana tabacum and N. nesophila, two species incapable of conventional sexual hybridization. Sexual hybrids, though, could be produced between these two species by using ovule culture only when N. nesophila was female. Clones of somatic hybrids were compared with sexual hybrids. Statistically significant variation was observed between clones, but not between sexual hybrids, for pollen viability, flower morphology, leaf morphology, and trichome density. As all clones of somatic hybrids have 96 chromosomes, the variability could not be explained by interclonal variation in chromosome number. Variation between somatic hybrids could be the result of cytoplasmic segregation or recombination, mitotic recombination or small chromosomal rearrangements prior to plant regeneration. Variation between clones could be exploited as these interspecies hybrids are now being used to incorporate disease resistance into cultivated tobacco.