Expression of nuclear and chloroplastic genes coding for fraction-1 protein in somatic hybrids of Nicotiana tabacum + rustica

Genetic transfer in plants through interspecific protoplast fusion

Protoplasts of sexually incompatible species have been fused and in some combinations have given rise to somatic hybrid plants. Partial elimination of parental chromosomes from either species is common in such hybrids, but total chromosome loss has generally occurred only with phylogenetically unrelated pairings. Genetic function of one parent may be retained despite a complete loss of its chromosomes, suggesting that genetic introgression is possible in the absence of complete donor chromosomes. A model interspecific combination for such studies is the potato-tomato somatic hybrid for which numerous phenotypes and karyotypes are encountered at the outset, with a broader range observed in the second somatic generation.

Non-random chloroplast segregation inNicotiana tabacum (+)N. rustica somatic hybrids selected by dual nuclear-encoded resistance

Nicotiana tabacum (+)N. rustica interspecific somatic hybrids were produced by fusion of leaf mesophyll protoplasts of transgenic methotrexate-resistantNicotiana tabacum L. with leaf mesophyll protoplasts of transgenic kanamycin-resistantN. rustica L. Somatic hybrids were selected on the basis of resistance to both methotrexate and kanamycin. Evidence for nuclear hybridization was obtained for 21 hybrids by restriction-fragment-length-polymorphism (RFLP) analysis using a heterologous wheat nuclear ribosomal-DNA (rDNA) probe and by analysis of glutamate-oxaloacetate transaminase (GOT) isoenzymes. Chloroplasts segregated non-randomly as 20 of the somatic hybrids possessedN. rustica chloroplasts and only one hadN. tabacum chloroplasts. Patterns of mitochondrial inheritance were examined by hybridization of a heterologous wheat cytochrome oxidase subunit II (coxII) gene with genomic DNA of the somatic hybrids. Four somatic hybrids with hybridization patterns similar toN. rustica and 17 with hybridization patterns consistent with mitochondrial DNA (mtDNA) rearrangement or recombination were obtained. None of the somatic hybrids had patterns ofcoxll hybridization identical withN. tabacum. Male-fertility levels in the hybrids ranged from undetectable to 87% and only nine hybrids produced a limited amount of viable seed. There was no apparent correlation between the patterns of organelle inheritance in the somatic hybrids and the relative degree of fertility.

Transmission of organelles in triploid hybrids produced by gametosomatic fusions of two Nicotiana species

Gametosomatic hybrids produced by the fusion of microspore protoplasts of Nicotiana tabacum Km(+)Sr(+) with somatic cell protoplasts of N. rustica were analysed for their organelle composition. For the analysis of mitochondrial (mt)DNA, species-specific patterns were generated by Southern hybridization of restriction endonuclease digests of total DNA and mtDNA with four DNA probes of mitochondrial origin: cytochrome oxidase subunit I, cytochrome oxidase subunit II, 26s rDNA and 5s-18s rDNA. Of the 22 hybrids analyzed, some had parental-type pattern for some probes and novel-type for the others, indicating interaction between mtDNA of the two parent species. For chloroplast (cp)DNA analysis, species-specific patterns were generated by Southern hybridization of restriction endonuclease digests of total DNA with large subunits of ribulose bisphosphate carboxylase and cpDNA as probes. All the hybrids had N. rustica-specific patterns. Hybrids were not resistant to streptomycin, a trait encoded by the chloroplast genome of N. tabacum. In gametosomatic fusions of the two Nicotiana species, mitochondria but not the chloroplasts are transmitted from the parent contributing microspore protoplasts.

Analysis of fertility in somatic hybrids of Nicotiana rustica and N. tabacum and progeny over two sexual generations

Somatic hybrid plants, produced between Nicotiana rustica and N. tabacum by heterokaryon isolation and culture and also by mutant complementation, were examined regarding their ability to set seed. From a total of seventeen independent somatic hybrids, three were found to be partially self-fertile while the others did not set seed. Differences regarding the methods of hybrid selection, parental varieties and chloroplast composition of hybrids did not appear to be significant regarding the ability of plants to set seed. Much variation in fertility was observed in subsequent generations and by recurrent selection of the most fertile, over two generations, it was possible to increase the level of self-fertility in some of the progeny. One R2 derivative possessed approximately a tenfold higher level of self-fertility than it's somatic hybrid parent. The presence of genetic markers from both parents were observed in all progeny indicating their hybrid nature.

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.

Intersectional cytoplasmic hybrids in Nicotiana : Identification of plastomes and chondriomes in N. sylvestris + N. rustica cybrids having N. sylvestris nuclear genomes

Cybrid plants having the nuclear genomes of one species and either or both plastomes and chondriomes of another species were obtained by fusing protoplasts of Nicotiana sylvestris, as "recipients", with X-irradiated protoplasts of N. rustica as "donors" of chloroplasts and mitochondria. Forty-nine flowering plants, derived from 28 calli, were analysed. As expected, they all had N. sylvestris (i.e. "recipients") morphology. Chloroplast DNA restriction patterns indicated that 8 and 41 plants had N. rustica and N. sylvestris plastomes, respectively. Some of the plants with either type of plastomes produced sterile pollen but none showed anther malformation typical to alloplasmic male sterility. Chondriome identification by mitochondrial DNA restriction analysis of cybrid plants revealed only restriction patterns which were either similar or identical to those of N. sylvestris while no cybrids with N. rustica restriction patterns were detected.

Plastid number and plastid structural changes associated with tobacco mesophyll protoplast culture and plant regeneration

Mesophyll protoplasts were isolated from Nicotiana tabacum L. cv. Xanthi, and cell-colony formation induced in liquid culture. The plastid changes associated with the morphogenetic sequence from mesophyll protoplast to whole plant were examined. Minor ultrastructural changes in the plastids were evident after 1 d of culture, but by 8 d (four-to-eight-cell stage) the plastids were small, there was much less thylakoid membrane appression, and many prominent plastoglobuli were also present. Plastid-division figures were evident at this point of time and it was common to find plastids clustered around the nucleus. A typical proplastid was the dominant plastid type in the cultured cells from about 11 d until about five weeks when large amyloplasts and pregranal plastids were observed. Normally structured chloroplasts were present in the regenerated plant. There was no plastid division until the four-cell stage, with plastid numbers per cell approximately halving at each cell division, then stabilising around 12 per cell during cell-colony development, a number typical of meristematic cells. Though nucleoids were always present, their numbers in the plastids were reduced by the eight-cell stage.

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.

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.

Cytoplasmic hybridization in Nicotiana: mitochondrial DNA analysis in progenies resulting from fusion between protoplasts having different organelle constitutions

Our previous studies indicated that fusion products with one functional nucleus but organelles of the two fusion partners (i.e. heteroplastomic cybrids) could be obtained by fusing X-irradiated (cytoplasmic donor) with non-irradiated (recipient) Nicotiana protoplasts. The present report deals with the analysis of mitochondria in cybrid populations resulting from the fusion of donor Nicotiana tabacum protoplasts with recipient protoplasts having a N. Sylvestris nucleus but chloroplasts of an alien Nicotiana species, and exhibiting cytoplasmic male sterility. The two fusion parents showed significant differences in restriction patterns of their chloroplast and mitochondrial DNA. Four groups of cybrid plants were obtained by this fusion. All had N. sylvestris nuclei but contained either donor or recipient chloroplasts and had either sterile or fertile anthers. There was no correlation between anther fertility and chloroplast type. The mitochondrial DNA restriction patterns of sterile cybrids were similar to the respective patterns of the sterile fusion partner while the mitochondrial DNA restriction patterns of the fertile cybrids were similar to the respective patterns of the fertile fusion partner. The results indicate an independent assortment of chloroplasts and mitochondria from the heteroplastomic fusion products.

Somatic hybridization in Nicotiana: Segregation of organellar traits among hybrid and cybrid plants

Protoplasts from a nitrate reductase-deficient mutant of Nicotiana tabacum L. were fused with protoplasts from a stamen-less, cytoplasmically malesterile cultivar of tobacco containing the cytoplasm from N. suaveolens Lehm. Plants were regenerated from the fused protoplasts and characterized with respect to stamen development, chromosome number, and chloroplast composition. Of 29 regenerated plants, stamen production was restored in 26 plants and pollen production in 22. One plant was male sterile and two plants have never flowered. Analysis of the electrophoretic mobility of ribulose-1,5-bisphosphate carboxylase (RuBPcase) showed that 19 of the plants contained RuBPcase of the N. suaveolens type, six plants contained enzyme of the N. tabacum type, and four plants contained both types. Analysis of resistance to tentoxin in seedlings from 20 of the plants demonstrated that 14 had N. suaveolens-type chloroplasts, three had N. tabacum type, and three contained both types. Many of the plants which produced stamens and pollen still contained chloroplasts of the N. suaveolens type. Thus, the trait of cytoplasmic male sterility in tobacco is not an expression of the type of chloroplast genetic material.

Somatic hybridization in Nicotiana: Restoration of photoautotrophy to an albino mutant with defective plastids

Protoplasts of a cytoplasmic albino mutant of Nicotiana tabacum L. characterized by a deficient chloroplast genome were fused with protoplasts of a nitrate-reductase deficient mutant (NR(-)) of N. tabacum. Somatic hybrids were obtained where the genome of the NR(-) mutant was complemented by the cytoplasmic albino mutant which could synthesize an active nitrate reductase, and the chlorophyll deficiency in the albino mutant was restored by the chloroplasts from the NR(-) mutant. Cybrids were also obtained in which the deficient plastids of the cytoplasmic albino mutant were replaced by normal chloroplasts from the NR(-) mutant. The system used permitted a simple selection of the hybrids and the cybrids. The NR(-) mutant was excluded at the cellular level by transfer of the cells to medium deficient in reduced nitrogen. The cytoplasmic albino mutant grew well on the selective nitrate medium. However, during callus formation, clear differences in the morphology and pigmentation of the calli were found which permitted selection for photoautotrophy at the callus level. The hybrid or cybrid nature of the plants was confirmed by examination of their morphology and chromosome number. Although the fusion partners come from the same species, only one plant showed the white-green variegated pattern typical of that of the cytoplasmic albino parent, indicating that segregation of plastids occurred during development of the calli and regeneration of the plants.

Detection of the Nicotiana rustica chloroplast genome coding for the large subunit of Fraction I protein in a somatic hybrid in which only the N. tabacum chloroplast genome appeared to have been expressed

Nine plants were produced from anthers of a somatic hybrid which had been obtained by fusion of Nicotiana tabacum L. and N. rustica L. protoplants. As determined by electrofocusing, the Fraction I protein of the original somatic hybrid had largesubunit polypeptides exclusively of the N. tabacum type. Two of the plants regenerated from anthers contained Fraction-I-protein large subunits exclusively of the N. rustica type. Since each plant was regenerated from a single cell, the somatic hybrid must have had cells containing both the N. tabacum and N. rustica chloroplast genome although the latter was not expressed. Possibilities to account for this non-expression of a chloroplast genome in the somatic hybrid are discussed.