Asymmetric hybridization between cytoplasmic male-sterile (CMS) and fertile rice (Oryza safiva L.) protoplasts

Small brown planthopper resistance loci in wild rice (Oryza officinalis)

Host-plant resistance is the most practical and economical approach to control the rice planthoppers. However, up to date, few rice germplasm accessions that are resistant to the all three kinds of planthoppers (1) brown planthopper (BPH; Nilaparvata lugens Stål), (2) the small brown planthopper (SBPH; Laodelphax striatellus Fallen), and (3) the whitebacked planthopper (WBPH, Sogatella furcifera Horvath) have been identified; consequently, the genetic basis for host-plant broad spectrum resistance to rice planthoppers in a single variety has been seldom studied. Here, one wild species, Oryza officinalis (Acc. HY018, 2n = 24, CC), was detected showing resistance to the all three kinds of planthoppers. Because resistance to WBPH and BPH in O. officinalis has previously been reported, the study mainly focused on its SBPH resistance. The SBPH resistance gene(s) was (were) introduced into cultivated rice via asymmetric somatic hybridization. Three QTLs for SBPH resistance detected by the SSST method were mapped and confirmed on chromosomes 3, 7, and 12, respectively. The allelic/non-allelic relationship and relative map positions of the three kinds of planthopper resistance genes in O. officinalis show that the SBPH, WBPH, and BPH resistance genes in O. officinalis were governed by multiple genes, but not by any major gene. The data on the genetics of host-plant broad spectrum resistance to planthoppers in a single accession suggested that the most ideally practical and economical approach for rice breeders is to screen the sources of broad spectrum resistance to planthoppers, but not to employ broad spectrum resistance gene for the management of planthoppers. Pyramiding these genes in a variety can be an effective way for the management of planthoppers.

Production of bacterial blight resistant lines from somatic hybridization between Oryza sativa L. and Oryza meyeriana L

Novel bacterial blight (BB) resistance gene(s) for rice was (were) introduced into a cultivated japonica rice variety Oryza sativa (cv. 8411), via somatic hybridization using the wild rice Oryza meyeriana as the donor of the resistance gene(s). Twenty-nine progenies of somatically hybridized plants were obtained. Seven somatically hybridized plants and their parents were used for AFLP (amplified fragment length polymorphism) analysis using 8 primer pairs. Results confirmed that these plants were somatic hybrids containing the characteristic bands of both parents. The morphology of the regenerated rice showed characters of both O. sativa and O. meyeriana. Two somatic hybrids showed highest BB resistance and the other 8 plants showed moderate resistance. The new germplasms with highest resistance have been used in the rice breeding program for the improvement of bacterial blight resistance.

[Nuclear and cytoplasmic genome analysis of somatic hybrid of Triticum aestivum L. and Leymus chinensis (Trin.) Tzvel]

Intergeneric somatic hybrids were obtained by fusion between protoplasts of Triticum aestivum L. cv. Jinan 177 and Leymus chinensis (Trin.) Tzvel. Protoplasts of L. chinensis were exposed to UV (300 microW/cm(2)) for 30 s, 45 s and 1 min before fusion. The results of morphological and chromosomal observation, isozyme pattern as well as RAPD analysis and the 5S rDNA space sequence analysis showed the hybrid nature of the regenerated colonies of fusion combination T (+) L (UV 30 s). Restriction Fragment Length Po1ymorphism (RFLP) analysis using mitochondrion-specific atp6 and chloroplast-specific rbcL probes showed that most of the organelle genomes of the hybrids originated from L. chinensis and some recombination happened between wheat and L. chinensis. The effects of elimination of the nuclear genome of receptor protoplast on hybrid regeneration and receptor organelle genome elimination are discussed.

Asymmetric somatic hybridization between tall fescue (Festuca arundinacea Schreb.) and irradiated Italian ryegrass (Lolium multiflorum Lam.) protoplasts

Intergeneric asymmetric somatic hybrids have been obtained by the fusion of metabolically inactivated protoplasts from embryogenic suspension cultures ofFestuca arundinacea (recipient) and protoplasts from a non-morphogenic cell suspension ofLolium multiflorum (donor) irradiated with 10, 25, 50, 100, 250 and 500 Gy of X-rays. Regenerating calli led to the recovery of genotypically and phenotypically different asymmetric somatic hybridFestulolium plants. The genome composition of the asymmetric somatic hybrid clones was characterized by quantitative dot-blot hybridizations using dispersed repetitive DNA sequences specific to tall fescue and Italian ryegrass. Data from dot-blot hybridizations using two cloned Italian ryegrass-specific sequences as probes showed that irradiation favoured a unidirectional elimination of most or part of the donor chromosomes in asymmetric somatic hybrid clones obtained from fusion experiments using donor protoplasts irradiated at doses ≤ 250 Gy. Irradiation of cells of the donor parent with 500 Gy prior to protoplast fusion produced highly asymmetric nuclear hybrids with over 80% elimination of the donor genome as well as clones showing a complete loss of donor chromosomes. Further information on the degree of asymmetry in regenerated hybrid plants was obtained from chromosomal analysis including in situ hybridizations withL. multiflorum-specific repetitive sequences. A Southern blot hybridization analysis using one chloroplast and six mitochondrial-specific probes revealed preferentially recipient-type organelles in asymmetric somatic hybrid clones obtained from fusion experiments with donor protoplasts irradiated with doses higher than 100 Gy. It is concluded that the irradiation of donor cells before fusion at different doses can be used for producing both nuclear hybrids with limited donor DNA elimination or highly asymmetric nuclear hybrid plants in an intergeneric graminaceous combination. For a wide range of radiation doses tested (25-250Gy), the degree of the species-specific genome elimination from the irradiated partner seems not to be dose dependent. A bias towards recipient-type organelles was apparent when extensive donor nuclear genome elimination occurred.

Asymmetric protoplast fusion aimed at intraspecific transfer of cytoplasmic male sterility (CMS) in Lolium perenne L

Techniques have been developed for the production of cybrids in Lolium perenne (perennial ryegrass). Gamma-irradiated protoplasts of a cytoplasmically male-sterile breeding line of perennial ryegrass (B200) were fused with iodoacetamide-treated protoplasts of a fertile breeding line (Jon 401). After fusion 25 putative cybrid calli were characterized to determine mitochondrion type and composition of the nuclear genome. Analysis of phosphoglucoisomerase isozyme profiles and determination of the ploidy level by flow cytometry indicated that all of the calli tested essentially contained the nuclear DNA of the fertile line. However, the presence of parts of the nuclear DNA from the sterile line could not be excluded. Southern blotting of total DNA isolated from the parental lines and putative cybrids combined with hybridizations using the mitochondrial probes cox1 and atp6 revealed that the mitochondria of the calli originated from the fertile line (5 calli), the sterile line (5 calli) or from both parental lines (15 calli). The hybridization patterns of the mtDNA from the cybrid calli showed extensive quantitative and qualitative variation, suggesting that fusion-induced inter- or intramolecular mitochondrial recombination had taken place.

Highly asymmetric intergeneric nuclear hybrids between Nicotiana and Petunia: evidence for recombinogenic and translocation events in somatic hybrid plants after "gamma"-fusion

Extremely asymmetric nuclear hybrids have been obtained via protoplast fusion in an intergeneric combination. Irradiated (cobalt(60),100 krad) kanamycinresistant Petunia hybrida mesophyll protoplasts were chemically fused with wild-type mesophyll protoplasts of Nicotiana plumbaginifolia. Eighty-six hybrid colonies were selected on kanamycin-containing medium, and twenty-four of these could be induced to regenerate numerous shoots. Cytological analysis of the regenerants showed the presence of a few chromosome fragments in some lines, and even a metacentric chromosome in yet another line. Besides additional chromosome fragments some lines only possessed typical Nicotiana chromosomes, and this at the diploid (2n = 2X = 20) as well as the tetraploid (2n = 2X = 40) level. Biochemical analysis showed that all regenerants had neomycin phosphotransferase activity (NPTII), which suggests that intergenomic recombination and or translocation events took place at least in those lines where no additional chromosome fragments could be detected. The presence of the NPTII gene was shown by Southern hybridization. All regenerants tested were fertile, and the segregation ratios for the kanamycin gene (for self and backcross pollinations to the recipient partner) for some of the regenerants correspond with Mendelian rules for a monogenic dominant marker. Most of the regenerants showed abnormal segregation ratios; in this case, no correlation could be made between segregation ratio and chromosome composition.Our results demonstrate the existence of intergenomic recombination and translocations evens in nuclear somatic hybrid plants obtained via "gamma"-fusion.

Stability of mitochondrial DNA in tissue-cultured cells of rice

Restriction analysis of mitochondrial (mt) DNA from 3-month-old callus cultures of the cytoplasmic male sterile rice, V41A, which contains S2 or "wild abortive" cytoplasm, and its fertile maintainer, V41B, showed the same BamHI restriction profiles as mtDNA from the corresponding leaf material. Similarly, mtDNA of rice (var. Taipei 309) from leaves, a 2-month-old cell suspension (T3MS2/A), a totipotent suspension (T3MS) and a 19-month-old suspension, which had lost its protoplast regeneration ability (LB3), showed indistinguishable BamHI restriction profiles. However, clear differences in mtDNA restriction profiles were observed between LB3 and a 30-month-old suspension culture of Taipei 309 (LB1), which appeared to reflect substantial changes in the relative abundance of specific DNA sequences. Hybridisation of a maizecoxII gene probe to blots of restricted mtDNA confirmed that, while the relative abundance of certain mtDNA sequences was preserved during long-term tissue culture of rice, major changes in abundance were observed with other sequences.

Plant regeneration from cytoplasmic hybrids of rice (Oryza sativa L.)

We obtained cybrid plants by electrofusing γ-irradiated protoplasts of a cytoplasmic male-sterile line "A-58 CMS" (Oryza sativa L.) and iodoacetamide (IOA)-treated protoplasts of the fertile (normal) rice cultivar "Fujiminori". The cybridity of the plants was confirmed by mitochondrial (mt) DNA restriction endonuclease, and plasmid-like DNA analyses, and by isozyme, cytological and morphological investigations. The chromosome number of the cybrid plants is 24.