Sister chromatid exchanges in cultured immature embryos of wheat species and regenerants

Immature embryos of Triticum aestivum (ten cultivars and lines), T. durum, T. dicoccum and T. monococcum were cultured in vitro on MS medium supplemented with 1 or 2 mg/l of 2,4-D and 20 or 30 g/l of sucrose for 3 days and processed to score sister chromatid exchanges (SCEs) per chromosome. Media components affect DNA replication from the start of the culture. The SCE frequencies were dependent on the genotype and were not correlated with the degree of ploidy. They increased after doubling of the concentration of 2,4-D and/or sucrose, except in one cultivar of T. aestivum. The mean numbers were lower than observed in root meristems of T. aestivum (two cultivars) and T. dicoccum. Immature embryos of regenerants of T. aestivum (one cultivar) and T. durum demonstrated variable SCE frequencies, which may have been caused by mutations in the parental cell cultures. In the T. aestivum embryos the lowest frequencies were found in regenerants obtained from explants with the highest frequencies.

The nad6 gene and the exon d of nad1 are co-transcribed in wheat mitochondria

The exon d of nad1 is located 993 bp upstream of the nad6 gene in the wheat mitochondrial genome. Transcription analyses of both sequences (nad1 exon d and the nad6 gene) were done by Northern hybridization using RNA from wheat seedlings and tissue cultures derived from immature embryos. A complicated pattern was generated with a probe including exon d of nad1 and the whole nad6 gene. An 0.71-kb transcript is specific to nad1 exon d whereas a 1.2-kb transcript is specific to the nad6 gene. Three larger transcripts hybridize to both probes suggesting that nad1 exon d and nad6 are co-transcribed. This co-transcription has been directly demonstrated by cDNA synthesis on mtRNAs and sequencing of the PCR amplification product.

Mitochondrial DNA variation in long-term tissue cultured rice lines

The effects of long-term tissue culture on mitochondrial DNAs were examined using rice (Oryza sativa) cell suspension cultures. Mitochondrial DNAs were isolated from P. I. 353705 (an indica subspecies of rice similar to 'Asam 5'), its anther-culture-derived line BL2 (an 8-year-old cell suspension culture), and five other cell lines (A1, A7, A11, A13, and A23), also derived from BL2 and independently selected for resistance to the lysine analog, S-(2-amino)-ethyl-L-cysteine. Mitochondrial DNAs of the rice lines were digested with ten restriction endonucleases (BamHI, BglII, EcoRI, EcoRV, HindIII, PstI, PvuII, SalI, SmaI, and XhoI), electrophoresed, and transferred to nylon membranes. Southern blots were hybridized with one rice and five maize probes containing mitochondrial genes. The restriction patterns of ten Southern blots and hybridization patterns of 60 endonuclease/probe combinations were analyzed. DNAs from all sources produced unique restriction patterns when digested with HindIII or BglII; with the other endonucleases an array of similarities and differences was observed. Lines BL2 and A11 showed unique patterns with all restriction endonucleases tested. No hybridization pattern differences were observed among the lines when probes containing apt9 and atpA were used. However, extensive hybridization pattern differences were observed with coxI, coxII, rrn18-rrn5, and atp6 probes. Both restriction and hybridization patterns revealed variation due to tissue culture effect. Coxll was most efficient in revealing the uniqueness of BL2. Among the analog selected lines A11 was most divergent, and probes rrn18-rrn5 and atp6 were most efficient in revealing its distinctiveness. Unique mitochondrial genomic organizations were found to be associated with long-term tissue culture.