nucleolus organizer region activation by 5-azacytidine in wheat x rye hybrids. Genome, 33: 707-712. Nucleolar activity was studied in several lines of Triticum aestivum cv. Chinese Spring, Triticum turgidum cv. Durum, and F1 hybrids from euploid and aneuploid lines of T. aestivum and Secale cereale cv. Centeio d o Alto, in cells from root tips of seeds germinated in water or in 5-azacytidine. 5-Azacytidine, an analog of cytidine modified in the 5 position of the pyrimidine ring, inhibits DNA methylation. By using silver staining to determine the number of nucleolus organizer regions and the average number of nucleoli per root-tip cell from seeds germinated in both situations, it became apparent that the presence of 5-azacytidine during germination allowed for the expression of the nucleolus organizer region locus belonging to the rye genome, in contrast to the usual observed cytological absence of the rye nucleolus organizer region in wheat-rye hybrids. It is suggested that wheat nucleolar dominance in wheat-rye hybrids is mainly a consequence of methylation of rRNA genes or its regulators located on the 1R chromosome of rye.
The expression of rRNA genes located in the nucleolar organizing region (NOR) present on the short arm of chromosome 1R from rye (Secale cereale L.) was examined in several hexaploid (Triticum aestivum L.) and tetraploid wheats (Triticum turgidum L.) containing the entire chromosome 1R from rye (disomic substitution 1B(1R)), its full haploid genome (hexaploid wheat–rye F1 hybrid), or only its short arm translocated to the long arms of wheat chromosomes from the homoeologous group 1 (disomic translocations 1AL/1RS, 1BL/1RS, or 1DL/1RS) or added to the complete hexaploid wheat genotype (monotelosomic addition 1RS). By silver staining and determination of the number of Ag-NORs and the average number of nucleoli per root-tip cell it became apparent that the expression of 1R NORs, in the presence of wheat genomes, depends on the absence of the long arm of rye chromosome 1R. In wheat-rye F1 hybrids and in hexaploid wheat with a disomic substitution 1B(1R), 1R NOR was morphologically absent, even when only one wheat major NOR was present, in contrast with its frequent expression in wheat–rye translocation or addition lines where only its short arm was added. It is suggested that wheat nucleolar dominance over rye as expressed by heterochromatic and silent NOR in 1RS is under a complex genetic control which involves interaction between 1RL and unidentified wheat genes.Key words: 1R nucleolus organizer region, gene activity, amphiplasty.
Regulation of nondisjunction of the rye B chromosome was investigated using the cytidine analog 5-azacytidine, which affects DNA methylation. The B chromosome of rye normally undergoes nondisjunction at first pollen grain mitosis and is stable at all other mitotic nuclear divisions. Observations on mitosis in the control root cells showed normal chromosome behaviour, whereas the treated roots were characterized by a high frequency of anaphases with lagging chromosomes showing evidence of failure of chromatid separation. The occurrence of nondisjunction was confirmed by the presence of variable numbers of B chromosomes between cells within roots, whereas the A-chromosome number remained constant. These results suggest that the epigenetic process by which somatic cells maintain the inactivity of the gene(s) responsible for B chromosome nondisjunction, between fertilization and meiosis, is mediated through DNA methylation.Key words: B chromosome, DNA methylation, meiotic reprogramming, 5-azacytidine, rye.
Nullisomic 5D – tetrasomic 5B wheat plants were grown at 20 and 16 °C, and the level of paired and unpaired lateral elements in synaptonemal complex (SC) spreads was quantified in early-mid prophase I cells to ascertain the origin of the low frequency of metaphase I associations observed in plants held at 16 °C in the absence of the Ltp1 gene located on the long arm of chromosome 5D. The distribution of chromosome arm associations in metaphase I cells was studied using the C-banding technique. The results obtained confirm that asynapsis was the main cause of the reduction in the level of association in metaphase I at low temperature. Although a nonrandom distribution of chromosome arm associations at metaphase I was observed at both temperatures, this tendency was more visible at 16 °C, leading to the suggestion that at low temperatures differences in the specific ability of chromosome arms to form SCs and chiasmata are enhanced.Key words: synaptonemal complex, Ltp1 gene, metaphase I bonds, wheat, C-banding.
Hops (Humulus lupulus L.) is economically relevant in the brewing industry. Although Brazil has a big beer market, the unfavorable climate makes the hop growing difficult. Micropropagation is an alternative way to clone plants, but in this process somaclonal subculture variations may occur. In order to reduce consecutive subcultures, the slow growth technique can be used. To develop a slow growth protocol for the conservation of hop plants and Cascade and Nugget cultivars in vitro, Murashige and Skoog (MS) medium was supplemented with doses of osmotic agents: sucrose and sorbitol. The plants were kept at 25°C or 4°C during 120 days. After 4 months, the plants were transferred to a new MS medium containing sucrose (30 g.L-1) and kept for 30 days under optimal growing conditions. After evaluation, the plants were transplanted and underwent a 30-day acclimatization period, and then the ex vitro survival was evaluated. For both cultivars, reduced growth was observed at 4°C, with a satisfactory survival rate. There was no significant interaction between the osmotic agent and temperature for Cascade cultivar Conversely, the Nugget cultivar did have interaction, and the treatment with sucrose (60 g.L-1) at 4 ºC reduced plant length, brought largest root length and fresh mass.
In somatic cells of intergencric hybrids Truicum uestirum (mono‐isosomic 5BL, 2n=6x=4f)x trinordum 12n = 6x = 42, amphiploid Hordeum clulense x Triticum turgidnm)it was observed that high dosage of the long arm of 5B induced chromosome instability in hybrids 2n=42, in hybrids 2n=41 with only one dose of 5BL from the normal 5B genome of the teteaploed wheat, all cells have consistently 2n=41 chromosomes and no rmal 5B genome of the tetraploid wheat all cells have consistently 2n=41 plant differenttation in plants with 2n=42 which carry three doses of 5BL (one isochromosome 5BL and one 5B chromosome)most of the metaphase cells had 2n=42 chromosomes. However other cells in a reasonable frequency varying from 19% to 40% carried from 2n=6 to 2n=44. and showed marked desturbances in all phases of the cell cycle leading to final failure in plant development. It is suggested that the Ph1 geng of wheat. Located on 5BL regulates chromosome stability in the somatic cells of those hybrids.
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