Nucleolar vacuolation in soybean root meristematic cells during recovery after chilling

The nucleolar vacuole formation in soybean root meristematic cells from seedlings grown 3 d at temperature 25 °C (control), 3 d at temperature 25 °C and then transferred to 10 °C (chilling) for 4 d, and after recovery for 1.5, 3, 6, 12 and 24 h at 25 °C were observed on semi-thin sections. Simultaneously, autoradiographic studies with ³H-uridine on squashed preparations were carried out. During recovery of plants, the number of vacuolated nucleoli increased gradually from 24 % after 1.5 h up to 40 % after 24 h, while in the control there were 18 % of nucleoli with vacuoles and after 4-d chilling only 5 %. Labelling of cells during 20-min incubation in ³H-uridine and during 80-min post-incubation in non-radioactive medium was increased in recovered plants in comparison with the control and chilled plants. The conclusion has been drawn that nucleolar vacuoles in soybean plants are formed as a result of migration of granular component accumulated in nucleolus during 4-d chilling.     

Fidelity of ribosomal ribonucleic acid synthesis by nucleoli and nucleolar chromatin.

Isolated nucleoli, nucleolar chromatin, and nucleolar DNA were used as templates for DNA synthesis in appropriately supplemented systems in which RNA polymerases other than RNA polymerase I were blocked by alpha-amanitin. With the aid of nucleotide analysis, DNA-RNA hybridization, and homochromatography fingerprinting, it was found that isolated nucleoli and nucleolar chromatin serve primarily as templates for synthesis of rRNA. However, the products formed with purified nucleolar DNA as a template do not contain the specific rRNA oligonucleotides nor are they appreciably hybridized to the rDNA region on cesium chloride gradients. These results indicate that whole nucleoli and nucleolar chromatin contain control mechanisms that restrict readouts by RNA polymerase I of nucleolar DNA to rDNA.     

Ultrastructural and autoradiographic studies of the role of nucleolar vacuoles in soybean root meristem

Ultrastructural and autoradiographic studies of nucleoli in soybean root meristematic cells in seedlings: (1) grown for 3 days at 25 degrees C (control), (2) grown for three days at 25 degrees C and for 4 days at 10 degrees C, and (3) grown as in (2) and recovered for 1 day at 25 degrees C were carried out. Control nucleoli had dense structure and a few small nucleolar vacuoles. Chilled plant nucleoli had less dense structure and no vacuoles. Nucleoli of plants recovered at 25 degrees C had big nucleolar vacuoles. In autoradiograms of squashed preparations, the labeling of nucleoli and cytoplasm after 20-min incubation in 3H-uridine was 5- and 6-fold stronger, respectively, in control than in chilled roots. Following recovery, the labeling of nucleoli and cytoplasm was much stronger than after chilling or even than in control roots. After 80-min postincubation in non-radioactive medium, average labeling of particular areas of cells was the highest in recovered plants which indicated intensification of rRNA synthesis, maturation and transport into cytoplasm resulting from the resumption of optimal conditions which was correlated with the appearance of big nucleolar vacuoles. In autoradiograms of semi-thin sections from roots of seedlings chilled for 4 days then recovered and incubated for 20 min in 3H-uridine, practically only extravacuolar parts of nucleoli were labeled. After 80-min postincubation, the labeling of nucleolar vacuoles was observed. Thus, during postincubation the labeled molecules were translocated from the nucleolar periphery into nucleolar vacuoles in cells where intensive transport of these molecules to the cytoplasm takes place. On the basis of these results, a hypothesis has been put forward that nucleolar vacuoles may be involved in the intensification of pre-ribosome transport outside nucleolus.     

玉米rRNA基因的组织和表达模式

玉米(Zea mays)只有1对45S rDNA位点并在分裂期染色体形成次缢痕, 是研究植物细胞rRNA基因组织和表达模式的简单模型。采用荧光原位杂交(fluorescence in situ hybridization, FISH)、CPD(PI与DAPI组合)染色和银染技术, 分析了玉米根尖分生细胞rRNA基因的组织和表达模式。45S rDNA探针在所有间期细胞核中显示2种杂交信号: 荧光强烈地位于核仁周边的纽, 而相对较弱地分布于核仁内的点。在部分细胞中可观察到点与纽相连或从纽发出; 点的数目越多, 纽变得越小; 点的数目多少与细胞的活性呈正相关。研究结果表明, 纽代表了处于凝缩状态的非活性的rDNA染色质, 纽解凝缩形成的点是rRNA基因活跃转录的细胞学表现; 不同阶段间期核的点的数目变化反映了被活化的rRNA基因数目不同。间期和前期细胞的CPD染色和相继的银染结果显示, 大部分rDNA染色质没有参与核仁的形成。rDNA FISH显示, 同一间期细胞的2个同源rDNA位点的表达水平存在差异, 同源染色体次缢痕的长度差异以及Ag-NOR和银染核仁的异态性进一步证实了这种差异的存在。FISH结果显示, 早中期细胞的rDNA染色质相对解凝缩, 银染在所有早中期细胞和部分中期细胞显示了明显的核仁, 表明玉米的rRNA基因在有丝分裂早中期有较活跃的转录, 其转录在晚中期才停止。     

玉米rRNA基因的组织和表达模式

玉米（Zea mays）只有1对45S rDNA位点并在分裂期染色体形成次缢痕,是研究植物细胞rRNA基因组织和表达模式的简单模型。采用荧光原位杂交（fluorescence in situ hybridization,FISH）、CPD（PI与DAPI组合）染色和银染技术,分析了玉米根尖分生细胞rRNA基因的组织和表达模式。45S rDNA探针在所有间期细胞核中显示2种杂交信号：荧光强烈地位于核仁周边的纽,而相对较弱地分布于核仁内的点。在部分细胞中可观察到点与纽相连或从纽发出;点的数目越多,纽变得越小;点的数目多少与细胞的活性呈正相关。研究结果表明,纽代表了处于凝缩状态的非活性的rDNA染色质,纽解凝缩形成的点是rRNA基因活跃转录的细胞学表现;不同阶段间期核的点的数目变化反映了被活化的rRNA基因数目不同。间期和前期细胞的CPD染色和相继的银染结果显示,大部分rDNA染色质没有参与核仁的形成。rDNA FISH显示,同一间期细胞的2个同源rDNA位点的表达水平存在差异,同源染色体次缢痕的长度差异以及Ag-NOR和银染核仁的异态性进一步证实了这种差异的存在。FISH结果显示,早中期细胞的rDNA染色质相对解凝缩,银染在所有早中期细胞和部分中期细胞显示了明显的核仁,表明玉米的rRNA基因在有丝分裂早中期有较活跃的转录,其转录在晚中期才停止。