Plant Nucleolar Dynamic Structure

Nucleoli are the domains in which transcription and processing of rRNA and assembly of ribosomes take place. The nucleolus has three basic ultrastructural subcompartments: fibrillar centers (FCs), dense fibrillar components (DFCs), and granular components (GCs). The studies on nucleolus focus on the sites of transcription and distribution of rDNA, assembly of ribosomes, and proteomics of nucleolus. Up to now, there are still controversial documents on whether the transcription of rDNA takes place in either FCs or DFCs. By using routine EM and terbium citrate single-stranded RNA (ssRNA) special staining, we investigated the morphological changes of the nucleolus and the distribution of ssRNAs in it. In the nucleolus cycle, due to newly synthesized rRNAs cannot form a secondary structure, the distribution of some ssRNAs represents the distribution of newly synthesized ssRNAs, which is beneficial for solving the controversy about where are the transcription sites of rDNAs. The results from this study are as follows:First, by conventional EM, we found that:In the cell cycle, the nucleoli begin to emerge at the end of mitotic telophase. At the earlier stage of G1, the nucleoli are very small, and they don't form FCs and DFCs. With the cell grow up, the nucleoli become bigger gradually, and the structures of FCs and DFCs turn obvious and then the nucleoli begin to fuse. In the mid- and late-G1, the cell usually has one or two nucleoli. In the time of the mid- and late-G1, S and G2, the size of nucleolus has no obvious change. DNA doubling in S and G2 phases has no obvious effects on nucleolus volume. In S and S to G2, the nucleolus rim shows hairy and thus more linking of the nucleolus rim to the peripheral chromatins become apparent. In the typical G1 or G2, the morphology changes of the nucleolus rim and the increased linking to the peripheral chromatins are not observed.3. The changes and the distributions of FCs and DFCs in nucleoli are not synchronous to the changes of the cell cycles. In the phase of G1, S and G2, there exist both bigger FCs and smaller FCs in the nucleoli. Generally, cells whose nucleoli have bigger FCs but in lower number and cells whose nucleoli have smaller FCs but in higher number But in some cells, we can find that one nucleolus has both bigger and smaller FCs. DFCs become clear at the mid-G1 and exist in the nucleolus until the early mitotic prophase. Whether the nulceolous has bigger or smaller FCs and the FCs' distributions may be related to a single cell, but not to the nucleolus cycle of the cell. Whether FCs are heterogeneous or homogeneous cannot be necessarily related to the sizes and ratios of DFCs around it. In some cells, FCs show regionalized and centralized distribution, suggesting the tandem characters of rDNAs and reflecting the regionalism of FCs and DFCs when many NORs organizing one nucleolus.4. It is a process that the nucleoli begin to disappear at prophase. At the early stage, the structures of FCs and DFCs are not obviously changed in the nucleoli. And then, the nucleolus begins to disintegrate in to several small nucleoli. At this moment, structures of FCs and DFCs still exist. Later, the structures of FCs and DFCs disappear. Before the nucleoli turn into NORs, there have been no structures of FCs and DFCs.5.If nucleoli have the nucleolus lacunae, a nucleolus has generally only one. The lacunae structures appear in S and they originate from multiple sites.Second, in Allium cepa, with the ion terbium (III), we found that ssRNAs mainly distribute in DFCs and their peripheral regions, and these distribution further extents in the regions of GCs. ssRNA are not found in FCs S and G2. So when nucleolus rDNAs are transcribed, newly synthesized RNAs should be located in DFCs. That means that the transcription sites of rDNAs take place in DFCs. In DFCs, distribution of ssRNAs shows regional, reflecting the distribution of rDNAs is also regional in DFCs.