Development of the Nucleolus of Tobacco BY-2 Cells Accompanies the Multiplication of the Fibrillar Centers by Fragmentation

Abstract

The nucleolus of rapidly growing tobacco BY-2 cells displayed several distinctive features at the electron microscopic level such as remarkable development of the dense fibrillar component and vigorous dispersion of the granular component from the nucleolar surface toward the cytoplasm. The fibrillar centers (FCs) where resting ribosomal RNA genes (rDNA) reside were found as low electron-dense patches. These FCs were detected as dots at the light microscopic level by fluorescence in situ hybridization (FISH). We examined the relationship between appearances of the FCs and the development of the nucleolus. There was a positive correlation between the multiplication of the FCs and the nucleolar size : The nucleolus became large as the number of the FCs increased while the nucleolus degenerated concurrently with a numerical reduction and an enlargement of the FCs, probably due to fusion among them. These results are discussed in favor of the concept that the multiplication of the FCs produces many sites for actively transcribing rDNA.