Effect of Hypoxia on Nucleoli in Excised Root Tips of Vicia faba : An Electron Microscopic Study

Abstract

Actively growing root tips of Vicia faba excised to about 5 mm length were exposed to hypoxia and the structure of nucleoli were examined under electron microscopy. The nucleolar segregation was first discerned under hypoxia. Then, the fibrillar centers (FCs) enlarged at the 4th hr, accompanying many unusual granules about 35 nm in diameter (35 nm granules). Most FCs appeared as a channel-like configuration at the 4th and 8th hr. The 35 nm granules became ambiguous toward the 8th hr. In parallel with development of the FCs, the GC dispersed into the space between dense chromatin masses. Then, collapse of the DFC followed and eventually, the FCs coated with thin layer of the DFC were exposed. In the control specimens, the FCs also developed a little. However, no 35 nm granule arose in the FCs and no conspicuous degradation of the nucleoli occurred. These results strongly suggest that the hypoxic shock lowers the rate of rRNA synthesis to retract active rRNA genes from the DFC, leaving some nucleolar proteins in the FCs as the 35 nm granules.