Changes in Cytoplasmic rRNA Density in the Root Apical Meristem Observed by High Resolution in situ Hybridization.

Changes in cytoplasmic rRNA density during cell differentiation were studied in the root tip of Lactuca sativa L. and Arabidopsis thaliana (L)., especially near the quiescent centre (QC). The density in cytoplasm was investigated semi-quantitatively by a new in situ hybridization method which provides very high resolution to study cell specific gene expression. Samples were embedded in Technovit 7100 resin, cut into thin sections, and hybridized with a digoxigenin-labelled probe. The hybridized probe was detected by indirect immunofluorescence microscopy using anti-digoxigenin antibody. The fluorescence intensity in each cell was measured using microphotometry. The intensity in the QC was less than that in the surrounding meristematic cells. The density of rRNA was relatively higher in the meristematic cells, and declined in the differentiated cells of the elongation zone and root cap as the distance from the QC increased. The density of cytoplasmic rRNA in the elongation zone was five fold lower than that in the active meristem in root of Arabidopsis. This suggests that the density of cytoplasmic rRNA is closely related to mitotic activity. This method also enabled to observe the subcellular localization of rRNA with a light microscope. The rRNA level of organelles was also observed in the root apical meristem. The organelles in the root tip contain large amounts of DNA, but the rRNA level in the organelles was low.