Localization of organelle DNA synthesis within the root apical meristem of rice

Abstract

DNA synthesis in cell nuclei and organelles in the root apical meristem of rice was analysed by anti-BrdU immunofluorescence microscopy to determine whether there is a specific order of these events in monocot roots. In the root meristem, organelle DNAs were synthesized in a specific region in the distal part of the root apical meristem, and were not synthesized in the root meristem's proximal region or the elongation zone. In contrast, cell nuclear DNA was synthesized throughout the root apical meristem, except in the quiescent centre. In the root cap of rice, DNA synthesis in both cell nuclei and organelle nucleoids was detected only in the two layers of cells at the proximal end, which is a striking characteristic of monocotyledonous plants. Moreover, to determine quantitatively the activity of DNA synthesis in cell nuclei and organelle nucleoids in micro-scale sections of plant tissues, we developed novel techniques for micro-scale hybridization and immuno-detection analysis. At the distal end of the root apical meristem, DNA levels of plastids and mitochondria were 4-fold and 5-fold greater than those in the elongation zone, respectively. Intracellular organelle DNA levels dropped rapidly as the distance from the root tip increased. The activity of organelle DNA synthesis in the distal end of the root apical meristem was about 10-fold greater than that in the elongation zone. Our present results confirm that nuclear and organelle DNA synthesis are not synchronized, but the latter occurs preferentially before multiple cell divisions