Activation of organelle DNA synthesis during the initial phase of proliferation of BY-2 cultured tobacco cells after medium renewal

Abstract

Summary: Several lines of evidence have indicated that plastid and mitochondrial DNAs are preferentially synthesized during the initial phase of cell proliferation, when plant cells undergo multiple cell divisions successively. In the present study, we investigated the conditions necessary for and intracellular processes involved in the preferential activation of organelle DNA synthesis by using BY-2 tobacco(Nicotiana tabacum L.)cells as a model system for plant cell proliferation.When stationary-phase BY-2 cells were transferred to fresh medium, the amounts of plastid and mitochondrial DNA per cell increased immediately after the transfer(during lag to early logphases)and decreased thereafter(during late log-to stationary phases). In vitro DNA synthesis assays using isolated organelle-nuclei demonstrated that the initial increases in the organelle DNA levels were accompanied by transient activation of organelle DNA synthesis. Quantitative Southern hybridization analyses demonstrated that activation of organelle DNA synthesis occurred within 3 to 9 h after renewal of the medium. We found sucrose and auxin to be the most important elements in the fresh culture medium for stimulating organelle DNA synthesis. Experiments with various inhibitors revealed that de novo transcription and translation of cell-nuclear genes were required for the activation of organelle DNA synthesis.