Studies on the behavior of organelles and their nucleoids in the root apical meristem of Arabidopsis thaliana (L.) Col. |
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Authors: | Makoto Fujie Haruko Kuroiwa Shigeyuki Kawano Tsuneyoshi Kuroiwa |
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Institution: | (1) Department of Biology, Faculty of Science, University of Tokyo, Hongo, 113 Tokyo, Japan;(2) Kyoritsu Women's Junior College, 102 Tokyo, Japan |
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Abstract: | The behavior of cell nuclei, mitochondrial nucleoids (mt-nucleoids) and plastid nucleoids (ptnucleoids) was studied in the
root apical meristem of Arabidopsis thaliana. Samples were embedded in Technovit 7100 resin, cut into thin sections and stained with 4′-6-diamidino-2-phenylindole for
light-microscopic autoradiography and microphotometry. Synthesis of cell nuclear DNA and cell division were both active in
the root apical meristem between 0 μm and 300 μm from the central cells. It is estimated that the cells generated in the lower
part of the root apical meristem enter the elongation zone after at least four divisions. Throughout the entire meristematic
zone, individual cells had mitochondria which contained 1–5 mt-nucleoids. The number of mitochondria increased gradually from
65 to 200 in the meristem of the central cylinder. Therefore, throughout the meristem, individual mitochondria divided either
once or twice per mitotic cycle. By contrast, based on the incorporation of 3H]thymidine into organelle nucleoids, syntheses of mitochondrial DNA (mtDNA) and plastid DNA (ptDNA) occurred independently
of the mitotic cycle and mainly in a restricted region (i.e., the lower part of the root apical meristem). Fluorimetry, using
a videointensified microscope photon-counting system, revealed that the amount of mtDNA per mt-nucleoid in the cells in the
lower part of the meristem, where mtDNA synthesis was active, corresponded to more than 1 Mbp. By contrast, in the meristematic
cells just below the elongation zone of the root tip, the amount of mtDNA per mt-nucleoid fell to approximately 170 kbp. These
findings strongly indicate that the amount of mtDNA per mitochondrion, which has been synthesized in the lower part of the
meristem, is gradually reduced as a result of continual mitochondrial divisions during low levels of mtDNA synthesis. This
phenomenon would explain why differentiated cells in the elongation zone have mitochondria that contain only extremely small
amounts of mtDNA.
This work was supported by a Grant-in Aid (T.K.) for Special Research on Priority Areas (Project No. 02242102, Cellular and
Molecular Basis for Reproduction Processes in Plants) from the Ministry of Education, Science and Culture of Japan and by
a Grant-in Aid (T.K.) for Original and Creative Research Project on Biotechnology from the Research Council, Ministry of Agriculture,
Forestry and Fisheries of Japan. |
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Keywords: | Apical meristem (root) Arabidopsis (cell cycle) Mitochondrion (nucleoid) Mitotic cycle Nucleoid (organelle) Root (apical meristem) |
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