Ultrastructure of vascular cambial cell cytokinesis in pine seedlings preserved by cryofixation and substitution |
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Authors: | Rensing K H Samuels A L Savidge R A |
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Institution: | (1) Department of Botany, University of British Columbia, Vancouver, British Columbia,;(2) Faculty of Forestry and Environmental Management, University of New Brunswick, Fredericton, New Brunswick, |
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Abstract: | Summary. Trees depend on the secondary vascular cambium to produce cells for new xylem and phloem. The fusiform cells of this lateral
meristem are long and narrow, presenting special challenges for arranging the mitotic spindle and phragmoplast. Fusiform cambial
cells of Pinus ponderosa and Pinus contorta were studied by cryofixation and cryosubstitution which preserved ultrastructure and phases of cytokinesis with a resolution
not previously attained. Membranous structures including the plasma membrane, tonoplast, and those of other organelles were
smooth and unbroken, indicating that they were preserved while the protoplasm was in a fully turgid state. Mitotic spindles
separated daughter chromosomes diagonally across the radial width of the cells. The cell plate was initiated at an angle to
the cell axis between the anaphase chromosomes by a microtubule array which organized vesicles at the phragmoplast midline.
Within the phragmoplast, vesicles initially joined across thin tubular projections and then amalgamated into a tubulo-vesicular
network. Axial expansion of the cell plate generated two opposing phragmoplasts connected by a thin, extended bridge of cell
plate and cytoplasm that was oriented along the cell axis. In the cytoplasmic bridge trailing each phragmoplast, the callose-rich
tubular network gradually consolidated into a fenestrated plate and then a complete cell wall. Where new membrane merged with
old, the parent plasmalemma appeared to be loosened from the cell wall and the membranes joined via a short tubulo-vesicular
network. These results have not been previously reported in cambial tissue, but the same phases of cytokinesis have been observed
in cryofixed root tips and suspension-cultured cells of tobacco.
Received February 11, 2002; accepted May 31, 2002; published online October 31, 2002
RID="*"
ID="*" Correspondence and reprints: Department of Botany, University of British Columbia, 6270 University Boulevard, Vancouver,
BC V6T 1Z4, Canada.
Abbreviations: CFS cryofixation and cryosubstitution; ER endoplasmic reticulum; HPF high-pressure freezing; PPB preprophase band. |
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Keywords: | : Secondary vascular cambium Pinus spp High-pressure freezing Freeze substitution Cell plate Phragmoplast |
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