ISSN:
1615-6102
Keywords:
Bud formation
;
Cytokinin
;
Funaria
;
Morphometry
;
Polarity
;
Tip growth
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
Notes:
Summary Cytokinin stimulates caulonemata ofFunaria to undergo an asymmetric division leading to the gametophore. The earliest detectable event is a small protuberance at the distal portion of the cell accompanied by the reorganization of the underlying organelles into a polarized distribution reminiscent of a tip growing cell. Dictyosomes and associated vesicles accumulate in the protuberance directly beneath the plasma membrane with mitochondria subjacent to the vesicular layer. Endoplasmic reticulum lies beneath the mitochondrial zone directly above the large central vacuole, while chloroplasts are outside the bud. As development continues the bud elongates causing the outer cell wall to exfoliate. During the above events the nucleus migrates toward the bud site concomitant with an increase in the number of microtubules between the nucleus and the base of the outgrowth. Nucleoli, extruded from the nucleus during a previous division, persist as diffuse fragments within the protuberance. Upon reaching the bud site, division occurs with the developing phragmoplast being initiated distal to the caulonema tip cell. The former polarized distribution of the cytoplasm is altered as mitochondria, chloroplasts and small vacuoles become evenly dispersed throughout the cytoplasm; dicytosomes and endoplasmic reticulum occupy a cortical position. These events indicate a change from 2-D tip growth to 3-D diffuse growth. To quantify the ultrastructural changes associated with bud formation we performed a morphometric analysis of cells in various stages of budding. The relative volumes of dictyosomes and vesicles adjacent to the bud apex decrease during bud development coincident with an increase in these organelles in lower portions of the cytoplasm. Mitochondria and chloroplasts follow this same pattern although their highest relative volumes initially are 4 μm from the bud apex and outside the bud site, respectively. These data, as well as density profile topographic maps for vesicle fractions, support the contention that cytokinin induces a change in morphological symmetry and polarity in the fine structure ofFunaria.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01282984
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