Localization of Ca++-containing antimonate precipitates during mitosis.

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Abstract
Intracellular bound Ca2+ was localized throughout mitosis and cytokinesis in 2 plant species by means of in situ precipitation with potassium antimonate and EM visualization. Identification of Ca2+ as the major cation precipitated was made by comparing solubility properties in water, EDTA and EGTA [ethyleneglycol-bis(.beta.-aminethyl ether) N,N,N'',N''-tetraacetic acid] of the intracellular deposits with respect to those of K+-, Mg2+- and Ca2+-antimonate standards. In spermatogenous cells of the water fern, Marsilea vestita, and stomatal complex cells of barley, Hordeum vulgare, antimonate deposits were found associated with the endoplasmic reticulum (ER), vacuoles, euchromatin/nucleoplasm and mitochondria. The last contain a much higher density of precipitates in Marsilea than in Hordeum. Dictyosomes and the nuclear envelope of Marsilea also contain antimonate deposits, as do the plasmalemma, cell wall, and phragmoplast vesicles of Hordeum. Microtubule-organizing centers such as kinetochores and the blepharoplast of Marsilea do not stain. In spite of differences in associated antimonate between certain organelles of the 2 spp., the presence of antimonate along the ER throughout the cell cycle is common to both. Of particular interest are those precipitates seen along the tubules and cisternae of the extensive smooth ER that surrounds and invades the mitotic spindle in both species. The ability to bind divalent cations makes the mitotic apparatus-associated ER a likely candidate for regulation of free Ca2+ levels in the immediate vicinity of structural components and processes that are Ca2+-sensitive and proposed to be Ca2+-regulated.