In a previous article, it was reported that, under stress conditions caused by Ca2 ions, isolated protoplasts of Betula platyphylla leaves release a bundle of hollow fibrils as a stress-induced response. In the present article, details of this phenomenon have been investigated. As a special method, light microscopy combined with a 'through focus variation' technique was applied. In a regular culture medium, protoplasts could regenerate the cell walls, whereas protoplasts submitted to Ca2 stress could not. Under stress conditions, the protoplasts inflated and formed a huge vacuole and, after 28 days, secreted the peculiar gigantic fibril with a diameter of 150 μm. The giant fibril is composed of (1→3)-β-glucan chains, i.e., the material of callose tissue.
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