The current study attempts to verify secretion of a callose hollow fiber from plant protoplasts, which was previously reported in woody plants, as a common stressed culture-induced phenomenon among plants, by re-examining herbaceous plant cultured cells of tobacco BY-2 (Nicotiana tabacum L. cv. Bright Yellow 2) and Arabidopsis thaliana allowing inhibition of cell wall formation. Then, inhibition of cell wall formation was found to induce the secretion of callose fibers by adding isoxaben, an inhibitor of cellulose synthesis, into the stressed culture medium. The inducing period for the secretion from both tobacco BY-2 and A. thaliana was shorter than that previously reported in Betula platyphylla (white birch). The secreted fibers were observed by confocal laser scanning microscopy and atomic force microscopy. The microscopic images indicated that the callose fibers from herbaceous plants did not necessarily possess a similar structure to hollow fibers from white birch. Eventually, it has been demonstrated that the secretion of callose fibers from protoplasts can be a stress-induced phenomenon commonly occurring in plants, but their hierarchical 3D-fiber structures are likely to depend on species.
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