Embryology ofCymbidium sinense:the Microtubule Organization of Early Embryos

Abstract

InCymbidium sinense, the pattern of embryo development is unusual in that oblique cell divisions result in the formation of several suspensor cells prior to the development of the embryo proper. Characteristic changes in microtubular distribution can be found within the zygote and the proembryo during their development. After fertilization, the ellipsoid-shaped zygote has randomly distributed microtubules within its cytoplasm. As the zygote takes on a more rounded appearance, microtubules organize into a dense meshwork. Furthermore, microtubule bundles appear at the chalazal region of the cell prior to the first mitotic division of the zygote. At the preprophase stage of mitosis, a preprophase band of microtubules appears in the cytoplasm of the zygote. The zygote divides obliquely and unequally and gives rise to an apical cell and a slightly larger basal cell. Many randomly-aligned microtubules can be found in the cortex of the basal cell. The increase in the abundance of microtubules coincides with the isotropic expansion of the basal cell. The early division of the basal cell and subsequent division of the apical cell results in the formation of a four-celled embryo, of which three cells near the micropylar pole develop as suspensor cells. In the suspensor cells, the microtubules tend to orient in the same direction as the long axis of the cell. In addition, prominent microtubules can also be found near the adjoining cell walls of the four-celled embryo. The terminal cell is highly cytoplasmic with abundant microtubules within the cell. Subsequent divisions of the terminal cell give rise to additional suspensor cells and the embryo proper. In the mature embryo, five suspensor cells are usually present; one eventually grows through the micropyle of the inner integument and four grow towards the chalazal pole. The cortical microtubules of suspensor cells redistribute from a longitudinal to a transverse direction as they grow towards their respective poles.