EMBRYOLOGY OF CYMBIDIUM-SINENSE - THE MICROTUBULE ORGANIZATION OF EARLY EMBRYOS

In Cymbidium sinense; the pattern of embryo development is unusual in that oblique cell divisions result in the formation of several suspens or cells prior to the development of the embryo proper. Characteristic changes in microtubular distribution can be found within the zygote a nd the proembryo during their development. After fertilization, the el lipsoid-shaped zygote has randomly distributed microtubules within its cytoplasm. As the zygote takes on a more rounded appearance, microtub ules organize into a dense meshwork. Furthermore, microtubule bundles appear at the chalazal region of the cell prior to the first mitotic d ivision of the zygote. At the preprophase stage of mitosis, a preproph ase band of microtubules appears in the cytoplasm of the zygote. The z ygote 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 abun dance of microtubules coincides with the isotropic expansion of the ba sal 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 cel ls. In the suspensor cells, the microtubules tend to orient in the sam e direction as the long axis of the cell. In addition, prominent micro tubules can also be found near the adjoining cell walls of the four-ce lled embryo. The terminal cell is highly cytoplasmic with abundant mic rotubules within the cell. Subsequent divisions of the terminal cell g ive rise to additional suspensor cells and the embryo proper. In the m ature embryo, five suspensor cells are usually present; one eventually grows through the micropyle of the inner integument and four grow tow ards the chalazal pole. The cortical microtubules of suspensor cells r edistribute from a longitudinal to a transverse direction as they grow towards their respective poles. (C) 1998 Annals of Botany Company.