Changes in the pattern of organization of the microtubular cytoskeleton during megasporogenesis in Cymbidium sinense

Abstract

InCymbidium sinense embryo sac development follows the bisporic pattern. This pattern of development is rare in orchids. Changes in the structure and organization of the microtubular cytoskeleton during megasporogenesis in this orchid were followed using an immunofluorescence technique and confocal microscopy. At the initial stage of development the microtubules in the archespore are randomly oriented throughout the cortex and cytoplasm. Later, microtubules undergo reorganization with more microtubules distributed in the cortex and around the nucleus. Megasporocyte formation is marked by the elongation of the archesporial cell. The cytoskeleton in the elongated megasporocyte is radially organized with microtubules extending from the nucleus to the peripheral region of the cell. In the megasporocyte some of the microtubules also show a polarized pattern of distribution with more microtubules at the chalazal end than the micropylar end. After meiosis I, a dyad is formed. Each dyad member possesses a complement of randomly oriented microtubules. But some microtubules in the dyad also show radial orientation. Soon afterwards microtubules in the degenerating micropylar dyad cell disappear. The functional (chalazal) dyad cell proceeds to meiosis II giving rise to a two-nucleate embryo sac. The pattern of microtubule organization in the two-nucleate embryo sac is quite complex. Each nucleus is surrounded by a set of randomly distributed microtubules. But the vacuolar region between the two nuclei is surrounded by a parallel array of cytoplasmic microtubules. No preprophase bands were observed during the development of the megasporocyte. This is the first description of microtubules in bisporic embryo sac development.