Structural and Functional Features of the Antipodal Cells of Wheat Embryo Sac during the Differentiation
The detailed study of the structural features of the multilevel antipodal complex of wheat Triticum aestivum L. embryo sac was performed at different stages of the differentiation of the complex after double fertilization. The heterogeneity of the ploidy of the nuclei of individual antipodal complexes, due to the asynchrony of the endoreduplication rounds of the DNA of the nuclei, was revealed. The ploidy of the nuclei of basal, middle, and apical layers of the complexes was measured at the early, middle, and late stages of differentiation. At the early stage of differentiation, the ploidy of the nuclei of the basal layer of the antipodal complex, adjacent to the chalasal region of the nucellus, reaches 13 C, the nuclei of the cells of the apical layer, which contacts with the endosperm syncytium, reaches 63 C, and the nuclei of the middle layer, located between the basal and apical layers, reaches 30 C. At the middle stage of differentiation, the ploidy of the nuclei of the basal layer increases to 17 C, the nuclei of the cells of the apical layer increases to 95 C, and the nucleus of the middle layer increases to 45 C. At the stage of late differentiation, the ploidy of nuclei the basal layer increases to 24 C; the apical layer increases to 215 C; the middle layer increases to 63 C. Changes in the shape and structure of the nuclei during differentiation were revealed, which is manifested in their heterogeneity in shape, size and structure of chromatin, the formation of individual polytene chromosomes, nuclear membrane invaginations and the variation in the number of nucleoli. The data about the distribution and structure of cytoplasmic organelles of the antipodal cells, endoplasmic reticulum, dictyosomes, mitochondria and microtubules, at different stages of differentiation of the antipodal complexes are fundamentally new. The number of cytoplasmic organelles increased during the differentiation. During the differentiation, prolong cisterns of the granular reticulum are replaced by concentric rings, extended and cupped shape mitochondria and plastids appear, and the microtubule network is rebuilt. The features of the antipodal cell structure may reflect changes in the functions of the antipodal complex during the differentiation. At the early stage all cells of the complex perform an osmoregulatory function, at the middle stage of differentiation cells of different layers of the complex specialize. The level of ploidy of cell nuclei with polytene chromosomes reflects their functional significance in the formation of the endosperm at the nuclear stage of development, and, subsequently, of normal full-fledged grain.