Structural and Functional Features of the Wheat Embryo Sac’s Antipodal Cells during Differentiation

A detailed study into the structural features of the multilevel antipodal complex of wheat Triticum aestivum L. embryo sac was performed at different stages of the complex’s differentiation after double fertilization. The heterogeneity of nuclei ploidy in individual antipodal complexes caused by the asynchrony of the endoreduplication rounds of the nuclear DNA was revealed. The nuclei ploidy 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 nuclei ploidy of the antipodal complex’s basal layer adjacent to the chalasal region of the nucellus of the embryo sac reaches 13 C, the nuclei of the apical layer cells that contact the endosperm syncytium reaches 63 C, and the nuclei of the middle layer located between the basal and apical layers reach 30 C. At the middle stage of differentiation, the nuclei ploidy in the basal layer increases to 17 C. The nuclei ploidy of the apical layer cells increases to 95 C, and nuclei ploidy of the middle layer increases to 45 C. At the stage of late differentiation, the nuclei ploidy in the basal layer increases to 24 C; the apical layer ploidy increases to 215 C; the middle layer ploidy increases to 63 C. Changes in the shape and structure of the nuclei during differentiation were revealed. They manifest themselves in 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. Data on 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 increased number of cytoplasmic organelles was revealed. During the differentiation, prolong cisterns of the granular reticulum are replaced by concentric rings, mitochondria and plastids of extended and cupped shape appear, and the microtubule network is rebuilt. The features of the antipodal complex’s 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, and cells of different layers of the complex specialize at the middle stage of differentiation. The ploidy level of cell nuclei with polytene chromosomes reflects their functional significance in the formation of endosperm at the nuclear stage of development, and, subsequently, of normal full-fledged grain.