The ultrastructure of antipodal cells of the Triticum aestivum embryo sac was studied at different stages of differentiation and programmed cell death. The importance of cell function in the antipodal complex is evidenced by the fact that it is fully formed before double fertilization, past the stages of proliferation of three initial cells, and several rounds of genome endoreduplication during differentiation. In this study, we showed that the actively synthesizing organelles, the granular reticulum, and Golgi apparatus, alter their structure during differentiation and death. The polymorphism of the shape of the mitochondria and plastids was demonstrated. For the first time, the actin filaments of the cytoskeleton and numerous multivesicular bodies associated with the plasma membrane were detected in the cytoplasm. The transfer of cytoplasm and organelles between antipodal cells and into the coenocyte of the endosperm was confirmed. DNA breaks and the release of cytochrome c at various stages of death were revealed. To understand the function of the antipodal cells, a quantitative PCR analysis of the expression of wheat genes involved in protective, antistress, and metabolic processes was carried out. We found that gene expression in the antipodal cell fraction was increased compared with that in the whole embryo sac. On the basis of the data, we assume that antipodal cells produce both nutrients and numerous antistress factors that ensure the normal development of the endosperm of the grain, which, in turn, further ensures the development of the embryo.

MAIN CONCLUSION:During antipodal cells PCD, polytene chromosomes rearrangement, segregation of nucleoli components and extrusion of nuclear components occur, cytochrome c is released from the mitochondria and DNA breaks appear. We studied in detail the nuclei of cells of the antipodal complex of wheat embryo sac (Triticum aestivum L.) during programmed cell death (PCD). The antipodal complex has been reported to be formed before double fertilisation of the embryo sac. Polyploidisation leads to the formation of giant polytene chromosomes in the nuclei of antipodal cells. These chromosomes are involved in secretory functions and are important for the development of cellular endosperm. Terminal deoxynucleotidyl transferase dUTP nick end labelling assay and immunodetection revealed DNA breaks in the nuclei and release of cytochrome c from mitochondria into the cytoplasm of antipodal cells during PCD. We used transmission electron microscopy, immunodetection and histochemistry to analyse the characteristic structural changes in the nuclei of antipodal cells during PCD. These included sequential structural changes in the nuclei containing polytene chromosomes, segregation of some components of the nucleolus into the bodies of polytene chromosomes, extrusion of nucleolar components and parts of chromosomes into the cytoplasm of antipodal cells and then into the endosperm coenocyte. The obtained results expand the understanding of the structural changes of plant cells with giant polytene chromosomes during PCD.

The process of programmed cell death is essential for plant ontogenesis. Seed development reveals the key role of programmed cell death in cell elimination and formation of new structures. Morphological data and biochemical regulators of programmed cell death during formation of the embryo sac and seed are discussed in the review.

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.