Ultrastructural and cytochemical aspects of female gametophyte development in Sedum hispanicum L. (Crassulaceae)

Callose deposition analysis with special emphasis on plasmodesmata ultrastructure during megasporogenesis in Sedum (Crassulaceae)

In this study, the results of the first detection of callose within the ovules of the representatives of the family Crassulaceae are presented. This study was carried out on three species of the genus Sedum. Data analysis showed differences in the callose deposition pattern between Sedum hispanicum and Sedum ser. Rupestria species during megasporogenesis. Callose was present mostly in the transversal walls of dyads and tetrads in S. hispanicum. Furthermore, a complete loss of callose from the cell walls of the linear tetrad and a gradual and simultaneous deposition of callose within the nucellus of S. hispanicum were observed. The findings of this study showed the presence of hypostase with callose in the ovules of S. hispanicum, which is not common in other angiosperms. The remaining species tested in this study-Sedum sediforme and Sedum rupestre-showed a typical, well-known callose deposition pattern for plants with the monospore type of megasporogenesis and the Polygonum type of embryo sac. The functional megaspore (FM) in all studied species was located most chalazally. FM is a mononuclear cell, which wall is callose-free in the chalazal pole. The study presents the causes of different patterns of callose deposition within Sedum and their relationship with the systematic position of the study species. Moreover, embryological studies present an argument for excluding callose as a substance that forms an electron-dense material near the plasmodesmata in megaspores of S. hispanicum. This research expands the knowledge about the embryological processes of succulent plants from the family Crassulaceae.

Developmental, ultrastructural and cytochemical investigations of the female gametophyte in Sedum rupestre L. (Crassulaceae)

This article describes the development of female gametophyte in Sedum rupestre L. New embryological information about the processes of megasporogenesis and megagametogenesis provided in this paper expand the current knowledge about the embryology of the studied species. S. rupestre is characterized by monosporic megasporogenesis and the formation of Polygonum-type embryo sac. The process of megasporogenesis is initiated by one megaspore mother cell, resulting in the formation of a triad of cells after meiosis and cytokinesis. The functional megaspore, which is located chalazally, is a mononuclear cell present next to the megaspore in the centre of the triad. Only one of the two non-functional cells of the triad is binucleate, which occur at the micropylar pole. In this paper, we explain the functional ultrastructure of the female gametophytic cells in S. rupestre. Initially, the cytoplasm of the gametophytic cells does not differ from each other; however, during differentiation, the cells reveal different morphologies. The antipodals and the synergids gradually become organelle-rich and metabolically active. The antipodal cells participate in the absorption and transport of nutrients from the nucellar cells towards the megagametophyte. Their ultrastructure shows the presence of plasmodesmata with electron-dense material, which is characteristic of Crassulaceae, and wall ingrowths in the outer walls. The ultrastructure of synergid cells is characterized by the presence of filiform apparatus and cytoplasm with active dictyosomes, abundant profiles of endoplasmic reticulum and numerous vesicles, which agrees with their main function-the secretion of pollen tube attractants. Reported data can be used to resolve the current taxonomic problems within the genus Sedum ser. Rupestria.

Development of Embryo Suspensors for Five Genera of Crassulaceae with Special Emphasis on Plasmodesmata Distribution and Ultrastructure

The suspensor in the majority of angiosperms is an evolutionally conserved embryonic structure functioning as a conduit that connects ovule tissues with the embryo proper for nutrients and growth factors flux. This is the first study serving the purpose of investigating the correlation between suspensor types and plasmodesmata (PD), by the ultrastructure of this organ in respect of its full development. The special attention is paid to PD in representatives of Crassulaceae genera: Sedum, Aeonium, Monanthes, Aichryson and Echeveria. The contribution of the suspensor in transporting nutrients to the embryo was confirmed by the basal cell structure of the suspensor which produced, on the micropylar side of all genera investigated, a branched haustorium protruding into the surrounding ovular tissue and with wall ingrowths typically associated with cell transfer. The cytoplasm of the basal cell was rich in endoplasmic reticulum, mitochondria, dictyosomes, specialized plastids, microtubules, microbodies and lipid droplets. The basal cell sustained a symplasmic connection with endosperm and neighboring suspensor cells. Our results indicated the dependence of PD ultrastructure on the type of suspensor development: (i) simple PD are assigned to an uniseriate filamentous suspensor and (ii) PD with an electron-dense material are formed in a multiseriate suspensor. The occurrence of only one or both types of PD seems to be specific for the species but not for the genus. Indeed, in the two tested species of Sedum (with the distinct uniseriate/multiseriate suspensors), a diversity in the structure of PD depends on the developmental pattern of the suspensor. In all other genera (with the multiseriate type of development of the suspensor), the one type of electron-dense PD was observed.

Female gametophyte development in Sedum sediforme (Jacq.) Pau (Crassulaceae): an anatomical, cytochemical and ultrastructural analysis

Available documentation about the development of the female gametophyte of Crassulaceae is very limited. The aim of this study was to extend the embryological knowledge of Crassulaceae by analysing the development of the embryo sac in Sedum sediforme. Transmission electron microscopy and light microscopy including Nomarski optics (DIC) were used to observe individual stages of female gametophyte development. Cytochemical staining enabled detection of lipids, insoluble polysaccharides and proteins in gametophyte cells during their formation. Their increased accumulation was observed during nucellar cell and unfunctional cell degeneration in the embryo sac at the coenocytic and cellular stages (megagametogenesis). The female gametophyte develops in anatropous, bitegmic and crassinucellate ovules. The mature embryo sac is built of seven cells but after antipodes degeneration it is formed by the egg apparatus and a central cell. The monosporic Polygonum type was observed. One megaspore mother cell (MMC) formed three cells after meiosis. A triad was formed from a functional megaspore (placed chalazally), one uninucleate megaspore and a binucleate cell located at the micropylar end. Plasmodesmata with adhering electron-dense dome were noticed in walls of the coenocytic embryo sac and in the outer walls of ephemeral antipodes. Moreover, similar to synergids, antipodes form wall ingrowths. Here, we report new structural features of the antipodal cells (the presence of plasmodesmata with an electron-dense dome) which have not been described before. This new structural observation indicates that these cells participate in substance transport and that this process can probably be additionally regulated.