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.

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.

Until now, development of the female gametophyte has been investigated only in some species of Crassulaceae using a light microscope. To the best of our knowledge, this is the first report that describes the process of megasporogenesis and megagametogenesis in Crassulaceae in detail. To achieve this, we performed embryological studies on Sedum hispanicum L. (Crassulaceae). Cytochemical analysis detected the presence of proteins, lipids, and insoluble polysaccharides in individual cells of the gametophyte. The development of the embryo sac conforms to the monosporic or Polygonum-type in anatropous, crassinucellate, and bitegmic ovules. One megaspore mother cell initiates the process of megasporogenesis. Prior to the first meiotic division, the nucleus is centrally located within the meiocyte. Other organelles seem to be distributed evenly over the micropylar and chalazal parts during the development. Most storage reserves detected during megasporogenesis were observed in the megaspore mother cell. Three mitotic divisions within the chalazal functional megaspore resulted in the enlargement of the eight-nucleated embryo sac. In the seven-celled gametophyte, three chalazally located antipodes degenerated. A mature embryo sac was formed by the egg apparatus and central cell. When the antipodes degenerated, both synergids became organelle-rich and more active. The concentration of lipid droplets, starch grains, and proteins increased during megagametogenesis in the growing gametophyte. In the cellular embryo sac, the central cell can be distinguished by its largest accumulation. Our data confirm the hypothesis that plasmodesmata with electron-dense dome are formed during development of the female gametophyte in S. hispanicum and not just during the stages of embryogenesis. We observed these structures in megaspores and coenocytic embryo sac walls. Functions of observed plasmodesmata are discussed.

No abstract

Crepidium is a large genus of mainly pantropical orchids. The lips of its flowers are upwardly directed and do not serve as landing platforms for pollinators. This role is assumed by the dorsal sepal and/or gynostemium. Information about the pollination and floral morphology of this genus is scarce. To date, no papers have been published on these topics. Field observations have revealed that the flowers are visited by small flies, midges, fruit flies, other small dipterans, ants, spiders, and mites. Preliminary observations revealed at least two forms of small liquid droplets secreted on the lip surface of Crepidium species: simple secretions from epidermal cells, and cell sap released upon the rupturing of raphide-producing cells. Further research revealed that this was the first time liquid secretion was recorded in this genus. Floral secretions were subjected to sequential organic solvent extraction and gas chromatography–mass spectrometry (GC–MS). Floral parts were investigated by means of scanning (SEM) and transmission electron microscopy (TEM), and histochemical tests. The presence of liquid droplets on the lip of Crepidium, the presence of a food reward, and the sequence of raphide development are reported here for the first time.