Suspensor Development in Gagea Lutea (L.) Ker Gawl., with Emphasis on the Cytoskeleton

The study used fluorescence microscopy to examine changes in cytoskeleton configuration during development of the embryo suspensor in Gagea lutea and to describe them in tandem with the development of the embryo proper. During the early phase of embryo suspensor development, tubulin and actin filaments were observed in the cytoplasm of the basal cell from the micropylar to the chalazal ends of the cell. Around the nucleus of the basal cell were clusters of numerous microtubules. These accumulations of tubulin arrays congregated near the nucleus surface; numerous bundles of microtubules radiated from the nucleus envelope. At this time, microfil-aments formed a delicate network in the cytoplasm of the basal cell. In the fully differentiated embryo suspensor, microtubules were observed at the chalazal end of the basal cell. Numerous bundles of microtubules were visualized in the cytoplasm adjacent to the wall separating the basal cell from the embryo proper. Microfilaments formed a dense network which uniformly filled the basal cell cytoplasm. There were some foci of F-actin material in the vicinity of the nucleus surface and at the chalazal end of the basal cell. In all studied phases of embryo suspensor development a prominent cortical network of actin and tubulin skeleton was observed in embryo proper cells.

Alisma plantago-aquatica L.: the cytoskeleton of the suspensor development

The actin and the tubulin cytoskeleton organization during the differentiation of the embryo-suspensor in <em>Alisma plantago-aquatica</em> was studied in comparison with the development of embryo, using immunofluorescence detection and rhodamine-phalloidin assay. At the early stage of the suspensor basal cell development (from 2- to ~10-celled embryos) microfilaments form an abundant network in the cytoplasm of the basal cell, while the microtubules form a delicate network. At the mature stage of development (from a dozen to several dozen-celled embryos), in the suspensor basal cell, the microfilaments and microtubules were localized from micropylar to chalazal pole of the cell. At the micropylar end of the basal cell a high amount of actin and tubulin material was observed. The microfilaments were mainly arranged parallel whereas numerous bundles of microtubules distributed longitudinally or transversally to the long axis of the cell. At this stage of basal cell functioning, some bundles of microtubules appeared to pass close to the nucleus surface. Microtubules were also observed distributed at the chalazal pole of the basal cell. At the senescence stage of the suspensor basal cell (&gt;100-celled embryos) the actin and tubulin filaments disorganize, some disrupted microfilaments and microtubules were observed in the cytoplasm of the basal cell. At all stages of the suspensor basal cell development in the embryo cells an extensive actin and tubulin network was observed.

Two waves of programmed cell death occur during formation and development of somatic embryos in the gymnosperm, Norway spruce

In the animal life cycle, the earliest manifestations of programmed cell death (PCD) can already be seen during embryogenesis. The aim of this work was to determine if PCD is also involved in the elimination of certain cells during plant embryogenesis. We used a model system of Norway spruce somatic embryogenesis, which represents a multistep developmental pathway with two broad phases. The first phase is represented by proliferating proembryogenic masses (PEMs). The second phase encompasses development of somatic embryos, which arise from PEMs and proceed through the same sequence of stages as described for their zygotic counterparts. Here we demonstrate two successive waves of PCD, which are implicated in the transition from PEMs to somatic embryos and in correct embryonic pattern formation, respectively. The first wave of PCD is responsible for the degradation of PEMs when they give rise to somatic embryos. We show that PCD in PEM cells and embryo formation are closely interlinked processes, both stimulated upon withdrawal or partial depletion of auxins and cytokinins. The second wave of PCD eliminates terminally differentiated embryo-suspensor cells during early embryogeny. During the dismantling phase of PCD, PEM and embryo-suspensor cells exhibit progressive autolysis, resulting in the formation of a large central vacuole. Autolytic degradation of the cytoplasm is accompanied by lobing and budding-like segmentation of the nucleus. Nuclear DNA undergoes fragmentation into both large fragments of about 50 kb and multiples of approximately 180 bp. The tonoplast rupture is delayed until lysis of the cytoplasm and organelles, including the nucleus, is almost complete. The protoplasm then disappears, leaving a cellular corpse represented by only the cell wall. This pathway of cell dismantling suggests overlapping of apoptotic and autophagic types of PCD during somatic embryogenesis in Norway spruce.