Activation of rape (Brassica napus L.) embryo during seed germination. IV. Germinating embryo. The first zones of mitoses, starch and DNA synthesis and their expansion pattern

The rape radicle is completely covered by the root cap. The successive lateral cell layers of the root cap are terminated by T-forming walls in the dermatogen layer in a statistically constant position. T-walls in dermatogen were utilized for delimitation of successive root sectors on longitudinal microtome sections at the succeeding germination stages. The length and the cell number of the corresponding sectors were studied and the starch and DNA synthesis sites as well as cell divisions localized. All these processes are initiated in a constant sequence and in specific embryo zones. The first symptom of activation of the embryo is starch synthesis. It begins in two centres: in the apical part of the radicle columella and in the hypoctyl dermalogen it moves deep into the cortex and in both directions along the columella the activation of starch synthesis shifts basipetally into the whole columella and the initial centre as well as the lateral parts of the root cap. From the hypocotyl dermatogen it moves into the cortex and in both directions along the embryo axis. In the root dermatogen and periblem the activation zone is first located in the basal sector and, then, gradually, in the lower ones. Just before germination the basal and apical zones of activation meet. Starch can then, be found throughout the root. During starch synthesis, in the basal part of the radicle. DNA synthesis and cell growth begin. In the root sectors which have already begun to grow and synthesize DNA, cell divisions start. The boundary of the dividing cells zone shifts acropetally at some distance above the lower boundary of DNA synthesis and the zone of cell premitotic growth. The acropetal shift of the mitotic activation zones can be described as wave expansion. Before the first mitotic wave reaches the promeristem it is followed by at least three acropetal waves, arising in the already activated basal sectors. and then the mitoses are asynchronised. Mitotic.: activation of the root cap is partly independent of the acropetal wave of cell activation and results from the expanding weaker basipetal wave

Ultrastructural changes during megasporogenesis in Epipactis (Orchidaceae)

Plastids were temporarily localized within the micropylar portion of the early first prophase Epipactis meiocyte. Some of these plastids were observed in close proximity to the nuclear envelope. With the exception of this short period plastids were distributed randomly in the meiocyte. During late prophase, starch-containing plastids become cup-shaped and depleted of starch. Plastids were found within both dyad cells and all cells of the tetrad. Elongated segments of ER cisternae in various configurations were present. The chalazal wall of the prophase meiocyte differed from other walls in the presence of the ingrowths and plasmodesmata. The micropylar portion of the nuclear envelope at some stages of the I prophase seemed to be devoid of pores whereas the chalazal part contained numerous pores. These structural characters reflect a polar differentiation of the meiocyte along a micropylar-chalazal axis

Cytological changes in meristematic cells of Allium cepa L. root tip treated with extracts from callus of Catharanthus roseus (L.) G. Don

The effect of an ethanolic extract from callus of Catharanthus roseus on Allium cepa root cells divisions was investigated. Two lines: white and green callus, were established on solid B5 medium with IAA 1 mg/l and kinetin 0.1 mg/l. The HPLC analysis of callus extracts showed the presence of indole alkaloids, however not known pharmacologically active alkaloids or derivatives used in semisynthesis (vinblastine, ajmalicine, serpentine, yohimbine, vindoline and catharanthine) have been found. The ethanolic extract of C. roseus callus inhibited the number of mitoses in Allium cepa root tip cells. Short (1-3 hours) treatment resulted in an increase in the index of late prophases, with characteristic light spaces, and the index of metaphases with twisted chromosomes forming an equatorial plate or irregular structures of c-metaphases. At the same time, the percentage of anaphases and telophases decreased significantly. Longer treatment of the root tip inhibited mitotic activity, stopping it completely already after 12 hours. Interphase nuclei became at first denser and homogeneous, eventually their structure became , partitioned into zones and formation of chromatin territories, with distinct large nucleoli has been observed. Electron microscope observations revealed well developed rough endoplasmic reticulum and thick, invaginating cell membrane

Influence of hydroxyurea on cell divisions and microtubular cytoskeleton in Allium cepa root meristem

In onion roots, hydroxyurea (HU) causes a gradual depression of mitotic activity which ceases after 24-36 hrs. The effect is reversible; divisions begin after several hours of recovery and after 12-14 hrs about 90% cells undergo mitosis. Mitotic activity commences in the distal region of the apical meristem, and as a wave it spreads towards the apex. In the roots treated with HU for a short time, the tubulin immunofluorescence method reveals normal arrays of microtubules (MTs). After 36 hrs of HU treatment there are only cortical and endocytoplasmatic MTs. In the recovering roots, preprophase bands (PB) mitotic spindles and phragmoplasts appear. Some PBs are split into two parallel rings. These abnormal PBs mostly occur in elongated cells. Apart from this, HU does not appear to have any significant influence on microtubular organization