The formation of integuments, megasporogenesis and megagametogenesis in Dendrobium catenatum, with special discussions on embryo sac types and section techniques

The formation of integuments, megasporogenesis and megagametogenesis in Dendrobium catenatum, an economically important orchid, are observed. After pollination, mitotic cell divisions of the placental epidermis result in the formation of a branching system of outgrowths. The tip of each branch consists of an archesporial cell derived from the differentiation of the terminal subepidermal nucellar cell. It differentiates directly into a megasporocyte. The first division of the meiosis of the megasporocyte produces a dyad approximately equal in size, in which the micropylar cell promptly degenerates. The second meiotic division of the remaining dyad cell results in the formation of two megaspores of unequal size. The larger chalazal cell becomes functional and eventually develops into a mature megagametophyte. The development of the megagametophyte conforms to the Monosporic Polygonum type. The final arrangement of the mature embryo sac conforms to a sevencelled/ eight-nucleate structure. The mature ovule is bitegmic, tenuinucellate and has an anatropous orientation. In the present study, we also discuss the differences between three main types of embryo sac development and the improvement of section techniques.

Spatiotemporal restriction of FUSCA3 expression by class I BPC promotes ovule development and coordinates embryo and endosperm growth

ABSTRACTSpatiotemporal regulation of gene expression plays an important role in developmental timing in plants and animals. FUSCA3 regulates the transition between different phases of development by acting as a link between different hormonal pathways in Arabidopsis. However, the mechanisms governing its spatiotemporal expression patterns are poorly understood. Here, we show that FUS3 is expressed in the chalaza and funiculus of the mature ovule and seed, but is repressed in the embryo sac, integuments and endosperm. FUS3 repression requires class I BASIC PENTACYSTEINE (BPC) proteins, which directly bind to the FUS3 locus and restrict its expression pattern. During vegetative and reproductive development, derepression of FUS3 in bpc1/2 or pML1:FUS3 misexpression lines results in dwarf plants carrying defective flowers and aborted ovules. Post-fertilization, ectopic FUS3 expression in the endosperm increases endosperm nuclei proliferation and seed size and delays or arrests embryo development. These phenotypes are rescued in bpc1/2 fus3-3. Lastly, class I BPCs interact with FIS-PRC2 (FERTILIZATION-INDEPENDENT SEED-Polycomb Repressive Complex 2), which represses FUS3 in the endosperm. We propose that BPC1/2 promotes the transition from reproductive to seed development by repressing FUS3 in ovule integuments. After fertilization, BPC1/2 and FIS-PRC2 repress FUS3 in the endosperm to coordinate endosperm and embryo growth.

On the Embryology of Two Species of Genus Lepidium (Brassicaceae)

Some species of genus Lepidium of the family Brassicaceae are ruderal plants, and they can grow well on less fertile soils and may have potential as oilseed crops for marginal lands. To develop cultivation techniques for wild species, the reproductive capacity of the species needs to be revealed. The objective of this work was embryological study of two Lepidium species (L. campestre and L. ruderale). As a result of the study, the main features of male and female generative spheres were established. Male generative sphere: The anther is tetrasporangiate and its wall, the development of which follows the monocotyledonous-type, consists of epidermis, endothecium, one middle layer, and glandular tapetum. Predominantly, tetrahedral microspore tetrads form after simultaneous type of microsporogenesis. The mature pollen grains are two-celled. Female generative sphere: The mature ovule is ana-amphytropous, crassinucellate, and bitegmic with unicellular archesporium that functions as a megaspore mother cell without cutting off of parietal cells. The development of the embryo sac follows the polygonum-type development. The embryo and endosperm develop after the onagrad-type embryogenesis. The established peculiarities of the reproductive biology characterize the studied species as sexually reproducing taxa that guarantee the stability of size of their populations. This is important for the conservation of these species as part of the Bulgarian flora biodiversity given their status of valuable medicinal plants. The data obtained will contribute to the knowledge of the embryological characteristic of genus Lepidium. The results contribute to the understanding of Lepidium biology and potential development of Lepidium species as oilseed cash crops for marginal lands.

Ovule Structure of Scotch thistle Onopordum acanthium L. (Cynareae, Asteraceae)

Studies concerning the ultrastructure of the periendothelial zone integumentary cells of Asteraceae species are scarce. The aim was to check whether and/or what kinds of integument modifications occur inOnopordum acanthium. Ovule structure was investigated using light microscopy, scanning electron microscopy, transmission electron microscopy and histochemistry. For visualization of calcium oxalate crystals, the polarizing microscopy was used. The periendothelial zone of integument inO. acanthiumis well developed and composed of mucilage cells near the integumentary tapetum and large, highly vacuolated cells at the chalaza and therefore they differ from other integumentary cells. The cells of this zone lack starch and protein bodies. Periendothelial zone cells do not have calcium oxalate crystals, in contrast to other integument cells. The disintegration of periendothelial zone cells was observed in a mature ovule. The general ovule structure ofO. acanthiumis similar to other members of the subfamily Carduoideae, although it is different to “Taraxacum”, “Galinsoga” and “Ratibida” ovule types.

Floral organogenesis in Urophysa rockii, a rediscovered endangered and rare species of Ranunculaceae

Urophysa is an Asian endemic genus in the Ranunculaceae, but data on floral organogenesis, which would be a useful complement to molecular data in clarifying the relationship with closely related taxa (Aquilegia and Semiaquilegia) in Ranunculaceae, are completely lacking. We used scanning electron microscopy and light microscopy to study the floral development of Urophysa rockii Ulbrich, a recently rediscovered species in this genus. The sepals are initiated spirally, whereas other organs are nonsimultaneously whorled; the floral phyllotaxis is whorled. Primordia of the sepals are lunular and truncate, but those of the petals and stamens are hemispherical and rounded. After sepal initiation, there is a delay in development, but the initiation of petals and stamens is continuous. The developmental sequence of the microspores in the stamens is centrifugal, although the stamens are initiated centripetally. The early developmental stages of the staminodes are similar to those of the stamens, although much smaller, so they may be phylogenetically homologous organs. The carpel primordia are lunular in shape and plicate. The mature ovule is anatropous and bitegmic. Urophysa shows similar floral development features as Aquilegia and Semiaquilegia, although with some differences, which supports the relationship inferred by DNA sequence data.

Embryology of the dioecious Australian endemic Lomandra longifolia (Lomandraceae)

Microsporogenesis, embryogeny and endosperm development of Lomandra longifolia Labill. are described in detail. The formation of the anther wall is the basic type composed of four cell layers, namely an epidermis, an endothecium, one middle layer and a tapetum. The tapetum layer has glandular, uninucleate cells. Successive cytokinesis follows meiosis, subsequently forming a tetrahedral tetrad of microspores. The ovule in each carpel is hemitropous, crassinucellate and bitegmic, with the micropyle formed by the inner integument. The archesporial cell divides periclinally to form the primary parietal and primary sporogenous cells. The sporogenous cell functions as the megaspore mother cell, whereas the parietal cell divides to give rise to two parietal layers. The mature megagametophyte, which has enlarged synergids and antipodals, is of the Polygonum type, with the normal complement of seven cells and eight nuclei. Nucellar tissue in the mature ovule consists of enlarged dermal cells and irregular subdermal cells surrounding a central strand of markedly smaller cells. Endosperm development is of the nuclear type. Embryo development is of the Graminad type, characterised by oblique zygotic and early pro-embryonic divisions.

Dissection of sexual organ ontogenesis: a genetic analysis of ovule development in Arabidopsis thaliana

Understanding organogenesis remains a major challenge in biology. Specification, initiation, pattern formation and cellular morphogenesis, have to be integrated to generate the final three-dimensional architecture of a multicellular organ. To tackle this problem we have chosen the ovules of the flowering plant Arabidopsis thaliana as a model system. In a first step towards a functional analysis of ovule development, we performed a large-scale genetic screen and isolated a number of sterile mutants with aberrant ovule development, We provide indirect genetic evidence for the existence of proximal-distal pattern formation in the Arabidopsis ovule primordium. The analysis of the mutants has identified genes that act at an intermediate regulatory level and control initiation of morphogenesis in response to proximal-distal patterning. A second group of genes functions at a subordinate control level and regulates general cellular processes of morphogenesis. A large group of male and female sterile mutants shows defects restricted to early or late gametogenesis. In addition, we propose that the mature ovule obtains its overall curved shape by at least three different processes that act in only one domain of the ovule.

Floral Biology and Embryo Development in Chestnut (Castanea sativa Mill.)

Floral biology of chestnut, from sporogenesis to mature embryo, is described. Microsporogenesis in flowers of unisexual catkins occurred in the first week of June 1991. Anthesis started in mid-June (≈70 days after budbreak) and lasted 2 weeks. In mid-June, in each pistillate flower, six to eight styles began to emerge, and 4 to 7 days later, they were extended fully (i.e., full bloom). In each flower, 10 to 16 anatropous ovules developed from the ovary axis. The megaspore mother cell had formed by the end of bloom. The mature ovule consisted of two integuments and a long, narrow nucellus with a small embryo sac of the Polygonum type. Zygotes were found 15 to 20 days after pistillate flower full bloom. Embryo development followed the Onagrad type, Trifolium variation. Seeds attained full size in mid-September, and fruit were mature in early November. The embryonal axis averaged 4.5 mm long × 2.1 mm wide. An apical meristem and the radicle were evident at opposite ends of the axis.

Development of the strophiole in seeds of white clover (Trifolium repens L.)

The seed coat of Trifolium repens L. was studied with special emphasis on the development of the strophiole, which is the site for water entry during imbibition in leguminous seeds. The epidermal cells of the strophiole are longer than the cells in the remainder of the seed epidermis in the mature ovule. During seed development the median cells of the strophiolar epidermis divide periclinally into an outer layer of palisade cells and an inner layer of isodiametric cells. Prior to maturity a fissure is formed between some of the palisade cells in the centre of the strophiole. It is suggested that tension develops between the palisade cells and the iso-diametrical cells during later maturation stages causing the formation of the fissure which it is believed functions in water uptake. It is indicated that the ‘light line’ is caused by alteration of cellulose microfibrillar orientation in palisade cell walls. It is confirmed that removal of the epicuticular wax from hard seeds by rinsing in absolute alcohol or hexane does not induce water imbibition. Only when seed coats are mechanically abraded do hard seeds germinate.

Ovule Fibers as a Possible Source of Neps and Motes in Pima Cotton

The appearance of green motes in Pima cotton makes what is already a serious problem particularly difficult, since the color is resistant to the normal bleaching process. The source of the colored motes was traced directly to the formation of the ovule fibers and indirectly to ineffective pollination of the cotton plants. Unpollinated ovules produce very fine, long, mature “ovule” fibers bound to the ovule coat, which is easily crushed, thus yielding the “seeds” for the formation of motes and probably neps as well. The formation of the green motes is only one example of ovule motes, and the color of some fibers could be traced to a specific variant of a hybrid. The green ovule motes constitute thus only a particular case of general ovule mote formation. Differentiating between the usual motes and the ovule motes is possible on the basis of the appearance and dimensions of the fibers constituting the motes. Better pollination, as suggested, should thus improve the quality of cotton lint.