Ephemeral and non-ephemeral structures during seed development in two Cytisus species (Papilionoideae,Leguminosae)

Abstract

Seed formation involves not only the embryo and endosperm development, but also the formation of a series of either ephemeral or non-ephemeral structures. In this article, we study several of those structures in Cytisus multiflorus and Cytisus striatus. The endosperm development is first nuclear and later cellular, except for the chalazal area, whose development is always nuclear. It generates, in the early developmental stages, a sac-like haustorium. As the seed develops, two structures seem to be closely related to nutrient mobilization to the embryo sac: on the one hand, a group of cells and a channel, located in the chalazal area and closely related between them and to the endosperm haustorium, which could be interpreted as a hypostase and on the other hand, an endothelium, derived from the inner integument, which later degenerates leaving no trace in the mature seed. All of these structures would be associated with the directionality of assimilates from ovule tissues to embryo sac. In mature seed and surrounding the embryo appears a unicellular layer of cells rich in proteins (aleurone layer), which is the origin of the outermost layer of the cellular endosperm. The seed coat is made up only of the outer integument.     

<Emphasis Type="Italic">Gypsy embryo</Emphasis> specifies ovule curvature by regulating ovule/integument development in rice

The embryo position in a seed is stable in most plant species, indicating the existence of a strict regulatory mechanism that specifies the embryo position in the seed. To elucidate this mechanism, we analyzed the gypsy embryo (gym) mutant of rice, in which the position of the mature embryo in the seed is altered at a low frequency. Analyses of early embryogenesis and ovule development showed that the ectopic embryo was derived from an ill-positioned egg cell, which resulted from the incomplete curvature of the ovule. Although the development of both the inner and outer integuments was impaired, the ovule curvature was associated closely with the extent of inner integument growth. Therefore, inner integument development controls ovule curvature in rice. The expression patterns of OSH1 and OsMADS13 indicated that, in gym, a small number of indeterminate cells are maintained on the style side of the ovule and then in the integument primordium at a low frequency. The prolonged survival of these indeterminate cells disturbs normal integument development. The gym fon2 double mutant suggests that GYM and FON2 are involved redundantly in floral meristem determinacy. Possible functions of the GYM gene and the ovule developmental mechanism are discussed.     

Embryogenesis and seed development in Sinomanglietia glauca (Magnoliaceae)

The development of the floral bud, especially the ovule and seed coat, of Sinomanglietia glauca was observed. Floral buds were covered by eight to nine hypsophyll pieces. The hypsophyll nearest the tepal was closed completely and characterized by two arrays of densely stained cells with dense cytoplasm, which split longitudinally at flowering. The perianth consisted of 16 tepals arranged in three whorls. The gynoecium was composed of numerous apocarpous carpels; the ovule was anatropous with two integuments. Embryogenesis was of the Polygonum type, and the endosperm was nuclear. The inner integument degenerated during seed development. The seed of S. glauca had an endotestal seed coat comprised of a sclerotic layer derived from the inner adaxial epidermis of the outer integument and a sarcotesta derived mainly from the middle cells between the inner and outer epidermis of the outer integument. The embryo developed normally, so embryogenesis is not the cause of difficult regeneration.     

Embryology of the dioecious Woonyoungia septentrionalis (Magnoliaceae)

The embryological characteristics and ovular integument development of the dioecious species Woonyoungia septentrionalis (Dandy) Law (Magnoliaceae), which are poorly understood, were investigated under laser scanning confocal microscope (LSCM) and light microscope (LM). The embryological characteristics conform to most of the previously studied species in Magnoliaceae. The anther has 4 microsporangia, and the anther wall develops according to the dicotyledonous type. Cytokinesis at meiosis of the microspore mother cells follows a modified simultaneous type, giving rise to isobilateral or decussate tetrads, and a cell plate is absent, but a membrane was observed. Mature pollen grains are 2‐cellular and have high germination rates. The ovule is anatropous, crassinucellate and bitegmic, and meiotic result in linear tetrads of megaspores, the one at the chalazal end functions directly as an embryo‐sac cell. The development of the embryo sac is of the Polygonum‐type and endosperm formation is of the nuclear type. The outer integument of the ovule differentiates into an outer fleshy and an inner stony layer while the inner integument is reduced to a tanniniferous layer. The normal embryological development, high germination rates of pollen and high seed set indicate that the primary reason for the decline of the species is not to be found in these developmental processes.     

Morphology and development of anthers and ovules in Croton and Astraea (Euphorbiaceae)

We examined the embryological development of anthers and ovules from Astraea (A. lobata and A. praetervisa) and Croton (C. floribundus, C. fuscescens, C. glandulosus, C. lundianus, C. piptocalyx, C. urucurana and C. triqueter) focusing on features with systematic significance for the group. Some of these features are common in Euphorbiaceae including: a dicotyledonous type of anther wall formation, a secretory tapetum, a mixed origin of the outer ovule integument, an epidermal origin of the inner ovule integument, the occurrence of many archesporial cells inside the ovules and a megagametophyte of the Polygonum‐type. Other features, such as the presence of styloid crystals in the tapetum, an idioblast with a druse in the endothecium, simultaneous and successive microsporogenesis, and a functional micropylar megaspore, have not previously been reported in the family. These characters appear to distinguish Croton and Astraea from other Euphorbiaceae and possibly represents autapomorphies for the tribe Crotoneae.     

<Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> outer ovule integument morphogenesis: Ectopic expression of <Emphasis Type="Italic">KNAT1</Emphasis>reveals a compensation mechanism

Background  

The Arabidopsis outer ovule integument is a simple two-cell layered structure that grows around the developing embryo and develops into the outer layer of the seed coat. As one of the functions of the seed coat is the protection of the plant embryo, the outer ovule integument is an example for a plant organ whose morphogenesis has to be precisely regulated.     

The embryology ofStegnospermataceae,with a discussion on its status,affinities and systematic position

The embryology ofStegnosperma halimifolium andS. watsonii has been studied in detail. The tapetum is of the secretory type and its cells become multinucleate. Simultaneous cytokinesis in the pollen mother cells follows meiosis. The ripe pollen grains are 3-celled. The ovule is crassinucellate, bitegmic and amphitropous, with the micropyle formed by the inner integument alone. The female archesporium is one celled, and the parietal tissue 3–5 layered. The embryo sac development conforms to thePolygonum type. A central strand, 6 or 7 cells thick, differentiates inside the nucellus and extends from the base of the embryo sac to the chalazal region. The endosperm is nuclear. The embryogeny conforms to the Caryophyllad type. The seed coat is formed by the outer epidermis of the outer integument and the inner epidermis of the inner integument. Based on this evidence and other data, the status of the genus as an independent family,Stegnospermataceae (Stegnospermaceae) is confirmed. Apparently, it forms a connecting link betweenPhytolaccaceae andCaryophyllaceae.     

SOME HISTOCHEMICAL,ULTRASTRUCTURAL, AND NUTRITIONAL ASPECTS OF THE OVULE OF QUERCUS GAMBELII

Histochemical analyses of the ovule of Quercus gambelii show that the major food reserves (starch grains and lipids) are located almost exclusively within the outer integument. Vascular traces are present only within this integument which contains numerous, well-developed plasmodesmata. The inner integument is virtually devoid of any food reserves and has very few plasmodesmata. The ovule has a persistent chalazal extension of residual nucellar cells (called the postament) which projects into the embryo sac. Due to the above information and the fact that the synergids rarely contain starch and no plasmodesmata are present in the walls of any of the cells of the egg apparatus (Mogensen, 1972), it is concluded that the synergids play little or no role in embryo sac nutrition. Rather, it is proposed that the pathway of available food materials in the young ovule is from the outer integument to the chalaza and through the postament into the embryo sac.     

Embryology ofCrocus thomasii (Iridaceae)

The embryology ofCrocus thomasii is described. Male meiosis is of simultaneous type, and gives rise to starchy microspores which develop into lipoid pollen grains; these are two-celled and show a spinulate acolpate, abaculate exine lacking apertures. The tapetum is glandular and its cells become bi- or sometimes multinucleate. The ovule is anatropous and bitegmic; the inner integument forms the micropyle. Megasporogenesis is heteropolar with starch accumulation in the functional chalazal megaspore. Embryo sac development conforms to thePolygonum type. The endosperm development is nuclear. The embryo develops according to the Caryophyllad type. In the ripe seed it is differentiated and enveloped by a starchy cellular endosperm. The embryological characters observed strongly favour a close relation betweenC. thomasii andC. sativus.     

Ovular development and morphology in some magnoliaceae species

Floral phenology and ovular development ofLiriodendron tulipifera are described. The ovule primordia are initiated in December, followed by prominent development in March, and the ovules are mature in May. The inner integument is formed as an annular rim on the incurving ovule primordia, but the outer integument develops as a semi-annular rim interrupted on the concave side of the funicle. Later, an outgrowth, which is interpreted here as an obturator, arises on the concave side of the funicle. The funicular outgrowth arises far from the inner integument, while the outer integument is close to the inner. The outer integument and the funicular outgrowth together form an envelope complex. Later the outer integument produces two distal lobes, which disappear at maturity. Mature ovules of the threeMagnolia species examined have similar lobes. It is suggested that the hood-shaped outer integument is primitive in angiosperms.     

Resolving the systematic and phylogenetic position of isolated ovules: a case study on a new genus from the Permian of China

Isolated ovules occur in many fossil plant assemblages, where they provide important insights into seed‐plant diversity and evolution. However, in many cases, the ovules cannot be attributed to individual groups of seed plants, restricting systematic and evolutionary assessments that can be made from otherwise well‐characterized fossil taxa. In the present paper, we describe a new kind of ovule discovered in tuffaceous sediments from the Permian‐aged Xuanwei Formation in Guizhou Province, China. This ovule has 180° rotational symmetry and an integument comprising a variably thick sarcotesta, a uniformly thick sclerotesta and a uniformly thin endotesta. The nucellus is attached to the integument at least basally and contains a collapsed seed megaspore; a nucellar apex is absent. Both the integument and nucellus are vascularized by paired bundles in the major plane of the ovule; the integumentary bundles are considerably larger than the nucellar bundles and the nucellar bundles emerge from a conical vascular pad. Generation of a three‐dimensional reconstruction based on serial peels revealed the gross morphology and organization of the ovule and highlighted the presence of features consistent with cardiocarpalean‐type ovules (ovule shape, histological features of the integument) and also features more typical of lagenostomalean‐ and trigonocarpalean‐type ovules (large integumentary bundles, presence of nucellar bundles). To assess the affinity and evolutionary significance of the ovule, it has been included in a cladistic matrix of cardiocarpalean‐, lagenostomalean‐ and trigonocarpalean‐type ovules. Results place the ovule within the cardiocarpalean group of ovules known to have been produced by several plant groups, including cordaitean coniferophytes, pteridosperms and Palaeozoic conifers. The cladistic topology supports generic level distinction of the present species, requiring the establishment of Muricosperma guizhouensis Seyfullah & J.Hilton gen. & sp. nov . Lagenostomalean ovules produced by hydrasperman pteridosperms form a basal paraphyletic grade, whereas trigonocarpalean ovules produced by medullosan pteridosperms form a monophyletic group in which Stephanospermum is paraphyletic with respect to Rhynchosperma and Pachytesta. The results also place the Mississippian ovule Mitrospermum bulbosum apart from all of the Pennsylvanian species of Mitrospermum that form a strongly supported clade. Consequently, M. bulbosum is transferred to the new genus Whitaddera Seyfullah & J.Hilton as W. bulbosa (Long) Seyfullah & J.Hilton. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 164 , 84–108.     

The origin of the ovule

Of several theories for the origin of the ovule advanced in this century and based largely on fossil evidence, the telomic concept is widely considered the most plausible. Its principal tenet is the evolution of the integument through fusion of sterile branches or telomes around a terminal megasporangium. The only point of agreement in these theories is that the entire nucellus is a megasporangium that retains a single megaspore and the endosporic female gametophyte. Their differences center on the origin of the integument. A new concept offered here on evidence from ovules of both Paleozoic and extant seed plants significantly alters the telomic theory. It proposes that the nucellus is a sporangiophore of stem origin that bears a terminal megasporangium; that at least some of the fused integumentary telomes were fertile; and that among all features cited to characterize ovules, the unique nature of the retained megaspore alone defines the seed habit. Changes in the seed plant megaspore that extended the period of nutrient absorption over the whole course of female gametophyte development, along with complex physiological changes in the nucellus, were probably achieved along a single phylogenetic line beginning in a Late Devonian population of progymnosperms. For such a combination of events to have occurred more than once is highly unlikely, and, therefore, a monophyletic origin for seed plants is proposed. Several primitive features in ovule structure, some not evidenced since the Lower Carboniferous Period, occur in a mutant form of Arabidopsis thaliana isolated from genetically transformed plants. Their recurrence provides additional support for the proposed concept of ovule origin and also suggests that the genetic mechanisms for expression of primitive features in advanced taxa could be initiated in each case by mutation of a single homeotic gene.     

Comparative embryology of threeGentianaceae species from the Central Caucasus and the European Alps

A comparative investigation was carried out on the ovule and seed development of three mountain species ofGentianaceae, the perennial speciesGentiana pyrenaica, and the two short-lived monocarpic speciesGentianella caucasea andG. germanica. In all three species most embryological characters conform to those generally found in the family of theGentianaceae. In some features, however,G. pyrenaica and the twoGentianella species differ from each other. InG. pyrenaica the ovule is anatropous, the integument 8–10 layered and the three reduced antipodals degenerate soon after fertilization. In contrast,G. caucasea andG. germanica form a hemitropous ovule, a 4–5 layered integument and up to 16 antipodal cells by secondary multiplication. All three species exhibit differences in synchronization between embryogenesis and endosperm development. Functional relations between the antipodal structure and the dynamics of seed development of the investigated species are postulated.     

依兰(番荔枝科)雌配子体发育研究

荔枝科是被子植物的基部类群之一,依兰属是番荔枝科较为原始的类群,其有性生殖过程,特别是胚胎发生与发育的研究结果,可以补充被子植物胚胎学原始特征的相关基础资料。该研究利用常规石蜡切片技术,对依兰胚珠、大孢子和雌配子体的发生发育过程进行了观察。结果表明:依兰的胚珠为倒生胚珠、厚珠心、三层珠被,第三层珠被(中间珠被)发生在大孢子母细胞时期,于外珠被与内珠被之间、胚珠合点端两侧发生并隆起;雌配子体为蓼型。此外,依兰的个别胚珠中存在双雌配子体现象,且两个雌配子体均由大孢子母细胞发育而来,大小、形状相近,呈线形排列。该研究结果对于揭示原始被子植物胚胎发育特征具有重要意义。     

多花地宝兰胚囊和胚发育的细胞学研究

为探讨多花地宝兰(Geodorum recurvum)胚胎发育的系统分类学意义,采用石蜡制片法对多花地宝兰胚囊和胚的发育进行解剖学观察。结果表明,在开花前,多花地宝兰胚珠原基发育缓慢,开花授粉后胚珠原基快速发育成"树状二杈分枝结构",随后在"分枝结构"末端形成孢原细胞,开始胚囊发育。多花地宝兰的胚囊发育属于单孢蓼型胚囊,胚珠具有双层珠被。孢原细胞形成后,经过细胞膨大延长发育形成胚囊母细胞,胚囊母细胞经过减数分裂形成线性四分体,在珠孔端形成1个功能大孢子,功能大孢子经过3次有丝分裂形成8核胚囊。多花地宝兰的胚发育具有藜型和紫苑型两种方式。双受精完成后,多花地宝兰合子进行一次橫裂后形成基细胞和顶细胞;基细胞经过多次分裂形成细胞团,细胞团中的细胞向不同方向膨大延长形成多个胚柄细胞;顶细胞有两种分裂方式,一种是横裂形成藜型胚,一种是纵裂形成紫苑型胚。因此,推测多花地宝兰在兰科植物系统分类学上属于较为原始种。     

小叶兜兰的果实生长和雌配子体发育

为了解濒危兰科植物小叶兜兰(Paphiopedilum barbigerum Tang et Wang)胚珠和雌配子体的发育过程,采用常规石蜡切片技术对其果实的生长动态进行了研究。结果表明,授粉后60~75 d的蒴果内种子数量迅速增加,到授粉后120 d时种子充满整个蒴果。授粉后40 d的胎座上分化形成多数由1层表皮细胞包被1列细胞的胚珠原基;授粉后60 d时位于胎座指状结构末端处紧靠表皮细胞下方的孢原细胞分化为大孢子母细胞。之后,大孢子母细胞经过减数分裂和有丝分裂最终形成成熟胚囊;授粉后135 d胚囊发育成熟,附着在胎座上的种子个体分化明显。小叶兜兰胚囊的发育类型为双孢子葱型,胚珠为倒生胚珠,薄珠心,单珠被,成熟胚囊为8核。这为小叶兜兰的生殖生物学及繁殖体系的建立提供理论依据。     

Embryology ofMalaxis saprophyta,with comments on the systematic position ofMalaxis (Orchidaceae)

InMalaxis saprophyta, anther wall development corresponds to the Monocotyledonous type. The uninucleate tapetum is of secretory type and the endothecium develops U- and V-shaped thickenings on the inner tangential and radial walls. Cytokinesis is simultaneous; tetrahedral, isobilateral and T-shaped tetrads are formed which are compactly aggregated in pollinia. At anthesis the microspore tetrads are 2-celled. The ovule is anatropous, bitegmic and both integuments are dermal in origin. A single hypodermal cell develops directly into a megaspore mother cell. Embryo sac development is predominantly monosporic and less often bisporic. Irrespective of the type of development, the mature embryo sac is 6-nucleate. Although double fertilization occurs, the primary endosperm nucleus degenerates. Embryogeny is of the Onagrad type. The mature embryo lacks differentiation into cotyledon, plumule and radicle. The reticulate seed coat is formed entirely by the outer layer of outer integument. There are three sterile and three fertile valves in the ovary. Although initially parenchymatous, the entire three sterile valves in the ovary and the upper half of the three fertile valves become sclerified after fertilization. The embryological characters support the disputed systematic position ofMalaxis within subtribeMalaxidinae ofEpidendreae.     

Embryological observations on Helichrysum rupestre (Rafin.) DC. var. errerae (Tineo) Pignatti

Abstract

The ovule and anther development of Helichrysum rupestre var. errerae from Pantelleria is studied. The ovule is anatropous, tenuinucellate, unitegmic, endotheliate and is derived from a two-zonate primordium. The archespore is one- or two-celled and all the megaspores show a functional tendency, although only the chalazal one is able to form the gametophyte according to the Polygonum type. The polar nuclei fuse before fertilization and the three antipodal cells divide further. Anthers are tetrasporangiate and their wall development conforms to the Dicotyledonous type. The epidermis persists at maturity and the endothecial cells thicken their walls. The tapetum is of the periplasmodial type and its cells become plurinucleate during microsporogenesis. The number of microsporocytes varies along the length of the same locule. Arrangement of the resulting microspores is tetrahedral, isobilateral or decussate. Pollen grains are three-celled when shed. Degenerating microsporocytes and microspores have frequently been found.