Megagametophyte Development Research Articles

Expression Profiling of MSP1 Gene Involved in Mega-gametophyte Development in Ploidy Series of Guinea Grass (Panicum maximum Jacq.)

Hybrid seeds are playing important role in increasing agricultural production. However, process of hybrid seed development involves high cost for most of the major crops. Under such situation apomixis can play crucial role in creation of one-time hybrid and maintain it for multiple generations. Apomixis and polyploidy are interrelated with each other and expression of apomixis completely relies on polyploidy. Guinea grass (Panicum maximum Jacq.) provides ideal system for research on apomixis and polyploidy because of its extensive diversity in morphological, cytological, biochemical, and molecular characteristics. The experiment was carried out in the eight-ploidy series i.e. 3x, 4x, 5x, 6x, 7x, 8x, 9x, 11x and one obligate sexual Guinea grass genotype SRP75. Considering role of MSP1 gene in apomeiosis, its expression was studied in guinea grass at three developmental stages of spikelet viz., pre-meiotic, meiotic and post-meiotic. Findings of this study highlight the complex regulation of MSP-1 expression during different developmental stages and across various ploidy levels in guinea grass. At pre-meiotic stage, except 4x, downregulation pattern of MSP1 was observed in all the ploidy levels. But opposite was observed in the post-meiotic stage.

Arabidopsis ribosomal RNA processing meerling mutants exhibit suspensor-derived polyembryony due to direct reprogramming of the suspensor.

Embryo development in Arabidopsis (Arabidopsis thaliana) starts off with an asymmetric division of the zygote to generate the precursors of the embryo proper and the supporting extraembryonic suspensor. The suspensor degenerates as the development of the embryo proper proceeds beyond the heart stage. Until the globular stage, the suspensor maintains embryonic potential and can form embryos in the absence of the developing embryo proper. We report a mutant called meerling-1 (mrl-1), which shows a high penetrance of suspensor-derived polyembryony due to delayed development of the embryo proper. Eventually, embryos from both apical and suspensor lineages successfully develop into normal plants and complete their life cycle. We identified the causal mutation as a genomic rearrangement altering the promoter of the Arabidopsis U3 SMALL NUCLEOLAR RNA-ASSOCIATED PROTEIN 18 (UTP18) homolog that encodes a nucleolar-localized WD40-repeat protein involved in processing 18S preribosomal RNA. Accordingly, root-specific knockout of UTP18 caused growth arrest and accumulation of unprocessed 18S pre-rRNA. We generated the mrl-2 loss-of-function mutant and observed asynchronous megagametophyte development causing embryo sac abortion. Together, our results indicate that promoter rearrangement decreased UTP18 protein abundance during early stage embryo proper development, triggering suspensor-derived embryogenesis. Our data support the existence of noncell autonomous signaling from the embryo proper to prevent direct reprogramming of the suspensor toward embryonic fate.

Megasporogenesis and megagametogenesis in Hydrocleys nymphoides, Alisma plantago-aquatica, and Sagittaria montevidensis (Alismataceae).

Ovule morphology, megasporogenesis, and megagametogenesis processes were examined in Hydrocleys nymphoides, Alisma plantago-aquatica, and Sagittaria montevidensis. Each of these species belongs to a different clade within the Alismataceae family. It is worth mentioning that the genus Hydrocleys previously belonged to the Limnocharitaceae family but is now classified within the Alismataceae. Flowers in different developmental stages were processed following classical histological methods for their observation with bright-field microscope. The three species present an anatropous and bitegmic mature ovule. This is tenuinucellate in A. plantago-aquatica and S. montevidensis and pseudo-crassinucellate in H. nymphoides. Although all three species have the same type of megasporogenesis, they differ in the megagametogenesis and in the total number of nuclei and cells that form the mature gametophyte. H. nymphoides has a female gametophyte composed of four cells and four nuclei, while A. plantago-aquatica and S. montevidensis have a female gametophyte of five cells and six nuclei. The results are discussed according to the phylogenetic position of each of the species. Moreover, new types of megagametophyte development are described: Hydrocleys and Sagittaria types. The reduction of the female gametophyte with respect to the Polygonum type is found in families belonging to the ANA grade and in other aquatic families within the order Alismatales. We infer that the reduction in the number of cells and nuclei in the female gametophyte is characteristic of species that inhabit aquatic environments. Future studies in aquatic species belonging to other families would be necessary to confirm this hypothesis.

MircroRNA Profiles of Early Rice Inflorescence Revealed a Specific miRNA5506 Regulating Development of Floral Organs and Female Megagametophyte in Rice.

The developmental process of inflorescence and gametophytes is vital for sexual reproduction in rice. Multiple genes and conserved miRNAs have been characterized to regulate the process. The changes of miRNAs expression during the early development of rice inflorescence remain unknown. In this study, the analysis of miRNAs profiles in the early stage of rice inflorescence development identified 671 miRNAs, including 67 known and 44 novel differentially expressed miRNAs (DEMs). Six distinct clusters of miRNAs expression patterns were detected, and Cluster 5 comprised 110 DEMs, including unconserved, rice-specific osa-miR5506. Overexpression of osa-miR5506 caused pleiotropic abnormalities, including over- or under-developed palea, various numbers of floral organs and spikelet indeterminacy. In addition, the defects of ovaries development were frequently characterized by multiple megasporocytes, ovule-free ovary, megasporocyte degenerated and embryo sac degenerated in the transgenic lines. osa-miR5506 targeted REM transcription factor LOC_Os03g11370. Summarily, these results demonstrated that rice-specific osa-miR5506 plays an essential role in the regulation of floral organ number, spikelet determinacy and female gametophyte development in rice.

Development of male and female gametophytes and embryogenesis in the Arabidopsis thaliana

Sporogenesis and gametophytes development in Arabidopsis thaliana (Brassicaceae) was studied. Arabidopsis thaliana was selected for this study as a biogenetic model with a small genome and a short life cycle. Flowers and buds in different developmental stages were removed, fixed in an FAA70, stored in 70 % ethanol, embedded in paraffin and sectioned at 7 − 10 µm with a microtome, stained with Hematoxylin – Eosin and analyzed with a photomicroscope. The results showed the anthers are tetrasporangiate, anther tapetum is of secretory type at the beginning and plasmodial at the end of anther development. Also, microspore tetrads are tetrahedral and rarely tetragonal. In addition, mature pollen grains are spherical, tricolporate, scabrate, and tricellular. The gynoecium is bicarpellate and the ovule is amphitropous, bitegmic, and tenuinucellate. Hypostasis was also observed in basis of nucellus. Endothelium is composed of one cell row and cell divisions of megaspore mother cell results in T-shaped tetrad. The chalazal cell is a functional megaspore that survives and functions in megagametophyte development. The embryo sac development belonged to the Polygonum type and it is curved. The mature embryo sac is composed of 7 cells, one central cell containing polar nuclei, two elongated synergids, a triangular oosphere cell, and three antipodal cells that are degenerated immediately by developing the embryogenesis stage in the mature embryo.

Embryology and fertility of the natural tetraploid Lessingianthus plantaginoides (Asteraceae, Vernonieae): taxonomic implications

Abstract Lessingianthus plantaginoides (Vernonieae, Asteraceae) is a small natural tetraploid shrub that inhabits rocky highlands from South America. The population studied inhabits and covers an extensive region of a private reserve with high local biodiversity and animal and plant endemisms. With the purpose of providing insights into the cyto-embryology of this tetraploid species, the aims of this study were: to perform an ontogenetic study of the male and female gametophytes of L. plantaginoides; to carry out detailed meiotic analysis and evaluate the fertility of this species; to document and provide highlights on taxonomic implications of their reproductive aspects. Lessingianthus plantaginoides presented the following male and female gametophyte traits: dicotyledonous type of anther wall development, tetrahedral tetrads, 3-celled mature pollen grains; development of the chalazal megaspore, monosporic embryo sac and Polygonum type of megagametophyte development. The meiotic behavior was regular, the spores were tetrads of equal size and the pollen grains were highly stainable. Lessingianthus plantaginoides is a highly diplodized autotetraploid that reproduces sexually and has high meiotic regularity; which is apparently responsible for its colonization potential. It now seems certain that polyploid speciation plays a significant role in the establishment and diversification of the genus.

The effect of seasonality on developmental stages of anthetic ovule integuments in Mediterranean orchids.

Orchids, differently from most flowering plants, have under-differentiated ovules at anthesis that require pollination to complete differentiation. This ovule developmental stage has been often observed in tropical species in which the absence of an evident seasonality may allow plants to extend their phenology beneficiating of a long time for post-pollination events. Here, we used scanning electron microscopy (SEM) to detect ovule integument developmental stages in 21 species of Mediterranean Orchidoideae and Epidendroideae and in 11 tropical Epidendroideae with the aim of understanding whether species with a seasonal constraint and shorter time for post-pollination ovule maturation are characterized by different stages of ovule development at anthesis. We found that Mediterranean orchids (both Orchidoideae and Epidendroideae) have more developed ovule integuments than tropical orchids. Most species show partially to fully developed ovules at anthesis with the exception of Cephalanthera where ovules are arrested in a very early developmental stage. Despite the phylogenetic signal, anthetic ovule integument development differs between related species (with different pollination strategies or blooming times), suggesting the presence of some ecological constraints. The synchronization between ovule integuments and megagametophyte development, as found in tropical orchids, is also confirmed in Mediterranean orchids. Our data show that Mediterranean and tropical orchids clearly differ in anthetic ovule developmental stages, likely depending on seasonality.

Megagametophyte development and female sterility in Maytenus obtusifolia Mart. (Celastraceae).

Reproduction in flowering plants is closely related to the megagametophyte, since the megagametophyte is involved in pollen tube reception and contains the two female gametes-egg cell and central cell. Previous conventional light microscopy methods have shown that female sterility in perfect flowers of Maytenus obtusifolia is associated with the occurrence of sterile ovules whose megagametophytes have hypertrophied synergids. Here, using transmission electron microscopy and cytochemical methods, we compare the megagametophytes in fertile and sterile ovules from perfect and pistillate flowers, and investigate the cellular events that result in the degradation of the megagametophyte cells from sterile ovules. In fertile ovules of perfect and pistillate flowers, mature megagametophytes have two synergids, egg cell and central cell. In fertile ovules, the synergids present an extensive rough endoplasmic reticulum (RER) profile, large populations of mitochondria, when compared to egg cells, vesicles, Golgi bodies, plastids and a nucleus with heterochromatin. Besides that, the egg cell has a small population of organelles and the central cell exhibits cytoplasm with free ribosomes, RER, vesicles originating from the RER, Golgi bodies and oil inclusions. In mature megagametophytes from sterile ovules of perfect and pistillate flowers, massive autophagy occurs by tonoplast rupture promoting hydrolase release, leading to protoplast and cell wall degradation-typical evidence of programmed cell death (PCD). Therefore, female sterility in the majority of M. obtusifolia sterile ovules is the result of PCD by massive autophagy in the megagametophyte cells. In a few other sterile ovules, sterility is due to the delayed or the absence of megagametophyte development.

Coenocytic Growth Phases in Land Plant Development: A Paleo-Evo-Devo Perspective

Premise of research. The evolutionary origin of the seed habit coincided with profound physiological and structural changes associated with underlying developmental patterns. A coenocyte is formed during megagametophyte development in many vascular plants, including some lycophytes and all spermatophytes; this structure compares closely with similar free-nuclear growth phases during embryo development in gymnosperms and endosperm development in angiosperms. Methodology. We review the various free-nuclear growth phases that occur during development of the megaspore and of the ensuing seed of land plants in the context of phylogenies of extant embryophytes and recent studies of the genes that underlie plant development. Pivotal results. Two contrasting types of coenocyte are controlled by different genetic systems: coenocytic growth phases that subsequently become cellularized and contribute to the plant body (type a) and terminally differentiated multinucleate structures such as haustorial suspensor cells (type b). Conclusions. Coding decisions made for coenocytic characters may have contributed to the ongoing instability that characterizes morphological cladistic analyses of land plants. Coenocytic growth phases in lycophytes and spermatophytes probably have independent evolutionary origins. However, the strong similarity between them indicates similar underlying genetic machinery; we hypothesize that this type of growth is controlled by ancient regulatory factors that were recruited during the development of novel structures. Combined with heterospory and endospory, a free-nuclear proembryo likely represented a preadaptive factor that facilitated the evolutionary origin of the seed habit. We highlight a positive correlation between a coenocytic growth phase and increased ovule/seed size in early spermatophytes. In general terms, the relatively derived seed plant lineages—including extant conifers, gnetaleans, and angiosperms—have reverted to a smaller seed size and concomitantly restricted coenocytic growth phases that reflect heterochronic change. A positive correlation between coenocytic growth phases and the absence of a preprophase band indicates that suppression of cytokinesis is a significant aspect of early growth phases in spermatophyte life histories.

Integrative Paleobotany: Affirming the Role of Fossils in Modern Plant Biology—Introduction and Dedication

Integrative Paleobotany: Affirming the Role of Fossils in Modern Plant Biology—Introduction and Dedication

Embryological peculiarities of interspecific hybridization in Pinus sibirica

Cytoembryological research of the ovules in experiments with interspecific hybridization of Pinus sibirica (pollination be the pollen of P. koraiensis, P. armandii, P. parviflora, P. strobus, P. hokkaidensis, P. wallichiana, P. monticola, and P. сembra) revealed that the development of megagametophytes occurred in them by the usual scenario and resulted in the formation of mature archegonia. Pollen successfully germinated on the nucellus of ovules. However, disturbances were observed in the process of male gametophyte development, and pollen tubes on the nucellus were not visible by the period of archegonia maturation. Fertilization was usually absent. The development of embryonic channel is determined by egg cell maturity. The only exception was the variant of the controlled pollination of Pinus sibirica × P. сembra, in which the embryo has been formed.

Ovule, Fruit, and Seed Development ofOrectanthe sceptrumand Its Systematic Relevance to Xyridaceae (Poales)

Premise of research. Orectanthe consists of only two species with a distribution limited to the Guayana Highlands, in locations where sample collection is challenging. The genus belongs to Abolbodoideae, one of the two subfamilies of Xyridaceae. The development of the ovule, fruit, and seed of Orectanthe sceptrum was studied using a comparative approach to provide useful information for the taxonomy and phylogeny of the family.Methodology. Flowers and fruits at different developmental stages were analyzed by light and scanning electron microscopy. Optimization of select characters on a previously published phylogeny was conducted to evaluate the evolution of some embryological characters in Poales. The seed weights of O. sceptrum and species of Abolboda and Xyris were measured for comparison.Pivotal results. Orectanthe sceptrum presents anatropous, tenuinucellate, and bitegmic ovules with a micropyle bounded by both integuments. The multilayered outer integument forms the seed wing, which is a unique feature of this genus. The development of megagametophyte is of the Polygonum type. The seed has a tanniferous hypostase, a starchy endosperm, and a reduced and undifferentiated embryo. The seed coat is composed of a tanniferous tegmen (endotegmen and exotegmen) and testa (endotesta and exotesta). The mechanical layer of the seed coat is the exotesta. The fruit has a sclerenchymatous exocarp. The ancestral character-state reconstructions show that anatropous, crassinucellate ovules, megagametophyte development of the Polygonum type, and nuclear endosperm are the ancestral conditions in Poales. Orthotropous, tenuinucellate ovules appear in the most recent common ancestor of the xyrid, restiid, and graminid clades. The seed weight of O. sceptrum is one to two orders of magnitude greater than the seed weight of Abolboda and Xyris species.Conclusions. Anatropous ovules and a seed coat formed by endotegmen, exotegmen, endotesta, and exotesta link Orectanthe to Abolboda (Abolbodoideae). Such characteristics are not found in Xyris (Xyridoideae), which is consistent with the subfamilial division. The presence of anatropous ovules in Abolbodoideae most likely represents a character reversal. The greater seed weight of O. sceptrum may explain the presence of wings in the seeds.

The MADS Box Genes ABS, SHP1, and SHP2 Are Essential for the Coordination of Cell Divisions in Ovule and Seed Coat Development and for Endosperm Formation in Arabidopsis thaliana.

Seed formation is a pivotal process in plant reproduction and dispersal. It begins with megagametophyte development in the ovule, followed by fertilization and subsequently coordinated development of embryo, endosperm, and maternal seed coat. Two closely related MADS-box genes, SHATTERPROOF 1 and 2 (SHP1 and SHP2) are involved in specifying ovule integument identity in Arabidopsis thaliana. The MADS box gene ARABIDOPSIS BSISTER (ABS or TT16) is required, together with SEEDSTICK (STK) for the formation of endothelium, part of the seed coat and innermost tissue layer formed by the maternal plant. Little is known about the genetic interaction of SHP1 and SHP2 with ABS and the coordination of endosperm and seed coat development. In this work, mutant and expression analysis shed light on this aspect of concerted development. Triple tt16 shp1 shp2 mutants produce malformed seedlings, seed coat formation defects, fewer seeds, and mucilage reduction. While shp1 shp2 mutants fail to coordinate the timely development of ovules, tt16 mutants show less peripheral endosperm after fertilization. Failure in coordinated division of the innermost integument layer in early ovule stages leads to inner seed coat defects in tt16 and tt16 shp1 shp2 triple mutant seeds. An antagonistic action of ABS and SHP1/SHP2 is observed in inner seed coat layer formation. Expression analysis also indicates that ABS represses SHP1, SHP2, and FRUITFUL expression. Our work shows that the evolutionary conserved Bsister genes are required not only for endothelium but also for endosperm development and genetically interact with SHP1 and SHP2 in a partially antagonistic manner.

A transcriptomic resource for Douglas-fir seed development and analysis of transcription during late megagametophyte development.

Douglas-fir transcriptomics. Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) is economically important with extensive breeding programs and seed trade. However, the molecular genetics of its seed development are largely unknown. We developed a transcriptome resource covering key developmental stages of megagametophytes over time: prefertilization, fertilization, embryogenesis, and early, unfertilized abortion. RNA sequencing reads were assembled de novo into 105,505 predicted high-confidence transcripts derived from 34,521 predicted genes. Expression levels were estimated based on alignment of the original reads to the reference. Megagametophytes express a distinct set of genes compared to those of vegetative tissues. Transcripts related to signaling, protein turnover, and RNA biogenesis have lower expression values in vegetative tissues, whereas cell wall remodeling, solute transport, and seed storage protein transcripts have higher expression values in megagametophytes. Seed storage protein transcripts become very abundant in both pollinated and unpollinated megagametophytes over time, even in aborting ovules. However, the absence of protein storage bodies in unfertilized megagametophytes suggests extensive posttranscriptional mechanisms that either inhibit storage protein translation or their aggregation into protein bodies. This novel transcriptome resource provides a foundation for further important insights into conifer seed development.

Ovule and megagametophyte development in selected species of Apeibeae and Grewieae (Malvaceae–Grewioideae) from South America and its systematic implications

This is the first embryological report of the Grewioideae subfamily, which is meant to contribute to the characterisation of the genera Corchorus, Luehea and Triumfetta. Ovule and female gametophyte development in C. argutus, L. divaricata and T. semitriloba was analysed. The ovules of all species are anatropous, bitegmic and of crassinucellate mixed type. The micropyle of the studied species is formed by the outer integument (exostome). The ovule of L. divaricata differs from those of the other two species because the chalazal tissue expands forming a cap, which gives rise to a wing in the seed. All species present one hypostase. The megaspore mother cell gives rise to a linear megaspore triad in C. argutus and L. divaricata, whereas in T. semitriloba, triads and diads can be observed in the same ovule. The chalazal megaspore develops a seven-celled and eight-nucleate female gametophyte corresponding to the Polygonum type. The synergids of L. divaricata have hooks and a conspicuous filiform apparatus. The antipodal cells in C. argutus are persistent, whereas in the other species, they are small and ephemeral. The embryological characters are compared with those of other taxa within the family and the megagametophyte formation in these species is discussed.

Ovule, fruit and seed development inAbolboda(Xyridaceae, Poales): implications for taxonomy and phylogeny

Xyridaceae belongs to the xyrid clade of Poales, but the phylogenetic position of the xyrid families is only weakly supported. Xyridaceae is divided into two subfamilies and five genera, the relationships of which remain unclear. The development of the ovule, fruit and seed of Abolboda spp. was studied to identify characteristics of taxonomic and phylogenetic value. All of the studied species share anatropous, tenuinucellate and bitegmic ovules with a micropyle formed by the inner and outer integuments, megagametophyte development of the Polygonum type, seeds with a tanniferous hypostase, a helobial and starchy endosperm and an undifferentiated embryo, seed coat derived from both integuments with a tanniferous tegmen and a micropylar operculum, and fruits with a parenchymatous endocarp and mesocarp and a sclerenchymatous exocarp. Most of the ovule and seed characteristics described for Abolboda are also present in Xyris and may represent a pattern for the family. Abolboda is distinguished by the ovule type, endosperm formation and the number of layers in the seed coat, in agreement with its classification in Abolbodoideae. The following characteristics link Xyridaceae to Eriocaulaceae and Mayacaceae, supporting the xyrid clade: tenuinucellate, bitegmic ovules; seeds with a tanniferous hypostase, a starchy endosperm and an undifferentiated embryo; and a seed coat with a tanniferous tegmen. A micropylar operculum in the seeds of Abolboda is described for the first time here and may represent a synapomorphy for the xyrids. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 175, 144–154.

Megasporogenesis and Development of Megagametophyte of Flueggea suffruticosa(Pall.) Baill.

Megasporogenesis and development of megagametophyte of the pistillate flower in Flueggea suffruticosa were observed by using the method of paraffin sections. The results showed that the pistillate flower of F. suffruticosa had epigynous ovary which was axile placenta; each ovary had 3 locules and bearing 2 anatropous bi-integument ovules in each locules. The ovules were crassinucellate and had nucellar beak and obturator. The development of embryo sac was of the Polygonum type. The mature embryo sac was consisted of two synergids and an egg cell near the micropyle,a central cell with two nucleus in the center,three antipodal cells by the side of chalaza. These results could provide basic data for the research of reproductive biology and pollination biology in Euphorbiaceae,meanwhile,for the research of plant taxonomy in Euphorbiaceae.

Female gametophyte development and embryogenesis in Taxus baccata L.

Crassinucellate ovules are initiated in <em>Taxus</em>, directly from the shoot apex. The rudimentary pollen chamber is formed in the nucellus. A linear tetrad of megaspores with a functional chalazal megaspore is formed. A free-nuclear stage is charac-teristic at the beginning of megagametophyte development. Archegonia without ventral canal cell are solitary or in complexes. The embryo has a very long suspensor even after maturation. Two types of polyembryony have been revealed: i) embryogenic redifferentiation of suspensor cells and ii) cleavage of embryonic region in the early embryo. In the northern temperate climate of St. Petersburg one month delay in development of reproductive structures has been noted.

Chromosome Number and Reproductive Attributes ForErigeron lemmonii(Asteraceae), A Cliff-Dwelling Endemic of Southeastern Arizona

Abstract Erigeron lemmonii A. Gray, restricted to Scheelite Canyon in the Huachuca Range, Arizona, has previously been proposed for federal listing as an endangered species but basic cytological and reproductive information has been wanting. The first chromosome count for the species is 2n = 18, which is the common diploid number in Erigeron. Analyses of eight plants from five disparate sites within the population show that pollen averages 85.8% staining in cotton blue in lactophenol. Microscopic observation using differential interference contrast optics shows that E. lemmonii combines about equally monosporic and bisporic megagametophyte development within a single capitulum. Despite variability in developmental route, the egg apparatus among mature megagametophytes appears to be nearly uniform in structure. In greenhouse culture, isolated plants fail to set seed indicating that plants are probably self-incompatible. Controlled crosses yield seed, but variation in seed set intimates the possible prese...

Embryological studies of Oxalis debilis Kunth

The study of pollen and female gametophyte development of Oxalis debilis was carried out to elucidate the reasons for the absence of seed production in this species. The formation of an incomplete callosic wall separating the microspores of the tetrad was observed; therefore, cytoplasm connections were present between the microspores. Microspores of different sizes and with different grades of vacuolation were observed in the mature anther. Only few microspores divide mitotically and form the generative and vegetative cell. The cytoplasm of the vegetative cell may accumulate different substances in reserve. Pollen grains are 8–12 colpate, and the morphology and electron density of the exine varies. Because flowers in full anthesis have all ovules at the megasporocyte stage with the nucellar epidermis intact, fecundation could not occur. The production of only a few viable pollen grains and the absence of successful megagametophyte development prevent fertilization and account for the absence of fruits and seeds formation.