Apomictic development during different flower development stages in Crataegus tanacetifolia (Lam.) Pers., endemic to Turkey

In this study, aposporic apomictic development and its relation to the different flower development stages were investigated by light and fluorescence microscopy in Crataegus tanacetifolia (Lam.) Pers. (Rosaceae). At pre-anthesis stage, aposporic initial cell differentiated at the somatic nucellus tissue shortly after the megaspore mother cell formation. The volume of aposporic initial cell increased during the generation of dyad and megaspore tetrad respectively by regular meiosis. At this stage, linear megaspore tetrad and vacuolated aposporic initial cell were located side by side into the same ovule. At anthesis stage, before pollination, four nucleated aposporic embryo sac was formed while sexual development came to end by atrophy of megaspores completely. At this stage, atrophied megaspores and two nucleated aposporic embryo sac were located side by side into the same ovule. At post-anthesis stage, pollination still had not begun and ovule contained only eight nucleated aposporic embryo sac. Mature aposporic embryo sac was composed of two synergid cells and one egg cell on the micropylar side, three antipodal cells on the chalazal side and a central cell with two polar nuclei in the middle of the sac. The absence of filiform apparatus in the synergid cells was quite remarkable. No callose accumulation around the aposporic initial cell was observed in any development stage. Pollination started shortly after the proembryo formation. Embryo and endosperm developed without fertilization due to the problems encountered in reaching pollen tubes to the ovary.

Developmental, ultrastructural and cytochemical investigations of the female gametophyte in Sedum rupestre L. (Crassulaceae)

This article describes the development of female gametophyte in Sedum rupestre L. New embryological information about the processes of megasporogenesis and megagametogenesis provided in this paper expand the current knowledge about the embryology of the studied species. S. rupestre is characterized by monosporic megasporogenesis and the formation of Polygonum–type embryo sac. The process of megasporogenesis is initiated by one megaspore mother cell, resulting in the formation of a triad of cells after meiosis and cytokinesis. The functional megaspore, which is located chalazally, is a mononuclear cell present next to the megaspore in the centre of the triad. Only one of the two non-functional cells of the triad is binucleate, which occur at the micropylar pole. In this paper, we explain the functional ultrastructure of the female gametophytic cells in S. rupestre. Initially, the cytoplasm of the gametophytic cells does not differ from each other; however, during differentiation, the cells reveal different morphologies. The antipodals and the synergids gradually become organelle-rich and metabolically active. The antipodal cells participate in the absorption and transport of nutrients from the nucellar cells towards the megagametophyte. Their ultrastructure shows the presence of plasmodesmata with electron-dense material, which is characteristic of Crassulaceae, and wall ingrowths in the outer walls. The ultrastructure of synergid cells is characterized by the presence of filiform apparatus and cytoplasm with active dictyosomes, abundant profiles of endoplasmic reticulum and numerous vesicles, which agrees with their main function—the secretion of pollen tube attractants. Reported data can be used to resolve the current taxonomic problems within the genus Sedum ser. Rupestria.

Pollen tube-triggered accumulation of NORTIA at the filiform apparatus facilitates fertilization in Arabidopsis thaliana

During gamete delivery in Arabidopsis thaliana, intercellular communication between the attracted pollen tube and the receptive synergid cell leads to subcellular events in both cells culminating in the rupture of the tip-growing pollen tube and release of the sperm cells to achieve double fertilization. Live imaging of pollen tube reception revealed dynamic subcellular changes that occur in the female synergid cells. Pollen tube arrival triggers the trafficking of NORTIA (NTA) MLO protein from Golgi-associated compartments and the accumulation of endosomes at or near the synergid filiform apparatus, a membrane-rich region that acts as the site of communication between the pollen tube and synergids. Domain swaps and site-directed mutagenesis reveal that NTA’s C-terminal cytoplasmic tail with its calmodulin-binding domain influences the subcellular localization and function of NTA in pollen tube reception and that accumulation of NTA at the filiform apparatus is necessary and sufficient for MLO function in pollen tube reception.

CrRLK1L receptor-like kinases HERCULES RECEPTOR KINASE 1 and ANJEA are female determinants of pollen tube reception

Communication between the gametophytes is vital for angiosperm fertilisation. Multiple CrRLK1L-type receptor kinases prevent premature pollen tube burst, while another CrRLK1L protein, FERONIA (FER), is required for pollen tube burst in the female gametophyte. We report here the identification of two additional CrRLK1L homologues, HERCULES RECEPTOR KINASE 1 (HERK1) and ANJEA (ANJ), which act redundantly to promote pollen tube burst at the synergid cells. HERK1 and ANJ localise to the filiform apparatus of the synergid cells in unfertilised ovules, and in herk1 anj mutants a majority of ovules remain unfertilised due to pollen tube overgrowth, together indicating that HERK1 and ANJ act as female determinants for fertilisation. As in fer mutants, the synergid cell-specific, endomembrane protein NORTIA (NTA) is not relocalised after pollen tube reception; however, unlike fer mutants, reactive oxygen species levels are unaffected in herk1 anj double mutants. Both ANJ and HERK1 associate with FER and its proposed co-receptor LORELEI (LRE) in planta. Together, our data indicate that HERK1 and ANJ act with FER to mediate female-male gametophyte interactions during plant fertilisation.

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.

Female gametophyte and pollen tube of Epilobium palustre L.

The monosporic, tetranucleate embryo sac of <i>Epilobium palustre (Onagraceae)</i> develops from the micropylar megaspore in a linear tetrad. In mononucleate embryo sacs a peculiar chromatic structure associated with a nucleolus appears in the nucleus. This structure seems to be formed by fibrillar material and is not visible in the subsequant stages of development. A large amount of rough ER cisternae occurs in the late mononucleate stage, during the binucleate stage their contents become optically dense. It the early tetranucleate stage the amount of ER is small, it increases again in the developing synergids and central cell. Numerous amyloplasts present in the mononucleate embryo sac loose their starch grains and some are transformed into cup-shaped plastids or proplastids. They are passed on to each of the embryo sac cells. The growth of the pollen tube ceases immediately after the penetration through the filiform apparatus of a synergid. At the apex of the tube a pore is formed. At the last stages of the penetration the apical part of the pollen tube becomes separated by a transverse partition from the distal part of the tube. The contents of the both parts differ in their internal structure. The distal part contains cytoplasm with numerous organoids, while the apical part is mainly filled with spherical bodies.

Ovule and female gametophyte development in the Bromeliaceae: an embryological study of Pitcairnia encholirioides

Pitcairnia encholirioides L.B.Sm. is an endangered species endemic to the Brazilian Atlantic Forest. This species exhibits limited flowering, late seed germination, and preference for clonal growth. Because little is known about its life cycle and female gametophyte development, the ovule development, gynosporogenesis, and gynogametogenesis were analysed to advance knowledge of the species’ life cycle and structural alterations during ovule and female gametophyte development. Also, identification of embryological characters contributing to systematics of Pitcairnioideae is relevant. The ovules are anatropous, bitegmic, and crassinucellate, the gynosporogenesis is monosporic, and the female gametophyte is a Polygonum type. Different patterns in development of the integuments, nucellus, chalazal appendage, and micropylar channel indicate the potential of these characters for subfamily systematics. In the filiform apparatus, a range of glycan-directed monoclonal antibodies was used; the filiform exhibited a biphasic structure. While only arabinogalactan proteins (AGPs) occurred in the translucent matrix, mannans were the most prevalent glycan in the denser matrix. These phases may have distinct mechanical or signalling properties, as they showed different cell wall component distributions. The distinct spatial distribution between AGPs and other glycans showed that the filiform apparatus is heterogeneous and has a common polymer assemblage for both synergids.

Ultrastructure of Synergids in Sugar Beet

We use TEM to study the synergids in sugar beet, so as to provide more information for reproductive biology of angiosperm. Results were as follows: two synergids were similar in flower bud stage. Both of them show polarity with developed filiform apparatus (FA) at micropyle end and lacked cell wall at chalazal end. Then electron density in one synergid increased which suggested cell degeneration began. Complete degeneration finished before pollination. Organelles including mitochondrium, plastids and ribosomes gradually increased in the other synergid (persistent synergid). Metabolism of persistent synergid gradually enhanced. It began to degenerate when zygote had alveolate cell wall at the chalazal end, while completely degeneration and disappearance of FA took place at late stage of zygote. The results suggested that degeneration of one synergid in sugar beet must be triggered by other stimulation than pollination and pollen tube growth. As a transfer cell, persistent synergid transported nutrition for the development of embryo sac.

Ultrastructure of the egg apparatus of Spinacia

The egg apparatus of <em>Spinacia</em> was studied from the time the embryo sac reaches its maximal size to just before fertilization, i.e., until about 8-9 hours after pollination. At maturity each synergid has a large elongated nucleus and prominent chalazal vacuoles, Numerous mitochondria, plastids, dictyosomes, free ribosomes, rough endoplasmic reticulum (RER), and lipid bodies are present. The cell wall exists only around the micropylar half of the synergids and each cell has a distinct, striated filiform apparatus. In general, degeneration of one synergid starts after pollination. The egg cell has a spherical nucleus and nucleolus and a large micropylar vacuole. Numerous mitochondria, some plastids with starch grains, dictyosomes, free ribosomes, and HER are present. A continuous cell wall is absent around the chalazal end of the egg cell.