The female gametophyte and the endosperm control cell proliferation and differentiation of the seed coat in Arabidopsis

Double fertilization of the female gametophyte produces the endosperm and the embryo enclosed in the maternal seed coat. Proper seed communication necessitates exchanges of signals between the zygotic and maternal components of the seed. However, the nature of these interactions remains largely unknown. We show that double fertilization of the Arabidopsis thaliana female gametophyte rapidly triggers sustained cell proliferation in the seed coat. Cell proliferation and differentiation of the seed coat occur in autonomous seeds produced in the absence of fertilization of the multicopy suppressor of ira1 (msi1) mutant. As msi1 autonomous seeds mostly contain autonomous endosperm, our results indicate that the developing endosperm is sufficient to enhance cell proliferation and differentiation in the seed coat. We analyze the effect of autonomous proliferation in the retinoblastoma-related1 (rbr1) female gametophyte on seed coat development. In contrast with msi1, supernumerary nuclei in rbr1 female gametophytes originate mainly from the endosperm precursor lineage but do not express an endosperm fate marker. In addition, defects of the rbr1 female gametophyte also reduce cell proliferation in the ovule integuments before fertilization and prevent further differentiation of the seed coat. Our data suggest that coordinated development of the seed components relies on interactions before fertilization between the female gametophyte and the surrounding maternal ovule integuments and after fertilization between the endosperm and the seed coat.     

Differentiation of mucilage secretory cells of the Arabidopsis seed coat

In some plant species, including Arabidopsis, fertilization induces the epidermal cells of the outer ovule integument to differentiate into a specialized seed coat cell type with a unique morphology and containing large quantities of polysaccharide mucilage (pectin). Such seed coat mucilage cells are necessary for neither viability nor germination under normal laboratory conditions. Thus, the Arabidopsis seed coat offers a unique system with which to use genetics to identify genes controlling cell morphogenesis and complex polysaccharide biosynthesis and secretion. As a first step in the application of this system, we have used microscopy to investigate the structure and differentiation of Arabidopsis seed coat mucilage cells, including cell morphogenesis and the synthesis, secretion, and extrusion of mucilage. During seed coat development in Arabidopsis, the epidermal cells of the outer ovule integument grow and differentiate into cells that produce large quantities of mucilage between the primary cell wall and plasma membrane. Concurrent with mucilage production, the cytoplasm is shaped into a column in the center of the cell. Following mucilage secretion the cytoplasmic column is surrounded by a secondary cell wall to form a structure known as the columella. Thus, differentiation of the seed coat mucilage cells involves a highly regulated series of events including growth, morphogenesis, mucilage biosynthesis and secretion, and secondary cell wall synthesis.     

Expression of TaCYP78A3, a gene encoding cytochrome P450 CYP78A3 protein in wheat (Triticum aestivum L.), affects seed size

Several studies have described quantitative trait loci (QTL) for seed size in wheat, but the relevant genes and molecular mechanisms remain largely unknown. Here we report the functional characterization of the wheat TaCYP78A3 gene and its effect on seed size. TaCYP78A3 encoded wheat cytochrome P450 CYP78A3, and was specifically expressed in wheat reproductive organs. TaCYP78A3 activity was positively correlated with the final seed size. Its silencing caused a reduction of cell number in the seed coat, resulting in an 11% decrease in wheat seed size, whereas TaCYP78A3 over‐expression induced production of more cells in the seed coat, leading to an 11–48% increase in Arabidopsis seed size. In addition, the cell number in the final seed coat was determined by the TaCYP78A3 expression level, which affected the extent of integument cell proliferation in the developing ovule and seed. Unfortunately, TaCYP78A3 over‐expression in Arabidopsis caused a reduced seed set due to an ovule developmental defect. Moreover, TaCYP78A3 over‐expression affected embryo development by promoting embryo integument cell proliferation during seed development, which also ultimately affected the final seed size in Arabidopsis. In summary, our results indicated that TaCYP78A3 plays critical roles in influencing seed size by affecting the extent of integument cell proliferation. The present study provides direct evidence that TaCYP78A3 affects seed size in wheat, and contributes to an understanding of the cellular basis of the gene influencing seed development.     

Evolution and patterning of the ovule in seed plants

The ovule and its developmental successor, the seed, together represent a highly characteristic feature of seed plants that has strongly enhanced the reproductive and dispersal potential of this diverse group of taxa. Ovules encompass multiple tissues that perform various roles within a highly constrained space, requiring a complex cascade of genes that generate localized cell proliferation and programmed cell death during different developmental stages. Many heritable morphological differences among lineages reflect relative displacement of these tissues, but others, such as the second (outer) integuments of angiosperms and Gnetales, represent novel and apparently profound and independent innovations. Recent studies, mostly on model taxa, have considerably enhanced our understanding of gene expression in the ovule. However, understanding its evolutionary history requires a comparative and phylogenetic approach that is problematic when comparing extant angiosperms not only with phylogenetically distant extant gymnosperms but also with taxa known only from fossils. This paper reviews ovule characters across a phylogenetically broad range of seed plants in a dynamic developmental context. It discusses both well-established and recent theories of ovule and seed evolution and highlights potential gaps in comparative data that will usefully enhance our understanding of evolutionary transitions and developmental mechanisms.     

Family life at close quarters: communication and constraint in angiosperm seed development

The formation of viable angiosperm seeds involves the co-ordinated growth and development of three genetically distinct organisms, the maternally derived seed coat and the zygotic embryo and endosperm. The physical relationships of these tissues are initially established during the specification and differentiation of the female gametophyte within the tissues of the developing ovule. The molecular programmes implicated in both ovule and seed development involve elements of globally important pathways (such as auxin signalling), as well as ovule- and seed-specific pathways. Recurrent themes, such as the precisely controlled death of specific cell types and the regulation of cell–cell communication and nutrition by the selective establishment of symplastic and apoplastic barriers, appear to play key roles in both pre- and post-fertilization seed development. Much of post-fertilization seed growth occurs during a key developmental window shortly after fertilization and involves the dramatic expansion of the young endosperm, constrained by surrounding maternal tissues. The complex tissue-specific regulation of carbohydrate metabolism in specific seed compartments has been shown to provide a driving force for this early seed expansion. The embryo, which is arguably the most important component of the seed, appears to be only minimally involved in early seed development. Given the evolutionary and agronomic importance of angiosperm seeds, the complex combination of communication pathways which co-ordinate their growth and development remains remarkably poorly understood.     

Isolation and Characterization of Mutants Defective in Seed Coat Mucilage Secretory Cell Development in Arabidopsis

In Arabidopsis, fertilization induces the epidermal cells of the outer ovule integument to differentiate into a specialized seed coat cell type producing extracellular pectinaceous mucilage and a volcano-shaped secondary cell wall. Differentiation involves a regulated series of cytological events including growth, cytoplasmic rearrangement, mucilage synthesis, and secondary cell wall production. We have tested the potential of Arabidopsis seed coat epidermal cells as a model system for the genetic analysis of these processes. A screen for mutants defective in seed mucilage identified five novel genes (MUCILAGE-MODIFIED [MUM]1–5). The seed coat development of these mutants, and that of three previously identified ones (TRANSPARENT TESTA GLABRA1, GLABRA2, and APETALA2) were characterized. Our results show that the genes identified define several events in seed coat differentiation. Although APETALA2 is needed for differentiation of both outer layers of the seed coat, TRANSPARENT TESTA GLABRA1, GLABRA2, and MUM4 are required for complete mucilage synthesis and cytoplasmic rearrangement. MUM3 and MUM5 may be involved in the regulation of mucilage composition, whereas MUM1 and MUM2 appear to play novel roles in post-synthesis cell wall modifications necessary for mucilage extrusion.     

Programmed cell death during development of cowpea (Vigna unguiculata (L.) Walp.) seed coat

The seed coat develops primarily from maternal tissues and comprises multiple cell layers at maturity, providing a metabolically dynamic interface between the developing embryo and the environment during embryogenesis, dormancy and germination of seeds. Seed coat development involves dramatic cellular changes, and the aim of this research was to investigate the role of programmed cell death (PCD) events during the development of seed coats of cowpea [Vigna unguiculata (L.) Walp.]. We demonstrate that cells of the developing cowpea seed coats undergo a programme of autolytic cell death, detected as cellular morphological changes in nuclei, mitochondria, chloroplasts and vacuoles, DNA fragmentation and oligonucleosome accumulation in the cytoplasm, and loss of membrane viability. We show for the first time that classes 6 and 8 caspase‐like enzymes are active during seed coat development, and that these activities may be compartmentalized by translocation between vacuoles and cytoplasm during PCD events.     

Seed coat development in Leucospermum cordifolium (Knight) Fourcade (Proteaceae) and a clarification of the seed covering structures in Proteaceae

MANNING, J. C. & BRITS, G. J., 1993. Seed coat development in Leucospermum cordifolium (Knight) Fourcade (Proteaceae) and a clarification of the seed covering structures in Proteaceae . The development of the seed coat and pericarp is studied in Leucospermum cordifolium from ovule to mature seed. The ovule and seed are characterized by a tegmic pachychalaza. The pericarp is adnate to the integuments from anthesis and remains unthickened to maturity. The outer integument forms the seed coat and the seed is endotestal: the outer epidermis becomes tanniniferous and the inner epidermis develops into a crystalliferous palisade. The inner integument degenerates at an early stage. Examination of the literature reveals that the crystal palisade layer of the outer integument has been erroneously assumed to constitute an endocarp. This finding indicates that a re-interpretation of all published information on the seed coat in indehiscent Proteaceae is necessary before any speculations on the phylogenetic significance of the seed coat can be entertained.     

Turgor-sensitive sucrose and amino acid transport into developing seeds of Pisum sativum. Effect of a high sucrose or mannitol concentration in experiments with empty ovules

Sugar and amino acid transport into empty ovules of Pisum sativum L. cv. Marzia was examined. In fruits containing 4–6 developing seeds, the embryo was removed from four ovules. After this surgical treatment, each empty seed coat was filled with a solution (pH 5.5) containing a low (0, 50 or 200 m M ), medium (350, 400 or 500 m M ) or high (0.7 or 1 M ) concentration of sucrose and/or mannitol. In pulse-labelling experiments with sucrose and α-aminoisobutyric acid (AIB), transport of sucrose and AIB into an empty ovule filled with a solution containing a high sucrose concentration was the same as transport into an ovule filled with a mannitol solution of similar osmolarity, demonstrating that a high sucrose concentration in the seed coat apoplast affects phloem transport of sucrose and AIB into the seed coat only by the osmotic effect. The osmolarity of a given solution filling the seed coat cavity appeared to be important for phloem transport of sucrose and AIB into empty ovules.
In our experiments, 350 m M appeared to be the optimal concentration for sucrose and AIB transport into the cavity within an empty ovule, giving results comparable with transport into intact ovules. A lower osmolarity of the solution induced less transport. Very high sucrose or mannitol concentrations caused a strong inhibition of sucrose and AIB unloading from the seed coat, so that transport into the empty ovules was inhibited. A low (strongly negative) but not too low osmotic potential of the solution in the seed coat apoplast seems necessary to maintain a normal rate of phloem transport into developing seeds. Apparently, the "sink strength" of developing seeds is turgor-sensitive.     

Seed Coat Unloading in Pisum sativum--Osmotic Effects in Attached versus Excised Empty Ovules

Experiments were undertaken with embryo-less ovules of Pisumsativum to study the influence of apoplastic osmolality on seedcoat import and seed coat unloading.¹¹CO₂ pulse labelling alongwith collimated monitoring of plant tissues were used with attachedovules to measure continuously and simultaneously total podimport, import into a modified ovule and photo-assimilate washoutfrom the seed coat of the ovule into a flow-through bathingsolution.Our results indicated that seed coat import was immediatelyaffected by a change in the applied bathing solution osmolality,with a decrease in osmolality lowering seed coat import andan increase in osmolality increasing import. ¹¹C-photo-assimilatewashout from attached ovules was found to respond in a similarmanner to the apoplastic osmolality. However, the osmotic effecton ¹¹C-washout was a delayed response and it appears that themajority of this observed response was due to the alterationin seed coat tracer import. Further experiments with ¹⁴C-labelled,excised seed coat halves (i.e. no further import) supportedthis hypothesis by demonstrating that seed coat unloading (measuredas ¹⁴C-photo-assimilate washout) was actually enhanced at alow solution osmolality. PCMBS had no effect on seed coat importor washout in attached, modified ovules, suggesting that photo-assimilateunloading from seed coats of Pisum does not involve a carrierprotein. Studies of the spatial distribution of imported ¹⁴Cin Pisum seed coats further suggest that this unloading, intothe apoplast, occurs from non-phloem cell types, and that themovement of photo-assimilates from the sieve elements to theterminal unloading site occurs via symplastic transport. Key words: Pisum sativum, seed coat, seed coat unloading, phloem unloading     

Development and structure of the seed of Ozoroa paniculosa (Anacardiaceae) and taxonomic notes

The anatropous, bitegmic and crassinucellar ovule has a nuclear endosperm development. It is further characterized by a hypostase sensu lato. This hypostase being an integral part of the chalaza undergoes a secondary extension with it. At maturity the exalbuminous seed is partially pachychalazal and therefore two anatomically distinct larger parts can be distinguished in the mature seed coat. An endotegmen typifies the integumentary seed coat, while a saddle-shaped hypostase characterizes the chalazal seed coat. This seed coat shows several characteristics of the typical anacardiaceous pachychalazal seed. The cotyledons store lipids and protein as nutrient reserveS. A well-developed cuticle, cuticular layer, cutin and callose in the hypostase cell walls, as well as tannin-like deposits in the seed coat, protect the physiologically ripe seed against dehydration.     

Embryology of <Emphasis Type="Italic">Cardiopteris</Emphasis> (Cardiopteridaceae,Aquifoliales), with emphasis on unusual ovule and seed development

Cardiopteris (Cardiopteridaceae), a twining herb of two or three species distributed from Southeast Asia to Northern Australia, requires an embryological study for better understanding of its reproductive features. The present study of C. quinqueloba showed that the ovule and seed development involves a number of unusual structures, most of which are unknown elsewhere in angiosperms. The ovule pendant from the apical placenta is straight (not orthotropous), ategmic, and tenuinucellate, developing a monosporic seven-celled/eight-nucleate female gametophyte with an egg apparatus on the funicular side. Fertilization occurs by a pollen tube entering from the funicular side, resulting in a zygote on the funicular side. The endosperm is formed by the cell on the funicular side in the two endosperm cell stage. While retaining a (pro)embryo/endosperm as it is, the raphe (differentiating late in pre-fertilization stages) elongates toward the antiraphal side during post-fertilization stages, resulting in an anatropous seed. The two-cell-layered nucellar epidermis (belatedly forming by periclinal divisions), along with the raphe, envelops the embryo/endosperm entirely as the seed coat. The possibility was discussed that the arrested integument development triggers a series of the subsequent unusual structures of ovule and seed development. The fertilization mode in Cardiopteris underpins the hypothesis that the Polygonum?type female gametophyte comprises two four-celled archegonia.     

A vacuolar processing enzyme, deltaVPE, is involved in seed coat formation at the early stage of seed development

Vacuolar processing enzyme (VPE) is a Cys proteinase responsible for the maturation of vacuolar proteins. Arabidopsis thaliana deltaVPE, which was recently found in the database, was specifically and transiently expressed in two cell layers of the seed coat (ii2 and ii3) at an early stage of seed development. At this stage, cell death accompanying cell shrinkage occurs in the ii2 layer followed by cell death in the ii3 layer. In a deltaVPE-deficient mutant, cell death of the two layers of the seed coat was delayed. Immunocytochemical analysis localized deltaVPE to electron-dense structures inside and outside the walls of seed coat cells that undergo cell death. Interestingly, deltaVPE in the precipitate fraction from young siliques exhibits caspase-1-like activity, which has been detected in various types of plant cell death. Our results suggest that, at the early stage of seed development, deltaVPE is involved in cell death of limited cell layers, the purpose of which is to form a seed coat.     

Regulation of anti‐apoptotic BCL2‐proteins by non‐canonical interactions: The next step forward or two steps back?

All aspects of cellular biology affect the process of regulated cell death, or apoptosis, and disruption of this process is a causative event in many diseases. Therefore, a comprehensive understanding of all pathways that regulate apoptosis would increase our knowledge of basic cellular functions, as well as the etiologies of many diseases. In turn, we may be able to use this knowledge to better treat patients with diseases, including cancer. Although the basic signaling pathway that regulates apoptosis has been known for over 10 years, we still have much to learn about the upstream signaling components that can directly regulate the core apoptosis machinery. The focus of this review will be to direct attention to non-canonical regulators of the BCL2-family of proteins, especially our void of understanding of such interactions, and the controversy that surrounds some such interactions.     

More than just a coating: Ecological importance,taxonomic occurrence and phylogenetic relationships of seed coat mucilage

Studies on the ecological importance of seed coat mucilage have provided valuable information about its roles in critical stages of the plant life cycle. Seed mucilage may, by providing a moist environment and maintaining metabolic activity in the seed, promote seed development. In seed dispersal, seed mucilage influences topochory, epizoochory, endozoochory and hydrochory by anchorage of seeds to soil surface, lubrication or changing the specific weight of the seed. In arid environments, seed mucilage can prevent seeds from drying or initiate DNA repair mechanisms, thereby maintaining the soil seed bank. Seed mucilage reduces oxygen diffusion to the seed and thus has a role in regulating seed dormancy. Due to it being hydrophilous, acting as a physical barrier and containing chemicals, seed mucilage is proposed to promote seed germination in favorable environments. In seedling growth, seed mucilage may lubricate the radicle as it penetrates the soil and be degraded by soil microfloras and thus promote seedling growth. Further investigation of seed mucilage for more species in diverse habitats from the perspectives of evolution, genetics, proteomics, phylogeny and plant–microbe interactions would contribute substantially to our understanding about its ecological importance.     

<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.