Ovule Integument Research Articles

Alasemenia, the earliest ovule with three wings and without cupule.

The ovules or seeds (fertilized ovules) with wings are widespread and especially important for wind dispersal. However, the earliest ovules in the Famennian of the Late Devonian are rarely known about the dispersal syndrome and usually surrounded by a cupule. From Xinhang, Anhui, China, we now report a new taxon of Famennian ovules, Alasemenia tria gen. et sp. nov. Each ovule of this taxon possesses three integumentary wings evidently extending outwards, folding inwards along abaxial side and enclosing most part of nucellus. The ovule is borne terminally on smooth dichotomous branches and lacks a cupule. Alasemenia suggests that the integuments of the earliest ovules without a cupule evolved functions in probable photosynthetic nutrition and wind dispersal. It indicates that the seed wing originated earlier than other wind dispersal mechanisms such as seed plume and pappus, and that three- or four-winged seeds were followed by seeds with less wings. Mathematical analysis shows that three-winged seeds are more adapted to wind dispersal than seeds with one, two or four wings under the same condition.

Alasemenia, the earliest ovule with three wings and without cupule

The ovules or seeds (fertilized ovules) with wings are widespread and especially important for wind dispersal. However, the earliest ovules in the Famennian of the Late Devonian are rarely known about the dispersal syndrome and usually surrounded by a cupule. From Xinhang, Anhui, China, we now report a new taxon of Famennian ovules, Alasemenia tria gen. et sp. nov. Each ovule of this taxon possesses three integumentary wings evidently extending outwards, folding inwards along abaxial side and enclosing most part of nucellus. The ovule is borne terminally on smooth dichotomous branches and lacks a cupule. Alasemenia suggests that the integuments of the earliest ovules without a cupule evolved functions in probable photosynthetic nutrition and wind dispersal. It indicates that the seed wing originated earlier than other wind dispersal mechanisms such as seed plume and pappus, and that three- or four-winged seeds were followed by seeds with less wings. Mathematical analysis shows that three-winged seeds are more adapted to wind dispersal than seeds with one, two or four wings under the same condition.

Principles of amyloplast replication in the ovule integuments of Arabidopsis thaliana.

Plastids in vascular plants have various differentiated forms, among which amyloplasts are crucial for starch storage and plant productivity. Despite the vast knowledge of the binary-fission mode of chloroplast division, our understanding of the replication of non-photosynthetic plastids, including amyloplasts, remains limited. Recent studies have suggested the involvement of stromules (stroma-filled tubules) in plastid replication when the division apparatus is faulty. However, details of the underlying mechanism(s) and their relevance to normal processes have yet to be elucidated. Here, we developed a live analysis system for studying amyloplast replication using Arabidopsis (Arabidopsis thaliana) ovule integuments. We showed the full sequence of amyloplast development and demonstrated that wild-type amyloplasts adopt three modes of replication, binary fission, multiple fission, and stromule-mediated fission, via multi-way placement of the FtsZ ring. The minE mutant, with severely inhibited chloroplast division, showed marked heterogeneity in amyloplast size, caused by size-dependent but wild-type modes of plastid fission. The dynamic properties of stromules distinguish the wild-type and minE phenotypes. In minE cells, extended stromules from giant amyloplasts acquired stability, allowing FtsZ ring assembly and constriction, as well as the growth of starch grains therein. Despite hyper-stromule formation, amyloplasts did not proliferate in the ftsZ null mutant. These data clarify the differences between amyloplast and chloroplast replication and demonstrate that the structural plasticity of amyloplasts underlies the multiplicity of their replication processes. Furthermore, this study shows that stromules can generate daughter plastids via the assembly of the FtsZ ring.

JAGGER localization and function are dependent on GPI anchor addition

Key messageGPI anchor addition is important for JAGGER localization and in vivo function. Loss of correct GPI anchor addition in JAGGER, negatively affects its localization and function.In flowering plants, successful double fertilization requires the correct delivery of two sperm cells to the female gametophyte inside the ovule. The delivery of a single pair of sperm cells is achieved by the entrance of a single pollen tube into one female gametophyte. To prevent polyspermy, Arabidopsis ovules avoid the attraction of multiple pollen tubes to one ovule–polytubey block. In Arabidopsisjagger mutants, a significant number of ovules attract more than one pollen tube to an ovule due to an impairment in synergid degeneration. JAGGER encodes a putative arabinogalactan protein which is predicted to be anchored to the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor. Here, we show that JAGGER fused to citrine yellow fluorescent protein (JAGGER-cYFP) is functional and localizes mostly to the periphery of ovule integuments and transmitting tract cells. We further investigated the importance of GPI-anchor addition domains for JAGGER localization and function. Different JAGGER proteins with deletions in predicted ω-site regions and GPI attachment signal domain, expected to compromise the addition of the GPI anchor, led to disruption of JAGGER localization in the cell periphery. All JAGGER proteins with disrupted localization were also not able to rescue the polytubey phenotype, pointing to the importance of GPI-anchor addition to in vivo function of the JAGGER protein.

DELLA proteins positively regulate seed size in Arabidopsis.

Human and animal nutrition is mainly based on seeds. Seed size is a key factor affecting seed yield and has thus been one of the primary objectives of plant breeders since the domestication of crop plants. Seed size is coordinately regulated by signals of maternal and zygotic tissues that control the growth of the seed coat, endosperm, and embryo. Here, we provide novel evidence of the role of DELLA proteins, key repressors of gibberellin responses, in the maternal control of seed size. The gain-of-function della mutant gai-1 produces larger seeds due to an increase in the cell number in ovule integuments. This leads to an increase in ovule size and, in turn, an increase in seed size. Moreover, DELLA activity promotes increased seed size by inducing the transcriptional activation of AINTEGUMENTA, a genetic factor that controls cell proliferation and organ growth, in the ovule integuments of gai-1. Overall, our results point to DELLA proteins as new players in control of seed size and suggest that modulation of the DELLA-dependent pathway could be used to improve crop yield.

Reproductive Mechanisms in Ginkgo and Cycas: Sisters but not Twins

The study of reproductive mechanisms is of particular interest for a real understanding of seed plant evolution. Spermatophytes comprise angiosperms and four orders of gymnosperms (Cycadales, Ginkgoales, Coniferales, and Gnetales) whose main characteristic is the reproduction via seeds. Ginkgo and cycads form a sister clade to the other gymnosperms and occupy a key phylogenetic position in-between the extinct Paleozoic seed ferns and the other extant gymnosperms. This review focuses on the similarities and differences between the reproductive mechanisms of Ginkgo and Cycas, from the morphogenesis of the male and female organs to the pollination and fertilization events. Together with the morphological and cytological description, the latest available molecular data on the reproductive organ development of the two plant genera are discussed. This will, hopefully, pave the path for new studies aiming at filling the gaps in our understanding of the hormonal and genetic regulation of their reproductive mechanisms. The whole reproductive process is presented in detail, providing a comprehensive and organic picture together with complete illustrations and photographic material. Each phase of the reproductive process is dissected, pointing out the main similarities and differences found among the two genera. The comparison comprises the male and female reproductive organs development, with a focus on pollen ontogeny, shape, ultrastructure, and germination as well as ovule development and patterning, female gametophyte formation, and ovule integument differentiation. Particular attention is given to the pollination and fertilization events focusing on the role of reproductive fluids as well as zoogamy.

Fertility loss in senescing Arabidopsis ovules is controlled by the maternal sporophyte via a NAC transcription factor triad

Flowers have a species-specific fertile period during which pollination and fertilization have to occur to initiate seed and fruit development. Unpollinated flowers remain receptive for mere hours in some species, and up to several weeks in others before flower senescence terminates fertility. As such, floral longevity is a key trait subject to both natural selection and plant breeding. Within the flower, the life span of the ovule containing the female gametophyte is decisive for fertilization and the initiation of seed development. Here, we show that unfertilized ovules in Arabidopsis thaliana undergo a senescence program that generates morphological and molecular hallmarks of canonical programmed cell death processes in the sporophytically derived ovule integuments. Transcriptome profiling of isolated aging ovules revealed substantial transcriptomic reprogramming during ovule senescence, and identified up-regulated transcription factors as candidate regulators of these processes. Combined mutation of three most-up-regulated NAC (NAM, ATAF1/2, and CUC2) transcription factors, NAP/ANAC029, SHYG/ANAC047, and ORE1/ANAC092, caused a substantial delay in ovule senescence and an extension of fertility in Arabidopsis ovules. These results suggest that timing of ovule senescence and duration of gametophyte receptivity are subject to genetic regulation controlled by the maternal sporophyte.

Cell-specific clock-controlled gene expression program regulates rhythmic fiber cell growth in cotton

BackgroundThe epidermis of cotton ovule produces fibers, the most important natural cellulose source for the global textile industry. However, the molecular mechanism of fiber cell growth is still poorly understood.ResultsHere, we develop an optimized protoplasting method, and integrate single-cell RNA sequencing (scRNA-seq) and single-cell ATAC sequencing (scATAC-seq) to systematically characterize the cells of the outer integument of ovules from wild type and fuzzless/lintless (fl) cotton (Gossypium hirsutum). By jointly analyzing the scRNA-seq data from wildtype and fl, we identify five cell populations including the fiber cell type and construct the development trajectory for fiber lineage cells. Interestingly, by time-course diurnal transcriptomic analysis, we demonstrate that the primary growth of fiber cells is a highly regulated circadian rhythmic process. Moreover, we identify a small peptide GhRALF1 that circadian rhythmically controls fiber growth possibly through oscillating auxin signaling and proton pump activity in the plasma membrane. Combining with scATAC-seq, we further identify two cardinal cis-regulatory elements (CREs, TCP motif, and TCP-like motif) which are bound by the trans factors GhTCP14s to modulate the circadian rhythmic metabolism of mitochondria and protein translation through regulating approximately one third of genes that are highly expressed in fiber cells.ConclusionsWe uncover a fiber-specific circadian clock-controlled gene expression program in regulating fiber growth. This study unprecedentedly reveals a new route to improve fiber traits by engineering the circadian clock of fiber cells.

KNOX transcription factor VvHB63 affects grape seed development by interacting with protein VvHB06

The fast-growing demand for seedless table grapes has attracted the attention of scientists for the development of new seedless cultivars. Various genes and pathways have been identified which affect seedlessness. However, the detail of the mechanism(s) regulating seedless traits in grape is still unclear, and genes related to seedlessness in grape require further study. Transcriptomic and genomic analyses of Homeobox (HB) transcription factors have suggested the involvement of HB genes, especially of HB-KNOX members, in grape seed development. Here, we functionally characterize VvHB63 gene in grape and report its role in fruit and seed development. VvHB63 showed higher expressions levels in the chalaza and integument of ovules in seedless grapes, than in seeded ones. However, no differences were observed in the sequences of seedless and seeded grape cultivars. In situ hybridization (ISH) analysis showed that VvHB63 gene was expressed in the episperm cells and ovules of 'Thompson Seedless'. Conserved domains KNOX1 and KNOX2 were important for the interaction of VvHB63 with VvHB06. Heterologous over-expression of VvHB63 (35 S::VvHB63-OE) in tomato induced smaller fruits and seeds than in wild type or SlTkn1-KO. The synergistic cooperation between VvHB63 and related proteins play an important role in ovule development.

The Arabidopsis INNER NO OUTER (INO) gene acts exclusively and quantitatively in regulation of ovule outer integument development.

The INNER NO OUTER (INO) gene is essential for formation of the outer integument of ovules in Arabidopsis thaliana. Initially described lesions in INO were missense mutations resulting in aberrant mRNA splicing. To determine the null mutant phenotype, we generatedframeshift mutations and found, in confirmation of results on another recently identified frameshift mutation, that such mutants have a phenotype identical to the most severe splicing mutant (ino-1), with effects specific to outer integument development. We show that the altered protein of an ino mRNA splicing mutant with a less severe phenotype (ino-4) does not have INO activity, and the mutant is partial because it produces a small amount of correctly spliced INO mRNA. Screening for suppressors of ino-4 in a fast neutron-mutagenized population identified a translocated duplication of the ino-4 gene, leading to an increase in the amount of this mRNA. The increased expression led to a decrease in the severity of the mutant effects, indicating that the amount of INO activity quantitatively regulates outer integument growth. The results further confirm that the role of INO in Arabidopsis development is specific to the outer integument of ovules where it quantitatively affects the growth of this structure.

Ovule cell wall composition is a maternal determinant of grain size in barley.

In cereal species, seed and grain size is influenced by growth of the ovule integuments (seed coat), the spikelet hull (lemma and palea) and the filial endosperm. It has remained unclear whether a highly conserved ovule tissue, the nucellus, has any impact on grain size. Immunolabelling revealed that the barley nucellus comprises two distinct cell types that differ in terms of cell wall homogalacturonan (HG) accumulation. Transcriptional profiling of the nucellus identified two pectin methylesterase genes, OVULE PECTIN MODIFIER 1 (OPM1) and OPM2, which are expressed in the ovule but absent from the seed. Ovules from an opm1 opm2 mutant, and plants expressing an ovule-specific pectin methylesterase inhibitor (PMEI), exhibit reduced HG accumulation. This results in changes to ovule cell size and shape, and ovules that are longer than wild-type controls. At grain maturity, this is manifested as significantly longer grain. These findings indicate that cell wall composition during ovule development acts to limit ovule and seed growth. The investigation of ovule PME and PMEI activity reveals an unexpected role of maternal tissues in controlling grain growth prior to fertilisation, one that has been lacking from models exploring improvements in grain size.

Novelties in the floral morphoanatomy of three Psychotria L. species (Rubiaceae)

The Psychotrieae alliance is a monophyletic group belonging to the Rubiaceae family, composed of nine tribes, with a problematic internal delimitation. The genus Psychotria L. is the most representative in the alliance, with about 2,000 species, and in the current circumscription is considered paraphyletic. Since Psychotria is morphologically uniform, the accurate circumscription of the genus is difficult to establish due to the large number of species and the absence of diagnostic characters. In this context, we studied the floral morphoanatomy of Psychotria leiocarpa Cham. & Schltdl., Psychotria carthagenensis Jacq., and Psychotria brachyceras Mull. Arg., aiming to provide informative characteristics for ongoing phylogenetic studies within the genus and the alliance. Our results showed that all species have cymose and terminal inflorescences, forming a dichasium. The anatomical structure of the floral organs demonstrated potentially informative traits, including: the presence of a set of secretory cells in the petals, raphides in the ovule integument, and papillose cells in the outer epidermis of the ovary in P. brachyceras; the presence of prismatic crystals in the ovary of P. brachyceras and P. leiocarpa; as well as the presence of non-glandular trichomes in the outer epidermis of the ovary, presence of phenolic compounds, and absence of prismatic crystals in all floral organs of P. carthagensis. Regarding the other tribes of the alliance, our results indicate that the traits related to the gynoecium seem to have more relevance in superior taxonomic categories, such as the structure of the ovary, number of ovules per locule, placentation, ovule curvature and presence/absence of an obturator. This is the first comprehensive attempt to clearly identify informative floral morphoanatomical traits among the three species of the genus. Psychotria L.

Morphological and Structural Details of Tomato Seed Coat Formation: A Different Functional Role of the Inner and Outer Epidermises in Unitegmic Ovule

In order to understand how and what structures of the tomato ovule with a single integument form the seed coat of a mature seed, a detailed study of the main development stages of the tomato ovule integument was carried out using the methods of light and electron microscopy. The integument itself it was shown to transform in the course of development into the coat (skin) of a mature seed, but the outer and inner epidermises of the integument and some layers of the integument parenchyma are mainly involved in this process. The outer epidermis cells are highly modified in later stages; their walls are thickened and lignified, creating a unique relatively hard outer coat. The fate of the inner epidermis of integument is completely different. It is separated from the other parenchyma cells of integument and is transformed into an independent new secretory tissue, an endothelium, which fences off the forming embryo and endosperm from the death zone. Due to the secretory activity of the endothelium, the dying inner parenchyma cells of the integument are lysed. Soon after the cuticle covers the endosperm, the lysis of dead integument cells stops and their flattened remnants form dense layers, which then enter the final composition of the coat of mature tomato seed. The endothelium itself returns to the location of the integument inner epidermis.

The role of pollination in controlling Ginkgo biloba ovule development.

Summary Generally, in gymnosperms, pollination and fertilization events are temporally separated and the developmental processes leading the switch from ovule integument into seed coat are still unknown. The single ovule integument of Ginkgo biloba acquires the typical characteristics of the seed coat long before the fertilization event. In this study, we investigated whether pollination triggers the transformation of the ovule integument into the seed coat.Transcriptomics and metabolomics analyses performed on ovules just prior and after pollination lead to the identification of changes occurring in Ginkgo ovules during this specific time.A morphological atlas describing the developmental stages of ovule development is presented. The metabolic pathways involved in the lignin biosynthesis and in the production of fatty acids are activated upon pollination, suggesting that the ovule integument starts its differentiation into a seed coat before the fertilization.Omics analyses allowed an accurate description of the main changes that occur in Ginkgo ovules during the pollination time frame, suggesting the crucial role of the pollen arrival on the progression of ovule development.

Fleshy Structures Associated with Ovule Protection and Seed Dispersal in Gymnosperms: A Systematic and Evolutionary Overview

Fleshy structures associated with the ovule/seed arose independently several times during gymnosperm evolution. Fleshy structures are linked to ovule/seed protection and dispersal, and are present in all the four lineages of extant gymnosperms. The ontogenetic origin of the fleshy structures could be different, and spans from the ovule funiculus in the Taxus baccata aril, the ovule integument in Ginkgo biloba, to modified bracts as in case of Ephedra species. This variability in ontogeny is reflected in the morphology and characteristics that these tissues display among the different species. This review aims to provide a complete overview of these ovule/seed-associated fleshy structures in living gymnosperms, reporting detailed descriptions for every genus. The evolution of these independently evolved structures is still unclear, and different hypotheses have been presented—protection for the seeds, protection to desiccation—each plausible but no one able to account for all their independent origins. Our purpose is to offer an extensive discussion on these fleshy structures, under different points of view (morphology, evolution, gene involvement), to stimulate further studies on their origin and evolution on both ecological and molecular levels.

Guarding tomato fruit setting in adverse temperatures through the miRNA166-SlHB15A regulatory module

Guarding tomato fruit setting in adverse temperatures through the miRNA166-SlHB15A regulatory module

Mesozoic cupules and the origin of the angiosperm second integument.

The second integument of the angiosperm ovule is unique among seed plants, with developmental genetics that are distinct from those of the inner integument1. Understanding how the second integument should be compared to structures in other seed plants is therefore crucial to resolving the long-standing question of the origin of angiosperms2-6. Attention has focused on several extinct plants with recurved cupules that are reminiscent of the anatropous organization of the basic bitegmic ovules of angiosperms1-6, but interpretations have been hampered by inadequate information on the relevant fossils. Here we describe abundant exceptionally well-preserved recurved cupules from a newly discovered silicified peat dating to the Early Cretaceous epoch (around 125.6million years ago) in Inner Mongolia, China. The new material, combined with re-examination of potentially related fossils, indicates that the recurved cupules of several groups of Mesozoic plants are all fundamentally comparable, and that their structure is consistent with the recurved form and development of the second integument in the bitegmic anatropous ovules of angiosperms. Recognition of these angiosperm relatives (angiophytes) provides a partial answer to the question of angiosperm origins, will help to focus future work on seed plant phylogenetics and has important implications for ideas on the origin of the angiosperm carpel.

A tale of two morphs: developmental patterns and mechanisms of seed coat differentiation in the dimorphic diaspore model Aethionema arabicum (Brassicaceae)

SUMMARYThe developmental transition from a fertilized ovule to a dispersed diaspore (seed or fruit) involves complex differentiation processes of the ovule's integuments leading to the diversity in mature seed coat structures in angiosperms. In this study, comparative imaging and transcriptome analysis were combined to investigate the morph‐specific developmental differences during outer seed coat differentiation and mucilage production in Aethionema arabicum, the Brassicaceae model for diaspore dimorphism. One of the intriguing adaptations of this species is the production and dispersal of morphologically distinct, mucilaginous and non‐mucilaginous diaspores from the same plant (dimorphism). The dehiscent fruit morph programme producing multiple mucilaginous seed diaspores was used as the default trait combination, similar to Arabidopsis thaliana, and was compared with the indehiscent fruit morph programme leading to non‐mucilaginous diaspores. Synchrotron‐based radiation X‐ray tomographic microscopy revealed a co‐ordinated framework of morph‐specific early changes in internal anatomy of developing A. arabicum gynoecia including seed abortion in the indehiscent programme and mucilage production by the mucilaginous seed coat. The associated comparative analysis of the gene expression patterns revealed that the unique seed coat dimorphism of Ae. arabicum provides an excellent model system for comparative study of the control of epidermal cell differentiation and mucilage biosynthesis by the mucilage transcription factor cascade and their downstream cell wall and mucilage remodelling genes. Elucidating the underlying molecular framework of the dimorphic diaspore syndrome is key to understanding differential regulation of bet‐hedging survival strategies in challenging environments, timely in the face of global climatic change.

Deciphering the evolution of the ovule genetic network through expression analyses in Gnetum gnemon.

The ovule is a synapomorphy of all seed plants (gymnosperms and angiosperms); however, there are some striking differences in ovules among the major seed plant lineages, such as the number of integuments or the orientation of the ovule. The genetics involved in ovule development have been well studied in the model species Arabidopsis thaliana, which has two integuments and anatropous orientation. This study is approached from what is known in arabidopsis, focusing on the expression patterns of homologues of four genes known to be key for the proper development of the integuments in arabidopsis: AINTEGUMENTA (ANT), BELL1, (BEL1), KANADIs (KANs) and UNICORN (UCN). We used histology to describe the morphoanatomical development from ovules to seeds in Gnetum gnemon. We carried out spatiotemporal expression analyses in G. gnemon, a gymnosperm, which has a unique ovule morphology with an integument covering the nucellus, two additional envelopes where the outermost becomes fleshy as the seed matures, and an orthotropous orientation. Our anatomical and developmental descriptions provide a framework for expression analyses in the ovule of G. gnemon. Our expression results show that although ANT, KAN and UCN homologues are expressed in the inner integument, their spatiotemporal patterns differ from those found in angiosperms. Furthermore, all homologues studied here are expressed in the nucellus, revealing major differences in seed plants. Finally, no expression of the studied homologues was detected in the outer envelopes. Altogether, these analyses provide significant comparative data that allows us to better understand the functional evolution of these gene lineages, providing a compelling framework for evolutionary and developmental studies of seeds. Our findings suggest that these genes were most likely recruited from the sporangium development network and became restricted to the integuments of angiosperm ovules.

The miR166–SlHB15A regulatory module controls ovule development and parthenocarpic fruit set under adverse temperatures in tomato

Fruit set is inhibited by adverse temperatures, with consequences on yield. We isolated a tomato mutant producing fruits under non-permissive hot temperatures and identified the causal gene as SlHB15A, belonging to class III homeodomain leucine-zipper transcription factors. SlHB15A loss-of-function mutants display aberrant ovule development that mimics transcriptional changes occurring in fertilized ovules and leads to parthenocarpic fruit set under optimal and non-permissive temperatures, in field and greenhouse conditions. Under cold growing conditions, SlHB15A is subjected to conditional haploinsufficiency and recessive dosage sensitivity controlled by microRNA 166 (miR166). Knockdown of SlHB15A alleles by miR166 leads to a continuum of aberrant ovules correlating with parthenocarpic fruit set. Consistent with this, plants harboring an Slhb15a–miRNA166-resistant allele developed normal ovules and were unable to set parthenocarpic fruit under cold conditions. DNA affinity purification sequencing and RNA-sequencing analyses revealed that SlHB15A is a bifunctional transcription factor expressed in the ovule integument. SlHB15A binds to the promoters of auxin-related genes to repress auxin signaling and to the promoters of ethylene-related genes to activate their expression. A survey of tomato genetic biodiversity identified pat and pat-1, two historical parthenocarpic mutants, as alleles of SlHB15A. Taken together, our findings demonstrate the role of SlHB15A as a sentinel to prevent fruit set in the absence of fertilization and provide a mean to enhance fruiting under extreme temperatures.