Transition from two to one integument in Prunus species: expression pattern of INNER NO OUTER (INO), ABERRANT TESTA SHAPE (ATS) and ETTIN (ETT)

A model worker: Multifaceted modulation of AUXIN RESPONSE FACTOR3 orchestrates plant reproductive phases

The key phytohormone auxin is involved in practically every aspect of plant growth and development. Auxin regulates these processes by controlling gene expression through functionally distinct AUXIN RESPONSE FACTORs (ARFs). As a noncanonical ARF, ARF3/ETTIN (ETT) mediates auxin responses to orchestrate multiple developmental processes during the reproductive phase. The arf3 mutation has pleiotropic effects on reproductive development, causing abnormalities in meristem homeostasis, floral determinacy, phyllotaxy, floral organ patterning, gynoecium morphogenesis, ovule development, and self-incompatibility. The importance of ARF3 is also reflected in its precise regulation at the transcriptional, posttranscriptional, translational, and epigenetic levels. Recent studies have shown that ARF3 controls dynamic shoot apical meristem (SAM) maintenance in a non-cell autonomous manner. Here, we summarize the hierarchical regulatory mechanisms by which ARF3 is regulated and the diverse roles of ARF3 regulating developmental processes during the reproductive phase.

BELL1 interacts with CRABS CLAW and INNER NO OUTER to regulate ovule and seed development in pomegranate

Pomegranate (Punica granatum) flowers are classified as bisexual flowers and functional male flowers. Functional male flowers have sterile pistils that show abnormal ovule development. In previous studies, we identified INNER NO OUTER (INO), CRABS CLAW (CRC), and BELL1 (BEL1), which were specifically expressed in bisexual and functional male flowers. However, the functions of ovule identity genes and the mechanism underlying ovule sterility in pomegranate remain unknown. Here, we found that the integument primordia formed and then ceased developing in the ovules of functional male flowers with a vertical diameter of 8.1-13.0 mm. Megaspore mother cells were observed in bisexual flowers when the vertical diameters of flowers were 10.1-13.0 mm, but not in functional male flowers. We analyzed the expression patterns of ovule-related genes in pomegranate ovule sterility and found that PgCRC mRNA was highly expressed at a critical stage of ovule development in bisexual flowers. Ectopic expression of PgCRC and PgINO was sufficient to increase seed number in transgenic lines. PgCRC partially complemented the Arabidopsis (Arabidopsis thaliana) crc mutant, and PgINO successfully rescued the seeds set in the Arabidopsis ino mutant. The results of yeast two-hybrid assays, bimolecular fluorescence complementation assays, and genetic data analyses showed that PgCRC and PgINO directly interact with PgBEL1. Our results also showed that PgCRC and PgINO could not interact directly with MADS-box proteins and that PgBEL1 interacted with SEPALLATA proteins. We report the function of PgCRC and PgINO in ovule and seed development and show that PgCRC and PgINO interact with PgBEL1. Thus, our results provide understanding of the genetic regulatory networks underlying ovule development in pomegranate.

Does integument arise de novo or from pre-existing structures? ── Insights from the key regulatory genes controlling integument development

The origin of seeds is one of the key innovations in land plant evolution. Ovules are the developmental precursors of seeds. The integument is the envelope structure surrounding the nucellus within the ovule and developing into the seed coat when ovules mature upon fertilization. The question of whether the integument arise de novo or evolve from elaboration of pre-existing structures has caused much debate. By exploring the origin and evolution of the key regulatory genes controlling integument development and their functions during both individual and historical developmental processes, we showed the widespread presence of the homologs of ANT, CUC, BEL1, SPL, C3HDZ, INO, ATS, and ETT in seedless plant genomes. All of these genes have undergone duplication-divergence events in their evolutionary history, with most of the descendant paralogous suffering motif gain and/or loss in the coding regions. Expression and functional characterization have shown that these genes are key components of the genetic program that patterns leaf-like lateral organs. Serial homology can thus be postulated between integuments and other lateral organs in terms of the shared master regulatory genes. Given that the genetic program patterning leaf-like lateral organs formed in seedless plants, and was reused during seed origin, the integument is unlikely to arise de novo but evolved from the stem segment-specific modification of pre-existing serially homologous structures. The master 'switches' trigging the modification to specify the integument identity remain unclear. We propose a successive transformation model of integument origin.

Formation of Unique Placental Seed Capsules in the Maturation Process of the Tomato Fruit

The morphological and anatomical study of the seed formation features in a juicy tomato fruit was carried out. The ovules, which form on the placenta, have been shown to be gradually enveloped by the protrusions of placental tissue that arises simultaneously with them. As a result of this process, each seed is enclosed in an individual capsule. These seed capsules have been shown in vivo to be airtight and air-filled. Tomato seeds, as has been shown in this study, develop inside these capsules until the full maturity of the fruit and do not come into contact with the detached and moldered cells of the placenta protrusions, which convert into a gel (pulp). Using scanning electron microscopy, it was possible to reveal the details of a ribbon-like “pubescence” formation of the tomato seed, as well as to understand the mechanism of cracking of the outer layer cells in the seed coat, associated with the detection of calcium oxalate crystals in these cells. The unique outer layer of the tomato seed coat seems to play the role of a scaffold that maintains a constant volume of the protective capsule.

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.

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

BACKGROUND AND AIMS

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).

METHODS

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.

KEY RESULTS

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.

CONCLUSIONS

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.

Ovary Signals for Pollen Tube Guidance in Chalazogamous Mangifera indica L

Most flowering plants show porogamy in which the pollen tubes reach the egg apparatus through the micropyle. However, several species show chalazogamy, an unusual pollen tube growth, in which the pollen tubes reach the embryo sac through the chalaza. While ovary signals for pollen tube growth and guidance have been extensively studied in porogamous species, few studies have addressed the process in chalazogamous species such as mango (Mangifera indica L.), one of the five most important fruit crops worldwide in terms of production. In this study, we characterize pollen-pistil interaction in mango, paying special attention to three key players known to be involved in the directional pollen tube growth of porogamous species such as starch, arabinogalactan proteins (AGPs), and γ-aminobutyric acid (GABA). Starch grains were observed in the style and in the ponticulus at anthesis, but their number decreased 1 day after anthesis. AGPs, revealed by JIM8 and JIM13 antibodies, were homogenously observed in the style and ovary, but were more conspicuous in the nucellus around the egg apparatus. GABA, revealed by anti-GABA antibodies, was specifically observed in the transmitting tissue, including the ponticulus. Moreover, GABA was shown to stimulate in vitro mango pollen tube elongation. The results support the heterotrophic growth of mango pollen tubes in the style at the expense of starch, similarly to the observations in porogamous species. However, unlike porogamous species, the micropyle of mango does not show high levels of GABA and starch, although they were observed in the ponticulus and could play a role in supporting the unusual pollen tube growth in chalazogamous species.

Functional conservation of the grapevine candidate gene INNER NO OUTER for ovule development and seed formation

Seedlessness represents a highly appreciated trait in table grapes. Based on an interesting case of seedless fruit production described in the crop species Annona squamosa, we focused on the Vitis vinifera INNER NO OUTER (INO) gene as a candidate. This gene encodes a transcription factor belonging to the YABBY family involved in the determination of abaxial identity in several organs. In Arabidopsis thaliana, this gene was shown to be essential for the formation and asymmetric growth of the ovule outer integument and its mutation leads to a phenotypic defect of ovules and failure in seed formation. In this study, we identified in silico the V. vinifera orthologue and investigated its phylogenetic relationship to INO genes from other species and its expression in different organs in seeded and seedless varieties. Applying cross-species complementation, we have tested its functionality in the Arabidopsis ino-1 mutant. We show that the V. vinifera INO successfully rescues the ovule outer integument growth and seeds set and also partially complements the outer integument asymmetric growth in the Arabidopsis mutant, differently from orthologues from other species. These data demonstrate that VviINO retains similar activity and protein targets in grapevine as in Arabidopsis. Potential implications for grapevine breeding are discussed.

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.

Phylogenetic analyses of key developmental genes provide insight into the complex evolution of seeds

Gene duplication plays a decisive role in organismal diversification and in the appearance of novel structures. In plants the megagametophyte covered by the integuments, which after fertilization becomes the seed constitutes a novel structure: the ovule. In Arabidopsis thaliana, genetic mechanisms regulating ovule development, including the genetics underlying ovule initiation, ovule patterning and integument development, have been identified. Among seed plants, integuments are not only a novelty in evolution, but integuments also present an enormous morphological variation. This study is focused on the evolution of gene families that play a role in the proper morphological development of the integuments, BELL1 (BEL1), KANADIs (KAN1, KAN2, and KAN4/ATS), UNICORN (UCN) and SHORT INTEGUMENTS1 (SIN1). In Arabidopsis, BEL1 establishes the initiation of integument development. KAN1 and 2 act in the proper development of the outer integument. While ABERRANT TESTA SHAPE (ATS), is involved in the correct separation of both integuments. UCN acts in planar growth of the outer integument repressing ATS. SIN1 is involved in cell elongation in the integuments. The results of our analyses show that each of these genes has a different evolutionary history and that while gymnosperms appear to have a simpler ovule morphology, they have more homologues of these candidate genes than angiosperms. In addition, we present the conserved and novel motifs for each of these genes among seed plants and their selection constraints, which may be related to functional changes and to the diversity of ovule morphologies.

Translating auxin responses into ovules, seeds and yield: Insight from Arabidopsis and the cereals

Grain production in cereal crops depends on the stable formation of male and female gametes in the flower. In most angiosperms, the female gamete is produced from a germline located deep within the ovary, protected by several layers of maternal tissue, including the ovary wall, ovule integuments and nucellus. In the field, germline formation and floret fertility are major determinants of yield potential, contributing to traits such as seed number, weight and size. As such, stimuli affecting the timing and duration of reproductive phases, as well as the viability, size and number of cells within reproductive organs can significantly impact yield. One key stimulant is the phytohormone auxin, which influences growth and morphogenesis of female tissues during gynoecium development, gametophyte formation, and endosperm cellularization. In this review we consider the role of the auxin signaling pathway during ovule and seed development, first in the context of Arabidopsis and then in the cereals. We summarize the gene families involved and highlight distinct expression patterns that suggest a range of roles in reproductive cell specification and fate. This is discussed in terms of seed production and how targeted modification of different tissues might facilitate improvements.

The role of the integuments in pollen tube guidance in flowering plants

In angiosperms, pollen tube entry into the ovule generally takes place through the micropyle, but the exact role of the micropyle in pollen tube guidance remains unclear. A limited number of studies have examined eudicots with bitegmic micropyles, but information is lacking in ovules of basal/early-divergent angiosperms with unitegmic micropyles. We have evaluated the role of the micropyle in pollen tube guidance in an early-divergent angiosperm (Annona cherimola) and the evolutionarily derived Arabidopsis thaliana by studying γ-aminobutyric acid (GABA) and arabinogalactan proteins (AGPs) in wild-type plants and integument-defective mutants. A conserved inhibitory role of GABA in pollen tube growth was shown in A. cherimola, in which AGPs surround the egg apparatus. In Arabidopsis, the micropyle formed only by the outer integument in wuschel-7 mutants caused a partial defect in pollen tube guidance. Moreover, pollen tubes were not observed in the micropyle of an inner no outer (ino) mutant in Arabidopsis, but were observed in homologous ino mutants in Annona. The similar distribution of GABA and AGPs observed in the micropyle of Arabidopsis and Annona, together with the anomalies from specific integument mutants, support the role of the inner integument in preventing multiple tube entrance (polytubey) in these two phylogenetically distant genera.

Development and evolution of the unique ovules of flowering plants

Ovules are the precursors to seeds and as such are critical to plant propagation and food production. Mutant studies have led to the identification of numerous genes regulating ovule development. Genes encoding transcription factors have been shown to direct ovule spacing, ovule identity and integument formation. Particular co-regulators have now been associated with activities of some of these transcription factors, and other protein families including cell surface receptors have been shown to regulate ovule development. Hormone levels and transport, especially of auxin, have also been shown to play critical roles in ovule emergence and morphogenesis and to interact with the transcriptional regulators. Ovule diversification has been studied using orthologs of regulatory genes in divergent angiosperm groups. Combining modern genetic evidence with expanding knowledge of the fossil record illuminates the possible origin of the unique bitegmic ovules of angiosperms.

Seed coat thickness in the evolution of angiosperms

The seed habit represents a remarkable evolutionary advance in plant sexual reproduction. Since the Paleozoic, seeds carry a seed coat that protects, nourishes and facilitates the dispersal of the fertilization product(s). The seed coat architecture evolved to adapt to different environments and reproductive strategies in part by modifying its thickness. Here, we review the great natural diversity observed in seed coat thickness among angiosperms and its molecular regulation in Arabidopsis.

Evidence for the Extensive Conservation of Mechanisms of Ovule Integument Development Since the Most Recent Common Ancestor of Living Angiosperms

The ovules and seeds of most angiosperm groups are enclosed by two integuments, whose evolutionary origins are considerably separated in time, as the inner integument arose over 300 million years ago (MYA) in an ancestor of all living seed plants, while the outer integument arose, perhaps as recently as 164 MYA, in an ancestor of all living angiosperms. Studies of the model angiosperm Arabidopsis thaliana indicate that the mechanisms of development of the inner and outer integuments depend on largely different sets of molecular players. However, it was not known, in most cases, whether these differences were already present in early flowering plants, or arose later in the Arabidopsis lineage. Here, we analyze the expression patterns of integument regulators in Amborella trichopoda, the likely sister to all other living angiosperms. The data obtained indicate that regulators of the YABBY, KANADI, and homeodomain-leucine zipper class III transcription factor families have largely conserved their integument-specific expression profiles in the Amborella and Arabidopsis lineages since the most recent common ancestor (MRCA) of living angiosperms. We identified only one case, involving the paralogous genes ETTIN and AUXIN RESPONSE FACTOR4, in which integument-specific expression patterns had clearly diverged between Amborella and Arabidopsis. We use the data obtained to partially reconstruct molecular mechanisms of integument development in the MRCA of living angiosperms and discuss our findings in the context of alternative hypotheses for the origin of the angiosperm outer integument.

The transition from somatic to germline identity shows conserved and specialized features during angiosperm evolution

How and why specific plant cells adopt germline identity during ovule development has proved challenging to address, and the pathways that are active in the ovules of basal/early-divergent angiosperms possessing a multilayered nucellus are still unclear. Here, we compare megasporogenesis between two early-divergent angiosperms (Annona cherimola and Persea americana) and the evolutionarily derived Arabidopsis thaliana, studying the three-dimensional spatial position of the megaspore mother cell (MMC), the compositional details of the MMC wall and the location of PIN1 expression. Specific wall polymers distinguished the central position of the MMC and its meiotic products from surrounding tissues in early-divergent angiosperms, whereas, in A. thaliana, only callose (in mature MMCs) and arabinogalactan proteins (AGPs) (in megaspores) distinguished the germline. However, PIN1 expression, which regulates polar auxin transport, was observed around the MMC in the single-layer nucellus of A. thaliana and in the multilayered nucellus of A. cherimola, or close to the MMC in P. americana. The data reveal a similar microenvironment in relation to auxin during megasporogenesis in all three species. However, the different wall polymers that mark MMC fate in early-divergent angiosperms may reflect a specific response to mechanical stress during differentiation, or the specific recruitment of polymers to sustain MMC growth.

Conservation of the role of INNER NO OUTER in development of unitegmic ovules of the Solanaceae despite a divergence in protein function

BACKGROUND

The INNER NO OUTER (INO) gene is expressed in the outermost cell layer of the outer integument of bitegmic ovules and is essential for this organ's growth. The role and cross-species functional conservation of INO orthologs were examined in members of the Solanaceae, which have unitegmic ovules. Unitegmy has evolved several times in disparate angiosperm lineages. INO expression has been observed in the outermost cell layers of all examined unitegmic ovules, but the functional role of INO in unitegmic ovules has not previously been evaluated.

RESULTS

INO orthologs were unambiguously identified in tobacco and tomato by sequence homology. Expression of the tomato INO gene was limited to the outer cell layer of the single integument indicating that this single integument has properties of the outer integument. Expression occurred only after integument initiation, later than observed in ovules of other examined angiosperms. Virus-induced knock-down of expression of the INO ortholog in tobacco inhibited growth of the outer cell layer of the integument leading to a decrease in both integument extension and curvature of the ovule. The altered ovules closely resemble those of the aberrant testa shape (ats) ino mutant combination in Arabidopsis where we see the effect of the ino mutation on a single fused integument produced by the ats mutation. Despite significant sequence identity and similar expression patterns, the tomato INO coding region was not able to complement the Arabidopsis ino mutant.

CONCLUSIONS

The similarity of effects of ino mutations on the unitegmic ovules of tobacco and the fused integuments of the Arabidopsis ats mutant show that: 1) INO orthologs play the same role in promoting integument growth in ovules of tobacco and Arabidopsis; and 2) the unitegmic ovules of tobacco (and hence other solanaceous species) are most likely the result of a congenital fusion of two ancestral integuments. Our results further indicate that INO has a conserved role in growth of the outermost cell layer of integuments. The curvature of solanaceous ovules is driven by unequal growth of the outer layers of the single integument that likely correspond to an ancestral outer integument.

The Diversity of the Pollen Tube Pathway in Plants: Toward an Increasing Control by the Sporophyte

Plants, unlike animals, alternate multicellular diploid, and haploid generations in their life cycle. While this is widespread all along the plant kingdom, the size and autonomy of the diploid sporophyte and the haploid gametophyte generations vary along evolution. Vascular plants show an evolutionary trend toward a reduction of the gametophyte, reflected both in size and lifespan, together with an increasing dependence from the sporophyte. This has resulted in an overlooking of the importance of the gametophytic phase in the evolution of higher plants. This reliance on the sporophyte is most notorious along the pollen tube journey, where the male gametophytes have to travel a long way inside the sporophyte to reach the female gametophyte. Along evolution, there is a change in the scenery of the pollen tube pathway that favors pollen competition and selection. This trend, toward apparently making complicated what could be simple, appears to be related to an increasing control of the sporophyte over the gametophyte with implications for understanding plant evolution.