Pattern formation during early ovule development in Arabidopsis thaliana

The evolution and development of sporangia-The fundamental reproductive organ of land plant sporophytes

A key innovation of land plants is the origin and evolution of the sporangium, the fundamental reproductive structure of the diploid sporophyte. In vascular plants, whether the structure is a cone, fertile leaf, or flower-all are clusters of sporangia. The evolution of morphologically distinct sporangia (heterospory) and retention of the gametophyte evolved three times independently as a prerequisite for the evolution of seeds. This review summarizes the development of vascular plant sporangia, molecular genetics of angiosperm sporangia, and provides a framework to investigate evolution and development in vascular plant sporangia.

ALTERED MERISTEM PROGRAM1 sustains cellular differentiation by limiting HD-ZIP III transcription factor gene expression

Plants show remarkable developmental and regenerative plasticity through the sustained activity of stem cells in meristems. Under certain conditions, pluripotency can even be reestablished in cells that have already entered differentiation. Mutation of the putative carboxypeptidase ALTERED MERISTEM PROGRAM1 (AMP1) in Arabidopsis (Arabidopsis thaliana) causes a set of hypertrophic phenotypes, indicating a defect in the suppression of pluripotency. A role of AMP1 in the miRNA-mediated inhibition of translation has previously been reported; however, how this activity is related to its developmental functions is unclear. Here, we examined the functional interaction between AMP1 and the Class III homeodomain-leucine zipper (HD-ZIP III) transcription factors, which are miRNA-controlled determinants of shoot meristem specification. We found that the HD-ZIP III transcriptional output is enhanced in the amp1 mutant and that plant lines with increased HD-ZIP III activity not only developed amp1 mutant-like phenotypes but also showed a synergistic genetic interaction with the mutant. Conversely, the reduction of HD-ZIP III function suppressed the shoot hypertrophy defects of the amp1 mutant. We further provide evidence that the expression domains of HD-ZIP III family members are expanded in the amp1 mutant and that this misexpression occurs at the transcriptional level and does not involve the function of miRNA165/166. Finally, amp1 mutant-specific phenotypes cannot be mimicked by a general inhibition of miRNA function in the AMP1 expression domain. These findings lead us to a model in which AMP1 restricts cellular pluripotency upstream of HD-ZIP III proteins, and this control appears to be not directly mediated by the canonical miRNA pathway.

Germline β-1,3-glucan deposits are required for female gametogenesis in Arabidopsis thaliana

Correct regulation of intercellular communication is a fundamental requirement for cell differentiation. In Arabidopsis thaliana, the female germline differentiates from a single somatic ovule cell that becomes encased in β-1,3-glucan, a water insoluble polysaccharide implicated in limiting pathogen invasion, regulating intercellular trafficking in roots, and promoting pollen development. Whether β-1,3-glucan facilitates germline isolation and development has remained contentious, since limited evidence is available to support a functional role. Here, transcriptional profiling of adjoining germline and somatic cells revealed differences in gene expression related to β-1,3-glucan metabolism and signalling through intercellular channels (plasmodesmata). Dominant expression of a β-1,3-glucanase in the female germline transiently perturbed β-1,3-glucan deposits, allowed intercellular movement of tracer molecules, and led to changes in germline gene expression and histone marks, eventually leading to termination of germline development. Our findings indicate that germline β-1,3-glucan fulfils a functional role in the ovule by insulating the primary germline cell, and thereby determines the success of downstream female gametogenesis.

Transcriptome analyses of leaf architecture in Sansevieria support a common genetic toolkit in the parallel evolution of unifacial leaves in monocots

Planar structures dramatically increase the surface-area-to-volume ratio, which is critically important for multicellular organisms. In this study, we utilize naturally occurring phenotypic variation among three Sansivieria species (Asperagaceae) to investigate leaf margin expression patterns that are associated with mediolateral and adaxial/abaxial development. We identified differentially expressed genes (DEGs) between center and margin leaf tissues in two planar-leaf species Sansevieria subspicata and Sansevieria trifasciata and compared these with expression patterns within the cylindrically leaved Sansevieria cylindrica. Two YABBY family genes, homologs of FILAMENTOUS FLOWER and DROOPING LEAF, are overexpressed in the center leaf tissue in the planar-leaf species and in the tissue of the cylindrical leaves. As mesophyll structure does not indicate adaxial versus abaxial differentiation, increased leaf thickness results in more water-storage tissue and enhances resistance to aridity. This suggests that the cylindrical-leaf in S. cylindrica is analogous to the central leaf tissue in the planar-leaf species. Furthermore, the congruence of the expression patterns of these YABBY genes in Sansevieria with expression patterns found in other unifacial monocot species suggests that patterns of parallel evolution may be the result of similar solutions derived from a limited developmental toolbox.

The EPFL-ERf-SERK signaling controls integument development in Arabidopsis

As the seed precursor, the ovule produces the female gametophyte (or embryo sac), and the subsequent double fertilization occurs in it. The integuments emerge sequentially from the integument primordia at the early stages of ovule development and finally enwrap the embryo sac gradually during gametogenesis, protecting and nursing the embryo sac. However, the mechanisms regulating integument development are still obscure. In this study, we show that SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASES (SERKs) play essential roles during integument development in Arabidopsis thaliana. The serk1/2/3 triple mutant shows arrested integuments and abnormal embryo sacs, similar defects also found in the triple loss-of-function mutants of ERECTA family (ERf) genes. Ovules of serk1/2/3 er erl1/2 show defects similar to er erl1/2 and serk1/2/3. Results of yeast two-hybrid analyses, bimolecular fluorescence complementation (BiFC) analyses, and co-immunoprecipitation assays demonstrated that SERKs interact with ERf, which depends on EPIDERMAL PATTERNING FACTOR-LIKE (EPFL) family small peptides. The sextuple mutant epfl1/2/3/4/5/6 shows integument defects similar to both of er erl1/2 and serk1/2/3. Our results demonstrate that ERf-SERK-mediated EPFL signaling orchestrates the development of the female gametophyte and the surrounding sporophytic integuments.

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 generated frameshift 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.

Transcription factors KNAT3 and KNAT4 are essential for integument and ovule formation in Arabidopsis

Integuments form important protective cell layers surrounding the developing ovules in gymno- and angiosperms. Although several genes have been shown to influence the development of integuments, the transcriptional regulatory mechanism is still poorly understood. In this work, we report that the Class II KNOTTED1-LIKE HOMEOBOX (KNOX II) transcription factors KNOTTED1-LIKE HOMEBOX GENE 3 (KNAT3) and KNAT4 regulate integument development in Arabidopsis (Arabidopsis thaliana). KNAT3 and KNAT4 were co-expressed in inflorescences and especially in young developing ovules. The loss-of-function double mutant knat3 knat4 showed an infertility phenotype, in which both inner and outer integuments of the ovule are arrested at an early stage and form an amorphous structure as in the bell1 (bel1) mutant. The expression of chimeric KNAT3- and KNAT4-EAR motif repression domain (SRDX repressors) resulted in severe seed abortion. Protein-protein interaction assays demonstrated that KNAT3 and KNAT4 interact with each other and also with INNER NO OUTER (INO), a key transcription factor required for the outer integument formation. Transcriptome analysis showed that the expression of genes related with integument development is influenced in the knat3 knat4 mutant. The knat3 knat4 mutant also had a lower indole-3-acetic acid (IAA) content, and some auxin signaling pathway genes were downregulated. Moreover, transactivation analysis indicated that KNAT3/4 and INO activate the auxin signaling gene IAA INDUCIBLE 14 (IAA14). Taken together, our study identified KNAT3 and KNAT4 as key factors in integument development in Arabidopsis.

Pivotal role of STIP in ovule pattern formation and female germline development in Arabidopsis thaliana

In spermatophytes the sporophytic (diploid) and the gametophytic (haploid) generations co-exist in ovules, and the coordination of their developmental programs is of pivotal importance for plant reproduction. To achieve efficient fertilization, the haploid female gametophyte and the diploid ovule structures must coordinate their development to form a functional and correctly shaped ovule. WUSCHEL-RELATED HOMEOBOX (WOX) genes encode a family of transcription factors that share important roles in a wide range of processes throughout plant development. Here, we show that STIP is required for the correct patterning and curvature of the ovule in Arabidopsis thaliana. The knockout mutant stip-2 is characterized by a radialized ovule phenotype due to severe defects in outer integument development. In addition, alteration of STIP expression affects the correct differentiation and progression of the female germline. Finally, our results reveal that STIP is required to tightly regulate the key ovule factors INNER NO OUTER, PHABULOSA and WUSCHEL, and they define a novel genetic interplay in the regulatory networks determining ovule development.

AP1G2 Affects Mitotic Cycles of Female and Male Gametophytes in Arabidopsis

During sexual reproduction in flowering plants, haploid spores are formed from meiosis of spore mother cells. The spores then undergo mitosis, develop into female and male gametophytes, and give rise to seeds after fertilization. We identified a female sterile mutant ap1g2-4 from EMS mutagenesis, and analyses of two T-DNA insertion mutants, ap1g2-1 ^+/- and ap1g2-3 ^-/-, and detected a partial female and male sterility. The ap1g2 mutant gametophyte development was arrested at one nuclear stage. A complementation test using a genomic sequence of AP1G2 with its native promoter restored the function in the three ap1g2 mutant lines. Transcriptome profiling of ap1g2 ovules revealed that four genes encoding clathrin assembly proteins PICALM5A/B and PICALM9A/B, which were involved in endocytosis, were downregulated, which were confirmed to interact with AP1G2 through yeast two-hybrid assays and BIFC analysis. Our result also demonstrated that RALFL4-8-15-19-26 CML16 and several calcium-dependent protein kinases, including CPK14-16-17, were all downregulated in the ovules of ap1g2-1 ^+/-. Moreover, Ca²⁺ concentration was low in impaired gametophytes. Therefore, we proposed that through interaction with PICALM5A/B and PICALM9A/B, AP1G2 may mediate gametogenesis accompanied by Ca²⁺ signaling in Arabidopsis. Our findings revealed a crucial role of AP1G2 in female and male gametogenesis in Arabidopsis and enhanced our understanding of the molecular mechanisms underpinning sexual reproduction in flowering plants.

The annotation and analysis of complex 3D plant organs using 3DCoordX

A fundamental question in biology concerns how molecular and cellular processes become integrated during morphogenesis. In plants, characterization of 3D digital representations of organs at single-cell resolution represents a promising approach to addressing this problem. A major challenge is to provide organ-centric spatial context to cells of an organ. We developed several general rules for the annotation of cell position and embodied them in 3DCoordX, a user-interactive computer toolbox implemented in the open-source software MorphoGraphX. 3DCoordX enables rapid spatial annotation of cells even in highly curved biological shapes. Using 3DCoordX, we analyzed cellular growth patterns in organs of several species. For example, the data indicated the presence of a basal cell proliferation zone in the ovule primordium of Arabidopsis (Arabidopsis thaliana). Proof-of-concept analyses suggested a preferential increase in cell length associated with neck elongation in the archegonium of Marchantia (Marchantia polymorpha) and variations in cell volume linked to central morphogenetic features of a trap of the carnivorous plant Utricularia (Utricularia gibba). Our work demonstrates the broad applicability of the developed strategies as they provide organ-centric spatial context to cellular features in plant organs of diverse shape complexity.

Big Role of Small RNAs in Female Gametophyte Development

In living organisms, sexual reproduction relies on the successful development of the gametes. Flowering plants produce gametes in the specialized organs of the flower, the gametophytes. The female gametophyte (FG), a multicellular structure containing female gametes (egg cell and central cell), is often referred to as an embryo sac. Intriguingly, several protein complexes, molecular and genetic mechanisms participate and tightly regulate the female gametophyte development. Recent evidence indicates that small RNA (sRNA) mediated pathways play vital roles in female gametophyte development and specification. Here, we present an insight into our understanding and the recent updates on the molecular mechanism of different players of small RNA-directed regulatory pathways during ovule formation and growth.

Interspecific complementation-restoration of phenotype in Arabidopsis cuc2cuc3 mutant by sugarcane CUC2 gene

BACKGROUND

In plants, a critical balance between differentiation and proliferation of stem cells at the shoot apical meristem zone is essential for proper growth. The spatiotemporal regulation of some crucial genes dictates the formation of a boundary within and around budding organs. The boundary plays a pivotal role in distinguishing one tissue type from another and provides a defined shape to the organs at their developed stage. NAM/CUC subfamily of the NAC transcription factors control the boundary formation during meristematic development.

RESULTS

Here, we have identified the CUP-SHAPED COTYLEDON (CUC) genes in sugarcane and named SsCUC2 (for the orthologous gene of CUC1 and CUC2) and SsCUC3. The phylogenetic reconstruction showed that SsCUCs occupy the CUC2 and CUC3 clade together with monocots, whereas eudicot CUC2 and CUC3 settled separately in the different clade. The structural analysis of CUC genes showed that most of the CUC3 genes were accompanied by an intron gain during eudicot divergence. Besides, the study of SsCUCs expression in the RNA-seq obtained during different stages of ovule development revealed that SsCUCs express in developing young tissues, and the expression of SsCUC2 is regulated by miR164. We also demonstrate that SsCUC2 (a monocot) could complement the cuc2cuc3 mutant phenotype of Arabidopsis (eudicot).

CONCLUSIONS

This study further supports that CUC2 has diverged in CUC1 and CUC2 during the evolution of monocots and eudicots from ancestral plants. The functional analysis of CUC expression patterns during sugarcane ovule development and ectopic expression of SsCUC2 in Arabidopsis showed that SsCUC2 has a conserved role in boundary formation. Overall, these findings improve our understanding of the functions of sugarcane CUC genes. Our results reveal the crucial functional role of CUC genes in sugarcane.

The emerging role of small RNAs in ovule development, a kind of magic

In plants, small RNAs have been recognized as key genetic and epigenetic regulators of development. Small RNAs are usually 20 to 30 nucleotides in length and they control, in a sequence specific manner, the transcriptional or post-transcriptional expression of genes. In this review, we present a comprehensive overview of the most recent findings about the function of small RNAs in ovule development, including megasporogenesis and megagametogenesis, both in sexual and apomictic plants. We discuss recent studies on the role of miRNAs, siRNAs and trans-acting RNAs (ta-siRNAs) in early female germline differentiation. The mechanistic complexity and unique regulatory features are reviewed, and possible directions for future research are provided.

Expression analyses in Ginkgo biloba provide new insights into the evolution and development of the seed

Although the seed is a key morphological innovation, its origin remains unknown and molecular data outside angiosperms is still limited. Ginkgo biloba, with a unique place in plant evolution, being one of the first extant gymnosperms where seeds evolved, can testify to the evolution and development of the seed. Initially, to better understand the development of the ovules in Ginkgo biloba ovules, we performed spatio-temporal expression analyses in seeds at early developing stages, of six candidate gene homologues known in angiosperms: WUSCHEL, AINTEGUMENTA, BELL1, KANADI, UNICORN, and C3HDZip. Surprisingly, the expression patterns of most these ovule homologues indicate that they are not wholly conserved between angiosperms and Ginkgo biloba. Consistent with previous studies on early diverging seedless plant lineages, ferns, lycophytes, and bryophytes, many of these candidate genes are mainly expressed in mega- and micro-sporangia. Through in-depth comparative transcriptome analyses of Ginkgo biloba developing ovules, pollen cones, and megagametophytes we have been able to identify novel genes, likely involved in ovule development. Finally, our expression analyses support the synangial or neo-synangial hypotheses for the origin of the seed, where the sporangium developmental network was likely co-opted and restricted during integument evolution.

Characterization of germline development and identification of genes associated with germline specification in pineapple

Understanding germline specification in plants could be advantageous for agricultural applications. In recent decades, substantial efforts have been made to understand germline specification in several plant species, including Arabidopsis, rice, and maize. However, our knowledge of germline specification in many agronomically important plant species remains obscure. Here, we characterized the female germline specification and subsequent female gametophyte development in pineapple using callose staining, cytological, and whole-mount immunolocalization analyses. We also determined the male germline specification and gametophyte developmental timeline and observed male meiotic behavior using chromosome spreading assays. Furthermore, we identified 229 genes that are preferentially expressed at the megaspore mother cell (MMC) stage during ovule development and 478 genes that are preferentially expressed at the pollen mother cell (PMC) stage of anther development using comparative transcriptomic analysis. The biological functions, associated regulatory pathways and expression patterns of these genes were also analyzed. Our study provides a convenient cytological reference for exploring pineapple germline development and a molecular basis for the future functional analysis of germline specification in related plant species.

Dosage-Dependent Gynoecium Development and Gene Expression in Brassica napus-Orychophragmus violaceus Addition Lines

Distant hybridization usually leads to female sterility of the hybrid but the mechanism behind this is poorly understood. Complete pistil abortion but normal male fertility was shown by one Brassica napus-Orychophragmus violaceus monosomic alien addition line (MA, AACC + 1 IO, 2n = 39) produced previously. To study the effect of a single O. violaceus chromosome addition on pistil development in different genetic backgrounds, hybrids between the MA and B. carinata (BBCC), B. juncea (AABB), and two synthetic hexaploids (AABBCC) were firstly produced in this study which show complete female sterility. A microspore culture was further performed to produce the haploid monosomic alien addition line (HMA, AC + 1 IO, 2n = 20) and disomic addition line (DA, AACC + 2 IO, 2n = 40) together with haploid (H, AC, 2n = 19) and double haploid (DH, AACC, 2n = 38) plants of B. napus from MA to investigate the dosage effect of the alien O. violaceus chromosome on pistil development and gene expression. Compared to MA, the development of the pistils of DA and HMA was completely or partially recovered, in which the pistils could swell and elongate to a normal shape after open pollination, although no seeds were produced. Comparative RNA-seq analyses revealed that the numbers of the differentially expressed genes (DEGs) were significantly different, dosage-dependent, and consistent with the phenotypic difference in pairwise comparisons of HMA vs. H, DA vs. DH, MA vs. DH, MA vs. DA, and MA vs. HMA. The gene ontology (GO) enrichment analysis of DEGs showed that a number of genes involved in the development of the gynoecium, embryo sac, ovule, and integuments. Particularly, several common DEGs for pistil development shared in HMA vs. H and DA vs. DH showed functions in genotoxic stress response, auxin transport, and signaling and adaxial/abaxial axis specification. The results provided updated information for the molecular mechanisms behind the gynoecium development of B. napus responding to the dosage of alien O. violaceus chromosomes.

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.

Why does only one embryo sac develop in the Paeonia ovule with multiple archesporium?

Structure of the multiple archesporium in an ovule, time and place of archesporial cell differentiation and their developmental potential have not been studied in detail. In Paeonia species supernumerary archesporial cells are formed and differentiate as multiple megasporocytes, but only one embryo sac usually develops into an ovule. The reasons leading to development of one gametophyte and the death of most megasporocytes are unknown. The morphological structure of the multiple archesporium in Paeonia veitchii and P. caucasica was studied using cytoembryological methods. We used staining with aniline blue and fluorescence microscopy for visualization of callose on the megasporocyte walls. All cells of the ovule in investigated Paeonia species are uniform and meristematic at the earliest development stage. The onset of archesporium differentiation correlates with inner integument initiation. The sporogenous complex includes ten to 25 cells which develop asynchronously. The cell located in the central part of the sporogenous complex is differentiated into a megasporocyte earlier than in neighbouring cells. Only this megasporocyte is enveloped in callose; it develops further through to meiosis and forms a female gametophyte. The other megasporocytes degenerate during ovule development. We consider that callose participates in the mechanism of 'lateral inhibition' during megasporocyte maturation. The cell located in the central part of the Paeonia ovule is the first to receive signals that stimulate the onset of megasporogenesis and formation of the callose wall. It is possible that callose participates in blocking of development signals to neighbouring megasporocytes, leading to the arrest of their 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.

A digital 3D reference atlas reveals cellular growth patterns shaping the Arabidopsis ovule

A fundamental question in biology is how morphogenesis integrates the multitude of processes that act at different scales, ranging from the molecular control of gene expression to cellular coordination in a tissue. Using machine-learning-based digital image analysis, we generated a three-dimensional atlas of ovule development in Arabidopsis thaliana, enabling the quantitative spatio-temporal analysis of cellular and gene expression patterns with cell and tissue resolution. We discovered novel morphological manifestations of ovule polarity, a new mode of cell layer formation, and previously unrecognized subepidermal cell populations that initiate ovule curvature. The data suggest an irregular cellular build-up of WUSCHEL expression in the primordium and new functions for INNER NO OUTER in restricting nucellar cell proliferation and the organization of the interior chalaza. Our work demonstrates the analytical power of a three-dimensional digital representation when studying the morphogenesis of an organ of complex architecture that eventually consists of 1900 cells.

Comparative Transcriptomic and Metabolic Analyses Reveal the Molecular Mechanism of Ovule Development in the Orchid, Cymbidium sinense

Ovule development is pivotal to plant reproduction and seed development. Cymbidium sinense (Orchidaceae) has high ornamental value due to its pleasant aroma and elegant floral morphology. The regulatory mechanism underlying ovule development in orchids, especially C. sinense, is largely unknown and information on the C. sinense genome is very scarce. In this study, a combined analysis was performed on the transcriptome and non-targeted metabolomes of 18 C. sinense 'Qi Jian Hei Mo' ovule samples. Transcriptome analysis assembled gene-related information related to six growth stages of C. sinense ovules (S1-S6, equivalent to 30, 35, 42, 46, 53, and 60 days after pollination). Illumina sequencing technology was used to obtain the complete set of transcriptome sequences of the 18 samples. A total of 81,585 unigene sequences were obtained after assembly, 24,860 (30.47%) of which were functionally annotated. Using transcriptome sequencing technology, a total of 9845 differentially expressed unigenes (DEUs) were identified in C. sinense ovules that were assigned to specific metabolic pathways according to the Kyoto Encyclopedia of Genes and Genomes (KEGG). DEUs associated with transcription factors (TFs) and phytohormones were identified and analyzed. The TFs homeobox and MADS-box were associated with C. sinense ovule development. In particular, the phytohormones associated with DEUs such as indole-3-acetic acid (IAA), cytokinin (CK), gibberellin (GA), abscisic acid (ABA), brassinosteroid (BR), and jasmonate (JA), may have important regulatory effects on C. sinense ovule development. Metabolomic analysis showed an inconsistent number of KEGG annotations of differential metabolites across comparisons (S2_vs_S4, S2_vs_S5, and S4_vs_S5 contained 23, 26, and 3 annotations, respectively) in C. sinense ovules. This study provides a valuable foundation for further understanding the regulation of orchid ovule development and formation, and establishes a theoretical background for future practical applications during orchid cultivation.

AMP1 and CYP78A5/7 act through a common pathway to govern cell fate maintenance in Arabidopsis thaliana

Higher plants can continuously form new organs by the sustained activity of pluripotent stem cells. These stem cells are embedded in meristems, where they produce descendants, which undergo cell proliferation and differentiation programs in a spatiotemporally-controlled manner. Under certain conditions, pluripotency can be reestablished in descending cells and this reversion in cell fate appears to be actively suppressed by the existing stem cell pool. Mutation of the putative carboxypeptidase ALTERED MERISTEM PROGRAM1 (AMP1) in Arabidopsis causes defects in the suppression of pluripotency in cells normally programmed for differentiation, giving rise to unique hypertrophic phenotypes during embryogenesis as well as in the shoot apical meristem. A role of AMP1 in the miRNA-dependent control of translation has recently been established, however, how this activity is connected to its developmental functions is not resolved. Here we identify members of the cytochrome P450 clade CYP78A to act in parallel with AMP1 to control cell fate in Arabidopsis. Mutation of CYP78A5 and its close homolog CYP78A7 in a cyp78a5,7 double mutant caused suspensor-to-embryo conversion and ectopic stem cell pool formation in the shoot meristem, phenotypes characteristic for amp1. The tissues affected in the mutants showed pronounced expression levels of AMP1 and CYP78A5 in wild type. A comparison of mutant transcriptomic responses revealed an intriguing degree of overlap and highlighted alterations in protein lipidation processes. Moreover, we also found elevated protein levels of selected miRNA targets in cyp78a5,7. Based on comprehensive genetic interaction studies we propose a model in which both enzyme classes act on a common downstream process to sustain cell fate decisions in the early embryo and the shoot apical meristem.

HUA ENHANCER1 Mediates Ovule Development

Ovules are female reproductive organs of angiosperms, containing sporophytic integuments and gametophytic embryo sacs. After fertilization, embryo sacs develop into embryos and endosperm whereas integuments into seed coat. Ovule development is regulated by transcription factors (TF) whose expression is often controlled by microRNAs. Mutations of Arabidopsis DICER-LIKE 1 (DCL1), a microRNA processing protein, caused defective ovule development and reduced female fertility. However, it was not clear whether other microRNA processing proteins participate in this process and how defective ovule development influenced female fertility. We report that mutations of HUA ENHANCER1 (HEN1) and HYPONASTIC LEAVES 1 (HYL1) interfered with integument growth. The sporophytic defect caused abnormal embryo sac development and inability of mutant ovules to attract pollen tubes, leading to reduced female fertility. We show that the role of HEN1 in integument growth is cell-autonomous. Although AUXIN RESPONSE FACTOR 6 (ARF6) and ARF8 were ectopically expressed in mutant ovules, consistent with the reduction of microRNA167 in hen1, introducing arf6;arf8 did not suppress ovule defects of hen1, suggesting the involvement of more microRNAs in this process. Results presented indicate that the microRNA processing machinery is critical for ovule development and seed production through multiple microRNAs and their targets.

Integrated Analysis of Small RNA, Transcriptome and Degradome Sequencing Provides New Insights into Floral Development and Abscission in Yellow Lupine (Lupinus luteus L.)

The floral development in an important legume crop yellow lupine (Lupinus luteus L., Taper cv.) is often affected by the abscission of flowers leading to significant economic losses. Small non-coding RNAs (sncRNAs), which have a proven effect on almost all developmental processes in other plants, might be of key players in a complex net of molecular interactions regulating flower development and abscission. This study represents the first comprehensive sncRNA identification and analysis of small RNA, transcriptome and degradome sequencing data in lupine flowers to elucidate their role in the regulation of lupine generative development. As shedding in lupine primarily concerns flowers formed at the upper part of the inflorescence, we analyzed samples from extreme parts of raceme separately and conducted an additional analysis of pedicels from abscising and non-abscising flowers where abscission zone forms. A total of 394 known and 28 novel miRNAs and 316 phased siRNAs were identified. In flowers at different stages of development 59 miRNAs displayed differential expression (DE) and 46 DE miRNAs were found while comparing the upper and lower flowers. Identified tasiR-ARFs were DE in developing flowers and were strongly expressed in flower pedicels. The DEmiR-targeted genes were preferentially enriched in the functional categories related to carbohydrate metabolism and plant hormone transduction pathways. This study not only contributes to the current understanding of how lupine flowers develop or undergo abscission but also holds potential for research aimed at crop improvement.

Large-scale phylogenomic analysis suggests three ancient superclades of the WUSCHEL-RELATED HOMEOBOX transcription factor family in plants

The adaptation of plants to land required multiple morphological innovations. Among these include a variety of lateral organs that are initiated from apical meristems, in which the mantainance of undifferentiated stem cells is regulated by the homeodomain WUSCHEL-RELATED (WOX) transcription factors. Expansion of the WOX gene family has been associated with whole genome duplication (WGD) events and postulated to have been pivotal to the evolution of morphological complexity in land plants. Previous studies have classified the WOX gene family into three superclades (e.g., the ancient clade, the intermediate clade, and the modern clade). In order to improve our understanding of the evolution of the WOX gene family, we surveyed the WOX gene sequences from 38 genomes and 440 transcriptomes spanning the Viridiplantae and Rhodophyta. The WOX phylogeny inferred from 1039 WOX proteins drawn from 267 species with improved support along the backbone of the phylogeny suggests that the plant-specific WOX family contains three ancient superclades, which we term Type 1 (T1WOX, the WOX10/13/14 clade), Type 2 (T2WOX, the WOX8/9 and WOX11/12 clades), and Type 3 (T3WOX, the WUS, WOX1/6, WOX2, WOX3, WOX4 and WOX5/7 clades). Divergence of the T1WOX and T2WOX superclades may predate the diversification of vascular plants. Synteny analysis suggests contribution of WGD to expansion of the WOX family. Promoter analysis finds that the capacity of the WOX genes to be regulated by the auxin and cytokinin signaling pathways may be deeply conserved in the Viridiplantae. This study improves our phylogenetic context for elucidating functional evolution of the WOX gene family, which has likely contributed to the morphological complexity of land plants.

Revisiting the Female Germline and Its Expanding Toolbox

The Arabidopsis thaliana ovule arises as a female reproductive organ composed solely of somatic diploid cells. Among them, one cell will acquire a unique identity and initiate female germline development. In this review we explore the complex network that facilitates differentiation of this single cell, and consider how it becomes committed to a distinct developmental program. We highlight recent progress towards understanding the role of intercellular communication, cell competency, and cell-cycle regulation in the ovule primordium, and we discuss the possibility that distinct pathways restrict germline development at different stages. Importantly, these recent findings suggest a renaissance in plant ovule research, restoring the female germline as an attractive model to study cell communication and cell fate establishment in multicellular organs.

The AGC protein kinase UNICORN controls planar growth by attenuating PDK1 in Arabidopsis thaliana

Tissue morphogenesis critically depends on the coordination of cellular growth patterns. In plants, many organs consist of clonally distinct cell layers, such as the epidermis, whose cells undergo divisions that are oriented along the plane of the layer. The developmental control of such planar growth is poorly understood. We have previously identified the Arabidopsis AGCVIII-class protein kinase UNICORN (UCN) as a central regulator of this process. Plants lacking UCN activity show spontaneous formation of ectopic multicellular protrusions in integuments and malformed petals indicating that UCN suppresses uncontrolled growth in those tissues. In the current model UCN regulates planar growth of integuments in part by directly repressing the putative transcription factor ABERRANT TESTA SHAPE (ATS). Here we report on the identification of 3-PHOSPHOINOSITIDE-DEPENDENT PROTEIN KINASE 1 (PDK1) as a novel factor involved in UCN-mediated growth control. PDK1 constitutes a basic component of signaling mediated by AGC protein kinases throughout eukaryotes. Arabidopsis PDK1 is implied in stress responses and growth promotion. Here we show that loss-of-function mutations in PDK1 suppress aberrant growth in integuments and petals of ucn mutants. Additional genetic, in vitro, and cell biological data support the view that UCN functions by repressing PDK1. Furthermore, our data indicate that PDK1 is indirectly required for deregulated growth caused by ATS overexpression. Our findings support a model proposing that UCN suppresses ectopic growth in integuments through two independent processes: the attenuation of the protein kinase PDK1 in the cytoplasm and the repression of the transcription factor ATS in the nucleus.

Tissue-specific NIA1 and NIA2 expression in Arabidopsis thaliana

Nitrogen (N) is an essential macronutrient for optimal plant growth and ultimately for crop productivity Nitrate serves as the main N source for most plants. Although it seems a well-established fact that nitrate concentration affects flowering, its molecular mode of action in flowering time regulation was poorly understood. We recently found how nitrate, present at the shoot apical meristem (SAM), controls flowering time In this short communication, we present data on the tissue-specific expression patterns of NITRATE REDUCTASE 1 (NIA1) and NIA2 in planta. We show that transcripts of both genes are present throughout the life cycle of Arabidopsis thaliana plants with NIA1 being predominantly active in leaves and NIA2 in meristematic tissues.

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.

The WUSCHEL-related homeobox1 gene of cucumber regulates reproductive organ development

The WUSCHEL-related homeobox1 (WOX1) transcription factor plays an important role in lateral growth of plant organs; however, the underlying mechanisms in the regulation of reproductive development are largely unknown. Cucumber (Cucumis sativus) has separate male and female flowers, facilitating the study of the role of WOX1 in stamen and carpel development. Here, we identified a mango fruit (mf) mutant in cucumber, which displayed multiple defects in flower growth as well as male and female sterility. Map-based cloning showed that Mf encodes a WOX1-type transcriptional regulator (CsWOX1), and that the mf mutant encodes a truncated protein lacking the conserved WUS box. Further analysis showed that elevated expression of CsWOX1 was responsible for the mutant phenotype in cucumber and Arabidopsis. Comparative transcriptome profiling revealed certain key players and CsWOX1-associated networks that regulate reproductive development. CsWOX1 directly interacts with cucumber SPOROCYTELESS (CsSPL), and many genes in the CsSPL-mediated pathway were down-regulated in plants with the mutant allele at the Mf locus. In addition, auxin distribution was affected in both male and female flowers of the mutant. Taking together, these data suggest that CsWOX1 may regulate early reproductive organ development and be involved in sporogenesis via the CsSPL-mediated pathway and/or modulate auxin signaling in cucumber.

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.

Functionally Diversified Members of the MIR165/6 Gene Family Regulate Ovule Morphogenesis in Arabidopsis thaliana

The ovules of flowering plants consist of a central embryo sac and surrounding layers of the inner and outer integument. As these structural units eventually give rise to the embryo/endosperm and seed coat, respectively, a precisely organized ovule structure is essential for successful fertilization and seed production. In Arabidopsis thaliana, correct ovule patterning depends on the restricted expression of the CLASS III HOMEODOMAIN LEUCINE ZIPPER (HD-ZIP III) gene PHABULOSA (PHB) in the apical region of the incipient inner integument, which in turn is regulated via post-transcriptional suppression by miR165 and miR166 (miR165/6) derived from multiple MIR165/6 genes. While a common subset of MIR165/6 genes regulate PHB expression in the root meristem, leaf primordium and embryo, it is unknown whether the same MIR165/6 subset also regulate PHB expression during ovule development. Furthermore, it is unclear where in the ovule primordia miR165/6 are produced. Here, we show that a distinct set of MIR165/6 genes that are highly expressed in the small regions of early ovule primordia restrict the PHB expression domain to promote integument formation. MIR165/6 genes that function in ovule development are phylogenetically distinct from those acting in roots and leaf primordia. Taken together, our data suggest that members of the MIR165/6 gene family are diversified in their expression capacity to establish elaborate PHB expression patterns depending on the developmental context, thereby allowing HD-ZIP III transcription factors to regulate multiple aspects of plant development.

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.

miR160 and miR166/165 Contribute to the LEC2-Mediated Auxin Response Involved in the Somatic Embryogenesis Induction in Arabidopsis

MicroRNAs are non-coding small RNA molecules that are involved in the post-transcriptional regulation of the genes that control various developmental processes in plants, including zygotic embryogenesis (ZE). miRNAs are also believed to regulate somatic embryogenesis (SE), a counterpart of the ZE that is induced in vitro in plant somatic cells. However, the roles of specific miRNAs in the regulation of the genes involved in SE, in particular those encoding transcription factors (TFs) with an essential function during SE including LEAFY COTYLEDON2 (LEC2), remain mostly unknown. The aim of the study was to reveal the function of miR165/166 and miR160 in the LEC2-controlled pathway of SE that is induced in in vitro cultured Arabidopsis explants.In ZE, miR165/166 controls the PHABULOSA/PHAVOLUTA (PHB/PHV) genes, which are the positive regulators of LEC2, while miR160 targets the AUXIN RESPONSE FACTORS (ARF10, ARF16, ARF17) that control the auxin signaling pathway, which plays key role in LEC2-mediated SE. We found that a deregulated expression/function of miR165/166 and miR160 resulted in a significant accumulation of auxin in the cultured explants and the spontaneous formation of somatic embryos. Our results show that miR165/166 might contribute to SE induction via targeting PHB, a positive regulator of LEC2 that controls embryogenic induction via activation of auxin biosynthesis pathway (Wójcikowska et al., 2013). Similar to miR165/166, miR160 was indicated to control SE induction through auxin-related pathways and the negative impact of miR160 on ARF10/ARF16/ARF17 was shown in an embryogenic culture. Altogether, the results suggest that the miR165/166- and miR160-node contribute to the LEC2-mediated auxin-related pathway of embryogenic transition that is induced in the somatic cells of Arabidopsis. A model summarizing the suggested regulatory interactions between the miR165/166-PHB and miR160-ARF10/ARF16/ARF17 nodes that control SE induction in Arabidopsis was proposed.

The pomegranate (Punica granatum L.) genome and the genomics of punicalagin biosynthesis

Pomegranate (Punica granatum L.) is a perennial fruit crop grown since ancient times that has been planted worldwide and is known for its functional metabolites, particularly punicalagins. We have sequenced and assembled the pomegranate genome with 328 Mb anchored into nine pseudo-chromosomes and annotated 29 229 gene models. A Myrtales lineage-specific whole-genome duplication event was detected that occurred in the common ancestor before the divergence of pomegranate and Eucalyptus. Repetitive sequences accounted for 46.1% of the assembled genome. We found that the integument development gene INNER NO OUTER (INO) was under positive selection and potentially contributed to the development of the fleshy outer layer of the seed coat, an edible part of pomegranate fruit. The genes encoding the enzymes for synthesis and degradation of lignin, hemicelluloses and cellulose were also differentially expressed between soft- and hard-seeded varieties, reflecting differences in their accumulation in cultivars differing in seed hardness. Candidate genes for punicalagin biosynthesis were identified and their expression patterns indicated that gallic acid synthesis in tissues could follow different biochemical pathways. The genome sequence of pomegranate provides a valuable resource for the dissection of many biological and biochemical traits and also provides important insights for the acceleration of breeding. Elucidation of the biochemical pathway(s) involved in punicalagin biosynthesis could assist breeding efforts to increase production of this bioactive compound.

The THO Complex Non-Cell-Autonomously Represses Female Germline Specification through the TAS3-ARF3 Module

In most sexually reproducing plants, a single somatic, sub-epidermal cell in an ovule is selected to differentiate into a megaspore mother cell, which is committed to giving rise to the female germline. However, it remains unclear how intercellular signaling among somatic cells results in only one cell in the sub-epidermal layer differentiating into the megaspore mother cell. Here we uncovered a role of the THO complex in restricting the megaspore mother cell fate to a single cell. Mutations in TEX1, HPR1, and THO6, components of the THO/TREX complex, led to the formation of multiple megaspore mother cells, which were able to initiate gametogenesis. We demonstrated that TEX1 repressed the megaspore mother cell fate by promoting the biogenesis of TAS3-derived trans-acting small interfering RNA (ta-siRNA), which represses ARF3 expression. The TEX1 protein was present in epidermal cells, but not in the germline, and, through TAS3-derived ta-siRNA, restricted ARF3 expression to the medio domain of ovule primordia. Expansion of ARF3 expression into lateral epidermal cells in a TAS3 ta-siRNA-insensitive mutant led to the formation of supernumerary megaspore mother cells, suggesting that TEX1- and TAS3-mediated restriction of ARF3 expression limits excessive megaspore mother cell formation non-cell-autonomously. Our findings reveal the role of a small-RNA pathway in the regulation of female germline specification in Arabidopsis.

The cell wall-localized atypical β-1,3 glucanase ZERZAUST controls tissue morphogenesis in Arabidopsis thaliana

Orchestration of cellular behavior in plant organogenesis requires integration of intercellular communication and cell wall dynamics. The underlying signaling mechanisms are poorly understood. Tissue morphogenesis in Arabidopsis depends on the receptor-like kinase STRUBBELIG. Mutations in ZERZAUST were previously shown to result in a strubbelig-like mutant phenotype. Here, we report on the molecular identification and functional characterization of ZERZAUST We show that ZERZAUST encodes a putative GPI-anchored β-1,3 glucanase suggested to degrade the cell wall polymer callose. However, a combination of in vitro, cell biological and genetic experiments indicate that ZERZAUST is not involved in the regulation of callose accumulation. Nonetheless, Fourier-transformed infrared-spectroscopy revealed that zerzaust mutants show defects in cell wall composition. Furthermore, the results indicate that ZERZAUST represents a mobile apoplastic protein, and that its carbohydrate-binding module family 43 domain is required for proper subcellular localization and function whereas its GPI anchor is dispensable. Our collective data reveal that the atypical β-1,3 glucanase ZERZAUST acts in a non-cell-autonomous manner and is required for cell wall organization during tissue morphogenesis.

3D Imaging of Whole-Mount Ovules at Cellular Resolution to Study Female Germline Development in Rice

Recent advances in fluorescence-based staining of cellular compartments coupled with confocal microscopy imaging have allowed the visualization of three-dimensional (3D) structures with cellular resolution in various intact plant tissues and species. Such approaches are of particular interest for the analysis of the reproductive lineage in plants including the meiotic precursor cells deeply embedded within the ovary of the gynoecium enclosed in the flower. Yet, their relative inaccessibility and the lack of optical clarity of plant tissues prevent robust staining and imaging across several cell layers. Several whole-mount tissue staining and clearing techniques are available. One of them specifically allows staining of cellular boundaries in thick tissue samples while providing extreme optical clarity, using an acidic treatment followed by a modified Pseudo-Schiff propidium iodide (mPS-PI) method. While commonly used for Arabidopsis tissues, its application to other species like the model crop rice required protocol adaptations for obtaining robust staining that we present here. The procedure comprises six steps: (a) Material sampling; (b) Material fixation; (c) Tissue preparation; (d) Staining; (e) Sample mounting; and (d) Microscopy imaging. Particularly, we use ethanol and acetic anhydride as fixative reagents. A modified enzymatic treatment proved essential for starch degradation influencing optical clarity hence allowing acquisition of images at high resolution. This improved protocol is efficient for analyzing the megaspore mother cells in rice (Oryza sativa) ovary but is broadly applicable to other crop tissues of complex composition, without the need for tissue sectioning.

SEXUAL STERILITY is Essential for Both Male and Female Gametogenesis in Tomato

Gametogenesis is a key step in the production of ovules or pollen in higher plants. The molecular aspects of gametogenesis are well characterized in the model plant Arabidopsis; however, little information is known in tomato, which is a model plant for fleshy fruit development. In this study, we characterized a tomato (Solanum lycopersicum L.) γ-ray mutant, sexual sterility (Slses), that exhibited both male and female sterility. Morphological analysis revealed that the Slses mutant forms incomplete ovules and wilted anthers devoid of pollen grains at the anthesis stage. Genetic and next-generation sequencing analyses revealed that the Slses mutant carried a 13 bp deletion within the first exon of a homolog of SPOROCYTELESS/NOZZLE (SPL/NZZ), which plays an important role in gametogenesis in Arabidopsis. Complementation analysis in which the complete SlSES genomic region was introduced into the Slses mutant fully restored normal phenotypes, demonstrating that Solyc07g063670 is responsible for the Slses mutation. SlSES probably act as a transcriptional repressor because of an EAR motif at the C-terminal region. Gene expression levels of WUSCHEL (SlWUS) and INNER NO OUTER (SlINO), both of which are required for ovule development, were dramatically reduced in the early stages of pistil development in the Slses mutant, suggesting a positive regulatory role for SlSES in the transcription of gametogenesis genes and differences in the regulation of INO (SlINO) and integument development by SPL/NZZ (SLSES) between Arabidopsis and tomato. Taken together, our results indicate that SlSES is a novel tomato gametogenesis gene essential for both male and female gametogenesis.

Perspectives for a Framework to Understand Aril Initiation and Development

A differentiated structure called "aril" has been described in seeds of several plant species during the course of evolution and might be considered as a supernumerary integument. Besides its ecological function in seed dispersal, the structure also represents a relevant character for systematic classification and exhibits important properties that impart agronomic value in certain species. Little is known about the molecular pathways underlying this morphological innovation because it is absent in currently used model species. A remarkable feature of the seeds of Passiflora species is the presence of a conspicuous aril. This genus is known for the ornamental, medicinal, and food values of its species. In view of the molecular resources and tools available for some Passiflora species, we highlight the potential of these species as models for developmental studies of the aril.

Transcriptome analyses of seed development in grape hybrids reveals a possible mechanism influencing seed size

Background

Seedlessness in grape (Vitis vinifera) is of considerable commercial importance for both the table grape and processing industries. Studies to date of grape seed development have been made certain progress, but many key genes have yet to be identified and characterized.

Results

In this study we analyzed the seed transcriptomes of progeny derived from the V. vinifera seeded maternal parent ‘Red Globe’ and the seedless paternal parent ‘Centennial seedless’ to identify genes associated with seedlessness. A total of 6,607 differentially expressed genes (DEGs) were identified and examined from multiple perspectives, including expression patterns, Gene Ontology (GO) annotations, pathway enrichment, inferred hormone influence and epigenetic regulation. The expression data of hormone-related genes and hormone level measurement reveals the differences during seed development between seedless and seeded progeny. Based on both our results and previous studies of A. thaliana seed development, we generated network maps of grape seed-related DEGs, with particular reference to hormone balance, seed coat and endosperm development, and seed identity complexes.

Conclusion

In summary, the major differences identified during seed development of seedless and seeded progeny were associated with hormone and epigenetic regulation, the development of the seed coat and endosperm, and the formation of seed identity complexes. Overall the data provides insights into the possible molecular mechanism controlling grape seed size, which is of great importance for both basic research and future translation applications in the grape industry.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3193-1) contains supplementary material, which is available to authorized users.

Genome-wide transcriptome analysis of female-sterile rice ovule shed light on its abortive mechanism

The comprehensive transcriptome analysis of rice female-sterile line and wild-type line ovule provides an important clue for exploring the regulatory network of the formation of rice fertile female gametophyte. Ovules are the female reproductive tissues of rice (Oryza sativa L.) and play a major role in sexual reproduction. To investigate the potential mechanism of rice female gametophyte fertility, we used RNA sequencing, combined with genetic subtraction, to compare the transcriptome of the ovules of a high-frequency female-sterile line (fsv1) and a rice wild-type line (Gui 99) during ovule development. Ovules were harvested at three developmental stages: ovule containing megaspore mother cell in meiosis process (stage 1), ovule containing functional megaspore in mitosis process (stage 2), and ovule containing mature female gametophyte (stage 3). Six cDNA libraries generated a total of 42.2 million high-quality clean reads that aligned with 30,204 genes. The comparison between the fsv1 and Gui 99 ovules identified a large number of differentially expressed genes (DEGs), i.e., 45, 495, and 932 DEGs at the three ovule developmental stages, respectively. From the comparison of the two rice lines, Gene Ontology, Kyoto Encyclopedia of Genes and Genomes, and MapMan analyses indicated that a large number of DEGs associated with starch and sucrose metabolism, plant hormone signal transduction, protein modification and degradation, oxidative phosphorylation, and receptor kinase. These DEGs might play roles in ovule development and fertile female gametophyte formation. Many transcription factor genes and epigenetic-related genes also exhibit different expression patterns and significantly different expression levels in two rice lines during ovule development, which might provide important information regarding the abortive mechanism of the female gametophyte in rice.

Homeobox Is Pivotal for OsWUS Controlling Tiller Development and Female Fertility in Rice

OsWUS has recently been shown to be a transcription factor gene critical for tiller development and fertility in rice. The OsWUS protein consists of three conserved structural domains, but their biological functions are still unclear. We discovered a new rice mutant resulting from tissue culture, which hardly produced tillers and exhibited complete female sterility. The male and female floral organs of the mutant were morphologically indistinguishable from those of the wild type. We named the mutant srt1 for completely sterile and reduced tillering 1. Map-based cloning revealed that the mutant phenotypes were caused by a mutation in OsWUS. Compared with the two previously reported null allelic mutants of OsWUS (tab1-1 and moc3-1), which could produce partial N-terminal peptides of OsWUS, the srt1 protein contained a deletion of only seven amino acids within the conserved homeobox domain of OsWUS. However, the mutant phenotypes (monoculm and female sterility) displayed in srt1 were as typical and severe as those in tab1-1 and moc3-1. This indicates that the homeobox domain of SRT1 is essential for the regulation of tillering and sterility in rice. In addition, srt1 showed an opposite effect on panicle development to that of the two null allelic mutants, implying that the srt1 protein might still have partial or even new functions on panicle development. The results of this study suggest that the homeobox domain is pivotal for OsWUS function.

Amino Acid Change in an Orchid Desaturase Enables Mimicry of the Pollinator's Sex Pheromone

Mimicry illustrates the power of selection to produce phenotypic convergence in biology [1]. A striking example is the imitation of female insects by plants that are pollinated by sexual deception of males of the same insect species [2-4]. This involves mimicry of visual, tactile, and chemical signals of females [2-7], especially their sex pheromones [8-11]. The Mediterranean orchid Ophrys exaltata employs chemical mimicry of cuticular hydrocarbons, particularly the 7-alkenes, in an insect sex pheromone to attract and elicit mating behavior in its pollinators, males of the cellophane bee Colletes cunicularius [11-13]. A difference in alkene double-bond positions is responsible for reproductive isolation between O. exaltata and closely related species, such as O. sphegodes [13-16]. We show that these 7-alkenes are likely determined by the action of the stearoyl-acyl-carrier-protein desaturase (SAD) homolog SAD5. After gene duplication, changes in subcellular localization relative to the ancestral housekeeping desaturase may have allowed proto-SAD5's reaction products to undergo further biosynthesis to both 7- and 9-alkenes. Such ancestral coproduction of two alkene classes may have led to pollinator-mediated deleterious pleiotropy. Despite possible evolutionary intermediates with reduced activity, amino acid changes at the bottom of the substrate-binding cavity have conferred enzyme specificity for 7-alkene biosynthesis by preventing the binding of longer-chained fatty acid (FA) precursors by the enzyme. This change in desaturase function enabled the orchid to perfect its chemical mimicry of pollinator sex pheromones by escape from deleterious pleiotropy, supporting a role of pleiotropy in determining the possible trajectories of adaptive evolution.

CORONA, PHABULOSA and PHAVOLUTA collaborate with BELL1 to confine WUSCHEL expression to the nucellus in Arabidopsis ovules

Angiosperm ovules consist of three proximal-distal domains - the nucellus, chalaza and funiculus - demarcated by developmental fate and specific gene expression. Mutation in three paralogous class III homeodomain leucine zipper (HD-ZIPIII) genes leads to aberrations in ovule integument development. Expression of WUSCHEL (WUS) is normally confined to the nucellar domain, but in this triple mutant expression expands into the chalaza. MicroRNA-induced suppression of this expansion partially suppresses the effects of the HD-ZIPIII mutations on ovule development, implicating ectopic WUS expression as a component of the mutant phenotype. bell1 (bel1) mutants produce aberrant structures in place of the integuments and WUS is ectopically expressed in these structures. Combination of bel1 with the HD-ZIPIII triple mutant leads to a striking phenotype in which ectopic ovules emerge from nodes of ectopic WUS expression along the funiculi of the primary ovules. The synergistic phenotype indicates that BEL1 and the HD-ZIPIII genes act in at least partial independence in confining WUS expression to the nucellus and maintaining ovule morphology. The branching ovules of the mutant resemble those of some fossil gymnosperms, implicating BEL1 and HD-ZIPIII genes as players in the evolution of the unbranched ovule form in extant angiosperms.

Gene Expression Profiles in Rice Developing Ovules Provided Evidence for the Role of Sporophytic Tissue in Female Gametophyte Development

The development of ovule in rice (Oryza sativa) is vital during its life cycle. To gain more understanding of the molecular events associated with the ovule development, we used RNA sequencing approach to perform transcriptome-profiling analysis of the leaf and ovules at four developmental stages. In total, 25,401, 23,343, 23,647 and 23,806 genes were identified from the four developmental stages of the ovule, respectively. We identified a number of differently expressed genes (DEGs) from three adjacent stage comparisons, which may play crucial roles in ovule development. The DEGs were then conducted functional annotations and Kyoto encyclopedia of genes and genomes (KEGG) pathway analyses. Genes related to cellular component biogenesis, membrane-bounded organelles and reproductive regulation were identified to be highly expressed during the ovule development. Different expression levels of auxin-related and cytokinin-related genes were also identified at various stages, providing evidence for the role of sporophytic ovule tissue in female gametophyte development from the aspect of gene expression. Generally, an overall transcriptome analysis for rice ovule development has been conducted. These results increased our knowledge of the complex molecular and cellular events that occur during the development of rice ovule and provided foundation for further studies on rice ovule development.

Identification and Characterization of MicroRNAs in Ginkgo biloba var. epiphylla Mak

Ginkgo biloba, a dioecious plant known as a living fossil, is an ancient gymnosperm that stands distinct from other gymnosperms and angiosperms. Ginkgo biloba var. epiphylla (G. biloba var. epiphylla), with ovules borne on the leaf blade, is an unusual germplasm derived from G. biloba. MicroRNAs (miRNAs) are post-transcriptional gene regulators that play critical roles in diverse biological and metabolic processes. Currently, little is known about the miRNAs involved in the key stage of partly epiphyllous ovule germination in G. biloba var. epiphylla. Two small RNA libraries constructed from epiphyllous ovule leaves and normal leaves of G. biloba var. epiphylla were sequenced on an Illumina/Solexa platform. A total of 82 miRNA sequences belonging to 23 families and 53 putative novel miRNAs were identified in the two libraries. Differential expression analysis showed that 25 conserved and 21 novel miRNAs were differentially expressed between epiphyllous ovule leaves and normal leaves. The expression patterns of partially differentially expressed miRNAs and the transcript levels of their predicted target genes were validated by quantitative real time RT-PCR. All the expression profiles of the 21 selected miRNAs were similar to those detected by Solexa deep sequencing. Additionally, the transcript levels of almost all the putative target genes of 9 selected miRNAs were opposite to those of the corresponding miRNAs. The putative target genes of the differentially expressed miRNAs were annotated with Gene Ontology terms related to reproductive process, metabolic process and responding to stimulus. This work presents a broad range of small RNA transcriptome data obtained from epiphyllous ovule and normal leaves of G. biloba var. epiphylla, which may provide insights into the miRNA-mediated regulation in the epiphyllous ovule germination process.

Emergence of apospory and bypass of meiosis via apomixis after sexual hybridisation and polyploidisation

Hybridisation and polyploidy are major forces contributing to plant speciation. Homoploid (2x) and heteroploid (3x) hybrids, however, represent critical stages for evolution due to disturbed meiosis and reduced fertility. Apomixis--asexual reproduction via seeds--can overcome hybrid sterility, but requires several concerted alterations of developmental pathways to result in functional seed formation. Here, we analyse the reproductive behaviours of homo- and heteroploid synthetic hybrids from crosses between sexual diploid and tetraploid Ranunculus auricomus species to test the hypothesis that developmental asynchrony in hybrids triggers the shift to apomictic reproduction. Evaluation of male and female gametophyte development, viability and functionality of gametes shows developmental asynchrony, whereas seed set and germinability indicate reduced fitness in synthetic hybrids compared to sexual parents. We present the first experimental evidence for spontaneous apospory in most hybrids as an alternative pathway to meiosis, and the appearance of functional apomictic seeds in triploids. Bypassing meiosis permits these triploid genotypes to form viable seed and new polyploid progeny. Asynchronous development causes reduced sexual seed set and emergence of apospory in synthetic Ranunculus hybrids. Apomixis is functional in triploids and associated with drastic meiotic abnormalities. Selection acts to stabilise developmental patterns and to tolerate endosperm dosage balance shifts which facilitates successful seed set and establishment of apomictic lineages.

The molecular mechanism of sporocyteless/nozzle in controlling Arabidopsis ovule development

Ovules are essential for plant reproduction and develop into seeds after fertilization. Sporocyteless/nozzle (SPL/NZZ) has been known for more than 15 years as an essential factor for ovule development in Arabidopsis, but the biochemical nature of SPL function has remained unsolved. Here, we demonstrate that SPL functions as an adaptor-like transcriptional repressor. We show that SPL recruits topless/topless-related (TPL/TPR) co-repressors to inhibit the Cincinnata (CIN)-like Teosinte branched1/cycloidea/PCF (TCP) transcription factors. We reveal that SPL uses its EAR motif at the C-terminal end to recruit TPL/TPRs and its N-terminal part to bind and inhibit the TCPs. We demonstrate that either disruption of TPL/TPRs or overexpression of TCPs partially phenocopies the defects of megasporogenesis in spl. Moreover, disruption of TCPs causes phenotypes that resemble spl-D gain-of-function mutants. These results define the action mechanism for SPL, which along with TPL/TPRs controls ovule development by repressing the activities of key transcription factors. Our findings suggest that a similar gene repression strategy is employed by both plants and fungi to control sporogenesis.