Maternal control of PIN1 is required for female gametophyte development in Arabidopsis

Control of cellularization, nuclear localization, and antipodal cell cluster development in maize embryo sacs

The maize female gametophyte contains four cell types: two synergids, an egg cell, a central cell, and a variable number of antipodal cells. In maize, these cells are produced after three rounds of free-nuclear divisions followed by cellularization, differentiation, and proliferation of the antipodal cells. Cellularization of the eight-nucleate syncytium produces seven cells with two polar nuclei in the central cell. Nuclear localization is tightly controlled in the embryo sac. This leads to precise allocation of the nuclei into the cells upon cellularization. Nuclear positioning within the syncytium is highly correlated with their identity after cellularization. Two mutants are described with extra polar nuclei, abnormal antipodal cell morphology, and reduced antipodal cell number, as well as frequent loss of antipodal cell marker expression. Mutations in one of these genes, indeterminate gametophyte2 encoding a MICROTUBULE ASSOCIATED PROTEIN65-3 homolog, shows a requirement for MAP65-3 in cellularization of the syncytial embryo sac as well as for normal seed development. The timing of the effects of ig2 suggests that the identity of the nuclei in the syncytial female gametophyte can be changed very late before cellularization.

Insights into the role of phytohormones in plant female germline cell specification

Germline specification is a fundamental process in plant reproduction, and the Megaspore Mother Cell (MMC), is a critical cell that differentiates and develops into the female gametophyte. While numerous studies have investigated the molecular mechanisms underlying female germline specification, previous reviews have mainly focused on gene regulatory networks, epigenetic pathways, and small RNAs, neglecting the potential contribution of phytohormones to this process. This review aims to address this gap by highlighting recent advances in MMC formation and discussing the roles of specific phytohormones in female germline specialization. Here, we provide a comprehensive overview of the functions of phytohormones in the formation of MMC and their effects on female gametophyte development. Specifically, it examines the roles of gibberellins (GAs), brassinosteroids (BRs), auxins, and cytokinin, in MMC development. Understanding the function of phytohormones in MMC development is essential for comprehending the complex mechanisms underlying plant reproduction. This review adds valuable insights to the existing knowledge on MMC development, providing a new perspective for future research in the field of plant reproduction.

AUX1, PIN3, and TAA1 collectively maintain fertility in Arabidopsis

MAIN CONCLUSION

We found that auxin synthesis gene TAA1 and auxin polar transport genes AUX1 and PIN3 collectively maintain fertility and seed size in Arabidopsis. Auxin plays a vital role in plant gametophyte development and embryogenesis. The auxin synthesis gene TAA1 and the auxin polar transport genes AUX1 and PIN3 are expressed during Arabidopsis gametophyte and seed development. However, aux1, pin3, and taa1 single mutants only exhibit mild reproductive defects. We, therefore, generated aux1-T pin3 taa1-k2 and aux1-T pin3-2 taa1-k1 triple mutants by crossing or CRISPR/Cas9 technique. These triple mutants displayed severe reproductive defects with approximately 70% and 77%, respectively, of the siliques failing to elongate after anthesis. Reciprocal crosses and microscopy analyses showed that the development of pollen and ovules in the aux1 pin3 taa1 mutants was normal, whereas the filaments were remarkably short, which might be the cause of the silique sterility. Further analyses indicated that the development and morphology of aux1 pin3 taa1 seeds were normal, but their size was smaller compared with that of the wild type. These results indicate that AUX1, PIN3, and TAA1 act in concert to maintain fertility and seed size in Arabidopsis.

Biosynthesis- and transport-mediated dynamic auxin distribution during seed development controls seed size in Arabidopsis

Auxin is indispensable to the fertilization-induced coordinated development of the embryo, endosperm, and seed coat. However, little attention has been given to the distribution pattern, maintenance mechanism, and function of auxin throughout the process of seed development. In the present study, we found that auxin response signals display a dynamic distribution pattern during Arabidopsis seed development. Shortly after fertilization, strong auxin response signals were observed at the funiculus, chalaza, and micropylar integument where the embryo attaches. Later, additional signals appeared at the middle layer of the inner integument (ii1') above the chalaza and the whole inner layer of the outer integument (oi1). These signals peaked when the seed was mature, then declined upon desiccation and disappeared in the dried seed. Auxin biosynthesis genes, including ASB1, TAA1, YUC1, YUC4, YUC8, and YUC9, contributed to the accumulation of auxin in the funiculus and seed coat. Auxin efflux carrier PIN3 and influx carrier AUX1 also contributed to the polar auxin distribution in the seed coat. PIN3 was expressed in the ii1 (innermost layer of the inner integument) and oi1 layers of the integument and showed polar localization. AUX1 was expressed in both layers of the outer integument and the endosperm and displayed a uniform localization. Further research demonstrated that the accumulation of auxin in the seed coat regulates seed size. Transgenic plants that specifically express the YUC8 gene in the oi2 or ii1 seed coat produced larger seeds. These results provide useful tools for cultivating high-yielding crops.

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.

Cytokinin regulates female gametophyte development by cell cycle modulation in Arabidopsis thaliana

Male and female gametophyte development, double fertilization, and embryogenesis are key to alternating generations in angiosperms. The female gametophyte of Arabidopsis is an eight-nucleate haploid structure developed from functional megaspores (FMs) through three flawless mitoses regulated by a series of cell cycle genes. Cytokinin, an important phytohormone, plays a critical role in the regulation of plant growth and development. However, the mechanisms by which cytokinins regulate female gametophyte development remain largely unknown. In this study, we constructed transgenic plants (pES1::CKX1) with low cytokinin levels in the embryo sac. Phenotypic analysis showed that pES1::CKX1 inhibits female gametophyte development. Microscopic observation revealed that female gametophyte development of pES1::CKX1 was delayed. The promoters of all cell cycle genes were cloned and transformed into wild-type (WT). We crossed these transgenic plants of cell cycle genes expressed in ovules with pES1::CKX1 and compared the expression level of β-glucuronidase (GUS) in pES1::CKX1 and WT. Many cell cycle-regulated genes were up or downregulated in pES1::CKX1 compared with WT, and the embryo sac development cell cycle in cycd2;1/+ cycd3;3 was defective. Our results demonstrated that cytokinin affects cell division in the female gametophyte by affecting the expression of cell cycle genes.

Tonoplast proton pumps regulate nuclear spacing of female gametophytes via mediating polar auxin transport in Arabidopsis

The vacuole is an important organelle with multiple functions in plants, and the tonoplast that wraps the vacuole also plays essential roles in intracellular trafficking and ion homeostasis. Previous studies found that tonoplast proton pumps regulate embryo development and morphogenesis through their effects on vacuole biogenesis and distribution, as well as polar auxin transport and concomitant auxin gradient. However, the precise roles of the tonoplast proton pumps in gametophyte development remain unclear. Here we demonstrated that the lack of two types of tonoplast proton pumps or the absence of V-ATPase alone leads to abnormal development and nuclear localization of female gametophyte (FG), and slowed endosperm nuclei division after fertilization of the central cell. We further revealed that V-ATPase regulates auxin levels in ovules through coordinating the content and localization of PIN-FORMED 1 (PIN1) protein, hence influencing nuclear spacing between centra cell and egg cell, and subsequent endosperm development. Collectively, our findings revealed a crucial role of V-ATPase in auxin-mediated FG development in Arabidopsis and expanded our understanding of the functions of tonoplast proton pumps in seed plants reproductive development.

Ovule initiation: the essential step controlling offspring number in Arabidopsis

Seed is the offspring of angiosperms. Plants produce large numbers of seeds to ensure effective reproduction and survival in varying environments. Ovule is a fundamentally important organ and is the precursor of the seed. In Arabidopsis and other plants characterized by multi-ovulate ovaries, ovule initiation determines the maximal ovule number, thus greatly affecting seed number per fruit and seed yield. Investigating the regulatory mechanism of ovule initiation has both scientific and economic significance. However, the genetic and molecular basis underlying ovule initiation remains unclear due to technological limitations. Very recently, rules governing the multiple ovules initiation from one placenta have been identified, the individual functions and crosstalk of phytohormones in regulating ovule initiation have been further characterized, and new regulators of ovule boundary are reported, therefore expanding the understanding of this field. In this review, we present an overview of current knowledge in ovule initiation and summarize the significance of ovule initiation in regulating the number of plant offspring, as well as raise insights for the future study in this field that provide potential routes for the improvement of crop yield.

Function of Rice High-Affinity Potassium Transporters in Pollen Development and Fertility

Plant High-affinity K+ transporters/K+ uptake permeases/K+ transporters (HAK/KUP/KT) transporters have been predicted as membrane H+-K+ symporters in facilitating K+ uptake and distribution, while their role in seed production remains to be elucidated. In this study, we report that OsHAK26 is preferentially expressed in anthers and seed husks and located in the Golgi apparatus. Knockout of either OsHAK26 or plasma membrane located H+-K+ symporter gene OsHAK1 or OsHAK5 in both Nipponbare and Dongjin cultivars caused distorted anthers, reduced number and germination rate of pollen grains. Seed-setting rate assay by reciprocal cross-pollination between the mutants of oshak26, oshak1, oshak5 and their wild types confirmed that each HAK transporter is foremost for pollen viability, seed-setting and grain yield. Intriguingly, the pollens of oshak26 showed much thinner wall and were more vulnerable to desiccation than those of oshak1 or oshak5. In vitro assay revealed that the pollen germination rate of oshak5 was dramatically affected by external K+ concentration. The results suggest that the role of OsHAK26 in maintaining pollen development and fertility may relate to its proper cargo sorting for construction of pollen walls, while the role of OsHAK1 and OsHAK5 in maintaining seed production likely relates to their transcellular K+ transport activity.

Transcriptome Profiling Identifies Candidate Genes Contributing to Male and Female Gamete Development in Synthetic Brassica Allohexaploids

Polyploidy plays a crucial role in plant evolution and speciation. The development of male and female gametes is essential to the reproductive capacity of polyploids, but their gene expression pattern has not been fully explored in newly established polyploids. The present study aimed to reveal a detailed atlas of gene expression for gamete development in newly synthetic Brassica allohexaploids that are not naturally existing species. Comparative transcriptome profiling between developing anthers (staged from meiosis to mature pollen) and ovules (staged from meiosis to mature embryo sac) was performed using RNA-Seq analysis. A total of 8676, 9775 and 4553 upregulated differentially expressed genes (DEGs) were identified for the development of both gametes, for male-only, and for female-only gamete development, respectively, in the synthetic Brassica allohexaploids. By combining gene ontology (GO) biological process analysis and data from the published literature, we identified 37 candidate genes for DNA double-strand break formation, synapsis and the crossover of homologous recombination during male and female meiosis and 51 candidate genes for tapetum development, sporopollenin biosynthesis and pollen wall development in male gamete development. Furthermore, 23 candidate genes for mitotic progression, nuclear positioning and cell specification and development were enriched in female gamete development. This study lays a good foundation for revealing the molecular regulation of genes related to male and female gamete development in Brassica allohexaploids and provides more resourceful genetic information on the reproductive biology of Brassica polyploid breeding.

Gibberellins regulate ovule number through a DELLA-CUC2 complex in Arabidopsis

Ovule development is a key process for plant reproduction, helping to ensure correct seed production. Several molecular factors and plant hormones such as gibberellins are involved in ovule initiation and development. Gibberellins control ovule development by the destabilization of DELLA proteins, whereas DELLA activity has been shown to act as a positive factor for ovule primordia emergence. But the molecular mechanism by which DELLA acts in ovule primordia initiation remained unknown. In this study we report that DELLA proteins participate in ovule initiation by the formation of a protein complex with the CUC2 transcription factor. The DELLA protein GAI requires CUC2 to promote ovule primordia formation, through the direct GAI-CUC2 interaction in placental cells that would determine the boundary regions between ovules during pistil development. Analysis of GAI-CUC2 interaction and co-localization in the placenta supports this hypothesis. Moreover, molecular analysis identified a subset of the loci for which the GAI protein may act as a transcriptional co-regulator in a CUC2-dependent manner. The DELLA-CUC2 complex is a component of the gene regulatory network controlling ovule primordia initiation in Arabidopsis.

PIN3 positively regulates the late initiation of ovule primordia in Arabidopsis thaliana

Ovule initiation determines the maximum ovule number and has great impact on seed number and yield. However, the regulation of ovule initiation remains largely elusive. We previously reported that most of the ovule primordia initiate asynchronously at floral stage 9 and PINFORMED1 (PIN1) polarization and auxin distribution contributed to this process. Here, we further demonstrate that a small amount of ovule primordia initiate at floral stage 10 when the existing ovules initiated at floral stage 9 start to differentiate. Genetic analysis revealed that the absence of PIN3 function leads to the reduction in pistil size and the lack of late-initiated ovules, suggesting PIN3 promotes the late ovule initiation process and pistil growth. Physiological analysis illustrated that, unlike picloram, exogenous application of NAA can’t restore these defective phenotypes, implying that PIN3-mediated polar auxin transport is required for the late ovule initiation and pistil length. qRT-PCR results indicated that the expression of SEEDSTICK (STK) is up-regulated under auxin analogues treatment while is down-regulated in pin3 mutants. Meanwhile, overexpressing STK rescues pin3 phenotypes, suggesting STK participates in PIN3-mediated late ovule initiation possibly by promoting pistil growth. Furthermore, brassinosteroid influences the late ovule initiation through positively regulating PIN3 expression. Collectively, this study demonstrates that PIN3 promotes the late ovule initiation and contributes to the extra ovule number. Our results give important clues for increasing seed number and yield of cruciferous and leguminous crops.

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.

Molecular Network for Regulation of Ovule Number in Plants

In seed-bearing plants, the ovule ("small egg") is the organ within the gynoecium that develops into a seed after fertilization. The gynoecium located in the inner compartment of the flower turns into a fruit. The number of ovules in the ovary determines the upper limit or the potential of seed number per fruit in plants, greatly affecting the final seed yield. Ovule number is an important adaptive characteristic for plant evolution and an agronomic trait for crop improvement. Therefore, understanding the mechanism and pathways of ovule number regulation becomes a significant research aspect in plant science. This review summarizes the ovule number regulators and their regulatory mechanisms and pathways. Specially, an integrated molecular network for ovule number regulation is constructed, in which phytohormones played a central role, followed by transcription factors, enzymes, other protein and micro-RNA. Of them, AUX, BR and CK are positive regulator of ovule number, whereas GA acts negatively on it. Interestingly, many ovule number regulators have conserved functions across several plant taxa, which should be the targets of genetic improvement via breeding or gene editing. Many ovule number regulators identified to date are involved in the diverse biological process, such as ovule primordia formation, ovule initiation, patterning, and morphogenesis. The relations between ovule number and related characteristics/traits especially of gynoecium/fruit size, ovule fertility, and final seed number, as well as upcoming research questions, are also discussed. In summary, this review provides a general overview of the present finding in ovule number regulation, which represents a more comprehensive and in-depth cognition on it.

Integrated Bioinformatics Analyses of PIN1, CKX, and Yield-Related Genes Reveals the Molecular Mechanisms for the Difference of Seed Number Per Pod Between Soybean and Cowpea

There is limited advancement on seed number per pod (SNPP) in soybean breeding, resulting in low yield in China. To address this issue, we identified PIN1 and CKX gene families that regulate SNPP in Arabidopsis, analyzed the differences of auxin and cytokinin pathways, and constructed interaction networks on PIN1, CKX, and yield-related genes in soybean and cowpea. First, the relative expression level (REL) of PIN1 and the plasma membrane localization and phosphorylation levels of PIN1 protein were less in soybean than in cowpea, which make auxin transport efficiency lower in soybean, and its two interacted proteins might be involved in serine hydrolysis, so soybean has lower SNPP than cowpea. Then, the CKX gene family, along with its positive regulatory factor ROCK1, had higher REL and less miRNA regulation in soybean flowers than in cowpea ones. These lead to higher cytokinin degradation level, which further reduces the REL of PIN1 and decreases soybean SNPP. We found that VuACX4 had much higher REL than GmACX4, although the two genes essential in embryo development interact with the CKX gene family. Next, a tandem duplication experienced by legumes led to the differentiation of CKX3 into CKX3a and CKX3b, in which CKX3a is a key gene affecting ovule number. Finally, in the yield-related gene networks, three cowpea CBP genes had higher RELs than two soybean CBP genes, low RELs of three soybean-specific IPT genes might lead to a decrease in cytokinin synthesis, and some negative and positive SNPP regulation were found, respectively, in soybean and cowpea. These networks may explain the SNPP difference in the two crops. We deduced that ckx3a or ckx3a ckx6 ckx7 mutants, interfering CYP88A, and over-expressed DELLA increase SNPP in soybean. This study reveals the molecular mechanism for the SNPP difference in the two crops, and provides an important idea for increasing soybean yield.

Plant nitrate supply regulates Erwinia amylovora virulence gene expression in Arabidopsis

We showed previously that nitrogen (N) limitation decreases Arabidopsis resistance to Erwinia amylovora (Ea). We show that decreased resistance to bacteria in low N is correlated with lower apoplastic reactive oxygen species (ROS) accumulation and lower jasmonic acid (JA) pathway expression. Consistently, pretreatment with methyl jasmonate (Me-JA) increased the resistance of plants grown under low N. In parallel, we show that in planta titres of a nonvirulent type III secretion system (T3SS)-deficient Ea mutant were lower than those of wildtype Ea in low N, as expected, but surprisingly not in high N. This lack of difference in high N was consistent with the low expression of the T3SS-encoding hrp virulence genes by wildtype Ea in plants grown in high N compared to plants grown in low N. This suggests that expressing its virulence factors in planta could be a major limiting factor for Ea in the nonhost Arabidopsis. To test this hypothesis, we preincubated Ea in an inducing medium that triggers expression of hrp genes in vitro, prior to inoculation. This preincubation strongly enhanced Ea titres in planta, independently of the plant N status, and was correlated to a significant repression of JA-dependent genes. Finally, we identify two clusters of metabolites associated with resistance or with susceptibility to Ea. Altogether, our data showed that high susceptibility of Arabidopsis to Ea, under low N or following preincubation in hrp-inducing medium, is correlated with high expression of the Ea hrp genes in planta and low expression of the JA signalling pathway, and is correlated with the accumulation of specific metabolites.

The Glycine-Rich Domain Protein GRDP2 Regulates Ovule Development via the Auxin Pathway in Arabidopsis

The glycine-rich domain proteins (GRDP) have been functionally implicated in the cell wall structure, biotic, and abiotic stress responses. However, little is known about GRDP genes in female gametophyte development of Arabidopsis. This study shows that GRDP2, a GRDP, plays a crucial role in female gametophyte development. In GRDP2 overexpression lines, grdp2-3, the embryo sacs were arrested at FG1 and no nucleus stages. Furthermore, callose staining shows that cell plate formation during megasporogenesis is disturbed in grdp2-3. In contrast, the pollen development is not affected in grdp2-3. The expression patterns of auxin-specific marker lines in female gametophytes showed that the auxin distribution and transport were significantly changed during megagametogenesis in grdp2-3. In addition, compared with the membrane-localized pattern of PIN1, PIN2, and PIN7 in WT, the signals were detected in the cytoplasm in grdp2-3. Together, our data suggest that GRDP2 plays an essential role in auxin-mediated female gametophyte development.

High-throughput single-cell transcriptomics reveals the female germline differentiation trajectory in Arabidopsis thaliana

Female germline cells in flowering plants differentiate from somatic cells to produce specialized reproductive organs, called ovules, embedded deep inside the flowers. We investigated the molecular basis of this distinctive developmental program by performing single-cell RNA sequencing (scRNA-seq) of 16,872 single cells of Arabidopsis thaliana ovule primordia at three developmental time points during female germline differentiation. This allowed us to identify the characteristic expression patterns of the main cell types, including the female germline and its surrounding nucellus. We then reconstructed the continuous trajectory of female germline differentiation and observed dynamic waves of gene expression along the developmental trajectory. A focused analysis revealed transcriptional cascades and identified key transcriptional factors that showed distinct expression patterns along the germline differentiation trajectory. Our study provides a valuable reference dataset of the transcriptional process during female germline differentiation at single-cell resolution, shedding light on the mechanisms underlying germline cell fate determination.

Zhimin Hou, Yanhui Liu et al. used single cell RNA-seq to analyze the model organism, Arabidopsis thaliana, at three stages during female germline differentiation. They reconstructed the continuous trajectory of female germline differentiation, providing a valuable reference for future investigation of germline cell fate determination in A. thaliana.

Expression and Functional Analyses of Nymphaea caerulea MADS-Box Genes Contribute to Clarify the Complex Flower Patterning of Water Lilies

Nymphaeaceae are early diverging angiosperms with large flowers characterized by showy petals and stamens not clearly whorled but presenting a gradual morphological transition from the outer elements to the inner stamens. Such flower structure makes these plant species relevant for studying flower evolution. MADS-domain transcription factors are crucial components of the molecular network that controls flower development. We therefore isolated and characterized MADS-box genes from the water lily Nymphaea caerulea. RNA-seq experiments on floral buds have been performed to obtain the transcript sequences of floral organ identity MADS-box genes. Maximum Likelihood phylogenetic analyses confirmed their belonging to specific MADS-box gene subfamilies. Their expression was quantified by RT-qPCR in all floral organs at two stages of development. Protein interactions among these transcription factors were investigated by yeast-two-hybrid assays. We found especially interesting the involvement of two different AGAMOUS-like genes (NycAG1 and NycAG2) in the water lily floral components. They were therefore functionally characterized by complementing Arabidopsis ag and shp1 shp2 mutants. The expression analysis of MADS-box genes across flower development in N. caerulea described a complex scenario made of numerous genes in numerous floral components. Their expression profiles in some cases were in line with what was expected from the ABC model of flower development and its extensions, while in other cases presented new and interesting gene expression patterns, as for instance the involvement of NycAGL6 and NycFL. Although sharing a high level of sequence similarity, the two AGAMOUS-like genes NycAG1 and NycAG2 could have undergone subfunctionalization or neofunctionalization, as only one of them could partially restore the euAG function in Arabidopsis ag-3 mutants. The hereby illustrated N. caerulea MADS-box gene expression pattern might mirror the morphological transition from the outer to the inner floral organs, and the presence of transition organs such as the petaloid stamens. This study is intended to broaden knowledge on the role and evolution of floral organ identity genes and the genetic mechanisms causing biodiversity in angiosperm flowers.

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.

The Rab Geranylgeranyl Transferase Beta Subunit Is Essential for Embryo and Seed Development in Arabidopsis thaliana

Auxin is a key regulator of plant development affecting the formation and maturation of reproductive structures. The apoplastic route of auxin transport engages influx and efflux facilitators from the PIN, AUX and ABCB families. The polar localization of these proteins and constant recycling from the plasma membrane to endosomes is dependent on Rab-mediated vesicular traffic. Rab proteins are anchored to membranes via posttranslational addition of two geranylgeranyl moieties by the Rab Geranylgeranyl Transferase enzyme (RGT), which consists of RGTA, RGTB and REP subunits. Here, we present data showing that seed development in the rgtb1 mutant, with decreased vesicular transport capacity, is disturbed. Both pre- and post-fertilization events are affected, leading to a decrease in seed yield. Pollen tube recognition at the stigma and its guidance to the micropyle is compromised and the seed coat forms incorrectly. Excess auxin in the sporophytic tissues of the ovule in the rgtb1 plants leads to an increased tendency of autonomous endosperm formation in unfertilized ovules and influences embryo development in a maternal sporophytic manner. The results show the importance of vesicular traffic for sexual reproduction in flowering plants, and highlight RGTB1 as a key component of sporophytic-filial signaling.

Auxin efflux controls orderly nucellar degeneration and expansion of the female gametophyte in Arabidopsis

The nucellus tissue in flowering plants provides nutrition for the development of the female gametophyte (FG) and young embryo. The nucellus degenerates as the FG develops, but the mechanism controlling the coupled process of nucellar degeneration and FG expansion remains largely unknown. The degeneration process of the nucellus and spatiotemporal auxin distribution in the developing ovule before fertilization were investigated in Arabidopsis thaliana. Nucellar degeneration before fertilization occurs through vacuolar cell death and in an ordered degeneration fashion. This sequential nucellar degeneration is controlled by the signalling molecule auxin. Auxin efflux plays the core role in precisely controlling the spatiotemporal pattern of auxin distribution in the nucellus surrounding the FG. The auxin efflux carrier PIN1 transports maternal auxin into the nucellus while PIN3/PIN4/PIN7 further delivers auxin to degenerating nucellar cells and concurrently controls FG central vacuole expansion. Notably, auxin concentration and auxin efflux are controlled by the maternal tissues, acting as a key communication from maternal to filial tissue.

Rab-dependent vesicular traffic affects female gametophyte development in Arabidopsis

Eukaryotic cells rely on the accuracy and efficiency of vesicular traffic. In plants, disturbances in vesicular trafficking are well studied in quickly dividing root meristem cells or polar growing root hairs and pollen tubes. The development of the female gametophyte, a unique haploid reproductive structure located in the ovule, has received far less attention in studies of vesicular transport. Key molecules providing the specificity of vesicle formation and its subsequent recognition and fusion with the acceptor membrane are Rab proteins. Rabs are anchored to membranes by covalently linked geranylgeranyl group(s) that are added by the Rab geranylgeranyl transferase (RGT) enzyme. Here we show that Arabidopsis plants carrying mutations in the gene encoding the β-subunit of RGT (rgtb1) exhibit severely disrupted female gametogenesis and this effect is of sporophytic origin. Mutations in rgtb1 lead to internalization of the PIN1 and PIN3 proteins from the basal membranes to vesicles in provascular cells of the funiculus. Decreased transport of auxin out of the ovule is accompanied by auxin accumulation in tissue surrounding the growing gametophyte. In addition, female gametophyte development arrests at the uni- or binuclear stage in a significant portion of the rgtb1 ovules. These observations suggest that communication between the sporophyte and the developing female gametophyte relies on Rab-dependent vesicular traffic of the PIN1 and PIN3 transporters and auxin efflux out of the ovule.

Auxin and Flower Development: A Blossoming Field

The establishment of the species-specific floral organ body plan involves many coordinated spatiotemporal processes, which include the perception of positional information that specifies floral meristem and floral organ founder cells, coordinated organ outgrowth coupled with the generation and maintenance of inter-organ and inter-whorl boundaries, and the termination of meristem activity. Auxin is integrated within the gene regulatory networks that control these processes and plays instructive roles at the level of tissue-specific biosynthesis and polar transport to generate local maxima, perception, and signaling. Key features of auxin function in several floral contexts include cell nonautonomy, interaction with cytokinin gradients, and the central role of MONOPTEROS and ETTIN to regulate canonical and noncanonical auxin response pathways, respectively. Arabidopsis flowers are not representative of the enormous angiosperm floral diversity; therefore, comparative studies are required to understand how auxin underlies these developmental differences. It will be of great interest to compare the conservation of auxin pathways among flowering plants and to discuss the evolutionary role of auxin in floral 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.

The RNA-dependent DNA methylation pathway is required to restrict SPOROCYTELESS/NOZZLE expression to specify a single female germ cell precursor in Arabidopsis

In higher plants, the female germline is formed from the megaspore mother cell (MMC), a single cell in the premeiotic ovule. Previously, it was reported that mutants in the RNA-dependent DNA methylation (RdDM) pathway might be involved in restricting the female germline to a single nucellus cell. We show that the DRM methyltransferase double mutant drm1drm2 also presents ectopic enlarged cells, consistent with supernumerary MMC-like cells. In wild-type ovules, MMC differentiation requires SPOROCYTELESS/NOZZLE (SPL/NZZ), as demonstrated by the spl/nzz mutant failing to develop an MMC. We address the poorly understood upstream regulation of SPL/NZZ in ovules, showing that the RdDM pathway is important to restrict SPL/NZZ expression. In ago9, rdr6 and drm1drm2 mutants, SPL/NZZ is expressed ectopically, suggesting that the multiple MMC-like cells observed might be attributable to the ectopic expression of SPL/NZZ. We show that the ovule identity gene, SEEDSTICK, directly regulates AGO9 and RDR6 expression in the ovule and therefore indirectly regulates SPL/NZZ expression. A model is presented describing the network required to restrict SPL/NZZ expression to specify a single MMC.

Comparative transcriptome analysis of gynoecious and monoecious inflorescences reveals regulators involved in male flower development in the woody perennial plant Jatropha curcas

ABCE model genes along with genes related to GA biosynthesis and auxin signalling may play significant roles in male flower development in Jatropha curcas. Flowering plants exhibit extreme reproductive diversity. Jatropha curcas, a woody plant that is promising for biofuel production, is monoecious. Here, two gynoecious Jatropha mutants (bearing only female flowers) were used to identify key genes involved in male flower development. Using comparative transcriptome analysis, we identified 17 differentially expressed genes (DEGs) involved in floral organ development between monoecious plants and the two gynoecious mutants. Among these DEGs, five floral organ identity genes, Jatropha AGAMOUS, PISTILLATA, SEPALLATA 2-1 (JcSEP2-1), JcSEP2-2, and JcSEP3, were downregulated in ch mutant inflorescences; two gibberellin (GA) biosynthesis genes, Jatropha GA REQUIRING 1 and GIBBERELLIN 3-OXIDASE 1, were downregulated in both the ch and g mutants; and two genes involved in the auxin signalling pathway, Jatropha NGATHA1 and STYLISH1, were downregulated in the ch mutant. Furthermore, four hub genes involved in male flower development, namely Jatropha SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE 1, CRYPTOCHROME 2, SUPPRESSOR OF OVEREXPRESSION OF CO 1 and JAGGED, were identified using weighted gene correlation network analysis. These results suggest that floral organ identity genes and genes involved in GA biosynthesis and auxin signalling may participate in male flower development in Jatropha. This study will contribute to understanding sex differentiation in woody perennial plants.

Plant Development: Regular Ovule Spacing Is Controlled by Process-Specific Receptor-Ligand Combinations

Ovules are initiated in a highly regular pattern along the placenta inside the gynoecia of flowering plants. New research reveals that two separate EPFL peptide-mediated pathways functioning through a set of ERECTA-family receptors coordinate ovule patterning with fruit growth.

Auxin: Hormonal Signal Required for Seed Development and Dormancy

The production of viable seeds is a key event in the life cycle of higher plants. Historically, abscisic acid (ABA) and gibberellin (GAs) were considered the main hormones that regulate seed formation. However, auxin has recently emerged as an essential player that modulates, in conjunction with ABA, different cellular processes involved in seed development as well as the induction, regulation and maintenance of primary dormancy (PD). This review examines and discusses the key role of auxin as a signaling molecule that coordinates seed life. The cellular machinery involved in the synthesis and transport of auxin, as well as their cellular and tissue compartmentalization, is crucial for the development of the endosperm and seed-coat. Thus, auxin is an essential compound involved in integuments development, and its transport from endosperm is regulated by AGAMOUS-LIKE62 (AGL62) whose transcript is specifically expressed in the endosperm. In addition, recent biochemical and genetic evidence supports the involvement of auxins in PD. In this process, the participation of the transcriptional regulator ABA INSENSITIVE3 (ABI3) is critical, revealing a cross-talk between auxin and ABA signaling. Future experimental aimed at advancing knowledge of the role of auxins in seed development and PD are also discussed.

Gynoecium size and ovule number are interconnected traits that impact seed yield

Angiosperms form the largest group of land plants and display an astonishing diversity of floral structures. The development of flowers greatly contributed to the evolutionary success of the angiosperms as they guarantee efficient reproduction with the help of either biotic or abiotic vectors. The female reproductive part of the flower is the gynoecium (also called pistil). Ovules arise from meristematic tissue within the gynoecium. Upon fertilization, these ovules develop into seeds while the gynoecium turns into a fruit. Gene regulatory networks involving transcription factors and hormonal communication regulate ovule primordium initiation, spacing on the placenta, and development. Ovule number and gynoecium size are usually correlated and several genetic factors that impact these traits have been identified. Understanding and fine-tuning the gene regulatory networks influencing ovule number and pistil length open up strategies for crop yield improvement, which is pivotal in light of a rapidly growing world population. In this review, we present an overview of the current knowledge of the genes and hormones involved in determining ovule number and gynoecium size. We propose a model for the gene regulatory network that guides the developmental processes that determine seed yield.

Genome-Wide Association Analysis Identifies Candidate Genes Regulating Seed Number Per Silique in Arabidopsis thaliana

Seed weight and number ultimately determine seed yield. Arabidopsis seed number comprised of silique number and seed number per silique (SNS). Comparing seed development and weight, determinants of seed number remain largely uncharacterized. In this study, taking advantage of 107 available Arabidopsis accessions, genome-wide association analysis (GWAS) was employed to identify the candidate genes regulating SNS. GWAS-based genotype and phenotype association analysis identified 38 most significant SNPs marker sites that were mapped to specific chromosomal positions and allowed us to screen for dozens of candidate genes. One of them (PIN3) was selected for functional validation based on gene expression analysis. It is a positive regulator of Arabidopsis SNS. Although silique length of PIN3 loss of function mutant was not significantly changed, its SNS and seed density (SD) were significantly reduced as compared with the wild type. Notably, PIN3 overexpression lines driven by a placenta-specific promoter STK exhibited significantly shorter siliques, slightly reduced SNS, but significant increased SD compared with wild type, suggesting that PIN3 positively regulates SD through inducing ovule primordia initiation regardless of the placenta size. Ovule initiation determines the maximal possibility of SNS, and new genes and mechanism regulating SNS through modulating ovule initiation is worth further investigated.

Reproductive Multitasking: The Female Gametophyte

Fertilization of flowering plants requires the organization of complex tasks, many of which become integrated by the female gametophyte (FG). The FG is a few-celled haploid structure that orchestrates division of labor to coordinate successful interaction with the sperm cells and their transport vehicle, the pollen tube. As reproductive outcome is directly coupled to evolutionary success, the underlying mechanisms are under robust molecular control, including integrity check and repair mechanisms. Here, we review progress on understanding the development and function of the FG, starting with the functional megaspore, which represents the haploid founder cell of the FG. We highlight recent achievements that have greatly advanced our understanding of pollen tube attraction strategies and the mechanisms that regulate plant hybridization and gamete fusion. In addition, we discuss novel insights into plant polyploidization strategies that expand current concepts on the evolution of flowering plants.

Comparative Expression Profiling Reveals Genes Involved in Megasporogenesis

Megasporogenesis is a key step during ovule development in angiosperms, but the small number and inaccessibility of these cells have hampered molecular and genome-wide studies. Thus, many questions remain regarding the molecular basis of cell specification, differentiation, and development in the female gametophyte. Here, taking advantage of the correlation between spikelet length and ovule development in rice (Oryza sativa), we studied the transcriptome dynamics of young ovules at three stages, the archesporial cell, the megaspore mother cell before meiosis, and the functional megaspore after meiosis, using expression profiling based on RNA sequencing. Our analysis showed that 5,274 genes were preferentially expressed in ovules during megasporogenesis as compared to ovules at the mature female gametophyte stage. Out of these, 958 (18.16%) genes were archesporial cell- and/or megaspore mother cell-preferential genes, and represent a significant enrichment of genes involved in hormone signal transduction and plant pathogen interaction pathways, as well as genes encoding transcription factors. The expression patterns of nine genes that were preferentially expressed in ovules of different developmental stages, including the OsERECTA2 (OsER2) receptor-like kinase gene, were confirmed by in situ hybridization. We further characterized the OsER2 loss-of-function mutant, which had an excessive number of female germline cells and an abnormal female gametophyte, suggesting that OsER2 regulates germline cell specification during megasporogenesis in rice. These results expand our understanding of the molecular control of megasporogenesis in rice and contribute to the functional studies of genes involved in megasporogenesis.

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.

Maternal regulation of seed growth and patterning in flowering plants

The plant haploid generation is specified late in higher plant development, and post-meiotic haploid plant cells divide mitotically to produce a haploid gametophyte, in which a subset of cells differentiates into the gametes. The immediate mother of the angiosperm seed is the female gametophyte, also called the embryo sac. In most flowering plants the embryo sac is comprised of two kinds of gametes (egg and central cell) and two kinds of subsidiary cells (antipodals and synergids) all of which descend from a single haploid spore produced by meiosis. The embryo sac develops within a specialized organ of the flower called the ovule, which supports and controls many steps in the development of both the embryo sac and the seed. Double fertilization of the central cell and egg cell by the two sperm cells of a pollen grain produce the endosperm and embryo of the seed, respectively. The endosperm and embryo develop under the influence of their precursor gametes and the surrounding tissues of the ovule and the gametophyte. The final size and pattern of the angiosperm seed then is the result of complex interactions across multiple tissues of three different generations (maternal sporophyte, maternal gametophyte, and the fertilization products) and three different ploidies (haploid gametophyte, diploid parental sporophyte and embryo, and triploid endosperm).

Different expression pattern of flowering pathway genes contribute to male or female organ development during floral transition in the monoecious weed Ambrosia artemisiifolia L. (Asteraceae)

The highly allergenic and invasive weed Ambrosia artemisiifolia L. is a monoecius plant with separated male and female flowers. The genetic regulation of floral morphogenesis is a less understood field in the reproduction biology of this species. Therefore the objective of this work was to investigate the genetic control of sex determination during floral organogenesis. To this end, we performed a genome-wide transcriptional profiling of vegetative and generative tissues during the plant development comparing wild-growing and in vitro cultivated plants. RNA-seq on Illumina NextSeq 500 platform with an integrative bioinformatics analysis indicated differences in 80 floral gene expressions depending on photoperiodic and endogenous initial signals. Sex specificity of genes was validated based on RT-qPCR experiments. We found 11 and 16 uniquely expressed genes in female and male transcriptomes that were responsible particularly to maintain fertility and against abiotic stress. High gene expression of homologous such as FD, FT, TFL1 and CAL, SOC1, AP1 were characteristic to male and female floral meristems during organogenesis. Homologues transcripts of LFY and FLC were not found in the investigated generative and vegetative tissues. The repression of AP1 by TFL1 homolog was demonstrated in male flowers resulting exclusive expression of AP2 and PI that controlled stamen and carpel formation in the generative phase. Alterations of male and female floral meristem differentiation were demonstrated under photoperiodic and hormonal condition changes by applying in vitro treatments.

Refinement of a clearing protocol to study crassinucellate ovules of the sugar beet (Beta vulgaris L., Amaranthaceae)

BACKGROUND

Clearing methods allow relatively quick processing of plant material and examination of cellular structures by rendering tissues and organs translucent. They have been adapted for plant embryology, primarily to study ovule development, megasporogenesis, megagametogenesis and embryogenesis. Such clearing methods overcome several disadvantages of the conventional embedding-sectioning techniques that are arduous and time-consuming. Although numerous protocols with different clearing solutions have been described, there have been no reports to date proposing a reliable method to clear the crassinucellate ovules of the sugar beet (Beta vulgaris L.), an economically important crop. Therefore, this study aims to find a suitable approach to improve the tissue transparency of sugar beet ovules at different developmental stages.

RESULTS

We established a methyl salicylate-based protocol that significantly improved the transparency of the B. vulgaris ovule structures, which allowed us to observe the megagameto- and embryogenesis of that species. This was achieved by (1) chemical softening of the tissues; (2) vacuum pump-assisted infiltration step; (3) shaking-assisted incubation with clearing mixtures; and (4) manual removal of the chemically softened seed coat.

CONCLUSIONS

The effectiveness of our method is due to the strategy combining various approaches at different stages of the procedure aiming at increasing the accessibility of the internal ovule structures to the clearing solution. The results of this study may be applied in sugar beet breeding programs, and it will provide a basis for further investigation of numerous aspects of the species' embryology. Moreover, that unique approach may be easily adapted to other species developing crassinucellate ovules.

Annotation and Expression of IDN2-like and FDM-like Genes in Sexual and Aposporous Hypericum perforatum L. accessions

The protein IDN2, together with the highly similar interactors FDM1 and FDM2, is required for RNA-directed DNA methylation (RdDM) and siRNA production. Epigenetic regulation of gene expression is required to restrict cell fate determination in A. thaliana ovules. Recently, three transcripts sharing high similarity with the A. thaliana IDN2 and FDM1-2 were found to be differentially expressed in ovules of apomictic Hypericum perforatum L. accessions. To gain further insight into the expression and regulation of these genes in the context of apomixis, we investigated genomic, transcriptional and functional aspects of the gene family in this species. The H. perforatum genome encodes for two IDN2-like and 7 FDM-like genes. Differential and heterochronic expression of FDM4-like genes was found in H. perforatum pistils. The involvement of these genes in reproduction and seed development is consistent with the observed reduction of the seed set and high variability in seed size in A. thaliana IDN2 and FDM-like knockout lines. Differential expression of IDN2-like and FDM-like genes in H. perforatum was predicted to affect the network of potential interactions between these proteins. Furthermore, pistil transcript levels are modulated by cytokinin and auxin but the effect operated by the two hormones depends on the reproductive phenotype.

Establishing a framework for female germline initiation in the plant ovule

Female gametogenesis in flowering plants initiates in the ovule, where a single germline progenitor differentiates from a pool of somatic cells. Germline initiation is a fundamental prerequisite for seed development but is poorly understood at the molecular level due to the location of the cells deep within the flower. Studies in Arabidopsis have shown that regulators of germline development include transcription factors such as NOZZLE/SPOROCYTELESS and WUSCHEL, components of the RNA-dependent DNA methylation pathway such as ARGONAUTE9 and RNA-DEPENDENT RNA POLYMERASE 6, and phytohormones such as auxin and cytokinin. These factors accumulate in a range of cell types from where they establish an environment to support germline differentiation. Recent studies provide fresh insight into the transition from somatic to germline identity, linking chromatin regulators, cell cycle genes, and novel mobile signals, capitalizing on cell type-specific methodologies in both dicot and monocot models. These findings are providing unique molecular and compositional insight into the mechanistic basis and evolutionary conservation of female germline development in plants.

Importin β4 Mediates Nuclear Import of GRF-Interacting Factors to Control Ovule Development in Arabidopsis

Ovule development is critical for seed development and plant reproduction. Multiple transcription factors (TFs) have been reported to mediate ovule development. However, it is not clear which intracellular components regulate these TFs during ovule development. After their synthesis, TFs are transported into the nucleus a process regulated by karyopherins commonly known as importin alpha and β. Around half of Arabidopsis (Arabidopsis thaliana) importin β-coding genes have been functionally characterized but only two with specific cargos have been identified. We report here that Arabidopsis IMPORTIN β4 (IMB4) regulates ovule development through nucleocytoplasmic transport of transcriptional coactivator growth regulating factors-interacting factors (GIFs). Mutations in IMB4 impaired ovule development by affecting integument growth. imb4 mutants were also defective in embryo sac development, leading to partial female sterility. IMB4 directly interacts with GIFs and is critical for the nucleocytoplasmic transport of GIF1. Finally, functional loss of GIFs resulted in ovule defects similar to those in imb4 mutants, whereas enhanced expression of GIF1 partially restored the fertility of imb4 The results presented here uncover a novel genetic pathway regulating ovule development and reveal the upstream regulator of GIFs.

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 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.

Developmental Roles of AUX1/LAX Auxin Influx Carriers in Plants

Plant hormone auxin regulates several aspects of plant growth and development. Auxin is predominantly synthesized in the shoot apex and developing leaf primordia and from there it is transported to the target tissues e.g. roots. Auxin transport is polar in nature and is carrier-mediated. AUXIN1/LIKE-AUX1 (AUX1/LAX) family members are the major auxin influx carriers whereas PIN-FORMED (PIN) family and some members of the P-GLYCOPROTEIN/ATP-BINDING CASSETTE B4 (PGP/ABCB) family are major auxin efflux carriers. AUX1/LAX auxin influx carriers are multi-membrane spanning transmembrane proteins sharing similarity to amino acid permeases. Mutations in AUX1/LAX genes result in auxin related developmental defects and have been implicated in regulating key plant processes including root and lateral root development, root gravitropism, root hair development, vascular patterning, seed germination, apical hook formation, leaf morphogenesis, phyllotactic patterning, female gametophyte development and embryo development. Recently AUX1 has also been implicated in regulating plant responses to abiotic stresses. This review summarizes our current understanding of the developmental roles of AUX1/LAX gene family and will also briefly discuss the modelling approaches that are providing new insight into the role of auxin transport in plant development.

REM34 and REM35 Control Female and Male Gametophyte Development in Arabidopsis thaliana

The REproductive Meristem (REM) gene family encodes for transcription factors belonging to the B3 DNA binding domain superfamily. In Arabidopsis thaliana, the REM gene family is composed of 45 members, preferentially expressed during flower, ovule, and seed developments. Only a few members of this family have been functionally characterized: VERNALIZATION1 (VRN1) and, most recently, TARGET OF FLC AND SVP1 (TFS1) regulate flowering time and VERDANDI (VDD), together with VALKYRIE (VAL) that control the death of the receptive synergid cell in the female gametophyte. We investigated the role of REM34, REM35, and REM36, three closely related and linked genes similarly expressed in both female and male gametophytes. Simultaneous silencing by RNA interference (RNAi) caused about 50% of the ovules to remain unfertilized. Careful evaluation of both ovule and pollen developments showed that this partial sterility of the transgenic RNAi lines was due to a postmeiotic block in both female and male gametophytes. Furthermore, protein interaction assays revealed that REM34 and REM35 interact, which suggests that they work together during the first stages of gametogenesis.

Nitrogen Limitation Alters the Response of Specific Genes to Biotic Stress

In their natural environment, plants are generally confronted with multiple co-occurring stresses. However, the interaction between stresses is not well known and transcriptomic data in response to combined stresses remain scarce. This study aims at characterizing the interaction between transcriptomic responses to biotic stress and nitrogen (N) limitation. Plants were grown in low or full N, infected or not with Erwinia amylovora (Ea) and plant gene expression was analyzed through microarray and qRT-PCR. Most Ea-responsive genes had the same profile (induced/repressed) in response to Ea in low and full N. In response to stress combination, one third of modulated transcripts responded in a manner that could not be deduced from their response to each individual stress. Many defense-related genes showed a prioritization of their response to biotic stress over their response to N limitation, which was also observed using Pseudomonas syringae as a second pathosystem. Our results indicate an interaction between transcriptomic responses to N and biotic stress. A small fraction of transcripts was prioritized between antagonistic responses, reflecting a preservation of the plant defense program under N limitation. Furthermore, this interaction also led to a complex and specific response in terms of metabolism and cellular homeostasis-associated genes.

RNA-seq Analysis Reveals Gene Expression Profiling of Female Fertile and Sterile Ovules of PinusTabulaeformis Carr. during Free Nuclear Mitosis of the Female Gametophyte

The development of the female gametophyte (FG) is one of the key processes of life cycle alteration between the haploid gametophyte and the diploid sporophytes in plants and it is required for successful seed development after fertilization. It is well demonstrated that free nuclear mitosis (FNM) of FG is crucial for the development of the ovule. However, studies of the molecular mechanism of ovule and FG development focused mainly on angiosperms, such as Arabidopsis thaliana and further investigation of gymnosperms remains to be completed. Here, Illumina sequencing of six transcriptomic libraries obtained from developing and abortive ovules at different stages during free nuclear mitosis of magagametophyte (FNMM) was used to acquire transcriptome data and gene expression profiles of Pinus tabulaeformis. Six cDNA libraries generated a total of 71.0 million high-quality clean reads that aligned with 63,449 unigenes and the comparison between developing and abortive ovules identified 7174 differentially expressed genes (DEGs). From the functional annotation results, DEGs involved in the cell cycle and phytohormone regulation were highlighted to reveal their biological importance in ovule development. Furthermore, validation of DEGs from the phytohormone signal transduction pathway was performed using quantitative real-time PCR analysis, revealing the dynamics of transcriptional networks and potential key components in the regulation of FG development in P. tabulaeformis were identified. These findings provide new insights into the regulatory mechanisms of ovule development in woody gymnosperms.

Gibberellins negatively modulate ovule number in plants

Ovule formation is a complex developmental process in plants, with a strong impact on the production of seeds. Ovule primordia initiation is controlled by a gene network, including components of the signaling pathways of auxin, brassinosteroids and cytokinins. By contrast, gibberellins (GAs) and DELLA proteins, the negative regulators of GA signaling, have never been shown to be involved in ovule initiation. Here, we provide molecular and genetic evidence that points to DELLA proteins as novel players in the determination of ovule number in Arabidopsis and in species of agronomic interest, such as tomato and rapeseed, adding a new layer of complexity to this important developmental process. DELLA activity correlates positively with ovule number, acting as a positive factor for ovule initiation. In addition, ectopic expression of a dominant DELLA in the placenta is sufficient to increase ovule number. The role of DELLA proteins in ovule number does not appear to be related to auxin transport or signaling in the ovule primordia. Possible crosstalk between DELLA proteins and the molecular and hormonal network controlling ovule initiation is also discussed.