Female gametophyte development

RNA HELICASE 32 is essential for female gametophyte development in Arabidopsis

The normal progression of mitotic cycles and synchronized development within female reproductive organs are pivotal for sexual reproduction in plants. Nevertheless, our understanding of the genetic regulation governing mitotic cycles during the haploid phase of higher plants remains limited. In this study, we characterized RNA HELICASE 32 (RH32), which plays an essential role in female gametogenesis in Arabidopsis. The rh32 heterozygous mutant was semi-sterile, whereas the homozygous mutant was nonviable. The rh32 mutant allele could be transmitted through the male gametophyte, but not the female gametophyte. Phenotypic analysis revealed impaired mitotic progression, synchronization, and cell specification in rh32 female gametophytes, causing the arrest of embryo sacs. In the delayed pollination test, none of the retarded embryo sacs developed into functional female gametophytes, and the vast majority of rh32 female gametophytes were defective in the formation of the large central vacuole. RH32 is strongly expressed in the embryo sac. Knock-down of RH32 resulted in the accumulation of unprocessed 18 S pre-rRNA, implying that RH32 is involved in ribosome synthesis. Based on these findings, we propose that RH32 plays a role in ribosome synthesis, which is critical for multiple processes in female gametophyte development.

The BNB-GLID module regulates germline fate determination in Marchantia polymorpha

Germline fate determination is a critical event in sexual reproduction. Unlike animals, plants specify the germline by reprogramming somatic cells at the late stages of their development. However, the genetic basis of germline fate determination and how it evolved during the land plant evolution are still poorly understood. Here, we report that the plant homeodomain finger protein GERMLINE IDENTITY DETERMINANT (GLID) is a key regulator of the germline specification in liverwort, Marchantia polymorpha. Loss of the MpGLID function causes failure of germline initiation, leading to the absence of sperm and egg cells. Remarkably, the overexpression of MpGLID in M. polymorpha induces the ectopic formation of cells with male germline cell features exclusively in male thalli. We further show that MpBONOBO (BNB), with an evolutionarily conserved function, can induce the formation of male germ cell-like cells through the activation of MpGLID by directly binding to its promoter. The Arabidopsis (Arabidopsis thaliana) MpGLID ortholog, MALE STERILITY1 (AtMS1), fails to replace the germline specification function of MpGLID in M. polymorpha, demonstrating that a derived function of MpGLID orthologs has been restricted to tapetum development in flowering plants. Collectively, our findings suggest the presence of the BNB-GLID module in complex ancestral land plants that has been retained in bryophytes, but rewired in flowering plants for male germline fate determination.

JAGGER localization and function are dependent on GPI anchor addition

KEY MESSAGE

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

Polycomb proteins RING1A/B promote H2A monoubiquitination to regulate female gametophyte development in Arabidopsis

A functional female gametophyte is the basis of successful sexual reproduction in flowering plants. During female gametophyte development, the megaspore mother cell (MMC), which differentiates from a single subepidermal somatic cell in the nucellus, undergoes meiosis to produce four megaspores; only the one at the chalazal end, referred to as the functional megaspore (FM), then undergoes three rounds of mitosis and develops into a mature embryo sac. Here, we report that RING1A and RING1B (RING1A/B), two functionally redundant Polycomb proteins in Arabidopsis, are critical for female gametophyte development. Mutations of RING1A/B resulted in defects in the specification of the MMC and the FM, and in the subsequent mitosis of the FM, thereby leading to aborted ovules. Detailed analysis revealed that several genes essential for female gametophyte development were ectopically expressed in the ring1a ring1b mutant, including Argonaute (AGO) family genes and critical transcription factors. Furthermore, RING1A/B bound to some of these genes to promote H2A monoubiquitination (H2Aub). Taken together, our study shows that RING1A/B promote H2Aub modification at key genes for female gametophyte development, suppressing their expression to ensure that the development progresses correctly.

Unlocking fertility in the female gametophyte: a DEAD-box RNA helicase is essential for embryo sac development and seed setting

This article comments on:

Huang J, Qiao Z, Yu H, Lu Z, Chen W, Lu J, Wu J, Bao Y, Shahid MQ, Liu X. 2024. OsRH52A, a DEAD-box protein, regulates functional megaspore specification and is required for embryo sac development in rice. Journal of Experimental Botany 75, 4802–4821. https://doi.org/10.1093/jxb/erae180

Eight hydroxyproline-O-galactosyltransferases play essential roles in female reproductive development

In angiosperms, ovules give rise to seeds upon fertilization. Thus, seed formation is dependent on both successful ovule development and tightly controlled communication between female and male gametophytes. During establishment of these interactions, cell walls play a pivotal role, especially arabinogalactan-proteins (AGPs). AGPs are highly glycosylated proteins decorated by arabinogalactan side chains, representing 90 % of the AGP molecule. AGP glycosylation is initiated by a reaction catalysed by hydroxyproline-O-galactosyltransferases (Hyp-GALTs), specifically eight of them (GALT2-9), which add the first galactose to Hyp residues. Five Hyp-GALTs (GALT2, 5, 7, 8 and 9) were previously described as essential for AGP functions in pollen and ovule development, pollen-pistil interactions, and seed morphology. In the present work, a higher order Hyp-GALT mutant (23456789) was studied, with a high degree of under-glycosylated AGPs, to gain deeper insight into the crucial roles of these eight enzymes in female reproductive tissues. Notably, the 23456789 mutant demonstrated a high quantity of unfertilized ovules, displaying abnormal callose accumulation both at the micropylar region and, sometimes, throughout the entire embryo sac. Additionally, this mutant displayed ovules with abnormal embryo sacs, had a disrupted spatiotemporal distribution of AGPs in female reproductive tissues, and showed abnormal seed and embryo development, concomitant with a reduction in AGP-GlcA levels. This study revealed that at least three more enzymes exhibit Hyp-O-GALT activity in Arabidopsis (GALT3, 4 and 6), and reinforces the crucial importance of AGP carbohydrates in carrying out the biological functions of AGPs during plant reproduction.

A conserved GRAS-domain transcriptional regulator links meristem indeterminacy to sex determination in Ceratopteris gametophytes

Most land plants alternate between generations of sexual gametophytes and asexual sporophytes. Unlike seed plants, fern gametophytes are free living and grow independently of their sporophytes. In homosporous ferns such as Ceratopteris, gametophytes derived from genetically identical spores exhibit sexual dimorphism, developing as either males or hermaphrodites. Males lack meristems and promote cell differentiation into sperm-producing antheridia. In contrast, hermaphrodites initiate multicellular meristems that stay undifferentiated, sustain cell division and prothallus expansion, and drive the formation of egg-producing archegonia. Once initiating the meristem, hermaphrodites secrete the pheromone antheridiogen, which triggers neighboring slower-growing gametophytes to develop as males, while the hermaphrodites themselves remain insensitive to antheridiogen. This strategy promotes outcrossing and prevents all individuals in the colony from becoming males. This study reveals that an evolutionarily conserved GRAS-domain transcriptional regulator (CrHAM), directly repressed by Ceratopteris microRNA171 (CrmiR171), promotes meristem development in Ceratopteris gametophytes and determines the male-to-hermaphrodite ratio in the colony. CrHAM preferentially accumulates within the meristems of hermaphrodites but is excluded from differentiated antheridia. CrHAM sustains meristem proliferation and cell division through conserved hormone pathways. In the meantime, CrHAM inhibits the antheridiogen-induced conversion of hermaphrodites to males by suppressing the male program expression and preventing meristem cells from differentiating into sperm-producing antheridia. This finding establishes a connection between meristem indeterminacy and sex determination in ferns, suggesting both conserved and diversified roles of meristem regulators in land plants.

Are vacuolar dynamics crucial factors for plant cell division and differentiation?

Vacuoles are the largest membrane-bound organelles in plant cells, critical for development and environmental responses. Vacuolar dynamics indicate reversible changes of vacuoles in morphology, size, or numbers. In this review, we summarize current understandings of vacuolar dynamics in different types of plant cells, biological processes associated with vacuolar dynamics, and regulators controlling vacuolar dynamics. Specifically, we point out the possibility that vacuolar dynamics play key roles in cell division and differentiation, which are controlled by the nucleus. Finally, we propose three routes through which vacuolar dynamics actively participate in nucleus-controlled cellular activities.

Morphological Study on the Differentiation of Flower Buds and the Embryological Stages of Male and Female Floral Organs in Lespedeza davurica (Laxm.) Schindl. cv. JinNong (Fabaceae)

Lespedeza davurica (Laxm.) is a leguminous plant with significant ecological benefits, but its embryonic development mechanism remains unclear. We investigated the flower bud differentiation, megaspore and microspore formation, gametophyte development, and embryo and endosperm development in L. davurica. Our aim was to elucidate the relationship between the external morphology and internal development processes of male and female floral organs during growth, as well as the reproductive factors influencing fruiting. The results indicated that although the pistil develops later than the stamen during flower bud differentiation, both organs mature synchronously before flowering. L. davurica pollen exhibits three germination grooves, a reticulate outer wall, and papillary structures on the anther surface. In vivo pollination experiments revealed abnormal spiral growth of L. davurica pollen tubes within the style and the occurrence of callus plugs, which may reduce the seed setting rate. The anther wall development follows the dicotyledonous type, with tetrads formed through microspore meiosis exhibiting both left-right symmetry and tetrahedral arrangements. L. davurica has a single ovule, and the embryo sac develops in the monosporic polygonum type. After dormancy, the zygote undergoes multiple divisions, progressing through spherical, heart-shaped, and torpedo-shaped embryo stages, culminating in a mature embryo. A mature seed comprises cotyledons, hypocotyl, embryo, radicle, and seed coat. Phylogenetic tree analysis reveals a close genetic relationship between L. davurica and other leguminous plants from the genera Lespedeza and Medicago. This study provides valuable insights into the regulation of flowering and hybrid breeding in leguminous plants and offers a new perspective on the development of floral organs and seed setting rates.

Reactive oxygen species are signaling molecules that modulate plant reproduction

Reactive oxygen species (ROS) can serve as signaling molecules that are essential for plant growth and development but abiotic stress can lead to ROS increases to supraoptimal levels resulting in cellular damage. To ensure efficient ROS signaling, cells have machinery to locally synthesize ROS to initiate cellular responses and to scavenge ROS to prevent it from reaching damaging levels. This review summarizes experimental evidence revealing the role of ROS during multiple stages of plant reproduction. Localized ROS synthesis controls the formation of pollen grains, pollen-stigma interactions, pollen tube growth, ovule development, and fertilization. Plants utilize ROS-producing enzymes such as respiratory burst oxidase homologs and organelle metabolic pathways to generate ROS, while the presence of scavenging mechanisms, including synthesis of antioxidant proteins and small molecules, serves to prevent its escalation to harmful levels. In this review, we summarized the function of ROS and its synthesis and scavenging mechanisms in all reproductive stages from gametophyte development until completion of fertilization. Additionally, we further address the impact of elevated temperatures induced ROS on impairing these reproductive processes and of flavonol antioxidants in maintaining ROS homeostasis to minimize temperature stress to combat the impact of global climate change on agriculture.

Distribution of exchangeable Ca2+ during the process of Larix decidua Mill. pollination and germination

The involvement of Ca2+ ions in angiosperms sexual processes is well established, while in gymnosperms, such knowledge remains limited and is still a topic of discussion. In this study, we focused on Larix decidua, using Alizarin-red S staining and the pyroantimonate method to examine the tissue and subcellular distribution of free and loosely bound Ca2+ ions at different stages of the male gametophyte's development and its interaction with the ovule. Our findings show that in larch, both the germination of pollen grains and the growth of pollen tubes occur in an environment rich in Ca2+. These ions play a crucial role in the adhesion of the pollen grain to the stigmatic tip and its subsequent movement to the micropylar canal. There is a significant presence of free and loosely bound Ca2+ ions in both the fluid of the micropylar canal and the extracellular matrix of the nucellus. As the pollen tube extends through the nucellus, we observed a notable accumulation of Ca2+ ions just above the entry to the mature archegonium, a region likely crucial for the male gametophyte's directional growth. Meanwhile, the localized presence of free and loosely bound Ca2+ ions within the egg cell cytoplasm may inhibit the pollen tubes growth and rupture, playing an important role in fertilization.

Reactive oxygen species: Multidimensional regulators of plant adaptation to abiotic stress and development

Reactive oxygen species (ROS) are produced as undesirable by-products of metabolism in various cellular compartments, especially in response to unfavorable environmental conditions, throughout the life cycle of plants. Stress-induced ROS production disrupts normal cellular function and leads to oxidative damage. To cope with excessive ROS, plants are equipped with a sophisticated antioxidative defense system consisting of enzymatic and non-enzymatic components that scavenge ROS or inhibit their harmful effects on biomolecules. Nonetheless, when maintained at relatively low levels, ROS act as signaling molecules that regulate plant growth, development, and adaptation to adverse conditions. Here, we provide an overview of current approaches for detecting ROS. We also discuss recent advances in understanding ROS signaling, ROS metabolism, and the roles of ROS in plant growth and responses to various abiotic stresses.

Characteristic of GEX1 genes reveals the essential roles for reproduction in cotton

GEX1 (gamete expressed 1) proteins are critical membrane proteins conserved among flowering plants that are involved in the nuclear fusion and embryonic development. Herein, we identified the 32 GEX1 proteins from representative land plants. In cotton, GEX1 genes expressed in various tissues across all stages of the life cycle, especially in pollen. Subcellular localization indicated the position of GhGEX1 protein was localized in the endoplasmic reticulum. Experimental research has demonstrated that GhGEX1 has the potential to improve the partial abortion phenotype in Arabidopsis. CRISPR/Cas9-mediated knockout of GhGEX1 exhibited the seed abortion. Paraffin section of the ovule revealed that the polar nuclear fusion of ghgex1 plants remains at a standstill when the wild type has developed into a normal embryo. Comparative transcriptome analysis showed that the DEGs of reproductive-related processes and membrane-related processes were repressed in the pollen of knockout lines. The predicted protein interactions showed that GhGEX1 probably functioned through interactions with proteins related to reproduction and membrane. From all these investigations, it was possible to conclude that the GEX1 proteins are evolutionarily conserved in flowering plants and elucidated the pivotal roles during fertilization and early embryonic development in cotton.

The impact of heat stress in plant reproduction

The increment in global temperature reduces crop productivity, which in turn threatens food security. Currently, most of our food supply is produced by plants and the human population is estimated to reach 9 billion by 2050. Gaining insights into how plants navigate heat stress in their reproductive phase is essential for effectively overseeing the future of agricultural productivity. The reproductive success of numerous plant species can be jeopardized by just one exceptionally hot day. While the effects of heat stress on seedlings germination and root development have been extensively investigated, studies on reproduction are limited. The intricate processes of gamete development and fertilization unfold within a brief timeframe, largely concealed within the flower. Nonetheless, heat stress is known to have important effects on reproduction. Considering that heat stress typically affects both male and female reproductive structures concurrently, it remains crucial to identify cultivars with thermotolerance. In such cultivars, ovules and pollen can successfully undergo development despite the challenges posed by heat stress, enabling the completion of the fertilization process and resulting in a robust seed yield. Hereby, we review the current understanding of the molecular mechanisms underlying plant resistance to abiotic heat stress, focusing on the reproductive process in the model systems of Arabidopsis and Oryza sativa.

Bacteria existing in pre-pollinated styles (silks) can defend the exposed male gamete fertilization channel of maize against an environmental Fusarium pathogen

In flowering plants, fertilization requires exposing maternal style channels to the external environment to capture pollen and transmit its resident sperm nuclei to eggs. This results in progeny seed. However, environmental fungal pathogens invade developing seeds through the style. We hypothesized that prior to environmental exposure, style tissue already possesses bacteria that can protect styles and seed from such pathogens. We further hypothesized that farmers have been inadvertently selecting immature styles over many generations to have such bacteria. We tested these hypotheses in maize, a wind-pollinated crop, which has unusually long styles (silks) that are invaded by the economically-important fungal pathogen Fusarium graminearum (Fg). Here, unpollinated silk-associated bacteria were cultured from a wild teosinte ancestor of maize and diverse maize landraces selected by indigenous farmers across the Americas, grown in a common Canadian field for one season. The bacteria were taxonomically classified using 16S rRNA sequencing. In total, 201 bacteria were cultured, spanning 29 genera, 63 species, and 62 unique OTUs, dominated by Pseudomonas, Pantoea and Microbacterium. These bacteria were tested for their ability to suppress Fg in vitro which identified 10 strains belonging to 6 species: Rouxiella badensis, Pantoea ananatis, Pantoea dispersa, Pseudomonas koreensis, Rahnella aquatilis, and Ewingella americana. Two anti-Fg strains were sprayed onto silks before/after Fg inoculation, resulting in ≤90% reductions in disease (Gibberella ear rot) and 70-100% reductions in associated mycotoxins (deoxynivalenol and zearalenone) in progeny seeds. These strains also protected progeny seeds post-harvest. Confocal fluorescent imaging showed that one silk bacterium (Rouxiella AS112) colonized susceptible entry points of Fg on living silks including stigmatic trichomes, wounds, and epidermal surfaces where they formed thick biofilms. Post-infection, AS112 was associated with masses of dead Fg hyphae. These results suggest that the maize style (silk) is endowed with potent bacteria from the mother plant to protect itself and progeny from Fusarium. The evidence suggests this trait may have been selected by specific indigenous peoples, though this interpretation requires further study.

Nuclear Fusion in Yeast and Plant Reproduction

Nuclear fusion is essential for the sexual reproduction of various organisms, including plants, animals, and fungi. During the life cycle of flowering plants, nuclear fusion occurs three times: once during female gametogenesis and twice during double fertilization, when two sperm cells fertilize the egg and the central cell. Haploid nuclei migrate in an actin filament-dependent manner to become in close contact and, then, two nuclei fuse. The nuclear fusion process in plant reproduction is achieved through sequential nuclear membrane fusion events. Recent molecular genetic analyses using Arabidopsis thaliana showed the conservation of nuclear membrane fusion machinery between plants and the budding yeast Saccharomyces cerevisiae. These include the heat-shock protein 70 in the endoplasmic reticulum and the conserved nuclear membrane proteins. Analyses of the A. thaliana mutants of these components show that the completion of the sperm nuclear fusion at fertilization is essential for proper embryo and endosperm development.

Signaling between sporophytic integuments and developing female gametophyte during ovule development

Ovules are precursors of seeds and contain sporophytic integuments and gametophytic embryo sac. In Arabidopsis, embryo sac development requires highly synchronized morphogenesis of integument such that defects in integument growth often accompanies with a block in megagametogenesis, indicating that integument instructs the development of female gametophytes. In this mini review, we discuss signaling pathways through which integument cells mediate embryo sac development. We also propose ways to identify key signaling factors for the communication between integument and developing female gametophyte.

Gene Expression Analysis of Different Organs and Identification of AP2 Transcription Factors in Flax (Linum usitatissimum L.)

Flax (Linum usitatissimum L.) is an important oilseed crop widely cultivated for its oil and fiber. This study conducted transcriptome analysis to analyze the gene expression profiles of roots, leaves, stamens, pistils, and fruits in the flax cultivar Longya10. A total of 43,471 genes were detected in the RNA-seq data, with 34,497 genes showing differential expression levels between different organs. Gene expression patterns varied across different organs, with differences observed in expression-regulating genes within specific organs. However, 23,448 genes were found to be commonly expressed across all organs. Further analysis revealed organ-specific gene expressions, with 236, 690, 544, 909, and 1212 genes identified in pistils, fruits, leaves, roots, and stamens, respectively. Gene Ontology (GO) enrichment analysis was performed on these organ-specific genes, and significant enrichment was observed in various biological processes, cellular components, and molecular functions, providing new insights for the specific growth patterns of flax organs. Furthermore, we investigated the expression differences of AP2 transcription factors in various tissues and organs of Longya10. We identified 96 AP2 genes that were differentially expressed in different organs and annotated them into various biological pathways. Our results suggest that AP2 transcription factors may play important roles in regulating the growth and development of flax organs including stress response. In summary, our study provides a comprehensive analysis of gene expression patterns in different organs and tissues of flax plant and identifies potential critical regulators of flax organ growth and development. These findings contribute to a better understanding of the molecular mechanisms underlying flax organ development and may have important implications for the genetic improvement of flax crops.

Arabidopsis TCP4 transcription factor inhibits high temperature-induced homeotic conversion of ovules

Abnormal high temperature (HT) caused by global warming threatens plant survival and food security, but the effects of HT on plant organ identity are elusive. Here, we show that Class II TEOSINTE BRANCHED 1/CYCLOIDEA/ PCF (TCP) transcription factors redundantly protect ovule identity under HT. The duodecuple tcp2/3/4/5/10/13/17/24/1/12/18/16 (tcpDUO) mutant displays HT-induced ovule conversion into carpelloid structures. Expression of TCP4 in tcpDUO complements the ovule identity conversion. TCP4 interacts with AGAMOUS (AG), SEPALLATA3 (SEP3), and the homeodomain transcription factor BELL1 (BEL1) to strengthen the association of BEL1 with AG-SEP3. The tcpDUO mutant synergistically interacts with bel1 and the ovule identity gene seedstick (STK) mutant stk in tcpDUO bel1 and tcpDUO stk. Our findings reveal the critical roles of Class II TCPs in maintaining ovule identity under HT and shed light on the molecular mechanisms by which ovule identity is determined by the integration of internal factors and environmental temperature.

Mutation of DEFECTIVE EMBRYO SAC1 results in a low seed-setting rate in rice by regulating embryo sac development

The seed-setting rate has a significant effect on grain yield in rice (Oryza sativa L.). Embryo sac development is essential for seed setting; however, the molecular mechanism underlying this process remains unclear. Here, we isolated defective embryo sac1 (des1), a rice mutant with a low seed-setting rate. Cytological examination showed degenerated embryo sacs and reduced fertilization capacity in des1. Map-based cloning revealed a nonsense mutation in OsDES1, a gene that encodes a putative nuclear envelope membrane protein (NEMP)-domain-containing protein that is preferentially expressed in pistils. The OsDES1 mutation disrupts the normal formation of functional megaspores, which ultimately results in a degenerated embryo sac in des1. Reciprocal crosses showed that fertilization is abnormal and that the female reproductive organ is defective in des1. OsDES1 interacts with LONELY GUY (LOG), a cytokinin-activating enzyme that acts in the final step of cytokinin synthesis; mutation of LOG led to defective female reproductive organ development. These results demonstrate that OsDES1 functions in determining the rice seed-setting rate by regulating embryo sac development and fertilization. Our study sheds light on the function of NEMP-type proteins in rice reproductive development.

I Choose You: Selecting Accurate Reference Genes for qPCR Expression Analysis in Reproductive Tissues in Arabidopsis thaliana

Quantitative real-time polymerase chain reaction (qPCR) is a widely used method to analyse the gene expression pattern in the reproductive tissues along with detecting gene levels in mutant backgrounds. This technique requires stable reference genes to normalise the expression level of target genes. Nonetheless, a considerable number of publications continue to present qPCR results normalised to a single reference gene and, to our knowledge, no comparative evaluation of multiple reference genes has been carried out in specific reproductive tissues of Arabidopsis thaliana. Herein, we assessed the expression stability levels of ten candidate reference genes (UBC9, ACT7, GAPC-2, RCE1, PP2AA3, TUA2, SAC52, YLS8, SAMDC and HIS3.3) in two conditional sets: one across flower development and the other using inflorescences from different genotypes. The stability analysis was performed using the RefFinder tool, which combines four statistical algorithms (geNorm, NormFinder, BestKeeper and the comparative ΔCt method). Our results showed that RCE1, SAC52 and TUA2 had the most stable expression in different flower developmental stages while YLS8, HIS3.3 and ACT7 were the top-ranking reference genes for normalisation in mutant studies. Furthermore, we validated our results by analysing the expression pattern of genes involved in reproduction and examining the expression of these genes in published mutant backgrounds. Overall, we provided a pool of appropriate reference genes for expression studies in reproductive tissues of A. thaliana, which will facilitate further gene expression studies in this context. More importantly, we presented a framework that will promote a consistent and accurate analysis of gene expression in any scientific field. Simultaneously, we highlighted the relevance of clearly defining and describing the experimental conditions associated with qPCR to improve scientific reproducibility.

Cell Division and Meristem Dynamics in Fern Gametophytes

One of the most important questions in all multicellular organisms is how to define and maintain different cell fates during continuous cell division and proliferation. Plant meristems provide a unique research system to address this fundamental question because meristems dynamically maintain themselves and sustain organogenesis through balancing cell division and cell differentiation. Different from the gametophytes of seed plants that depend on their sporophytes and lack meristems, the gametophytes of seed-free ferns develop different types of meristems (including apical cell-based meristems and multicellular apical and marginal meristems) to promote independent growth and proliferation during the sexual gametophyte phase. Recent studies combining confocal time-lapse imaging and computational image analysis reveal the cellular basis of the initiation and proliferation of different types of meristems in fern gametophytes, providing new insights into the evolution of meristems in land plants. In this review, we summarize the recent progress in understanding the cell growth dynamics in fern gametophytes and discuss both conserved and diversified mechanisms underlying meristem cell proliferation in seed-free vascular plants.

The type-B response regulators ARR10, ARR12, and ARR18 specify the central cell in Arabidopsis

In Arabidopsis thaliana, the female gametophyte consists of two synergid cells, an egg cell, a diploid central cell, and three antipodal cells. CYTOKININ INDEPENDENT 1 (CKI1), a histidine kinase constitutively activating the cytokinin signaling pathway, specifies the central cell and restricts the egg cell. However, the mechanism regulating CKI1-dependent central cell specification is largely unknown. Here, we showed that the type-B ARABIDOPSIS RESPONSE REGULATORS10, 12, and 18 (ARR10/12/18) localize at the chalazal pole of the female gametophyte. Phenotypic analysis showed that the arr10 12 18 triple mutant is female sterile. We examined the expression patterns of embryo sac marker genes and found that the embryo sac of arr10 12 18 plants had lost central cell identity, a phenotype similar to that of the Arabidopsis cki1 mutant. Genetic analyses demonstrated that ARR10/12/18, CKI1, and ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN2, 3, and 5 (AHP2/3/5) function in a common pathway to regulate female gametophyte development. In addition, constitutively activated ARR10/12/18 in the cki1 embryo sac partially restored the fertility of cki1. Results of transcriptomic analysis supported the conclusion that ARR10/12/18 and CKI1 function together to regulate the identity of the central cell. Our results demonstrated that ARR10/12/18 function downstream of CKI1-AHP2/3/5 as core factors to determine cell fate of the female gametophyte.

Genic male and female sterility in vegetable crops

Vegetable crops are greatly appreciated for their beneficial nutritional and health components. Hybrid seeds are widely used in vegetable crops for advantages such as high yield and improved resistance, which require the participation of male (stamen) and female (pistil) reproductive organs. Male- or female-sterile plants are commonly used for production of hybrid seeds or seedless fruits in vegetables. In this review we will focus on the types of genic male sterility and factors affecting female fertility, summarize typical gene function and research progress related to reproductive organ identity and sporophyte and gametophyte development in vegetable crops [mainly tomato (Solanum lycopersicum) and cucumber (Cucumis sativus)], and discuss the research trends and application perspectives of the sterile trait in vegetable breeding and hybrid production, in order to provide a reference for fertility-related germplasm innovation.

The egg cell is preferentially fertilized in Arabidopsis double fertilization

In flowering plants (angiosperms), fertilization of the egg cell by one sperm cell produces an embryo, whereas fusion of a second sperm cell with the central cell generates the endosperm. In most angiosperms like Arabidopsis, a pollen grain contains two isomorphic sperm cells required for this double fertilization process. A long-standing unsolved question is whether the two fertilization events have any preference. A tool to address this question is the usage of the cyclin-dependent kinase a1 (cdka;1) mutant pollen, which produces a single sperm-like cell (SLC). Here, we first adopt a complementation-based fluorescence-labeling method to successfully separate and collect cdka;1 mutant pollen containing a single SLC. Single-cell RNA-sequencing analysis revealed that cdka;1 SLCs show a gene expression profile highly similar to that of sperm cells and not to the generative cell, precursor of the two sperm cells. Pollination assays using a limited number of cdka;1 mutant pollen revealed that in 98.2% of the ovules, single fertilization of the egg cell occurred. Pollination of pistils with excessive cdka;1 mutant pollen allowed the delivery of a second SLC via fertilization recovery, which fertilized the central cell, resulting in 20.7% double-fertilized ovules. This indicates that cdka;1 SLCs are able to fertilize both the egg and the central cell. Taken together, our findings have answered a long-standing question and support that preferential fertilization of the egg cell is evident in Arabidopsis.

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.

Expression of GEX1 Orthologs of Brassica rapa and Oryza sativa Rescued the Nuclear Fusion Defect of the Arabidopsis GEX1 Mutant

Nuclear fusion is required for the sexual reproduction of various organisms, including angiosperms. During the life cycle of angiosperms, nuclear fusion occurs three times: once during female gametogenesis, when the two polar nuclei fuse in the central cell, and twice during double fertilization. Nuclear fusion in plant reproduction is achieved by sequential nuclear fusion events: outer and inner nuclear membrane fusion. Arabidopsis gamete expressed 1 (GEX1) is a nuclear membrane protein of gametes that is required for nuclear fusion during reproduction. Although orthologs of GEX1 have been identified in various land plants, sequence identities are not high, even between angiosperm GEX1 orthologs; the sequence identity between Arabidopsis GEX1 and Oryza sativa GEX1 ortholog is lower than 50%. Here, we found that the expression of GEX1 orthologs of O. sativa, as well as of Brassica rapa from the Arabidopsis GEX1 promoter, rescued the polar nuclear fusion defect of the gex1 mutant. We also found that the expression of these GEX1 orthologs rescued the lethality of the gex1 homozygous mutant, which is proposed to be caused by the sperm nuclear fusion defects upon fertilization. Our results indicate a functional conservation between Arabidopsis and O. sativa GEX1 orthologs, despite their relatively low sequence identities.

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.

Positional cues and cell division dynamics drive meristem development and archegonium formation in Ceratopteris gametophytes

Fern gametophytes are autotrophic and independent of sporophytes, and they develop pluripotent meristems that drive prothallus development and sexual reproduction. To reveal cellular dynamics during meristem development in fern gametophytes, we performed long-term time-lapse imaging and determined the real-time lineage, identity and division activity of each single cell from meristem initiation to establishment in gametophytes of the fern Ceratopteris richardii. Our results demonstrate that in Ceratopteris gametophytes, only a few cell lineages originated from the marginal layer contribute to meristem initiation and proliferation, and the meristem lacks a distinguishable central zone or apical cell with low division activity. Within the meristem, cell division is independent of cell lineages and cells at the marginal layer are more actively dividing than inner cells. Furthermore, the meristem triggers differentiation of adjacent cells into egg-producing archegonia in a position-dependent manner. These findings advance the understanding of diversified meristem and gametophyte development in land plants.

Time-lapse imaging of the fern Ceratopteris richardii during meristem initiation and proliferation provides insights into the lineage, identity and division activity of each cell throughout the growth of gametophytes.

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.

ROS homeostasis mediated by MPK4 and SUMM2 determines synergid cell death

Sexual plant reproduction depends on the attraction of sperm-cell delivering pollen tubes (PT) by two synergids, followed by their programmed cell death (PCD) in Arabidopsis. Disruption of the mitogen-activated protein kinase 4 (MPK4) by pathogenic effectors activates the resistance protein (R) SUMM2-mediated immunity and cell death. Here we show that synergid preservation and reactive oxygen species (ROS) homeostasis are intimately linked and maintained by MPK4. In mpk4, ROS levels are increased and synergids prematurely undergo PCD before PT-reception. However, ROS scavengers and the disruption of SUMM2, in mpk4, restore ROS homeostasis, synergid maintenance and PT perception, demonstrating that the guardian of MPK4, SUMM2, triggers synergid-PCD. In mpk4/summ2, PTs show a feronia-like overgrowth phenotype. Our results show that immunity-associated PCD and synergid cell death during plant reproduction are regulated by MPK4 underscoring an underlying molecular mechanism for the suppression of plant reproduction during systemic R-mediated immunity.

Synergid cells undergo programmed cell death following pollen tube reception and successful fertilization. Here the authors show that premature synergid cell death is prevented by the mitogen activated protein kinase MPK4 and the R protein SUMM2 which maintain ROS homeostasis in Arabidopsis.

A procedure for Dex-induced gene transactivation in Arabidopsis ovules

BACKGROUND

Elucidating the genetic and molecular control of plant reproduction often requires the deployment of functional approaches based on reverse or forward genetic screens. The loss-of-function of essential genes, however, may lead to plant lethality prior to reproductive development or to the formation of sterile structures before the organ-of-interest can be analyzed. In these cases, inducible approaches that enable a spatial and temporal control of the genetic perturbation are extremely valuable. Genetic induction in reproductive organs, such as the ovule, deeply embedded in the flower, is a delicate procedure that requires both optimization and validation.

RESULTS

Here we report on a streamlined procedure enabling reliable induction of gene expression in Arabidopsis ovule and anther tissues using the popular pOP/LhGR Dex-inducible system. We demonstrate its efficiency and reliability using fluorescent reporter proteins and histochemical detection of the GUS reporter gene.

CONCLUSION

The pOP/LhGR system allows for a rapid, efficient, and reliable induction of transgenes in developing ovules without compromising developmental progression. This approach opens new possibilities for the functional analysis of candidate regulators in sporogenesis and gametogenesis, which is otherwise affected by early lethality in conventional, stable mutants.

The SEEL motif and members of the MYB-related REVEILLE transcription factor family are important for the expression of LORELEI in the synergid cells of the Arabidopsis female gametophyte

Synergid cells in the micropylar end of the female gametophyte are required for critical cell-cell signaling interactions between the pollen tube and the ovule that precede double fertilization and seed formation in flowering plants. LORELEI (LRE) encodes a putative GPI-anchored protein that is expressed primarily in the synergid cells, and together with FERONIA, a receptor-like kinase, it controls pollen tube reception by the receptive synergid cell. Still, how LRE expression is controlled in synergid cells remains poorly characterized. We identified candidate cis-regulatory elements enriched in LRE and other synergid cell-expressed genes. One of the candidate motifs ('TAATATCT') in the LRE promoter was an uncharacterized variant of the Evening Element motif that we named as the Short Evening Element-like (SEEL) motif. Deletion or point mutations in the SEEL motif of the LRE promoter resulted in decreased reporter expression in synergid cells, demonstrating that the SEEL motif is important for expression of LRE in synergid cells. Additionally, we found that LRE expression is decreased in the loss of function mutants of REVEILLE (RVE) transcription factors, which are clock genes known to bind the SEEL and other closely related motifs. We propose that RVE transcription factors regulate LRE expression in synergid cells by binding to the SEEL motif in the LRE promoter. Identification of cis-regulatory elements and transcription factors involved in the expression of LRE will serve as a foundation to characterize the gene regulatory networks in synergid cells.

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.

Viability of Embryo Sacs and Fruit Set in Different Plum (Prunus domestica L.) Cultivars Grown under Norwegian Climatic Conditions

Compatibility and synchrony between specialized tissues of the pistil, female gametophytes and male gametophytes, are necessary for successful pollination, fertilization, and fruit set in angiosperms. The aim of the present work was to study the development and viability of embryo sacs, as well as fertilization success, in relation to the fruit set of the cultivars ‘Mallard’, ‘Edda’, ‘Jubileum’, and ‘Reeves’, under specific Norwegian climatic conditions. Emasculated, unpollinated, and open-pollinated flowers were collected at the beginning of flowering, and on the 3rd, 6th, 9th, and 12th days after flowering, from all four plum cultivars over two years (2018/2019). Ovaries were dehydrated, embedded in paraffin wax, sectioned, stained, and observed under a light microscope. Results showed the existence of synchronization between successive phases in the development of the embryo sac and individual phases of flowering. All plum cultivars had higher percentages of viable embryo sacs, fertilized embryo sacs, and fruit set in 2018 than in 2019. These differences may be related to the very low temperatures during the post-full-flowering period in 2019, and to the low adaptation of some studied cultivars to unfavorable conditions. In our study, the cultivar ‘Jubileum’ showed the highest percentage of viable embryo sacs, fertilized embryo sacs, and fruit set compared to other cultivars, i.e., the best low-temperature adaptation.

Aerially Applied Zinc Oxide Nanoparticle Affects Reproductive Components and Seed Quality in Fully Grown Bean Plants (Phaseolus vulgaris L.)

The development of reproductive components in plant species is susceptible to environmental stresses. The extensive application of zinc oxide nanoparticles (nZnO) in various agro-industrial processes has jeopardized the performance and functionality of plants. To understand the response of the developmental (gametogenesis and sporogenesis) processes to nanoparticles (NPs) exposure, the aerial application of nZnO and their ionic counterpart of ZnSO₄ at four different levels were examined on bean plants (Phaseolus vulgaris) before the flowering stage. To evaluate the mentioned processes, briefly, flowers in multiple sizes were fixed in paraffin, followed by sectioning and optical analysis. The possibility of alteration in reproductive cells was thoroughly analyzed using both light and electron microscopes. Overall, our results revealed the histological defects in male and female reproductive systems of mature plants depend on NPs levels. Furthermore, NPs caused tapetum abnormalities, aberrations in carbohydrate accumulation, and apoptosis. The nZnO induced abnormal alterations right after meiosis and partly hindered the microspore development, leading to infertile pollens. The seed yield and dry weight were reduced to 70 and 82% at 2,000 mg L^-1 nZnO foliar exposure, respectively. The sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis pattern showed the increased expression of two proteins at the molecular weight of 28 and 42 kDa at various concentrations of nZnO and ZnSO₄. Overall, our results provided novel insights into the negative effect of nano-scaled Zn on the differential mechanism involved in the reproductive stage of the plants compared with salt form.

Ecological and phylogenetic constraints determine the stage of anthetic ovule development in orchids

PREMISE

Unlike most flowering plants, orchid flowers have under-developed ovules that complete development only after pollination. Classical studies reported variation in the stage in which ovule development is arrested, but the extent of this variation and its evolutionary and ecological significance are unclear.

METHODS

Here, we used light microscopy to observe ovule development at anthesis for 39 species not previously studied and surveyed the literature gaining information on 94 orchid species. Tropical and temperate members of all five orchid subfamilies as well as species with contrasting pollination strategies (rewarding versus deceptive) and life forms (epiphytic versus terrestrial) were represented. We analyzed the data using statistical comparisons and a phylogenetic generalized least square (PGLS) analysis.

RESULTS

Apostasioideae, the sister to the rest of the orchids, have mature ovules similar to other Asparagales, while under-differentiated ovules are present in the other subfamilies. Ovule developmental stages showed high variation even among closely related groups. Ovules were more developed in terrestrial than in epiphytic, in temperate than in tropical, and in rewarding than in deceptive pollination orchid species. This latter comparison was also significant in the PGLS analysis.

CONCLUSIONS

These results suggest that ovule developmental stage in orchids can be shaped by ecological factors, such as seasonality and pollination strategy, and can be selected for optimizing female reproductive investment.

Global Survey of Alternative Splicing in Rice by Direct RNA Sequencing During Reproductive Development: Landscape and Genetic Regulation

Alternative splicing is a widespread phenomenon, which generates multiple isoforms of the gene product. Reproductive development is the key process for crop production. Although numerous forms of alternative splicing have been identified in model plants, large-scale study of alternative splicing dynamics during reproductive development in rice has not been conducted. Here, we investigated alternative splicing of reproductive development of young panicles (YP), unfertilized florets (UF) and fertilized florets (F) in rice using direct RNA sequencing, small RNA sequencing, and degradome sequencing. We identified a total of 35,317 alternative splicing (AS) events, among which 67.2% splicing events were identified as novel alternative splicing events. Intron retention (IR) was the most abundant alternative splicing subtype. Splicing factors that differentially expressed and alternatively spliced could result in global alternative splicing. Global analysis of miRNAs-targets prediction revealed that alternative spliced transcripts affected miRNAs' targets during development. Degradome sequencing detected only 6.8% of the differentially alternative splicing transcripts, suggesting a productive transcripts generation during development. In addition, alternative splicing isoforms of Co-like, a transcription factor, interacted with Casein kinase 1-like protein HD1 (CKI) examined in luciferase assay, which could modulate normal male-floral organs development and flowering time. These results reveal that alternative splicing is intensely associated with developmental stages, and a high complexity of gene regulation.

Structure of antipodal cells nuclei of wheat embryo sac during programmed cell death

During antipodal cells PCD, polytene chromosomes rearrangement, segregation of nucleoli components and extrusion of nuclear components occur, cytochrome c is released from the mitochondria and DNA breaks appear. We studied in detail the nuclei of cells of the antipodal complex of wheat embryo sac (Triticum aestivum L.) during programmed cell death (PCD). The antipodal complex has been reported to be formed before double fertilisation of the embryo sac. Polyploidisation leads to the formation of giant polytene chromosomes in the nuclei of antipodal cells. These chromosomes are involved in secretory functions and are important for the development of cellular endosperm. Terminal deoxynucleotidyl transferase dUTP nick end labelling assay and immunodetection revealed DNA breaks in the nuclei and release of cytochrome c from mitochondria into the cytoplasm of antipodal cells during PCD. We used transmission electron microscopy, immunodetection and histochemistry to analyse the characteristic structural changes in the nuclei of antipodal cells during PCD. These included sequential structural changes in the nuclei containing polytene chromosomes, segregation of some components of the nucleolus into the bodies of polytene chromosomes, extrusion of nucleolar components and parts of chromosomes into the cytoplasm of antipodal cells and then into the endosperm coenocyte. The obtained results expand the understanding of the structural changes of plant cells with giant polytene chromosomes during PCD.

Divergence among rice cultivars reveals roles for transposition and epimutation in ongoing evolution of genomic imprinting

Parent-of-origin-dependent gene expression in mammals and flowering plants results from differing chromatin imprints (genomic imprinting) between maternally and paternally inherited alleles. Imprinted gene expression in the endosperm of seeds is associated with localized hypomethylation of maternally but not paternally inherited DNA, with certain small RNAs also displaying parent-of-origin-specific expression. To understand the evolution of imprinting mechanisms in Oryza sativa (rice), we analyzed imprinting divergence among four cultivars that span both japonica and indica subspecies: Nipponbare, Kitaake, 93-11, and IR64. Most imprinted genes are imprinted across cultivars and enriched for functions in chromatin and transcriptional regulation, development, and signaling. However, 4 to 11% of imprinted genes display divergent imprinting. Analyses of DNA methylation and small RNAs revealed that endosperm-specific 24-nt small RNA-producing loci show weak RNA-directed DNA methylation, frequently overlap genes, and are imprinted four times more often than genes. However, imprinting divergence most often correlated with local DNA methylation epimutations (9 of 17 assessable loci), which were largely stable within subspecies. Small insertion/deletion events and transposable element insertions accompanied 4 of the 9 locally epimutated loci and associated with imprinting divergence at another 4 of the remaining 8 loci. Correlating epigenetic and genetic variation occurred at key regulatory regions-the promoter and transcription start site of maternally biased genes, and the promoter and gene body of paternally biased genes. Our results reinforce models for the role of maternal-specific DNA hypomethylation in imprinting of both maternally and paternally biased genes, and highlight the role of transposition and epimutation in rice imprinting evolution.

Spatiotemporal control of miR398 biogenesis, via chromatin remodeling and kinase signaling, ensures proper ovule development

The coordinated development of sporophytic and gametophytic tissues is essential for proper ovule patterning and fertility. However, the mechanisms regulating their integrated development remain poorly understood. Here, we report that the Swi2/Snf2-Related1 (SWR1) chromatin-remodeling complex acts with the ERECTA receptor kinase-signaling pathway to control female gametophyte and integument growth in Arabidopsis thaliana by inhibiting transcription of the microRNA gene MIR398c in early-stage megagametogenesis. Moreover, pri-miR398c is transcribed in the female gametophyte but is then translocated to and processed in the ovule sporophytic tissues. Together, SWR1 and ERECTA also activate ARGONAUTE10 (AGO10) expression in the chalaza; AGO10 sequesters miR398, thereby ensuring the expression of three AGAMOUS-LIKE (AGL) genes (AGL51, AGL52, and AGL78) in the female gametophyte. In the context of sexual organ morphogenesis, these findings suggest that the spatiotemporal control of miRNA biogenesis, resulting from coordination between chromatin remodeling and cell signaling, is essential for proper ovule development in Arabidopsis.

The neglected other half - role of the pistil in plant heat stress responses

Heat stress coinciding with reproductive stage leads to a significant loss in reproductive organs viability, resulting in lower seed-set and crop productivity. Successful fertilization and seed formation are determined by the viability of male and female reproductive organs. The impact of heat stress on the male reproductive organ (pollen) is studied more often compared to the female reproductive organ (pistil). This is attributed to easier accessibility of the pollen coupled with the notion that the pistil's role in fertilization and seed-set under heat stress is negligible. However, depending on species and developmental stages, recent studies reveal varying degrees of sensitivity of the pistil to heat stress. Remarkably, in some cases, the vulnerability of the pistil is even greater than the pollen. This article summarizes the current knowledge of the impact of heat stress on three critical stages of pistil for successful seed-set, that is, female reproductive organ development (gametogenesis), pollen-pistil interactions including pollen capture on stigma and pollen tube growth in style, as well as fertilization and early embryogenesis. Further, future research directions are suggested to unravel molecular basis of heat stress tolerance in pistil, which is critical for sustaining crop yields under predicted warming scenarios.

DEFECTIVE EMBRYO AND MERISTEMS genes are required for cell division and gamete viability in Arabidopsis

The DEFECTIVE EMBRYO AND MERISTEMS 1 (DEM1) gene encodes a protein of unknown biochemical function required for meristem formation and seedling development in tomato, but it was unclear whether DEM1’s primary role was in cell division or alternatively, in defining the identity of meristematic cells. Genome sequence analysis indicates that flowering plants possess at least two DEM genes. Arabidopsis has two DEM genes, DEM1 and DEM2, which we show are expressed in developing embryos and meristems in a punctate pattern that is typical of genes involved in cell division. Homozygous dem1 dem2 double mutants were not recovered, and plants carrying a single functional DEM1 allele and no functional copies of DEM2, i.e. DEM1/dem1 dem2/dem2 plants, exhibit normal development through to the time of flowering but during male reproductive development, chromosomes fail to align on the metaphase plate at meiosis II and result in abnormal numbers of daughter cells following meiosis. Additionally, these plants show defects in both pollen and embryo sac development, and produce defective male and female gametes. In contrast, dem1/dem1 DEM2/dem2 plants showed normal levels of fertility, indicating that DEM2 plays a more important role than DEM1 in gamete viability. The increased importance of DEM2 in gamete viability correlated with higher mRNA levels of DEM2 compared to DEM1 in most tissues examined and particularly in the vegetative shoot apex, developing siliques, pollen and sperm. We also demonstrate that gamete viability depends not only on the number of functional DEM alleles inherited following meiosis, but also on the number of functional DEM alleles in the parent plant that undergoes meiosis. Furthermore, DEM1 interacts with RAS-RELATED NUCLEAR PROTEIN 1 (RAN1) in yeast two-hybrid and pull-down binding assays, and we show that fluorescent proteins fused to DEM1 and RAN1 co-localize transiently during male meiosis and pollen development. In eukaryotes, RAN is a highly conserved GTPase that plays key roles in cell cycle progression, spindle assembly during cell division, reformation of the nuclear envelope following cell division, and nucleocytoplasmic transport. Our results demonstrate that DEM proteins play an essential role in cell division in plants, most likely through an interaction with RAN1.

Up to half of the genes predicted from genome projects lack a known biological and biochemical function. Many of these genes are likely to play essential roles but it is difficult to reveal their function because minor changes in the genetic sequence can result in lethality and genetic redundancy can obscure analysis. Genome projects predict that flowering plants have at least two DEM genes that encode a protein of unknown cellular and biochemical function. In this paper, we use multiple combinations of dem mutants in Arabidopsis to show that DEM genes are essential for cell division and gamete viability. Interestingly, gamete viability depends not only on the number of functional copies of DEM genes in the gametes, but also on the number of functional copies of DEM genes in the parent plant that produces the gametes. We also show that DEM proteins interact with RAN, a highly conserved protein that controls cell division in all eukaryotic organisms.

Developmental, ultrastructural and cytochemical investigations of the female gametophyte in Sedum rupestre L. (Crassulaceae)

This article describes the development of female gametophyte in Sedum rupestre L. New embryological information about the processes of megasporogenesis and megagametogenesis provided in this paper expand the current knowledge about the embryology of the studied species. S. rupestre is characterized by monosporic megasporogenesis and the formation of Polygonum-type embryo sac. The process of megasporogenesis is initiated by one megaspore mother cell, resulting in the formation of a triad of cells after meiosis and cytokinesis. The functional megaspore, which is located chalazally, is a mononuclear cell present next to the megaspore in the centre of the triad. Only one of the two non-functional cells of the triad is binucleate, which occur at the micropylar pole. In this paper, we explain the functional ultrastructure of the female gametophytic cells in S. rupestre. Initially, the cytoplasm of the gametophytic cells does not differ from each other; however, during differentiation, the cells reveal different morphologies. The antipodals and the synergids gradually become organelle-rich and metabolically active. The antipodal cells participate in the absorption and transport of nutrients from the nucellar cells towards the megagametophyte. Their ultrastructure shows the presence of plasmodesmata with electron-dense material, which is characteristic of Crassulaceae, and wall ingrowths in the outer walls. The ultrastructure of synergid cells is characterized by the presence of filiform apparatus and cytoplasm with active dictyosomes, abundant profiles of endoplasmic reticulum and numerous vesicles, which agrees with their main function-the secretion of pollen tube attractants. Reported data can be used to resolve the current taxonomic problems within the genus Sedum ser. Rupestria.

The nucleolar protein SAHY1 is involved in pre-rRNA processing and normal plant growth

Although the nucleolus is involved in ribosome biogenesis, the functions of numerous nucleolus-localized proteins remain unclear. In this study, we genetically isolated Arabidopsis thaliana salt hypersensitive mutant 1 (sahy1), which exhibits slow growth, short roots, pointed leaves, and sterility. SAHY1 encodes an uncharacterized protein that is predominantly expressed in root tips, early developing seeds, and mature pollen grains and is mainly restricted to the nucleolus. Dysfunction of SAHY1 primarily causes the accumulation of 32S, 18S-A3, and 27SB pre-rRNA intermediates. Coimmunoprecipitation experiments further revealed the interaction of SAHY1 with ribosome proteins and ribosome biogenesis factors. Moreover, sahy1 mutants are less sensitive to protein translation inhibitors and show altered expression of structural constituents of ribosomal genes and ribosome subunit profiles, reflecting the involvement of SAHY1 in ribosome composition and ribosome biogenesis. Analyses of ploidy, S-phase cell cycle progression, and auxin transport and signaling indicated the impairment of mitotic activity, translation of auxin transport carrier proteins, and expression of the auxin-responsive marker DR5::GFP in the root tips or embryos of sahy1 plants. Collectively, these data demonstrate that SAHY1, a nucleolar protein involved in ribosome biogenesis, plays critical roles in normal plant growth in association with auxin transport and signaling.

The Arabidopsis R-SNARE protein YKT61 is essential for gametophyte development

Gametophyte development is a pre-requisite for plant reproduction and seed yield; therefore, studies of gametophyte development help us understand fundamental biological questions and have potential applications in agriculture. The biogenesis and dynamics of endomembrane compartments are critical for cell survival, and their regulatory mechanisms are just beginning to be revealed. Here, we report that the Arabidopsis thaliana SNARE (soluble N-ethylmaleimide sensitive factor attachment protein receptor) protein YKT61 is essential for both male and female gametogenesis. By using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9)-based genome editing, we demonstrated that male and female gametophytes carrying YKT61 loss-of-function alleles do not survive. Specifically, loss of YKT61 function resulted in the arrest of male gametophytic development at pollen mitosis I and the degeneration of female gametophytes. A three-base-pair deletion in YKT61 in the ykt61-3 mutant resulted in a single-amino acid deletion in the longin domain of YKT61; the resulting mutant protein does not interact with multiple SNAREs and showed substantially reduced membrane association, suggesting that the N-terminal longin domain of YKT61 plays multiple roles in its function. This study demonstrates that Arabidopsis YKT61 is essential for male and female gametogenesis and sets an example for functional characterization of essential genes with the combination of Cas9-mediated editing and expression from a Cas9-resistant transgene.

Dynamics of the cell fate specifications during female gametophyte development in Arabidopsis

The female gametophytes of angiosperms contain cells with distinct functions, such as those that enable reproduction via pollen tube attraction and fertilization. Although the female gametophyte undergoes unique developmental processes, such as several rounds of nuclear division without cell plate formation and final cellularization, it remains unknown when and how the cell fate is determined during development. Here, we visualized the living dynamics of female gametophyte development and performed transcriptome analysis of individual cell types to assess the cell fate specifications in Arabidopsis thaliana. We recorded time lapses of the nuclear dynamics and cell plate formation from the 1-nucleate stage to the 7-cell stage after cellularization using an in vitro ovule culture system. The movies showed that the nuclear division occurred along the micropylar–chalazal (distal–proximal) axis. During cellularization, the polar nuclei migrated while associating with the forming edge of the cell plate, and then, migrated toward each other to fuse linearly. We also tracked the gene expression dynamics and identified that the expression of MYB98pro::GFP–MYB98, a synergid-specific marker, was initiated just after cellularization in the synergid, egg, and central cells and was then restricted to the synergid cells. This indicated that cell fates are determined immediately after cellularization. Transcriptome analysis of the female gametophyte cells of the wild-type and myb98 mutant revealed that the myb98 synergid cells had egg cell–like gene expression profiles. Although in myb98, egg cell–specific gene expression was properly initiated in the egg cells only after cellularization, but subsequently expressed ectopically in one of the 2 synergid cells. These results, together with the various initiation timings of the egg cell–specific genes, suggest complex regulation of the individual gametophyte cells, such as cellularization-triggered fate initiation, MYB98-dependent fate maintenance, cell morphogenesis, and organelle positioning. Our system of live-cell imaging and cell type–specific gene expression analysis provides insights into the dynamics and mechanisms of cell fate specifications in the development of female gametophytes in plants.

The female gametophytes of angiosperms contain cells with distinct functions, such as those that enable reproduction via pollen tube attraction and fertilization. Live-cell imaging and transcriptome analysis of single female gametophyte cell reveal novel insights into the dynamics and mechanisms of cell fate specifications in the model plant Arabidopsis.

Paving the Way for Fertilization: The Role of the Transmitting Tract

Angiosperm reproduction relies on the precise growth of the pollen tube through different pistil tissues carrying two sperm cells into the ovules’ embryo sac, where they fuse with the egg and the central cell to accomplish double fertilization and ultimately initiate seed development. A network of intrinsic and tightly regulated communication and signaling cascades, which mediate continuous interactions between the pollen tube and the sporophytic and gametophytic female tissues, ensures the fast and meticulous growth of pollen tubes along the pistil, until it reaches the ovule embryo sac. Most of the pollen tube growth occurs in a specialized tissue—the transmitting tract—connecting the stigma, the style, and the ovary. This tissue is composed of highly secretory cells responsible for producing an extensive extracellular matrix. This multifaceted matrix is proposed to support and provide nutrition and adhesion for pollen tube growth and guidance. Insights pertaining to the mechanisms that underlie these processes remain sparse due to the difficulty of accessing and manipulating the female sporophytic tissues enclosed in the pistil. Here, we summarize the current knowledge on this key step of reproduction in flowering plants with special emphasis on the female transmitting tract tissue.