Auxin Orchestrates Germ Cell Specification in Arabidopsis

The initiation and specification of germline cells are crucial for plant reproduction and the continuity of species. In Arabidopsis thaliana, auxin plays a vital role in guiding the transition of somatic cells into germline fate, orchestrating the specification of both male archesporial cells and female megaspore mother cells. This process is regulated through interaction with the transcription factor Sporocyteless/Nozzle, which forms a feedback mechanism that modulates germ cell specialization. Auxin biosynthesis, polar transport, and signal transduction pathways collectively ensure the accurate determination of germ cell fate. Furthermore, the coordination of auxin signaling with epigenetic regulation and miRNA-mediated control fine-tunes the differentiation between germline and somatic cells. This review discusses the mechanisms underlying auxin-guided germ cell specification. It proposes future research directions, including studies on PIN-FORMED-mediated polar transport, the role of the YUCCA family in auxin biosynthesis, and the involvement of the Transport Inhibitors Response 1/Auxn Signaling F-Box-Auxin Response Factor (TIR1/AFB-ARF) signaling pathway in germ cell fate determination. These insights will enhance our understanding of plant reproductive biology and provide new strategies for crop breeding.

Cell proliferation suppressor RBR1 interacts with ARID1 to promote pollen mitosis via stabilizing DUO1 in Arabidopsis

In plants, sperm cell formation involves two rounds of pollen mitoses, in which the microspore initiates the first pollen mitosis (PMI) to produce a vegetative cell and a generative cell, then the generative cell continues the second mitosis (PMII) to produce two sperm cells. DUO1, a R2R3 Myb transcription factor, is activated in the generative cell to promote S-G2/M transition during PMII. Loss-of-function of DUO1 caused a complete arrest of PMII. Despite the importance of DUO1, how DUO1 is regulated is largely unexplored. We previously demonstrated that ARID1, an ARID transcription factor, stimulates DUO1 transcription. Here, we show that cell proliferation suppressor RBR1 interacts with ARID1 to stabilize DUO1. While the C-terminus of RBR1 is dispensable for vegetative growth, it plays a crucial role in reproductive development and facilitates interaction with ARID1. Moreover, DUO1 is a short-lived protein, ARID1 promotes the RBR1-DUO1 interaction, and RBR1 stabilizes DUO1 in a proteasome-dependent manner. Thus, RBR1 promotes DUO1-dependent PMII progression via antagonizing its repressive role in the cell cycle factors CDKA;1 and CYCB1;1. Collectively, we uncover that ARID1 and RBR1 act in concert to regulate DUO1 at both the transcriptional and posttranscriptional levels, balancing cell specification and cell division.

The DNA methylation-demethylation balance prevents development of multiple megaspore mother cells in Arabidopsis

Female gametophyte development initiates from the megaspore mother cell (MMC), and only 1 somatic cell develops into an MMC. Here, we report that the balance between DNA methylation and demethylation ensures the maintenance of a single MMC in Arabidopsis (Arabidopsis thaliana). Surprisingly, a decrease or an increase of all 3 DNA methylation types (mCG, mCHG, and mCHH) caused abnormal enlargement of MMC-like cells and re-balancing DNA methylation rescued the enlarged MMC-like cells in the mutants of regulators for DNA methylation and demethylation. Systematic quantification of DNA methylation at the single-cell level demonstrated that mCHH levels begin to decrease from the central precursor MMC, preceding expression of the MMC marker KNUCKLES(KNU). Disrupting the regulation of DNA methylation caused the mCHH levels to become similar in the MMC and its neighboring cells, and these neighbors usually developed into MMC-like cells. Levels of the de novo DNA methyltransferase DOMAINS REARRANGED METHYLTRANSFERASE2 (DRM2) in the precursor MMC decreased before KNU expression, but the levels of the DNA glycosylase DEMETER decreased after KNU expression. Re-introduction of DRM2 or knockdown of DEMETER specifically in neighboring cells rescued the defects in drm1 drm2 double mutants. Collectively, our findings demonstrate that the balance of DNA methylation, rather than total methylation levels, facilitates maintenance of a single MMC.

Regulation of cell cycle in plant gametes: when is the right time to divide?

Cell division is a fundamental process shared across diverse life forms, from yeast to humans and plants. Multicellular organisms reproduce through the formation of specialized types of cells, the gametes, which at maturity enter a quiescent state that can last decades. At the point of fertilization, signalling lifts the quiescent state and triggers cell cycle reactivation. Studying how the cell cycle is regulated during plant gamete development and fertilization is challenging, and decades of research have provided valuable, yet sometimes contradictory, insights. This Review summarizes the current understanding of plant cell cycle regulation, gamete development, quiescence, and fertilization-triggered reactivation.

Revisiting the female germline cell development

The formation of the female germline is the fundamental process in most flowering plants' sexual reproduction. In Arabidopsis, only one somatic cell obtains the female germline fate, and this process is regulated by different pathways. Megaspore mother cell (MMC) is the first female germline, and understanding MMC development is essential for comprehending the complex mechanisms of plant reproduction processes. Recently, more advanced technologies such as whole-mount single-molecule fluorescence in situ hybridization (smFISH), laser-assisted microdissection (LCM), chromatin immunoprecipitation/sequencing, and CRISPR gene editing have provided opportunities to reveal the mechanism of female germline development at different stages. Single-cell transcriptome/spatial transcriptomics analysis helps to investigate complex cellular systems at the single-cell level, reflecting the biological complexity of different cell types. In this review, we highlight recent progress that facilitates the development of the female germline to explore the roles of crucial gene regulatory networks, epigenetic pathways, cell-cycle regulators, and phytohormones in this process. This review discusses three key phases in female germline development and provides the possibility of distinct pathways restricting germline development in the future.

Cell fate determination during sexual plant reproduction

The flowering plant life cycle is completed by an alternation of diploid and haploid generations. The diploid sporophytes produce initial cells that undergo meiosis and produce spores. From haploid spores, male or female gametophytes, which produce gametes, develop. The union of gametes at fertilization restores diploidy in the zygote that initiates a new cycle of diploid sporophyte development. During this complex process, cell fate determination occurs at each of the critical stages and necessarily underpins successful plant reproduction. Here, we summarize available knowledge on the regulatory mechanism of cell fate determination at these critical stages of sexual reproduction, including sporogenesis, gametogenesis, and early embryogenesis, with particular emphasis on regulatory pathways of both male and female gametes before fertilization, and both apical and basal cell lineages of a proembryo after fertilization. Investigations reveal that cell fate determination involves multiple regulatory factors, such as positional information, differential distribution of cell fate determinants, cell-to-cell communication, and cell type-specific transcription factors. These factors temporally and spatially act for different cell type differentiation to ensure successful sexual reproduction. These new insights into regulatory mechanisms underlying sexual cell fate determination not only updates our knowledge on sexual plant reproduction, but also provides new ideas and tools for crop breeding.

Developmental cues are encoded by the combinatorial phosphorylation of Arabidopsis RETINOBLASTOMA-RELATED protein RBR1

RETINOBLASTOMA-RELATED (RBR) proteins orchestrate cell division, differentiation, and survival in response to environmental and developmental cues through protein-protein interactions that are governed by multisite phosphorylation. Here we explore, using a large collection of transgenic RBR phosphovariants to complement protein function in Arabidopsis thaliana, whether differences in the number and position of RBR phosphorylation events cause a diversification of the protein's function. While the number of point mutations influence phenotypic strength, phosphosites contribute differentially to distinct phenotypes. RBR pocket domain mutations associate primarily with cell proliferation, while mutations in the C-region are linked to stem cell maintenance. Both phospho-mimetic and a phospho-defective variants promote cell death, suggesting that distinct mechanisms can lead to similar cell fates. We observed combinatorial effects between phosphorylated T406 and phosphosites in different protein domains, suggesting that specific, additive, and combinatorial phosphorylation events fine-tune RBR function. Suppression of dominant phospho-defective RBR phenotypes with a mutation that inhibits RBR interacting with LXCXE motifs, and an exhaustive protein-protein interaction assay, not only revealed the importance of DREAM complex members in phosphorylation-regulated RBR function but also pointed to phosphorylation-independent RBR roles in environmental responses. Thus, combinatorial phosphorylation defined and separated developmental, but not environmental, functions of RBR.

SCI1, a flower regulator of cell proliferation, and its partners NtCDKG2 and NtRH35 interact with the splicing machinery

Successful plant reproduction depends on the adequate development of floral organs controlled by cell proliferation and other processes. The Stigma/style cell-cycle inhibitor 1 (SCI1) gene regulates cell proliferation and affects the final size of the female reproductive organ. To unravel the molecular mechanism exerted by Nicotiana tabacum SCI1 in cell proliferation control, we searched for its interaction partners through semi-in vivo pull-down experiments, uncovering a cyclin-dependent kinase, NtCDKG;2. Bimolecular fluorescence complementation and co-localization experiments showed that SCI1 interacts with NtCDKG;2 and its cognate NtCyclin L in nucleoli and splicing speckles. The screening of a yeast two-hybrid cDNA library using SCI1 as bait revealed a novel DEAD-box RNA helicase (NtRH35). Interaction between the NtCDKG;2-NtCyclin L complex and NtRH35 is also shown. Subcellular localization experiments showed that SCI1, NtRH35, and the NtCDKG;2-NtCyclin L complex associate with each other within splicing speckles. The yeast two-hybrid screening of NtCDKG;2 and NtRH35 identified the conserved spliceosome components U2a', NF-κB activating protein (NKAP), and CACTIN. This work presents SCI1 and its interactors, the NtCDKG;2-NtCyclin L complex and NtRH35, as new spliceosome-associated proteins. Our findings reveal a network of interactions and indicate that SCI1 may regulate cell proliferation through the splicing process, providing new insights into the intricate molecular pathways governing plant development.

Brassinosteroid signaling represses ZINC FINGER PROTEIN11 to regulate ovule development in Arabidopsis

Brassinosteroid (BR) signaling and the C-class MADS-box gene AGAMOUS (AG) play important roles in ovule development in Arabidopsis (Arabidopsis thaliana). However, how BR signaling integrates with AG functions to control the female reproductive process remains elusive. Here, we showed that the regulatory role of BR signaling in proper ovule development is mediated by the transcriptional repressor gene ZINC FINGER PROTEIN 11 (ZFP11), which is a direct target of AG. ZFP11 expression initiates from the placenta upon AG induction and becomes prominent in the funiculus of ovule primordia. Plants harboring zfp11 mutations showed reduced placental length with decreased ovule numbers and some aborted ovules. During ovule development, the transcription factor BRASSINAZOLE-RESISTANT 1 (BZR1), which functions downstream of BR signaling, inhibits ZFP11 expression in the chalaza and nucellus. Weakened BR signaling leads to stunted integuments in ovules, resulting from the direct repression of INNER NO OUTER (INO) and WUSCHEL (WUS) by extended ZFP11 expression in the chalaza and nucellus, respectively. In addition, the zfp11 mutant shows reduced sensitivity to BR biosynthesis inhibitors and can rescue outer integument defects in brassinosteroid insensitive 1 (bri1) mutants. Thus, the precise spatial regulation of ZFP11, which is activated by AG in the placenta and suppressed by BR signaling in the central and distal regions of ovules, is essential for ensuring sufficient ovule numbers and proper ovule formation.

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.

Cell death in bryophytes: emerging models to study core regulatory modules and conserved pathways

This review summarizes recent progress in our current understanding of the mechanisms underlying the cell death pathways in bryophytes, focusing on conserved pathways and particularities in comparison to angiosperms. Regulated cell death (RCD) plays key roles during essential processes along the plant life cycle. It is part of specific developmental programmes and maintains homeostasis of the organism in response to unfavourable environments. Bryophytes could provide valuable models to study developmental RCD processes as well as those triggered by biotic and abiotic stresses. Some pathways analogous to those present in angiosperms occur in the gametophytic haploid generation of bryophytes, allowing direct genetic studies. In this review, we focus on such RCD programmes, identifying core conserved mechanisms and raising new key questions to analyse RCD from an evolutionary perspective.

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

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

RETINOBLASTOMA RELATED 1 switches mitosis to meiosis in rice

The transition from mitosis to meiosis is a critical event in the reproductive development of all sexually reproducing species. However, the mechanisms that regulate this process in plants remain largely unknown. Here, we find that the rice (Oryza sativa L.) protein RETINOBLASTOMA RELATED 1 (RBR1) is essential to the transition from mitosis to meiosis. Loss of RBR1 function results in hyper-proliferative sporogenous-cell-like cells (SCLs) in the anther locules during early stages of reproductive development. These hyper-proliferative SCLs are unable to initiate meiosis, eventually stagnating and degrading at late developmental stages to form pollen-free anthers. These results suggest that RBR1 acts as a gatekeeper of entry into meiosis. Furthermore, cytokinin content is significantly increased in rbr1 mutants, whereas the expression of type-B response factors, particularly LEPTO1, is significantly reduced. Given the known close association of cytokinins with cell proliferation, these findings imply that hyper-proliferative germ cells in the anther locules may be attributed to elevated cytokinin concentrations and disruptions in the cytokinin pathway. Using a genetic strategy, the association between germ cell hyper-proliferation and disturbed cytokinin signaling in rbr1 has been confirmed. In summary, we reveal a unique role of RBR1 in the initiation of meiosis; our results clearly demonstrate that the RBR1 regulatory module is connected to the cytokinin signaling pathway and switches mitosis to meiosis in rice.

Differential expression and evolutionary diversification of RNA helicases in Boechera sexual and apomictic reproduction

In higher plants, sexual reproduction is characterized by meiosis of the first cells of the germlines, and double fertilization of the egg and central cell after gametogenesis. In contrast, in apomicts of the genus Boechera, meiosis is omitted or altered and only the central cell requires fertilization, while the embryo forms parthenogenetically from the egg cell. To deepen the understanding of the transcriptional basis underlying these differences, we applied RNA-seq to compare expression in reproductive tissues of different Boechera accessions. This confirmed previous evidence of an enrichment of RNA helicases in plant germlines. Furthermore, few RNA helicases were differentially expressed in female reproductive ovule tissues harboring mature gametophytes from apomictic and sexual accessions. For some of these genes, we further found evidence for a complex recent evolutionary history. This included a homolog of Arabidopsis thaliana FASCIATED STEM4 (FAS4). In contrast to AtFAS4, which is a single-copy gene, FAS4 is represented by three homologs in Boechera, suggesting a potential for subfunctionalization to modulate reproductive development. To gain first insights into functional roles of FAS4, we studied Arabidopsis lines carrying mutant alleles. This identified the crucial importance of AtFAS4 for reproduction, as we observed developmental defects and arrest during male and female gametogenesis.

Identification of short protein-destabilizing sequences in Arabidopsis cyclin-dependent kinase inhibitors, ICKs

Plants have a family of cyclin-dependent kinase (CDK) inhibitors called interactors/inhibitors of CDK (ICKs) or Kip-related proteins (KRPs). ICK proteins have important functions in cell proliferation, endoreduplication, plant growth, and reproductive development, and their functions depend on the protein levels. However, understanding of how ICK protein levels are regulated is very limited. We fused Arabidopsis ICK sequences to green fluorescent protein (GFP) and determined their effects on the fusion proteins in plants, yeast, and Escherichia coli. The N-terminal regions of ICKs drastically reduced GFP fusion protein levels in Arabidopsis plants. A number of short sequences of 10-20 residues were found to decrease GFP fusion protein levels when fused at the N-terminus or C-terminus. Three of the four short sequences from ICK3 showed a similar function in yeast. Intriguingly, three short sequences from ICK1 and ICK3 caused the degradation of the fusion proteins in E. coli. In addition, computational analyses showed that ICK proteins were mostly disordered and unstructured except for the conserved C-terminal region, suggesting that ICKs are intrinsically disordered proteins. This study has identified a number of short protein-destabilizing sequences, and evidence suggests that some of them may cause protein degradation through structural disorder and instability.

Epigenetic regulation of female germline development through ERECTA signaling pathway

Germline development is a key step in sexual reproduction. Sexual plant reproduction begins with the formation of haploid spores by meiosis of megaspore mother cells (MMCs). Although many evidences, directly or indirectly, show that epigenetics plays an important role in MMC specification, how it controls the commitment of the MMC to downstream stages of germline development is still unclear. Electrophoretic mobility shift assay (EMSA), western blot, immunofluorescence, and chromatin immunoprecipitation coupled with quantitative PCR analyses were performed. Genetic interactions between BZR1 transcription factor family and the SWR1-SDG2-ER pathway in the control of female germline development were further studied. The present findings showed in Arabidopsis that two epigenetic factors, the chromatin remodeling complex SWI2/SNF2-RELATED 1 (SWR1) and a writer for H3K4me3 histone modification SET DOMAIN GROUP 2 (SDG2), genetically interact with the ERECTA (ER) receptor kinase signaling pathway and regulate female germline development by restricting the MMC cell fate to a single cell in the ovule primordium and ensure that only that single cell undergoes meiosis and subsequent megaspore degeneration. We also showed that SWR1-SDG2-ER signaling module regulates female germline development by promoting the protein accumulation of BZR1 transcription factor family on the promoters of primary miRNA processing factors, HYPONASTIC LEAVES 1 (HYL1), DICER-LIKE 1 (DCL1), and SERRATE (SE) to activate their expression. Our study elucidated a Gene Regulation Network that provides new insights for understanding how epigenetic factors and receptor kinase signaling pathways function in concert to control female germline development in Arabidopsis.

The canonical E2Fs together with RETINOBLASTOMA-RELATED are required to establish quiescence during plant development

Maintaining stable and transient quiescence in differentiated and stem cells, respectively, requires repression of the cell cycle. The plant RETINOBLASTOMA-RELATED (RBR) has been implicated in stem cell maintenance, presumably by forming repressor complexes with E2F transcription factors. Surprisingly we find that mutations in all three canonical E2Fs do not hinder the cell cycle, but similarly to RBR silencing, result in hyperplasia. Contrary to the growth arrest that occurs when exit from proliferation to differentiation is inhibited upon RBR silencing, the e2fabc mutant develops enlarged organs with supernumerary stem and differentiated cells as quiescence is compromised. While E2F, RBR and the M-phase regulatory MYB3Rs are part of the DREAM repressor complexes, and recruited to overlapping groups of targets, they regulate distinct sets of genes. Only the loss of E2Fs but not the MYB3Rs interferes with quiescence, which might be due to the ability of E2Fs to control both G1-S and some key G2-M targets. We conclude that collectively the three canonical E2Fs in complex with RBR have central roles in establishing cellular quiescence during organ development, leading to enhanced plant growth.

Three-Dimensional Morphological Analysis Revealed the Cell Patterning Bases for the Sexual Dimorphism Development in the Liverwort Marchantia polymorpha

In land plants, sexual dimorphism can develop in both diploid sporophytes and haploid gametophytes. While developmental processes of sexual dimorphism have been extensively studied in the sporophytic reproductive organs of model flowering plants such as stamens and carpels of Arabidopsis thaliana, those occurring in gametophyte generation are less well characterized due to the lack of amenable model systems. In this study, we performed three-dimensional morphological analyses of gametophytic sexual branch differentiation in the liverwort Marchantia polymorpha, using high-depth confocal imaging and a computational cell segmentation technique. Our analysis revealed that the specification of germline precursors initiates in a very early stage of sexual branch development, where incipient branch primordia are barely recognizable in the apical notch region. Moreover, spatial distribution patterns of germline precursors differ between males and females from the initial stage of primordium development in a manner dependent on the master sexual differentiation regulator MpFGMYB. At later stages, distribution patterns of germline precursors predict the sex-specific gametangia arrangement and receptacle morphologies seen in mature sexual branches. Taken together, our data suggest a tightly coupled progression of germline segregation and sexual dimorphism development in M. polymorpha.

Overcoming roadblocks for in vitro nurseries in plants: induction of meiosis

Efforts to increase genetic gains in breeding programs of flowering plants depend on making genetic crosses. Time to flowering, which can take months to decades depending on the species, can be a limiting factor in such breeding programs. It has been proposed that the rate of genetic gain can be increased by reducing the time between generations by circumventing flowering through the in vitro induction of meiosis. In this review, we assess technologies and approaches that may offer a path towards meiosis induction, the largest current bottleneck for in vitro plant breeding. Studies in non-plant, eukaryotic organisms indicate that the in vitro switch from mitotic cell division to meiosis is inefficient and occurs at very low rates. Yet, this has been achieved with mammalian cells by the manipulation of a limited number of genes. Therefore, to experimentally identify factors that switch mitosis to meiosis in plants, it is necessary to develop a high-throughput system to evaluate a large number of candidate genes and treatments, each using large numbers of cells, few of which may gain the ability to induce meiosis.

3D cellular morphometrics of ovule primordium development in Zea mays reveal differential division and growth dynamics specifying megaspore mother cell singleness

Introduction

Differentiation of spore mother cells marks the somatic-to-reproductive transition in higher plants. Spore mother cells are critical for fitness because they differentiate into gametes, leading to fertilization and seed formation. The female spore mother cell is called the megaspore mother cell (MMC) and is specified in the ovule primordium. The number of MMCs varies by species and genetic background, but in most cases, only a single mature MMC enters meiosis to form the embryo sac. Multiple candidate MMC precursor cells have been identified in both rice and Arabidopsis, so variability in MMC number is likely due to conserved early morphogenetic events. In Arabidopsis, the restriction of a single MMC per ovule, or MMC singleness, is determined by ovule geometry. To look for potential conservation of MMC ontogeny and specification mechanisms, we undertook a morphogenetic description of ovule primordium growth at cellular resolution in the model crop maize.

Methods

We generated a collection of 48 three-dimensional (3D) ovule primordium images for five developmental stages, annotated for 11 cell types. Quantitative analysis of ovule and cell morphological descriptors allowed the reconstruction of a plausible developmental trajectory of the MMC and its neighbors.

Results

The MMC is specified within a niche of enlarged, homogenous L2 cells, forming a pool of candidate archesporial (MMC progenitor) cells. A prevalent periclinal division of the uppermost central archesporial cell formed the apical MMC and the underlying cell, a presumptive stack cell. The MMC stopped dividing and expanded, acquiring an anisotropic, trapezoidal shape. By contrast, periclinal divisions continued in L2 neighbor cells, resulting in a single central MMC.

Discussion

We propose a model where anisotropic ovule growth in maize drives L2 divisions and MMC elongation, coupling ovule geometry with MMC fate.

Genetic Regulation of Mitosis-Meiosis Fate Decision in Plants: Is Callose an Oversighted Polysaccharide in These Processes?

Timely progression of the meiotic cell cycle and synchronized establishment of male meiosis in anthers are key to ascertaining plant fertility. With the discovery of novel regulators of the plant cell cycle, the mechanisms underlying meiosis initiation and progression appear to be more complex than previously thought, requiring the conjunctive action of cyclins, cyclin-dependent kinases, transcription factors, protein-protein interactions, and several signaling components. Broadly, cell cycle regulators can be classified into two categories in plants based on the nature of their mutational effects: (1) those that completely arrest cell cycle progression; and (2) those that affect the timing (delay or accelerate) or synchrony of cell cycle progression but somehow complete the division process. Especially the latter effects reflect evasion or obstruction of major steps in the meiosis but have sometimes been overlooked due to their subtle phenotypes. In addition to meiotic regulators, very few signaling compounds have been discovered in plants to date. In this review, we discuss the current state of knowledge about genetic mechanisms to enter the meiotic processes, referred to as the mitosis-meiosis fate decision, as well as the importance of callose (β-1,3 glucan), which has been unsung for a long time in male meiosis in plants.

RETINOBLASTOMA-RELATED interactions with key factors of the RNA-directed DNA methylation (RdDM) pathway and its influence on root development

MAIN CONCLUSION

Our study presents evidence for a novel mechanism for RBR function in transcriptional gene silencing by interacting with key players of the RdDM pathway in Arabidopsis and several plant clades. Transposable elements and other repetitive elements are silenced by the RNA-directed DNA methylation pathway (RdDM). In RdDM, POLIV-derived transcripts are converted into double-stranded RNA (dsRNA) by the activity of RDR2 and subsequently processed into 24 nucleotide short interfering RNAs (24-nt siRNAs) by DCL3. 24-nt siRNAs serve as guides to direct AGO4-siRNA complexes to chromatin-bound POLV-derived transcripts generated from the template/target DNA. The interaction between POLV, AGO4, DMS3, DRD1, RDM1 and DRM2 promotes DRM2-mediated de novo DNA methylation. The Arabidopsis Retinoblastoma protein homolog (RBR) is a master regulator of the cell cycle, stem cell maintenance, and development. We in silico predicted and explored experimentally the protein-protein interactions (PPIs) between RBR and members of the RdDM pathway. We found that the largest subunits of POLIV and POLV (NRPD1 and NRPE1), the shared second largest subunit of POLIV and POLV (NRPD/E2), RDR1, RDR2, DCL3, DRM2, and SUVR2 contain canonical and non-canonical RBR binding motifs and several of them are conserved since algae and bryophytes. We validated experimentally PPIs between Arabidopsis RBR and several of the RdDM pathway proteins. Moreover, seedlings from loss-of-function mutants in RdDM and RBR show similar phenotypes in the root apical meristem. We show that RdDM and SUVR2 targets are up-regulated in the 35S:AmiGO-RBR background.

Signaling by the EPFL-ERECTA family coordinates female germline specification through the BZR1 family in Arabidopsis

In most flowering plants, the female germline is initiated in the subepidermal L2 layer of ovule primordia forming a single megaspore mother cell (MMC). How signaling from the L1 (epidermal) layer could contribute to the gene regulatory network (GRN) restricting MMC formation to a single cell is unclear. We show that EPIDERMAL PATTERNING FACTOR-like (EPFL) peptide ligands are expressed in the L1 layer, together with their ERECTA family (ERf) receptor kinases, to control female germline specification in Arabidopsis thaliana. EPFL-ERf dependent signaling restricts multiple subepidermal cells from acquiring MMC-like cell identity by activating the expression of the major brassinosteroid (BR) receptor kinase BRASSINOSTEROID INSENSITIVE 1 and the BR-responsive transcription factor BRASSINOZOLE RESISTANT 1 (BZR1). Additionally, BZR1 coordinates female germline specification by directly activating the expression of a nucleolar GTP-binding protein, NUCLEOSTEMIN-LIKE 1 (NSN1), which is expressed in early-stage ovules excluding the MMC. Mutants defective in this GRN form multiple MMCs resulting in a strong reduction of seed set. In conclusion, we uncovered a ligand/receptor-like kinase-mediated signaling pathway acting upstream and coordinating BR signaling via NSN1 to restrict MMC differentiation to a single subepidermal cell.

Characterization of the soybean KRP gene family reveals a key role for GmKRP2a in root development

Kip-related proteins (KRPs), as inhibitory proteins of cyclin-dependent kinases, are involved in the growth and development of plants by regulating the activity of the CYC-CDK complex to control cell cycle progression. The KRP gene family has been identified in several plants, and several KRP proteins from Arabidopsis thaliana have been functionally characterized. However, there is little research on KRP genes in soybean, which is an economically important crop. In this study, we identified nine GmKRP genes in the Glycine max genome using HMM modeling and BLASTP searches. Protein subcellular localization and conserved motif analysis showed soybean KRP proteins located in the nucleus, and the C-terminal protein sequence was highly conserved. By investigating the expression patterns in various tissues, we found that all GmKRPs exhibited transcript abundance, while several showed tissue-specific expression patterns. By analyzing the promoter region, we found that light, low temperature, an anaerobic environment, and hormones-related cis-elements were abundant. In addition, we performed a co-expression analysis of the GmKRP gene family, followed by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) set enrichment analysis. The co-expressing genes were mainly involved in RNA synthesis and modification and energy metabolism. Furthermore, the GmKRP2a gene, a member of the soybean KRP family, was cloned for further functional analysis. GmKRP2a is located in the nucleus and participates in root development by regulating cell cycle progression. RNA-seq results indicated that GmKRP2a is involved in cell cycle regulation through ribosome regulation, cell expansion, hormone response, stress response, and plant pathogen response pathways. To our knowledge, this is the first study to identify and characterize the KRP gene family in soybean.

Specification of female germline by microRNA orchestrated auxin signaling in Arabidopsis

Germline determination is essential for species survival and evolution in multicellular organisms. In most flowering plants, formation of the female germline is initiated with specification of one megaspore mother cell (MMC) in each ovule; however, the molecular mechanism underlying this key event remains unclear. Here we report that spatially restricted auxin signaling promotes MMC fate in Arabidopsis. Our results show that the microRNA160 (miR160) targeted gene ARF17 (AUXIN RESPONSE FACTOR17) is required for promoting MMC specification by genetically interacting with the SPL/NZZ (SPOROCYTELESS/NOZZLE) gene. Alterations of auxin signaling cause formation of supernumerary MMCs in an ARF17- and SPL/NZZ-dependent manner. Furthermore, miR160 and ARF17 are indispensable for attaining a normal auxin maximum at the ovule apex via modulating the expression domain of PIN1 (PIN-FORMED1) auxin transporter. Our findings elucidate the mechanism by which auxin signaling promotes the acquisition of female germline cell fate in plants.

Epigenetic regulation in the shoot apical meristem

Epigenetic mechanisms form the basis of cellular memory, developmental decisions, and the cellular immune system that defends against transposons and viruses. Organs develop from the shoot apical meristem (SAM) to shape the plant's areal phenotype, and stem cells in the SAM serve as a functional germline. While many details on the regulation of stem cell pool size, organ initiation, and patterning at the meristem periphery are known, we know surprisingly little about the molecular characteristics of SAM cells, including their epigenome and how it changes during development. Here, we summarize information on epigenetic regulation of selected genes necessary for stem cell maintenance. As recent evidence suggests that SAM stem cells might be a hotspot of transposon activation, we discuss this aspect of epigenetic control in the meristem and speculate on mechanisms that maintain the flexibility of SAM stem cells in response to developmental or environmental cues.

Killing me softly - Programmed cell death in plant reproduction from sporogenesis to fertilization

Regulated or programmed cell death (RCD or PCD) is a fundamental biological principle integral to a considerable variety of functions in multicellular organisms. In plants, different PCD processes are part of biotic and abiotic stress responses, but also occur as an essential aspect of unperturbed plant development. PCD is particularly abundant during plant reproduction, eliminating unwanted or no longer needed cells, tissues, or organs in a precisely controlled manner. Failure in reproductive PCD can have detrimental consequences for plant reproduction. Here we shed a light on the latest research into PCD mechanisms in plant reproduction from sex determination over sporogenesis to pollination and fertilization.

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

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

KIL1 terminates fertility in maize by controlling silk senescence

Plant flowers have a functional life span during which pollination and fertilization occur to ensure seed and fruit development. Once flower senescence is initiated, the potential to set seed or fruit is irrevocably lost. In maize, silk strands are the elongated floral stigmas that emerge from the husk-enveloped inflorescence to intercept airborne pollen. Here we show that KIRA1-LIKE1 (KIL1), an ortholog of the Arabidopsis NAC (NAM (NO APICAL MERISTEM), ATAF1/2 (Arabidopsis thaliana Activation Factor1 and 2) and CUC (CUP-SHAPED COTYLEDON 2)) transcription factor KIRA1, promotes senescence and programmed cell death (PCD) in the silk strand base, ending the window of accessibility for fertilization of the ovary. Loss of KIL1 function extends silk receptivity and thus strongly increases kernel yield following late pollination. This phenotype offers new opportunities for possibly improving yield stability in cereal crops. Moreover, despite diverging flower morphologies and the substantial evolutionary distance between Arabidopsis and maize, our data indicate remarkably similar principles in terminating floral receptivity by PCD, whose modulation offers the potential to be widely used in agriculture.

Epigenetic Regulation of Megaspore Mother Cell Formation

In flowering plants, the female gametophyte (FG) initiates from the formation of the megaspore mother cell (MMC). Among a pool of the somatic cells in the ovule primordium, only one hypodermal cell undergoes a transition of cell fate to become the MMC. Subsequently, the MMC undergoes a series of meiosis and mitosis to form the mature FG harboring seven cells with eight nuclei. Although SPL/NZZ, the core transcription factor for MMC formation, was identified several decades ago, which and why only one somatic cell is chosen as the MMC have long remained mysterious. A growing body of evidence reveal that MMC formation is associated with epigenetic regulation at multiple layers, including dynamic distribution of histone variants and histone modifications, small RNAs, and DNA methylation. In this review, we summarize the progress of epigenetic regulation in the MMC formation, emphasizing the roles of chromosome condensation, histone variants, histone methylation, small RNAs, and DNA methylation.

Cycling in a crowd: Coordination of plant cell division, growth, and cell fate

The reiterative organogenesis that drives plant growth relies on the constant production of new cells, which remain encased by interconnected cell walls. For these reasons, plant morphogenesis strictly depends on the rate and orientation of both cell division and cell growth. Important progress has been made in recent years in understanding how cell cycle progression and the orientation of cell divisions are coordinated with cell and organ growth and with the acquisition of specialized cell fates. We review basic concepts and players in plant cell cycle and division, and then focus on their links to growth-related cues, such as metabolic state, cell size, cell geometry, and cell mechanics, and on how cell cycle progression and cell division are linked to specific cell fates. The retinoblastoma pathway has emerged as a major player in the coordination of the cell cycle with both growth and cell identity, while microtubule dynamics are central in the coordination of oriented cell divisions. Future challenges include clarifying feedbacks between growth and cell cycle progression, revealing the molecular basis of cell division orientation in response to mechanical and chemical signals, and probing the links between cell fate changes and chromatin dynamics during the cell cycle.

Heat stress reveals a specialized variant of the pachytene checkpoint in meiosis of Arabidopsis thaliana

Plant growth and fertility strongly depend on environmental conditions such as temperature. Remarkably, temperature also influences meiotic recombination and thus, the current climate change will affect the genetic make-up of plants. To better understand the effects of temperature on meiosis, we followed male meiocytes in Arabidopsis thaliana by live cell imaging under three temperature regimes: at 21°C; at heat shock conditions of 30°C and 34°C; after an acclimatization phase of 1 week at 30°C. This work led to a cytological framework of meiotic progression at elevated temperature. We determined that an increase from 21°C to 30°C speeds up meiosis with specific phases being more amenable to heat than others. An acclimatization phase often moderated this effect. A sudden increase to 34°C promoted a faster progression of early prophase compared to 21°C. However, the phase in which cross-overs mature was prolonged at 34°C. Since mutants involved in the recombination pathway largely did not show the extension of this phase at 34°C, we conclude that the delay is recombination-dependent. Further analysis also revealed the involvement of the ATAXIA TELANGIECTASIA MUTATED kinase in this prolongation, indicating the existence of a pachytene checkpoint in plants, yet in a specialized form.

Caught in the Act: Live-Cell Imaging of Plant Meiosis

Live-cell imaging is a powerful method to obtain insights into cellular processes, particularly with respect to their dynamics. This is especially true for meiosis, where chromosomes and other cellular components such as the cytoskeleton follow an elaborate choreography over a relatively short period of time. Making these dynamics visible expands understanding of the regulation of meiosis and its underlying molecular forces. However, the analysis of meiosis by live-cell imaging is challenging; specifically in plants, a temporally resolved understanding of chromosome segregation and recombination events is lacking. Recent advances in live-cell imaging now allow the analysis of meiotic events in plants in real time. These new microscopy methods rely on the generation of reporter lines for meiotic regulators and on the establishment of ex vivo culture and imaging conditions, which stabilize the specimen and keep it alive for several hours or even days. In this review, we combine an overview of the technical aspects of live-cell imaging in plants with a summary of outstanding questions that can now be addressed to promote live-cell imaging in Arabidopsis and other plant species and stimulate ideas on the topics that can be addressed in the context of plant meiotic recombination.

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

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

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

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

A CENH3 mutation promotes meiotic exit and restores fertility in SMG7-deficient Arabidopsis

Meiosis in angiosperm plants is followed by mitotic divisions to form multicellular haploid gametophytes. Termination of meiosis and transition to gametophytic development is, in Arabidopsis, governed by a dedicated mechanism that involves SMG7 and TDM1 proteins. Mutants carrying the smg7-6 allele are semi-fertile due to reduced pollen production. We found that instead of forming tetrads, smg7-6 pollen mother cells undergo multiple rounds of chromosome condensation and spindle assembly at the end of meiosis, resembling aberrant attempts to undergo additional meiotic divisions. A suppressor screen uncovered a mutation in centromeric histone H3 (CENH3) that increased fertility and promoted meiotic exit in smg7-6 plants. The mutation led to inefficient splicing of the CENH3 mRNA and a substantial decrease of CENH3, resulting in smaller centromeres. The reduced level of CENH3 delayed formation of the mitotic spindle but did not have an apparent effect on plant growth and development. We suggest that impaired spindle re-assembly at the end of meiosis limits aberrant divisions in smg7-6 plants and promotes formation of tetrads and viable pollen. Furthermore, the mutant with reduced level of CENH3 was very inefficient haploid inducer indicating that differences in centromere size is not the key determinant of centromere-mediated genome elimination.

Meiosis is a reductional cell division that halves number of chromosomes during two successive rounds of chromosome segregation without intervening DNA replication. Such mode of chromosome segregation requires extensive reprogramming of the cell division machinery at the entry to meiosis, and inactivation of the meiotic program upon the formation of haploid spores. Here we showed that Arabidopsis partially deficient in the RNA decay factor SMG7 fail to exit meiosis and continue with attempts to undergo additional cycles of post-meiotic chromosome segregations without genome replication. This results in a reduced number of viable pollen and diminished fertility. To find genes involved in meiotic exit, we performed a suppressor screen for the SMG7-deicient plants that re-gain fertility. We found that reducing the amount of centromeric histone partially restores pollen formation and fertility in smg7 mutants. This is likely due to inefficient formation of centromere-microtubule interactions that impairs spindle reassembly and re-entry into aberrant rounds of post-meiotic chromosome segregation.

RH17 restricts reproductive fate and represses autonomous seed coat development in sexual Arabidopsis

Plant sexual and asexual reproduction through seeds (apomixis) is tightly controlled by complex gene regulatory programs, which are not yet fully understood. Recent findings suggest that RNA helicases are required for plant germline development. This resembles their crucial roles in animals, where they are involved in controlling gene activity and the maintenance of genome integrity. Here, we identified previously unknown roles of Arabidopsis RH17 during reproductive development. Interestingly, RH17 is involved in repression of reproductive fate and of elements of seed development in the absence of fertilization. In lines carrying a mutant rh17 allele, development of supernumerary reproductive cell lineages in the female flower tissues (ovules) was observed, occasionally leading to formation of two embryos per seed. Furthermore, seed coat, and putatively also endosperm development, frequently initiated autonomously. Such induction of several features phenocopying distinct elements of apomixis by a single mutation is unusual and suggests that RH17 acts in regulatory control of plant reproductive development. Furthermore, an in-depth understanding of its action might be of use for agricultural applications.

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.

How to Switch from Mitosis to Meiosis: Regulation of Germline Entry in Plants

One of the major cell fate transitions in eukaryotes is entry into meiosis. While in single-celled yeast this decision is triggered by nutrient starvation, in multicellular eukaryotes, such as plants, it is under developmental control. In contrast to animals, plants have only a short germline and instruct cells to become meiocytes in reproductive organs late in development. This situation argues for a fundamentally different mechanism of how plants recruit meiocytes, and consistently, none of the regulators known to control meiotic entry in yeast and animals are present in plants. In recent years, several factors involved in meiotic entry have been identified, especially in the model plant Arabidopsis, and pieces of a regulatory network of germline control in plants are emerging. However, the corresponding studies also show that the mechanisms of meiotic entry control are diversified in flowering plants, calling for further analyses in different plant species. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The Formation of Bivalents and the Control of Plant Meiotic Recombination

During the first meiotic division, the segregation of homologous chromosomes depends on the physical association of the recombined homologous DNA molecules. The physical tension due to the sites of crossing-overs (COs) is essential for the meiotic spindle to segregate the connected homologous chromosomes to the opposite poles of the cell. This equilibrated partition of homologous chromosomes allows the first meiotic reductional division. Thus, the segregation of homologous chromosomes is dependent on their recombination. In this review, we will detail the recent advances in the knowledge of the mechanisms of recombination and bivalent formation in plants. In plants, the absence of meiotic checkpoints allows observation of subsequent meiotic events in absence of meiotic recombination or defective meiotic chromosomal axis formation such as univalent formation instead of bivalents. Recent discoveries, mainly made in Arabidopsis, rice, and maize, have highlighted the link between the machinery of double-strand break (DSB) formation and elements of the chromosomal axis. We will also discuss the implications of what we know about the mechanisms regulating the number and spacing of COs (obligate CO, CO homeostasis, and interference) in model and crop plants.

Plant sexual reproduction: perhaps the current plant two-sex model should be replaced with three- and four-sex models?

The two-sex model makes the assumption that there are only two sexual reproductive states: male and female. However, in land plants (embryophytes) the application of this model to the alternation of generations life cycle requires the subtle redefinition of several common terms related to sexual reproduction, which seems to obscure aspects of one or the other plant generation: For instance, the homosporous sporophytic plant is treated as being asexual, and the gametophytes of angiosperms treated like mere gametes. In contrast, the proposal is made that the sporophytes of homosporous plants are indeed sexual reproductive organisms, as are the gametophytes of heterosporous plants. This view requires the expansion of the number of sexual reproductive states we accept for these plant species; therefore, a three-sex model for homosporous plants and a four-sex model for heterosporous plants are described and then contrasted with the current two-sex model. These new models allow the use of sexual reproductive terms in a manner largely similar to that seen in animals, and may better accommodate the plant alternation of generations life cycle than does the current plant two-sex model. These new models may also help stimulate new lines of research, and examples of how they might alter our view of events in the flower, and may lead to new questions about sexual determination and differentiation, are presented. Thus it is suggested that land plant species have more than merely two sexual reproductive states and that recognition of this may promote our study and understanding of them.

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

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

Organ geometry channels reproductive cell fate in the Arabidopsis ovule primordium

In multicellular organisms, sexual reproduction requires the separation of the germline from the soma. In flowering plants, the female germline precursor differentiates as a single spore mother cell (SMC) as the ovule primordium forms. Here, we explored how organ growth contributes to SMC differentiation. We generated 92 annotated 3D images at cellular resolution in Arabidopsis. We identified the spatio-temporal pattern of cell division that acts in a domain-specific manner as the primordium forms. Tissue growth models uncovered plausible morphogenetic principles involving a spatially confined growth signal, differential mechanical properties, and cell growth anisotropy. Our analysis revealed that SMC characteristics first arise in more than one cell but SMC fate becomes progressively restricted to a single cell during organ growth. Altered primordium geometry coincided with a delay in the fate restriction process in katanin mutants. Altogether, our study suggests that tissue geometry channels reproductive cell fate in the Arabidopsis ovule primordium.

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.

ABAP1 Plays a Role in the Differentiation of Male and Female Gametes in Arabidopsis thaliana

The correct development of a diploid sporophyte body and a haploid gametophyte relies on a strict coordination between cell divisions in space and time. During plant reproduction, these divisions have to be temporally and spatially coordinated with cell differentiation processes, to ensure a successful fertilization. Armadillo BTB Arabidopsis protein 1 (ABAP1) is a plant exclusive protein that has been previously reported to control proliferative cell divisions during leaf growth in Arabidopsis. Here, we show that ABAP1 binds to different transcription factors that regulate male and female gametophyte differentiation, repressing their target genes expression. During male gametogenesis, the ABAP1-TCP16 complex represses CDT1b transcription, and consequently regulates microspore first asymmetric mitosis. In the female gametogenesis, the ABAP1-ADAP complex represses EDA24-like transcription, regulating polar nuclei fusion to form the central cell. Therefore, besides its function during vegetative development, this work shows that ABAP1 is also involved in differentiation processes during plant reproduction, by having a dual role in regulating both the first asymmetric cell division of male gametophyte and the cell differentiation (or cell fusion) of female gametophyte.

Plant cell cycle regulators: Mitogen-activated protein kinase, a new regulating switch?

Cell cycle is essential for the maintenance of genetic material and continuity of a species. Its regulation involves a complex interplay between multiple proteins with diverse molecular functions such as the kinases, transcription factors, proteases and phosphatases. Every step of this cycle requires a certain combination of these protein regulators which paves the way for the next stage. It is now evident that plants have their own unique features in the context of cell cycle regulation. Cell cycle in plants is not only necessary for maintenance of its physio-morphological parameter but it also regulates traits important for mankind like grain or fruit size. This makes it even more important to understand how plants regulate its cell cycle amidst various a/biotic stresses it is subjected to during its lifetime. The association of MAPK signaling pathways with every major developmental and stress response pathways in plants raises the question of its potential role in cell cycle regulation. There are number of cell cycle regulating proteins with putative sites for MAPK phosphorylation. The MAPK signaling pathway may directly or in a parallel pathway regulate the plant cell cycle. Unraveling the role of MAPK in cell cycle will open up new arenas to explore.

H3.1 Eviction Marks Female Germline Precursors in Arabidopsis

In flowering plants, germline precursors are differentiated from somatic cells. The female germline precursor of Arabidopsis thaliana is located in the internal (nucellar) tissue of the ovule, and is known as the Megaspore Mother Cell (MMC). MMC differentiation in Arabidopsis occurs when a cell in the subepidermal layer of the nucellar apex enters the meiotic program. Increasing evidence has demonstrated that MMC specification is a plastic process where the number and developmental outcome of MMCs are variable. During its differentiation, the MMC displays specific chromatin hallmarks that distinguish it from other cells within the primordium. To date, these signatures have been only analyzed at developmental stages where the MMC is morphologically conspicuous, and their role in reproductive fate acquisition remains to be elucidated. Here, we show that the histone 3 variant H3.1 HISTONE THREE RELATED 13 (HTR13) can be evicted in multiple subepidermal cells of the nucellus, but that H3.1 eviction persists only in the MMC. This pattern is established very early in ovule development and is reminiscent of the specific eviction of H3.1 that marks cell cycle exit in other somatic cell types, such as the root quiescent center (QC) of Arabidopsis. Our findings suggest that cell cycle progression in the subepidermal region of the ovule apex is modified very early in development and is associated with plasticity of reproductive fate acquisition.

Roles of plant retinoblastoma protein: cell cycle and beyond

The human retinoblastoma (RB1) protein is a tumor suppressor that negatively regulates cell cycle progression through its interaction with members of the E2F/DP family of transcription factors. However, RB-related (RBR) proteins are an early acquisition during eukaryote evolution present in plant lineages, including unicellular algae, ancient plants (ferns, lycophytes, liverworts, mosses), gymnosperms, and angiosperms. The main RBR protein domains and interactions with E2Fs are conserved in all eukaryotes and not only regulate the G1/S transition but also the G2/M transition, as part of DREAM complexes. RBR proteins are also important for asymmetric cell division, stem cell maintenance, and the DNA damage response (DDR). RBR proteins play crucial roles at every developmental phase transition, in association with chromatin factors, as well as during the reproductive phase during female and male gametes production and embryo development. Here, we review the processes where plant RBR proteins play a role and discuss possible avenues of research to obtain a full picture of the multifunctional roles of RBR for plant life.

Beyond What Your Retina Can See: Similarities of Retinoblastoma Function between Plants and Animals, from Developmental Processes to Epigenetic Regulation

The Retinoblastoma protein (pRb) is a key cell cycle regulator conserved in a wide variety of organisms. Experimental analysis of pRb's functions in animals and plants has revealed that this protein participates in cell proliferation and differentiation processes. In addition, pRb in animals and its orthologs in plants (RBR), are part of highly conserved protein complexes which suggest the possibility that analogies exist not only between functions carried out by pRb orthologs themselves, but also in the structure and roles of the protein networks where these proteins are involved. Here, we present examples of pRb/RBR participation in cell cycle control, cell differentiation, and in the regulation of epigenetic changes and chromatin remodeling machinery, highlighting the similarities that exist between the composition of such networks in plants and animals.