COMET Functions as a PCH2 Cofactor in Regulating the HORMA Domain Protein ASY1

Arabidopsis KNL1 recruits type one protein phosphatase to kinetochores to silence the spindle assembly checkpoint

Proper chromosome segregation during cell division is essential for genomic integrity and organismal development. This process is monitored by the spindle assembly checkpoint (SAC), which delays anaphase onset until all chromosomes are properly attached to the mitotic spindle. The kinetochore protein KNL1 plays a critical role in recruiting SAC proteins. Here, we reveal that Arabidopsis KNL1 regulates SAC silencing through the direct recruitment of type one protein phosphatase (TOPP) to kinetochores. We show that KNL1 interacts with all nine TOPPs via a conserved RVSF motif in its N terminus, and this interaction is required for the proper localization of TOPPs to kinetochores during mitosis. Disrupting KNL1-TOPP interaction leads to persistent SAC activation, resulting in a severe metaphase arrest and defects in plant growth and development. Our findings highlight the evolutionary conservation of KNL1 in coordinating kinetochore-localized phosphatase to ensure timely SAC silencing and faithful chromosome segregation in Arabidopsis.

The plant early recombinosome: a high security complex to break DNA during meiosis

KEY MESSAGE

The formacion of numerous unpredictable DNA Double Strand Breaks (DSBs) on chromosomes iniciates meiotic recombination. In this perspective, we propose a 'multi-key lock' model to secure the risky but necesary breaks as well as a 'one per pair of cromatids' model for the topoisomerase-like early recombinosome. During meiosis, homologous chromosomes recombine at few sites of crossing-overs (COs) to ensure correct segregation. The initiation of meiotic recombination involves the formation of DNA double strand breaks (DSBs) during prophase I. Too many DSBs are dangerous for genome integrity: if these DSBs are not properly repaired, it could potentially lead to chromosomal fragmentation. Too few DSBs are also problematic: if the obligate CO cannot form between bivalents, catastrophic unequal segregation of univalents lead to the formation of sterile aneuploid spores. Research on the regulation of the formation of these necessary but risky DSBs has recently advanced in yeast, mammals and plants. DNA DSBs are created by the enzymatic activity of the early recombinosome, a topoisomerase-like complex containing SPO11. This opinion paper reviews recent insights on the regulation of the SPO11 cofactors necessary for the introduction of temporally and spatially controlled DSBs. We propose that a 'multi-key-lock' model for each subunit of the early recombinosome complex is required to secure the formation of DSBs. We also discuss the hypothetical implications that the established topoisomerase-like nature of the SPO11 core-complex can have in creating DSB in only one of the two replicated chromatids of early prophase I meiotic chromosomes. This hypothetical 'one per pair of chromatids' DSB formation model could optimize the faithful repair of the self-inflicted DSBs. Each DSB could use three potential intact homologous DNA sequences as repair template: one from the sister chromatid and the two others from the homologous chromosomes.

Role of TRIP13 in human cancer development

As an AAA + ATPase, thyroid hormone receptor interacting protein 13 (TRIP13) primarily functions in DNA double-strand break repair, chromosome recombination, and cell cycle checkpoint regulation; aberrant expression of TRIP13 can result in chromosomal instability (CIN). According to recent research, TRIP13 is aberrantly expressed in a variety of cancers, and a patient's poor prognosis and tumor stage are strongly correlated with high expression of TRIP13. Tumor cell and subcutaneous xenograft growth can be markedly inhibited by TRIP13 knockdown or TRIP13 inhibitor administration. In the initiation and advancement of human malignancies, TRIP13 seems to function as an oncogene. Based on available data, TRIP13 may function as a biological target and biomarker for cancer. The creation of inhibitors that specifically target TRIP13 may present novel approaches to treating cancer.

TRIP13 localizes to synapsed chromosomes and functions as a dosage-sensitive regulator of meiosis

Meiotic progression requires coordinated assembly and disassembly of protein complexes involved in chromosome synapsis and meiotic recombination. Mouse TRIP13 and its ortholog Pch2 are instrumental in remodeling HORMA domain proteins. HORMAD proteins are associated with unsynapsed chromosome axes but depleted from the synaptonemal complex (SC) of synapsed homologs. Here we report that TRIP13 localizes to the synapsed SC in early pachytene spermatocytes and to telomeres throughout meiotic prophase I. Loss of TRIP13 leads to meiotic arrest and thus sterility in both sexes. Trip13-null meiocytes exhibit abnormal persistence of HORMAD1 and HOMRAD2 on synapsed SC and chromosome asynapsis that preferentially affects XY and centromeric ends. These major phenotypes are consistent with reported phenotypes of Trip13 hypomorph alleles. Trip13 heterozygous mice exhibit meiotic defects that are less severe than the Trip13-null mice, showing that TRIP13 is a dosage-sensitive regulator of meiosis. Localization of TRIP13 to the synapsed SC is independent of SC axial element proteins such as REC8 and SYCP2/SYCP3. Terminal FLAG-tagged TRIP13 proteins are functional and recapitulate the localization of native TRIP13 to SC and telomeres. Therefore, the evolutionarily conserved localization of TRIP13/Pch2 to the synapsed chromosomes provides an explanation for dissociation of HORMA domain proteins upon synapsis in diverse organisms.

Chromatin binding by HORMAD proteins regulates meiotic recombination initiation

The meiotic chromosome axis coordinates chromosome organization and interhomolog recombination in meiotic prophase and is essential for fertility. In S. cerevisiae, the HORMAD protein Hop1 mediates the enrichment of axis proteins at nucleosome-rich islands through a central chromatin-binding region (CBR). Here, we use cryoelectron microscopy to show that the Hop1 CBR directly recognizes bent nucleosomal DNA through a composite interface in its PHD and winged helix-turn-helix domains. Targeted disruption of the Hop1 CBR-nucleosome interface causes a localized reduction of axis protein binding and meiotic DNA double-strand breaks (DSBs) in axis islands and leads to defects in chromosome synapsis. Synthetic effects with mutants of the Hop1 regulator Pch2 suggest that nucleosome binding delays a conformational switch in Hop1 from a DSB-promoting, Pch2-inaccessible state to a DSB-inactive, Pch2-accessible state to regulate the extent of meiotic DSB formation. Phylogenetic analyses of meiotic HORMADs reveal an ancient origin of the CBR, suggesting that the mechanisms we uncover are broadly conserved.

TRIP13 localizes to synapsed chromosomes and functions as a dosage-sensitive regulator of meiosis

Meiotic progression requires coordinated assembly and disassembly of protein complexes involved in chromosome synapsis and meiotic recombination. The AAA+ ATPase TRIP13 and its orthologue Pch2 are instrumental in remodeling HORMA domain proteins. Meiosis-specific HORMAD proteins are associated with unsynapsed chromosome axes but depleted from the synaptonemal complex (SC) of synapsed chromosome homologues. Here we report that TRIP13 localizes to the synapsed SC in early pachytene spermatocytes and to telomeres throughout meiotic prophase I. Loss of TRIP13 leads to meiotic arrest and thus sterility in both sexes. Trip13-null meiocytes exhibit abnormal persistence of HORMAD1 and HOMRAD2 on synapsed SC and chromosome asynapsis that preferentially affects XY and centromeric ends. These findings confirm the previously reported phenotypes of the Trip13 hypomorph alleles. Trip13 heterozygous (Trip13+/-) mice also exhibit meiotic defects that are less severe than the Trip13-null mice, showing that TRIP13 is a dosage-sensitive regulator of meiosis. Localization of TRIP13 to the synapsed SC is independent of SC axial element proteins such as REC8 and SYCP2/SYCP3. The N- or C-terminal FLAG-tagged TRIP13 proteins are functional and recapitulate the localization of native TRIP13 to SC and telomeres in knockin mice. Therefore, the evolutionarily conserved localization of TRIP13/Pch2 to the synapsed chromosomes provides an explanation for dissociation of HORMA domain proteins upon chromosome synapsis in diverse organisms.

Exportin-mediated nucleocytoplasmic transport maintains Pch2 homeostasis during meiosis

The meiotic recombination checkpoint reinforces the order of events during meiotic prophase I, ensuring the accurate distribution of chromosomes to the gametes. The AAA+ ATPase Pch2 remodels the Hop1 axial protein enabling adequate levels of Hop1-T318 phosphorylation to support the ensuing checkpoint response. While these events are localized at chromosome axes, the checkpoint activating function of Pch2 relies on its cytoplasmic population. In contrast, forced nuclear accumulation of Pch2 leads to checkpoint inactivation. Here, we reveal the mechanism by which Pch2 travels from the cell nucleus to the cytoplasm to maintain Pch2 cellular homeostasis. Leptomycin B treatment provokes the nuclear accumulation of Pch2, indicating that its nucleocytoplasmic transport is mediated by the Crm1 exportin recognizing proteins containing Nuclear Export Signals (NESs). Consistently, leptomycin B leads to checkpoint inactivation and impaired Hop1 axial localization. Pch2 nucleocytoplasmic traffic is independent of its association with Zip1 and Orc1. We also identify a functional NES in the non-catalytic N-terminal domain of Pch2 that is required for its nucleocytoplasmic trafficking and proper checkpoint activity. In sum, we unveil another layer of control of Pch2 function during meiosis involving nuclear export via the exportin pathway that is crucial to maintain the critical balance of Pch2 distribution among different cellular compartments.

ASYNAPSIS 1 ensures crossover fidelity in polyploid wheat by promoting homologous recombination and suppressing non-homologous recombination

During meiosis, the chromosome axes and synaptonemal complex mediate chromosome pairing and homologous recombination to maintain genomic stability and accurate chromosome segregation. In plants, ASYNAPSIS 1 (ASY1) is a key component of the chromosome axis that promotes inter-homolog recombination, synapsis and crossover formation. Here, the function of ASY1 has been cytologically characterized in a series of hypomorphic wheat mutants. In tetraploid wheat, asy1 hypomorphic mutants experience a reduction in chiasmata (crossovers) in a dosage-specific manner, resulting in failure to maintain crossover (CO) assurance. In mutants with only one functional copy of ASY1, distal chiasmata are maintained at the expense of proximal and interstitial chiasmata, indicating that ASY1 is required to promote chiasma formation away from the chromosome ends. Meiotic prophase I progression is delayed in asy1 hypomorphic mutants and is arrested in asy1 null mutants. In both tetraploid and hexaploid wheat, single asy1 mutants exhibit a high degree of ectopic recombination between multiple chromosomes at metaphase I. To explore the nature of the ectopic recombination, Triticum turgidum asy1b-2 was crossed with wheat-wild relative Aegilops variabilis. Homoeologous chiasmata increased 3.75-fold in Ttasy1b-2/Ae. variabilis compared to wild type/Ae. variabilis, indicating that ASY1 suppresses chiasma formation between divergent, but related chromosomes. These data suggest that ASY1 promotes recombination along the chromosome arms of homologous chromosomes whilst suppressing recombination between non-homologous chromosomes. Therefore, asy1 mutants could be utilized to increase recombination between wheat wild relatives and elite varieties for expediting introgression of important agronomic traits.

Engineering apomixis in crops

Apomixis is an asexual mode of reproduction through seeds where progeny are clones of the mother plants. Naturally apomictic modes of reproduction are found in hundreds of plant genera distributed across more than 30 plant families, but are absent in major crop plants. Apomixis has the potential to be a breakthrough technology by allowing the propagation through seed of any genotype, including F1 hybrids. Here, we have summarized the recent progress toward synthetic apomixis, where combining targeted modifications of both the meiosis and fertilization processes leads to the production of clonal seeds at high frequencies. Despite some remaining challenges, the technology has approached a level of maturity that allows its consideration for application in the field.

TurboID-based proteomic profiling of meiotic chromosome axes in Arabidopsis thaliana

During meiotic prophase I, sister chromatids are arranged in a loop-base array along a proteinaceous structure, called the meiotic chromosome axis. This structure is essential for synapsis and meiotic recombination progression and hence formation of genetically diverse gametes. Proteomic studies in plants aiming to unravel the composition and regulation of meiotic axes are constrained by limited meiotic cells embedded in floral organs. Here we report TurboID (TbID)-based proximity labelling (PL) in meiotic cells of Arabidopsis thaliana. TbID fusion to the two meiotic chromosome axis proteins ASY1 and ASY3 enabled the identification of their proximate 'interactomes' based on affinity purification coupled with mass spectrometry. We identified 39 ASY1 and/or ASY3 proximate candidates covering most known chromosome axis-related proteins. Functional studies of selected candidates demonstrate that not only known meiotic candidates but also new meiotic proteins were uncovered. Hence, TbID-based PL in meiotic cells enables the identification of chromosome axis proximate proteins in A. thaliana.

ZmASY1 interacts with ZmPRD3 and is crucial for meiotic double-strand break formation in maize

During meiosis, recombination-mediated pairing and synapsis of homologous chromosomes begin with programmed DNA double-strand breaks (DSBs). In yeast and mice, DSBs form in a tethered loop-axis complex, in which DSB sites are located within chromatin loops and tethered to the proteinaceous axial element (AE) by DSB-forming factors. In plants, the molecular connection between DSB sites and chromosome axes is poorly understood. By integrating genetic analysis, immunostaining technology, and protein-protein interaction studies, the putative factors linking DSB formation to chromosome axis were explored in maize meiosis. Here, we report that the AE protein ZmASY1 directly interacts with the DSB-forming protein ZmPRD3 in maize (Zea mays) and mediates DSB formation, synaptonemal complex assembly, and homologous recombination. ZmPRD3 also interacts with ZmPRD1, which plays a central role in organizing the DSB-forming complex. These results suggest that ZmASY1 and ZmPRD3 may work as a key module linking DSB sites to chromosome axes during DSB formation in maize. This mechanism is similar to that described in yeast and recently Arabidopsis involving the homologs Mer2/ZmPRD3 and HOP1/ZmASY1, thus indicating that the process of tethering DSBs in chromatin loops to the chromosome axes may be evolutionarily conserved in diverse taxa.

ZYP1-mediated recruitment of PCH2 to the synaptonemal complex remodels the chromosome axis leading to crossover restriction

Chromosome axis-associated HORMA domain proteins (HORMADs), e.g. ASY1 in Arabidopsis, are crucial for meiotic recombination. ASY1, as other HORMADs, is assembled on the axis at early meiosis and depleted when homologous chromosomes synapse. Puzzlingly, both processes are catalyzed by AAA+ ATPase PCH2 together with its cofactor COMET. Here, we show that the ASY1 remodeling complex is temporally and spatially differently assembled. While PCH2 and COMET appear to directly interact in the cytoplasm in early meiosis, PCH2 is recruited by the transverse filament protein ZYP1 and brought to the ASY1-bound COMET assuring the timely removal of ASY1 during chromosome synapsis. Since we found that the PCH2 homolog TRIP13 also binds to the ZYP1 homolog SYCP1 in mouse, we postulate that this mechanism is conserved among eukaryotes. Deleting the PCH2 binding site of ZYP1 led to a failure of ASY1 removal. Interestingly, the placement of one obligatory crossover per homologous chromosome pair, compromised by ZYP1 depletion, is largely restored in this separation-of-function zyp1 allele suggesting that crossover assurance is promoted by synapsis. In contrast, this zyp1 allele, similar to the zyp1 null mutant, showed elevated type I crossover numbers indicating that PCH2-mediated eviction of ASY1 from the axis restricts crossover formation.

Checkpoint control in meiotic prophase: Idiosyncratic demands require unique characteristics

Chromosomal transactions such as replication, recombination and segregation are monitored by cell cycle checkpoint cascades. These checkpoints ensure the proper execution of processes that are needed for faithful genome inheritance from one cell to the next, and across generations. In meiotic prophase, a specialized checkpoint monitors defining events of meiosis: programmed DNA break formation, followed by dedicated repair through recombination based on interhomolog (IH) crossovers. This checkpoint shares molecular characteristics with canonical DNA damage checkpoints active during somatic cell cycles. However, idiosyncratic requirements of meiotic prophase have introduced unique features in this signaling cascade. In this review, we discuss the unique features of the meiotic prophase checkpoint. While being related to canonical DNA damage checkpoint cascades, the meiotic prophase checkpoint also shows similarities with the spindle assembly checkpoint (SAC) that guards chromosome segregation. We highlight these emerging similarities in the signaling logic of the checkpoints that govern meiotic prophase and chromosome segregation, and how thinking of these similarities can help us better understand meiotic prophase control. We also discuss work showing that, when aberrantly expressed, components of the meiotic prophase checkpoint might alter DNA repair fidelity and chromosome segregation in cancer cells. Considering checkpoint function in light of demands imposed by the special characteristics of meiotic prophase helps us understand checkpoint integration into the meiotic cell cycle machinery.

ZmPRD1 is essential for double-strand break formation, but is not required for bipolar spindle assembly during maize meiosis

Homologs of PUTATIVE RECOMBINATION INITIATION DEFECT 1 (PRD1) are known to be essential for meiotic double-strand break (DSB) formation in mouse (Mus musculus), Arabidopsis, and rice (Oryza sativa). Recent research has shown that rice PRD1 also plays an unanticipated role in meiotic bipolar spindle assembly, revealing that PRD1 has multiple functions in plant meiosis. In this study, we characterize the meiotic function of PRD1 in maize (Zea mays; ZmPRD1). Our results show that Zmprd1 mutant plants display normal vegetative growth but have complete male and female sterility. Meiotic DSB formation is fully abolished in mutant meiocytes, leading to failure in homologous pairing, synapsis, and recombination. ZmPRD1 exhibits a different pattern of chromosome localization compared to its rice homologs. The ZmPRD1 protein interacts with several DSB-forming proteins, but does not directly interact with the kinetochore proteins REC8 and SGO1. Possibly as a result of this, there are no significant abnormalities of bipolar spindle assembly in Zmprd1 meiocytes. Overall, our results demonstrate that ZmPRD1 is essential for DSB formation and homologous recombination in maize meiosis. However, the recently-identified function of PRD1 in bipolar spindle assembly during rice meiosis is not conserved in maize.

Engineering Apomixis: Clonal Seeds Approaching the Fields

Apomixis is a form of reproduction leading to clonal seeds and offspring that are genetically identical to the maternal plant. While apomixis naturally occurs in hundreds of plant species distributed across diverse plant families, it is absent in major crop species. Apomixis has a revolutionary potential in plant breeding, as it could allow the instant fixation and propagation though seeds of any plant genotype, most notably F₁ hybrids. Mastering and implementing apomixis would reduce the cost of hybrid seed production, facilitate new types of hybrid breeding, and make it possible to harness hybrid vigor in crops that are not presently cultivated as hybrids. Synthetic apomixis can be engineered by combining modifications of meiosis and fertilization. Here, we review the current knowledge and recent major achievements toward the development of efficient apomictic systems usable in agriculture.

Functions and Regulation of Meiotic HORMA-Domain Proteins

During meiosis, homologous chromosomes must recognize, pair, and recombine with one another to ensure the formation of inter-homologue crossover events, which, together with sister chromatid cohesion, promote correct chromosome orientation on the first meiotic spindle. Crossover formation requires the assembly of axial elements, proteinaceous structures that assemble along the length of each chromosome during early meiosis, as well as checkpoint mechanisms that control meiotic progression by monitoring pairing and recombination intermediates. A conserved family of proteins defined by the presence of a HORMA (HOp1, Rev7, MAd2) domain, referred to as HORMADs, associate with axial elements to control key events of meiotic prophase. The highly conserved HORMA domain comprises a flexible safety belt sequence, enabling it to adopt at least two of the following protein conformations: one closed, where the safety belt encircles a small peptide motif present within an interacting protein, causing its topological entrapment, and the other open, where the safety belt is reorganized and no interactor is trapped. Although functional studies in multiple organisms have revealed that HORMADs are crucial regulators of meiosis, the mechanisms by which HORMADs implement key meiotic events remain poorly understood. In this review, we summarize protein complexes formed by HORMADs, discuss their roles during meiosis in different organisms, draw comparisons to better characterize non-meiotic HORMADs (MAD2 and REV7), and highlight possible areas for future research.

Does the Pachytene Checkpoint, a Feature of Meiosis, Filter Out Mistakes in Double-Strand DNA Break Repair and as a side-Effect Strongly Promote Adaptive Speciation?

This essay aims to explain two biological puzzles: why eukaryotic transcription units are composed of short segments of coding DNA interspersed with long stretches of non-coding (intron) DNA, and the near ubiquity of sexual reproduction. As is well known, alternative splicing of its coding sequences enables one transcription unit to produce multiple variants of each encoded protein. Additionally, padding transcription units with non-coding DNA (often many thousands of base pairs long) provides a readily evolvable way to set how soon in a cell cycle the various mRNAs will begin being expressed and the total amount of mRNA that each transcription unit can make during a cell cycle. This regulation complements control via the transcriptional promoter and facilitates the creation of complex eukaryotic cell types, tissues, and organisms. However, it also makes eukaryotes exceedingly vulnerable to double-strand DNA breaks, which end-joining break repair pathways can repair incorrectly. Transcription units cover such a large fraction of the genome that any mis-repair producing a reorganized chromosome has a high probability of destroying a gene. During meiosis, the synaptonemal complex aligns homologous chromosome pairs and the pachytene checkpoint detects, selectively arrests, and in many organisms actively destroys gamete-producing cells with chromosomes that cannot adequately synapse; this creates a filter favoring transmission to the next generation of chromosomes that retain the parental organization, while selectively culling those with interrupted transcription units. This same meiotic checkpoint, reacting to accidental chromosomal reorganizations inflicted by error-prone break repair, can, as a side effect, provide a mechanism for the formation of new species in sympatry. It has been a long-standing puzzle how something as seemingly maladaptive as hybrid sterility between such new species can arise. I suggest that this paradox is resolved by understanding the adaptive importance of the pachytene checkpoint, as outlined above.

Evolutionary Dynamics and Molecular Mechanisms of HORMA Domain Protein Signaling

Controlled assembly and disassembly of multi-protein complexes is central to cellular signaling. Proteins of the widespread and functionally diverse HORMA family nucleate assembly of signaling complexes by binding short peptide motifs through a distinctive safety-belt mechanism. HORMA proteins are now understood as key signaling proteins across kingdoms, serving as infection sensors in a bacterial immune system and playing central roles in eukaryotic cell cycle, genome stability, sexual reproduction, and cellular homeostasis pathways. Here, we describe how HORMA proteins' unique ability to adopt multiple conformational states underlies their functions in these diverse contexts. We also outline how a dedicated AAA+ ATPase regulator, Pch2/TRIP13, manipulates HORMA proteins' conformational states to activate or inactivate signaling in different cellular contexts. The emergence of Pch2/TRIP13 as a lynchpin for HORMA protein action in multiple genome-maintenance pathways accounts for its frequent misregulation in human cancers and highlights TRIP13 as a novel therapeutic target. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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.

Meiosis in crops: from genes to genomes

Meiosis is a key feature of sexual reproduction. During meiosis homologous chromosomes replicate, recombine, and randomly segregate, followed by the segregation of sister chromatids to produce haploid cells. The unique genotypes of recombinant gametes are an essential substrate for the selection of superior genotypes in natural populations and in plant breeding. In this review we summarize current knowledge on meiosis in diverse monocot and dicot crop species and provide a comprehensive resource of cloned meiotic mutants in six crop species (rice, maize, wheat, barley, tomato, and Brassica species). Generally, the functional roles of meiotic proteins are conserved between plant species, but we highlight notable differences in mutant phenotypes. The physical lengths of plant chromosomes vary greatly; for instance, wheat chromosomes are roughly one order of magnitude longer than those of rice. We explore how chromosomal distribution for crossover recombination can vary between species. We conclude that research on meiosis in crops will continue to complement that in Arabidopsis, and alongside possible applications in plant breeding will facilitate a better understanding of how the different stages of meiosis are controlled in plant species.

Conservation and divergence of meiotic DNA double strand break forming mechanisms in Arabidopsis thaliana

In the current meiotic recombination initiation model, the SPO11 catalytic subunits associate with MTOPVIB to form a Topoisomerase VI-like complex that generates DNA double strand breaks (DSBs). Four additional proteins, PRD1/AtMEI1, PRD2/AtMEI4, PRD3/AtMER2 and the plant specific DFO are required for meiotic DSB formation. Here we show that (i) MTOPVIB and PRD1 provide the link between the catalytic sub-complex and the other DSB proteins, (ii) PRD3/AtMER2, while localized to the axis, does not assemble a canonical pre-DSB complex but establishes a direct link between the DSB-forming and resection machineries, (iii) DFO controls MTOPVIB foci formation and is part of a divergent RMM-like complex including PHS1/AtREC114 and PRD2/AtMEI4 but not PRD3/AtMER2, (iv) PHS1/AtREC114 is absolutely unnecessary for DSB formation despite having a conserved position within the DSB protein network and (v) MTOPVIB and PRD2/AtMEI4 interact directly with chromosome axis proteins to anchor the meiotic DSB machinery to the axis.

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.

Pch2 orchestrates the meiotic recombination checkpoint from the cytoplasm

During meiosis, defects in critical events trigger checkpoint activation and restrict cell cycle progression. The budding yeast Pch2 AAA+ ATPase orchestrates the checkpoint response launched by synapsis deficiency; deletion of PCH2 or mutation of the ATPase catalytic sites suppress the meiotic block of the zip1Δ mutant lacking the central region of the synaptonemal complex. Pch2 action enables adequate levels of phosphorylation of the Hop1 axial component at threonine 318, which in turn promotes activation of the Mek1 effector kinase and the ensuing checkpoint response. In zip1Δ chromosomes, Pch2 is exclusively associated to the rDNA region, but this nucleolar fraction is not required for checkpoint activation, implying that another yet uncharacterized Pch2 population must be responsible for this function. Here, we have artificially redirected Pch2 to different subcellular compartments by adding ectopic Nuclear Export (NES) or Nuclear Localization (NLS) sequences, or by trapping Pch2 in an immobile extranuclear domain, and we have evaluated the effect on Hop1 chromosomal distribution and checkpoint activity. We have also deciphered the spatial and functional impact of Pch2 regulators including Orc1, Dot1 and Nup2. We conclude that the cytoplasmic pool of Pch2 is sufficient to support the meiotic recombination checkpoint involving the subsequent Hop1-Mek1 activation on chromosomes, whereas the nuclear accumulation of Pch2 has pathological consequences. We propose that cytoplasmic Pch2 provokes a conformational change in Hop1 that poises it for its chromosomal incorporation and phosphorylation. Our discoveries shed light into the intricate regulatory network controlling the accurate balance of Pch2 distribution among different cellular compartments, which is essential for proper meiotic outcomes.

During gametogenesis, the number of chromosomes is reduced by half and it returns to the normal ploidy when the two gametes fuse during fertilization. Meiosis lies at the heart of gametogenesis because it is the specialized cell division making possible the reduction in ploidy. The fidelity in this process is essential to maintain the chromosome complement characteristic of the species and to avoid aneuploidies. Meiotic cells possess an intricate surveillance network that monitors crucial meiotic events. In response to defects in synapsis and recombination, the meiotic recombination checkpoint blocks meiotic cell cycle progression, thus avoiding aberrant chromosome segregation and formation of defective gametes. The AAA+ ATPase Pch2 is an essential component of the checkpoint response triggered by the recombination defects occurring in the zip1Δ mutant lacking the central region of the synaptonemal complex. Pch2 supports proper chromosomal localization and phosphorylation of the Hop1 axial component required for the ensuing checkpoint response. We reveal here the biological relevance of a cytoplasmic population of Pch2 that is necessary for meiotic events occurring on chromosomes. Using a variety of strategies, we demonstrate that the checkpoint activating function of Pch2 takes place outside the nucleus, whereas the nuclear accumulation of Pch2 has deleterious consequences. Our work highlights the importance of nucleocytoplasmic communication for a balanced distribution of Pch2 among different subcellular compartments and how it impinges on Hop1 dynamics, which is crucial for proper completion of the meiotic program.

REV7 directs DNA repair pathway choice

REV7 is a small multifunctional protein that participates in multiple DNA repair pathways, most notably translesion DNA synthesis and double-strand break (DSB) repair. While the role of REV7 in translesion synthesis has been known for several decades, its function in DSB repair is a recent discovery. Investigations into the DSB repair function of REV7 have led to the discovery of a new DNA repair complex known as Shieldin. Recent studies have also highlighted the importance of REV7's HORMA domain, an ancient structural motif, in REV7 function and have identified the HORMA regulators, TRIP13 and p31, as novel DNA repair factors. In this review, we discuss these recent findings and their implications for repair pathway choice, at both DSBs and replication forks. We suggest that REV7, in particular the activation state of its HORMA domain, can act as a critical determinant of mutagenic versus error-free repair in multiple contexts.

Moving to and fro between Arabidopsis and its crop relatives confirms the role of chromosome remodelling on meiotic recombination

This article comments on:

Cuacos M, Lambing C, Pachon-Penalba M, Osman K, Armstrong SJ, Henderson IR, Sanchez-Moran E, Franklin FCH, Heckmann S. 2021. Meiotic chromosome axis remodelling is critical for meiotic recombination in Brassica rapa. Journal of Experimental Botany 72, 3012–3027.

ZYP1 is required for obligate cross-over formation and cross-over interference in Arabidopsis

The synaptonemal complex (SC) is a meiosis-specific proteinaceous ultrastructure required to ensure cross-over (CO) formation in the majority of sexually reproducing eukaryotes. It is composed of two lateral elements adjoined by transverse filaments. Even though the general structure of the SC is conserved throughout kingdoms, phenotypic differences between mutants perpetuate the enigmatic role of the SC. Here, we have used genetic and cytogenetic approaches to show that the transverse filament protein, ZYP1, acts on multiple pathways of meiotic recombination in Arabidopsis. ZYP1 is required for CO assurance, thus ensuring that every chromosome pair receives at least one CO. ZYP1 limits the number of COs and mediates CO interference, the phenomenon that reduces the probability of multiple COs forming close together.

The synaptonemal complex is a tripartite proteinaceous ultrastructure that forms between homologous chromosomes during prophase I of meiosis in the majority of eukaryotes. It is characterized by the coordinated installation of transverse filament proteins between two lateral elements and is required for wild-type levels of crossing over and meiotic progression. We have generated null mutants of the duplicated Arabidopsis transverse filament genes zyp1a and zyp1b using a combination of T-DNA insertional mutants and targeted CRISPR/Cas mutagenesis. Cytological and genetic analysis of the zyp1 null mutants reveals loss of the obligate chiasma, an increase in recombination map length by 1.3- to 1.7-fold and a virtual absence of cross-over (CO) interference, determined by a significant increase in the number of double COs. At diplotene, the numbers of HEI10 foci, a marker for Class I interference-sensitive COs, are twofold greater in the zyp1 mutant compared to wild type. The increase in recombination in zyp1 does not appear to be due to the Class II interference-insensitive COs as chiasmata were reduced by ∼52% in msh5/zyp1 compared to msh5. These data suggest that ZYP1 limits the formation of closely spaced Class I COs in Arabidopsis. Our data indicate that installation of ZYP1 occurs at ASY1-labeled axial bridges and that loss of the protein disrupts progressive coalignment of the chromosome axes.

Getting there: understanding the chromosomal recruitment of the AAA+ ATPase Pch2/TRIP13 during meiosis

The generally conserved AAA+ ATPase Pch2/TRIP13 is involved in diverse aspects of meiosis, such as prophase checkpoint function, DNA break regulation, and meiotic recombination. The controlled recruitment of Pch2 to meiotic chromosomes allows it to use its ATPase activity to influence HORMA protein-dependent signaling. Because of the connection between Pch2 chromosomal recruitment and its functional roles in meiosis, it is important to reveal the molecular details that govern Pch2 localization. Here, we review the current understanding of the different factors that control the recruitment of Pch2 to meiotic chromosomes, with a focus on research performed in budding yeast. During meiosis in this organism, Pch2 is enriched within the nucleolus, where it likely associates with the specialized chromatin of the ribosomal (r)DNA. Pch2 is also found on non-rDNA euchromatin, where its recruitment is contingent on Zip1, a component of the synaptonemal complex (SC) that assembles between homologous chromosomes. We discuss recent findings connecting the recruitment of Pch2 with its association with the Origin Recognition Complex (ORC) and reliance on RNA Polymerase II-dependent transcription. In total, we provide a comprehensive overview of the pathways that control the chromosomal association of an important meiotic regulator.

The Mad2-Binding Protein p31comet as a Potential Target for Human Cancer Therapy

The spindle assembly checkpoint (SAC) is a surveillance mechanism that prevents mitotic exit at the metaphase-to-anaphase transition until all chromosomes have established correct bipolar attachment to spindle microtubules. Activation of SAC relies on the assembly of the mitotic checkpoint complex (MCC), which requires conformational change from inactive open Mad2 (OMad2) to the active closed Mad2 (C-Mad2) at unattached kinetochores. The Mad2-binding protein p31^comet plays a key role in controlling timely mitotic exit by promoting SAC silencing, through preventing Mad2 activation and promoting MCC disassembly. Besides, increasing evidences highlight the p31^comet potential as target for cancer therapy. Here, we provide an updated overview of the functional significance of p31^comet in mitotic progression, and discuss the potential of deregulated expression of p31^comet in cancer and in therapeutic strategies.