PAIR3, an axis-associated protein, is essential for the recruitment of recombination elements onto meiotic chromosomes in rice

Genetic and molecular mechanisms underlying the male sterility in rice

Male reproductive development is a complex and highly ordered phenomenon which demands comprehensive understandings of underlying molecular mechanisms to expand its scope for crop improvement. Genetic manipulation of male fertility/sterility is critical for crop hybrid breeding. Although male sterility is not a good trait for the plant itself, its wider application in hybrid rice breeding has made it valuable. The currently widely used male sterile line breeding systems mainly include the following: three-line hybrid rice based on cytoplasmic male sterility and two-line hybrid rice based on environmentally sensitive gene male sterility. The study of male sterility is an excellent thoroughfare to critically understand the regulatory mechanisms essential for the complicated male reproductive developmental process. The unique trait of male sterility also provides valuable resources and convenience for the genetic improvement of rice hybrids. Therefore, deeper and broader understandings about the genetic causes of male sterility are necessary for both basic studies and rice genetic improvement.

m6A demethylase OsALKBH5 is required for double-strand break formation and repair by affecting mRNA stability in rice meiosis

N6-methyladenosine (m6A) RNA modification is the most prevalent messenger RNA (mRNA) modification in eukaryotes and plays critical roles in the regulation of gene expression. m6A is a reversible RNA modification that is deposited by methyltransferases (writers) and removed by demethylases (erasers). The function of m6A erasers in plants is highly diversified and their roles in cereal crops, especially in reproductive development essential for crop yield, are largely unknown. Here, we demonstrate that rice OsALKBH5 acts as an m6A demethylase required for the normal progression of male meiosis. OsALKBH5 is a nucleo-cytoplasmic protein, highly enriched in rice anthers during meiosis, that associates with P-bodies and exon junction complexes, suggesting that it is involved in regulating mRNA processing and abundance. Mutations of OsALKBH5 cause reduced double-strand break (DSB) formation, severe defects in DSB repair, and delayed meiotic progression, leading to complete male sterility. Transcriptome analysis and m6A profiling indicate that OsALKBH5-mediated m6A demethylation stabilizes the mRNA level of multiple meiotic genes directly or indirectly, including several genes that regulate DSB formation and repair. Our study reveals the indispensable role of m6A metabolism in post-transcriptional regulation of meiotic progression in rice.

Kinesin-1-like protein PSS1 is essential for full-length homologous pairing and synapsis in rice meiosis

The pairing and synapsis of homologous chromosomes are crucial for their correct segregation during meiosis. The LINC (Linker of Nucleoskeleton and Cytoskeleton) complex can recruit kinesin protein at the nuclear envelope, affecting telomere bouquet formation and homologous pairing. Kinesin-1-like protein Pollen Semi-Sterility1 (PSS1) plays a pivotal role in male meiotic chromosomal behavior and is essential for fertility in rice. However, its exact role in meiosis, especially as kinesin involved in homologous pairing and synapsis, has not been fully elucidated. Here, we generated three pss1 mutants by genome editing technology to dissect PSS1 biological functions in meiosis. The pss1 mutants exhibit alterations in the radial microtubule organization at pachytene and manifest a deficiency in telomere clustering, which is critical for full-length homologous pairing. We reveal that PSS1 serves as a key mediator between chromosomes and cytoskeleton, thereby regulating microtubule organization and transmitting the force to nuclei to facilitate homologous chromosome pairing and synapsis in meiosis.

ASYNAPSIS3 has diverse dosage-dependent effects on meiotic crossover formation in Brassica napus

Crossovers create genetic diversity and are required for equal chromosome segregation during meiosis. Crossover number and distribution are highly regulated by different mechanisms that are not yet fully understood, including crossover interference. The chromosome axis is crucial for crossover formation. Here, we explore the function of the axis protein ASYNAPSIS3. To this end, we use the allotetraploid species Brassica napus; due to its polyploid nature, this system allows a fine-grained dissection of the dosage of meiotic regulators. The simultaneous mutation of all 4 ASY3 alleles results in defective synapsis and drastic reduction of crossovers, which is largely rescued by the presence of only one functional ASY3 allele. Crucially, while the number of class I crossovers in mutants with 2 functional ASY3 alleles is comparable to that in wild type, this number is significantly increased in mutants with only one functional ASY3 allele, indicating that reducing ASY3 dosage increases crossover formation. Moreover, the class I crossovers on each bivalent in mutants with 1 functional ASY3 allele follow a random distribution, indicating compromised crossover interference. These results reveal the distinct dosage-dependent effects of ASY3 on crossover formation and provide insights into the role of the chromosome axis in patterning recombination.

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.

Crossover interference mechanism: New lessons from plants

Plants are the source of our understanding of several fundamental biological principles. It is well known that Gregor Mendel discovered the laws of Genetics in peas and that maize was used for the discovery of transposons by Barbara McClintock. Plant models are still useful for the understanding of general key biological concepts. In this article, we will focus on discussing the recent plant studies that have shed new light on the mysterious mechanisms of meiotic crossover (CO) interference, heterochiasmy, obligatory CO, and CO homeostasis. Obligatory CO is necessary for the equilibrated segregation of homologous chromosomes during meiosis. The tight control of the different male and female CO rates (heterochiasmy) enables both the maximization and minimization of genome shuffling. An integrative model can now predict these observed aspects of CO patterning in plants. The mechanism proposed considers the Synaptonemal Complex as a canalizing structure that allows the diffusion of a class I CO limiting factor linearly on synapsed bivalents. The coarsening of this limiting factor along the SC explains the interfering spacing between COs. The model explains the observed coordinated processes between synapsis, CO interference, CO insurance, and CO homeostasis. It also easily explains heterochiasmy just considering the different male and female SC lengths. This mechanism is expected to be conserved in other species.

Rice GLUCAN SYNTHASE-LIKE5 promotes anther callose deposition to maintain meiosis initiation and progression

Callose is a plant cell wall polysaccharide whose deposition is spatiotemporally regulated in various developmental processes and environmental stress responses. The appearance of callose in premeiotic anthers is a prominent histological hallmark for the onset of meiosis in flowering plants; however, the biological role of callose in meiosis remains unknown. Here, we show that rice (Oryza sativa) GLUCAN SYNTHASE LIKE5 (OsGSL5), a callose synthase, localizes on the plasma membrane of pollen mother cells (PMCs) and is responsible for biogenesis of callose in anther locules through premeiotic and meiotic stages. In Osgsl5 mutant anthers mostly lacking callose deposition, aberrant PMCs accompanied by aggregated, unpaired, or multivalent chromosomes were frequently observed and, furthermore, a considerable number of mutant PMCs had untimely progress into meiosis compared to that of wild-type PMCs. Immunostaining of meiosis-specific protein HOMOLOGOUS PAIRING ABERRATION IN RICE MEIOSIS2 in premeiotic PMCs revealed precocious meiosis entry in Osgsl5 anthers. These findings provide insights into the function of callose in controlling the timing of male meiosis initiation and progression, in addition to roles in microsporogenesis, in flowering plants.

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.

OsRAD51 Plays a Vital Role in Promoting Homologous Recombination in Rice Meiosis

Meiotic recombination plays a pivotal role in achieving accurate chromosomal segregation and increasing genetic diversity. In the homologous recombination pathway, the detailed mechanisms of how OsRAD51 and OsDMC1 work in rice meiosis remain to be explored. Here, we obtained different types of mutants for Osrad51a1, Osrad51a2, Osdmc1a, and Osdmc1b through CRISPR/Cas9. Both Osrad51a1 and Osrad51a2 exhibited normal vegetative growth and fertility. Osrad51 (Osrad51a1 Osrad51a2) mutant plants show normal vegetative growth but exhibit complete sterility, indicating that OsRAD51A1 and OsRAD51A2 are functionally redundant in rice fertility. In contrast to the wild type, Osrad51 chromosomes are not paired perfectly at pachytene and synaptonemal complex (SC) formation is deficient. Moreover, univalents and multivalent associations were observed at metaphase I, chromosome fragments presented at anaphase I, and crossover formation is basically suppressed in Osrad51 pollen mother cells (PMCs). OsRAD51 foci emerge at leptotene and disappear from late pachytene and chromosome localization of OsRAD51 depends on the formation of double-strand breaks (DSBs). Most OsRAD51 foci can co-localize with OsDMC1 signals. OsRAD51 is essential for the loading of OsDMC1 onto chromosomes, and vice versa. In addition, both OsRAD51 and OsDMC1 can interact with OsFIGL1 and OsBRCA2, two important components in rice meiosis. Moreover, the Osrad51 Osdmc1 (Osrad51a1 Osrad51a2 Osdmc1a Osdmc1b) quadruple mutant PMCs exhibited similar defective phenotypes as Osrad51 in homologous pairing, synapsis, and DSB repair. Taken together, our results suggest that the recombinases DMC1 and RAD51 may functionally depend on each other and play important roles in meiotic recombination during meiosis in rice.

Homologous chromosome associations in domains before meiosis could facilitate chromosome recognition and pairing in wheat

The increasing human population demands an increase in crop yields that must be implemented through breeding programmes to ensure a more efficient and sustainable production of agro-food products. In the framework of breeding, genetic crosses are developed between cultivated species such as wheat and their relative species that are used as genetic donors to transfer desirable agronomic traits into the crop. Unfortunately, interspecific associations between chromosomes from the donor species and the cultivar are rare during meiosis, the process to produce gametes in organisms with sexual reproduction, hampering the transfer of genetic variability into wheat. In addition, little is known about how homologous (equivalent) chromosomes initiate interaction and recognition within the cell nucleus to enter meiosis. In this context, we aim to get insight into wheat chromatin structure, particularly the distribution of homologous chromosomes within the cell nucleus and their putative interactions in premeiotic stages to facilitate chromosome associations and recombination at the beginning of meiosis. Cytogenetics allows the study of both the structure and the behaviour of chromosomes during meiosis and is key in plant breeding. In this study we visualized an extra pair of barley homologous chromosomes in a wheat genetic background to study the spatial distribution, arrangements and interactions occurring exclusively between this pair of homologous chromosomes during premeiosis using fluorescence in situ hybridization (FISH). Our results suggest that homologous chromosomes can initiate interactions in premeiotic stages that could facilitate the processes of specific chromosome recognition and association occurring at the onset of meiosis.

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.

The E3 ubiquitin ligase DESYNAPSIS1 regulates synapsis and recombination in rice meiosis

Synaptonemal complex (SC) assembly and homologous recombination, the most critical events during prophase I, are the prerequisite for faithful meiotic chromosome segregation. However, the underlying regulatory mechanism remains largely unknown. Here, we reveal that a functional RING finger E3 ubiquitin ligase, DESYNAPSIS1 (DSNP1), plays significant roles in SC assembly and homologous recombination during rice meiosis. In the dsnp1 mutant, homologous synapsis is discontinuous and aberrant SC-like polycomplexes occur independent of coaligned homologous chromosomes. Accompanying the decreased foci of HEI10, ZIP4, and MER3 on meiotic chromosomes, the number of crossovers (COs) decreases dramatically in dsnp1 meiocytes. Furthermore, the absence of central elements largely restores the localization of non-ZEP1 ZMM proteins and the number of COs in the dsnp1 background. Collectively, DSNP1 stabilizes the canonical tripartite SC structure along paired homologous chromosomes and further promotes the formation of COs.

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.

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.

Rewiring Meiosis for Crop Improvement

Meiosis is a specialized cell division that contributes to halve the genome content and reshuffle allelic combinations between generations in sexually reproducing eukaryotes. During meiosis, a large number of programmed DNA double-strand breaks (DSBs) are formed throughout the genome. Repair of meiotic DSBs facilitates the pairing of homologs and forms crossovers which are the reciprocal exchange of genetic information between chromosomes. Meiotic recombination also influences centromere organization and is essential for proper chromosome segregation. Accordingly, meiotic recombination drives genome evolution and is a powerful tool for breeders to create new varieties important to food security. Modifying meiotic recombination has the potential to accelerate plant breeding but it can also have detrimental effects on plant performance by breaking beneficial genetic linkages. Therefore, it is essential to gain a better understanding of these processes in order to develop novel strategies to facilitate plant breeding. Recent progress in targeted recombination technologies, chromosome engineering, and an increasing knowledge in the control of meiotic chromosome segregation has significantly increased our ability to manipulate meiosis. In this review, we summarize the latest findings and technologies on meiosis in plants. We also highlight recent attempts and future directions to manipulate crossover events and control the meiotic division process in a breeding perspective.

Meiotic chromosome axis remodelling is critical for meiotic recombination in Brassica rapa

Meiosis generates genetic variation through homologous recombination (HR) that is harnessed during breeding. HR occurs in the context of meiotic chromosome axes and the synaptonemal complex. To study the role of axis remodelling in crossover (CO) formation in a crop species, we characterized mutants of the axis-associated protein ASY1 and the axis-remodelling protein PCH2 in Brassica rapa. asy1 plants form meiotic chromosome axes that fail to synapse. CO formation is almost abolished, and residual chiasmata are proportionally enriched in terminal chromosome regions, particularly in the nucleolar organizing region (NOR)-carrying chromosome arm. pch2 plants show impaired ASY1 loading and remodelling, consequently achieving only partial synapsis, which leads to reduced CO formation and loss of the obligatory CO. PCH2-independent chiasmata are proportionally enriched towards distal chromosome regions. Similarly, in Arabidopsis pch2, COs are increased towards telomeric regions at the expense of (peri-) centromeric COs compared with the wild type. Taken together, in B. rapa, axis formation and remodelling are critical for meiotic fidelity including synapsis and CO formation, and in asy1 and pch2 CO distributions are altered. While asy1 plants are sterile, pch2 plants are semi-sterile and thus PCH2 could be an interesting target for breeding programmes.

PRD1, a homologous recombination initiation factor, is involved in spindle assembly in rice meiosis

The bipolar spindle structure in meiosis is essential for faithful chromosome segregation. PUTATIVE RECOMBINATION INITIATION DEFECT 1 (PRD1) previously has been shown to participate in the formation of DNA double strand breaks (DSBs). However, the role of PRD1 in meiotic spindle assembly has not been elucidated. Here, we reveal by both genetic analysis and immunostaining technology that PRD1 is involved in spindle assembly in rice (Oryza sativa) meiosis. We show that DSB formation and bipolar spindle assembly are disturbed in prd1 meiocytes. PRD1 signals display a dynamic pattern of localization from covering entire chromosomes at leptotene to congregating at the centromere region after leptotene. Centromeric localization of PRD1 signals depends on the organization of leptotene chromosomes, but not on DSB formation and axis establishment. PRD1 exhibits interaction and co-localization with several kinetochore components. We also find that bi-orientation of sister kinetochores within a univalent induced by mutation of REC8 can restore bipolarity in prd1. Furthermore, PRD1 directly interacts with REC8 and SGO1, suggesting that PRD1 may play a role in regulating the orientation of sister kinetochores. Taken together, we speculate that PRD1 promotes bipolar spindle assembly, presumably by modulating the orientation of sister kinetochores in rice meiosis.

State changes of the HORMA protein ASY1 are mediated by an interplay between its closure motif and PCH2

HORMA domain-containing proteins (HORMADs) play an essential role in meiosis in many organisms. The meiotic HORMADs, including yeast Hop1, mouse HORMAD1 and HORMAD2, and Arabidopsis ASY1, assemble along chromosomes at early prophase and the closure motif at their C-termini has been hypothesized to be instrumental for this step by promoting HORMAD oligomerization. In late prophase, ASY1 and its homologs are progressively removed from synapsed chromosomes promoting chromosome synapsis and recombination. The conserved AAA+ ATPase PCH2/TRIP13 has been intensively studied for its role in removing HORMADs from synapsed chromosomes. In contrast, not much is known about how HORMADs are loaded onto chromosomes. Here, we reveal that the PCH2-mediated dissociation of the HORMA domain of ASY1 from its closure motif is important for the nuclear targeting and subsequent chromosomal loading of ASY1. This indicates that the promotion of ASY1 to an ‘unlocked’ state is a prerequisite for its nuclear localization and chromosomal assembly. Likewise, we find that the closure motif is also necessary for the removal of ASY1 by PCH2 later in prophase. Our work results in a unified new model for PCH2 and HORMADs function in meiosis and suggests a mechanism to contribute to unidirectionality in meiosis.

Rice OsBRCA2 Is Required for DNA Double-Strand Break Repair in Meiotic Cells

The mammalian BREAST CANCER 2 (BRCA2) gene is a tumor suppressor that plays a crucial role in DNA repair and homologous recombination (HR). Here, we report the identification and characterization of OsBRCA2, the rice orthologue of human BRCA2. Osbrca2 mutant plants exhibit normal vegetative growth but experience complete male and female sterility as a consequence of severe meiotic defects. Pairing, synapsis and recombination are impaired in osbrca2 male meiocytes, leading to chromosome entanglements and fragmentation. In the absence of OsBRCA2, localization to the meiotic chromosome axes of the strand-invasion proteins OsRAD51 and OsDMC1 is severely reduced and in vitro OsBRCA2 directly interacts with OsRAD51 and OsDMC1. These results indicate that OsBRCA2 is essential for facilitating the loading of OsRAD51 and OsDMC1 onto resected ends of programmed double-strand breaks (DSB) during meiosis to promote single-end invasions of homologous chromosomes and accurate recombination. In addition, treatment of osbrca2-1 seedlings with mitomycin C (MMC) led to hypersensitivity. As MMC is a genotoxic agent that creates DNA lesions in the somatic cells that can only be repaired by HR, these results suggest that OsBRCA2 has a conserved role in DSB repair and HR in rice.

Dynamic localization of SPO11-1 and conformational changes of meiotic axial elements during recombination initiation of maize meiosis

Meiotic double-strand breaks (DSBs) are generated by the evolutionarily conserved SPO11 complex in the context of chromatin loops that are organized along axial elements (AEs) of chromosomes. However, how DSBs are formed with respect to chromosome axes and the SPO11 complex remains unclear in plants. Here, we confirm that DSB and bivalent formation are defective in maize spo11-1 mutants. Super-resolution microscopy demonstrates dynamic localization of SPO11-1 during recombination initiation, with variable numbers of SPO11-1 foci being distributed in nuclei but similar numbers of SPO11-1 foci being found on AEs. Notably, cytological analysis of spo11-1 meiocytes revealed an aberrant AE structure. At leptotene, AEs of wild-type and spo11-1 meiocytes were similarly curly and discontinuous. However, during early zygotene, wild-type AEs become uniform and exhibit shortened axes, whereas the elongated and curly AEs persisted in spo11-1 mutants, suggesting that loss of SPO11-1 compromised AE structural maturation. Our results reveal an interesting relationship between SPO11-1 loading onto AEs and the conformational remodeling of AEs during recombination initiation.

Meiosis is essential during sexual reproduction to produce haploid gametes. Recombination is the most crucial step during meiotic prophase I. It enables pairing of homologous chromosomes prior to their reductional division and generates new combinations of genetic alleles for transmission to the next generation. Meiotic recombination is initiated by generating DNA double-strand breaks (DSBs) via SPO11, a topoisomerase-related enzyme. The activity, timing and location of this DSB machinery must be controlled precisely, but how this is achieved remains obscure. Here, we show dynamic localization of SPO11-1 on chromatin during meiotic initiation in maize, yet a similar number of SPO11-1 is able to load onto axial elements (AEs), which accompanies a structural change of the AEs of wild-type meiotic chromosomes. Interestingly, loss of SPO11-1 not only affects DSB formation but also impairs structural alterations of AEs, resulting in abnormally long and curly AEs during early meiosis. Our study provides new insights into SPO11-1 localization during recombination initiation and suggests an intimate relationship between DSB formation and AE structural changes.

Tapetum-Dependent Male Meiosis Progression in Plants: Increasing Evidence Emerges

In higher plants, male meiosis is a key process during microsporogenesis and is crucial for male fertility and seed set. Meiosis involves a highly dynamic organization of chromosomes and cytoskeleton and specifically takes place within sexual cells. However, studies in multiple plant species have suggested that the normal development of tapetum, the somatic cell layer surrounding the developing male meiocytes, is indispensable for the completion of the male meiotic cell cycle. Disrupted tapetum development causes alterations in the expression of a large range of genes involved in male reproduction. Moreover, recent experiments suggest that small RNAs (sRNAs) present in the anthers, including microRNAs (miRNAs) and phased, secondary, small interfering RNAs (phasiRNAs), play a potential but important role in controlling male meiosis, either by influencing the expression of meiotic genes in the meiocytes or through other unclear mechanisms, supporting the hypothesis that male meiosis is non-cell autonomously regulated. In this mini review, we summarize the recorded meiotic defects that occur in plants with defective tapetum development in both Arabidopsis and crops. Thereafter, we outline the latest understanding on the molecular mechanisms that potentially underpin the tapetum-dependent regulation of male meiosis, and we especially discuss the regulatory role of sRNAs. At the end, we propose several outstanding questions that should be addressed in future studies.

Surface Spreading Technique in Plant Meiocytes for Analysis of Synaptonemal Complex by Electron Microscopy

An improved method of preparing two-dimensional surface spreads of synaptonemal complexes (SCs) in higher plants for examination by electron microscopy is described. This protocol produces clear, well-spread preparations of SCs and unpaired axial cores from a range of meiotic prophase I stages (leptotene to pachytene) from meiocytes of different plant species. Synaptonemal complex (SC) analyses have been widely used in plant cytogenetic studies to address the process of meiotic chromosome synapses, because of the high-resolution allowed by electron microscopy. Although the real role of SC is still enigmatic, its presence and structural conservation in the vast majority of organisms reflect the importance of this protein structure in the meiotic process.

Chromatin Immunoprecipitation of Meiotically Expressed Proteins from Arabidopsis thaliana Flowers

During meiosis recombination occurs between homologous chromosomes which can result in reciprocal exchanges of genetic information, called crossovers. Crossover rate is heterogeneous within the genome, with local regions having a significantly higher recombination rate relative to the genome average. These regions are termed hotspots and typically occur with widths of kilobases. Therefore, there is a need to profile recombination factors at a similar resolution during meiosis via techniques such as chromatin immunoprecipitation (ChIP). Here we describe a ChIP protocol, combined with high throughput sequencing (ChIP-seq) optimised for analysis of meiotically expressed proteins in Arabidopsis thaliana flowers. We provide methods to (1) isolate nuclei and prepare the chromatin for shearing, (2) immunoprecipitate DNA molecules cross-linked to a protein of interest, (3) to size-select and purify immunoprecipitated DNA molecules, and (4) to prepare DNA sequencing libraries suitable for high-throughput sequencing. Together, these methods allow the detection of binding sites for meiotic proteins in the Arabidopsis genome at high resolution, which will provide insights into relationships between meiotic chromosome organization, chromatin and recombination.

Analytical Methodology of Meiosis in Autopolyploid and Allopolyploid Plants

Meiosis is the cellular process responsible for producing gametes with half the genetic content of the parent cells. Integral parts of the process in most diploid organisms include the recognition, pairing, synapsis, and recombination of homologous chromosomes, which are prerequisites for balanced segregation of half-bivalents during meiosis I. In polyploids, the presence of more than two sets of chromosomes adds to the basic meiotic program of their diploid progenitors the possibility of interactions between more than two chromosomes and the formation of multivalents, which has implications on chromosome segregations and fertility. The mode of how chromosomes behave in meiosis in competitive situations has been the aim of many studies in polyploid species, some of which are considered here. But polyploids are also of interest in the study of meiosis because some of them tolerate the loss of chromosome segments or complete chromosomes as well as the addition of chromosomes from related species. Deletions allow to assess the effect of specific chromosome segments on meiotic behavior. Introgression lines are excellent materials to monitor the behavior of a given chromosome in the genetic background of the recipient species. We focus on this approach here as based on studies carried out in bread wheat, which is commonly used as a model species for meiosis studies. In addition to highlighting the relevance of the use of materials derived from polyploids in the study of meiosis, cytogenetics tools such as fluorescence in situ hybridization and the immunolabeling of proteins interacting with DNA are also emphasized.

A Multiprotein Complex Regulates Interference-Sensitive Crossover Formation in Rice

In most eukaryotes, a set of conserved proteins that are collectively termed ZMM proteins (named for molecular zipper 1 [ZIP1], ZIP2, ZIP3, and ZIP4, MutS homologue 4 [MSH4] and MSH5, meiotic recombination 3, and sporulation 16 [SPO16] in yeast [Saccharomyces cerevisiae]) are essential for the formation of the majority of meiotic crossovers (COs). Recent reports indicated that ZIP2 acts together with SPO16 and ZIP4 to control CO formation through recognizing and stabilizing early recombination intermediates in budding yeast. However, whether this mechanism is conserved in plants is not clear. Here, we characterized the functions of SHORTAGE OF CHIASMATA 1 (OsSHOC1; ZIP2 ortholog) and PARTING DANCERS (OsPTD; SPO16 ortholog) and their interactions with other ZMM proteins in rice (Oryza sativa). We demonstrated that disruption of OsSHOC1 caused a reduction of CO numbers to ∼83% of wild-type CO numbers, whereas synapsis and early meiotic recombination steps were not affected. Furthermore, OsSHOC1 interacts with OsPTD, which is responsible for the same set of CO formations as OsSHOC1. In addition, OsSHOC1 and OsPTD are required for the normal loading of other ZMM proteins, and conversely, the localizations of OsSHOC1 and OsPTD were also affected by the absence of OsZIP4 and human enhancer of invasion 10 in rice (OsHEI10). OsSHOC1 interacts with OsZIP4 and OsMSH5, and OsPTD interacts with OsHEI10. Furthermore, bimolecular fluorescence complementation and yeast-three hybrid assays demonstrated that OsSHOC1, OsPTD, OsHEI10, and OsZIP4 were able to form various combinations of heterotrimers. Moreover, statistical and genetic analysis indicated that OsSHOC1 and OsPTD are epistatic to OsHEI10 and OsZIP4 in meiotic CO formation. Taken together, we propose that OsSHOC1, OsPTD, OsHEI10, and OsZIP4 form multiple protein complexes that have conserved functions in promoting class I CO formation.

Ultrastructure and Dynamics of Synaptonemal Complex Components During Meiotic Pairing and Synapsis of Standard (A) and Accessory (B) Rye Chromosomes

During prophase I a meiosis-specific proteinaceous tripartite structure, the synaptonemal complex (SC), forms a scaffold to connect homologous chromosomes along their lengths. This process, called synapsis, is required in most organisms to promote recombination between homologs facilitating genetic variability and correct chromosome segregations during anaphase I. Recent studies in various organisms ranging from yeast to mammals identified several proteins involved in SC formation. However, the process of SC disassembly remains largely enigmatic. In this study we determined the structural changes during SC formation and disassembly in rye meiocytes containing accessory (B) chromosomes. The use of electron and super-resolution microscopy (3D-SIM) combined with immunohistochemistry and FISH allowed us to monitor the structural changes during prophase I. Visualization of the proteins ASY1, ZYP1, NSE4A, and HEI10 revealed an extensive SC remodeling during prophase I. The ultrastructural investigations of the dynamics of these four proteins showed that the SC disassembly is accompanied by the retraction of the lateral and axial elements from the central region of the SC. In addition, SC fragmentation and the formation of ball-like SC structures occur at late diakinesis. Moreover, we show that the SC composition of rye B chromosomes does not differ from that of the standard (A) chromosome complement. Our ultrastructural investigations indicate that the dynamic behavior of the studied proteins is involved in SC formation and synapsis. In addition, they fulfill also functions during desynapsis and chromosome condensation to realize proper recombination and homolog separation. We propose a model for the homologous chromosome behavior during prophase I based on the observed dynamics of ASY1, ZYP1, NSE4A, and HEI10.

OsHOP2 regulates the maturation of crossovers by promoting homologous pairing and synapsis in rice meiosis

Meiotic recombination is closely linked with homologous pairing and synapsis. Previous studies have shown that HOMOLOGOUS PAIRING PROTEIN2 (HOP2), plays an essential role in homologous pairing and synapsis. However, the mechanism by which HOP2 regulates crossover (CO) formation has not been elucidated. Here, we show that OsHOP2 mediates the maturation of COs by promoting homologous pairing and synapsis in rice (Oryza sativa) meiosis. We used a combination of genetic analysis, immunolocalization and super-resolution imaging to analyze the function of OsHOP2 in rice meiosis. We showed that full-length pairing, synapsis and CO formation are disturbed in Oshop2 meiocytes. Moreover, structured illumination microscopy showed that OsHOP2 localized to chromatin and displayed considerable co-localization with axial elements (AEs) and central elements (CEs). Importantly, the interaction between OsHOP2 and a transverse filament protein of synaptonemal complex (ZEP1), provided further evidence that OsHOP2 was involved in assembly or stabilization of the structure of the synaptonemal complex (SC). Although the initiation of recombination and CO designation occur normally in Oshop2 mutants, mature COs were severely reduced, and human enhancer of invasion 10 (HEI10)10 foci were only present on the synapsed region. Putting the data together, we speculate that OsHOP2 may serve as a global regulator to coordinate homologous pairing, synapsis and meiotic recombination in rice meiosis.

Identification of ASYNAPTIC4, a Component of the Meiotic Chromosome Axis

During the leptotene stage of prophase I of meiosis, chromatids become organized into a linear looped array via a protein axis that forms along the loop bases. Establishment of the axis is essential for the subsequent synapsis of the homologous chromosome pairs and the progression of recombination to form genetic crossovers. Here, we describe ASYNAPTIC4 (ASY4), a meiotic axis protein in Arabidopsis (Arabidopsis thaliana). ASY4 is a small coiled-coil protein that exhibits limited sequence similarity with the carboxyl-terminal region of the axis protein ASY3. We used enhanced yellow fluorescent protein-tagged ASY4 to show that ASY4 localizes to the chromosome axis throughout prophase I. Bimolecular fluorescence complementation revealed that ASY4 interacts with ASY1 and ASY3, and yeast two-hybrid analysis confirmed a direct interaction between ASY4 and ASY3. Mutants lacking full-length ASY4 exhibited defective axis formation and were unable to complete synapsis. Although the initiation of recombination appeared to be unaffected in the asy4 mutant, the number of crossovers was reduced significantly, and crossovers tended to group in the distal parts of the chromosomes. We conclude that ASY4 is required for normal axis and crossover formation. Furthermore, our data suggest that ASY3/ASY4 are the functional homologs of the mammalian SYCP2/SYCP3 axial components.

Variable Patterning of Chromatin Remodeling, Telomere Positioning, Synapsis, and Chiasma Formation of Individual Rye Chromosomes in Meiosis of Wheat-Rye Additions

Meiosis, the type of cell division that halves the chromosome number, shows a considerable degree of diversity among species. Unraveling molecular mechanisms of the meiotic machinery has been mainly based on meiotic mutants, where the effects of a change were assessed on chromosomes of the particular species. An alternative approach is to study the meiotic behavior of the chromosomes introgressed into different genetic backgrounds. As an allohexaploid, common wheat tolerates introgression of chromosomes from related species, such as rye. The behavior of individual pairs of rye homologues added to wheat has been monitored in meiotic prophase I and metaphase I. Chromosome 4R increased its length in early prophase I much more than other chromosomes studied, implying chromosome specific patterns of chromatin organization. Chromosome conformation affected clustering of telomeres but not their dispersion. Telomeres of the short arm of submetacentric chromosomes 4R, 5R, and 6R failed more often to be included in the telomere cluster either than the telomeres of the long arms or telomeres of metacentrics such as 2R, 3R, and 7R. The disturbed migration of the telomeres of 5RS and 6RS was associated with failure of synapsis and chiasma formation. However, despite the failed convergence of its telomere, the 4RS arm developed normal synapsis, perhaps because the strong increase of its length in early prophase I facilitated homologous encounters in intercalary regions. Surprisingly, chiasma frequencies in both arms of 4R were reduced. Similarly, the short arm of metacentric chromosome 2R often failed to form chiasmata despite normal synapsis. Chromosomes 1R, 3R, and 7R showed a regular meiotic behavior. These observations are discussed in the context of the behavior that these chromosomes show in rye itself.

Tackling Plant Meiosis: From Model Research to Crop Improvement

Genetic engineering and traditional plant breeding, which harnesses the natural genetic variation that arises during meiosis, will have key roles to improve crop varieties and thus deliver Food Security in the future. Meiosis, a specialized cell division producing haploid gametes to maintain somatic diploidy following their fusion, assures genetic variation by regulated genetic exchange through homologous recombination. However, meiotic recombination events are restricted in their total number and their distribution along chromosomes limiting allelic variations in breeding programs. Thus, modifying the number and distribution of meiotic recombination events has great potential to improve and accelerate plant breeding. In recent years much progress has been made in understanding meiotic progression and recombination in plants. Many genes and factors involved in these processes have been identified primarily in Arabidopsis thaliana but also more recently in crops such as Brassica, rice, barley, maize, or wheat. These advances put researchers in the position to translate acquired knowledge to various crops likely improving and accelerating breeding programs. However, although fundamental aspects of meiotic progression and recombination are conserved between species, differences in genome size and organization (due to repetitive DNA content and ploidy level) exist, particularly among plants, that likely account for differences in meiotic progression and recombination patterns found between species. Thus, tools and approaches are needed to better understand differences and similarities in meiotic progression and recombination among plants, to study fundamental aspects of meiosis in a variety of plants including crops and non-model species, and to transfer knowledge into crop species. In this article, we provide an overview of tools and approaches available to study plant meiosis, highlight new techniques, give examples of areas of future research and review distinct aspects of meiosis in non-model species.

ZmCom1 Is Required for Both Mitotic and Meiotic Recombination in Maize

CtIP/Ctp1/Sae2/Com1, a highly conserved protein from yeast to higher eukaryotes, is required for DNA double-strand break repair through homologous recombination (HR). In this study, we identified and characterized the COM1 homolog in maize. The ZmCom1 gene is abundantly expressed in reproductive tissues at meiosis stages. In ZmCom1-deficient plants, meiotic chromosomes are constantly entangled as a formation of multivalents and accompanied with chromosome fragmentation at anaphase I. In addition, the formation of telomere bouquet, homologous pairing and synapsis were disturbed. The immunostaining assay showed that the localization of ASY1 and DSY2 was normal, while ZYP1 signals were severely disrupted in Zmcom1 meiocytes, indicating that ZmCom1 is critically required for the proper SC assembly. Moreover, RAD51 signals were almost completely absent in Zmcom1 meiocytes, implying that COM1 is required for RAD51 loading. Surprisingly, in contrast to the Atcom1 and Oscom1 mutants, Zmcom1 mutant plants exhibited a number of vegetative phenotypes under normal growth condition, which may be partly attributed to mitotic aberrations including chromosomal fragmentation and anaphase bridges. Taken together, our results suggest that although the roles of COM1 in HR process seem to be primarily conserved, the COM1 dysfunction can result in the marked dissimilarity in mitotic and meiotic outcomes in maize compared to Arabidopsis and rice. We suggest that this character may be related to the discrete genome context.

The F-Box Protein ZYGO1 Mediates Bouquet Formation to Promote Homologous Pairing, Synapsis, and Recombination in Rice Meiosis

Telomere bouquet formation, a highly conserved meiotic event, plays an important role in homologous pairing and therefore progression of meiosis; however, the underlying molecular mechanism remains largely unknown. Here, we identified ZYGOTENE1 (ZYGO1), a novel F-box protein in rice (Oryza sativa), and verified its essential role in bouquet formation during early meiosis. In zygo1 mutants, zygotene chromosome aggregation and telomere clustering failed to occur. The suppressed telomere clustering in homologous pairing aberration in rice meiosis1 (pair1) zygo1 and rice completion of meiotic recombination (Oscom1) zygo1 double mutants, together with the altered localization of OsSAD1 (a SUN protein associated with the nuclear envelope) in zygo1, showed that ZYGO1 has a significant function in bouquet formation. In addition, the interaction between ZYGO1 and rice SKP1-like protein 1 suggested that ZYGO1 might modulate bouquet formation as a component of the SKP1-Cullin1-F-box complex. Although double-strand break formation and early recombination element installation occurred normally, zygo1 mutants showed defects in full-length pairing and synaptonemal complex assembly. Furthermore, crossover (CO) formation was disturbed, and foci of Human enhancer of invasion 10 were restricted to the partially synapsed chromosome regions, indicating that CO reduction might be caused by the failure of full-length chromosome alignment in zygo1 Therefore, we propose that ZYGO1 mediates bouquet formation to efficiently promote homolog pairing, synapsis, and CO formation in rice meiosis.

Bivalent Formation 1, a plant-conserved gene, encodes an OmpH/coiled-coil motif-containing protein required for meiotic recombination in rice

Meiosis is essential for eukaryotic sexual reproduction and plant fertility. In comparison with over 80 meiotic genes identified in Arabidopsis, there are only ~30 meiotic genes characterized in rice (Oryza sativa L.). Many genes involved in the regulation of meiotic progression remain to be determined. In this study, we identified a sterile rice mutant and cloned a new meiotic gene, OsBVF1 (Bivalent Formation 1) by map-based cloning. Molecular genetics and cytological approaches were carried out to address the function of OsBVF1 in meiosis. Phylogenetic analyses were used to study the evolution of OsBVF1 and its homologs in plant species. Here we showed that the bvf1 male meiocytes were defective in formation of meiotic double strand break, thereby resulting in a failure of bivalent formation in diakinesis and unequal chromosome segregation in anaphase I. The causal gene, OsBVF1, encodes a unique OmpH/coiled-coil motif-containing protein and its homologs are highly conserved in the plant kingdom and seem to be a single-copy gene in the majority of plant species. Our study demonstrates that OsBVF1 is a novel plant-conserved factor involved in meiotic recombination in rice, providing a new insight into understanding of meiotic progression regulation.

Resolvase OsGEN1 Mediates DNA Repair by Homologous Recombination

Yen1/GEN1 are canonical Holliday junction resolvases that belong to the RAD2/XPG family. In eukaryotes, such as budding yeast, mice, worms, and humans, Yen1/GEN1 work together with Mus81-Mms4/MUS81-EME1 and Slx1-Slx4/SLX1-SLX4 in DNA repair by homologous recombination to maintain genome stability. In plants, the biological function of Yen1/GEN1 remains largely unclear. In this study, we characterized the loss of function mutants of OsGEN1 and OsSEND1, a pair of paralogs of Yen1/GEN1 in rice (Oryza sativa). We first investigated the role of OsGEN1 during meiosis and found a reduction in chiasma frequency by ∼6% in osgen1 mutants, compared to the wild type, suggesting a possible involvement of OsGEN1 in the formation of crossovers. Postmeiosis, OsGEN1 foci were detected in wild-type microspore nuclei, but not in the osgen1 mutant concomitant with an increase in double-strand breaks. Persistent double-strand breaks led to programmed cell death of the male gametes and complete male sterility. In contrast, depletion of OsSEND1 had no effects on plant development and did not enhance osgen1 defects. Our results indicate that OsGEN1 is essential for homologous recombinational DNA repair at two stages of microsporogenesis in rice.

P31comet, a member of the synaptonemal complex, participates in meiotic DSB formation in rice

The human mitotic arrest-deficient 2 (Mad2) binding protein p31(comet) participates in the spindle checkpoint and coordinates cell cycle events in mitosis although its function in meiosis remains unknown in all organisms. Here, we reveal P31(comet) as a synaptonemal complex (SC) protein in rice (Oryza sativa L.). In p31(comet), homologous pairing and synapsis are eliminated, leading to the homologous nondisjunction and complete sterility. The failure in loading of histone H2AX phosphorylation (γH2AX) in p31(comet), together with the suppressed chromosome fragmentation in rice completion of meiotic recombination 1 (com1) p31(comet) and radiation sensitive 51c (rad51c) p31(comet) double mutants, indicates that P31(comet) plays an essential role in double-strand break (DSB) formation. Interestingly, the dynamic colocalization pattern between P31(comet) and ZEP1 (a transverse filament protein of SC) by immunostaining, as well as the interaction between P31(comet) and CENTRAL REGION COMPONENT 1 (CRC1) in yeast two-hybrid assays, suggests possible involvement of P31(comet) in SC installation. Together, these data indicate that P31(comet) plays a key role in DSB formation and SC installation, mainly through its cooperation with CRC1.

MEIOTIC F-BOX Is Essential for Male Meiotic DNA Double-Strand Break Repair in Rice

F-box proteins constitute a large superfamily in plants and play important roles in controlling many biological processes, but the roles of F-box proteins in male meiosis in plants remain unclear. Here, we identify the rice (Oryza sativa) F-box gene MEIOTIC F-BOX (MOF), which is essential for male meiotic progression. MOF belongs to the FBX subfamily and is predominantly active during leptotene to pachytene of prophase I. mof meiocytes display disrupted telomere bouquet formation, impaired pairing and synapsis of homologous chromosomes, and arrested meiocytes at late prophase I, followed by apoptosis. Although normal, programmed double-stranded DNA breaks (DSBs) form in mof mutants, foci of the phosphorylated histone variant γH2AX, a marker for DSBs, persist in the mutant, indicating that many of the DSBs remained unrepaired. The recruitment of Completion of meiosis I (COM1) and Radiation sensitive51C (RAD51C) to DSBs is severely compromised in mutant meiocytes, indicating that MOF is crucial for DSB end-processing and repair. Further analyses showed that MOF could physically interact with the rice SKP1-like Protein1 (OSK1), indicating that MOF functions as a component of the SCF E3 ligase to regulate meiotic progression in rice. Thus, this study reveals the essential role of an F-box protein in plant meiosis and provides helpful information for elucidating the roles of the ubiquitin proteasome system in plant meiotic progression.

MS5 Mediates Early Meiotic Progression and Its Natural Variants May Have Applications for Hybrid Production in Brassica napus

During meiotic prophase I, chromatin undergoes dynamic changes to establish a structural basis for essential meiotic events. However, the mechanism that coordinates chromosome structure and meiotic progression remains poorly understood in plants. Here, we characterized a spontaneous sterile mutant MS5(b)MS5(b) in oilseed rape (Brassica napus) and found its meiotic chromosomes were arrested at leptotene. MS5 is preferentially expressed in reproductive organs and encodes a Brassica-specific protein carrying conserved coiled-coil and DUF626 domains with unknown function. MS5 is essential for pairing of homologs in meiosis, but not necessary for the initiation of DNA double-strand breaks. The distribution of the axis element-associated protein ASY1 occurs independently of MS5, but localization of the meiotic cohesion subunit SYN1 requires functional MS5. Furthermore, both the central element of the synaptonemal complex and the recombination element do not properly form in MS5(b)MS5(b) mutants. Our results demonstrate that MS5 participates in progression of meiosis during early prophase I and its allelic variants lead to differences in fertility, which may provide a promising strategy for pollination control for heterosis breeding.

OsDMC1 Is Not Required for Homologous Pairing in Rice Meiosis

Meiotic homologous recombination is pivotal to sexual reproduction. DMC1, a conserved recombinase, is involved in directing single-end invasion between interhomologs during meiotic recombination. In this study, we identified OsDMC1A and OsDMC1B, two closely related proteins in rice (Oryza sativa) with high sequence similarity to DMC1 proteins from other species. Analysis of Osdmc1a and Osdmc1b Tos17 insertion mutants indicated that these genes are functionally redundant. Immunolocalization analysis revealed OsDMC1 foci occurred at leptotene, which disappeared from late pachytene chromosomes in wild-type meiocytes. According to cytological analyses, homologous pairing is accomplished in the Osdmc1a Osdmc1b double mutant, but synapsis is seriously disrupted. The reduced number of bivalents and abnormal OsHEI10 foci in Osdmc1a Osdmc1b establishes an essential role for OsDMC1 in crossover formation. In the absence of OsDMC1, early recombination events probably occur normally, leading to normal localization of γH2AX, PAIR3, OsMRE11, OsCOM1, and OsRAD51C. Moreover, OsDMC1 was not detected in pairing-defective mutants, such as pair2, pair3, Oscom1, and Osrad51c, while it was loaded onto meiotic chromosomes in zep1, Osmer3, Oszip4, and Oshei10 Taken together, these results suggest that during meiosis, OsDMC1 is dispensable for homologous pairing in rice, which is quite different from the DMC1 homologs identified so far in other organisms.

The multifaceted roles of the HORMA domain in cellular signaling

The HORMA domain is a multifunctional protein–protein interaction module found in diverse eukaryotic signaling pathways including the spindle assembly checkpoint, numerous DNA recombination/repair pathways, and the initiation of autophagy. In all of these pathways, HORMA domain proteins occupy key signaling junctures and function through the controlled assembly and disassembly of signaling complexes using a stereotypical “safety belt” peptide interaction mechanism. A recent explosion of structural and functional work has shed new light on these proteins, illustrating how strikingly similar structural mechanisms give rise to radically different functional outcomes in each family of HORMA domain proteins.

Contribution of Structural Chromosome Mutants to the Study of Meiosis in Plants

Dissection of the molecular mechanisms underlying the transition through the complex events of the meiotic process requires the use of gene mutants or RNAi-mediated gene silencing. A considerable number of meiotic mutants have been isolated in plant species such as Arabidopsis thaliana, maize or rice. However, structural chromosome mutants are also important for the identification of the role developed by different chromosome domains in the meiotic process. This review summarizes the contribution of studies carried out in plants using structural chromosome variations. Meiotic events concerning the search of the homologous partner, the control of number and distribution of chiasmata, the mechanism of pairing correction, and chromosome segregation are considered.

Proteomic and phosphoproteomic analyses reveal extensive phosphorylation of regulatory proteins in developing rice anthers

Anther development, particularly around the time of meiosis, is extremely crucial for plant sexual reproduction. Meanwhile, cell-to-cell communication between somatic (especial tapetum) cells and meiocytes are important for both somatic anther development and meiosis. To investigate possible molecular mechanisms modulating protein activities during anther development, we applied high-resolution mass spectrometry-based proteomic and phosphoproteomic analyses for developing rice (Oryza sativa) anthers around the time of meiosis (RAM). In total, we identified 4984 proteins and 3203 phosphoproteins with 8973 unique phosphorylation sites (p-sites). Among those detected here, 1544 phosphoproteins are currently absent in the Plant Protein Phosphorylation DataBase (P3 DB), substantially enriching plant phosphorylation information. Mapman enrichment analysis showed that 'DNA repair','transcription regulation' and 'signaling' related proteins were overrepresented in the phosphorylated proteins. Ten genetically identified rice meiotic proteins were detected to be phosphorylated at a total of 25 p-sites; moreover more than 400 meiotically expressed proteins were revealed to be phosphorylated and their phosphorylation sites were precisely assigned. 163 putative secretory proteins, possibly functioning in cell-to-cell communication, are also phosphorylated. Furthermore, we showed that DNA synthesis, RNA splicing and RNA-directed DNA methylation pathways are extensively affected by phosphorylation. In addition, our data support 46 kinase-substrate pairs predicted by the rice Kinase-Protein Interaction Map, with SnRK1 substrates highly enriched. Taken together, our data revealed extensive protein phosphorylation during anther development, suggesting an important post-translational modification affecting protein activity.

XRCC3 is essential for proper double-strand break repair and homologous recombination in rice meiosis

RAD51 paralogues play important roles in the assembly and stabilization of RAD51 nucleoprotein filaments, which promote homologous pairing and strand exchange reactions in organisms ranging from yeast to vertebrates. XRCC3, a RAD51 paralogue, has been characterized in budding yeast, mouse, and Arabidopsis. In the present study, XRCC3 in rice was identified and characterized. The rice xrcc3 mutant exhibited normal vegetative growth but complete male and female sterility. Cytological investigations revealed that homologous pairing and synapsis were severely disrupted in the mutant. Meiotic chromosomes were frequently entangled from diplotene to metaphase I, resulting in chromosome fragmentation at anaphase I. The immunostaining signals from γH2AX were regular, implying that double-strand break (DSB) formation was normal in xrcc3 meiocytes. However, COM1 was not detected on early prophase I chromosomes, suggesting that the DSB end-processing system was destroyed in the mutant. Moreover, abnormal chromosome localization of RAD51C, DMC1, ZEP1, ZIP4, and MER3 was observed in xrcc3. Taken together, the results suggest that XRCC3 plays critical roles in both DSB repair and homologous chromosome recombination during rice meiosis.

The Axial Element Protein DESYNAPTIC2 Mediates Meiotic Double-Strand Break Formation and Synaptonemal Complex Assembly in Maize

During meiosis, homologous chromosomes pair and recombine via repair of programmed DNA double-strand breaks (DSBs). DSBs are formed in the context of chromatin loops, which are anchored to the proteinaceous axial element (AE). The AE later serves as a framework to assemble the synaptonemal complex (SC) that provides a transient but tight connection between homologous chromosomes. Here, we showed that DESYNAPTIC2 (DSY2), a coiled-coil protein, mediates DSB formation and is directly involved in SC assembly in maize (Zea mays). The dsy2 mutant exhibits homologous pairing defects, leading to sterility. Analyses revealed that DSB formation and the number of RADIATION SENSITIVE51 (RAD51) foci are largely reduced, and synapsis is completely abolished in dsy2 meiocytes. Super-resolution structured illumination microscopy showed that DSY2 is located on the AE and forms a distinct alternating pattern with the HORMA-domain protein ASYNAPTIC1 (ASY1). In the dsy2 mutant, localization of ASY1 is affected, and loading of the central element ZIPPER1 (ZYP1) is disrupted. Yeast two-hybrid and bimolecular fluorescence complementation experiments further demonstrated that ZYP1 interacts with DSY2 but does not interact with ASY1. Therefore, DSY2, an AE protein, not only mediates DSB formation but also bridges the AE and central element of SC during meiosis.

OsSDS is essential for DSB formation in rice meiosis

SDS is a meiosis specific cyclin-like protein and required for DMC1 mediated double-strand break (DSB) repairing in Arabidopsis. Here, we found its rice homolog, OsSDS, is essential for meiotic DSB formation. The Ossds mutant is normal in vegetative growth but both male and female gametes are inviable. The Ossds meiocytes exhibit severe defects in homologous pairing and synapsis. No γH2AX immunosignals in Ossds meiocytes together with the suppression of chromosome fragmentation in Ossds-1 Osrad51c, both provide strong evidences that OsSDS is essential for meiotic DSB formation. Immunostaining investigations revealed that meiotic chromosome axes are normally formed but both SC installation and localization of recombination elements are failed in Ossds. We suspected that this cyclin protein has been differentiated pretty much between monocots and dicots on its function in meiosis.

Tomato Male sterile 1035 is essential for pollen development and meiosis in anthers

Male fertility in flowering plants depends on proper cellular differentiation in anthers. Meiosis and tapetum development are particularly important processes in pollen production. In this study, we showed that the tomato male sterile (ms10(35)) mutant of cultivated tomato (Solanum lycopersicum) exhibited dysfunctional meiosis and an abnormal tapetum during anther development, resulting in no pollen production. We demonstrated that Ms10(35) encodes a basic helix-loop-helix transcription factor that is specifically expressed in meiocyte and tapetal tissue from pre-meiotic to tetrad stages. Transgenic expression of the Ms10(35) gene from its native promoter complemented the male sterility of the ms10(35) mutant. In addition, RNA-sequencing-based transcriptome analysis revealed that Ms10(35) regulates 246 genes involved in anther development processes such as meiosis, tapetum development, cell-wall degradation, pollen wall formation, transport, and lipid metabolism. Our results indicate that Ms10(35) plays key roles in regulating both meiosis and programmed cell death of the tapetum during microsporogenesis.

Crossover formation during rice meiosis relies on interaction of OsMSH4 and OsMSH5

MSH4 encodes a MutS protein that plays a specialized role in meiosis. In eukaryotic species, such as budding yeast, mice, Caenorhabditis elegans, and Arabidopsis, msh4 mutants display meiotic defects with a reduced number of chiasmata. Here, we characterized rice MSH4 by map-based cloning. In Osmsh4 mutants, the chiasma frequency was dramatically decreased to ∼10% of the wild type, but the synaptonemal complex was normally installed. The double mutant analysis showed that in the Osmsh4 Osmsh5 mutant, the reduction of chiasmata was greater than other zmm mutants. This was consistent with the absence of localization for OsZIP4 and OsMER3 in Osmsh4 and suggests an earlier role for OsMSH4 and OsMSH5 than other ZMM proteins where they may be required to stabilize progenitor Holliday junctions. Using yeast two-hybrid and pull-down assays, we verified the direct physical association between OsMSH4 and OsMSH5 and OsMSH5 and HEI10 in plants for the first time. The MSH4-MSH5 heterodimer has been demonstrated in mammals to stabilize the formation of progenitor and double Holliday junctions that may be resolved as crossovers (COs). We propose that OsMSH4 interacts with OsMSH5 to promote formation of the majority of COs in rice.

Factors underlying restricted crossover localization in barley meiosis

Meiotic recombination results in the formation of cytological structures known as chiasmata at the sites of genetic crossovers (COs). The formation of at least one chiasma/CO between homologous chromosome pairs is essential for accurate chromosome segregation at the first meiotic division as well as for generating genetic variation. Although DNA double-strand breaks, which initiate recombination, are widely distributed along the chromosomes, this is not necessarily reflected in the chiasma distribution. In many species there is a tendency for chiasmata to be distributed in favored regions along the chromosomes, whereas in others, such as barley and some other grasses, chiasma localization is extremely pronounced. Localization of chiasma to the distal regions of barley chromosomes restricts the genetic variation available to breeders. Studies reviewed herein are beginning to provide an explanation for chiasma localization in barley. Moreover, they suggest a potential route to manipulating chiasma distribution that could be of value to plant breeders.

Molecular dissection of complex agronomic traits of rice: a team effort by Chinese scientists in recent years

Rice is a staple food for more than half of the worldwide population and is also a model species for biological studies on monocotyledons. Through a team effort, Chinese scientists have made rapid and important progresses in rice biology in recent years. Here, we briefly review these advances, emphasizing on the regulatory mechanisms of the complex agronomic traits that affect rice yield and grain quality. Progresses in rice genome biology and genome evolution have also been summarized.