Central region component1, a novel synaptonemal complex component, is essential for meiotic recombination initiation in rice

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.

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.

Deciphering the genetic basis of agronomic, yield, and nutritional traits in rice (Oryza sativa L.) using a saturated GBS-based SNP linkage map

Developing high-yielding rice varieties that possess favorable agronomic characteristics and enhanced grain Zn content is crucial in ensuring food security and addressing nutritional needs. This research employed ICIM, IM, and multi-parent population QTL mapping methods to identify important genetic regions associated with traits such as DF, PH, NT, NP, PL, YLD, TGW, GL, GW, Zn, and Fe. Two populations of recombinant inbred lines consisting of 373 lines were phenotyped for agronomic, yield and grain micronutrient traits for three seasons at IRRI, and genotyped by sequencing. Most of the traits demonstrated moderate to high broad-sense heritability. There was a positive relationship between Zn and Fe contents. The principal components and correlation results revealed a significant negative association between YLD and Zn/Fe. ICIM identified 81 QTLs, while IM detected 36 QTLs across populations. The multi-parent population analysis detected 27 QTLs with six of them consistently detected across seasons. We shortlisted eight candidate genes associated with yield QTLs, 19 genes with QTLs for agronomic traits, and 26 genes with Zn and Fe QTLs. Notable candidate genes included CL4 and d35 for YLD, dh1 for DF, OsIRX10, HDT702, sd1 for PH, OsD27 for NP, whereas WFP and OsIPI1 were associated with PL, OsRSR1 and OsMTP1 were associated to TGW. The OsNAS1, OsRZFP34, OsHMP5, OsMTP7, OsC3H33, and OsHMA1 were associated with Fe and Zn QTLs. We identified promising RILs with acceptable yield potential and high grain Zn content from each population. The major effect QTLs, genes and high Zn RILs identified in our study are useful for efficient Zn biofortification of rice.

Molecular mechanisms and regulation of recombination frequency and distribution in plants

KEY MESSAGE

Recent developments in understanding the distribution and distinctive features of recombination hotspots are reviewed and approaches are proposed to increase recombination frequency in coldspot regions. Recombination events during meiosis provide the foundation and premise for creating new varieties of crops. The frequency of recombination in different genomic regions differs across eukaryote species, with recombination generally occurring more frequently at the ends of chromosomes. In most crop species, recombination is rare in centromeric regions. If a desired gene variant is linked in repulsion with an undesired variant of a second gene in a region with a low recombination rate, obtaining a recombinant plant combining two favorable alleles will be challenging. Traditional crop breeding involves combining desirable genes from parental plants into offspring. Therefore, understanding the mechanisms of recombination and factors affecting the occurrence of meiotic recombination is important for crop breeding. Here, we review chromosome recombination types, recombination mechanisms, genes and proteins involved in the meiotic recombination process, recombination hotspots and their regulation systems and discuss how to increase recombination frequency in recombination coldspot regions.

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.

Targeting TRIP13 for overcoming anticancer drug resistance (Review)

Cancer is one of the greatest dangers to human wellbeing and survival. A key barrier to effective cancer therapy is development of resistance to anti‑cancer medications. In cancer cells, the AAA+ ATPase family member thyroid hormone receptor interactor 13 (TRIP13) is key in promoting treatment resistance. Nonetheless, knowledge of the molecular processes underlying TRIP13‑based resistance to anticancer therapies is lacking. The present study evaluated the function of TRIP13 expression in anticancer drug resistance and potential methods to overcome this resistance. Additionally, the underlying mechanisms by which TRIP13 promotes resistance to anticancer drugs were explored, including induction of mitotic checkpoint complex surveillance system malfunction, promotion of DNA repair, the enhancement of autophagy and the prevention of immunological clearance. The effects of combination treatment, which include a TRIP13 inhibitor in addition to other inhibitors, were discussed. The present study evaluated the literature on TRIP13 as a possible target and its association with anticancer drug resistance, which may facilitate improvements in current anticancer therapeutic options.

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.

The meiotic topoisomerase VI B subunit (MTOPVIB) is essential for meiotic DNA double-strand break formation in barley (Hordeum vulgare L.)

In barley (Hordeum vulgare), MTOPVIB is critical for meiotic DSB and accompanied SC and CO formation while dispensable for meiotic bipolar spindle formation. Homologous recombination during meiosis assures genetic variation in offspring. Programmed meiotic DNA double-strand breaks (DSBs) are repaired as crossover (CO) or non-crossover (NCO) during meiotic recombination. The meiotic topoisomerase VI (TopoVI) B subunit (MTOPVIB) plays an essential role in meiotic DSB formation critical for CO-recombination. More recently MTOPVIB has been also shown to play a role in meiotic bipolar spindle formation in rice and maize. Here, we describe a meiotic DSB-defective mutant in barley (Hordeum vulgare L.). CRISPR-associated 9 (Cas9) endonuclease-generated mtopVIB plants show complete sterility due to the absence of meiotic DSB, synaptonemal complex (SC), and CO formation leading to the occurrence of univalents and their unbalanced segregation into aneuploid gametes. In HvmtopVIB plants, we also frequently found the bi-orientation of sister kinetochores in univalents during metaphase I and the precocious separation of sister chromatids during anaphase I. Moreover, the near absence of polyads after meiosis II, suggests that despite being critical for meiotic DSB formation in barley, MTOPVIB seems not to be strictly required for meiotic bipolar spindle formation.

Synthetic apomixis: the beginning of a new era

Apomixis is a process of asexual reproduction that enables plants to bypass meiosis and fertilization to generate clonal seeds that are identical to the maternal genotype. Apomixis has tremendous potential for breeding plants with desired characteristics, given its ability to fix any elite genotype. However, little is known about the origin and dynamics of natural apomictic plant systems. The introgression of apomixis-related genes from natural apomicts has achieved limited success. Therefore, synthetic apomixis, engineered to include apomeiosis, autonomous embryo formation, and autonomous endosperm development, has been proposed as a promising platform to effectuate apomixis in any crop. In this study, we have summarized recent advances in the understanding of synthetic apomixis and discussed the limitations of current synthetic apomixis systems and ways to overcome them.

OsPRD2 is essential for double-strand break formation, but not spindle assembly during rice meiosis

Meiotic recombination starts with the programmed formation of double-strand breaks (DSB) in DNA, which are catalyzed by SPO11, a type II topoisomerase that is evolutionarily conserved, and several other accessary proteins. Homologs of MEIOSIS INHIBITOR 4 (MEI4/REC24/PRD2) are proteins that are also essential for the generation of meiotic DSBs in budding yeast, mice and Arabidopsis thaliana. In Arabidopsis, the protein ARABIDOPSIS THALIANA PUTATIVE RECOMBINATION INITIATION DEFECTS 2/MULTIPOLAR SPINDLE 1 (AtPRD2/MPS1) has been shown to have additional roles in spindle assembly, indicating a functional diversification. Here we characterize the role of the rice MEI4/PRD2 homolog in meiosis. The osprd2 mutant was completely male and female sterile. In male meiocytes of osprd2, no γH2AX foci were detected and twenty-four univalents were produced at diakinesis, suggesting that OsPRD2 is essential for DSB generation. OsPRD2 showed a dynamic localization during meiosis. For instance, OsPRD2 foci first appeared as discrete signals across chromosome at leptotene, and then became confined to the centromeres during zygotene, suggesting that they might be involved in assembly of the spindle. However we did not observe any obvious aberrant morphologies in neither the organization of the bipolar spindle nor in the orientation of the kinetochore in the mutant. These findings suggest that in rice PRD2 might not be required for spindle assembly and organization, as it does in Arabidopsis. Taken together our results indicate that plant MEI4/PRD2 homologs do play a conserved role in the formation of meiotic DSBs in DNA, but that their involvement in bipolar spindle assembly is rather species-specific.

Natural variation of codon repeats in COLD11 endows rice with chilling resilience

Abnormal temperature caused by global climate change threatens the rice production. Defense signaling network for chilling has been uncovered in plants. However, less is known about repairing DNA damage produced from overwhelmed defense and its evolution during domestication. Here, we genetically identified a major QTL, COLD11, using the data-merging genome-wide association study based on an algorithm combining polarized data from two subspecies, indica and japonica, into one system. Rice loss-of-function mutations of COLD11 caused reduced chilling tolerance. Genome evolution analysis of representative rice germplasms suggested that numbers of GCG sequence repeats in the first exon of COLD11 were subjected to strong domestication selection during the northern expansion of rice planting. The repeat numbers affected the biochemical activity of DNA repair protein COLD11/RAD51A1 in renovating DNA damage under chilling stress. Our findings highlight a potential way to finely manipulate key genes in rice genome and effectively improve chilling tolerance through molecular designing.

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.

OsMADS58 Stabilizes Gene Regulatory Circuits during Rice Stamen Development

Rice (Oryza sativa) OsMADS58 is a C-class MADS box protein, and characterization of a transposon insertion mutant osmads58 suggested that OsMADS58 plays a role in stamen development. However, as no null mutation has been obtained, its role has remained unclear. Here, we report that the CRISPR knockout mutant osmads58 exhibits complex altered phenotypes, including anomalous diploid germ cells, aberrant meiosis, and delayed tapetum degeneration. This CRISPR mutant line exhibited stronger changes in expression of OsMADS58 target genes compared with the osmads58 dSpm (transposon insertion) line, along with changes in multiple pathways related to early stamen development. Notably, transcriptional regulatory circuits in young panicles covering the stamen at stages 4-6 were substantially altered in the CRISPR line compared to the dSpm line. These findings strongly suggest that the pleiotropic effects of OsMADS58 on stamen development derive from a potential role in stabilizing gene regulatory circuits during early stamen development. Thus, this work opens new avenues for viewing and deciphering the regulatory mechanisms of early stamen development from a network perspective.

PCH-2 and meiotic HORMADs: A module for evolutionary innovation in meiosis?

Sexual reproduction and the specialized cell division it relies upon, meiosis, are biological processes that present an incredible degree of both evolutionary conservation and divergence. One clear example of this paradox is the role of the evolutionarily ancient PCH-2/HORMAD module during meiosis. On one hand, the complex, and sometimes disparate, meiotic defects observed when PCH-2 and/or the meiotic HORMADS are mutated in different model systems have prevented a straightforward characterization of their conserved functions. On the other hand, these functional variations demonstrate the impressive molecular rewiring that accompanies evolution of the meiotic processes these factors are involved in. While the defects observed in pch-2 mutants appear to vary in different systems, in this review, I argue that PCH-2 has a conserved meiotic function: to coordinate meiotic recombination with synapsis to ensure an appropriate number and distribution of crossovers. Further, given the dramatic variation in how the events of recombination and synapsis are themselves regulated in different model systems, the mechanistic differences in PCH-2 and meiotic HORMAD function make biological sense when viewed as species-specific elaborations layered onto this fundamental, conserved role.

An F-box protein ACOZ1 functions in crossover formation by ensuring proper chromosome compaction during maize meiosis

Meiosis is an essential reproductive process to create new genetic variation. During early meiosis, higher order chromosome organization creates a platform for meiotic processes to ensure the accuracy of recombination and chromosome segregation. However, little is known about the regulatory mechanisms underlying dynamic chromosome organization in plant meiosis. Here, we describe abnormal chromosome organization in zygotene1 (ACOZ1), which encodes a canonical F-box protein in maize. In acoz1 mutant meiocytes, chromosomes maintain a leptotene-like state and never compact to a zygotene-like configuration. Telomere bouquet formation and homologous pairing are also distorted and installation of synaptonemal complex ZYP1 protein is slightly defective. Loading of early recombination proteins RAD51 and DMC1 is unaffected, indicating that ACOZ1 is not required for double strand break formation or repair. However, crossover formation is severely disturbed. The ACOZ1 protein localizes on the boundary of chromatin, rather directly to chromosomes. Furthermore, we identified that ACOZ1 interacts with SKP1 through its C-terminus, revealing that it acts as a subunit of the SCF E3 ubiquitin/SUMO ligase complex. Overall, our results suggest that ACOZ1 functions independently from the core meiotic recombination pathway to influence crossover formation by controlling chromosome compaction during maize meiosis.

Meiotic chromosome organization and its role in recombination and cancer

Chromosomes adopt specific conformations to regulate various cellular processes. A well-documented chromosome configuration is the highly compacted chromosome structure during metaphase. More regional chromatin conformations have also been reported, including topologically associated domains encompassing mega-bases of DNA and local chromatin loops formed by kilo-bases of DNA. In this review, we discuss the changes in chromatin conformation taking place between somatic and meiotic cells, with a special focus on the establishment of a proteinaceous structure, called the chromosome axis, at the beginning of meiosis. The chromosome axis is essential to support key meiotic processes such as chromosome pairing, homologous recombination, and balanced chromosome segregation to transition from a diploid to a haploid stage. We review the role of the chromosome axis in meiotic chromatin organization and provide a detailed description of its protein composition. We also review the conserved and distinct roles between species of axis proteins in meiotic recombination, which is a major factor contributing to the creation of genetic diversity and genome evolution. Finally, we discuss situations where the chromosome axis is deregulated and evaluate the effects on genome integrity and the consequences from protein deregulation in meiocytes exposed to heat stress, and aberrant expression of genes encoding axis proteins in mammalian somatic cells associated with certain types of cancers.

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.

Low Female Gametophyte Fertility Contributes to the Low Seed Formation of the Diploid Loquat [Eriobotrya Japonica (Thunb.) Lindl.] Line H30-6

Loquat is a widely grown subtropic fruit because of its unique ripening season, nutrient content, and smooth texture of its fruits. However, loquat is not well-received because the fruits contain many large seeds. Therefore, the development of seedless or few-seed loquat varieties is the main objective of loquat breeding. Polyploidization is an effective approach for few-seed loquat breeding, but the resource is rare. The few-seed loquat line H30-6 was derived from a seedy variety. Additionally, H30-6 was systematically studied for its fruit characteristics, gamete fertility, pollen mother cell (PMC) meiosis, stigma receptivity, in situ pollen germination, fruit set, and karyotype. The results were as follows. (1) H30-6 produced only 1.54 seeds per fruit and the fruit edible rate was 70.77%. The fruit setting rate was 14.44% under open pollination, and the other qualities were equivalent to those of two other seedy varieties. (2) The in vitro pollen germination rate was only 4.04 and 77.46% of the H30-6 embryo sacs were abnormal. Stigma receptivity and self-compatibility in H30-6 were verified by in situ pollen germination and artificial pollination. Furthermore, the seed numbers in the fruits of H30-6 did not significantly differ among any of the pollination treatments (from 1.59 ±0.14 to 2 ± 0.17). (3) The chromosome configuration at meiotic diakinesis of H30-6 was 6.87I + 9.99II + 1.07III +0.69IV +0.24V (H30-6), and a total of 89.55% of H30-6 PMCs presented univalent chromosomes. Furthermore, chromosome lagging was the main abnormal phenomenon. Karyotype analysis showed that chromosomes of H30-6 had no recognizable karyotype abnormalities leading to unusual synapsis on the large scale above. (4) The abnormal embryo sacs of H30-6 could be divided into three main types: those remaining in the tetrad stage (13.38%), those remaining in the binucleate embryo sac stage (1.41%), and those without embryo sacs (52.82%). Therefore, we conclude that the loquat line H30-6 is a potential few-seed loquat resource. The diploid loquat line H30-6 was with low gametophyte fertility, which may be driven by abnormal meiotic synapses. The low female gamete fertility was the main reason for the few seeds. This diploid loquat line provides a new possibility for breeding a few-seed loquat at the diploid level.

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.

Reinforcement of CHH methylation through RNA-directed DNA methylation ensures sexual reproduction in rice

DNA methylation is an important epigenetic mark that regulates the expression of genes and transposons. RNA-directed DNA methylation (RdDM) is the main molecular pathway responsible for de novo DNA methylation in plants. Although the mechanism of RdDM has been well studied in Arabidopsis (Arabidopsis thaliana), most mutations in RdDM genes cause no remarkable developmental defects in Arabidopsis. Here, we isolated and cloned Five Elements Mountain 1 (FEM1), which encodes RNA-dependent RNA polymerase 2 (OsRDR2) in rice (Oryza sativa). Mutation in OsRDR2 abolished the accumulation of 24-nt small interfering RNAs, and consequently substantially decreased genome-wide CHH (H = A, C, or T) methylation. Moreover, male and female reproductive development was disturbed, which led to sterility in osrdr2 mutants. We discovered that OsRDR2-dependent DNA methylation may regulate the expression of multiple key genes involved in stamen development, meiosis, and pollen viability. In wild-type (WT) plants but not in osrdr2 mutants, genome-wide CHH methylation levels were greater in panicles, stamens, and pistils than in seedlings. The global increase of CHH methylation in reproductive organs of the WT was mainly explained by the enhancement of RdDM activity, which includes OsRDR2 activity. Our results, which revealed a global increase in CHH methylation through enhancement of RdDM activity in reproductive organs, suggest a crucial role for OsRDR2 in the sexual reproduction of rice.

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.

RNA N6-methyladenosine modification promotes auxin biosynthesis required for male meiosis in rice

N⁶-methyladenosine (m⁶A) RNA modification confers an essential layer of gene regulation in living organisms, including plants; yet, the underlying mechanisms of its deposition on specific target mRNAs involved in key plant developmental processes are so far unknown. Here, we show that a core component of the rice m⁶A methyltransferase complex, OsFIP37, is recruited by an RNA-binding protein, OsFIP37-associated protein 1 (OsFAP1), to mediate m⁶A RNA modification on an auxin biosynthesis gene, OsYUCCA3, during microsporogenesis. This stabilizes OsYUCCA3 mRNA and promotes local auxin biosynthesis in anthers during male meiosis, which is essential for meiotic division and subsequent pollen development in rice. Loss of function of OsFAP1 causes dissociation of OsFIP37 with OsYUCCA3 and the resulting abolished m⁶A deposition on OsYUCCA3. Our findings reveal that OsFAP1-dependent m⁶A deposition on OsYUCCA3 by OsFIP37 constitutes a hitherto unknown link between RNA modification and hormonal control of male meiosis in plant reproductive development.

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.

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.

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.

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.

Plant zygote development: recent insights and applications to clonal seeds

In flowering plants, haploid gametes - an egg cell and a sperm cell fuse to form the first diploid cell - the zygote. The zygote is the progenitor stem cell that gives rise to all the embryonic and post embryonic tissues and organs. Unlike animals, both maternal and paternal gene products participate in the initial development of zygotes in plants. Here, we discuss recent advances in understanding of the zygotic transition and embryo initiation in angiosperms, including the role of parental contributions to gene expression in the zygote. We further discuss utilization of this knowledge in agricultural biotechnology through synthetic apomixis. Parthenogenesis obtained by manipulation of embryogenic factors, combined with mutations that bypass meiosis, enables clonal propagation of hybrid crops through seeds.

Oryza sativa RNA-Dependent RNA Polymerase 6 Contributes to Double-Strand Break Formation in Meiosis

RNA-dependent RNA polymerase 6 (RDR6) is a core component of the small RNA biogenesis pathway, but its function in meiosis is unclear. Here, we report a new allele of OsRDR6 (Osrdr6-meiosis [Osrdr6-mei]), which causes meiosis-specific phenotypes in rice (Oryza sativa). In Osrdr6-mei, meiotic double-strand break (DSB) formation is partially blocked. We created a biallelic mutant with more severe phenotypes, Osrdr6-bi, by crossing Osrdr6-mei with a knockout mutant, Osrdr6-edit In Osrdr6-bi meiocytes, 24 univalents were observed, and no histone H2AX phosphorylation foci were detected. Compared with the wild type, the number of 21-nucleotide small RNAs in Osrdr6-mei was dramatically lower, while the number of 24-nucleotide small RNAs was significantly higher. Thousands of differentially methylated regions (DMRs) were discovered in Osrdr6-mei, implying that OsRDR6 plays an important role in DNA methylation. There were 457 genes downregulated in Osrdr6-mei, including three genes, CENTRAL REGION COMPONENT1, P31 comet , and O. sativa SOLO DANCERS, related to DSB formation. Interestingly, the downregulated genes were associated with a high level of 24-nucleotide small RNAs but less strongly associated with DMRs. Therefore, we speculate that the alteration in expression of small RNAs in Osrdr6 mutants leads to the defects in DSB formation during meiosis, which might not be directly dependent on RNA-directed DNA methylation.

OsATM Safeguards Accurate Repair of Meiotic Double-Strand Breaks in Rice

ATAXIA TELANGIECTASIA-MUTATED (ATM) protein has been well studied for its roles in the DNA damage response. However, its role in meiosis has not been fully explored. Here, we characterized the functions of the rice (Oryza sativa) ATM homolog during meiosis. Aberrant chromosome associations and DNA fragmentations were observed after the completion of homologous pairing and synapsis in Osatm pollen mother cells (PMCs). Aberrant chromosome associations disappeared in Osspo11-1 Osatm-1 double mutants and more severe defects were observed in Osdmc1 Osatm, suggesting that OsATM functions downstream of OsSPO11-1-catalyzed double-strand break formation and in parallel with OsDMC1-mediated homologous recombination. We further demonstrated that phosphorylation of H2AX in PMCs did not depend on OsATM, in contrast to the situation in somatic cells. Moreover, the removal of OsDMC1 from chromosomes in Osatm PMCs was delayed and the number of HEI10 foci (markers of interference-sensitive crossover intermediates) decreased. Together, these findings suggest that OsATM plays important roles in the accurate repair of meiotic double-strand breaks in rice.

Transcriptional trajectories of anther development provide candidates for engineering male fertility in sorghum

Sorghum is a self-pollinated crop with multiple economic uses as cereal, forage, and biofuel feedstock. Hybrid breeding is a cornerstone for sorghum improvement strategies that currently relies on cytoplasmic male sterile lines. To engineer genic male sterility, it is imperative to examine the genetic components regulating anther/pollen development in sorghum. To this end, we have performed transcriptomic analysis from three temporal stages of developing anthers that correspond to meiotic, microspore and mature pollen stages. A total of 5286 genes were differentially regulated among the three anther stages with 890 of them exhibiting anther-preferential expression. Differentially expressed genes could be clubbed into seven distinct developmental trajectories using K-means clustering. Pathway mapping revealed that genes involved in cell cycle, DNA repair, regulation of transcription, brassinosteroid and auxin biosynthesis/signalling exhibit peak expression in meiotic anthers, while those regulating abiotic stress, carbohydrate metabolism, and transport were enriched in microspore stage. Conversely, genes associated with protein degradation, post-translational modifications, cell wall biosynthesis/modifications, abscisic acid, ethylene, cytokinin and jasmonic acid biosynthesis/signalling were highly expressed in mature pollen stage. High concurrence in transcriptional dynamics and cis-regulatory elements of differentially expressed genes in rice and sorghum confirmed conserved developmental pathways regulating anther development across species. Comprehensive literature survey in conjunction with orthology analysis and anther-preferential accumulation enabled shortlisting of 21 prospective candidates for in-depth characterization and engineering male fertility in sorghum.

The plant-specific ABERRANT GAMETOGENESIS 1 gene is essential for meiosis in rice

Meiotic recombination plays a central role in maintaining genome stability and increasing genetic diversity. Although meiotic progression and core components are widely conserved across kingdoms, significant differences remain among species. Here we identify a rice gene ABERRANT GAMETOGENESIS 1 (AGG1) that controls both male and female gametogenesis. Cytological and immunostaining analysis showed that in the osagg1 mutant the early recombination processes and synapsis occurred normally, but the chiasma number was dramatically reduced. Moreover, OsAGG1 was found to interact with ZMM proteins OsHEI10, OsZIP4, and OsMSH5. These results suggested that OsAGG1 plays an important role in crossover formation. Phylogenetic analysis showed that OsAGG1 is a plant-specific protein with a highly conserved N-terminal region. Further genetic and protein interaction analyses revealed that the conserved N-terminus was essential for the function of the OsAGG1 protein. Overall, our work demonstrates that OsAGG1 is a novel and critical component in rice meiotic crossover formation, expanding our understanding of meiotic progression. This study identified a plant-specific gene ABERRANT GAMETOGENESIS 1 that is required for meiotic crossover formation in rice. The conserved N-terminus of the AGG1 protein was found to be essential for its function.

Fixation of hybrid vigor in rice: synthetic apomixis generated by genome editing

Apomixis is an asexual reproduction process in which clonal seeds are formed without meiosis and fertilization. Because of its potential in permanently preserving hybrid vigor, apomixis has attracted a great deal of interests from plant biologists and the seed industry. However, despite of decades of effort, introgression of apomixis traits from wild relatives into major crops has remained unsuccessful. Therefore, synthetic apomixis has been proposed as an alternative to fix hybrid vigor. In this article, I present the development of the MiMe (Mitosis instead of Meiosis), which turns meiosis into mitosis and leads to the production of clonal gametes. Apomixis-like clonal seeds are generated when MiMe plants are crossed to special genome elimination lines, which contain an altered centromere-specific histone 3 (CENH3). Furthermore, induction of haploid plants from egg cells can be achieved by either egg cell-specific expression of BABY BOOM1 (BBM1), or disruption of MATRILINEAL (MTL) using CRISPR/Cas9 gene-editing technology. Synthetic apomixis is established and clonal seeds are produced by simultaneous engineering MiMe with altering BBM1 expression or MTL disruption. Finally, I discuss how to further improve the apomixis strategy and its applications in crop breeding.

Engineering meiotic recombination pathways in rice

In the last 15 years, outstanding progress has been made in understanding the function of meiotic genes in the model dicot and monocot plants Arabidopsis and rice (Oryza sativa L.), respectively. This knowledge allowed to modulate meiotic recombination in Arabidopsis and, more recently, in rice. For instance, the overall frequency of crossovers (COs) has been stimulated 2.3- and 3.2-fold through the inactivation of the rice FANCM and RECQ4 DNA helicases, respectively, two genes involved in the repair of DNA double-strand breaks (DSBs) as noncrossovers (NCOs) of the Class II crossover pathway. Differently, the programmed induction of DSBs and COs at desired sites is currently explored by guiding the SPO11-1 topoisomerase-like transesterase, initiating meiotic recombination in all eukaryotes, to specific target regions of the rice genome. Furthermore, the inactivation of 3 meiosis-specific genes, namely PAIR1, OsREC8 and OsOSD1, in the Mitosis instead of Meiosis (MiMe) mutant turned rice meiosis into mitosis, thereby abolishing recombination and achieving the first component of apomixis, apomeiosis. The successful translation of Arabidopsis results into a crop further allowed the implementation of two breakthrough strategies that triggered parthenogenesis from the MiMe unreduced clonal egg cell and completed the second component of diplosporous apomixis. Here, we review the most recent advances in and future prospects of the manipulation of meiotic recombination in rice and potentially other major crops, all essential for global food security.

Advances Towards How Meiotic Recombination Is Initiated: A Comparative View and Perspectives for Plant Meiosis Research

Meiosis is an essential cell-division process for ensuring genetic diversity across generations. Meiotic recombination ensures the accuracy of genetic interchange between homolous chromosomes and segregation of parental alleles. Programmed DNA double-strand breaks (DSBs), catalyzed by the evolutionarily conserved topoisomerase VIA (a subunit of the archaeal type II DNA topoisomerase)-like enzyme Spo11 and several other factors, is a distinctive feature of meiotic recombination initiation. The meiotic DSB formation and its regulatory mechanisms are similar among species, but certain aspects are distinct. In this review, we introduced the cumulative knowledge of the plant proteins crucial for meiotic DSB formation and technical advances in DSB detection. We also summarized the genome-wide DSB hotspot profiles for different model organisms. Moreover, we highlighted the classical views and recent advances in our knowledge of the regulatory mechanisms that ensure the fidelity of DSB formation, such as multifaceted kinase-mediated phosphorylation and the consequent high-dimensional changes in chromosome structure. We provided an overview of recent findings concerning DSB formation, distribution and regulation, all of which will help us to determine whether meiotic DSB formation is evolutionarily conserved or varies between plants and other organisms.

The oncogenic role of TRIP13 in regulating proliferation, invasion, and cell cycle checkpoint in NSCLC cells

TRIP13 (thyroid hormone receptor interacting protein 13) AAA-ATPase has been reported to be involved in the metaphase checkpoint in human breast cancer, prostate cancer, and cervical cancer. However, the expression pattern and biologic role of TRIP13 in non-small cell lung cancer (NSCLC) remained unknown. In our present study, real-time PCR and western blot were used to detect the expression level of TRIP13 in NSCLC tissues and cell lines. We found that the expression levels of TRIP13 mRNA and protein were significantly upregulated in cell lines and lung tissues. Knockdown of TRIP13 by lentivirus inhibited cell proliferation and invasion in both A549 and H1299 cells. Furthermore, flow cytometry, western blot and immunoprecipitation showed that the MCC complex was disassembled and cells became arrested in metaphase, when TRIP13 was inhibited. In conclusion, here we first report that TRIP13 acts as a tumor promoter in regulating cell proliferation, invasion, and cell cycle checkpoint in NSCLC cells and may be a clinically useful marker for the diagnosis and treatment of lung cancer.

OsMTOPVIB is required for meiotic bipolar spindle assembly

The organization of microtubules into a bipolar spindle is essential for chromosome segregation. Both centrosome and chromatin-dependent spindle assembly mechanisms are well studied in mouse, Drosophila melanogaster, and Xenopus oocytes; however, the mechanism of bipolar spindle assembly in plant meiosis remains elusive. According to our observations of microtubule assembly in Oryza sativa, Zea mays, Arabidopsis thaliana, and Solanum lycopersicum, we propose that a key step of plant bipolar spindle assembly is the correction of the multipolar spindle into a bipolar spindle at metaphase I. The multipolar spindles failed to transition into bipolar ones in OsmtopVIB with the defect in double-strand break (DSB) formation. However, bipolar spindles were normally assembled in several other mutants lacking DSB formation, such as Osspo11-1, pair2, and crc1, indicating that bipolar spindle assembly is independent of DSB formation. We further revealed that the mono-orientation of sister kinetochores was prevalent in OsmtopVIB, whereas biorientation of sister kinetochores was frequently observed in Osspo11-1, pair2, and crc1 In addition, mutations of the cohesion subunit OsREC8 resulted in biorientation of sister kinetochores as well as bipolar spindles even in the background of OsmtopVIB Therefore, we propose that biorientation of the kinetochore is required for bipolar spindle assembly in the absence of homologous recombination.

Conformational dynamics of the Hop1 HORMA domain reveal a common mechanism with the spindle checkpoint protein Mad2

The HORMA domain is a highly conserved protein–protein interaction module found in eukaryotic signaling proteins including the spindle assembly checkpoint protein Mad2 and the meiotic HORMAD proteins. HORMA domain proteins interact with short ‘closure motifs’ in partner proteins by wrapping their C-terminal ‘safety belt’ region entirely around these motifs, forming topologically-closed complexes. Closure motif binding and release requires large-scale conformational changes in the HORMA domain, but such changes have only been observed in Mad2. Here, we show that Saccharomyces cerevisiae Hop1, a master regulator of meiotic recombination, possesses conformational dynamics similar to Mad2. We identify closure motifs in the Hop1 binding partner Red1 and in Hop1 itself, revealing that HORMA domain–closure motif interactions underlie both Hop1’s initial recruitment to the chromosome axis and its self-assembly on the axis. We further show that Hop1 adopts two distinct folded states in solution, one corresponding to the previously-observed ‘closed’ conformation, and a second more extended state in which the safety belt region has disengaged from the HORMA domain core. These data reveal strong mechanistic similarities between meiotic HORMADs and Mad2, and provide a mechanistic basis for understanding both meiotic chromosome axis assembly and its remodeling by the AAA+ ATPase Pch2/TRIP13.

Characterization of Pch2 localization determinants reveals a nucleolar-independent role in the meiotic recombination checkpoint

The meiotic recombination checkpoint blocks meiotic cell cycle progression in response to synapsis and/or recombination defects to prevent aberrant chromosome segregation. The evolutionarily conserved budding yeast Pch2^TRIP13 AAA+ ATPase participates in this pathway by supporting phosphorylation of the Hop1^HORMAD adaptor at T318. In the wild type, Pch2 localizes to synapsed chromosomes and to the unsynapsed rDNA region (nucleolus), excluding Hop1. In contrast, in synaptonemal complex (SC)-defective zip1Δ mutants, which undergo checkpoint activation, Pch2 is detected only on the nucleolus. Alterations in some epigenetic marks that lead to Pch2 dispersion from the nucleolus suppress zip1Δ-induced checkpoint arrest. These observations have led to the notion that Pch2 nucleolar localization could be important for the meiotic recombination checkpoint. Here we investigate how Pch2 chromosomal distribution impacts checkpoint function. We have generated and characterized several mutations that alter Pch2 localization pattern resulting in aberrant Hop1 distribution and compromised meiotic checkpoint response. Besides the AAA+ signature, we have identified a basic motif in the extended N-terminal domain critical for Pch2's checkpoint function and localization. We have also examined the functional relevance of the described Orc1-Pch2 interaction. Both proteins colocalize in the rDNA, and Orc1 depletion during meiotic prophase prevents Pch2 targeting to the rDNA allowing unwanted Hop1 accumulation on this region. However, Pch2 association with SC components remains intact in the absence of Orc1. We finally show that checkpoint activation is not affected by the lack of Orc1 demonstrating that, in contrast to previous hypotheses, nucleolar localization of Pch2 is actually dispensable for the meiotic checkpoint.

The Number of Meiotic Double-Strand Breaks Influences Crossover Distribution in Arabidopsis

Meiotic recombination generates genetic diversity and ensures proper chromosome segregation. Recombination is initiated by the programmed formation of double-strand breaks (DSBs) in chromosomal DNA by DNA Topoisomerase VI-A Subunit (SPO11), a topoisomerase-like enzyme. Repair of some DSBs leads to the formation of crossovers (COs). In most organisms, including plants, the number of DSBs greatly exceeds the number of COs and which DSBs become CO sites is tightly controlled. The CO landscape is affected by DNA sequence and epigenome features of chromosomes as well as by global mechanisms controlling recombination dynamics. The latter are poorly understood and their effects on CO distribution are not well elucidated. To study how recombination dynamics affects CO distribution, we engineered Arabidopsis thaliana plants to carry hypomorphic alleles of SPO11-1 Two independent transgenic lines showed ∼30% and 40% reductions in DSB numbers, which were commensurate with the dosage of the SPO11-1 transcript. The reduction in DSB number resulted in proportional, although smaller, reductions of the number of COs. Most interestingly, CO distribution along the chromosomes was dramatically altered, with substantially fewer COs forming in pericentromeric chromosome regions. These results indicate that SPO11 activity, and the resulting DSB numbers are major factors shaping the CO landscape.

Meiotic Recombination: Mixing It Up in Plants

Meiosis halves diploid chromosome numbers to haploid levels that are essential for sexual reproduction in most eukaryotes. Meiotic recombination ensures the formation of bivalents between homologous chromosomes (homologs) and their subsequent proper segregation. It also results in genetic diversity among progeny that influences evolutionary responses to selection. Moreover, crop breeding depends upon the action of meiotic recombination to rearrange elite traits between parental chromosomes. An understanding of the molecular mechanisms that drive meiotic recombination is important for both fundamental research and practical applications. This review emphasizes advances made during the past 5 years, primarily in Arabidopsis and rice, by summarizing newly characterized genes and proteins and examining the regulatory mechanisms that modulate their action.

Pervasive interactions of Sa and Sb loci cause high pollen sterility and abrupt changes in gene expression during meiosis that could be overcome by double neutral genes in autotetraploid rice

BACKGROUND

Intersubspecific autotetraploid rice hybrids possess high hybrid vigor; however, low pollen fertility is a critical hindrance in its commercial utilization. Our previous study demonstrated that polyploidy could increase the multi-loci interaction and cause high pollen abortion in autotetraploid rice hybrids. However, there is little known about the critical role of pollen sterility locus or loci in the intersubspecific hybrids. We developed autotetraploid rice hybrids harboring heterozygous genotypes (S i S i S j S j ) at different pollen sterility loci by using the near isogenic lines of Taichung65-4×. Moreover, autotetraploid lines carrying double neutral genes, Sa n and Sb n , were used to assess their effect on fertility restoration.

RESULTS

Cytological studies showed that the deleterious genetic interactions at Sa and Sb pollen sterility loci resulted in higher pollen sterility (76.83%) and abnormal chromosome behavior (24.59%) at metaphase I of meiosis in autotetraploid rice hybrids. Transcriptome analysis revealed 1092 differentially expressed genes (DEG) in a hybrid with the pervasive interactions at Sa and Sb pollen sterility loci, and most of the genes (about 83%) exhibited down regulation. Of the DEG, 60 were associated with transcription regulation and 18 genes were annotated as meiosis-related genes. Analysis on the hybrids developed by using autotetraploid rice harboring double neutral genes, Sa n and Sb n , revealed normal pollen fertility, and transcriptome analysis showed non-significant difference in number of DEG among different hybrids.

CONCLUSIONS

Our finding revealed that pervasive interactions at Sa and Sb pollen sterility loci cause high sterility in the autotetraploid hybrids that lead to the down-regulation of important meiosis-related genes and transcription regulation factors. Moreover, we also found that the hybrids sterility could be overcome by double neutral genes, Sa n and Sb n , in autotetraploid rice hybrids. The present study provided a strong evidence for the utilization of heterosis in autotetraploid rice hybrids.