FANCM-associated proteins MHF1 and MHF2, but not the other Fanconi anemia factors, limit meiotic crossovers

Recent advances in plant kinetochore research

Faithful chromosome segregation is crucial for cell division in eukaryotes, facilitated by the kinetochore, a multi-subunit protein complex that connects chromosomes to the spindle microtubules. Recent research has significantly advanced our understanding of kinetochore function in plants, including surprising findings about spindle assembly checkpoint, the composition of the inner kinetochore and unique kinetochore arrangement in holocentric Cuscuta species. Additionally, some kinetochore proteins in plants have been implicated in roles beyond chromosome segregation, such as cytokinesis regulation and involvement in developmental processes. This review summarizes recent insights into plant kinetochore biology, compares plant kinetochores with those of animals and fungi, and highlights key open questions and potential future directions in the field.

FIGL1 attenuates meiotic interhomolog repair and is counteracted by the RAD51 paralog XRCC2 and the chromosome axis protein ASY1 during meiosis

Two recombinases, RAD51 and DMC1, catalyze meiotic break repair to ensure crossovers (COs) between homologous chromosomes (interhomolog) rather than between sisters (intersister). FIDGETIN-LIKE-1 (FIGL1) downregulates both recombinases. However, the understanding of how FIGL1 functions in meiotic repair remains limited. Here, we discover new genetic interactions of Arabidopsis thaliana FIGL1 that are important in vivo determinants of meiotic repair outcome. In figl1 mutants, compromising RAD51-dependent repair, either through the loss of RAD51 paralogs (RAD51B or XRCC2) or RAD54 or by inhibiting RAD51 catalytic activity, results in either unrepaired breaks or meiotic CO defects. Further, XRCC2 physically interacts with FIGL1 and partially counteracts FIGL1 activity for RAD51 focus formation. Our data indicate that RAD51-mediated repair mechanisms compensate FIGL1 dysfunction. FIGL1 is not necessary for intersister repair in dmc1 but is essential for the completion of meiotic repair in mutants such as asy1 that have impaired DMC1 functions and interhomolog bias. We show that FIGL1 attenuates interhomolog repair, and ASY1 counteracts FIGL1 to promote interhomolog recombination. Altogether, this study underlines that multiple factors can counteract FIGL1 activity to promote accurate meiotic repair.

Enhanced recombination empowers the detection and mapping of Quantitative Trait Loci

Modern plant breeding, such as genomic selection and gene editing, is based on the knowledge of the genetic architecture of desired traits. Quantitative trait loci (QTL) analysis, which combines high throughput phenotyping and genotyping of segregating populations, is a powerful tool to identify these genetic determinants and to decipher the underlying mechanisms. However, meiotic recombination, which shuffles genetic information between generations, is limited: Typically only one to two exchange points, called crossovers, occur between a pair of homologous chromosomes. Here we test the effect on QTL analysis of boosting recombination, by mutating the anti-crossover factors RECQ4 and FIGL1 in Arabidopsis thaliana full hybrids and lines in which a single chromosome is hybrid. We show that increasing recombination ~6-fold empowers the detection and resolution of QTLs, reaching the gene scale with only a few hundred plants. Further, enhanced recombination unmasks some secondary QTLs undetected under normal recombination. These results show the benefits of enhanced recombination to decipher the genetic bases of traits.

Synthetically induced Arabidopsis thaliana autotetraploids provide insights into the analysis of meiotic mutants with altered crossover frequency

Mutations affecting crossover (CO) frequency and distribution lead to the presence of univalents during meiosis, giving rise to aneuploid gametes and sterility. These mutations may have a different effect after chromosome doubling. The combination of altered ploidy and mutations could be potentially useful to gain new insights into the mechanisms and regulation of meiotic recombination; however, studies using autopolyploid meiotic mutants are scarce. Here, we have analyzed the cytogenetic consequences in colchicine-induced autotetraploids (colchiploids) from different Arabidopsis mutants with an altered CO frequency. We have found that there are three types of mutants: mutants in which chiasma frequency is doubled after chromosome duplication (zip4, mus81), as in the control; mutants in which polyploidy leads to a higher-than-expected increase in chiasma frequency (asy1, mer3, hei10, and mlh3); and mutants in which the rise in chiasma frequency produced by the presence of two extrachromosomal sets is less than doubled (msh5, fancm). In addition, the proportion of class I/class II COs varies after chromosome duplication in the control. The results obtained reveal the potential of colchiploid meiotic mutants for better understanding of the function of key proteins during plant meiosis. This is especially relevant considering that most crops are polyploids.

Research progress of the Fanconi anemia pathway and premature ovarian insufficiency†

The Fanconi anemia pathway is a key pathway involved in the repair of deoxyribonucleic acidinterstrand crosslinking damage, which chiefly includes the following four modules: lesion recognition, Fanconi anemia core complex recruitment, FANCD2-FANCI complex monoubiquitination, and downstream events (nucleolytic incision, translesion synthesis, and homologous recombination). Mutations or deletions of multiple Fanconi anemia genes in this pathway can damage the interstrand crosslinking repair pathway and disrupt primordial germ cell development and oocyte meiosis, thereby leading to abnormal follicular development. Premature ovarian insufficiency is a gynecological clinical syndrome characterized by amenorrhea and decreased fertility due to decreased oocyte pool, accelerated follicle atresia, and loss of ovarian function in women <40 years old. Furthermore, in recent years, several studies have detected mutations in the Fanconi anemia gene in patients with premature ovarian insufficiency. In addition, some patients with Fanconi anemia exhibit symptoms of premature ovarian insufficiency and infertility. The Fanconi anemia pathway and premature ovarian insufficiency are closely associated.

FANCM interacts with the MHF1-MHF2 complex to limit crossover frequency during rice meiosis

Crossovers (COs) are necessary for generating genetic diversity that breeders can select, but there are conserved mechanisms that regulate their frequency and distribution. Increasing CO frequency may raise the efficiency of selection by increasing the chance of integrating more desirable traits. In this study, we characterize rice FANCM and explore its functions in meiotic CO control. FANCM mutations do not affect fertility in rice, but they cause a great boost in the overall frequency of COs in both inbred and hybrid rice, according to genetic analysis of the complete set of fancm zmm (hei10, ptd, shoc1, mer3, zip4, msh4, msh5, and heip1) mutants. Although the early homologous recombination events proceed normally in fancm, the meiotic extra COs are not marked with HEI10 and require MUS81 resolvase for resolution. FANCM depends on PAIR1, COM1, DMC1, and HUS1 to perform its functions. Simultaneous disruption of FANCM and MEICA1 synergistically increases CO frequency, but it is accompanied by nonhomologous chromosome associations and fragmentations. FANCM interacts with the MHF complex, and ablation of rice MHF1 or MHF2 could restore the formation of 12 bivalents in the absence of the ZMM gene ZIP4. Our data indicate that unleashing meiotic COs by mutating any member of the FANCM-MHF complex could be an effective procedure to accelerate the efficiency of rice breeding.

Fancm has dual roles in the limiting of meiotic crossovers and germ cell maintenance in mammals

Meiotic crossovers are required for accurate chromosome segregation and producing new allelic combinations. Meiotic crossover numbers are tightly regulated within a narrow range, despite an excess of initiating DNA double-strand breaks. Here, we reveal the tumor suppressor FANCM as a meiotic anti-crossover factor in mammals. We use unique large-scale crossover analyses with both single-gamete sequencing and pedigree-based bulk-sequencing datasets to identify a genome-wide increase in crossover frequencies in Fancm-deficient mice. Gametogenesis is heavily perturbed in Fancm loss-of-function mice, which is consistent with the reproductive defects reported in humans with biallelic FANCM mutations. A portion of the gametogenesis defects can be attributed to the cGAS-STING pathway after birth. Despite the gametogenesis phenotypes in Fancm mutants, both sexes are capable of producing offspring. We propose that the anti-crossover function and role in gametogenesis of Fancm are separable and will inform diagnostic pathways for human genomic instability disorders.

Control of meiotic crossing over in plant breeding

Meiotic crossing over is the main mechanism for constructing a new allelic composition of individual chromosomes and is necessary for the proper distribution of homologous chromosomes between gametes. The parameters of meiotic crossing over that have developed in the course of evolution are determined by natural selection and do not fully suit the tasks of selective breeding research. This review summarizes the results of experimental studies aimed at increasing the frequency of crossovers and redistributing their positions along chromosomes using genetic manipulations at different stages of meiotic recombination. The consequences of inactivation and/or overexpression of the SPO11 genes, the products of which generate meiotic double-strand breaks in DNA, for the redistribution of crossover positions in the genome of various organisms are discussed. The results of studies concerning the effect of inactivation or overexpression of genes encoding RecA-like recombinases on meiotic crossing over, including those in cultivated tomato (Solanum lycopersicum L.) and its interspecific hybrids, are summarized. The consequences of inactivation of key genes of the mismatch repair system are discussed. Their suppression made it possible to significantly increase the frequency of meiotic recombination between homeologues in the interspecific hybrid yeast Saccharomyces cerevisiae × S. paradoxus and between homologues in arabidopsis plants (Arabidopsis thaliana L.). Also discussed are attempts to extrapolate these results to other plant species, in which a decrease in reproductive properties and microsatellite instability in the genome have been noted. The most significant results on the meiotic recombination frequency increase upon inactivation of the FANCM, TOP3α, RECQ4, FIGL1 crossover repressor genes and upon overexpression of the HEI10 crossover enhancer gene are separately described. In some experiments, the increase of meiotic recombination frequency by almost an order of magnitude and partial redistribution of the crossover positions along chromosomes were achieved in arabidopsis while fully preserving fecundity. Similar results have been obtained for some crops.

Repair of DNA double-strand breaks in plant meiosis: role of eukaryotic RecA recombinases and their modulators

Homologous recombination during meiosis is crucial for the DNA double-strand breaks (DSBs) repair that promotes the balanced segregation of homologous chromosomes and enhances genetic variation. In most eukaryotes, two recombinases RAD51 and DMC1 form nucleoprotein filaments on single-stranded DNA generated at DSB sites and play a central role in the meiotic DSB repair and genome stability. These nucleoprotein filaments perform homology search and DNA strand exchange to initiate repair using homologous template-directed sequences located elsewhere in the genome. Multiple factors can regulate the assembly, stability, and disassembly of RAD51 and DMC1 nucleoprotein filaments. In this review, we summarize the current understanding of the meiotic functions of RAD51 and DMC1 and the role of their positive and negative modulators. We discuss the current models and regulators of homology searches and strand exchange conserved during plant meiosis. Manipulation of these repair factors during plant meiosis also holds a great potential to accelerate plant breeding for crop improvements and productivity.

Why do plants need the ZMM crossover pathway? A snapshot of meiotic recombination from the perspective of interhomolog polymorphism

At the heart of meiosis is crossover recombination, i.e., reciprocal exchange of chromosome fragments between parental genomes. Surprisingly, in most eukaryotes, including plants, several recombination pathways that can result in crossover event operate in parallel during meiosis. These pathways emerged independently in the course of evolution and perform separate functions, which directly translate into their roles in meiosis. The formation of one crossover per chromosome pair is required for proper chromosome segregation. This "obligate" crossover is ensured by the major crossover pathway in plants, and in many other eukaryotes, known as the ZMM pathway. The secondary pathways play important roles also in somatic cells and function mainly as repair mechanisms for DNA double-strand breaks (DSBs) not used for crossover formation. One of the consequences of the functional differences between ZMM and other DSB repair pathways is their distinct sensitivities to polymorphisms between homologous chromosomes. From a population genetics perspective, these differences may affect the maintenance of genetic variability. This might be of special importance when considering that a significant portion of plants uses inbreeding as a predominant reproductive strategy, which results in loss of interhomolog polymorphism. While we are still far from fully understanding the relationship between meiotic recombination pathways and genetic variation in populations, recent studies of crossovers in plants offer a new perspective.

The FANCC-FANCE-FANCF complex is evolutionarily conserved and regulates meiotic recombination

At meiosis, programmed meiotic DNA double-strand breaks are repaired via homologous recombination, resulting in crossovers (COs). From a large excess of DNA double-strand breaks that are formed, only a small proportion gets converted into COs because of active mechanisms that restrict CO formation. The Fanconi anemia (FA) complex proteins AtFANCM, MHF1 and MHF2 were previously identified in a genetic screen as anti-CO factors that function during meiosis in Arabidopsis thaliana. Here, pursuing the same screen, we identify FANCC as a new anti-CO gene. FANCC was previously only identified in mammals because of low primary sequence conservation. We show that FANCC, and its physical interaction with FANCE-FANCF, is conserved from vertebrates to plants. Further, we show that FANCC, together with its subcomplex partners FANCE and FANCF, regulates meiotic recombination. Mutations of any of these three genes partially rescues CO-defective mutants, which is particularly marked in female meiosis. Functional loss of FANCC, FANCE, or FANCF results in synthetic meiotic catastrophe with the pro-CO factor MUS81. This work reveals that FANCC is conserved outside mammals and has an anti-CO role during meiosis together with FANCE and FANCF.

Creating novel ornamentals via new strategies in the era of genome editing

Ornamental breeding has traditionally focused on improving novelty, yield, quality, and resistance to biotic or abiotic stress. However, achieving these goals has often required laborious crossbreeding, while precise breeding techniques have been underutilized. Fortunately, recent advancements in plant genome sequencing and editing technology have opened up exciting new frontiers for revolutionizing ornamental breeding. In this review, we provide an overview of the current state of ornamental transgenic breeding and propose four promising breeding strategies that have already proven successful in crop breeding and could be adapted for ornamental breeding with the help of genome editing. These strategies include recombination manipulation, haploid inducer creation, clonal seed production, and reverse breeding. We also discuss in detail the research progress, application status, and feasibility of each of these tactics.

Characterization and acceleration of genome shuffling and ploidy reduction in synthetic allopolyploids by genome sequencing and editing

Polyploidy and the subsequent ploidy reduction and genome shuffling are the major driving forces of genome evolution. Here, we revealed short-term allopolyploid genome evolution by sequencing a synthetic intergeneric hybrid (Raphanobrassica, RRCC). In this allotetraploid, the genome deletion was quick, while rearrangement was slow. The core and high-frequency genes tended to be retained while the specific and low-frequency genes tended to be deleted in the hybrid. The large-fragment deletions were enriched in the heterochromatin region and probably derived from chromosome breaks. The intergeneric translocations were primarily of short fragments dependent on homoeology, indicating a gene conversion origin. To accelerate genome shuffling, we developed an efficient genome editing platform for Raphanobrassica. By editing Fanconi Anemia Complementation Group M (FANCM) genes, homoeologous recombination, chromosome deletion and secondary meiosis with additional ploidy reduction were accelerated. FANCM was shown to be a checkpoint of meiosis and controller of ploidy stability. By simultaneously editing FLIP genes, gene conversion was precisely introduced, and mosaic genes were produced around the target site. This intergeneric hybrid and genome editing platform not only provides models that facilitate experimental evolution research by speeding up genome shuffling and conversion but also accelerates plant breeding by enhancing intergeneric genetic exchange and creating new genes.

Location and Identification on Chromosome 3B of Bread Wheat of Genes Affecting Chiasma Number

Understanding meiotic crossover (CO) variation in crops like bread wheat (Triticum aestivum L.) is necessary as COs are essential to create new, original and powerful combinations of genes for traits of agronomical interest. We cytogenetically characterized a set of wheat aneuploid lines missing part or all of chromosome 3B to identify the most influential regions for chiasma formation located on this chromosome. We showed that deletion of the short arm did not change the total number of chiasmata genome-wide, whereas this latter was reduced by ~35% while deleting the long arm. Contrary to what was hypothesized in a previous study, deletion of the long arm does not disturb the initiation of the synaptonemal complex (SC) in early meiotic stages. However, progression of the SC is abnormal, and we never observed its completion when the long arm is deleted. By studying six different deletion lines (missing different parts of the long arm), we revealed that at least two genes located in both the proximal (C-3BL2-0.22) and distal (3BL7-0.63-1.00) deletion bins are involved in the control of chiasmata, each deletion reducing the number of chiasmata by ~15%. We combined sequence analyses of deletion bins with RNA-Seq data derived from meiotic tissues and identified a set of genes for which at least the homoeologous copy on chromosome 3B is expressed and which are involved in DNA processing. Among these genes, eight (CAP-E1/E2, DUO1, MLH1, MPK4, MUS81, RTEL1, SYN4, ZIP4) are known to be involved in the recombination pathway.

Unravelling mechanisms that govern meiotic crossover formation in wheat

Wheat is a major cereal crop that possesses a large allopolyploid genome formed through hybridisation of tetraploid and diploid progenitors. During meiosis, crossovers (COs) are constrained in number to 1–3 per chromosome pair that are predominantly located towards the chromosome ends. This reduces the probability of advantageous traits recombining onto the same chromosome, thus limiting breeding. Therefore, understanding the underlying factors controlling meiotic recombination may provide strategies to unlock the genetic potential in wheat. In this mini-review, we will discuss the factors associated with restricted CO formation in wheat, such as timing of meiotic events, chromatin organisation, pre-meiotic DNA replication and dosage of CO genes, as a means to modulate recombination.

FIGNL1 Inhibits Non-homologous Chromosome Association and Crossover Formation

Meiotic crossovers (COs) not only generate genetic diversity but also ensure the accuracy of homologous chromosome segregation. Here, we identified FIGNL1 as a new inhibitor for extra crossover formation in rice. The fignl1 mutant displays abnormal interactions between non-homologous chromosomes at diakinesis, and chromosome bridges and fragmentation at subsequent stages of meiosis, but shows normal homologous chromosome pairing and synapsis during early prophase I. FIGNL1 participates in homologous chromosome recombination and functions downstream of DMC1. Mutation of FIGNL1 increases the number of bivalents in zip4 mutants, but does not change the number of HEI10 foci, indicating that FIGNL1 functions in limiting class II CO formation. FIGNL1 interacts with MEICA1, and colocalizes with MEICA1 in a dynamic pattern as punctate foci located between two linear homologous chromosomes. The localization of FIGNL1 depends on ZEP1-mediated assembly of the synaptonemal complex. Based on these results, we propose that FIGNL1 inhibits non-homologous chromosome interaction and CO formation during rice meiosis.

FANCM promotes class I interfering crossovers and suppresses class II non-interfering crossovers in wheat meiosis

FANCM suppresses crossovers in plants by unwinding recombination intermediates. In wheat, crossovers are skewed toward the chromosome ends, thus limiting generation of novel allelic combinations. Here, we observe that FANCM maintains the obligate crossover in tetraploid and hexaploid wheat, thus ensuring that every chromosome pair exhibits at least one crossover, by localizing class I crossover protein HEI10 at pachytene. FANCM also suppresses class II crossovers that increased 2.6-fold in fancm msh5 quadruple mutants. These data are consistent with a role for FANCM in second-end capture of class I designated crossover sites, whilst FANCM is also required to promote formation of non-crossovers. In hexaploid wheat, genetic mapping reveals that crossovers increase by 31% in fancm compared to wild type, indicating that fancm could be an effective tool to accelerate breeding. Crossover rate differences in fancm correlate with wild type crossover distributions, suggesting that chromatin may influence the recombination landscape in similar ways in both wild type and fancm.

The FANCM helicase functions in limiting crossovers (COs) by unwinding inter-homolog repair intermediates. Here, the authors generate null mutants of fancm in tetraploid and hexaploid wheat and show that FANCM promotes class I interfering COs and suppresses class II noninterfering COs in wheat meiosis.

Regulation of interference-sensitive crossover distribution ensures crossover assurance in Arabidopsis

During meiosis, crossovers (COs) are typically required to ensure faithful chromosomal segregation. Despite the requirement for at least one CO between each pair of chromosomes, closely spaced double COs are usually underrepresented due to a phenomenon called CO interference. Like Mus musculus and Saccharomyces cerevisiae, Arabidopsis thaliana has both interference-sensitive (Class I) and interference-insensitive (Class II) COs. However, the underlying mechanism controlling CO distribution remains largely elusive. Both AtMUS81 and AtFANCD2 promote the formation of Class II CO. Using both AtHEI10 and AtMLH1 immunostaining, two markers of Class I COs, we show that AtFANCD2 but not AtMUS81 is required for normal Class I CO distribution among chromosomes. Depleting AtFANCD2 leads to a CO distribution pattern that is intermediate between that of wild-type and a Poisson distribution. Moreover, in Atfancm, Atfigl1, and Atrmi1 mutants where increased Class II CO frequency has been reported previously, we observe Class I CO distribution patterns that are strikingly similar to Atfancd2. Surprisingly, we found that AtFANCD2 plays opposite roles in regulating CO frequency in Atfancm compared with either in Atfigl1 or Atrmi1. Together, these results reveal that although AtFANCD2, AtFANCM, AtFIGL1, and AtRMI1 regulate Class II CO frequency by distinct mechanisms, they have similar roles in controlling the distribution of Class I COs among chromosomes.

Replication protein A large subunit (RPA1a) limits chiasma formation during rice meiosis

Replication protein A (RPA), a single-stranded DNA-binding protein, plays essential role in homologous recombination. However, because deletion of RPA causes embryonic lethality in mammals, the exact function of RPA in meiosis remains unclear. In this study, we generated an rpa1a mutant using CRISPR/Cas9 technology and explored its function in rice (Oryza sativa) meiosis. In rpa1a, 12 bivalents were formed at metaphase I, just like in wild-type, but chromosome fragmentations were consistently observed at anaphase I. Fluorescence in situ hybridization assays indicated that these fragmentations were due to the failure of the recombination intermediates to resolve. Importantly, the mutant had a highly elevated chiasma number, and loss of RPA1a could completely restore the 12 bivalent formations in the zmm (for ZIP1-4, MSH4/5, and MER3) mutant background. Protein-protein interaction assays showed that RPA1a formed a complex with the methyl methansulfonate and UV sensitive 81 (and the Fanconi anemia complementation group M-Bloom syndrome protein homologs (RECQ4A)-Topoisomerase3α-RecQ-mediated genome instability 1 complex to regulate chiasma formation and processing of the recombination intermediates. Thus, our data establish a pivotal role for RPA1a in promoting the accurate resolution of recombination intermediates and in limiting redundant chiasma formation during rice meiosis.

Downregulation of Barley Regulator of Telomere Elongation Helicase 1 Alters the Distribution of Meiotic Crossovers

Programmed meiotic DNA double-strand breaks (DSBs), necessary for proper chromosomal segregation and viable gamete formation, are repaired by homologous recombination (HR) as crossovers (COs) or non-crossovers (NCOs). The mechanisms regulating the number and distribution of COs are still poorly understood. The regulator of telomere elongation helicase 1 (RTEL1) DNA helicase was previously shown to enforce the number of meiotic COs in Caenorhabditis elegans but its function in plants has been studied only in the vegetative phase. Here, we characterised barley RTEL1 gene structure and expression using RNA-seq data previously obtained from vegetative and reproductive organs and tissues. Using RNAi, we downregulated RTEL1 expression specifically in reproductive tissues and analysed its impact on recombination using a barley 50k iSelect SNP Array. Unlike in C. elegans, in a population segregating for RTEL1 downregulated by RNAi, high resolution genome-wide genetic analysis revealed a significant increase of COs at distal chromosomal regions of barley without a change in their total number. Our data reveal the important role of RTEL1 helicase in plant meiosis and control of recombination.

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.

Fanconi anemia ortholog FANCM regulates meiotic crossover distribution in plants

Meiotic recombination increases genetic diversity and manipulation of its frequency and distribution holds great promise in crop breeding. In Arabidopsis thaliana, FANCM (a homolog of mammalian Fanconi anemia complementation group M) suppresses recombination and its function seems conserved in other species including the rosids Brassica spp. and pea (Pisum sativum), and the monocot rice (Oryza sativa). To examine the role of FANCM during meiotic recombination in lettuce (Lactuca sativa, an asterid), we characterized the function of lettuce LsFANCM and found that it can functionally substitute for AtFANCM in transgenic Arabidopsis plants. Moreover, three independent CRISPR/Cas9-edited lettuce Lsfancm mutants showed reduced pollen viability and seed setting. Unexpectedly, analyses of chromosome behavior revealed that 77.8% of Lsfancm meiocytes exhibited univalents. The normal formation of double-strand breaks in DNA and the discontinuous assembly of synaptonemal complex in Lsfancm mutants supports the hypothesis that LsFANCM might be dispensable for the initiation of meiotic recombination but required for normal synapsis. Furthermore, the frequency of lettuce HEI10 (Human Enhancer of Invasion 10) foci, a marker for Class-I crossovers (COs), was similar between wild-type (WT) and Lsfancm. Strikingly, the distribution of LsHEI10 foci and chiasmata in Lsfancm meiotic chromosomes was markedly different from the WT. A similar alteration in the distribution of Class-I COs was also observed in the Arabidopsis Atfancm mutant. Taken together, these results demonstrate that FANCM is important for shaping the distribution of meiotic Class-I COs in plants, and reveal an evolutionarily divergent role for FANCM in meiotic bivalent formation between Arabidopsis and lettuce.

From Microscopy to Nanoscopy: Defining an Arabidopsis thaliana Meiotic Atlas at the Nanometer Scale

Visualization of meiotic chromosomes and the proteins involved in meiotic recombination have become essential to study meiosis in many systems including the model plant Arabidopsis thaliana. Recent advances in super-resolution technologies changed how microscopic images are acquired and analyzed. New technologies enable observation of cells and nuclei at a nanometer scale and hold great promise to the field since they allow observing complex meiotic molecular processes with unprecedented detail. Here, we provide an overview of classical and advanced sample preparation and microscopy techniques with an updated Arabidopsis meiotic atlas based on super-resolution microscopy. We review different techniques, focusing on stimulated emission depletion (STED) nanoscopy, to offer researchers guidance for selecting the optimal protocol and equipment to address their scientific question.

Shared ancestry of core-histone subunits and non-histone plant proteins containing the Histone Fold Motif (HFM)

The three helical Histone Fold Motif (HFM) of core histone proteins provides an evolutionarily favored site for the protein-DNA interface. Despite significant variation in sequence, the HFM retains a distinctive structural fold that has diversified into several non-histone protein families. In this work, we explore the ancestry of non-histone HFM containing families in the plant kingdom. A sequence search algorithm was developed using iterative profile Hidden Markov Models to identify remote homologs of core-histone proteins. The resulting hits were functionally annotated, classified into families, and subjected to comprehensive phylogenetic analyses via Maximum likelihood and Bayesian methods. We have identified 4390 HFM containing proteins in the plant kingdom that are not histones, mostly existing as diverse transcription factor families, distributed widely within and across taxonomic groups. Patterns of homology suggest that core histone subunit H2A has evolved into newer families like NF-YC and DRAP1, whereas the H2B subunit of core histones shares a common ancestry with NF-YB and DR1 class of TFs. Core histone subunits H3 and H4 were found to have evolved into DPE and TAF proteins, respectively. Taken together these results provide insights into diversification events during the evolution of the HFM, including sub-functionalization and neo-functionalization of the HFM.

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

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

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

A Genetic Screen to Identify New Molecular Players Involved in Photoprotection qH in Arabidopsis thaliana

Photosynthesis is a biological process which converts light energy into chemical energy that is used in the Calvin-Benson cycle to produce organic compounds. An excess of light can induce damage to the photosynthetic machinery. Therefore, plants have evolved photoprotective mechanisms such as non-photochemical quenching (NPQ). To focus molecular insights on slowly relaxing NPQ processes in Arabidopsis thaliana, previously, a qE-deficient line-the PsbS mutant-was mutagenized and a mutant with high and slowly relaxing NPQ was isolated. The mutated gene was named suppressor of quenching 1, or SOQ1, to describe its function. Indeed, when present, SOQ1 negatively regulates or suppresses a form of antenna NPQ that is slow to relax and is photoprotective. We have now termed this component qH and identified the plastid lipocalin, LCNP, as the effector for this energy dissipation mode to occur. Recently, we found that the relaxation of qH1, ROQH1, protein is required to turn off qH. The aim of this study is to identify new molecular players involved in photoprotection qH by a whole genome sequencing approach of chemically mutagenized Arabidopsis thaliana. We conducted an EMS-mutagenesis on the soq1 npq4 double mutant and used chlorophyll fluorescence imaging to screen for suppressors and enhancers of qH. Out of 22,000 mutagenized plants screened, the molecular players cited above were found using a mapping-by-sequencing approach. Here, we describe the phenotypic characterization of the other mutants isolated from this genetic screen and an additional 8000 plants screened. We have classified them in several classes based on their fluorescence parameters, NPQ kinetics, and pigment content. A high-throughput whole genome sequencing approach on 65 mutants will identify the causal mutations thanks to allelic mutations from having reached saturation of the genetic screen. The candidate genes could be involved in the formation or maintenance of quenching sites for qH, in the regulation of qH at the transcriptional level, or be part of the quenching site itself.

Excess crossovers impede faithful meiotic chromosome segregation in C. elegans

During meiosis, diploid organisms reduce their chromosome number by half to generate haploid gametes. This process depends on the repair of double strand DNA breaks as crossover recombination events between homologous chromosomes, which hold homologs together to ensure their proper segregation to opposite spindle poles during the first meiotic division. Although most organisms are limited in the number of crossovers between homologs by a phenomenon called crossover interference, the consequences of excess interfering crossovers on meiotic chromosome segregation are not well known. Here we show that extra interfering crossovers lead to a range of meiotic defects and we uncover mechanisms that counteract these errors. Using chromosomes that exhibit a high frequency of supernumerary crossovers in Caenorhabditis elegans, we find that essential chromosomal structures are mispatterned in the presence of multiple crossovers, subjecting chromosomes to improper spindle forces and leading to defects in metaphase alignment. Additionally, the chromosomes with extra interfering crossovers often exhibited segregation defects in anaphase I, with a high incidence of chromatin bridges that sometimes created a tether between the chromosome and the first polar body. However, these anaphase I bridges were often able to resolve in a LEM-3 nuclease dependent manner, and chromosome tethers that persisted were frequently resolved during Meiosis II by a second mechanism that preferentially segregates the tethered sister chromatid into the polar body. Altogether these findings demonstrate that excess interfering crossovers can severely impact chromosome patterning and segregation, highlighting the importance of limiting the number of recombination events between homologous chromosomes for the proper execution of meiosis.

Mutations of the AtYAK1 Kinase Suppress TOR Deficiency in Arabidopsis

The target of rapamycin (TOR) kinase is a conserved energy sensor that regulates growth in response to environmental cues. However, little is known about the TOR signaling pathway in plants. We used Arabidopsis lines affected in the lethal with SEC13 protein 8 (LST8-1) gene, a core element of the TOR complex, to search for suppressor mutations. Two suppressor lines with improved growth were isolated that carried mutations in the Yet Another Kinase 1 (AtYAK1) gene encoding a member of the dual-specificity tyrosine phosphorylation-regulated kinase (DYRK) family. Atyak1 mutations partly rescued the developmental defects of lst8-1-1 mutants and conferred resistance to the TOR inhibitor AZD-8055. Moreover, atyak1 mutations suppressed the transcriptomic and metabolic perturbations as well as the abscisic acid (ABA) hypersensitivity of the lst8-1-1 mutants. AtYAK1 interacted with the regulatory-associated protein of TOR (RAPTOR), a component of the TOR complex, and was phosphorylated by TOR. Thus, our findings reveal that AtYAK1 is a TOR effector that probably needs to be switched off to activate plant growth.

Counting on Crossovers: Controlled Recombination for Plant Breeding

Crossovers (COs), that drive genetic exchange between homologous chromosomes, are strongly biased toward subtelomeric regions in plant species. Manipulating the rate and positions of COs to increase the genetic variation accessible to breeders is a longstanding goal. Use of genome editing reagents that induce double-stranded breaks (DSBs) or modify the epigenome at desired sites of recombination, and manipulation of CO factors, are increasingly applicable approaches for achieving this goal. These strategies for 'controlled recombination' have potential to reduce the time and expense associated with traditional breeding, reveal currently inaccessible genetic diversity, and increase control over the inheritance of preferred haplotypes. Considerable challenges to address include translating knowledge from models to crop species and determining the best stages of the breeding cycle at which to control recombination.

CDKG1 Is Required for Meiotic and Somatic Recombination Intermediate Processing in Arabidopsis

The Arabidopsis (Arabidopsis thaliana) cyclin-dependent kinase G1 (CDKG1) is necessary for recombination and synapsis during male meiosis at high ambient temperature. In the cdkg1-1 mutant, synapsis is impaired and there is a dramatic reduction in the number of class I crossovers, resulting in univalents at metaphase I and pollen sterility. Here, we demonstrate that CDKG1 is necessary for the processing of recombination intermediates in the canonical ZMM recombination pathway and that loss of CDKG1 results in increased class II crossovers. While synapsis and events associated with class I crossovers are severely compromised in a cdkg1-1 mutant, they can be restored by increasing the number of recombination intermediates in the double cdkg1-1 fancm-1 mutant. Despite this, recombination intermediates are not correctly resolved, leading to the formation of chromosome aggregates at metaphase I. Our results show that CDKG1 acts early in the recombination process and is necessary to stabilize recombination intermediates. Finally, we show that the effect on recombination is not restricted to meiosis and that CDKG1 is also required for normal levels of DNA damage-induced homologous recombination in somatic tissues.

CRISPR/Cas inactivation of RECQ4 increases homeologous crossovers in an interspecific tomato hybrid

Crossover formation during meiosis in plants is required for proper chromosome segregation and is essential for crop breeding as it allows an (optimal) combination of traits by mixing parental alleles on each chromosome. Crossover formation commences with the production of a large number of DNA double-strand breaks, of which only a few result in crossovers. A small number of genes, which drive the resolution of DNA crossover intermediate structures towards non-crossovers, have been identified in Arabidopisis thaliana. In order to explore the potential of modification of these genes in interspecific hybrids between crops and their wild relatives towards increased production of crossovers, we have used CRISPR/Cas9-mutagenesis in an interspecific tomato hybrid to knockout RecQ4. A biallelic recq4 mutant was obtained in the F1 hybrid of Solanum lycopersicum and S. pimpinellifolium. Compared with the wild-type F1 hybrid, the F1 recq4 mutant was shown to have a significant increase in crossovers: a 1.53-fold increase when directly observing ring bivalents in male meiocytes microscopically and a 1.8-fold extension of the genetic map when measured by analysing SNP markers in the progeny (F2) plants. This is one of the first demonstrations of increasing crossover frequency in interspecific hybrids by manipulating genes in crossover intermediate resolution pathways and the first to do so by directed mutagenesis. SIGNIFICANCE STATEMENT: Increasing crossover frequency during meiosis can speed up or simplify crop breeding that relies on meiotic crossovers to introduce favourable alleles controlling important traits from wild relatives into crops. Here we show for the first time that knocking out an inhibitor of crossovers in an interspecific hybrid between tomato and its relative wild species using CRISPR/Cas9-mutagenesis results in increased recombination between the two genomes.

Redistribution of Meiotic Crossovers Along Wheat Chromosomes by Virus-Induced Gene Silencing

Meiotic recombination is the main driver of genetic diversity in wheat breeding. The rate and location of crossover (CO) events are regulated by genetic and epigenetic factors. In wheat, most COs occur in subtelomeric regions but are rare in centromeric and pericentric areas. The aim of this work was to increase COs in both "hot" and "cold" chromosomal locations. We used Virus-Induced gene Silencing (VIGS) to downregulate the expression of recombination-suppressing genes XRCC2 and FANCM and of epigenetic maintenance genes MET1 and DDM1 during meiosis. VIGS suppresses genes in a dominant, transient and non-transgenic manner, which is convenient in wheat, a hard-to-transform polyploid. F1 hybrids of a cross between two tetraploid lines whose genome was fully sequenced (wild emmer and durum wheat), were infected with a VIGS vector ∼ 2 weeks before meiosis. Recombination was measured in F2 seedlings derived from F1-infected plants and non-infected controls. We found significant up and down-regulation of CO rates along subtelomeric regions as a result of silencing either MET1, DDM1 or XRCC2 during meiosis. In addition, we found up to 93% increase in COs in XRCC2-VIGS treatment in the pericentric regions of some chromosomes. Silencing FANCM showed no effect on CO. Overall, we show that CO distribution was affected by VIGS treatments rather than the total number of COs which did not change. We conclude that transient silencing of specific genes during meiosis can be used as a simple, fast and non-transgenic strategy to improve breeding abilities in specific chromosomal regions.

DNA Helicases as Safekeepers of Genome Stability in Plants

Genetic information of all organisms is coded in double-stranded DNA. DNA helicases are essential for unwinding this double strand when it comes to replication, repair or transcription of genetic information. In this review, we will focus on what is known about a variety of DNA helicases that are required to ensure genome stability in plants. Due to their sessile lifestyle, plants are especially exposed to harmful environmental factors. Moreover, many crop plants have large and highly repetitive genomes, making them absolutely dependent on the correct interplay of DNA helicases for safeguarding their stability. Although basic features of a number of these enzymes are conserved between plants and other eukaryotes, a more detailed analysis shows surprising peculiarities, partly also between different plant species. This is additionally of high relevance for plant breeding as a number of these helicases are also involved in crossover control during meiosis and influence the outcome of different approaches of CRISPR/Cas based plant genome engineering. Thus, gaining knowledge about plant helicases, their interplay, as well as the manipulation of their pathways, possesses the potential for improving agriculture. In the long run, this might even help us cope with the increasing obstacles of climate change threatening food security in completely new ways.

Linked-read sequencing of gametes allows efficient genome-wide analysis of meiotic recombination

Meiotic crossovers (COs) ensure proper chromosome segregation and redistribute the genetic variation that is transmitted to the next generation. Large populations and the demand for genome-wide, fine-scale resolution challenge existing methods for CO identification. Taking advantage of linked-read sequencing, we develop a highly efficient method for genome-wide identification of COs at kilobase resolution in pooled recombinants. We first test this method using a pool of Arabidopsis F₂ recombinants, and recapitulate results obtained from the same plants using individual whole-genome sequencing. By applying this method to a pool of pollen DNA from an F₁ plant, we establish a highly accurate CO landscape without generating or sequencing a single recombinant plant. The simplicity of this approach enables the simultaneous generation and analysis of multiple CO landscapes, accelerating the pace at which mechanisms for the regulation of recombination can be elucidated through efficient comparisons of genotypic and environmental effects on recombination.

Meiotic crossovers (COs) generate genetic variation and ensure proper chromosome segregation. Here, the authors develop a method for identifying COs at kilobase resolution in pooled recombinants using linked-read sequencing data, and apply it to investigate genome-wide CO landscapes of Arabidopsis thaliana.

Linked-read sequencing of gametes allows efficient genome-wide analysis of meiotic recombination

Meiotic crossovers (COs) ensure proper chromosome segregation and redistribute the genetic variation that is transmitted to the next generation. Large populations and the demand for genome-wide, fine-scale resolution challenge existing methods for CO identification. Taking advantage of linked-read sequencing, we develop a highly efficient method for genome-wide identification of COs at kilobase resolution in pooled recombinants. We first test this method using a pool of Arabidopsis F₂ recombinants, and recapitulate results obtained from the same plants using individual whole-genome sequencing. By applying this method to a pool of pollen DNA from an F₁ plant, we establish a highly accurate CO landscape without generating or sequencing a single recombinant plant. The simplicity of this approach enables the simultaneous generation and analysis of multiple CO landscapes, accelerating the pace at which mechanisms for the regulation of recombination can be elucidated through efficient comparisons of genotypic and environmental effects on recombination.

DNA- and DNA-Protein-Crosslink Repair in Plants

DNA-crosslinks are one of the most severe types of DNA lesions. Crosslinks (CLs) can be subdivided into DNA-intrastrand CLs, DNA-interstrand CLs (ICLs) and DNA-protein crosslinks (DPCs), and arise by various exogenous and endogenous sources. If left unrepaired before the cell enters S-phase, ICLs and DPCs pose a major threat to genomic integrity by blocking replication. In order to prevent the collapse of replication forks and impairment of cell division, complex repair pathways have emerged. In mammals, ICLs are repaired by the so-called Fanconi anemia (FA) pathway, which includes 22 different FANC genes, while in plants only a few of these genes are conserved. In this context, two pathways of ICL repair have been defined, each requiring the interaction of a helicase (FANCJB/RTEL1) and a nuclease (FAN1/MUS81). Moreover, homologous recombination (HR) as well as postreplicative repair factors are also involved. Although DPCs possess a comparable toxic potential to cells, it has only recently been shown that at least three parallel pathways for DPC repair exist in plants, defined by the protease WSS1A, the endonuclease MUS81 and tyrosyl-DNA phosphodiesterase 1 (TDP1). The importance of crosslink repair processes are highlighted by the fact that deficiencies in the respective pathways are associated with diverse hereditary disorders.

Patronus is the elusive plant securin, preventing chromosome separation by antagonizing separase

Accurate chromosome segregation at mitosis and meiosis is crucial to prevent genome instability, birth defect, and cancer. Accordingly, separase, the protease that triggers chromosome distribution, is tightly regulated by a direct inhibitor, the securin. However, securin has not been identified, neither functionnally nor by sequence similarity, in other clades that fungi and animals. This raised doubts about the conservation of this mechanism in other branches of eukaryotes. Here, we identify and characterize the securin in plants. Despite extreme sequence divergence, the securin kept the same core function and is likely a universal regulator of cell division in eukaryotes.

Chromosome distribution at anaphase of mitosis and meiosis is triggered by separase, an evolutionarily conserved protease. Separase must be tightly regulated to prevent the untimely release of chromatid cohesion and disastrous chromosome distribution defects. Securin is the key inhibitor of separase in animals and fungi, but has not been identified in other eukaryotic lineages. Here, we identified PATRONUS1 and PATRONUS2 (PANS1 and PANS2) as the Arabidopsis homologs of securin. Disruption of PANS1 is known to lead to the premature separation of chromosomes at meiosis, and the simultaneous disruption of PANS1 and PANS2 is lethal. Here, we show that PANS1 targeting by the anaphase-promoting complex is required to trigger chromosome separation, mirroring the regulation of securin. We showed that PANS1 acts independently from Shugosins. In a genetic screen for pans1 suppressors, we identified SEPARASE mutants, showing that PANS1 and SEPARASE have antagonistic functions in vivo. Finally, we showed that the PANS1 and PANS2 proteins interact directly with SEPARASE. Altogether, our results show that PANS1 and PANS2 act as a plant securin. Remote sequence similarity was identified between the plant patronus family and animal securins, suggesting that they indeed derive from a common ancestor. Identification of patronus as the elusive plant securin illustrates the extreme sequence divergence of this central regulator of mitosis and meiosis.

Antagonism between BRCA2 and FIGL1 regulates homologous recombination

Homologous recombination (HR) maintains genome stability by promoting accurate DNA repair. Two recombinases, RAD51 and DMC1, are central to HR repair and form dynamic nucleoprotein filaments in vivo under tight regulation. However, the interplay between positive and negative regulators to control the dynamic assembly/disassembly of RAD51/DMC1 filaments in multicellular eukaryotes remains poorly characterized. Here, we report an antagonism between BRCA2, a well-studied positive mediator of RAD51/DMC1, and FIDGETIN-LIKE-1 (FIGL1), which we previously proposed as a negative regulator of RAD51/DMC1. Through forward genetic screen, we identified a mutation in one of the two Arabidopsis BRCA2 paralogs that suppresses the meiotic phenotypes of figl1. Consistent with the antagonistic roles of BRCA2 and FIGL1, the figl1 mutation in the brca2 background restores RAD51/DMC1 focus formation and homologous chromosome interaction at meiosis, and RAD51 focus formation in somatic cells. This study shows that BRCA2 and FIGL1 have antagonistic effects on the dynamics of RAD51/DMC1-dependent DNA transactions to promote accurate HR repair.

Antagonistic Actions of FPA and IBM2 Regulate Transcript Processing from Genes Containing Heterochromatin

Repressive epigenetic marks, such as DNA and histone methylation, are sometimes located within introns. In Arabidopsis (Arabidopsis thaliana), INCREASE IN BONSAI METHYLATION2 (IBM2), an RNA-binding protein containing a bromo-adjacent homology domain, is required to process functional transcript isoforms of genes carrying intronic heterochromatin. In a genetic screen for suppressors of the ibm2 mutation, we identified FPA, an RNA-binding protein that promotes use of proximal polyadenylation sites in genes targeted by IBM2, including IBM1 encoding an essential H3K9 histone demethylase and the disease resistance gene RECOGNITION OF PERONOSPORA PARASITICA7 Both IBM2 and FPA are involved in the processing of their common mRNA targets: Transcription of IBM2 target genes is restored when FPA is mutated in ibm2 and impaired in transgenic plants overexpressing FPA By contrast, transposons targeted by IBM2 and localized outside introns are not under this antagonistic control. The DNA methylation patterns of some genes and transposons are modified in fpa plants, including the large intron of IBM1, but these changes are rather limited and reversed when the mutant is complemented, indicating that FPA has a restricted role in mediating silencing. These data reveal a complex regulation by IBM2 and FPA pathways in processing mRNAs of genes bearing heterochromatic marks.

Manipulation of crossover frequency and distribution for plant breeding

The crossovers (COs) that occur during meiotic recombination lead to genetic diversity upon which natural and artificial selection can act. The potential of tinkering with the mechanisms of meiotic recombination to increase the amount of genetic diversity accessible for breeders has been under the research spotlight for years. A wide variety of approaches have been proposed to increase CO frequency, alter CO distribution and induce COs between non-homologous chromosomal regions. For most of these approaches, translational biology will be crucial for demonstrating how these strategies can be of practical use in plant breeding. In this review, we describe how tinkering with meiotic recombination could benefit plant breeding and give concrete examples of how these strategies could be implemented into breeding programs.

A TOR-YAK1 signaling axis controls cell cycle, meristem activity and plant growth in Arabidopsis

TARGET OF RAPAMYCIN (TOR) is a conserved eukaryotic phosphatidylinositol-3-kinase-related kinase that plays a major role in regulating growth and metabolism in response to environment in plants. We performed a genetic screen for Arabidopsis ethylmethane sulfonate mutants resistant to the ATP-competitive TOR inhibitor AZD-8055 to identify new components of the plant TOR pathway. We found that loss-of-function mutants of the DYRK (dual specificity tyrosine phosphorylation regulated kinase)/YAK1 kinase are resistant to AZD-8055 and, reciprocally, that YAK1 overexpressors are hypersensitive to AZD-8055. Significantly, these phenotypes were conditional on TOR inhibition, positioning YAK1 activity downstream of TOR. We further show that the ATP-competitive DYRK1A inhibitor pINDY phenocopies YAK1 loss of function. Microscopy analysis revealed that YAK1 functions to repress meristem size and induce differentiation. We show that YAK1 represses cyclin expression in the different zones of the root meristem and that YAK1 is essential for TOR-dependent transcriptional regulation of the plant-specific SIAMESE-RELATED (SMR) cyclin-dependent kinase inhibitors in both meristematic and differentiating root cells. Thus, YAK1 is a major regulator of meristem activity and cell differentiation downstream of TOR.

Interhomolog polymorphism shapes meiotic crossover within the Arabidopsis RAC1 and RPP13 disease resistance genes

During meiosis, chromosomes undergo DNA double-strand breaks (DSBs), which can be repaired using a homologous chromosome to produce crossovers. Meiotic recombination frequency is variable along chromosomes and tends to concentrate in narrow hotspots. We mapped crossover hotspots located in the Arabidopsis thaliana RAC1 and RPP13 disease resistance genes, using varying haplotypic combinations. We observed a negative non-linear relationship between interhomolog divergence and crossover frequency within the hotspots, consistent with polymorphism locally suppressing crossover repair of DSBs. The fancm, recq4a recq4b, figl1 and msh2 mutants, or lines with increased HEI10 dosage, are known to show increased crossovers throughout the genome. Surprisingly, RAC1 crossovers were either unchanged or decreased in these genetic backgrounds, showing that chromosome location and local chromatin environment are important for regulation of crossover activity. We employed deep sequencing of crossovers to examine recombination topology within RAC1, in wild type, fancm, recq4a recq4b and fancm recq4a recq4b backgrounds. The RAC1 recombination landscape was broadly conserved in the anti-crossover mutants and showed a negative relationship with interhomolog divergence. However, crossovers at the RAC1 5'-end were relatively suppressed in recq4a recq4b backgrounds, further indicating that local context may influence recombination outcomes. Our results demonstrate the importance of interhomolog divergence in shaping recombination within plant disease resistance genes and crossover hotspots.

Unleashing meiotic crossovers in crops

Improved plant varieties are important in our attempts to face the challenges of a growing human population and limited planet resources. Plant breeding relies on meiotic crossovers to combine favourable alleles into elite varieties1. However, meiotic crossovers are relatively rare, typically one to three per chromosome2, limiting the efficiency of the breeding process and related activities such as genetic mapping. Several genes that limit meiotic recombination were identified in the model species Arabidopsis thaliana2. Mutation of these genes in Arabidopsis induces a large increase in crossover frequency. However, it remained to be demonstrated whether crossovers could also be increased in crop species hybrids. We explored the effects of mutating the orthologues of FANCM3, RECQ44 or FIGL15 on recombination in three distant crop species, rice (Oryza sativa), pea (Pisum sativum) and tomato (Solanum lycopersicum). We found that the single recq4 mutation increases crossovers about three-fold in these crops, suggesting that manipulating RECQ4 may be a universal tool for increasing recombination in plants. Enhanced recombination could be used with other state-of-the-art technologies such as genomic selection, genome editing or speed breeding6 to enhance the pace and efficiency of plant improvement.

Unleashing meiotic crossovers in crops

Improved plant varieties are important in our attempts to face the challenges of a growing human population and limited planet resources. Plant breeding relies on meiotic crossovers to combine favourable alleles into elite varieties¹. However, meiotic crossovers are relatively rare, typically one to three per chromosome², limiting the efficiency of the breeding process and related activities such as genetic mapping. Several genes that limit meiotic recombination were identified in the model species Arabidopsis thaliana². Mutation of these genes in Arabidopsis induces a large increase in crossover frequency. However, it remained to be demonstrated whether crossovers could also be increased in crop species hybrids. We explored the effects of mutating the orthologues of FANCM³, RECQ4⁴ or FIGL1⁵ on recombination in three distant crop species, rice (Oryza sativa), pea (Pisum sativum) and tomato (Solanum lycopersicum). We found that the single recq4 mutation increases crossovers about three-fold in these crops, suggesting that manipulating RECQ4 may be a universal tool for increasing recombination in plants. Enhanced recombination could be used with other state-of-the-art technologies such as genomic selection, genome editing or speed breeding⁶ to enhance the pace and efficiency of plant improvement.

Two-way mixed-effects methods for joint association analysis using both host and pathogen genomes

Infectious diseases are often affected by specific pairings of hosts and pathogens and therefore by both of their genomes. The integration of a pair of genomes into genome-wide association mapping can provide an exquisitely detailed view of the genetic landscape of complex traits. We present a statistical method, ATOMM (Analysis with a Two-Organism Mixed Model), that maps a trait of interest to a pair of genomes simultaneously; this method makes use of whole-genome sequence data for both host and pathogen organisms. ATOMM uses a two-way mixed-effect model to test for genetic associations and cross-species genetic interactions while accounting for sample structure including interactions between the genetic backgrounds of the two organisms. We demonstrate the applicability of ATOMM to a joint association study of quantitative disease resistance (QDR) in the Arabidopsis thaliana-Xanthomonas arboricola pathosystem. Our method uncovers a clear host-strain specificity in QDR and provides a powerful approach to identify genetic variants on both genomes that contribute to phenotypic variation.

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.

FIGL1 and its novel partner FLIP form a conserved complex that regulates homologous recombination

Homologous recombination is central to repair DNA double-strand breaks, either accidently arising in mitotic cells or in a programed manner at meiosis. Crossovers resulting from the repair of meiotic breaks are essential for proper chromosome segregation and increase genetic diversity of the progeny. However, mechanisms regulating crossover formation remain elusive. Here, we identified through genetic and protein-protein interaction screens FIDGETIN-LIKE-1 INTERACTING PROTEIN (FLIP) as a new partner of the previously characterized anti-crossover factor FIDGETIN-LIKE-1 (FIGL1) in Arabidopsis thaliana. We showed that FLIP limits meiotic crossover together with FIGL1. Further, FLIP and FIGL1 form a protein complex conserved from Arabidopsis to human. FIGL1 interacts with the recombinases RAD51 and DMC1, the enzymes that catalyze the DNA strand exchange step of homologous recombination. Arabidopsis flip mutants recapitulate the figl1 phenotype, with enhanced meiotic recombination associated with change in counts of DMC1 and RAD51 foci. Our data thus suggests that FLIP and FIGL1 form a conserved complex that regulates the crucial step of strand invasion in homologous recombination.

Homologous recombination is a DNA repair mechanism that is essential to preserve the integrity of genetic information and thus to prevent cancer formation. Homologous recombination is also used during sexual reproduction to generate genetic diversity in the offspring by shuffling parental chromosomes. Here, we identified a novel protein complex (FLIP-FIGL1) that regulates homologous recombination and is conserved from plants to mammals. This suggests the existence of a novel mode of regulation at a central step of homologous recombination.

Plasticity of Meiotic Recombination Rates in Response to Temperature in Arabidopsis

Meiotic recombination shuffles genetic information from sexual species into gametes to create novel combinations in offspring. Thus, recombination is an important factor in inheritance, adaptation, and responses to selection. However, recombination is not a static parameter; meiotic recombination rate is sensitive to variation in the environment, especially temperature. That recombination rates change in response to both increases and decreases in temperature was reported in Drosophila a century ago, and since then in several other species. But it is still unclear what the underlying mechanism is, and whether low- and high-temperature effects are mechanistically equivalent. Here, we show that, as in Drosophila, both high and low temperatures increase meiotic crossovers in Arabidopsis thaliana We show that, from a nadir at 18°, both lower and higher temperatures increase recombination through additional class I (interfering) crossovers. However, the increase in crossovers at high and low temperatures appears to be mechanistically at least somewhat distinct, as they differ in their association with the DNA repair protein MLH1. We also find that, in contrast to what has been reported in barley, synaptonemal complex length is negatively correlated with temperature; thus, an increase in chromosome axis length may account for increased crossovers at low temperature in A. thaliana, but cannot explain the increased crossovers observed at high temperature. The plasticity of recombination has important implications for evolution and breeding, and also for the interpretation of observations of recombination rate variation among natural populations.