DNA polymerase epsilon interacts with SUVH2/9 to repress the expression of genes associated with meiotic DSB hotspot in Arabidopsis

Heterochromatin in plant meiosis

Heterochromatin is an organizational property of eukaryotic chromosomes, characterized by extensive DNA and histone modifications, that is associated with the silencing of transposable elements and repetitive sequences. Maintaining heterochromatin is crucial for ensuring genomic integrity and stability during the cell cycle. During meiosis, heterochromatin is important for homologous chromosome synapsis, recombination, and segregation, but our understanding of meiotic heterochromatin formation and condensation is limited. In this review, we focus on the dynamics and features of heterochromatin and how it condenses during meiosis in plants. We also discuss how meiotic heterochromatin influences the interaction and recombination of homologous chromosomes during prophase I.

HEI10 is subject to phase separation and mediates RPA1a degradation during meiotic interference-sensitive crossover formation

Reciprocal exchanges of DNA between homologous chromosomes during meiosis, or crossovers (COs), shuffle genetic information in gametes and progeny. In many eukaryotes, the majority of COs (class I COs) are sensitive to a phenomenon called interference, which influences the occurrence of closely spaced double COs. Class I COs depend on a group of factors called ZMM (Zip, Msh, Mer) proteins including HEI10 (Human Enhancer of Invasion-10). However, how these proteins are recruited to class I CO sites is unclear. Here, we show that HEI10 forms foci on chromatin via a liquid-liquid phase separation (LLPS) mechanism that relies on residue Ser70. A HEI10S70F allele results in LLPS failure and a defect in class I CO formation. We further used immunoprecipitation-mass spectrometry to identify RPA1a (Replication Protein A 1) as a HEI10 interacting protein. Surprisingly, we find that RPA1a also undergoes phase separation and its ubiquitination and degradation are directly regulated by HEI10. We also show that HEI10 is required for the condensation of other class I CO factors. Thus, our results provide mechanistic insight into how meiotic class I CO formation is controlled by HEI10 coupling LLPS and ubiquitination.

DNA polymerase ε harmonizes topological states and R-loops formation to maintain genome integrity in Arabidopsis

Genome topology is tied to R-loop formation and genome stability. However, the regulatory mechanism remains to be elucidated. By establishing a system to sense the connections between R-loops and genome topology states, we show that inhibiting DNA topoisomerase 1 (TOP1i) triggers the global increase of R-loops (called topoR-loops) and DNA damages, which are exacerbated in the DNA damage repair-compromised mutant atm. A suppressor screen identifies a mutation in POL2A, the catalytic subunit of DNA polymerase ε, rescuing the TOP1i-induced topoR-loop accumulation and genome instability in atm. Importantly we find that a highly conserved junction domain between the exonuclease and polymerase domains in POL2A is required for modulating topoR-loops near DNA replication origins and facilitating faithful DNA replication. Our results suggest that DNA replication acts in concert with genome topological states to fine-tune R-loops and thereby maintain genome integrity, revealing a likely conserved regulatory mechanism of TOP1i resistance in chemotherapy for ATM-deficient cancers.

Comprehensive dissection of meiotic DNA double-strand breaks and crossovers in cucumber

Meiotic recombination drives genetic diversity and crop genome optimization. In plant breeding, parents with favorable traits are crossed to create elite varieties. Different hybridizations produce diverse types of segment reshuffling between homologous chromosomes. However, little is known about the factors that cause hybrid-specific changes in crossovers (COs). Here, we constructed 2 F2 populations from crosses between a semiwild and 2 domesticated cucumber (Cucumis sativus) accessions and examined CO events. COs mainly occurred around genes and differed unevenly along chromosomes between the 2 hybrids. Fine-scale CO distributions were suppressed in regions of heterozygous structural variations (SVs) and were accelerated by high sequence polymorphism. C. sativus RADiation sensitive 51A (CsRAD51A) binding, histone H3 lysine 4 trimethylation (H3K4me3) modification, chromatin accessibility, and hypomethylation were positively associated with global CO landscapes and in local DNA double-strand break (DSB) hotspots and genes. The frequency and suppression of COs could be roughly predicted based on multiomic information. Differences in CO events between hybrids could be partially traced to distinct genetic and epigenetic features and were significantly associated with specific DSB hotspots and heterozygous SVs. Our findings identify the genomic and epigenetic features that contribute to CO formation and hybrid-specific divergence in cucumber and provide theoretical support for selecting parental combinations and manipulating recombination events at target genomic regions during plant breeding.

Crossover interference mechanism: New lessons from plants

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