SDG8 and HUB2 depositing euchromatin histone marks play important roles in meiosis and crossing-over regulation

Histone modifications play critical roles in plant growth and development. Crossing-over (CO) during meiosis, which constitutes a fundamental process ensuring sexual transmission of genetic material to the next generation and, meanwhile, generating diversity within species by creating new chromosome/allele combinations, occurs predominantly in euchromatin, which is enriched in active histone marks such as H3K4me3, H3K36me3, and H2Bub1. In plants, it is known that CO hotspots are correlated with H3K4me3 but the role of H3K36me3 and H2Bub1 during meiosis remains elusive so far. Here, we studied the Arabidopsis (Arabidopsis thaliana) sdg8-1 and hub2-2 mutants impeded in depositing H3K36me3 and H2Bub1, respectively. Chromosome spreading using 4',6-diamidino-2-phenylindole (DAPI) staining indicated that male meiotic stages are defective in the sdg8-1 mutant, and the defect increases synergistically in the sdg8-1hub2-2 double mutant. Defects in meiosis, seed formation, and silique length were also observed by RNAi-knockdown of SDG8 using the meiosis-specific gene DMC1 promoter. This corroborates to support a bona fide role of active histone marks during meiosis and plant reproduction. Using the tetrad-based visual reporter lines and immunostaining with antibodies against HEI10 and ZYP1, it was found that synapsis and pairing of homologous chromosomes are abnormal and CO rate increases in sdg8 mutants, pointing to a repressive role of SDG8 in Arabidopsis male meiotic homologous recombination.