ATXR5 and ATXR6 restrict meiotic crossover formation within heterochromatin in Arabidopsis

Meiotic crossover (CO) exchanges genetic information between homologs, thereby promoting genetic diversity among offspring. COs are non-randomly distributed across chromosomes, tending to occur in euchromatin, but rarely in heterochromatin. In plants, H3 lysine 27 monomethylation (H3K27me1) is crucial for maintaining heterochromatin condensation and genome stability in somatic cells; however, its role in germline cells remains to be determined. Here, we demonstrate that the plant-specific H3K27 mono-methyltransferases ATXR5/6 (ARABIDOPSIS TRITHORAX-RELATED PROTEIN 5/6) play an important role in inhibiting CO formation in meiotic heterochromatin. In atxr5 atxr6, both ZMM-dependent Type I COs and ZMM-independent Type II COs are significantly increased. We further observed decondensation, decreased H3K27me1 signals, and specifically compromised non-CG methylation in atxr5 atxr6 meiotic heterochromatin. Unexpectedly, in contrast to their roles in somatic cells, where ATXR5/6 primarily regulate heterochromatin condensation and gene silencing without influencing DNA methylation, in meiocytes, ATXR5/6 mainly function in suppressing recombination and preserving heterochromatic DNA methylation without directly regulating gene expression. Moreover, loss of Type II CO regulator MMS AND UV SENSITIVE 81 (MUS81) leads to pericentromeric fragmentation and polyad formation during meiosis in the absence of ATXR5/6, indicating that MUS81 is critical for resolving atypical recombination intermediates in pericentromeric heterochromatin. Taken together, our results provide insights into the roles of ATXR5/6 in repressing meiotic recombination within heterochromatin by regulating chromosome compaction and modifications.