Dynamic evolution of the heterochromatin sensing histone demethylase IBM1

Heterochromatin constitutes a fundamental aspect of genomes that is crucial for maintaining genome stability. In flowering plants, maintenance of heterochromatin relies on a positive feedback loop involving the histone 3 lysine nine methyltransferase (H3K9), KRYPTONITE (KYP), and the DNA methyltransferase, CHROMOMETHYLASE3 (CMT3). An H3K9 demethylase, INCREASED IN BONSAI METHYLATION 1 (IBM1), has evolved to modulate the activity of KYP-CMT3 within transcribed genes. The absence of IBM1 activity results in aberrant methylation of gene bodies, which is deleterious. This study demonstrates extensive genetic and gene expression variations in KYP, CMT3, and IBM1 within and between flowering plant species. IBM1 activity in Arabidopsis thaliana is uniquely regulated by the abundance of H3K9me2 in a repetitive sequence within an intron preceding the histone demethylase domain. This mechanism enables IBM1 to monitor global levels of H3K9me2. We discovered that the methylated intron is prevalent across flowering plants, however, its underlying sequence exhibits dynamic evolution. Its absence in species lacking gene body DNA methylation suggests its primary role in sensing H3K9me2 and preventing its integration into these constitutively expressed genes. Furthermore, our investigation uncovered Arabidopsis thaliana accessions resembling weak ibm1 mutants, several Brassicaceae species with reduced IBM1 expression, and an IBM1 deletion. Evolution towards reduced IBM1 activity in some flowering plants could explain the frequent natural occurrence of diminished or lost CMT3 activity, as cmt3 mutants in A. thaliana mitigate the deleterious effects of IBM1. ### Competing Interest Statement The authors have declared no competing interest.