Heterochromatin,being transcriptionally inactive,is the highly condensed form of chromatin that is associated with Histone H3 lysine 9 methylation (H3K9me) and DNA methylation in eukaryotic cells.In fission yeast and animals,Heterochromatin Protein 1 (HP1) plays a major role in forming and maintaining heterochromatin.HP1 has an N-terminal chromodomain that recognizes H3K9me3,and a C-terminal chromo shadow domain that mediates protein-protein interactions.In contrast,the Arabidopsis homolog of HP1,Like Heterochromatin Protein1 (LHP1),also known as TERMINAL FLOWER 2,recognizes and specifically co-localizes with chromatin regions enriched with H3K27me3 instead of H3K9me3,acting as a functional homolog of Polycomb protein (Wang et al.,2018).It is known that different from yeast and animals,H3K9me2,but not H3K9me3,is one of the epigenetic marks enriched at heterochromatin regions and is required for the silencing of transposable elements and other repetitive DNA in plants (Wendte and Schmitz,2018).However,it has remained a mystery for a long time whether plants encode genuine HP1 homologs.
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