Heterochromatin Protein 1 (HP1) recognizes histone H3 lysine 9 methylation (H3Kgme) through its conserved chromodomain and maintains heterochromatin from fission yeast to mammals.However,in Arabidopsis,Like Heterochromatin Protein 1 (LHP1) recognizes and colocalizes genome-wide with H3K27me3,and is the functional homolog of Polycomb protein.This raises the question whether genuine HP1 homologs exist in plants.Here,we report on the discovery of ADCP1,a plant-specific triple tandem Agenet protein,as a multivalent H3Kgme reader in Arabidopsis,and establish that ADCP1 is essential for heterochromatin formation and transposon silencing through modulating H3K9 and DNA methylation levels.Structural studies revealed the molecular basis underlying H3K9me-specific recognition by tandem Agenet of ADCP1.Similar to human HP1α and fly HP1a,ADCP1 mediates heterochromatin phase separation.Our results demonstrate that despite its distinct domain compositions,ADCP1 convergently evolves as an HP1-equivalent protein in plants to regulate heterochromatin formation.