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The juvenile-to-adult transition in plants involves changes in vegetative growth and plant architecture; the timing of this transition has important implications for ag-riculture. The microRNA miR156 regulates this transition and shoot maturation in plants. In Arabidopsis thaliana, deposi-tion of histone H3 trimethylation on lysine 27 (H3K27me3, a repressive mark) at the MIR156A/C loci is regulated by Pol-ycomb Repressive Complex 1 (PRC1) or PRC2, depending on the developmental stage. The levels of miR156 progressively decline during shoot maturation. The amount of H3K27me3 at MIR156A/C loci affects miR156 levels; however, whether this epigenetic regulation is conserved remains unclear. Here, we found that in rice (Oryza sativa), the putative PRC1 subunit LIKE HETEROCHROMATIN PROTEIN 1 (OsLHP1), with the miR156–SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) module, affects developmental phase transitions. Loss of OsLHP1 function results in ectopic ex-pression of MIR156B/C/I/E, phenocopy of miR156 over-expression, and reduced H3k27me3 levels at MIR156B/C/I/E. This indicates that OsLHP1 has functionally diverged from Arabidopsis LHP1. Genetic and transcriptome analyses of wild-type, miR156b/c-overexpression, and Oslhp1-2 mutant plants suggest that OsLHP1 acts upstream of miR156 and SPL during the juvenile-to-adult transition. Therefore, mod-ifying the OsLHP1–miR156–SPL pathway may enable alter-ation of the vegetative period and plant architecture.