The plant photoreceptor phytochrome B (phyB) mediates the responses of plants to red (R) light. Trimethylation of histone H3 at Lys27 (H3K27me3) plays a crucial role in governing gene expression and controlling the response of plants to environmental changes. However, how dynamic H3K27me3 mediates plant response to R light is poorly understood. Here, we report that RELATIVE OF EARLY FLOWERING 6 (REF6), an H3K27me3 demethylase, promotes hypocotyl elongation under R light in