The present invention relates to a novel protein interacting with phytochromes and use thereof, and more particularly, to a polypeptide having either an amino acid sequence set forth in SEQ ID NO: 4 or an amino acid sequence having at least 70% with said amino acid sequence, and use thereof. The polypeptide interacts with phytochromes A and B, and the TPR domain present at the N-terminal region of the polypeptide is involved in the interaction. Also, a PP2A catalytic domain (PP2Ac) having phosphatase activity is present at the C-terminal region of the polypeptide. The polypeptide can be used as a phosphatase, and is useful in the production of plants sensitive to light signal transduction. Furthermore, the TPR domain present in the polypeptide is useful in the production of dwarf plants.