There is provided a polynucleotide encoding a plant gene capable of controlling a signal transduction system for brassinosteroid hormone, the polynucleotide encoding an amino acid sequence from Met at position 1 to Arg at position 1057 of SEQ ID NO: 2 in the SEQUENCE LISTING, including any polynucleotide encoding an amino acid sequence in which one or more amino acids are deleted, substituted or added to the amino acid sequence.