Arabidopsis homeodomain transcription factor BRASSINOSTEROID-RELATED HOMEOBOX 1 represses BR responses under stress conditions

The brassinosteroid (BR) phytohormone is an important regulator of plant growth and stress responses. To identify novel transcription factors that regulate BR responses, we screened chimeric repressor gene silencing technology (CRES-T) plants, in which transcription factors were converted into chimeric repressors by the fusion of SRDX plant-specific repression domain, with brassinazole (Brz), an inhibitor of BR biosynthesis. We found a chimeric repressor of homeobox transcription factor, BRASSINOSTEROID-RELATED-HOMEOBOX-1 (BHB1-sx) induced a Brz-insensitive phenotype with longer hypocotyl, slender dwarf, and epinastic leaves and the expression of BR biosynthesis and inducible genes was enhanced in BHB1-sx. In contrast, ectopic expression of BHB1 (BHB1-ox) resulted in a Brz-hypersensitive phenotype with short hypocotyl, dwarf, and hyponastic leaves and the expression of BR associated genes was repressed in BHB1-ox. Transcriptome analysis revealed that the expression of auxin-related genes was downregulated, whereas that of stress-related genes were upregulated in BHB1-ox plants, indicating that BHB1 negatively regulates BR responses. The expression of BHB1 was induced by BR and stress condition, and BHB1-ox showed higher tolerance to salt stress, whereas bhb1 showed lower tolerance compared to the wild type. BHB1 may function as a hub factor mediating the crosstalk between salt stress and BR signaling.