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

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