Abstract
Brassinosteroid (BR) is known as the phytohormone that regulates plant height, although the regulatory system is not fully understood. To identify the novel regulator that control plant height, we screened the Arabidopsis CRES-T lines and found that the transgenic plants that express the chimeric repressor of HR0444 (HR0444SRDX) exhibited dwarf and dark green color phenotype. In HR0444SRDX plants, length of the each cell was much shorter than that of wild type, and the expression of BR inducible genes were decreased. In addition, HR0444SRDX plants were insensitive for BR treatment. Pro35S:HR0444 transgenic plants had similar phenotype to HR0444SRDX plants, and HR0444 does not show repressive activity. These results suggest that HR0444 act as a passive repressor. To approach further mechanism, we isolated transcriptional activators that interact with HR0444. The activators induce cell elongation when it was expressed ectopically. On the other hand, the expression of HR0444 was repressed by BZR1. These results indicate that BR controls the cell size via the repression of the expression of HR0444 that interacts and suppresses the activation activity of cell elongation activators.
