Endogenous level of abscisic acid down-regulated by brassinosteroids signaling via BZR1 to control the growth of .

The increased level of endogenous abscisic acid (ABA) in brassinosteroid (BR)-deficient mutants, such as and ×, suggests that ABA synthesis is inhibited by endogenous BRs in . Expression of the ABA biosynthesis gene () was negatively regulated by exogenously applied BR but up-regulated by the application of brassinazole and in and ×. In addition, expression decreased in , showing that ABA biosynthesis is inhibited by BR signaling via BZR1, intermediated by , in . Four -element sequences (E-boxes 1-4) in the putative promoter region of were identified as BZR1 binding sites. The electrophoretic mobility shift assay and chromatin immune precipitation analysis demonstrated that BZR1 directly binds to overlapped E-boxes (E-box 3/4) in the promoter region of . The level of endogenous ABA was decreased in compared to wild-type, indicating that binding of BZR1 to the promoter inhibits ABA synthesis in . Compared to wild-type, exhibited severely reduced growth and development. The abnormalities in were rescued by the application of ABA, suggesting that expression and ABA synthesis are necessary for the normal growth and development of . Finally, × exhibited inhibitory growth of primary roots compared to , verifying that is a downstream target of BZR1 in the plant. Taken together, the level of endogenous ABA is down-regulated by BR signaling via BZR1, controlling the growth of .