ABSTRACT

The increased level of endogenous abscisic acid (ABA) in brassinosteroid (BR)-deficient mutants, such as det2 and cyp85a1 × cyp85a2, suggests that ABA synthesis is inhibited by endogenous BRs in Arabidopsis thaliana. Expression of the ABA biosynthesis gene ABA-deficient 2 (ABA2) was negatively regulated by exogenously applied BR but up-regulated by the application of brassinazole and in det2 and cyp85a1 × cyp85a2. In addition, ABA2 expression decreased in bzr1-1D, showing that ABA biosynthesis is inhibited by BR signaling via BZR1, intermediated by ABA2, in Arabidopsis. Four cis-element sequences (E-boxes 1–4) in the putative promoter region of ABA2 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 ABA2. The level of endogenous ABA was decreased in bzr1-1D compared to wild-type, indicating that binding of BZR1 to the ABA2 promoter inhibits ABA synthesis in Arabidopsis. Compared to wild-type, aba2-1 exhibited severely reduced growth and development. The abnormalities in aba2-1 were rescued by the application of ABA, suggesting that ABA2 expression and ABA synthesis are necessary for the normal growth and development of A. thaliana. Finally, bzr1-KO × aba2-1 exhibited inhibitory growth of primary roots compared to bzr1-KO, verifying that ABA2 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 A. thaliana.

 抽象的

油菜素类固醇 (BR) 缺陷突变体（例如det2和cyp85a1 × cyp85a2 ）中内源脱落酸 (ABA) 水平增加，表明拟南芥中 ABA 合成受​​到内源 BR 的抑制。 ABA 生物合成基因ABA-deficient 2 ( ABA2 ) 的表达受到外源应用 BR 的负向调节，但通过应用黄铜硝唑以及det2和cyp85a1 × cyp85a2的表达上调。此外， bzr1-1D中的ABA2表达降低，表明拟南芥中 ABA 生物合成受到 BZR1 的 BR 信号传导（由ABA2介导）的抑制。 ABA2推定启动子区域中的四个顺式元件序列（E 盒 1-4）被鉴定为 BZR1 结合位点。电泳迁移率变动分析和染色质免疫沉淀分析表明，BZR1 直接结合ABA2启动子区域中重叠的 E-box（E-box 3/4）。与野生型相比， bzr1-1D中的内源 ABA 水平降低，表明 BZR1 与ABA2启动子的结合抑制了拟南芥中的 ABA 合成。与野生型相比， aba2-1的生长和发育严重降低。 ABA的应用可以挽救aba2-1的异常，表明ABA2的表达和ABA的合成是拟南芥正常生长发育所必需的。 最后，与bzr1-KO相比， bzr1-KO × aba2-1表现出初生根的抑制生长，证实ABA2是植物中 BZR1 的下游靶标。总之，内源 ABA 的水平通过 BZR1 被 BR 信号传导下调，从而控制拟南芥的生长。