Brassinosteroids regulate root growth by controlling reactive oxygen species homeostasis and dual effect on ethylene synthesis in Arabidopsis

The brassinosteroids (BRs; 油菜素内酯) represent a class of phytohormones, which regulate numerous aspects of growth and development. Here, a det2-9 mutant defective in BR synthesis was identified from an EMS mutant screening for defects in root length, and was used to investigate the role of BR in root development in Arabidopsis. The det2-9 mutant displays a short-root phenotype, which is result from the reduced cell number in root meristem and decreased cell size in root maturation zone (根的成熟区). Ethylene synthesis is highly increased in the det2-9 mutant compared with the wild type, resulting in the hyper-accumulation (超积累) of ethylene and the consequent (随之而来的) inhibition of root growth. The short-root phenotype of det2-9 was partially recovered in the det2-9/acs9 double mutant and det2-9/ein3/eil1-1 triple mutant which have defects either in ethylene synthesis or ethylene signaling, respectively. Exogenous application (外施) of BR showed that BRs either positively or negatively regulate ethylene biosynthesis in a concentration-dependent (浓度依赖性) manner. Different from the BR induced ethylene biosynthesis through stabilizing ACSs stability, we found that the BR signaling transcription factors BES1 and BZR1 directly interacted with the promoters of ACS7, ACS9 and ACS11 to repress their expression, indicating a native regulation mechanism under physiological levels of BR. In addition, the det2-9 mutant displayed over accumulated superoxide anions (O2-; 超氧阴离子) compared with the wild-type control, and the increased O2- level was shown to contribute to the inhibition of root growth. The BR-modulated control over the accumulation of O2- acted (起作用) via the peroxidase pathway (过氧化物酶途径) rather than via the NADPH oxidase pathway (NADPH氧化酶途径). This study reveals an important mechanism by which the hormone cross-regulation (交叉调节) between BRs and ethylene or/and ROS is involved in controlling root growth and development in Arabidopsis.

油菜素内酯（BRs）是一类调控多个植物生长和发育的植物激素。本文从筛选拟南芥根长度缺陷的EMS突变体库中鉴定到了一个det2-9突变体，其在BR的合成上存在缺陷，该突变体被进一步用来研究BR在根发育中的作用。det2-9突变体表现出短根的表型，该表型是由于根分生组织的细胞数量减少和根成熟区细胞尺寸的减小。与野生型相比，det2-9突变体中的乙烯合成高度增加，导致了乙烯的过量积累，进而抑制了根的生长。det2-9突变体的短根表型可以被det2-9/acs9双突和det2-9/ein3/eil1-1三突部分恢复，这两种突变体分别在乙烯的合成和信号转导方面存在缺陷。外施BR试验显示BR正向或反向调控乙烯的合成取决于外施BR的浓度。不同于BR通过稳定ACSs的稳定性来诱导乙烯的生物合成，本文发现BR信号转录因子BES1和BZR1可以直接结合到ACS7，ACS9和ACS11基因的启动子区以抑制它们的表达，表明在BR的生理水平下天然的调控机制。另外，相对于野生型，det2-9突变体还表现出超氧阴离子的过量积累，并且超氧阴离子水平的增加会导致根生长的抑制。BR控制超氧阴离子的积累主要通过过氧化物酶途径，而不是NADPH氧化酶途径。本文的研究揭示了拟南芥中BR和乙烯以及ROS之间的激素交叉调节控制根的生长和发育。

通讯：丁兆军 (http://www.lifesci.sdu.edu.cn/info/1062/1658.htm)

个人简介：1998年，聊城大学，学士；2003年，中国科学院植物研究所，博士；2004.2-2011.8，德国马普植物育种所和比利时根特大学，博士后。

doi: https://doi.org/10.1371/journal.pgen.1007144