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

摘要

The brassinosteroids (BRs) 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. Ethylene synthesis is highly increased in the det2-9 mutant compared with the wild type, resulting in the hyper-accumulation of ethylene and the consequent 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 of BR showed that BRs either positively or negatively regulate ethylene biosynthesis in a concentration-dependent 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 (O_(2)~(-)) compared with the wild-type control, and the increased O_(2)~(-)level was shown to contribute to the inhibition of root growth. The BR-modulated control over the accumulation of O_(2)~(-)acted via the peroxidase pathway rather than via the NADPH oxidase pathway. This study reveals an important mechanism by which the hormone cross-regulation between BRs and ethylene or/and ROS is involved in controlling root growth and development in Arabidopsis . Author summary Both brassinosteroids (BRs) and ethylene have been known to control root growth and development. ROS have been also reported to play an important role in root development. However, the relationship between BRs and ethylene or ROS in root growth and development was not addressed before. In this study, a det2-9 mutant defective in BR synthesis was identified from an EMS mutant screening, displaying a short-root phenotype which is result from the hyper-accumulation of ethylene and superoxide anions (O_(2)~(-)). Exogenous BR apply showed that BRs either positively or negatively regulate ethylene biosynthesis in a concentration-dependent manner. Different from the BR induced ethylene biosynthesis through stabilizing ACSs stability, we found that the BR signaling transcription factors BES1 and BZR1 interacted with promoters of ACS7 , ACS9 and ACS11 to repress their expression, indicating a native regulation mechanism under physiological levels of BR. The BR-modulated control over the accumulation of O_(2)~(-)acted via the peroxidase pathway rather than via the NADPH oxidase pathway. This study provides new insights into how brassinosteroids control root growth through the cross-regulation with ethylene synthesis and ROS.