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

Bingsheng Lv,Mingyi Bai,Feng Zhang,Jiajia Liu,Zhaojun Ding,Huiyu Tian,Chuanyou Li,Songchong Lu,Gloria K Muday

doi:10.1371/journal.pgen.1007144

Abstract

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 (O2-) 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 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.

Highlights

Roots are important plant ground organs, which absorb water and nutrients to control plant growth and development
A det2-9 mutant defective in BR synthesis was identified from an ethyl methane sulfonate (EMS) mutant screening, displaying a short-root phenotype which is result from the hyper-accumulation of ethylene and superoxide anions (O2-)
Different from the BR induced ethylene biosynthesis through stabilizing ACC synthase (ACS) 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

Summary

Introduction

Roots are important plant ground organs, which absorb water and nutrients to control plant growth and development. Recent studies in the Arabidopsis root have shown that different hormones control organ growth by regulating specific growth processes such as cell proliferation, differentiation or expansion in distinct tissues. Plant hormones such as auxin, cytokinin, abscisic acid, brassinosteroids, ethylene and gibberellins have been shown to be involved in root growth through a range of complex interactions. In Arabidopsis, cell division and cell differentiation largely determines root meristem size, which is under the control of cytokinin and auxin through an ARR1/ SHY2/PIN circuit [1] All these studies suggest that hormonal cross-talk plays a pivotal role in the regulation of root growth

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