The Short-Rooted Phenotype of thebrevis radixMutant Partly Reflects Root Abscisic Acid Hypersensitivity

Americo Rodrigues,Christian S Hardtke,Pedro L Rodriguez,Silvia Rubio,Angela Saez,Jose Gadea,Karen S Osmont,Julia Santiago

doi:10.1104/pp.108.133819

Abstract

To gain further insight into abscisic acid (ABA) signaling and its role in growth regulation, we have screened for Arabidopsis (Arabidopsis thaliana) mutants hypersensitive to ABA-mediated root growth inhibition. As a result, we have identified a loss-of-function allele of BREVIS RADIX (BRX) in the Columbia background, named brx-2, which shows enhanced response to ABA-mediated inhibition of root growth. BRX encodes a key regulator of cell proliferation and elongation in the root, which has been implicated in the brassinosteroid (BR) pathway as well as in the regulation of auxin-responsive gene expression. Mutants affected in BR signaling that are not impaired in root growth, such as bes1-D, bzr1-D, and bsu1-D, also showed enhanced sensitivity to ABA-mediated inhibition of root growth. Triple loss-of-function mutants affected in PP2Cs, which act as negative regulators of ABA signaling, showed impaired root growth in the absence of exogenous ABA, indicating that disturbed regulation of ABA sensitivity impairs root growth. In agreement with this result, diminishing ABA sensitivity of brx-2 by crossing it with a 35S:HAB1 ABA-insensitive line allowed significantly higher recovery of root growth after brassinolide treatment. Finally, transcriptomic analysis revealed that ABA treatment negatively affects auxin signaling in wild-type and brx-2 roots and that ABA response is globally altered in brx-2. Taken together, our results reveal an interaction between BRs, auxin, and ABA in the control of root growth and indicate that altered sensitivity to ABA is partly responsible for the brx short-root phenotype.

Highlights

Abscisic acid (ABA) is a universal stress hormone of higher plants that plays a key role as a regulator of growth and meristem function and in different plant developmental processes, such as embryo development, germination, vegetative development, flowering and organogenesis (Xu et al, 1998; Finkelstein et al, 2002; Barrero et al, 2005; Razem et al, 2006; de Smet et al, 2006; Liang et al, 2007)
Taking into account the root phenotype found in triple pp2c mutants, we reasoned that the enhanced ABA sensitivity of brx-2 might prevent a better rescue of its root growth defect by BL
The enhanced ABA response of brx-2 was specific for the root, as ABA-mediated inhibition of seed germination and water loss kinetics were similar to wild type

Summary

Introduction

Abscisic acid (ABA) is a universal stress hormone of higher plants that plays a key role as a regulator of growth and meristem function and in different plant developmental processes, such as embryo development, germination, vegetative development, flowering and organogenesis (Xu et al, 1998; Finkelstein et al, 2002; Barrero et al, 2005; Razem et al, 2006; de Smet et al, 2006; Liang et al, 2007) Both positive and negative effects of ABA on growth and development have been reported depending on tissue, concentration and interaction with the environment (Zeevaart and Creelman, 1988; Thompson et al, 2007). Our finding of brx-2 as hypersensitive to ABA-mediated inhibition of root growth reveals a novel phenotype for this mutant and suggests that BRX and BRs play important roles in modulating root response to ABA. This is corroborated by analysis of other mutants in the BR signaling pathway. Our results reveal a cross-talk between ABA and BRs for root growth control and suggest that a normal response to BRs is required to prevent enhanced sensitivity to ABA-mediated inhibition of root growth

RESULTS

DISCUSSION

Findings

MATERIALS AND METHODS