The Transcription Factor ABI4 Is a Regulator of Mitochondrial Retrograde Expression of ALTERNATIVE OXIDASE1a

Estelle Giraud,Olivier Van Aken,Lois H.M Ho,James Whelan

doi:10.1104/pp.109.139782

Abstract

Plant cells integrate signals from external sources and from organelles to regulate gene expression, referred to as anterograde and retrograde signaling, respectively. Functional characterization of the promoter of ALTERNATIVE OXIDASE1a (AOX1a) from Arabidopsis (Arabidopsis thaliana), a marker for mitochondrial retrograde response, was carried out by testing the ability of the AOX1a promoter to drive expression of the reporter gene GUS. This approach identified a strong repressor element, designated the B element, that was necessary for an increased promoter activity in response to the mitochondrial complex I inhibitor rotenone. This element overlaps with a previously identified potential binding site for the transcription factor ABSCISIC ACID INSENSITIVE4 (ABI4). AOX1a promoter activity was fully derepressed in abi4 mutants and was unresponsive to rotenone. Furthermore, deletion of the B element of the AOX1a promoter resulted in increased GUS staining activity compared to the wild-type promoter in transgenic plants. Binding of the ABI4 transcription factor to this region of the AOX1a promoter was demonstrated by electromobility shift and yeast one-hybrid assays. Analysis of transcript abundance for AOX1a in abi4 mutant lines revealed significantly increased levels of AOX1a mRNA that could not be further induced by rotenone, consistent with the role of ABI4 as a repressor that is derepressed in response to rotenone. These results show that ABI4 plays a central role in mediating mitochondrial retrograde signals to induce the expression of AOX1a. Furthermore, they provide a molecular link between mitochondrial and chloroplast retrograde signaling, as ABI4 has been previously shown to act downstream of at least two chloroplast retrograde signaling pathways.

Highlights

Plant cells integrate signals from external sources and from organelles to regulate gene expression, referred to as anterograde and retrograde signaling, respectively
The role of the B element was examined by deletion of this element located 21,589 to 21,594 nucleotides upstream of the transcriptional start site in the ALTERNATIVE OXIDASE1a (AOX1a)::GUS reporter construct, and the biolistic transformation assays demonstrated a 3-fold increase in promoter activity compared to the wild-type promoter (Fig. 1, A and B)
GUS staining of plants driven by the wild-type AOX1a promoter was barely detectable under normal conditions (Fig. 2), whereas deleting the B element resulted in plants with strong GUS staining patterns (Fig. 2)

Summary

Introduction

Plant cells integrate signals from external sources and from organelles to regulate gene expression, referred to as anterograde and retrograde signaling, respectively. Functional characterization of the promoter of ALTERNATIVE OXIDASE1a (AOX1a) from Arabidopsis (Arabidopsis thaliana), a marker for mitochondrial retrograde response, was carried out by testing the ability of the AOX1a promoter to drive expression of the reporter gene GUS This approach identified a strong repressor element, designated the B element, that was necessary for an increased promoter activity in response to the mitochondrial complex I inhibitor rotenone. Analysis of transcript abundance for AOX1a in abi mutant lines revealed significantly increased levels of AOX1a mRNA that could not be further induced by rotenone, consistent with the role of ABI4 as a repressor that is derepressed in response to rotenone These results show that ABI4 plays a central role in mediating mitochondrial retrograde signals to induce the expression of AOX1a. As ABI4 was originally characterized to play a role in abscisic acid (ABA) signaling (Finkelstein et al, 1998) and is involved in both sugar and developmental signaling (Rook et al, 2006), it can act as a point of convergence for both retrograde and anterograde signals

Methods

Results

Conclusion