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

Plant cells integrate signals from external sources and from organelles to regulate gene expression, referred to as anterograde andretrograde signaling, respectively. Functional characterization of the promoter of ALTERNATIVE OXIDASE1a (AOX1a) fromArabidopsis (Arabidopsis thaliana), a marker for mitochondrial retrograde response, was carried out by testing the ability of theAOX1a promoter to drive expression of the reporter gene GUS. This approach identified a strong repressor element, designatedthe B element, that was necessary for an increased promoter activity in response to the mitochondrial complex I inhibitorrotenone. This element overlaps with a previously identified potential binding site for the transcription factor ABSCISIC ACIDINSENSITIVE4 (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 wildtypepromoter in transgenic plants. Binding of the ABI4 transcription factor to this region of the AOX1a promoter wasdemonstrated by electromobility shift and yeast one-hybrid assays. Analysis of transcript abundance for AOX1a in abi4 mutantlines revealed significantly increased levels of AOX1a mRNA that could not be further induced by rotenone, consistent with therole of ABI4 as a repressor that is derepressed in response to rotenone. These results show that ABI4 plays a central role inmediating mitochondrial retrograde signals to induce the expression of AOX1a. Furthermore, they provide a molecular linkbetween mitochondrial and chloroplast retrograde signaling, as ABI4 has been previously shown to act downstream of at leasttwo chloroplast retrograde signaling pathways.