Mitochondrial proteotoxic stresses activate abscisic acid signaling in plants

The mitochondrial proteome is composed of both mitochondrial DNA (mtDNA)-encoded and nuclear DNA (nuDNA)-encoded proteins. The mitonuclear protein imbalance leads to the occurrence of mitochondrial pro-teotoxic stresses. As a response to the mitochondrial proteotoxic stress, mitochondrial unfolded protein response (UPRmt) is induced in cells to restore the mitochondrial functions. Although UPRmt has been extensively studied in animals, much less is known about the mechanisms of UPRmt signaling in plants. We report here that a DNA intercalating agent, ethidium bromide (EtBr) triggers UPRmt in Arabidopsis thaliana, with characteristic features similar to those caused by the mitochondrial translation inhibitor doxycycline (Dox). EtBr/Dox treatments activate abscisic acid (ABA) signaling. Then ABA promotes the expression of UPRmt genes, like alternative oxidase 1a (AOX1a). A key regulator of ABA signaling, ABA insensitive 5 (ABI5) directly binds to the promoter ofAOX1a, which enhances AOX1a gene transcription. Our study reveals a link between ABA and UPRmt in plants.