Heat Shock Factor HSFA6b Mediates Mitochondrial Unfolded Protein Response in Arabidopsis thaliana

Mitochondria are important organelles in eukaryotes and are involved in various metabolic processes. Mitochondrial proteotoxic stress triggers the mitochondrial unfolded protein response (UPR^mt) to restore mitochondrial protein homeostasis and maintain normal life activities. However, the regulatory mechanism of plant UPR^mt remains to be revealed in Arabidopsis. Based on the fact that UPR^mt activates heat shock protein (HSP) genes, we identified the heat shock transcription factor HSFA6b as a key regulator mediating UPR^mt through reverse genetics. HSFA6b responded to mitochondrial proteotoxic stress and regulated mitochondrial heat shock proteins' genes' (mtHSPs) expression. HSFA6b translocated to the nuclear after treatment with doxycycline (Dox)-a mitochondrial ribosome translation inhibitor. HSFA6b binds to the mtHSPs promoters and activates mtHSPs expression. The HSFA6b mutation blocked the UPR^mt signals to promote root growth under mitochondrial proteotoxic stress and accelerated leaf senescence during development. Our study reveals a novel signal-regulating mechanism in the UPR^mt pathways and provides new insights regarding the regulation of plant growth and development and stress resistance by the UPR^mt pathways.