Unveiling the Mitochondrial Unfolded Protein Response (UPRmt) in Arabidopsis thaliana

Mitochondria are key organelles involved in many cellular processes, from energy production to stress sensing and response. To maintain these crucial functions is necessary to maintain mitochondria proteostasis, which is usually done by an intricate network of proteases and chaperons constituting mitochondrial protein quality control (mtPQC). When damaged proteins accumulate beyond the capacity of the mtPQC, a specific response known as the mitochondrial unfolded protein response (UPRmt) is activated. UPRmt is a mitochondrial stress response by which stressed mitochondria promote the upregulation of nuclear protective genes to restore mitochondrial functionality. Despite being conversed in all eukaryotes, UPRmt is poorly understood in plants. In this Ph.D. thesis, we investigated proteotoxic stress and UPRmt in Arabidopsis thaliana. An efficient protocol of induction, using the antibiotic Doxycycline (DOX) has been set up and proven effective in inducing UPRmt in Arabidopsis seedlings, demonstrated by the inhibition of plant development and upregulation of UPRmt marker genes (AOX1a, mtHSC70-1, and mtHSC70-5). Moreover, phenotypic and molecular analysis of knock-out mutant lines identified two mutants, ftsh4 and mrpl1-3, exhibiting constitutively altered proteostasis and active UPRmt. Microscopy observations showed that proteotoxic damage heavily affects mitochondria morphology and dynamics, resulting in an active reorganization of mitochondrial populations to face the stress. Investigation of cytosolic and mitochondrial calcium dynamics showed disrupted dynamics in mutants and in conditions of disrupted mitochondrial proteostasis, suggesting calcium's involvement in signaling during the response. We investigated the role of UPRmt in physiological stress responses by subjecting Arabidopsis seedlings to heat-shock treatment demonstrating elevated expression of stress-responsive genes and increased thermotolerance in mutants, emphasizing the protective effect of UPRmt and mitochondrial proteotoxic damage. The crosstalk between chloroplasts and mitochondria during chloroplast proteotoxic stress has also been carried out in WT and gun1 mutants treated with LIN. Alterations in chloroplast proteostasis did not induce changes in mitochondrial morphology however were effective in altering respiratory activity and AOX activity in both genotypes suggesting an interconnected response between chloroplast functionality and mitochondrial activity in conditions of proteotoxic damage.