Cytosolic Class I J-domain proteins aid mitochondrial protein import and influence homeostasis in Arabidopsis thaliana.

Mitochondrial homeostasis heavily relies on import of numerous nuclear encoded proteins. Multiple chaperone machineries operate at the cytosolic and mitochondrial side of the bilayer to ensure the efficacy of this process. While the mechanisms and components involved in translocation of polypeptides into the mitochondria are known in great detail, the cytosolic events of mitochondrial protein import are poorly understood in plants. This study explores the role of two cytosolic Class I J-domain proteins (JDPs), atDjA1 and atDjA2, in mitochondrial homeostasis in Arabidopsis thaliana. We show that atDjA1 and atDjA2 uniquely interacted with outer mitochondrial surface receptors, Tom9, Metaxin, and OM64, which guide the import of numerous mitochondrial precursors. Knockout of atDjA1 or atDjA2 impeded seed germination under multiple abiotic stress conditions, with atDjA1 mutant being more sensitive. Both the mutants demonstrated decreased respiratory rates and enhanced accumulation of reactive oxygen species (ROS), indicating compromised mitochondrial function. Further, knockout of atDjA1 resulted in a significant decline in the number of active mitochondria as well as import of MTS-GFP. While atDjA2 mutants did not show a reduction in mitochondrial number, both the mutants had an activated mitochondrial retrograde signaling (MRS) further underscoring compromised mitochondrial homeostasis in these mutants. Our findings suggest that evolutionarily conserved cytosolic Class I JDPs preserve mitochondrial homeostasis by targeting specific receptors on the outer mitochondrial surface, facilitating the import of cytosolically synthesized mitochondrial precursors in Arabidopsis thaliana.