ABSTRACT: Mitochondrial redox homeostasis is essential for cellular metabolism and development. To investigate the consequences of disrupting redox homeostasis in this organelle in a metazoan organism, we generated a double mutant lacking mitochondrial glutathione reductase (gsr-1a) and thioredoxin reductase (trxr-2) genes in Caenorhabditis elegans. While gsr-1a or trxr-2 single mutants are phenotypically normal, double gsr-1a trxr-2 mutants displayed small body size, gonadal migration defects, reduced brood size, and prolonged egg-laying period, without developmental delay or lethality. Transcriptomic analysis revealed strong induction of stress and detoxification genes, prominently those regulated by the transcription factor ATFS-1, indicating activation of the mitochondrial unfolded protein response (UPRmt). Consistent with this, gsr-1a trxr-2 worms exhibited constitutive ATFS-1 nuclear localization and robust hsp-6::GFP expression. Triple gsr-1a trxr-2; atfs-1 mutants were nonviable, demonstrating that UPRmt activation is essential under mitochondrial redox stress. Despite the induction of a stress response at the transcriptional level, gsr-1a trxr-2 double mutants maintained normal respiration, ATP and ROS production, and were not more resistant to oxidative or pathogen stressors. In turn, mitochondrial morphology was altered in a tissue-specific manner, with elongated networks in muscle cells while more fragmented in hypodermis. These changes were independent of mitophagy but sensitive to disruption of mitochondrial fission or fusion machinery, highlighting the importance of dynamic remodeling of the mitochondrial network for survival in animals with impaired mitochondrial redox homeostasis. Functionally, gsr-1a trxr-2 mutants showed impaired motility, reduced calcium uptake upon carbachol stimulation, enhanced hypodermal wound repair, and decreased fertilization efficiency associated with lower muscle exopher production. Overall, our data show that mitochondrial GSR-1a and TRXR-2 act redundantly to preserve redox balance in this organelle. Their simultaneous loss triggers a constitutive ATFS-1–dependent UPRmt that sustains viability but compromises growth, fertility, and muscle performance, revealing mitochondrial redox control as a core determinant of organismal homeostasis.
