The study investigates the role of alternative oxidase (AOX) in reducing reactive oxygen species (ROS) production in plant mitochondria. Using transgenic tobacco cells with altered AOX levels, the researchers found that antisense suppression of AOX increased ROS levels, while overexpression of AOX decreased ROS abundance. Laser-scanning confocal microscopy confirmed that these changes were due to mitochondrial-specific ROS formation. The addition of antimycin A, which inhibits cytochrome electron transport, exacerbated mitochondrial ROS production. Overexpressing AOX cells also showed lower expression of genes encoding ROS-scavenging enzymes, such as superoxide dismutase (SodA and SodB) and glutathione peroxidase (GPX). Conversely, cells deficient in AOX had higher expression of salicylic acid-binding catalase (SA-Cat) and pathogenesis-related (PR) genes. These findings suggest that AOX plays a crucial role in maintaining low mitochondrial ROS levels, which may help protect plants from oxidative stress and enhance their resistance to biotic and abiotic stresses.The study investigates the role of alternative oxidase (AOX) in reducing reactive oxygen species (ROS) production in plant mitochondria. Using transgenic tobacco cells with altered AOX levels, the researchers found that antisense suppression of AOX increased ROS levels, while overexpression of AOX decreased ROS abundance. Laser-scanning confocal microscopy confirmed that these changes were due to mitochondrial-specific ROS formation. The addition of antimycin A, which inhibits cytochrome electron transport, exacerbated mitochondrial ROS production. Overexpressing AOX cells also showed lower expression of genes encoding ROS-scavenging enzymes, such as superoxide dismutase (SodA and SodB) and glutathione peroxidase (GPX). Conversely, cells deficient in AOX had higher expression of salicylic acid-binding catalase (SA-Cat) and pathogenesis-related (PR) genes. These findings suggest that AOX plays a crucial role in maintaining low mitochondrial ROS levels, which may help protect plants from oxidative stress and enhance their resistance to biotic and abiotic stresses.