Proline, an imino acid, plays a crucial role in stress adaptation across various organisms. Its widespread use as a stress adaptor molecule highlights its fundamental biological significance in stress response. Proline metabolism influences cellular signaling processes, promoting apoptosis or survival by increasing reactive oxygen species (ROS) formation in mitochondria via the electron transport chain. Enhanced ROS production due to proline metabolism has been implicated in plant hypersensitive responses, lifespan extension in worms, and apoptosis, tumor suppression, and cell survival in animals. The ability of proline to influence diverse cellular outcomes may be governed by ROS levels generated in mitochondria. Defining the threshold at which proline metabolic enzyme expression switches from inducing survival pathways to cellular apoptosis would provide insights into cellular redox regulation by proline. While ROS are key mediators of proline metabolic signaling, other factors may also be involved. Future research should focus on identifying other interacting partners of proline metabolic enzymes to uncover novel regulatory and signaling networks of cellular stress response. New evidence suggests that proline biosynthesis enzymes interact with redox proteins such as thioredoxin. Understanding these interactions will help elucidate the complex mechanisms by which proline protects cells during stress.Proline, an imino acid, plays a crucial role in stress adaptation across various organisms. Its widespread use as a stress adaptor molecule highlights its fundamental biological significance in stress response. Proline metabolism influences cellular signaling processes, promoting apoptosis or survival by increasing reactive oxygen species (ROS) formation in mitochondria via the electron transport chain. Enhanced ROS production due to proline metabolism has been implicated in plant hypersensitive responses, lifespan extension in worms, and apoptosis, tumor suppression, and cell survival in animals. The ability of proline to influence diverse cellular outcomes may be governed by ROS levels generated in mitochondria. Defining the threshold at which proline metabolic enzyme expression switches from inducing survival pathways to cellular apoptosis would provide insights into cellular redox regulation by proline. While ROS are key mediators of proline metabolic signaling, other factors may also be involved. Future research should focus on identifying other interacting partners of proline metabolic enzymes to uncover novel regulatory and signaling networks of cellular stress response. New evidence suggests that proline biosynthesis enzymes interact with redox proteins such as thioredoxin. Understanding these interactions will help elucidate the complex mechanisms by which proline protects cells during stress.