Abiotic stress triggers mitochondrial defense system: A comprehensive review

Mitochondria are small organelles widely distributed within the cells of living organisms. Their main function is the oxidative phosphorylation-coupled ATP synthesis. Under abiotic stress conditions mitochondrial function is significantly affected triggering the mitochondrial stress response. However, the stressful conditions could increase the chance of ROS generation in various cellular organelles. Plant cells struggle ROS generation through the induction of specific enzymatic and non-enzymatic defense systems. Nevertheless, the excessive accumulation of ROS within mitochondria induces mitochondrial signaling and the cellular responses to mitochondrial dysfunction in a process known as mitochondrial retrograde regulation. The exclusive accumulation of ROS causes severe disorders in ROS homoeostasis that stimulate programmed cell death. The inner mitochondrial membrane has uncoupling mitochondrial proteins (UMP) to keep mitochondrial integrity and function at both normal and stress conditions. Mitochondria can perform nonphosphorylating respiration under stress conditions, like the presence of respiration inhibitors, by the help of specific proteins called alternative oxidases (AOX). The expression of AOX or its mRNA has been reported to be induced by various abiotic stress conditions. The expression of AOX plays an important role in acclimation to many stress conditions like salinity, temperature and drought in various plant species.