Fig. 1.

Redox process of cellular state. Different reactive oxygen species (ROS) and reactive nitrogen species (RNS) are produced in plants under abiotic stress to regulate gene expression for stress tolerance. ROS and RNS generation are activated during abiotic stress in the apoplastic space, chloroplasts, mitochondria, and peroxisomes. For instance, first, superoxide (O₂^•−) is generated by plasma membrane NADPH (nicotinamide adenine dinucleotide phosphate reduced) oxidases, and O₂^•− is immediately dismutated to hydrogen peroxide (H₂O₂) by superoxide dismutases (SOD) in the corresponding organelles. Peroxidases (POX) and polyamine oxidases (POAX) are other sources of apoplastic H₂O₂ generation [14]. These organelles also have mechanisms to detoxify the H₂O; however, oxidative stress and programmed cell death (PCD) occur if ROS and RNS generation is beyond the homeostatic range.