ROS Production and Function at Plasma Membrane and Apoplast

The plasma membrane comprises cell boundary, which encloses intracellular components whereas the apoplast is the extracellular space, which extends outward from the plasma membrane, comprising the cell wall. Both plasma membrane and apoplast ensure the integrity of the cell and thus maintain cellular activities and facilitate cell communication with the outer environment. Like other cellular organelles, plasma membrane and apoplast are also involved in the production of reactive oxygen species (ROS) under both normal and stressed conditions. The generation of ROS in apoplast is the result of the activity of plasma membrane-localized enzyme NADPH (nicotinamide adenine dinucleotide phosphate) oxidase or respiratory burst oxidase homolog (RBOHs), cell wall-associated class III peroxidases (POXs), germin-like oxalate oxidase (OXOs), amine oxidases (AOs), and lipoxygenases (LOXs). The presence of ROS in the apoplast mediates cell growth via oxidative loosening and stiffening of the cell wall. Moreover, apoplastic ROS mediates the downstream signaling pathway, which helps to foster tolerance against existing stress. Under extreme adverse circumstances, the production and accumulation of ROS in apoplastic space increases exorbitantly, inducing oxidative damage to plasma membrane and cell wall-associated biomolecules, resulting in ions leakage, cell structure distortion, and ultimately cell death. Plasma membrane and apoplast also maintain an arsenal of antioxidants that partake in quenching of ROS, however, the concentration and activity of antioxidants in apoplastic space is too low, and therefore result in oxidative burst, eventually leading to signal transduction. Overall, the generation of extracellular ROS is imperative for plant growth and induces signaling under harsh conditions, but in certain cases, excess accumulation of ROS could induce detrimental effects. The chapter highlights the role of extracellular ROS in plant cells under prevailing environmental conditions.