Reactive oxygen species and their role in plant oxidative stress.

Abstract

Oxidative stress is a physiological response due to progressive accumulation under some circumstances of reactive oxygen species (ROS) and oxidized forms of biomolecules. This response is associated with damage of all components of the cell, leading to pathophysiological processes. At the mechanistic level, oxidative stress can be caused by: (i) external oxidizers (UV, O₃, halogens, gamma radiation, extreme light, some xenobiotics, etc.) and ^.OH-producing transition metals (Cu, Fe, Mn, Hg, Ni, etc.); (ii) cellular programmes inducing ROS generation as a part of a response to abiotic and biotic stresses; and (iii) ROS production for needs of normal physiology, such as programmed cell death and autophagy. The intensity and consequences of an oxidative stress depends on a biological system's ability to detoxify ROS and to repair oxidative damage. Antioxidants stop or delay the oxidation of biomolecules ameliorating oxidative stress-induced damage, and orchestrate ROS signalling. The origins of ROS generation leading to oxidative stress include electron-transport chains of chloroplasts, mitochondria and peroxisomes, NADPH oxidases, peroxidases, phospholipases, oxygenases and some other systems. These systems produce O₂^.-, singlet oxygen and NO as well as oxidized forms of organic molecules, which can give other ROS and free radicals in living cells. The most dangerous and highly oxidizing ROS is ^.OH, formed via Fenton-like reactions catalysed by transition metals. ROS are sensed via specific regulatory proteins, and are the reason for altered cell signalling and gene expression. They can trigger cytosolic Ca²⁺ elevation, K⁺ loss, autophagy and programmed cell death. Downstream ROS-Ca²⁺-regulated signalling cascades include regulatory systems with one (ion channels and transcription factors), two (Ca²⁺-activated NADPH oxidases and calmodulin) or multiple components (receptor-like and Ca²⁺-dependent protein kinases, mitogen-activated protein kinases, etc.). Currently, research into plant oxidative stress is at the forefront of developing stress-tolerant agricultural plants and designing strategies for adapting to global climate changes.