ROS signalling, and antioxidant defence network in halophytes.

Abstract Halophytes have evolved a range of adaptations to tolerate disturbances in reactive oxygen species (ROS) homeostasis caused by high salinity alone or in combination with other stresses. Singlet oxygen (1O2), superoxide anion (O2.-), hydroxyl radical (OH.) and hydrogen peroxide (H2O2) are the major ROS generated during plant metabolism. The main sources of ROS are chloroplasts, mitochondria, peroxisomes, apoplasts, plasma membranes and cell walls. Regulation of ROS levels in these compartments plays a key role in response to salt stress. Many various defence strategies have been discovered in chloroplasts. These include stress-avoidance mechanisms (e.g. alteration in proteins involved in electron transport chains, changes in lipid composition, harmless dissipation of excess energy) and ROS-scavenging systems (enzymatic and non-enzymatic antioxidants, some osmoprotectants). The efficient antioxidant mechanisms in halophytes can prepare plants to cope with subsequent stress factors by inducing a signal to activate the stress response and suppress the oxidative burst within a short time after the stimulus action. The interrelationship between ROS scavengers leads to spatio-temporal modulation of the ROS signalling network. Although the ROS level is modulated by both: (1) specific mechanisms enabling delay/reduction in the oxidative burst; and (2) ROS-scavenging systems in each cell compartment, interactions between cell organelle and cytoplasm are critical in adjusting redox homeostasis, and the ROS signalling network plays a role in inducing salinity tolerance at the cellular and whole-plant levels.