Reactive oxygen species regulation and antioxidant defence in halophytes

Production of reactive oxygen species (ROS), which are a by-product of normal cell metabolism in living organisms, is an inevitable consequence of aerobic life on Earth, and halophytes are no exception to this rule. The accumulation of ROS is elevated under different stress conditions, including salinity, due to a serious imbalance between their production and elimination. These ROS are highly toxic and, in the absence of protective mechanisms, can cause oxidative damage to lipids, proteins and DNA, leading to alterations in the redox state and further damage to the cell. Besides functioning as toxic by-products of stress metabolism, ROS are also important signal transduction molecules in controlling growth, development and responses to stress. Plants control the concentrations of ROS by an array of enzymatic and non-enzymatic antioxidants. Although a relation between enzymatic and non-enzymatic antioxidant defence mechanisms and tolerance to salt stress has been reported, little information is available on ROS-mediated signalling, perception and specificity in different halophytic species. Hence, in this review, we describe recent advances in ROS homeostasis and signalling in response to salt, and discuss current understanding of ROS involvement in stress sensing, stress signalling and regulation of acclimation responses in halophytes. We also highlight the role of genetic, proteomic and metabolic approaches for the successful study of the complex relationship among antioxidants and their functions in halophytes, which would be critical in increasing salt tolerance in crop plants.