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.

SummaryEndoplasmic reticulum stress, which is induced by tunicamycin, triggers reactive oxygen species signalling via NADPH oxidase activity and also regulates the antioxidant defence system in Arabidopsis thaliana.

a b s t r a c tDifferences between antioxidant responses to drought in C 3 and C 4 plants are rather scanty. Even, we are not aware of any research on comparative ROS formation and antioxidant enzymes in C 3 and C 4 species differing in carboxylation pathway of same genus which would be useful to prevent other differences in plant metabolism. With this aim, relative shoot growth rate, relative water content and osmotic potential, hydrogen peroxide (H 2 O 2 ) content and NADPH oxidase (NOX) activity, antioxidant defence system (superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX), glutathione reductase (GR) enzymes and their isoenzymes), CAT1 mRNA level, and lipid peroxidation in seedlings of Cleome spinosa (C 3 ) and Cleome gynandra (C 4 ) species of Cleome genus exposed to drought stress for 5 and 10 day (d) were comparatively investigated. Constitutive levels of antioxidant enzymes (except SOD) were consistently higher in C. spinosa than in C. gynandra under control conditions. CAT1 gene expression in C. spinosa was correlated with CAT activity but CAT1 gene expression in C. gynandra at 10 d did not show this correlation. Drought stress caused an increase in POX, CAT, APX and GR in both species. However, SOD activity was slightly decreased in C. gynandra while it was remained unchanged or increased on 5 and 10 d of stress in C. spinosa, respectively. Parallel to results of malon dialdehyde (MDA), H 2 O 2 content was also remarkably increased in C. spinosa as compared to C. gynandra under drought stress. These results suggest that in C. spinosa, antioxidant defence system was insufficient to suppress the increasing ROS production under stress condition. On the other hand, in C. gynandra, although its induction was lower as compared to C. spinosa, antioxidant system was able to cope with ROS formation under drought stress.

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Secretory proteins undergo modifications such as glycosylation and disulphide bond formation before proper folding, and move to their final destination via the endomembrane system. Accumulation of unfolded proteins in the endoplasmic reticulum (ER) due to suboptimal environmental conditions triggers a response called the unfolded protein response (UPR), which induces a set of genes that elevate protein folding capacity in the ER. This review aims to establish a connection among ER stress, UPR, and reactive oxygen species (ROS), which remains an unexplored topic in plants. For this, we focused on mechanisms of ROS production originating from ER stress, the interaction between ER stress and overall ROS signalling process in the cell, and the interaction of ER stress with other organellar ROS signalling pathways such as of the mitochondria and chloroplasts. The roles of the UPR during plant hormone signalling and abiotic and biotic stress responses are also discussed in connection with redox and ROS signalling.

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Chloroplast- and mitochondria-originated ROS have distinct roles in triggering the ER stress response. In general, low concentrations of ROS induced the transcription of ER stress-related genes, which can be attributed to the roles of ROS as secondary messengers. This is the first time that ROS production in organelles has been shown to affect the ER stress response in a plant system.

E. parvulum was able to adapt to osmotic stress by an efficient osmotic adjustment mechanism involving proline and was able to regulate its ion homeostasis. In addition, efficient induction of water-water cycle enzymes and other redox regulatory components such as TRXs and PRXs in chloroplasts were able to protect the chloroplasts from salinity-induced oxidative stress.