ions and to some extent Cl and SO 4 2 of Mg 2+ and nutrient imbalance caused by excess of Na + and Cl ions. Salinity stress response is multigenic, as a number of processes i n- volved in the tolerance mechanism are affected, such as var ious compatible solutes/osmolytes, polyamines, reactive oxygen species and antioxidant defence mecha- nism, ion transport and compartmentalization of inj u- rious ions. Various genes/cDNAs encoding proteins involved in the above-mentioned processes have been identified and isolated. The role of genes/cDNAs e n- coding proteins involved in regulating other genes/ pro- teins, signal transduction process involving hormones like ABA, JA and polyamines, and strategies to i mprove salinity stress tolerance have also been di scussed. EXCESS amount of salt in the soil adversely affects plant growth and development. Nearly 20% of the world's cul- tivated area and nearly half of the world's i rrigated lands are affected by salinity 1

Physiology and molecular biology of salinity stress tolerance in plants

Biotechnological approach of improving plant salt tolerance using antioxidants as markers.

Galactinol synthase (GolS) is a key enzyme in the synthesis of raffinose family oligosaccharides that function as osmoprotectants in plant cells. In leaves of Arabidopsis (Arabidopsis thaliana) plants overexpressing heat shock transcription factor A2 (HsfA2), the transcription of GolS1, -2, and -4 and raffinose synthase 2 (RS2) was highly induced; thus, levels of galactinol and raffinose increased compared with those in wild-type plants under control growth conditions. In leaves of the wild-type plants, treatment with 50 μm methylviologen (MV) increased the transcript levels of not only HsfA2, but also GolS1, -2, -3, -4, and -8 and RS2, -4, -5, and -6, the total activities of GolS isoenzymes, and the levels of galactinol and raffinose. GolS1- or GolS2-overexpressing Arabidopsis plants (Ox-GolS1-11, Ox-GolS2-8, and Ox-GolS2-29) had increased levels of galactinol and raffinose in the leaves compared with wild-type plants under control growth conditions. High intracellular levels of galactinol and raffinose in the transgenic plants were correlated with increased tolerance to MV treatment and salinity or chilling stress. Galactinol and raffinose effectively protected salicylate from attack by hydroxyl radicals in vitro. These findings suggest the possibility that galactinol and raffinose scavenge hydroxyl radicals as a novel function to protect plant cells from oxidative damage caused by MV treatment, salinity, or chilling.

Galactinol and raffinose constitute a novel function to protect plants from oxidative damage.

The present work describes, for the first time, the changes that take place in the leaf apoplastic antioxidant defenses in response to NaCl stress in two pea (Pisum sativum) cultivars (cv Lincoln and cv Puget) showing different degrees of sensitivity to high NaCl concentrations. The results showed that only superoxide dismutase, and probably dehydroascorbate reductase (DHAR), were present in the leaf apoplastic space, whereas ascorbate (ASC) peroxidase, monodehydroascorbate reductase (MDHAR), and glutathione (GSH) reductase (GR) seemed to be absent. Both ASC and GSH were detected in the leaf apoplastic space and although their absolute levels did not change in response to salt stress, the ASC/dehydroascorbate and GSH to GSH oxidized form ratios decreased progressively with the severity of the stress. Apoplastic superoxide dismutase activity was induced in NaCl-treated pea cv Puget but decreased in NaCl-treated pea cv Lincoln. An increase in DHAR and GR and a decrease in ASC peroxidase, MDHAR, ASC, and GSH levels was observed in the symplast from NaCl-treated pea cv Lincoln, whereas in pea cv Puget an increase in DHAR, GR, and MDHAR occurred. The results suggest a strong interaction between both cell compartments in the control of the apoplastic ASC content in pea leaves. However, this anti-oxidative response does not seem to be sufficient to remove the harmful effects of high salinity. This finding is more evident in pea cv Lincoln, which is characterized by a greater inhibition of the growth response and by a higher rise in the apoplastic hydrogen peroxide content, O(2)(.-) production and thiobarbituric acid-reactive substances, and CO protein levels. This NaCl-induced oxidative stress in the apoplasts might be related to the appearance of highly localized O(2)(.-)/H(2)O(2)-induced necrotic lesions in the minor veins in NaCl-treated pea plants. It is possible that both the different anti-oxidative capacity and the NaCl-induced response in the apoplast and in the symplast from pea cv Puget in comparison with pea cv Lincoln contributes to a better protection of pea cv Puget against salt stress.

Antioxidant systems and o2.?/h2o2 production in the apoplast of pea leaves. its relation with salt-induced necrotic lesions in minor veins

Halophytes are defined as plants that are adapted to live in soils containing high concentrations of salt and benefiting from it, and thus represent an ideal model to understand complex physiological and genetic mechanisms of salinity stress tolerance. It is also known that oxidative stress signalling and reactive oxygen species (ROS) detoxification are both essential components of salinity stress tolerance mechanisms. This paper comprehensively reviews the differences in ROS homeostasis between halophytes and glycophytes in an attempt to answer the questions of whether stress-induced ROS production is similar between halophytes and glycophytes; is the superior salinity tolerance in halophytes attributed to higher antioxidant activity; and is there something special about the specific ‘pool’ of enzymatic and non-enzymatic antioxidants in halophytes. We argue that truly salt-tolerant species possessing efficient mechanisms for Na + exclusion from the cytosol may not require a high level of antioxidant activity, as they simply do not allow excessive ROS production in the first instance. We also suggest that H 2O2 ‘signatures’ may operate in plant signalling networks, in addition to well-known cytosolic calcium ‘signatures’. According to the suggested concept, the intrinsically higher superoxide dismutase (SOD) levels in halophytes are required for rapid induction of the H2O2 ‘signature’, and to trigger a cascade of adaptive responses (both genetic and physiological), while the role of other enzymatic antioxidants may be in decreasing the basal levels of H2O2, once the signalling has been processed. Finally, we emphasize the importance of non-enzymatic antioxidants as the only effective means to prevent detrimental effects of hydroxyl radicals on cellular structures.

ROS homeostasis in halophytes in the context of salinity stress tolerance

http://directory.umm.ac.id/Data%20Elmu/jurnal/P/PlantScience/PlantScience_Elsevier/Vol148.Issue2.2000/5192.pdf

S Yokota

Papers

Salt tolerance of transgenic rice overexpressing yeast mitochondrial Mn-SOD in chloroplasts