Ionic homeostasis and reactive oxygen species control in leaves and xylem sap of two poplars subjected to NaCl stress

Abstract

We investigated the effects of increasing soil NaCl concentration on intracellular compartmentalization of salt and on the activities of antioxidant enzymes (superoxide dismutase (SOD), ascorbic peroxidase (APX), catalase (CAT) and glutathione reductase (GR)) and their role in the regulation of reactive oxygen species (ROS; O(2)(-*) and H(2)O(2)) in leaves and xylem sap of salt-tolerant Populus euphratica Oliv. and salt-sensitive P. popularis cv. 35-44. Mesophyll cells of P. euphratica exhibited a high capacity for NaCl exclusion and compartmentalization of salt in vacuoles compared with P. popularis. In P. popularis, the salt treatment resulted in large accumulations of Na(+) and Cl(-) in leaves that induced significant increases in O(2)(-*) and H(2)O(2) production despite marked increases in the activities of antioxidant enzymes in leaves and xylem sap. Separation of the isoforms of leaf SOD, APX and CAT by polyacrylamide gel electrophoresis followed by in-gel activity staining revealed that the salt-induced activities of APX and CAT were the result of increases in activities of all the isoenzymes. Leaf injury and shedding of aged leaves occurred following the oxidative burst in P. popularis, indicating that the increased activities of antioxidant enzymes in P. popularis were insufficient to counter the harmful effects of ROS at high soil NaCl concentrations. Unlike P. popularis plants, P. euphratica plants did not exhibit an oxidative burst in response to the NaCl treatments, because of (1) a high salt exclusion capacity and effective compartmentalization of salt in vacuoles, and (2) up-regulation of antioxidant enzymatic activities after the onset of salt stress. We conclude that P. euphratica plants subjected to saline conditions control ROS homeostasis through two pathways: (1) by maintaining cellular ionic homeostasis and thereby limiting the NaCl-induced enhancement of ROS production under long-term saline conditions; and (2) by rapidly up-regulating antioxidant defenses to prevent oxidative damage.

Effect of NaCl on leaf H+-ATPase and the relevance to salt tolerance in two contrasting poplar species

“…and Cl -in cell wall relative to the cytoplasm implies the active extrusion of salts from the cytosol, in addition to ion sequestration in the vacuole. This pattern of salt compartmentation suggests a reduction of the toxic effects of salts in the cytoplasm, thus limiting the over production of ROS and avoiding secondary oxidative stress in leaf cells of P. euphratica (Wang et al , 2008. Leaf respiration markedly decreased in P. popularis, but not in P. euphratica, after 2 weeks of salt treatment (Fig.…”

Plant Responses to Salt Stress: Adaptive Mechanisms

“…Furthermore, by using confocal laser microscopy, they found ROS accumulation in the cytoplasm and mitochondria. The effect of salinity on ROS levels (O2 .− and H2O2) was also monitored in the xylem sap in two poplar cultivars with different levels of salt sensitivity [210]. After 18 days of treatment (250 mM NaCl) these authors only observed significant O2 .− and H2O2 accumulation in the NaCl-sensitive plants.…”

Ion flux profiles and plant ion homeostasis control under salt stress

“…We found that salt-sensitive poplar species are usually unable to restrict the large accumulations of Na + and Cl -in root and shoot tissues, resulting in significant increases in ROS production despite marked increases in the activities of antioxidant enzymes in leaves and xylem sap. [4][5][6][7]27 As compared to salt-sensitive poplars, P. euphratica plants effectively restricted the salt accumulation and did not exhibit an oxidative burst in response to the NaCl treatments. 4,27 We hypothesized that salt-tolerant P. euphratica attenuates oxidative stress by NaCl exclusion, thus enabling ROS homeostasis to be maintained under saline conditions.…”

“…[4][5][6][7]27 As compared to salt-sensitive poplars, P. euphratica plants effectively restricted the salt accumulation and did not exhibit an oxidative burst in response to the NaCl treatments. 4,27 We hypothesized that salt-tolerant P. euphratica attenuates oxidative stress by NaCl exclusion, thus enabling ROS homeostasis to be maintained under saline conditions. 4 Generally, the less accumulated salts could occur for two possibilities: inhibition of uptake or enhancement of active extrusion.…”

“…In general, salinity causes direct and indirect stresses on plant tissues, e.g., (1) a reduced water availability, due to the high soluble salts in the soil; (2) ionspecific effects, resulting from the excessive accumulation of toxic ions in plants (Na + and Cl -); and, (3) the salt-induced secondary stress, e.g., oxidative bursts that caused by over-production of ROS (reactive oxygen species). [1][2][3][4] Buildup of salt ions contributes to decreasing cellular osmotic potentials but this makes plants confront ion toxicity and oxidative stress. Therefore, whether plants could survive salinity is dependent to a large extent on the ability to retain the ionic homeostasis under saline conditions.…”