Salicylic Acid-Altering Arabidopsis Mutants Response To Salt Stress

Salinity is a major limiting factor in agricultural production and exerts unfavorable influence on various physiological and biochemical processes associated with plant growth and development. Currently, soil salinification has become one of global environmental and ecological problems. Increasing evidence has shown that adverse stresses increase endogenous salicylic acid (SA) content in plants, and an application of moderate dose of SA protects plants against stress-induced damage. Therefore, SA has been widely implicated in plant responses to various adverse stresses.In this study, Arabidopsis wild type Columbia and its salicylic acid-altering mutants sncl with high SA level, transgenic line nahG with low SA level, npr1-1 and sid2 with SA signaling blockage, as well as ethylene insensitive mutant ein2-1 with ethylene signaling blockage and double mutant ein2-1/nprl-1 plants were used to reveal the mechanism by which Arabidopsis plants response to salt stress in controlled conditions. Six-week-old plants were randomly divided into four groups consisting of a control group and salt-exposed groups treated with three NaCl concentrations of 100 mmol/L,200 mmol/L, and 300 mmol/L, respectively. After 5 days of NaCl exposure, the plant leaves were harvested and data were collected on enzyme activities of superoxide dismutase (SOD) and peroxidase (POD), malondialdehyde (MDA) contents, electrolyte leakage, Chlorophyll contents, values of chlorophyll a/b and proline contents. The effects of NaCl exposure on the POD, SOD and CAT isozymes were studied by polyacrylamide gel electrophoresis (PAGE). Results showed that salt exposure inhibited all tested plant growth, caused chlorosis and wilting in some degrees. However, the salt-induced phenotype changes were the most significant in sncl plants among all the analysed mutants, as indicated by plant growth retardation, chlorophyll content decrease, mature leave etiolation, while nahG and sid2 plants both with low SA levels and nprl plants with a blockage in SA signaling exhibited increasing tolerance to NaCl exposure, as shown by more favorable exhibitions in above-mentioned parameters than those in wild-type plants. The salt exposure increased SOD and POD activities in all lines tested when compared to the control (no added NaCl), especially in sncl plants. In addition, some synergistic effect might exist between salicylic acid and ethylene in plant response to NaCl exposure, as indicated by SOD activities being lower in ein2-1/npr1-1 plants than in ein2-1 plants or nprl-1 plants. The isozyme activities of SOD, POD and CAT in all tested lines shown by PAGE also documented that the NaCl exposure had an induced effects, especially in sncl plants, which further suggested that snc1 plants suffered greater damage by NaCl stress. Also, the isozyme activities of SOD and POD shown by PAGE in all tested plants were correlated well to the values detected by ultraviolet spectrophotometer. Salt exposure increased MDA production in all the lines, with the maximum accumulation occurred only in snc1 at NaCl of 100 and 200 mmol/L and ein2-1/npr1-1 plants, whereas nahG, sid2 and npr1 had lower levels of MDA relative to the wild type. The electrolyte leakage was essentially consistent with the changed pattern of MDA. The contents of proline, as an important osmosis regulator, were gradually increased with the increasing of salt concentrations in all the tested plants. However, the elevated extends of proline in sncl plants were the least among all the tested lines, while the contents and elevated extends of proline were higher in nahG, sid2 and npr1 plants than in wild-type plants, which was consistent with the changes in phenotype and other parameters tested under NaCl exposure, indicating that proline played an crucial role in plant responses to NaCl exposure.In summary, this study demonstrated that the high accumulations of endogenous SA potentiated NaCl-induced oxidative damage to plants, as indicated by elevated lipid peroxidation, such as increases in MDA contents and electrolyte leakage. In addition, under NaCl exposure, the contents of chlorophyll and proline were decreased greater in the sncl plants than in wild-type plants, with accompanied by increases in antioxidative enzyme activities. On the contrary to snc1 plants, transgenic line nahG and sid2 plants both with low SA level, and npr1-1 plants with SA signaling blockage exhibited more resistant to NaCl than the wild-type plants did, as shown by better exhibitions in the most indices tested in this experiment. Taken together, these data suggested that SA high accumulations reinforced NaCl-induced damages to plants, while SA deficiency or signaling blockage alleviated the damage. To our knowledge, this is the first report in this sort.