Salicylic Acid-and Ethylene-altering Arabidopsis Mutant Response To Low Temperature Stress

Low temperature is a major factor inhibiting plant growth, and one of the most seriousnatural disasters in China. Low temperature-related plant damage mechanism is always ahot topic in plant biology. However, there are many scientific problems remaining to beresolved. Recent years, increasing evidence shows that salicylic acid (SA) plays animportant role in plant responses to low temperature, while the SA-related mechanism isnot fully understood. In addition, the most related studies are performed using exogenousapplication of SA, or measuring the endogenous SA levels, whereas the study withSA-altering plant mutants is scarce. As a traditional phytohormone, ethylene (ET) has beendemonstrated to be involved in plant response to low temperature. However, the studyusing ethylene-altering plant mutants is also scarce. As for the study using the combinationof SA-related and ET-related mutation, there are little available information so far.In the present study, Arabidopsis plants, including wild type (Columbia) and itstransgenic line nahG with SA deficiency, mutants sid2-1with SA deficiency, snc1(suppressor of npr1-1, constitutive) with SA high accumulation, npr1-1(nonexpressor ofPR gene) with a blockage in SA signaling, ein2-1(ethylene insensitive2-1), andein2-1/npr1-1were used to comparatively investigate the role of endogenous SA and ET inplant response to low temperature exposure. All the tested plants were randomly dividedinto control group and low temperature-stressed group exposed to5°C. After15days oflow temperature exposure, the plants were harvested, and determined the physiologicalindices including the activities of superoxide dismutase (SOD) and peroxidase (POD), lipidperoxidation indicated by the content of malondialdehyde (MDA), electrolyte leakage rates,contents of chlorophyll, and proline levels. The effects of low temperature on the SOD,POD and CAT isozymes were studied by polyacrylamide gel electrophoresis (PAGE).Results showed that exposure of low temperature inhibited all the tested plants growthretardation, and leaf dehydration, chlorisis and wilting. The exposure of low temperaturedecreased chlorophyll contents of all plants, with the least extent occurring in snc1plantsand most in other tested plants with no significant difference among them. Lowtemperature increased SOD activities in all plants, with the most extent in snc1plants. ThePOD activities were decreased in all plants under low temperature, with the least decreasein snc1plants. The activities of SOD and POD isozymes shown by PAGE were well correlated to their activities measured by spectrophotomentric method. However, thepatterns of the isozymes were very different among the mutants under low temperature.The expressive levels of CAT isozymes are inconsistent among the tested plants exposed tolow temperature, such as an increase level being observed in wild type plants, and adecrease in other plants. Based on the changes of these antioxidative enzymes and theirisozymes, it was proposed that they might play an important role in plant response to lowtemperature stress. In other word, the changes of antioxidative enzymes might reflect thelow temperature-induced damage to plants, because they are normally induced when plantsare subjected to adverse stresses. Furthermore, the different activities or patterns of theantioxidative enzymes among the SA-altering and ET-altering mutants also suggested thedifferent mechanisms by which SA or ET regulated plant responses to low temperature.The exposure of low temperature resulted in membrane lipid peroxidation in all the testedplants, as indicated by the content of malondialdehyde (MDA), a production of lipidperoxidation. Except the nahG plants in which the MDA contents were lower than those inwild type plants, the MDA contents in other tested plants were higher than the wild typeplants, in particular the MDA contents in ein2-1and ein2-1/npr1-1plants were muchhigher than other plants, suggesting a synergistic action existing between SA and ET inplant response to low temperature. The exposure of low temperature significantly increaseselectrolyte leakage rates of all tested plants, with snc1plants being most dramatic. Theexposure of low temperature increased proline levels of all plants, with the greatestaccumulation being observed in npr1-1plants, and the least in snc1plants.Taken together, these data showed that under exposure of low temperature, theactivities of antioxidative enzymes, levels of lipid peroxidation, contents of chlorophylland proline were differently affected among the SA-altering and ET-altering plants,suggesting that SA and ET play an important role in plant response to low temperaturestress, and they may synergistically regulate plant response to low temperature stress.However, this study did not show a well correlationship between these physiologicalindices and plant tolerance to low temperature.