Role Of Endogenous Salicylic Acid In Plant Response To Lead Stress

The social development, industrialization process and human activity, such as industrial wasteor automobile exhaust, release a lot of heavy metal wastes into the environments causing seriouspollution threatening the ecosystems and human health. Pb exposure can change theconformation of some large molecules, decrease the activities of some important enzymes, causeloss of cellular electrolyte leakage and induce membrane lipid peroxidation, therefore resultingin plant growth retardation, and development impairment, even death. Increasing evidencedemonstrates that acid (SA) plays an important role in plant responses to biotic and abioticstresses. Although there has been considerable of evidence to show that exogenous SA canimprove plant tolerance to Pb, using endogenous SA-altering plant mutants to evaluate the roleof SA in plant response to Pb exposure is scarce.In the present study, Arabidopsis plants, including wild type (Columbia) and its mutantsnpr1-1(nonexpressor of PR gene) with a blockage in SA signaling, transgenic line nahG withSA deficiency, and snc1(suppressor of npr1-1, constitutive) with SA high accumulation, wereused to comparatively investigate the role of endogenous SA in plant response to Pb exposure.All the tested plants were randomly divided into four groups including a control group and threePb stress groups treated with50μmol/L,100μmol/L and150μmol/L of lead nitrate solution,respectively. After7days of Pb exposure, the plants were harvested, and determined thephysiological and biochemical indexes including plant root length, dry weigh, activities ofsuperoxide dismutase (SOD), peroxidase (POD), catalase (CAT), lipid peroxidation indicated bythe content of malondialdehyde (MDA), electrolyte leakage rates and ratio of chlorophyll a/b,and proline levels. The effects of Pb exposure on the SOD, POD and CAT isozymes were studiedby polyacrylamide gel electrophoresis (PAGE). Results showed that Pb exposure caused alltested plants growth retardation, especially the root growth, with a dose-effect relationshiprelative to control. However, compared to wild type plants, a greater growth inhibition occurredin snc1, while a less inhibition was observed in nahG and npr1-1plants. With the Pbconcentration increases, plant dry biomass accumulation decreased. In view of the plantphenotype, Pb stress caused leaf dehydration, chlorisis and wilting to some degree. Among thetested plants, snc1plants were affected most dominant, with leaf serious yellowing, while nahGand npr1-1plants were damaged more slightly than the wild type. Compared to their respectivecontrols, Pb exposure increased the activities of SOD and POD in all the tested plants with a dose-effect relationship. Among all the plants, snc1had the highest SOD and POD activities, inparticular, the POD activity was much higher than other plants. The CAT activity was graduallydecreased with Pb increase in all the plants. The activities of SOD, POD and CAT isozymesshown by PAGE were well correlated to their activities measured by spectrophotomentricmethod. Pb exposure resulted in a decrease in chlorophyll contents and ratio of chlorophyll a/b inall plants, with the most decrease occurring in snc1plants and the least in nahG and npr1-1plants. Pb stress increased proline levels of all plants, with the most accumulation beingobserved in snc1plants. Under Pb exposure, membrane lipid peroxidation indicated by MDAcontents and electrolyte leakage rates were significantly enhanced in all plaints, with snc1plantsmost dramatically.Taken together, these data showed that under Pb exposure, SA high accumulations intensifiedoxidative damage to plants, inhibited plant root growth and dry weight increase. Endogenous SAdeficiency or SA signaling blockage was favorable for plant tolerance to Pb stress, as shown byobvious decrease of Pb-induced root growth retardation relative to wild type plants. Although theantioxidative enzymes SOD and POD significantly increased upon Pb exposure, they wereinsufficient to remove oxidative stress, especially in snc1plants, which resulted in membranelipid peroxidation. These data further confirmed that SOD or POD could be used as a biomarkerof metallic stress.