Multiple biological processes involved in the regulation of salicylic acid in Arabidopsis response to NO2 exposure

Salicylic acid (SA) has been extensively implicated in plant response to abiotic stresses. However, just a few studies addressed the regulatory role of SA in plants exposed to NO2. In this study, we used Arabidopsis wild type (WT) and its SA-related mutants, including snc1 (suppressor of npr1-1, constitutive 1) with high endogenous SA, transgenic line nahG with low SA level, and npr1-1 (nonexpressor of PR gene) with SA signaling blockage to explore the possible regulation mechanism of SA, focusing on NO2-induced gene expression patterns. Plants were fumigated with 10 ppm NO2 for 8 h. The results showed that NO2 fumigation led to oxidative stress damage to all the tested plants as indicated by productions of H2O2, superoxide anion (O2−) and malondialdehyde, and cell death shown by electrolyte leakage and Evans Blue staining. However, snc1 plants were injured to a much less degree, whereas nahG and npr1-1 plants to a larger degree as compared with WT plants. Correspondingly, the activities of superoxide dismutase, peroxidase, catalase and nitrate reductase were induced by NO2 fumigation to a higher extent in snc1 plants, and a less extent in nahG and npr1-1 plants relative to WT plants. The expressions of the genes related to antioxidative defense, disulfide bond reduction, SA-responsive and -biosynthesis pathway, flavonoid biosynthesis, nitrate metabolism and kinase were analyzed. The results showed that these genes were differentially expressed among the tested genotypes under NO2 fumigation, wherein the most were significantly up-regulated in snc1 plants relative to other tested genotypes. Based on the gene expression patterns, it was proposed that SA-mediated plant tolerance to NO2 stress was associated with multiple biological processes.