Salicylic acid-mediated alleviation in NO2 phytotoxicity correlated to increased expression levels of the genes related to photosynthesis and carbon metabolism in Arabidopsis

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Nitrogen dioxide (NO2) is one of the major air pollutants affecting plant growth, development and yields. Salicylic acid (SA)-mediated alleviation in NO2-induced plant damage has been reported in Arabidopsis. However, the involved mechanisms are far from clear. In this study, Arabidopsis wild type (WT) and its SA-altering mutants, snc1 (suppressor of npr1-1, constitutive 1) with high SA, npr1-1 (nonexpressor of PR gene) with SA signaling blockage, and transgenic line nahG with low SA, were used to decipher the SA action mechanisms. NO2 fumigation at a concentration of 10 ppm for 8 h led to more severe injuries on nahG and npr1-1 plants, whereas a lesser on snc1 plants compared with WT plants. The expressions of SA biosynthesis-related genes were up-regulated leading to more SA accumulations in all the tested plants after NO2 treatment. The expression patterns of ethylene, oxylipin and nitric oxide biosynthesis-related genes were regularly altered among NO2-treated genotypes, among which the expression of dioxygenase-encoding gene was dramatically increased in snc1 plants. Total chlorophyll contents, net photosynthetic rate, maximum quantum and actual quantum efficiency of PSII were decreased, to a greater degree in nahG and npr1-1 plants, whereas a lesser in snc1 plants compared with WT plants after NO2 treatment. We analyzed the expression patterns of the selected genes related to chlorophyll biosynthesis, photosystem I and II structure and functions, electron transport, ATP synthesis, Calvin cycle, as well as photorespiration, pentose phosphate pathway and starch degradation. The expression levels of the most genes were up-regulated in snc1 and WT plants, whereas down-regulated in nahG and npr1-1 plants after NO2 treatment, among which the expression pattern of a photorespiration-related gene was particularly specific. The plant tolerant phenotypes were positively correlated with the gene expressions, suggesting that SA-mediated alleviation in NO2 phytotoxicity was associated with photosynthesis and carbon metabolisms, also with other signal molecules.