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

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.