Ethylene insensitive mutation improves Arabidopsis plant tolerance to NO₂ exposure

Ethylene signaling was addressed, for the first time, in plant responses to nitrogen dioxide (NO₂) by comparatively analyzing the performance of Arabidopsis ethylene insensitive 2 (ein2-1) with wild-type (WT) plants. Following NO₂ fumigation, severe leaf wilting and chlorosis occurred in WT plants, but much less symptoms were observed in ein2-1. The activities of superoxide dismutase (SOD), peroxidase (PRX) and catalase (CAT) were 39%, 92%, and 11% higher, respectively, in ein2-1 than in WT following NO₂ exposure. Although glutathione contents and the ratio of its reduced form (GSH) to oxidized form (GSSG) were decreased by NO₂, an obviously alleviated degree was detected in ein2-1 relative to WT. Correspondingly, the contents of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), and electrolyte leakage were 25%, 24%, and 29% lower, respectively, in ein2-1 than in WT. The difference of oxidative stress between two tested genotypes was also revealed by the leaf staining regarding the production and distribution of H₂O₂, superoxide anion (O₂˙-), and cell death. The genes involved in antioxidation or oxidation-reduction processes mostly presented a stronger expression in ein2-1 than in WT under NO₂ stress. The photosynthesis-related parameters including chlorophyll and soluble sugar contents, net photosynthetic rate (Pn), and ribulose bisphosphate carboxylase/oxygenase (Rubisco) activity and gene expression, and chlorophyll fluorescence parameters were affected, generally, to a lesser degree in ein2-1 than in WT following NO₂ fumigation. The enzymatic activities and gene expressions of invertases mostly displayed a higher level in ein2-1 relative to WT following NO₂ fumigation. For example, the activities of cytoplasmic, cell wall and vacuolar invertases were 76%, 26%, and 26% higher, respectively, in ein2-1 than in WT. Together, these data suggest that ethylene signal insensitivity efficiently improves plant tolerance to NO₂ exposure, and the possible mechanisms might be correlated with leaf antioxidative defense, photosynthesis-related processes, and sucrose metabolisms.