Arbuscular mycorrhizal fungus-mediated amelioration of NO-induced phytotoxicity in tomato.

Atmospheric nitrogen dioxide (NO) negatively affects plant (crop) growth and development, as well the yield and quality in some regions or environments. Arbuscular mycorrhizal fungus (AMF)-mediated amelioration of NO-induced plant damage has been reported, but the underlying mechanisms remained unclear. This study explored the beneficial effect of AMF symbiosis on tomato plant responses to NO at physiology, biochemistry, and gene expression, with an emphasis on nitrate metabolism, antioxidative defense, and photosynthetic performance. Pot-grown plants were used in the experiments, which were performed in laboratory from February to November 2019. NO fumigation with a dose of 10 ± 1 ppm was carried out after 50 d of plant growth, and data were collected following 8 h of fumigation. NO fumigation (+NO) and AMF inoculation (+AMF), alone and especially in combination (NO + AMF), increased the gene expression of nitrate- and nitrite reductase, and their enzymatic activity in leaves, such as by 61%, 27%, and 126% for the activity of nitrate reductase, and by 95%, 37%, and 188% for nitrite reductase, respectively, in +NO, +AMF, and AMF + NO plants relative the control (-NO, -AMF) levels. Following NO exposure, +AMF leaves displayed stronger activities of superoxide dismutase, peroxidase and catalase, and higher content of glutathione and ratio of its reduced form to oxidized form, as compared with -AMF ones. Correspondingly, lesser oxidative damage was detected in +AMF than in -AMF plants, as indicated by the contents of HO and malondialdehyde, electrolyte leakage, also by in situ visualization for the formation of HO, superoxide anion, and dead cells. The increased antioxidative capacity in +AMF plants was correlated with enhanced expression of antioxidation-related genes. Exposure to NO substantially impaired photosynthetic processes in both + AMF and -AMF plants, but an obvious mitigation was observed in the former than in the latter. For example, the total chlorophyll, net photosynthetic rate, stomatal conductance, and ribulose-1,5-bisphosphate carboxylase activity were 18%, 27%, 26%, and 40% higher, respectively, in +AMF than in -AMF plants under NO stress. The differential photosynthetic performance was also revealed by chlorophyll fluorescence imaging. We analyzed the expression patterns of some genes related to photosynthesis and carbon metabolisms, and found that all of them exclusively presented a higher expression level in +AMF plants relative to -AMF ones under NO stress. Taken together, this study provided evidence that AMF symbiosis played a positively regulatory role in host plant responses to NO, probably by increasing leaf nitrate metabolism and antioxidative defense, and maintaining the photosynthetic efficiency to some extent, wherein the transcription regulation might be a main target.