Leaf nitrogen dioxide uptake coupling apoplastic chemistry, carbon/sulfur assimilation, and plant nitrogen status

Emission and plant uptake of atmospheric nitrogen oxides (NO + NO2) significantly influence regional climate change by regulating the oxidative chemistry of the lower atmosphere, species composition and the recycling of carbon and nutrients, etc. Plant uptake of nitrogen dioxide (NO2) is concentration-dependent and species-specific, and covaries with environmental factors. An important factor determining NO2 influx into leaves is the replenishment of the substomatal cavity. The apoplastic chemistry of the substomatal cavity plays crucial roles in NO2 deposition rates and the tolerance to NO2, involving the reactions between NO2 and apoplastic antioxidants, NO2-responsive germin-like proteins, apoplastic acidification, and nitrite-dependent NO synthesis, etc. Moreover, leaf apoplast is a favorable site for the colonization by microbes, which disturbs nitrogen metabolism of host plants. For most plant species, NO2 assimilation in a leaf primarily depends on the nitrate (NO3 (-)) assimilation pathway. NO2-N assimilation is coupled with carbon and sulfur (sulfate and SO2) assimilation as indicated by the mutual needs for metabolic intermediates (or metabolites) and the NO2-caused changes of key metabolic enzymes such as phosphoenolpyruvate carboxylase (PEPc) and adenosine 5'-phosphosulfate sulfotransferase, organic acids, and photorespiration. Moreover, arbuscular mycorrhizal (AM) colonization improves the tolerance of host plants to NO2 by enhancing the efficiency of nutrient absorption and translocation and influencing foliar chemistry. Further progress is proposed to gain a better understanding of the coordination between NO2-N, S and C assimilation, especially the investigation of metabolic checkpoints, and the effects of photorespiratory nitrogen cycle, diverse PEPc and the metabolites such as cysteine, O-acetylserine (OAS) and glutathione.