| Summary: | Exposing <i>Arabidopsis thaliana</i> (Arabidopsis) seedlings fed with soil nitrogen to 10−50 ppb nitrogen dioxide (NO<sub>2</sub>) for several weeks stimulated the uptake of major elements, photosynthesis, and cellular metabolisms to more than double the biomass of shoot, total leaf area and contents of N, C P, K, S, Ca and Mg per shoot relative to non-exposed control seedlings. The <sup>15</sup>N/<sup>14</sup>N ratio analysis by mass spectrometry revealed that N derived from NO<sub>2</sub> (NO<sub>2</sub>-N) comprised < 5% of the total plant N, showing that the contribution of NO<sub>2</sub>-N as N source was minor. Moreover, histological analysis showed that leaf size and biomass were increased upon NO<sub>2</sub> treatment, and that these increases were attributable to leaf age-dependent enhancement of cell proliferation and enlargement. Thus, NO<sub>2</sub> may act as a plant growth signal rather than an N source. Exposure of Arabidopsis leaves to 40 ppm NO<sub>2</sub> induced virtually exclusive nitration of PsbO and PsbP proteins (a high concentration of NO<sub>2</sub> was used). The PMF analysis identified the ninth tyrosine residue of PsbO1 (<sup>9</sup>Tyr) as a nitration site. <sup>9</sup>Tyr of PsbO1 was exclusively nitrated after incubation of the thylakoid membranes with a buffer containing NO<sub>2</sub> and NO<sub>2</sub><sup>−</sup> or a buffer containing NO<sub>2</sub><sup>−</sup> alone. Nitration was catalyzed by illumination and repressed by photosystem II (PSII) electron transport inhibitors, and decreased oxygen evolution. Thus, protein tyrosine nitration altered (downregulated) the physiological function of cellular proteins of Arabidopsis leaves. This indicates that NO<sub>2</sub>-induced protein tyrosine nitration may stimulate plant growth. We hypothesized that atmospheric NO<sub>2</sub> at ambient concentrations may induce tyrosine nitration of PYR/PYL/RCAR receptors in Arabidopsis leaves, followed by degradation of PYR/PYL/RCAR, upregulation of target of rapamycin (TOR) regulatory complexes, and stimulation of plant growth.
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