Nitrogen dioxide regulates organ growth by controlling cell proliferation and enlargement in Arabidopsis

To gain more insight into the physiological function of nitrogen dioxide (NO2), we investigated the effects of exogenous NO2 on growth in Arabidopsis thaliana. Plants were grown in air without NO2 for 1 wk after sowing and then grown for 1-4 wk in air with (designated treated plants) or without (control plants) NO2. Plants were irrigated semiweekly with a nutrient solution containing 19.7 mM nitrate and 10.3 mM ammonium. Five-week-old plants treated with 50 ppb NO2 showed a <= 2.8-fold increase in biomass relative to controls. Treated plants also showed early flowering. The magnitude of the effects of NO2 on leaf expansion, cell proliferation and enlargement was greater in developing than in maturing leaves. Leaf areas were 1.3-8.4 times larger on treated plants than corresponding leaves on control plants. The NO2-induced increase in leaf size was largely attributable to cell proliferation in developing leaves, but was attributable to both cell proliferation and enlargement in maturing leaves. The expression of different sets of genes for cell proliferation and/or enlargement was induced by NO2, but depended on the leaf developmental stage. Collectively, these results indicated that NO2 regulates organ growth by controlling cell proliferation and enlargement.