The main origin of endogenous NO in higher non-leguminous plants

The main origin of nitric oxide (NO) production in higher non-leguminous plants is studied in this paper. NO content in the leaf of wheat (Triticum aestivum L.), orchid (Malaxis monophyllos L.) and aloe (Aloe vera L.) was decreased over 90% by the treatment of heat or microwave. It indicates that NO is primarily enzymically produced in higher non-leguminous plants. Nitrate reductase (NR; EC 1.6.6.1) activity and NO content were about 74% and 56%, respectively, in wheat seedling grown in a medium lacking molybdenum. Sodium tungstate almost completely inhibited NR activity and decreased NO content near 90% in the wheat seedling extraction. Taking out inducing factors such as KNO3 and light of NR gene expression, the NR activity was less than 30% and the NO content was less than 20% of the control in the wheat seedling. These results suggest that NO is mainly produced through NR pathway in wheat seedlings. When NaNO2 was applied to wheat seedlings for 10 h in dark before harvest, the NO content vastly increased but dramatically decreased after treatment by heat or microwave. The provision of NaNO2 as above and added exogenous Cu2+ decreased both NR and nitrite reductase (NiR; EC 1.7.7.1) activity, but increased NO content. These results suggest that the conversion from NO2– to NO in wheat seedling is also enzymic and this process is independent of NR and NiR.