ABSTRACT

The aim of this work was to characterize the generation of nitric oxide (NO) as an endogenous product as well as the effect of nitric oxide supplementation to intact soybean (Glycine max var Hood) leaves. The EPR spectra of homogenates from soybean leaves were examined in the presence of MGD-Fe and substrates for the activities of the enzymes considered to be the feasible sources of NO in plants, nitrate reductase (NR) and nitric oxide synthase (NOS). A distinctive EPR signal was detected in leaves homogenates either in the presence of NADH and nitrite or arginine and NADPH. Soybean leaves and homogenates were exposed to S-nitrosoglutathione (GSNO) as NO donor. NO incorporation in soybean leaves was detected by the presence of the NO-MGD-Fe adduct by electron paramagnetic resonance (EPR). The activities of the heme enzymes, catalase and ascorbate peroxidase (AP), were inhibited by 74 and 71% when homogenates from soybean leaves were incubated in the presence of 1 mM GSNO. However, the activity of catalase was not significantly different between control and NO-exposed leaves. Under these conditions, the activity of AP was significantly inhibited (-17%) in soybean leaves immediately after exposure to NO as compared to control leaves (0.72±0.02 and 0.60±0.03 μmol ascorbate min^-1 mg^-1 prot, respectively). These results suggest that NO is an endogenous product in soybean leaves  and  that  under conditions where NO generation is increased it might be responsible of regulating cellular hydrogen peroxide levels, mainly through the reversible inhibition of AP activity.