Involvement of nitrate reductase in nitric oxide generation and in the induction of acclimation responses to phosphorus restriction in soybean plants.

Nitrate reductase (NR) is an essential enzyme because of its role in nitrogen metabolism and in key signaling events through the generation of the reactive nitrogen species, nitric oxide (NO). In this work, we evaluated changes in endogenous NO levels during the onset of P-restriction in soybean plants (Glycine max), focusing on the possible pathways involved in its generation, namely NR and NO synthase like activity, NOSlike, and the subsequent role of NR during low P-acclimation. During the first 96 h of P-starvation NO levels increased in the leaves. The treatment of plants with L-NAME (a known inhibitor of NOS) did not markedly affect NO levels, while the inhibition of NR activity with sodium tungstate (W) reduced NO-related DAF-FM fluorescence. In addition, P-restriction triggered typical acclimation responses such as the increase of acid phosphatase (AP) and ribonuclease (RNase) activities, and the accumulation of organic acids (malic and citric). Treatment of P starved plants with W reduced NR activity as well as the magnitude of the acclimation responses studied. Overall, the data here presented suggest a role for NR as NO source, influencing the signaling pathways leading to early acclimation responses following P-deprivation in soybean plants.