Involvement of nitric oxide in the inhibition of nitrogenase activity by nitrate in Lotus root nodules

Nitrogenase activity, as acetylene-reduction activity (ARA), in Lotus root nodules was clearly inhibited 27 h after the addition of nitrate. Nitric oxide (NO) production was detected at that time in nitrate-supplied root nodules using the NO-reactive fluorescent probe diaminofluorescein-2 diacetate. The involvement of NO production in the inhibition of nitrogenase activity by nitrate was investigated using the NO donor sodium nitroprusside (SNP) and the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO). SNP inhibited ARA at 1 mM, and c-PTIO suppressed the inhibition of ARA by nitrate. These results suggest that NO is involved in the inhibition of nitrogenase activity by nitrate in Lotus root nodules.