We explored the enzymatic source of nitric oxide (NO) in Medicago truncatula – Sinorhizobium meliloti nodules, under normoxic (normal oxygen concentration) and hypoxic (reduced oxygen content) conditions. When transferred from normoxia to hypoxia, nodule NO production rapidly increased, indicating that NO production capacity is present in functioning nodules and may be promptly up-regulated in response to decreased oxygen availability. NO production was stimulated by nitrate and nitrite, and inhibited by tungstate, a nitrate reductase inhibitor. Nodules obtained with either plant or bacteria deficient in nitrate reductase activities exhibited reduced NO production. Energy state (ATP/ADP) was reduced when nodules were incubated in the presence of tungstate. Our data indicate that both plant and bacterial nitrate reductases are involved in NO synthesis. We proposed the existence of a nitrate-NO respiration process in nodules which could play a role in the maintenance of the energy status required for nitrogen fixation under oxygen-limiting conditions.
Horchani, F. et al. (2011) Both Plant and Bacterial Nitrate Reductases Contribute to Nitric Oxide Production in Medicago truncatula Nitrogen-Fixing Nodules. Plant Physiology 155, 1023 –1036. DOI: 10.1104/pp.110.166140