Abstract
The interaction between legumes and rhizobia leads to the establishment of a symbiotic relationship characterized by the formation of new organs called nodules, in which bacteria have the ability to fix atmospheric nitrogen (N2) via the nitrogenase activity. Significant nitric oxide (NO) production was evidenced in the N2-fixing nodules suggesting that it may impact the symbiotic process. Indeed, NO was shown to be a potent inhibitor of nitrogenase activity and symbiotic N2 fixation. It has also been shown that NO production is increased in hypoxic nodules and this production was supposed to be linked – via a nitrate/NO respiration process – with improved capacity of the nodules to maintain their energy status under hypoxic conditions. Other data suggest that NO might be a developmental signal involved in the induction of nodule senescence. Hence, the questions were raised of the toxic effects versus signaling/metabolic functions of NO, and of the regulation of NO levels compatible with nitrogenase activity. The present review analyses the different roles of NO in functioning nodules, and discusses the role of plant and bacterial (flavo)hemoglobins in the control of NO level in nodules.
Highlights
Nitric oxide (NO) is a gaseous molecule which was found to be involved in plant development, and response to biotic or abiotic stresses (Besson-Bard et al, 2008)
The interaction between legumes and rhizobia leads to the establishment of a symbiotic relationship characterized by the formation of new differentiated organs called nodules, which provide a niche for bacterial N2 fixation
A bacterial strain of S. meliloti mutated in the f-Hb gene elicited nodules on M. truncatula roots with higher levels of NO, lower N2 fixation efficiency and earlier nodule senescence than the wild type (WT) (Cam et al, 2012), suggesting that the expression of the Hmp is essential for maintaining NO levels compatible with symbiosis even though plant Hbs are proficient
Summary
Nitric oxide (NO) is a gaseous molecule which was found to be involved in plant development, and response to biotic or abiotic stresses (Besson-Bard et al, 2008). NO: THE FOX TO MIND THE GEESE Nitric oxide concentration was roughly estimated to be in the micromolar range in Medicago nodules (Meilhoc et al, 2010), and its level was significantly increased under hypoxic conditions or when nitrate was applied to nodules (Kato et al, 2010; Sanchez et al, 2010; Horchani et al, 2011).
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