Nitric oxide, a multi-faceted regulator in legume-rhizobium symbiosis

Abstract

Nitric oxide (NO) is a gaseous intra- and intercellular signaling molecule with a broad spectrum of regulatory functions in plant growth and development. NO is involved in plant response to biotic and abiotic stresses, including hypoxia. NO production was also detected during symbiotic interactions, in the nitrogen-fixing symbiosis (NFS) between legumes and soil gram-negative bacteria called rhizobia. The interaction between legumes and rhizobia leads to the establishment of a symbiotic relationship characterized by the formation of new differentiated organs named nodules, which provide a niche for bacterial nitrogen fixation. In the nodules, bacteria differentiate into bacteroids with the ability to fix atmospheric nitrogen via nitrogenase activity. As nitrogenase is strongly inhibited by oxygen, nitrogen fixation is made possible by the microaerophilic conditions prevailing in the nodules. Thus, nodule development occurs in changing oxygen conditions, shifting from a normoxic environment during symbiosis establishment to a microoxic one in functioning nodules. Increasing evidence has been reporting the presence of NO during symbiosis, from early interaction steps between the plant and the bacterial partners, to nitrogen-fixing and senescence steps in mature nodules. Using a NO-specific fluorescent probe, a transient NO production was indeed detected on the surface of Medicago sativa and Lotus japonicus roots 4 hours after inoculation. NO was likewise present in M. truncatula about 4 days post-inoculation in infected root hairs, both in the colonized curl and along the infection thread, as well as in cells of nodule primordia, not yet infected by rhizobia. At later stages of the interaction, NO was observed to be produced in nitrogen-fixing nodules of Medicago truncatula and Medicago sativa, particularly in bacteroid-containing cells. NO was also detected in mature nodules of Lotus japonicus, soybean, and M. truncatula. Interestingly, hypoxic treatments were shown to significantly increase NO production in soybean and M. truncatula nodules. Both the plant and the bacterial partners participate significantly in NO synthesis.

The chemical nature, concentration and location of NO might determine its biological role. NO was found to be required for an optimal establishment of the symbiotic interaction. A transcriptomic analysis at early stages of the interaction showed that NO is potentially involved in the repression of plant defence reactions, favouring the establishment of the plant-microbe interaction. However, at high local concentration NO can become very toxic, and its inhibits the growth of Sinorhizobium meliloti in culture, as well as the symbiotic nitrogen fixation in legumes. Additionally, NO has also been demonstrated to play a beneficial metabolic function for the maintenance of the energy status under hypoxic conditions, or to have a regulatory role in the regulation of nitrogen metabolism in functioning nodules. Finally, it was established that an increase in NO level inside mature nodules of M. truncatula leads to early nodule senescence, and NO was hypothesized to be a signal for nodule senescence. This presentation will summarize the signalling roles of NO in the establishment and the breaking off of the symbiotic interaction, the toxic versus metabolic roles of NO in functioning nodules, and discuss the multi-faceted functions of NO in the regulation of the nitrogen-fixing symbiosis.