The Nitrate Assimilatory Pathway in Sinorhizobium meliloti: Contribution to NO Production

Abstract

The interaction between rhizobia and their legume host plants culminates in the formation of specialized root organs called nodules in which differentiated endosymbiotic bacteria (bacteroids) fix atmospheric nitrogen to the benefit of the plant. Interestingly, nitric oxide (NO) has been detected at various steps of the rhizobium-legume symbiosis where it has been shown to play multifaceted roles. It is recognized that both bacterial and plant partners of the Sinorhizobium meliloti – Medicago truncatula symbiosis are involved in NO synthesis in nodules. S. meliloti can also produce NO from nitrate when living as free cells in the soil. S. meliloti does not possess any NO synthase gene in its genome. Instead, the denitrification pathway is often described as the main driver of NO production with nitrate as substrate. This pathway includes the periplasmic nitrate reductase (Nap) which reduces nitrate into nitrite, and the nitrite reductase (Nir) which reduces nitrite into NO. However, additional genes encoding putative nitrate and nitrite reductases (called narB and nirB , respectively) have been identified in the S. meliloti genome. Here we examined the conditions where these genes are expressed, investigated their involvement in nitrate assimilation and NO synthesis in culture and their potential role in planta . We found that narB and nirB are expressed under aerobic conditions in absence of ammonium in the medium and most likely belong to the nitrate assimilatory pathway. Even though these genes are clearly expressed in the fixation zone of legume root nodule, they do not play a crucial role in symbiosis. Our results support the hypothesis that in S. meliloti , denitrification remains the main enzymatic way to produce NO while the assimilatory pathway involving NarB and NirB participates indirectly to NO synthesis by cooperating with the denitrification pathway.

Methane, arsenic, selenium and the origins of the DMSO reductase family

“…Further, this would argue for a later appearance of the NasC/NasA/AioA lineage as ammonium was the abundant form of nitrogen in the Archean 53 , obviating the need for an assimilatory nitrate reductase. A number of physiological studies have established that assimilatory nitrate reductase is expressed in both bacteria and archaea exclusively under oxic and suboxic conditions [54][55][56][57][58] , and in cyanobacteria assimilatory nitrate reduction occurs concomitantly with the evolution of O 2 during oxygenic photosynthesis 59 . Likewise, AioA functions predominantly to oxidize arsenite during aerobic respiration, even in arsenite oxidizing representatives occupying the deepest branches of the lineage (Fig.…”

Plant Nitrate Reductases Regulate Nitric Oxide Production and Nitrogen-Fixing Metabolism During the Medicago truncatula–Sinorhizobium meliloti Symbiosis

“…Indeed, the bacterial partner was shown to produce from 33 to 90% of NO in M. truncatula (Horchani et al, 2011) and soybean (Sanchez et al, 2010) nodules, respectively. If the bacterial denitrification pathway, including the periplasmic nitrate reductase (Nap) and nitrite reductase (Nir), has been described as the main enzymatic source of NO, additional genes encoding putative nitrate and nitrite reductase (called narB and NirN, respectively) have been recently identified that could also participate indirectly in NO synthesis (Ruiz et al, 2019). How the regulatory systems of the plant and the bacterial partners are coordinated to produce NO is one of the main issues to decipher the signaling and metabolic functions of NO at each stage of the symbiotic interaction.…”

Comparative Metagenomics Reveals Microbial Signatures of Sugarcane Phyllosphere in Organic Management

“…It also elevated the nitrogen metabolism of the phyllosphere microbes, including glutamine synthetase (K01915) and nitrite reductase (NADH) large subunit (K00362), which were identified as conventional-specific functions by LEfSe. The latter was reported to participate indirectly in nitric oxide (NO) production through the denitrification process ( Härtig and Zumft, 1999 ; Ruiz et al, 2019 ). Interestingly, human pathogenic genes involved in the amoebiasis pathway were specifically enriched under conventional farming, emphasizing the harmful effect of unsustainable practices not only to the environment but also to human health.…”