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Over the past decades, crop yields have risen in parallel with increasing use of fossil fuel-derived nitrogen (N) fertilizers but with concomitant negative impacts on climate and water resources. There is a need for more sustainable agricultural practices, and biological nitrogen fixation (BNF) could be part of the solution. A variety of nitrogen-fixing, epiphytic, and endophytic plant growth-promoting bacteria (PGPB) are known to stimulate plant growth. However, compared with the rhizobium-legume symbiosis, little mechanistic information is available as to how PGPB affect plant metabolism. Therefore, we investigated the metabolic changes in roots of the model grass species

Herbaspirillum seropedicae is a nitrogen-fixing β-proteobacterium that associates with roots of gramineous plants. In silico analyses revealed that H. seropedicae genome has genes encoding a putative respiratory (NAR) and an assimilatory nitrate reductase (NAS). To date, little is known about nitrate metabolism in H. seropedicae, and, as this bacterium cannot respire nitrate, the function of NAR remains unknown. This study aimed to investigate the function of NAR in H. seropedicae and how it metabolizes nitrate in a low aerated-condition. RNA-seq transcriptional profiling in the presence of nitrate allowed us to pinpoint genes important for nitrate metabolism in H. seropedicae, including nitrate transporters and regulatory proteins. Additionally, both RNA-seq data and physiological characterization of a mutant in the catalytic subunit of NAR (narG mutant) showed that NAR is not required for nitrate assimilation but is required for: (i) production of high levels of nitrite, (ii) production of NO and (iii) dissipation of redox power, which in turn lead to an increase in carbon consumption. In addition, wheat plants showed an increase in shoot dry weight only when inoculated with H. seropedicae wild type, but not with the narG mutant, suggesting that NAR is important to H. seropedicae-wheat interaction.