The diazotrophic bacteria Azospirillum baldaniorum and A. brasilense improve wheat seedlings' nitrogen budget through ammonia scavenging

Besides N 2 fixation, we consider that other diazotrophic traits can be explored to increase plants' nitrogen (N) budget. Here, we report initial results of the capacity of the diazotrophic plant growth promoting rhizobacteria Azospirillum baldaniorum and A. brasilense to improve wheat seedlings' N budget through ammonia (NH 3 ) scavenging. We inoculated wheat seedlings with two Azospirillum strains ( A. baldaniorum Sp245 and A. brasilense ARG2) and determined its effect on plant biomass, N content and N isotopic signatures (i.e., δ 15 N). Furthermore, using bipartite Petri dishes, we grew the Azospirillum strains under increasingly alkaline conditions (from pH 7.5 to 10.0), which created a gradient of atmospheric NH 3 concentrations ([NH 3 ]), and we used Saccharomyces cerevisiae mutants to explore the involvement of the AMT/MEP/Rh proteins in atmospheric NH 3 scavenging. Wheat seedlings inoculated with A. baldaniorum Sp245 and A. brasilense ARG2 increased their N content by 65 and 94 % (respectively), and their negative N isotopic signatures (around −10 ‰, which contrasted with positive signatures in control plants) were compatible with NH 3 transport through AMT/MEP/Rh transporters, but not with N 2 fixation. Furthermore, increasing the atmospheric [NH 3 ] stimulated the growth rate of the Azospirillum strains up to 5-fold in relation to ambient atmospheric [NH 3 ], showing that both Azospirillum strains scavenged the atmospheric NH 3 and used it to grow. Our data clearly show that: i) NH 3 scavenging by A. baldaniorum Sp245 and A. brasilense ARG2 is involved in increasing plant's N budget; and ii) NH 3 transport through AMT/MEP/Rh protein family transporters is involved in microbial NH 3 scavenging. This overlooked microbial trait can be an interesting tool to mitigate atmospheric [NH 3 ], especially in farming environments. • Azospirillum strains Sp245 and ARG2 can scavenge atmospheric NH 3 and use it as a N source. • Inoculation with Azospirillum strains Sp245 and ARG2 can improve plant's N budget through NH 3 scavenging. • The AMT/MEP/Rh proteins are involved in microbial atmospheric NH 3 scavenging.