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

Besides N₂ 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 (NH₃) 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., δ¹⁵N). 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 NH₃ concentrations ([NH₃]), and we used Saccharomyces cerevisiae mutants to explore the involvement of the AMT/MEP/Rh proteins in atmospheric NH₃ 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 NH₃ transport through AMT/MEP/Rh transporters, but not with N₂ fixation. Furthermore, increasing the atmospheric [NH₃] stimulated the growth rate of the Azospirillum strains up to 5-fold in relation to ambient atmospheric [NH₃], showing that both Azospirillum strains scavenged the atmospheric NH₃ and used it to grow. Our data clearly show that: i) NH₃ scavenging by A. baldaniorum Sp245 and A. brasilense ARG2 is involved in increasing plant's N budget; and ii) NH₃ transport through AMT/MEP/Rh protein family transporters is involved in microbial NH₃ scavenging. This overlooked microbial trait can be an interesting tool to mitigate atmospheric [NH₃], especially in farming environments.