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.

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