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Purpose: Auxins, especially indole-3-acetic acid (IAA), influences microbial physiology, but their effects on the plant microbiome are underreported. This study aimed to understand the impact of exogenously supplemented IAA or IAA produced by Azospirillum on maize rhizosphere microbiome. Methods: One-week-old maize seedlings were inoculated with Az39 (A. argentinense’s strain), Az39 + L-Trp (Azospirillum-produced IAA), L-Trp (rhizosphere-produced IAA), and exogenous IAA to study their effects on the maize microbiome. Rhizosphere samples were collected after 14 days for DNA extraction, sequencing via Illumina MiSeq, and bioinformatic analysis were made to explore prokaryotic community composition and predict metabolic functions. Results: Differences in the Shannon index were observed between Az39 inoculation and exogenous L-Trp application, and between Az39 inoculation and Az39 + L-Trp for phylogeny and observed features. Azospirillum inoculation influences on bacterial structure bacterial structure. Genus Actinospica and Bradyrhizobium were associated with IAA treatment, Rokubacteriales and Puia with L-Trp, and Cupriavidus and Pseudomonas with Az39 + L-Trp. Azospirillum and Sphingobium were linked to Az39 inoculation. We identified fifty microbial taxa following the exogenous application of IAA application and twenty-two with potential rhizosphere IAA production. Nitrogen fixation was the most abundant metabolic function in the prokaryotic rhizosphere. Conclusion: Our results show prokaryotic groups specifically increase in the maize rhizosphere following application of Azospirillum, the IAA produced by Azospirillum, or the rhizosphere community and the exogenous IAA. These groups could be considered specific markers of the IAA activity in the rhizosphere.

The aim of this work was to evaluate under diverse plant growth conditions the previous incubation between B. japonicumE109 (BjE109) and A. argentinenseAz39 (AaAz39) and elucidate their impact on the Bradyrhizobium-soybean symbiosis and plant growth. Five treatments were performed: (i) uninoculated seeds; (ii) seeds inoculated with BjE109; (iii) seeds inoculated with AaAz39; (iv) seeds co-inoculated at the seeds sowing with equal volume (1:1) of BjE109 and AaAz39; and (v) seeds inoculated with equal volume (1:1) of BjE109 and AaAz39 24 h before seed sowing. Each treatment was assessed through a seed recovery assay, glasshouse assays, and field assays. The single plant level differences were achieved under greenhouse conditions while differences at population level (crop) were achieved by a field assay. The previous incubation between BjE109 and AaAz39 improved the ability of BjE109 to survive on soybean seeds with 25% and 10% of cell recovery at 4 h and 6 days post-inoculation respectively. As a result of the greater bacterial survival, the symbiosis parameters like nodule number, size, and biomass and nodulation percentage also significantly increased. In agreement with these observations, the grain yield under field conditions showed 13.3 and 17.3% greater than immediate combination or single BjE109 inoculation respectively. The previous incubation between BjE109 and AaAz39 24 h before their inoculation improves the Bradyrhizobium-soybean symbiosis and increases both plant growth under culture controlled and crop productivity under field conditions, in comparison with the single inoculation with BjE109 or the immediate inoculation using both strains.