Auxin-Mediated Modulation of Maize Rhizosphere Microbiome: Insights from Azospirillum Inoculation and Indole-3-Acetic Acid Treatment

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.

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.

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.

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.