MibR and LibR are involved in the transcriptional regulation of the ipdC gene in Azospirillum brasilense Sp7

Azospirillum brasilense is a PGPR that produces the phytohormone IAA, a signaling molecule involved in bacteria-plant interaction processes. IAA biosynthesis in Azospirillum is mainly carried out via the IPyA pathway in which the enzyme phenylpyruvate decarboxylase encoded by the ipdC gene is the main. The promoter region of ipdC gene contains cis elements that are highly conserved among different Azospirillum strains. In this work, we identified two proteins that interact with the promoter region of the ipdC gene, named MibR and LibR that belong to the MarR and LuxR transcriptional regulators family, respectively. Both proteins have an HTH domain, and in the case of LibR, it has a REC domain, with aspartic acid residue conserved in positions 7, 8 and 54, this last as a possible phosphorylation target. To explore their participation in the regulation of the ipdC gene, mutants of libR, mibR, and libR-mibR double mutant were generated. The results showed a decrease in IAA biosynthesis that was related to the observed decrease in ipdC gene expression mostly in the doble mutant compared with the wild type. In this work we suggest that ipdC transcription is regulated by LibR and MibR, providing new findings insight into the mechanism employed by A. brasilense to control IAA biosynthesis.