Benzoxazinoids stimulate chemotaxis and act as a signaling molecule in Azospirillum brasilense Ab-V5, while showing minor effects on Pseudomonas protegens Pf-5.

Root colonization by plant growth-promoting bacteria (PGPB) involves recruiting beneficial partners from the rhizosphere. Among well-studied PGPB, Azospirillum brazilense Ab-V5 and Pseudomonas protegens Pf-5 are two well-known bacterial strains renowned for their growth-enhancing capacity and extensively used as bio-inputs. Many cereals, such as maize, produce indole-derived benzoxazinoids (BXs), specialized metabolites that shape root-associated microbiomes to promote colonization by plant-growth-promoting bacteria (PGPB). Although the mechanisms by which BXs recruit PGPB remain unclear, we hypothesize that BXs directly facilitate root colonization by favoring bacteria adapted to these metabolites in the soil environment. In this study, we investigated the impact of the relatively stable lactam BX-derivative, 6-methoxy-2-benzoxazolinone (MBOA), on two PGPB strains: Azospirillum brasilense Ab-V5 and Pseudomonas protegens Pf-5. Transcriptomic analysis revealed that MBOA had minimal effects on Pf-5, but triggered extensive gene expression changes in Ab-V5, particularly in pathways related to energy metabolism, chemotaxis, and biofilm formation. Subsequent assays confirmed that MBOA acts as a chemoattractant for Ab-V5 and, at moderate concentrations, enhances both biofilm formation and colonization of Arabidopsis roots. We propose that the chemotactic property of MBOA on Ab-V5 can enhance its establishment in the rhizosphere and that this metabolite can trigger the metabolic transition required for root colonization. IMPORTANCE: In this paper, we studied the impact of benzoxaziniods on root colonization mechanisms of two potent plant-growth- promoting bacterial strains. We explored these mechanisms by an RNA sequencing experiment and by microscopy. The paper highlights how biofilm is particularly affected and reports on chemotactic responses. Most of the results we obtained we could validate with phenotypic assays. We show that benzoxazinoids, produced by many cereals, profoundly affect bacterial behavior related to plant-bacterial interactions. The bacteria in this study are known for their ecological roles in the soil, being either in plant protection or as biofertilizers. Thus, this work holds significant socio-economic value for society.