# The phytohormone indole-3-acetic acid induces quorum sensing signal DSF turnover via a positive feedback biosynthetic loop in the phytopathogen Xanthomonas campestris

## TL;DR

This study shows that the plant hormone IAA helps the bacteria Xanthomonas campestris regulate its communication signals during infection.

## Contribution

The study reveals a new role for IAA in modulating bacterial quorum sensing through a positive feedback loop.

## Key findings

- IAA induces DSF turnover via RpfB-dependent mechanisms.
- IAA upregulates F0F1 ATP synthases and HepABCD to modulate pH-dependent DSF turnover.
- IAA downregulates IaepABCDE to modulate pH-independent DSF turnover.

## Abstract

Indole-3-acetic acid (IAA) accumulates in host plants following infection by Xanthomonas campestris pv. campestris (Xcc), the causal agent of cruciferous black rot. How exposure to IAA affects the invading Xcc remains unclear. Here, we demonstrate that either exogenous addition of IAA or endogenous production of IAA induced turnover of the quorum sensing (QS) signal diffusible signaling factor (DSF) in a RpfB-dependent manner. IAA addition prevented the cytoplasmic and culture pH decline. Transcriptomic analyses revealed four IAA-regulated gene clusters. Specifically, IAA induced the expression of trpB-A, enhancing tryptophan biosynthesis and intracellular IAA accumulation, and thereby establishing a self-reinforcing synthesis loop. IAA upregulated F0F1 ATP synthases and a resistance-nodulation-cell division (RND)-family efflux pump HepABCD to induce a pH-dependent DSF turnover. Moreover, IAA downregulated another RND family efflux pump IaepABCDE to induce a pH-independent DSF turnover. Finally, the IAA-regulated gene clusters were transcribed during the XC1 infection of cabbage. Collectively, these findings reveal a previously unrecognized role of IAA in modulating bacterial QS, underscoring the importance of IAA in the molecular dialogue between the pathogen and its host.

The online version contains supplementary material available at 10.1007/s44154-026-00298-1.