The AltR transcription factor responds to plant thiosulfinates to regulate gene expression in a bacterial pathogen of onion.

Pantoea ananatis, the causative agent of onion center rot, encounters potent antimicrobial thiosulfinates, volatile organosulfur compounds released from damaged Allium tissue during pathogen-induced necrosis. The allicin tolerance (alt) gene cluster allows P. ananatis to overcome this chemical barrier. We demonstrate that AltR, a TetR-family transcriptional repressor, specifically regulates expression of the alt cluster and thus thiosulfinate tolerance in vitro and fitness in vivo. We identified a putative AltR binding box both in the altR promoter and elsewhere in the alt cluster, show that AltR-mediated repression is relieved in response to thiosulfinates. Using cysteine to serine substitutions, we demonstrate that AltR Cys100 is essential for thiosulfinate-responsive de-repression, while other AltR cysteine residues tune responsivity. Strains expressing AltR alleles with reduced thiosulfinate responsivity have reduced fitness in planta. Our findings uncover a regulatory mechanism by which a plant antimicrobial secondary metabolite acts as an environmental cue to modulate bacterial gene expression, enabling pathogen survival and virulence.