Dual role of auxin in regulating plant defense and bacterial virulence gene expression duringPseudomonas syringae PtoDC3000 pathogenesis

Modification of host hormone biology is a common strategy used by plant pathogens to promote disease. For example, the bacterial pathogenPseudomonas syringaestrainPtoDC3000 produces the plant hormone auxin (Indole-3-acetic acid, or IAA) to promotePtoDC3000 growth in plant tissue. Previous studies suggest that auxin may promotePtoDC3000 pathogenesis through multiple mechanisms, including both suppression of salicylic acid (SA)-mediated host defenses and via an unknown mechanism that appears to be independent of SA. To test if host auxin signaling is important during pathogenesis, we took advantage ofArabidopsis thalianalines impaired in either auxin signaling or perception. We found that disruption of auxin signaling in plants expressing an inducible dominantaxr2-1mutation resulted in decreased bacterial growth, demonstrating that host auxin signaling is required for normal susceptibility toPtoDC3000, and this phenotype was dependent on SA-mediated defenses. However, despite exhibiting decreased auxin perception,tir1 afb1 afb4 afb5quadruple mutant plants lacking four of the six known auxin co-receptors supported increased levels of bacterial growth. This mutant also exhibited elevated IAA levels, suggesting that the increased IAA in these plants may promotePtoDC3000 growth independent of host auxin signaling, perhaps through a direct effect on the pathogen. In support of this, we found that IAA directly impacted the pathogen, by modulating expression of bacterial virulence genes, both in liquid culture and in planta. Thus, in addition to suppressing host defenses, IAA acts as a microbial signaling molecule that regulates bacterial virulence gene expression.