The auxin indole-3-acetic acid (IAA) is a plant hormone that not only regulates plant growth and development but also plays important roles in plant-microbe interactions. We previously reported that IAA alters expression of several virulence-related genes in the plant pathogen
Pseudomonas syringae
pv.
tomato strain DC3000 (
Pto
DC3000). To learn more about the impact of IAA on regulation of
Pto
DC3000 gene expression we performed a global transcriptomic analysis of bacteria grown in culture, in the presence or absence of exogenous IAA. We observed that IAA repressed expression of genes involved in the Type III secretion (T3S) system and motility and promoted expression of several known and putative transcriptional regulators. Several of these regulators are orthologs of factors known to regulate stress responses and accordingly expression of several stress response-related genes was also upregulated by IAA. Similar trends in expression for several genes were also observed by RT-qPCR. Using an
Arabidopsis thaliana
auxin receptor mutant that accumulates elevated auxin, we found that many of the
P. syringae
genes regulated by IAA
in vitro
were also regulated by auxin
in planta
. Collectively the data indicate that IAA modulates many aspects of
Pto
DC3000 biology, presumably to promote both virulence and survival under stressful conditions, including those encountered in or on plant leaves.
IMPORTANCE
Indole-3-acetic acid (IAA), a form of the plant hormone auxin, is used by many plant-associated bacteria as a cue to sense the plant environment. Previously, we showed that IAA can promote disease in interactions between the plant pathogen
Pseudomonas syringae
strain
Pto
DC000 and one of its hosts,
Arabidopsis thaliana
. However, the mechanisms by which IAA impacts the biology of
Pto
DC3000 and promotes disease are not well understood. Here we demonstrate that IAA is a signal molecule that regulates gene expression in
Pto
DC3000. The presence of exogenous IAA affects expression of over 700 genes in the bacteria, including genes involved in Type III secretion and genes involved in stress response. This work offers insight into the roles of auxin promoting pathogenesis.
Gene Expression and Stress Response Mediated by the Epigenetic Regulation of a Transposable Element Small RNA