The plant hormone auxin (e.g. IAA) promotes pathogenesis by both modulating host defense responses and by acting as a signal that modulates virulence gene expression in the plant pathogen P. syringae strain PtoDC3000. The primary research objectives of the Kunkel Lab are to elucidate the mechanisms through which PtoDC3000 senses and responds to IAA and investigate how IAA-responsive genes function to promote pathogen growth and disease. We are currently focusing on the following questions:
- How does PtoDC3000 perceive and respond to IAA?
- What functions do IAA-induced genes in PtoDC3000 have during pathogenesis?
- Do auxins impact the outcomes of interactions between other plant pathogens and their hosts?
How does PtoDC3000 perceive and respond to IAA?
Although several genes involved in responding to IAA have been identified in other bacterial species, only a few have been biochemically demonstrated to regulate responses to IAA, or to encode IAA receptors. The bacterial IAA receptors identified to date (e.g. MarR, IacR, and AdmX) regulate a variety of physiological and metabolic processes (Kunkel and Johnson, 2021; Conway et al, 2022), not host-microbe interactions. Thus, we have a limited understanding of how bacteria sense and respond to IAA in the context of pathogenesis. We are using a combination of forward and reverse genetic approaches to identify regulators of IAA responsiveness in PtoDC3000.
Investigating the functions of IAA-induced proteins and their roles during P. syringae pathogenesis.
We are beginning this project by focusing on several genes that were strongly up-regulated by IAA in our previous study (Djami-Tchatchou et al., 2022). Currently, of most interest are two proteins that are predicted to encode small secreted proteins of unknown function. Given that both of these genes encoding these proteins are strongly up-regulated in response to IAA and are expressed in plant tissue during infection, we hypothesize they may play an important role during pathogenesis. We are currently investigating the contribution of these proteins to pathogenesis and carrying out experiments to determine if they are secreted from the bacterial cell, either into the periplasm or into the extracellular (e.g. apoplastic) space.
Exploring the role of auxin in agriculturally important Pseudomonas/host interactions.
To broaden the impact of our studies, we are expanding our studies to include agriculturally significant Pseudomonas-plant interactions. This work should provide significant new insight into the roles of IAA and other auxins during pathogenesis and the signaling events regulating pathogen gene expression during infection. Ultimately, we anticipate that our findings could lead to development of strategies for reducing disease caused by Pseudomonas and other pathogens that manipulate auxin signaling as a virulence strategy.