Date of Award
11-9-2023
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Indole-3-acetic acid (IAA) is a molecule ubiquitous to the environment and is known to be produced across all three domains of life. It is most well-known for its role as an important plant hormone (an auxin) and until recently, its roles in other organisms have not been explored. It is well established that bacteria, and pathogens in particular, need to sense and respond to environmental signals. One model organism for studying plant pathogen interactions that is known to both produce and respond to IAA is Pseudomonas syringae pv. tomato strain DC3000 (PtoDC3000). Many plant metabolites have been explored as potential signals for pathogens during infection, but only recently have plant hormones been investigated as molecular signals sensed by pathogens. It remains unknown how bacteria sense and respond to IAA and what aspects of biology may be regulated in response. Here, I present data that gives insights into how PtoDC3000 senses and responds to IAA and how this response relates to infection. To begin my investigation, I first designed and carried out a mutant screen to identify candidate regulators of the PtoDC3000 IAA response. This screen unveiled several potential regulators and components of the IAA response. Much of my work focused on one candidate regulator, AefR, a TetR family repressor known to regulate several aspects of biology in other P. syringae strains, including motility and resistance to antimicrobial compounds. I determined that expression of several of the IAA responsive genes in PtoDC3000 are regulated through aefR by monitoring transcript levels. Next, I discovered that both motility and resistance to antimicrobial compounds are regulated by IAA, at least in part through aefR. I then carried out experiments to place this biology into the context of pathogenesis and found that aefR is a positive regulator of entry into the interior of the leaf, and a negative regulator of leaf interior (e.g. endophytic) colonization on Arabidopsis. My observations led to the development of a model for how this regulation by AefR might work. Interestingly, I discovered that expression of some genes in the aefR mutant were still IAA responsive, indicating that there are other as of yet unknown IAA response regulators in PtoDC3000. I also investigated if several IAA-regulated genes play a role in infection. My analysis revealed that the IAA response in PtoDC3000 appears to play multiple roles during infection. In summary, I have identified specific genes and biological processes regulated by IAA that play a role during infection, identified and characterized AefR, a key regulator of the PtoDC3000 IAA response, and described a novel method for how PtoDC3000 and potentially other pathogens regulate virulence genes to coordinate phases of infection.
Language
English (en)
Chair and Committee
Barabra Kunkel
Recommended Citation
Johnson, Joshua Michael Brian, "Uncovering the genetic basis underlying the Pseudomonas syringae pv. tomato strain DC3000’s response to IAA and its role in regulating PtoDC3000 biology" (2023). Arts & Sciences Electronic Theses and Dissertations. 3191.
https://openscholarship.wustl.edu/art_sci_etds/3191