Abstract
Microorganisms rely on intricate signaling networks to respond to environmental changes. For plant-pathogenic microbes like Pseudomonas syringae pv. tomato strain DC3000 (PtoDC3000) reprogram gene expression in response to host-derived signals such as the phytohormone auxin (indole-3-acetic acid, IAA). In Arabidopsis-PtoDC3000 interactions, IAA promotes bacterial pathogenesis by suppressing plant salicylic acid-mediated immunity and regulating bacterial gene expression. However, the molecular mechanisms underlying bacterial IAA perception and transcriptional regulation remain largely unexplored. This study investigates the regulatory role of pmeR, which encodes the TetR-like family transcriptional regulator PmeR, in regulating auxin-mediated bacterial gene expression and virulence. PmeR can sense host-derived signals, such as flavonoids, and derepress gene expression of both its own gene (pmeR) and the mexAB-oprM efflux operon. Pevious transcriptomic evidence suggested that both pmeR and the mexAB-oprM operon are upregulated by IAA; however, whether PmeR regulates this auxin signaling pathway was not known. Using genetic, molecular, and biochemical approaches, I demonstrated that IAA induces expression of pmeR and the mexAB-oprM operon via both pmeR-dependent derepression and pmeR-independent mechanisms. Interestingly, IAA itself does not interfere with the binding of PmeR to the DNA sequences, indicating that PmeR does not act as a direct IAA sensor. Instead, its conjugated form, IAA-lysine, prevents the formation of PmeR/DNA complex, suggesting it is a ligand of PmeR. In addition to regulating its known targets, I showed, using molecular and biochemical approaches, that pmeR positively regulates additional auxin-upregulated genes, including PSPTO_1824 and PSPTO_4297. However, this does not appear to be through direct DNA binding. Instead, pmeR likely modulates intracellular IAA levels via mexAB-oprM-mediated export, indirectly influencing gene expression. Finally, to further delineate the scope of PmeR’s regulatory network, I used bioinformatic and biochemical analyses to identify additional auxin-responsive PmeR regulons, suggesting a broader role of PmeR in auxin-responsive transcriptional regulation. In summary, this dissertation advances our understanding of how P. syringae integrates host-derived auxin signals through regulatory networks mediated by the TetR-like transcriptional regulator PmeR, ultimately contributing to colonization within plant tissue. This study provides a foundation for future studies on hormone-mediated host-pathogen interactions and bacterial signal transduction mechanisms. Additionally, these findings suggest potential directions for developing strategies to mitigate bacterial diseases in crops by targeting auxin-responsive regulatory pathways.
Committee Chair
Barbara Kunkel
Committee Members
Armando Bravo; Hani Zaher; Joshua Blodgett; Petra Levin
Degree
Doctor of Philosophy (PhD)
Author's Department
Biology & Biomedical Sciences (Plant & Microbial Biosciences)
Document Type
Dissertation
Date of Award
5-8-2025
Language
English (en)
DOI
https://doi.org/10.7936/gp27-7h86
Author's ORCID
https://orcid.org/0000-0001-8841-6139
Recommended Citation
Lee, Chia-Yun, "Characterizing the Regulatory Roles of pmeR in Indole-3-acetic acid (IAA) Responsiveness and Pathogenesis in Pseudomonas syringae pv. tomato strain DC3000." (2025). Arts & Sciences Theses and Dissertations. 3480.
The definitive version is available at https://doi.org/10.7936/gp27-7h86