Degree
Doctor of Philosophy (PhD)
Department
Department of Biological Sciences
Document Type
Dissertation
Abstract
Agrobacterium fabrum, the causative agent of crown gall disease, infects a wide range of plants, causing significant damage worldwide. In the rhizosphere, it is exposed to a variety of biotic and abiotic stressors. The A. fabrum-encoded transcription factor PecS was previously shown to confer fitness in the rhizosphere. The gene encoding pecS is divergent to pecM, which encodes an efflux pump. The regulation of the pecS-pecM gene pair is under the control of PecS. The dissertation describes the identification of the substrate for A. fabrum PecM as well as physiologically relevant properties of both PecM and PecS.
The substrate of the efflux pump PecM was identified as the plant exudate 4-hydroxybenzaldehyde (4HBA), as evidenced in part by untargeted metabolomics which showed that pecM::Km cells in which pecM is disrupted accumulate 4HBA. 4HBA was able to selectively induce pecM expression. However, in vitro assays suggested that 4HBA is not a ligand for PecS, implying the presence of a different 4HBA-responsive transcription factor.
A notable feature of pecM::Km cells included resistance to oxidative stress. The pecM::Km cells were otherwise indistinguishable from WT cells with respect to colony morphology, growth in rich media, sensitivity to antibiotics and metal ions, biofilm formation, siderophore production, motility and chemotaxis. RNA sequencing and proteomics data showed that the list of differentially expressed genes (DEGs) and proteins (DEPs) in pecM::Km cells compared to WT was surprisingly long, given the limited number of identifiable phenotypes. These data suggest that the accumulation of 4HBA resulted in pecM::Km cells mounting an oxidative stress response and significant metabolic adjustments to compensate for the loss of the efflux pump, implying that PecM is important for maintaining cellular homeostasis.
By contrast, the disruption of pecS resulted in fewer DEGs and DEPs, with a remarkably poor correlation. A notable observation was the marked depletion of the aminopeptidase AmpS. The ampS was highly expressed, which means that reduced AmpS levels were achieved by post transcriptional regulation. The data also revealed that the PecS regulon is not conserved between species.
Taken together, this work points to the roles of both PecS and PecM in maintaining bacterial fitness in the rhizosphere.
Date
7-23-2025
Recommended Citation
Ghosh, Arpita, "Understanding the Functional Role of the Efflux Pump PecM and Defining the Transcription Factor PecS Regulon in Agrobacterium fabrum" (2025). LSU Doctoral Dissertations. 6868.
https://repository.lsu.edu/gradschool_dissertations/6868
Committee Chair
Prof. Anne Grove
DOI
10.31390/gradschool_dissertations.6868