Biology subjects

Kunkel, B. N.

Publications and source records attributed to Kunkel, B. N..

4 recordsLinked to original sources

bioRxiv2025

PmeR, a TetR-like transcriptional regulator, is involved in both auxin signaling and virulence in the plant pathogen Pseudomonas syringae strain PtoDC3000.

Plant pathogenic bacteria, such as Pseudomonas syringae strain PtoDC3000, respond to host signals through complex signaling networks that regulate bacterial growth and virulence. The plant hormone indole-3-acetic acid (IAA), also known as auxin, promotes bacterial pathogenesis via multiple mechanisms, including through reprogramming bacterial transcription. However, the mechanisms that PtoDC3000 uses to sense and respond to auxin are not well understood. Here, we identify pmeR, which encodes a TetR-like family transcriptional repressor, as an important regulator of IAA-responsive gene expression in PtoDC3000. Using qRT-PCR and transcriptional reporter assays, we show that pmeR is induced by IAA and regulates a set of auxin-responsive genes, including itself as well as several genes known or proposed to be involved in virulence. Plant infection assays further show that the disruption of pmeR results in reduced bacterial growth in Arabidopsis thaliana. Notably, while PmeR de-represses transcription of pmeR upon IAA treatment, it does not appear to directly bind IAA. Rather, our biochemical results indicate that the auxin conjugate IAA-Lysine may serve as a ligand for PmeR. Our findings reveal a complex signaling network through which IAA modulates bacterial gene expression and emphasizes the role of PmeR in acclimating PtoDC3000 for growth in plant tissue. Author SummaryPlant pathogenic bacteria, such as Pseudomonas syringae strain PtoDC3000, respond to host signals through complex signaling networks that regulate bacterial growth and virulence. One key signal involved in these interactions is the plant hormone indole-3-acetic acid (IAA), which has been shown to promote bacterial pathogenicity in Arabidopsis thaliana and tomato. However, the mechanisms PtoDC3000 uses to sense and respond to IAA remain poorly understood. In this study, we explored the role of the TetR-like transcriptional regulator PmeR, encoded by the pmeR gene, in regulating bacterial responses to IAA. We found that pmeR is induced by IAA and regulates the expression of several additional auxin-responsive genes. Furthermore, we showed that pmeR is required for full virulence of PtoDC3000 in A. thaliana. These results suggest that PmeR is involved in regulating IAA-induced gene expression and that the ability to respond to auxin contributes to virulence of PtoDC3000. This work sheds new light on the molecular mechanisms through which IAA regulates bacterial pathogenesis, providing important insights into plant-microbe interactions and the role of auxin in regulating bacterial behavior. Our findings offer potential directions for developing strategies to mitigate bacterial diseases in crops by targeting auxin-responsive regulatory pathways.

microbiology↗

bioRxiv2024

Investigating the biosynthesis and roles of the auxin phenylacetic acid during Pseudomonas syringae-Arabidopsis thaliana pathogenesis

Several plant-associated microbes synthesize the auxinic plant growth regulator phenylacetic acid (PAA) in culture; however, the role of PAA in plant-pathogen interactions is not well understood. In this study, we investigate the role of PAA during interactions between the phytopathogenic bacterium Pseudomonas syringae strain PtoDC3000 (PtoDC3000) and the model plant host, Arabidopsis thaliana. Previous work demonstrated that indole-3-acetaldehyde dehydrogenase A (AldA) of PtoDC3000 converts indole-3-acetaldehyde (IAAld) to the auxin indole-3-acetic acid (IAA). Here, we further demonstrate the biochemical versatility of AldA, as it can use both IAAld and phenylacetaldehyde as substrates to produce IAA and PAA, respectively. We also show that during infection AldA-dependent synthesis of either IAA or PAA by PtoDC3000 does not contribute significantly to the increase in auxin levels in A. thaliana leaves. Using available arogenate dehydratase (adt) mutant lines of A. thaliana compromised for PAA synthesis, we observed that a reduction in PAA-Asp and PAA-Glu is correlated with elevated levels of IAA and increased susceptibility. These results provide evidence that PAA/IAA homeostasis in A. thaliana influences the outcome of plant-microbial interactions.

microbiology↗

bioRxiv2019

Dual role of auxin in regulating plant defense and bacterial virulence gene expression during Pseudomonas syringae PtoDC3000 pathogenesis.

Modification of host hormone biology is a common strategy used by plant pathogens to promote disease. For example, the bacterial pathogen Pseudomonas syringae strain PtoDC3000 produces the plant hormone auxin (Indole-3-acetic acid, or IAA) to promote PtoDC3000 growth in plant tissue. Previous studies suggest that auxin may promote PtoDC3000 pathogenesis through multiple mechanisms, including both suppression of salicylic acid (SA)-mediated host defenses and via an unknown mechanism that appears to be independent of SA. To test if host auxin signaling is important during pathogenesis, we took advantage of Arabidopsis thaliana lines impaired in either auxin signaling or perception. We found that disruption of auxin signaling in plants expressing an inducible dominant axr2-1 mutation resulted in decreased bacterial growth, demonstrating that host auxin signaling is required for normal susceptibility to PtoDC3000, and this phenotype was dependent on SA-mediated defenses. However, despite exhibiting decreased auxin perception, tir1 afb1 afb4 afb5 quadruple mutant plants lacking four of the six known auxin co-receptors supported increased levels of bacterial growth. This mutant also exhibited elevated IAA levels, suggesting that the increased IAA in these plants may promote PtoDC3000 growth independent of host auxin signaling, perhaps through a direct effect on the pathogen. In support of this, we found that IAA directly impacted the pathogen, by modulating expression of bacterial virulence genes, both in liquid culture and in planta. Thus, in addition to suppressing host defenses, IAA acts as a microbial signaling molecule that regulates bacterial virulence gene expression.

plant biology↗

bioRxiv2017

Indole-3-Acetaldehyde Dehydrogenase-dependent Auxin Synthesis Contributes to Virulence of Pseudomonas syringae Strain DC3000

The bacterial pathogen Pseudomonas syringae modulates plant hormone signaling to promote infection and disease development. P. syringae uses several strategies to manipulate auxin physiology in Arabidopsis thaliana to promote pathogenesis, including synthesis of indole-3-acetic acid (IAA), the predominant form of auxin in plants, and production of virulence factors that alter auxin responses in the host; however, the role of pathogen-derived auxin in P. syringae pathogenesis is not well understood. Here we demonstrate that P. syringae strain DC3000 produces IAA via a previously uncharacterized pathway and identify a novel indole-3-acetaldehyde dehydrogenase, AldA, that functions in IAA biosynthesis by catalyzing the NAD-dependent formation of IAA from indole-3-acetaldehyde (IAAld). Biochemical analysis and solving of the 1.9 [A] resolution x-ray crystal structure reveal key features of AldA for IAA synthesis, including the molecular basis of substrate specificity. Disruption of aldA and a close homolog, aldB, lead to reduced IAA production in culture and reduced virulence on A. thaliana. We use these mutants to explore the mechanism by which pathogen-derived auxin contributes to virulence and show that IAA produced by DC3000 suppresses salicylic acid-mediated defenses in A. thaliana. Thus, auxin is a DC3000 virulence factor that promotes pathogenicity by suppressing host defenses.\n\nAuthor SummaryPathogens have evolved multiple strategies for suppressing host defenses and modulating host physiology to promote colonization and disease development. For example, the plant pathogen Pseudomonas syringae uses several strategies to the manipulate hormone signaling of its hosts, including production of virulence factors that alter hormone responses in and synthesis of plant hormones or hormone mimics. Synthesis of indole-3-acetic acid (IAA), a common form of the plant hormone auxin, by many plant pathogens has been implicated in virulence. However, the role of pathogen-derived IAA during pathogenesis by leaf spotting pathogens such as P. syringae strain DC3000 is not well understood. Here, we demonstrate that P. syringae strain DC3000 uses a previously uncharacterized biochemical pathway to synthesize IAA, catalyzed by a novel aldehyde dehydrogenase, AldA, and carry out biochemical and structural studies of the AldA protein to investigate AldA activity and substrate specificity. We also generate an aldA mutant disrupted in IAA synthesis to show that IAA is a DC3000 virulence factor that promotes pathogenesis by suppressing host defense responses.

microbiology↗