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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 investigated the role of PAA during interactions between the phytopathogenic bacteriumPseudomonas syringaestrainPtoDC3000 (PtoDC3000) and the model plant host,Arabidopsis thaliana. Previous work demonstrated that indole-3-acetaldehyde dehydrogenase A (AldA) ofPtoDC3000 converts indole-3-acetaldehyde (IAAld) to the auxin indole-3-acetic acid (IAA). Here, we further demonstrated the biochemical versatility of AldA by conducting substrate screening and steady-state kinetic analyses, and showed that AldA can use both IAAld and phenylacetaldehyde as substrates to produce IAA and PAA, respectively. Quantification of auxin in infected plant tissue showed that AldA-dependent synthesis of either IAA or PAA byPtoDC3000 does not contribute significantly to the increase in auxin levels in infectedA. thalianaleaves. Using availablearogenate dehydratase(adt) mutant lines ofA. thalianacompromised 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 inA. thalianainfluences the outcome of plant-microbial interactions.
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This content will become publicly available on April 23, 2026
Indole-3-acetic acid (IAA) protects Azospirillum brasilense from indole-induced stress
ABSTRACT Azospirillum brasilenseis plant-growth promoting rhizobacteria that produces the phytohormone indole-3-acetic acid (IAA) to induce changes in plant root architecture. The major pathway for IAA biosynthesis inA. brasilenseconverts tryptophan into indole-3-pyruvic acid (I3P) and then, through the rate-limiting enzyme, indole-3-pyruvate decarboxylase (IpdC), into IAA. Here, we characterize the potential role for IAA biosynthesis in the physiology of these bacteria by characterizing the expression pattern of theipdCpromoter, analyzing anA. brasilense ipdCmutant using multiple physiological assays and characterizing the effect of I3P, which likely accumulates in the absence ofipdCand affects bacterial physiology. We found that theipdCmutant derivative has a reduced growth rate and an altered physiology, including reduced translation activity as well as a more depolarized membrane potential compared to the parent strain. Similar effects could be recapitulated in the parent strain by exposing these cells to increasing concentrations of I3P, as well as other indole intermediates of IAA biosynthesis. Our results also indicate a protective role for IAA against the harmful effects of indole derivatives, with exogenous IAA restoring the membrane potential of cells exposed to indole derivatives for prolonged periods. These protective effects appeared to restore cell physiology, including in the wheat rhizosphere. Together, our data suggest that the IAA biosynthesis pathway plays a major role inA. brasilensephysiology by maintaining membrane potential homeostasis and regulating translation, likely to mitigate the potential membrane-damaging effects of indoles that accumulate during growth under stressful conditions.IMPORTANCEIAA is widely synthesized in bacteria, particularly in soil and rhizosphere bacteria, where it functions as a phytohormone to modulate plant root architecture. IAA as a secondary metabolite has been shown to serve as a signaling molecule in several bacterial species, but the role of IAA biosynthesis in the physiology of the producing bacterium remains seldom explored. Results obtained here suggest that IAA serves to protectA. brasilensefrom the toxic effect of indoles, including metabolite biosynthetic precursors of IAA, on membrane potential homeostasis. Given the widespread production of IAA in soil bacteria, this protective effect of IAA may be conserved in diverse soil bacteria.
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- Award ID(s):
- 2130556
- PAR ID:
- 10613506
- Editor(s):
- Glass, Jennifer B
- Publisher / Repository:
- American society for Microbiology
- Date Published:
- Journal Name:
- Applied and Environmental Microbiology
- Volume:
- 91
- Issue:
- 4
- ISSN:
- 0099-2240
- Subject(s) / Keyword(s):
- Azospirillum, auxin, indoles, stress responses, plant root colonization
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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