RpfF is a bifunctional enzyme. Using a single active site, it performs two chemically distinct reactions. As an enoyl-acyl carrier protein (ACP) dehydratase, it helps convert intermediates of the bacterial fatty acid synthesis pathway, known as FASII, into the DSF family of signaling molecules, which include DSF, BDSF, IDSF, and CDSF. As an acyl-ACP thioesterase, it cleaves the thioester bond linking fatty acid chains to ACP, releasing free fatty acids. While the dehydratase role in quorum sensing has been studied extensively, the physiological purpose of the thioesterase activity remained a puzzle for more than a decade.

The clue that something more was going on came from earlier work showing that deleting the rpfF gene from Xoo caused a striking loss of membrane integrity. The mutant became hypersensitive to detergents, leaked proteins, and lost much of its ability to cause disease. But because rpfF deletion removes both enzymatic activities at once, it was unclear which function was responsible. The research team, led by Lizhen Luo, Zhe Hu, and Haihong Wang of South China Agricultural University together with John E. Cronan of the University of Illinois, resolved this ambiguity with an elegant trick: they replaced the missing thioesterase activity with unrelated enzymes borrowed from other organisms.

The team inserted into the chromosome of the rpfF deletion mutant two foreign thioesterase genes: tesA from the gut bacterium Escherichia coli and fatB1 from the camphor tree, Cinnamomum camphorum. Because these enzymes share no evolutionary history with RpfF and cannot make DSF signals, any rescue of the mutant’s defects could be attributed squarely to thioesterase chemistry. The results were unambiguous. Both foreign enzymes restored the mutant’s resistance to the detergents sodium dodecyl sulfate and Triton X-100, its tolerance of osmotic and oxidative stress, and its normal low permeability to fluorescent dyes. Adding purified DSF signal to the mutant, by contrast, did nothing for detergent sensitivity, proving that the membrane protection comes from thioesterase activity itself rather than from quorum sensing.

Microscopy drove the point home visually. Under scanning electron microscopy, untreated cells of the mutant looked normal, but exposure to detergent caused the rpfF deletion strain to undergo extensive lysis, spilling its intracellular contents, while wild-type cells and thioesterase-complemented strains remained largely intact. Transmission electron microscopy showed cytoplasmic aggregation and loss of electron density in the mutant under detergent stress, changes that were markedly milder in the complemented strains. Consistent with a repaired membrane, the mutant’s abnormal leakage of extracellular proteins, including cellulase, xylanase, and lipase, fell back toward wild-type levels when any of the three thioesterases was expressed, and an engineered green fluorescent protein that had escaped from the mutant’s cells was once again retained inside.

The lipid analysis explained why. In Gram-negative bacteria, membrane integrity depends on a delicate stoichiometric balance between phospholipids and lipopolysaccharide (LPS), as well as on the precise mix of fatty acyl chains within those lipids. Pulse-labeling experiments with radioactive acetate showed that the rpfF mutant accumulated excess phospholipids, particularly phosphatidylethanolamine and cardiolipin, while its LPS content dropped sharply: the lipid A-to-phospholipid ratio fell from 0.117 in the wild type to 0.037 in the mutant. Gas chromatography-mass spectrometry further revealed a skewed fatty acid profile, with the unsaturated C16:1 species rising and the saturated C16:0 and branched iso-C17:0 species falling. Expression of rpfF, tesA, or fatB1 corrected both the phospholipid-to-LPS ratio and the fatty acid composition.

The deepest defect turned out to be in the rate of fatty acid synthesis itself. Radiolabel incorporation showed that the rpfF mutant manufactured fatty acids at only about one-third of the wild-type rate, and specific synthesis rates measured by scintillation counting confirmed the slowdown. Complementation with any of the thioesterase genes restored synthetic flux to near-wild-type levels. The authors propose that when RpfF is absent, medium-chain acyl-ACPs accumulate inside the cell and feed back to inhibit the FASII machinery, throttling the supply of fatty acids needed to build phospholipids and LPS. By hydrolyzing excess acyl-ACPs, the thioesterase relieves this feedback inhibition and keeps membrane biogenesis running.

Crucially, the thioesterase also rescued virulence. In leaf-clipping assays on susceptible rice plants, the wild-type strain produced lesions averaging 17.8 centimeters at fourteen days after inoculation, while the rpfF mutant produced lesions of only 2.1 centimeters, an 88 percent reduction. Expressing the E. coli or plant thioesterase restored lesion lengths to 12.5 and 10.1 centimeters respectively, approaching the 13.3 centimeters achieved by restoring the native rpfF gene. Bacterial counts within lesions were only marginally lower in the mutant, indicating that the virulence defect reflects failed tissue colonization rather than poor growth inside the plant. Notably, exopolysaccharide production, which collapsed fourfold in the mutant, was restored only by native rpfF and by DSF signaling, not by the foreign thioesterases, showing that EPS is governed by quorum sensing while most other virulence traits ride on membrane health.

The study also showed that the thioesterase function is conserved across the genus and is itself wired into quorum sensing. Deleting rpfF in the related pathogen Xanthomonas campestris pv. campestris likewise caused detergent sensitivity and a skewed phospholipid-to-LPS ratio, and a foreign thioesterase repaired the damage. Moreover, overproducing the sensor kinase RpfC in wild-type Xoo mimicked the mutant’s detergent hypersensitivity, and the isolated receiver domain of RpfC inhibited purified RpfF’s thioesterase activity in vitro, extending to Xoo the known mechanism by which RpfC physically blocks RpfF in X. campestris. The authors propose a dual-function model in which RpfF simultaneously maintains lipid homeostasis through its thioesterase activity and produces DSF signals through its dehydratase activity. As cell density rises and DSF accumulates, RpfC’s grip on RpfF loosens, accelerating fatty acid synthesis and remodeling the membrane to withstand host-imposed stresses. In this way, the pathogen synchronizes virulence factor production with adaptive membrane restructuring, and the divergent phenotypes long observed between rpfF mutants of different Xanthomonas species are reinterpreted as the interplay of a conserved membrane-maintenance function with species-specific quorum-sensing circuitry. The work opens a potential avenue for anti-virulence strategies: disrupting RpfF’s thioesterase activity could strip the pathogen of both its signals and its structural resilience at once.

Subject of Research: The dual dehydratase and thioesterase activities of the RpfF enzyme in Xanthomonas oryzae pv. oryzae and their roles in fatty acid synthesis, membrane integrity, and virulence

Article Title: The RpfF Acyl-ACP thioesterase of major rice pathogen Xanthomonas oryzae pv . oryzae is essential for maintenance of membrane function and virulence

Article References: Luo, L., Hu, Z., Yan, M., Cronan, J. E., & Wang, H. (2026). The RpfF Acyl-ACP thioesterase of major rice pathogen Xanthomonas oryzae pv. oryzae is essential for maintenance of membrane function and virulence. PLOS Pathogens, 22(10), e1014669. https://doi.org/10.1371/journal.ppat.1014669

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014669

Keywords: Xanthomonas oryzae, RpfF, thioesterase, fatty acid synthesis, membrane integrity, quorum sensing, DSF, virulence, rice bacterial leaf blight, phospholipids, lipopolysaccharide, RpfC

The signal in question is the diffusible signaling factor, or DSF, chemically identified as cis-11-methyl-dodecenoic acid. DSF is the linchpin of quorum sensing in Xcc, the process by which bacterial populations gauge their density and synchronize the production of virulence factors, biofilms, and stress defenses. The molecule is synthesized by the enzyme RpfF through the fatty acid elongation cycle, sensed by the RpfC/RpfG two-component system, and funneled into the global regulator Clp. At late stages of growth, a fatty acyl-CoA ligase called RpfB activates the fatty acid beta-oxidation pathway to degrade DSF, effectively switching the conversation off. Because quorum sensing underpins nearly every aggressive behavior of the pathogen, any manipulation of DSF levels has profound consequences for disease.

To test whether IAA interferes with this system, the team, led by Si-Nan Li and Kai Song of Shanghai Jiao Tong University together with colleagues, grew the wild-type strain XC1 in Xylem-Sucrose medium, a formulation designed to mimic the nutrient environment inside plant xylem. Adding IAA at concentrations of 1 to 100 micromolar left bacterial growth untouched but slashed DSF levels dramatically. At 36 hours post-inoculation, DSF concentrations fell to 11.9, 9.2, and 1.4 percent of control levels at the three doses respectively. The effect held in a mutant that overproduces DSF, confirming that the hormone does not merely slow signal synthesis but actively accelerates its turnover.

Genetic dissection pinpointed RpfB as the indispensable executioner. In a strain lacking rpfB, 100 micromolar IAA failed to reduce DSF at all, while restoring a single chromosomal copy of the gene rescued the hormone’s effect. Intriguingly, IAA did not boost rpfB transcription, nor did it stimulate purified RpfB enzyme activity in vitro. The hormone was instead working through the cell’s physiology, and the next clue came from an unexpected quarter: pH.

Left alone in XYS medium, Xcc acidifies its surroundings, with culture pH dropping from 6.75 to 4.40 over 36 hours, and its cytoplasmic pH declining in parallel. IAA reversed this acidification in a dose-dependent manner. At 100 micromolar, the hormone significantly raised both culture and cytoplasmic pH by 24 hours, an effect sustained through 36 hours. This matters because prior work from the same group had shown that elevated cytoplasmic pH enhances RpfB-dependent DSF degradation. IAA, in other words, was not touching the degradation machinery directly; it was remodeling the intracellular environment so that the machinery works faster.

Transcriptome sequencing revealed how the hormone achieves this remodeling, identifying four IAA-regulated gene clusters. The hormone upregulated the atpABCDEFGH operon, encoding the F0F1 ATP synthase, by 2.6- to 5.2-fold. This bifunctional enzyme can run in reverse under acidic stress, hydrolyzing ATP to pump protons out of the cell and defend cytoplasmic pH. IAA also strongly induced hepABCD, an RND-family efflux pump shown previously to generate a proton motive force that raises cytoplasmic pH and stimulates RpfB activity. Digital PCR confirmed the RNA-seq patterns for representative genes, with hepB rising as much as 9.23-fold and atpB 4.96-fold at the highest IAA dose.

The most striking discovery, however, was a self-reinforcing biosynthetic loop. IAA induced the tryptophan synthase gene cluster trpB-A by 21- to 45-fold, driving production of L-tryptophan, the direct precursor of IAA in Xcc. Strains engineered to overexpress trpB-A accumulated more intracellular tryptophan and more IAA, and showed significantly reduced DSF levels, while deleting trpB in a DSF-overproducing background caused DSF to balloon to roughly 672 percent of control. The regulatory logic was completed by TrpI, a transcriptional activator adjacent to the operon: deletion of trpI abolished IAA’s induction of trpB-A, and electrophoretic mobility shift assays confirmed that TrpI binds the trpB promoter specifically. Yet adding IAA directly to the binding reaction changed nothing, meaning TrpI is essential but is not itself the IAA sensor, and the true receptor remains unidentified.

A fourth cluster added a second, pH-independent route to the same endpoint. IAA repressed iaepABCDE, an RND-family efflux pump the authors named the IAA-associated efflux pump, to as little as 0.06-fold of control levels. Deleting the cluster cut DSF levels by about 62 percent without altering culture or cytoplasmic pH, and the effect vanished in an rpfB mutant, showing that iaepABCDE normally suppresses RpfB-dependent turnover through a mechanism that does not involve proton pumping. Its substrate is still unknown, leaving a tantalizing gap in the pathway.

Critically, the laboratory findings translate to real infection. Using gusA reporter strains, the team detected promoter activity of all four IAA-regulated clusters, trpB-A, atpABCDEFGH, hepRABCD, and iaepABCDE, inside cabbage leaves five days after inoculation, confirming that the pathway operates during actual disease. The authors propose a dual-role model: Xcc exploits IAA to enhance its own virulence traits such as exopolysaccharide production and reactive oxygen species degradation, while the plant simultaneously deploys the hormone to erode quorum sensing and push the pathogen toward a quieter, more persistent lifestyle. The work also revealed synergy between IAA and the defense signal salicylic acid, hinting that multiple hormonal signals co-modulate bacterial behavior in planta. Because the study relied on a defined medium that simplifies the complex host environment, the authors caution that spatiotemporal IAA gradients, immune responses, and microbiome interactions still need validation in living plants, ideally using IAA-homeostasis mutants and real-time DSF reporters. Even so, the identification of a hormone-triggered feedback loop that amplifies its own precursor while dismantling bacterial communication opens a genuinely new front in the arms race between crops and one of their most devastating pathogens, and it suggests that auxin signaling could one day be manipulated to blunt quorum-sensing-dependent diseases across the roughly 400 plant species that Xanthomonas attacks.

Subject of Research: IAA-induced, RpfB-dependent turnover of the DSF quorum sensing signal in the phytopathogen Xanthomonas campestris pv. campestris

Article Title: The phytohormone indole-3-acetic acid induces quorum sensing signal DSF turnover via a positive feedback biosynthetic loop in the phytopathogen Xanthomonas campestris

Article References: Li, S.-N., Zhang, M.-L., Cui, Y., Li, L., Thawai, C., Jiang, L., Tian, D.-L., Gu, Y.-C., He, Y.-W., Zhou, L., & Song, K. (2026). The phytohormone indole-3-acetic acid induces quorum sensing signal DSF turnover via a positive feedback biosynthetic loop in the phytopathogen Xanthomonas campestris. Stress Biology, 6(1), Article 22. https://doi.org/10.1007/s44154-026-00298-1

Image Credits: AI Generated

DOI: 10.1007/s44154-026-00298-1

Keywords: Xanthomonas campestris, indole-3-acetic acid, quorum sensing, DSF, RpfB, tryptophan, cytoplasmic pH, efflux pumps, plant hormone, black rot, plant-pathogen interaction, positive feedback loop