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

Fusarium oxysporum f. sp. lycopersici, abbreviated Fol, is one of the most economically destructive soil-borne pathogens in tomato production worldwide. The fungus enters through the roots, colonizes the vascular tissue, and produces the signature symptoms of chlorosis, leaf wilting, and browning of the stem vasculature that ultimately kill the entire plant. Part of what makes Fol so difficult to manage is its chlamydospores, thick-walled dormant spores that persist in infested soil for years, waiting for a susceptible host. Despite cultural, biological, chemical, and resistance-based control strategies, disease management remains an uphill battle, particularly as new pathogenic races continue to emerge.

To invade a living plant, a fungus must contend with a hostile environment. As Fol grows through host tissue, it encounters reactive oxygen species unleashed by the plant immune system, along with nutrient limitation and other stresses. Surviving this barrage requires extensive transcriptional reprogramming, a process orchestrated by transcription factors that coordinate fungal development, stress adaptation, and virulence. Although many virulence-promoting transcription factors have been identified in F. oxysporum, far less is known about those that support disease indirectly, by safeguarding the basic cellular machinery of the pathogen itself.

The research team, led by Ching-Yu Chen and Ying-Lien Chen of National Taiwan University, began their investigation in the well-studied calcineurin signaling pathway, a conserved calcium- and calmodulin-activated phosphatase cascade that governs growth, morphogenesis, ion homeostasis, stress responses, and virulence in fungal pathogens. Their earlier work had shown that calcineurin controls hyphal growth, conidiation, chlamydospore formation, and virulence in Fol. The best-known calcineurin substrate in fungi is Crz1, a zinc finger transcription factor that, once dephosphorylated, moves into the nucleus and switches on genes involved in growth, stress tolerance, drug resistance, and pathogenicity. Yet across many fungi, deletion of crz1 produces far milder defects than deletion of calcineurin itself, hinting that other, poorly characterized substrates must carry out critical downstream functions.

Using protein similarity searches against the Fol 4287 genome, the researchers identified two candidate Crz1 homologs, encoded by the genes FOXG_00040 and FOXG_05246. Sequence and phylogenetic analyses told two very different stories. FOXG_00040, which they named FolCrz1, clustered tightly with canonical Crz1 orthologs from filamentous fungi and retained the hallmark architecture, including two C2H2 zinc finger motifs and the conserved PxIxIT and LxVP short linear motifs that calcineurin uses to recognize its substrates. FOXG_05246 was another matter entirely. Although it shares some structural features with Crz1, its overall sequence identity is low, it lacks the conserved PxIxIT motif at the corresponding position, and its closest homologs outside the genus Fusarium show only about 40 percent identity. The protein appears to be a Fusarium-specific invention, a regulatory pathway that evolved within this genus and nowhere else. The team designated it FolSrz1, short for stress resistance zinc finger 1.

Deleting each gene revealed sharply divergent roles. Mutants lacking FolCrz1 grew normally on standard medium and responded to calcium in a manner consistent with a conserved calcineurin-Crz1 pathway; the protein, tagged with green fluorescent protein, rushed into the nucleus within minutes of calcium treatment, and this movement was abolished by the calcineurin inhibitor cyclosporin A. Deleting FolSrz1, by contrast, slowed vegetative growth, thinned aerial mycelia, produced a pink colony appearance, and made the fungus hypersensitive to hydrogen peroxide and menadione, two oxidative stress-inducing agents. FolSrz1 mutants were also unusually sensitive to sodium dodecyl sulfate, pointing to a role in membrane integrity. Notably, FolSrz1-GFP sat mostly in the cytoplasm with only partial nuclear presence, and neither oxidative stress nor cyclosporin A changed this pattern, suggesting that its localization is governed by something other than canonical calcineurin-dependent translocation.

Both transcription factors proved important for producing chlamydospores, the fungus’s primary survival structures and field inoculum. Wild-type cultures generated roughly 1.28 billion chlamydospores per gram of dried mycelium, while the FolCrz1 and FolSrz1 mutants produced about 397 million and 77.5 million respectively, reductions that left spore morphology and lipid content unchanged. This indicates that both proteins participate in the induction of chlamydospore formation rather than in the structural development of the spores themselves. The FolCrz1 mutant additionally showed heightened sensitivity to tebuconazole, a widely used triazole fungicide, echoing findings in other Fusarium species and suggesting a conserved role for Crz1 in azole tolerance.

The most dramatic result emerged from tomato infection assays. Three-week-old seedlings dipped in spore suspensions developed severe wilting when inoculated with the wild type or the FolCrz1 mutant, with disease severity indices of 73 and 75 percent respectively at 28 days post-inoculation. Seedlings inoculated with the FolSrz1 mutant remained largely healthy, showing only mild yellowing of lower leaves, and the disease severity index plummeted to 22 percent. Vascular browning confirmed that the mutant could still invade the host; it simply could not do much damage once inside. FolCrz1, despite its conserved biochemistry, turned out to be dispensable for virulence in this pathosystem, reinforcing the idea that calcineurin must act through other substrates to support disease.

To uncover what FolSrz1 actually controls, the team compared gene expression between the wild type and the mutant using RNA sequencing, identifying 1,249 differentially expressed genes, 616 upregulated and 633 downregulated. The enriched pathways were strikingly mundane in the best sense: double-strand break repair, ubiquitin-mediated proteolysis, autophagy, nucleocytoplasmic transport, steroid biosynthesis, and copper ion transport. Crucially, genes encoding antioxidant enzymes, secreted effectors, cell wall-degrading enzymes, and mycotoxin biosynthesis proteins showed no significant differential expression. Quantitative PCR validation confirmed that FolSrz1’s influence was strongest under basal conditions; when the fungus was hit with hydrogen peroxide, expression differences between the strains largely dissolved, and the FolSRZ1 transcript itself was not induced by stress.

This pattern led the researchers to a counterintuitive conclusion. FolSrz1 is not a classic oxidative stress-responsive regulator that switches on detoxification genes when danger strikes. Rather, it maintains the steady-state machinery that keeps the cell functional: the ubiquitin-proteasome system and autophagy that clear damaged proteins and organelles, the non-homologous end-joining pathway that repairs broken DNA, and, importantly, the copper transporters Ctr1a and Ctr1b, orthologs of the yeast high-affinity copper importer Ctr1, which were significantly downregulated in the mutant. Copper is an essential cofactor for copper/zinc superoxide dismutase, so reduced copper uptake could impair superoxide detoxification, helping explain the mutant’s sensitivity to menadione, though not fully its sensitivity to hydrogen peroxide, which is detoxified primarily by catalases and peroxidases.

Measurements in infected tomato roots tied the molecular picture to disease. Hydrogen peroxide accumulation in roots at 24 hours post-inoculation was indistinguishable between wild-type and mutant infections, confirming that the mutant does not trigger an altered oxidative burst. Yet quantitative PCR at five days post-inoculation showed that the FolSrz1 mutant had achieved only about 51 percent of the wild-type fungal biomass in root tissue. The attenuated virulence, in other words, stems not from a failure to detoxify host reactive oxygen but from a general loss of fitness during colonization, a fitness that depends on intact genome maintenance, proteostasis, and copper homeostasis. This mechanism echoes recent work showing that copper acquisition, governed by the transcription factor Mac1, is itself essential for Fol to colonize tomato plants.

The study thus adds a new layer to the calcineurin story and offers a fresh conceptual angle on fungal virulence: a pathogen can be disarmed not only by stripping away its weapons but by sabotaging the cellular housekeeping that keeps those weapons functioning. Because FolSrz1 exists only within Fusarium, it also represents a potentially selective target for control strategies aimed at this genus without disrupting beneficial or unrelated soil microbes. As Fusarium wilt continues to spread through tomato-growing regions and chlamydospores stubbornly persist in field soils, understanding how a Fusarium-specific transcription factor quietly sustains the pathogen’s inner life may prove as important as cataloging the offensive arsenal it supports.

Subject of Research: Role of the Crz1-like transcription factor FolSrz1 in oxidative stress resistance and virulence of the tomato wilt fungus Fusarium oxysporum f. sp. lycopersici

Article Title: The Crz1-like protein FolSrz1 contributes to oxidative stress resistance and virulence through maintenance of cellular homeostasis in Fusarium oxysporum f. sp. lycopersici

Article References: Chen, C.-Y., Hsieh, Y.-J., Lin, C.-H., Hsu, L.-H., & Chen, Y.-L. (2026). The Crz1-like protein FolSrz1 contributes to oxidative stress resistance and virulence through maintenance of cellular homeostasis in Fusarium oxysporum f. sp. lycopersici. Journal of Agriculture and Food Research, 31, Article 103300. https://doi.org/10.1016/j.jafr.2026.103300

Image Credits: AI Generated

DOI: 10.1016/j.jafr.2026.103300

Keywords: Fusarium oxysporum, FolSrz1, transcription factor, calcineurin, oxidative stress, Fusarium wilt, tomato, virulence, chlamydospores, copper homeostasis, cellular homeostasis, plant pathology