A previously overlooked transcription factor in the fungus that causes Fusarium wilt of tomato has been shown to underpin the pathogen’s ability to survive oxidative assault and devastate its host, according to new research published in the Journal of Agriculture and Food Research. The protein, named FolSrz1, does not act through the familiar antioxidant enzyme systems that scientists have long targeted when studying plant-pathogenic fungi. Instead, it appears to keep the fungal cell running smoothly from the inside, quietly maintaining genome integrity, protein quality control, and trace-metal balance so that the pathogen can proliferate once inside tomato roots.
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
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Roger Howard. (September 20, 2026). Hidden zinc finger protein gives tomato wilt fungus its killer edge, study reveals. Scienmag. https://scienmag.com/hidden-zinc-finger-protein-gives-tomato-wilt-fungus-its-killer-edge-study-reveals/
Roger Howard. “Hidden zinc finger protein gives tomato wilt fungus its killer edge, study reveals.” Scienmag, 20 September 2026, https://scienmag.com/hidden-zinc-finger-protein-gives-tomato-wilt-fungus-its-killer-edge-study-reveals/. Accessed 20 September 2026.
Roger Howard. “Hidden zinc finger protein gives tomato wilt fungus its killer edge, study reveals.” Scienmag. September 20, 2026. https://scienmag.com/hidden-zinc-finger-protein-gives-tomato-wilt-fungus-its-killer-edge-study-reveals/
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Tags: calcineurincellular homeostasischlamydospore survival mechanismschlamydosporescopper homeostasisFolSrz1FolSrz1 transcription factorfungal oxidative stress resistancefungal protein quality controlfungal virulence factorsFusarium oxysporumFusarium wiltFusarium wilt pathogengenome integrity in fungiOxidative stressplant pathologyplant-pathogen interactionsoil-borne plant pathogenstomatotomato disease managementtomato plant diseasetrace-metal homeostasis in pathogenstranscription factorvirulence
