The new study, published in the journal Stress Biology, centers on the polarisome, a multiprotein complex that sits at the very tip of a growing fungal hypha. In filamentous fungi, growth is concentrated at the apex, where secretory vesicles ferry membrane and cell wall materials along cytoskeletal tracks to the site of expansion. The visual hallmark of this process is the spitzenkörper, a dense vesicle-rich structure at the hyphal tip, within which the polarisome resides. First characterized in budding yeast, where the proteins Spa2, Pea2 and Bud6 were shown to cosediment as a large 12S complex, the polarisome orchestrates actin remodeling at sites of polarized growth. In plant pathogenic fungi, this machinery is not merely a matter of cellular housekeeping; it is the engine of infection, since the fungus must extend its hyphae through vulnerable openings and stomata to colonize flowering wheat heads.

Previous work by the same group had established that the three core polarisome components of F. graminearum, FgSpa2, FgBud6 and FgPea2, are all critically required for polarized growth, development and virulence. But the downstream network of FgPea2 remained largely unmapped. To probe it, the researchers performed a pull-down assay on an FgPea2 protein tagged with green fluorescent protein, followed by mass spectrometry to identify binding partners. Among the proteins retrieved was FGSG_10016, an anillin-related protein showing strong similarity to the yeast polarity protein Boi2. The team named it FgBoi2. Co-immunoprecipitation experiments confirmed that FgBoi2 physically interacts with FgPea2 in vivo, and fluorescence microscopy revealed that FgBoi2 localizes to the tips of mycelia, conidiophores and conidia, a distribution strikingly reminiscent of polarisome components.

The localization studies yielded a subtle but important asymmetry. When FgBoi2 was tagged with GFP and co-expressed with FgPea2 fused to mCherry, the two proteins partially co-localized at hyphal tips, with line-scan analysis supporting the overlap. FgBoi2 also showed partial localization to the plasma membrane at the apex, hinting at a distinct functional niche from FgPea2 itself. Critically, when the researchers expressed FgBoi2-GFP in a mutant lacking FgPea2, the fluorescence signal became diffuse in the cytoplasm and around the plasma membrane rather than concentrated at the tip. The reverse experiment told the complementary story: FgPea2-GFP retained its polarisome-like localization even in the absence of FgBoi2. The team also found that FgBOI2 transcript levels dropped significantly in the FgPea2 deletion mutant, suggesting that FgPea2 regulates FgBoi2 both transcriptionally and by directing its polarized positioning. In short, FgPea2 acts upstream, likely recognizing and recruiting FgBoi2 to the spitzenkörper region.

What happens when FgBoi2 is removed entirely? The phenotypes were dramatic. Deletion mutants generated by targeted gene replacement and verified by Southern blot showed a significant reduction in vegetative growth across complete, starch-yeast and minimal media. Under the confocal microscope, the mutants displayed excessive hyphal branching and produced hyphal tips that were noticeably thinner than those of the wild type, classic signs of disrupted polarity. Asexual development suffered as well, with conidiation dropping sharply in liquid carboxymethylcellulose cultures. Sexual development presented a more nuanced picture: perithecia and ascospore formation proceeded largely normally, but the release of ascospores, the spores that launch infections of wheat heads in the field, was significantly impaired. Because both asexual and sexual stages are the principal phases at which the fungus attacks flowering wheat, these defects strike at the heart of the disease cycle.

Pathogenicity assays drove the point home. When flowering wheat heads were inoculated and observed for fourteen days, the deletion mutant still spread from the inoculated spikelet to its neighbors and induced typical head blight symptoms, but the average disease index, measured as diseased spikelets per head, was significantly lower than in the wild type or a complemented strain. Lesions on wheat seedling leaves were similarly shortened. The fungus’s chemical arsenal was blunted too: production of deoxynivalenol in trichothecene biosynthesis-inducing medium fell significantly in the mutant after seven days of incubation. Since DON facilitates fungal spread during infection, this reduction likely compounds the growth defects to diminish virulence. Together, the data establish FgBoi2 as a genuine virulence factor, not merely a growth accessory.

To understand how FgBoi2 performs its duties, the researchers turned to its domain architecture. Bioinformatic analysis revealed three conserved modules: an SH3 domain, a SAM domain and a PH domain. By constructing mutants lacking each domain in turn, the team found that the PH domain, spanning amino acids 708 to 843, is the linchpin. Removing it recapitulated the full deletion phenotype, producing slow growth, increased branching, thinner hyphae and impaired ascospore release, whereas loss of the SH3 or SAM domains caused only minor defects. Localization experiments were equally telling: without the PH domain, the FgBoi2-GFP signal abandoned the hyphal tip and accumulated as cytoplasmic puncta, drifting away from the plasma membrane. This mirrors findings in yeast, where Boi1 anchors to the bud membrane through its PH domain, and confirms that membrane anchorage is essential for FgBoi2’s role in polarized growth.

The study also uncovered an unexpected relationship with stress. Mutants lacking FgBoi2 were more resistant, not less, to cell wall damage inflicted by Congo Red and calcofluor white, to membrane disruption by SDS, and to oxidative stress from hydrogen peroxide. Quantitative PCR showed that expression of cell wall integrity pathway genes such as FgMGV1 and FgMKK1, along with several catalase, peroxidase and NADPH oxidase superfamily genes, was reduced in the mutant, indicating that FgBoi2 normally acts as a negative regulator of these stress response pathways. The PH domain proved critical for oxidative stress tolerance as well. In yeast, Boi1 and Boi2 are functionally redundant scaffolding proteins that promote the fusion of secretory vesicles with the plasma membrane, and losing both is lethal. F. graminearum, by contrast, carries only a single Boi protein, an evolutionary divergence that makes FgBoi2 uniquely vulnerable to disruption. Notably, the v-SNARE protein FgSnc1 still trafficked normally in the mutant, and brefeldin A did not perturb FgBoi2 localization, suggesting its mechanism differs from canonical secretory pathways and that it does not directly partner with FgBud6 as its yeast counterpart does.

Perhaps the most consequential finding concerns fungicides. Tebuconazole, carbendazim, phenamacril and difenoconazole are the mainstays of head blight control in the field, yet mutants lacking either FgPea2 or FgBoi2 showed reduced sensitivity to all four chemicals. The EC50 values tell the story: against difenoconazole, the wild type strain’s EC50 was 0.99 micrograms per milliliter, while the FgBoi2 mutant reached 1.45 and the FgPea2 mutant 1.29. The mechanism appears to lie in drug efflux, as transcript levels of the ABC transporter genes FgABC1 and FgABC6 and the major facilitator superfamily gene FgMFS1 rose in both mutants, while FgABC2 declined. The authors propose that disrupting hyphal tip growth broadly decreases fungicide sensitivity, and they suggest that future compounds designed to inhibit tip growth directly could offer a new strategy against head blight, one that sidesteps conventional resistance routes. As fungicide resistance continues to erode field control worldwide, a molecular map of the pathogen’s polarity machinery may prove to be exactly the blueprint that crop protection needs.

Subject of Research: Polarisome-mediated polarized growth and pathogenicity in Fusarium graminearum

Article Title: Polarisome core component FgPea2 regulates FgBoi2-mediated polarized growth, pathogenicity and environmental stress in Fusarium graminearum

Article References: Polarisome core component FgPea2 regulates FgBoi2-mediated polarized growth, pathogenicity and environmental stress in Fusarium graminearum. (n.d.). https://doi.org/10.1007/s44154-026-00300-w

Image Credits: AI Generated

DOI: 10.1007/s44154-026-00300-w

Keywords: Fusarium graminearum, polarisome, FgPea2, FgBoi2, polarized growth, Fusarium head blight, deoxynivalenol, pathogenicity, fungicide resistance, cell wall integrity, PH domain, hyphal tip growth

Fusarium verticillioides causes stalk rot and ear rot in corn across the globe, but its most insidious impact is chemical rather than physical. The fungus produces fumonisin B1, the most frequently detected mycotoxin in contaminated maize, and the World Health Organization has previously estimated that roughly half of the world’s maize and maize-derived products carry the toxin to varying degrees. The International Agency for Research on Cancer classifies fumonisin B1 as a potential carcinogen, and epidemiological studies have linked long-term dietary exposure to esophageal and liver cancers. At the molecular level, the toxin works as a potent inhibitor of ceramide synthase, a key enzyme in sphingolipid biosynthesis, disrupting essential signaling lipids in animal cells. Understanding how the fungus manufactures and deploys this compound has therefore been a central goal of plant pathology for decades.

MAP kinase pathways are the workhorses of cellular communication in fungi. Each cascade is built from three kinases, a MAP kinase kinase kinase, a MAP kinase kinase and a MAP kinase, that activate one another in sequence through phosphorylation, converting an external signal into a coordinated transcriptional response. In budding yeast, the Hog1 pathway responds to rising external osmolarity through two independent membrane sensors, Sln1 and Sho1, which funnel signals into the Ssk2-Pbs2-Hog1 cascade. Filamentous fungi retain these conserved components, but their functions have diverged in surprising ways. In Fusarium graminearum, Hog1 governs plant infection and deoxynivalenol production; in the rice blast fungus Magnaporthe oryzae, the Hog1 ortholog is dispensable for infection; and in Botrytis cinerea, the sensor proteins show functional redundancy. Until now, no comprehensive mutant analysis of the full pathway existed for F. verticillioides.

The research team, led by Haoxue Xia, Xulin Li, Yaru He, Wende Liu and Guangfei Tang, first identified all five pathway components in the fungus by searching its genome with yeast protein sequences as queries. The orthologs, named FvSln1, FvSho1, FvSsk2, FvPbs2 and FvHog1, encode proteins of 1150, 324, 1340, 654 and 306 amino acids respectively, and each carries the domain architecture expected of its yeast counterpart: FvSln1 has a transmembrane region, a histidine kinase domain and a response regulator; FvSho1 carries four transmembrane segments and an SH3 domain; FvSsk2 and FvHog1 contain serine-threonine kinase domains; and FvPbs2 holds a protein kinase domain. Phylogenetic comparison confirmed that these proteins are highly conserved across F. graminearum, M. oryzae, Zymoseptoria tritici, B. cinerea and yeast.

Using double-joint PCR and protoplast transformation, the team generated deletion mutants for each gene and complemented strains to verify that observed defects were genuinely attributable to the deleted sequences. On standard growth media, most mutants grew like the wild type, with one notable exception: the ΔFvSho1 strain grew significantly more slowly, implicating FvSho1 in vegetative growth. The most striking developmental phenotype, however, appeared in spore formation. Every deletion mutant, including ΔFvSln1 and ΔFvSho1, showed severe defects in conidiation compared with the wild type, revealing that the entire pathway, sensors included, is tightly linked to the production of the asexual spores that spread the fungus through cornfields.

Stress testing exposed a clear division of labor within the pathway. When the researchers grew the mutants on media containing 0.7 M sodium chloride or 1 M potassium chloride, the ΔFvSsk2, ΔFvPbs2 and ΔFvHog1 strains were severely stunted, confirming that the core cascade is essential for osmotic adaptation. Remarkably, deleting either sensor gene, FvSln1 or FvSho1, had no measurable effect on osmotic sensitivity, a result that diverges sharply from the yeast model and mirrors earlier findings in F. graminearum. The authors suggest that in Fusarium an additional, still-unidentified signaling route must activate the cascade during osmotic stress, decoupling the classical sensors from the pathway they were thought to control.

The picture inverted under other forms of stress. Whereas wild-type cultures were inhibited by hydrogen peroxide, the membrane-damaging detergent SDS and the cell wall-perturbing dye Congo red, the ΔFvSsk2, ΔFvPbs2 and ΔFvHog1 mutants tolerated all three stressors better than the wild type. This paradoxical gain of tolerance echoes observations in Candida albicans, where hog1 mutants resist cell wall inhibitors, and points to crosstalk between the Hog1 pathway and the cell wall integrity pathway. The researchers propose that losing the cascade may derepress compensatory stress networks, a hypothesis that will require further mechanistic work to confirm.

Virulence assays on corn ears delivered the study’s most consequential results. Seven days after inoculation with conidial suspensions, lesions caused by the ΔFvSho1, ΔFvSsk2, ΔFvPbs2 and ΔFvHog1 mutants barely spread beyond the wound site, while wild-type and complemented strains blanketed the ears in mycelium. Only ΔFvSln1 retained full virulence. Because fumonisin B1 acts as a virulence determinant during the seedling stage, the team also quantified toxin output in liquid culture using an ELISA assay. Production of FB1 dropped significantly in the ΔFvSho1, ΔFvSsk2, ΔFvPbs2 and ΔFvHog1 strains, and complementation restored wild-type levels, while FvSln1 deletion again had no effect. The core cascade, and unexpectedly the Sho1 sensor, thus sit upstream of both the fungus’s ability to invade maize and its capacity to poison it.

The fungicide experiments add an agronomically urgent dimension. Fludioxonil, a phenylpyrrole widely used in the field, is believed to kill fungi by hyperactivating the Hog1 pathway, so the researchers tested mutant sensitivity to fludioxonil, phenamacril and tebuconazole. Deleting FvSsk2, FvPbs2 or FvHog1 significantly increased resistance to fludioxonil, and the ΔFvPbs2 and ΔFvHog1 strains also resisted phenamacril, whereas sensitivity to the azole tebuconazole was unchanged. The result fits a broader pattern: in F. graminearum, Neurospora crassa and Colletotrichum lagenarium, disrupting Hog1-related kinases confers resistance to phenylpyrrole and dicarboximide fungicides. Because the drug’s lethal effect depends on the very pathway the fungus needs for virulence, the study suggests that components of this cascade, and the signaling networks connected to it, could serve as targets for next-generation fungicides designed to disarm the pathogen without simply selecting for resistant strains.

Taken together, the work establishes the FvSsk2-FvPbs2-FvHog1 cascade as a pivotal node controlling conidiation, toxin biosynthesis, plant infection and stress adaptation in F. verticillioides, while revealing that the upstream sensors FvSln1 and FvSho1 operate with unexpected independence from the core module. The species-specific quirks of the pathway, its dispensability for osmotic sensing at the sensor level, its paradoxical role in stress tolerance and its involvement in fungicide susceptibility, underscore why findings from yeast cannot simply be transplanted to crop pathogens. As fumonisin contamination continues to shadow global maize supplies, the molecular wiring mapped here offers both a deeper understanding of fungal pathogenesis and a shortlist of candidate targets for protecting one of the world’s most important food crops.

Subject of Research: The role of the Hog1-MAPK signaling pathway in stress responses, mycotoxin production, pathogenicity and fungicide resistance in Fusarium verticillioides

Article Title: The FvHOG1 pathway is essential for stress responses, fungicide resistance, fumonisin B1 production and pathogenesis in Fusarium verticillioides

Article References: Xia, H., Li, X., He, Y., Liu, W., & Tang, G. (2025). The FvHOG1 pathway is essential for stress responses, fungicide resistance, fumonisin B1 production and pathogenesis in Fusarium verticillioides. Crop Health, 3(1), Article 14. https://doi.org/10.1007/s44297-025-00052-5

Image Credits: AI Generated

DOI: 10.1007/s44297-025-00052-5

Keywords: Fusarium verticillioides, Hog1-MAPK pathway, fumonisin B1, mycotoxin, maize, plant pathogen, fungicide resistance, fludioxonil, MAP kinase, conidiation, virulence, stress response