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