How plant defenses can disable fungal infection pathways

Greg Howard
1st November, 2025

How plant defenses can disable fungal infection pathways

In response to a plant defense chemical (ferulic acid), a fungal stress protein (green) is sequestered into cytoplasmic clumps, preventing it from entering the nucleus (blue) as it normally would under osmotic stress.

Image adapted from: Zuchman et al. / CC BY (Source)

Key Findings

  • In the maize pathogen Cochliobolus heterostrophus, a defense compound called ferulic acid causes a key signaling protein, Hog1, to move from the cell nucleus to cytoplasmic spots
  • These cytoplasmic spots contain RNA and proteins involved in RNA processing and mitochondrial function, suggesting Hog1 is being actively relocated and sequestered
  • Ferulic acid suppresses the typical Hog1 response to osmotic stress, preventing its movement to the nucleus and hindering the fungus’s ability to respond to multiple stressors simultaneously
Fungal pathogens face numerous challenges when infecting plants, including exposure to potentially toxic compounds produced by the host as a defense mechanism. One such compound is ferulic acid (FA), a phenolic acid found in many plants. Understanding how fungi respond to these compounds is crucial for developing strategies to protect crops. Researchers from Technion - Israel Institute of Technology, MIGAL - Galilee Research Institute, Tel-Hai Academic College, and University of Cologne have recently investigated how the maize pathogen Cochliobolus heterostrophus responds to ferulic acid, focusing on a key signaling protein called Hog1[1]. Hog1 is a type of enzyme known as a mitogen-activated protein kinase (MAPK). MAPKs are involved in cellular responses to various stresses, and the Hog1 pathway is particularly important for osmoregulation – maintaining the correct water balance – in many organisms, including yeast[2]. In Candida albicans, another fungal pathogen, Hog1 plays a role in both stress resistance and virulence[3]. However, the way Hog1 is regulated can vary depending on the type of stress. The current study builds on previous work showing that in Cochliobolus heterostrophus, Hog1 activity is altered by plant compounds, specifically being dephosphorylated by ferulic acid[4]. Phosphorylation is a chemical modification that often activates a protein, so dephosphorylation effectively switches it off. The study revealed a surprising response to ferulic acid: Hog1 doesn’t simply become inactive, it relocates within the fungal cell. Normally, during osmotic stress, Hog1 moves into the nucleus – the cell’s control center – and becomes more phosphorylated[2]. However, when exposed to ferulic acid, Hog1 forms distinct spots, or foci, in the cytoplasm – the fluid inside the cell, but outside the nucleus. These foci also contain messenger RNA (mRNA), the molecule that carries genetic instructions from DNA to the protein-making machinery. This suggests that Hog1 is being sequestered into compartments containing RNA. Researchers used a technique called sm-FISH (single molecule fluorescence in situ hybridization) to visualize the mRNA within the cell and confirmed that these mRNA-containing foci extensively overlap with the Hog1 foci. Importantly, these foci are distinct from other cellular structures like the nucleus or mitochondria (the cell’s power plants), although some fragmented mitochondria were observed later in the response. To identify what other proteins are present in these Hog1-containing foci, the researchers used a technique called affinity purification. This allowed them to isolate the proteins that interact with Hog1 when ferulic acid is present. The analysis revealed a collection of proteins involved in RNA binding, translation (the process of making proteins from mRNA), and mitochondrial function. One protein, Puf2, which contains RNA-binding domains, was particularly enriched in the foci and was observed to accumulate alongside Hog1 when the fungus was exposed to ferulic acid. This finding suggests that ferulic acid triggers a unique response in Cochliobolus heterostrophus where Hog1 is not just deactivated, but actively moved into RNA-containing granules. This sequestration, coupled with dephosphorylation, may serve to prevent Hog1 from being overactivated by stress during infection. Overactivation of Hog1 could be detrimental to the fungus, and this mechanism might represent a survival strategy. Conversely, by preventing Hog1 from entering the nucleus and activating its target genes, ferulic acid interferes with the fungus’s ability to respond to other stresses. This study highlights a complex interplay between host defense and pathogen survival. It demonstrates that the response of Hog1 to ferulic acid is different from its response to osmotic stress, revealing a stress-contingent regulation of the Hog1 pathway[3]. The findings also build on previous research showing that fungal pathogens can evolve mechanisms to degrade plant defense compounds, such as stilbenes, through specialized enzymes[5], and that Hog1 is implicated in survival when exposed to plant phenolics[4]. The research suggests that the way fungi respond to plant compounds is not simply a matter of activating or deactivating signaling pathways, but involves dynamic changes in protein localization and interactions.

BiochemPlant ScienceMycology

References

Main Study

1) Cytoplasmic sequestering of a fungal stress-activated MAPK in response to a host plant phenolic acid

Published 30th October, 2025

https://doi.org/10.1371/journal.ppat.1013620

Related Studies

2) Hog1: 20 years of discovery and impact.

https://doi.org/10.1126/scisignal.2005458

3) Stress contingent changes in Hog1 pathway architecture and regulation in Candida albicans.

https://doi.org/10.1371/journal.ppat.1012314

4) Plant phenolic acids induce programmed cell death of a fungal pathogen: MAPK signaling and survival of Cochliobolus heterostrophus.

https://doi.org/10.1111/1462-2920.13528

5) Fungal adaptation to plant defences through convergent assembly of metabolic modules.

https://doi.org/10.1111/mec.14943


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