Plants do not surrender to fungal invasion without a fight. One of the earliest and most potent weapons in their innate immune arsenal is the oxidative burst, a rapid accumulation of reactive oxygen species, or ROS, at the site of infection. Hydrogen peroxide, the central player in this response, oxidatively damages fungal DNA, proteins, and lipid membranes while also restricting the nutrients available to the pathogen. ROS additionally act as signaling molecules that switch on defense-related genes throughout the plant. To counter this assault, fungal pathogens deploy an array of antioxidant enzymes, including catalases, peroxidases, and superoxide dismutases, alongside non-enzymatic reducing agents such as NADPH and glutathione, the most abundant intracellular thiol, which buffers the cell’s redox state and donates electrons to neutralize ROS.

The connection between sugar metabolism and this antioxidant machinery lies in the pentose phosphate pathway, the major source of NADPH in the cell. NADPH supplies the reducing equivalents that allow glutathione reductase to recycle oxidized glutathione back to its active form, sustaining the redox cycle that keeps ROS in check. In fungi such as the rice blast pathogen Magnaporthe oryzae and the yeast Saccharomyces cerevisiae, the trehalose 6-phosphate synthase Tps1 acts as a metabolic sensor for glucose-6-phosphate and regulates NADPH production through the pathway. ABL1, a gene encoding a protein resembling the beta subunit of AMP-activated protein kinase, was previously identified as a downstream component of this glucose-sensing circuit in M. oryzae. Whether an equivalent regulator governed the metabolism and virulence of C. chrysosperma was, until now, unknown.

Using protein sequence comparisons, the team identified a single Abl1 homolog in C. chrysosperma, designated CcAbl1, and found that related proteins are highly conserved across diverse filamentous fungi. Expression analysis showed that CcAbl1 is strongly glucose-responsive: in shake culture, its transcript levels rose roughly ninefold when glucose was added to the medium. During infection of poplar leaves, the gene was expressed at low levels one day after inoculation but became significantly upregulated by day three, a timing that suggested it participates in the early proliferation of the fungus inside host tissue.

To test its function directly, the researchers generated a deletion mutant lacking the entire CcAbl1 open reading frame, along with a genetically complemented strain in which the gene was restored. Pathogenicity assays on poplar branches told a striking story. Seven days after inoculation, the mutant produced markedly smaller necrotic lesions than the wild-type and complemented strains. Trypan blue staining of infected leaves revealed a greatly diminished ability of the mutant to induce host cell death, and quantitative PCR measurements of fungal biomass from one to six days post-infection showed that the mutant consistently accumulated far less tissue than either control strain. CcAbl1, the authors concluded, is essential for virulence.

The metabolic basis of this defect emerged from a series of growth experiments and metabolomic analyses. On minimal medium containing ten percent glucose, the wild-type and complemented strains grew robustly, while the deletion mutant showed pronounced growth defects, a phenotype that was much weaker on non-preferred carbon sources such as sucrose and cellulose. When glucose concentrations were varied from zero to twenty percent, wild-type colony growth peaked at five percent glucose, whereas the mutant faltered at five percent and above. Targeted glucose metabolomics using liquid chromatography-mass spectrometry, performed with five biological replicates per strain, revealed sharply segregated metabolic profiles: eighteen of twenty-seven detected metabolites differed significantly between the mutant and the wild type, with fourteen markedly reduced. Most of the depleted metabolites belonged to central energy metabolism and antioxidant defense pathways, and NADPH was the single most dramatically diminished metabolite in the mutant.

Because NADPH also fuels amino acid biosynthesis and nitrogen assimilation, the team examined how the mutant handled amino acids supplied as sole nitrogen sources. Wild-type hyphae grew significantly better on medium supplemented with cysteine, glutamic acid, glycine, alanine, or lysine, but the mutant failed to benefit. This was a telling result, since cysteine, glutamic acid, and glycine are the direct substrates for glutathione biosynthesis. Quantification confirmed the suspicion: total intracellular glutathione was significantly lower in the mutant under both standard culture conditions and infection-mimicking conditions using poplar branch extract, and the gap widened further under hydrogen peroxide stress. Transcript analysis of the glutathione biosynthetic genes CcGsh1 and CcGsh2, which encode gamma-glutamylcysteine synthetase and glutathione synthetase, showed that their expression was significantly reduced in the mutant under infection-mimicking conditions, and that the oxidative-stress-responsive induction of both genes seen in the wild type was substantially attenuated in the mutant.

The consequences for stress tolerance were comprehensive. The deletion mutant was hypersensitive to hydrogen peroxide and to methyl methanesulfonate, a DNA-damaging agent, and supplementing the medium with reduced glutathione partially alleviated these growth defects. Enzyme assays added a spatial dimension to the picture: intracellular catalase and superoxide dismutase activities were comparable across strains, but peroxidase activity was significantly reduced in the mutant, and under oxidative challenge the mutant showed markedly decreased extracellular catalase and superoxide dismutase activities. The authors interpret this as evidence that CcAbl1 governs antioxidant capacity principally through extracellular ROS-scavenging enzymes, while intracellular ROS removal depends predominantly on the glutathione-dependent pathway that collapses when NADPH runs short.

Perhaps the most compelling experiments concerned the host side of the interaction. When poplar leaves were infected with the mutant, DAB staining revealed significantly stronger ROS bursts than with the wild type, and defense-related genes, including the pathogenesis-related genes PR1 and PR3 and the signaling regulator MAPK6, were consistently upregulated. The team then asked whether the mutant’s weakness stemmed specifically from its inability to withstand host-derived oxidants. Pretreating host tissues with ethanol, which kills host cells and disrupts ROS generation, restored the mutant’s pathogenicity to nearly wild-type levels. Similarly, pharmacological inhibition of the host NADPH oxidase with diphenyleneiodonium reduced hydrogen peroxide accumulation in infected tissues and partially rescued mutant virulence. Together, these results directly tie the mutant’s attenuated disease capability to its heightened sensitivity to plant-generated oxidative stress.

The study establishes CcAbl1 as a central regulatory node that integrates glucose sensing, pentose phosphate pathway flux, NADPH-dependent glutathione metabolism, and oxidative stress tolerance into a single virulence program. Beyond its mechanistic significance for fungal physiology, the work carries practical implications: poplar Cytospora canker and related fungal diseases of economically important trees and crops might be controlled by targeting the metabolic-redox interface that pathogens rely upon to disarm plant immunity. As necrotrophic fungi are increasingly shown to harness sugar metabolism to fuel their antioxidant arsenals, the glucose-responsive NADPH-glutathione system identified here offers both a conceptual framework and a potential vulnerability in the arms race between trees and their fungal pathogens.

Subject of Research: Glucose-responsive regulation of NADPH-glutathione redox homeostasis and virulence in the poplar canker fungus Cytospora chrysosperma

Article Title: CcAbl1 is required for virulence via regulating NADPH–glutathione-mediated redox balance in Cytospora chrysosperma

Article References: CcAbl1 is required for virulence via regulating NADPH–glutathione-mediated redox balance in Cytospora chrysosperma. (n.d.). https://doi.org/10.1007/s44154-026-00302-8

Image Credits: AI Generated

DOI: 10.1007/s44154-026-00302-8

Keywords: Cytospora chrysosperma, CcAbl1, NADPH, glutathione, redox homeostasis, reactive oxygen species, pentose phosphate pathway, fungal virulence, poplar canker, oxidative stress, plant immunity, carbohydrate metabolism