Disruption of Oxaloacetate Acetylhydrolase Impairs Lignocellulose Utilization but Enhances Mycelial Material Properties in Perenniporia fraxinea

In the quiet, damp corners of a forest, a silent battle for resources plays out beneath the soil and within the rotting wood. Fungi are the great recyclers of the natural world, breaking down tough plant fibers that most other organisms cannot touch. To do this, they secrete a cocktail of chemicals, including a simple but powerful acid called oxalic acid. This acid acts like a chemical key, dissolving hard minerals in the ground so the fungus can drink them up, and it also lowers the pH of the environment to help the fungus's own enzymes work faster at tearing apart wood. However, this same acid has a double-edged nature. While it helps the fungus eat and grow, too much of it can weaken the very structure the fungus builds for itself, turning a sturdy material into something brittle and prone to breaking. Scientists have long wondered if they could tweak this chemical balance to create better materials from fungi, but doing so required a precise way to turn down the acid without killing the organism.

Researchers at Gyeongsang National University and Chosun University in South Korea decided to test this idea using a specific type of white-rot fungus called Perenniporia fraxinea. This fungus is known for forming tough, woody brackets on trees, making it an interesting candidate for creating sustainable building materials. The team focused on a specific gene that acts as a factory for producing oxalic acid. Using a modern gene-editing tool that works like molecular scissors, they disrupted this gene to see what would happen. They did not simply remove the gene entirely; instead, they broke the instructions so the fungus could no longer make the enzyme responsible for the bulk of its acid production. The goal was to observe how the fungus would react when its primary method of acidifying its environment was impaired, and whether this change would alter the strength of the mats it grows.

The results showed that the genetic edit worked exactly as intended, though not perfectly. The modified fungus produced significantly less oxalic acid, dropping from a concentration of 10.5 millimoles per liter in the normal fungus to 5.1 millimoles per liter in the edited version. Because there was less acid, the liquid surrounding the fungus became less acidic, with the pH rising from 4.8 to 5.4. This change had immediate consequences for how the fungus interacted with its food. When placed on a medium containing insoluble calcium phosphate—a form of mineral that is hard to dissolve—the edited fungus struggled to grow. It could not break down the mineral to access the nutrients inside. Similarly, when the fungus was placed on oak sawdust or a diet made only of lignin, the main component of wood, it grew very poorly compared to the unedited version. The researchers found that the enzymes responsible for digesting wood were less active in the edited fungus, but this was largely because the environment was not acidic enough for them to work efficiently. When the scientists manually lowered the pH of the liquid to match the natural acidity, the enzyme activity bounced back, proving that the fungus still had the tools to digest wood, it just lacked the chemical environment to use them.

Despite these struggles with eating and growing on tough substrates, the edited fungus produced something far more impressive: a much stronger physical material. The researchers grew large mats of the fungus, which are essentially thick blankets of fungal threads, and tested their strength. The mats made by the edited fungus were significantly tougher. They could withstand a breaking force that was nearly three times higher than the mats made by the normal fungus. They were also stiffer and could stretch further before snapping. This improvement in strength was not just a minor tweak; it was a fundamental shift in the material's properties. The edited fungus also grew slightly heavier and thicker mats, with a more robust upper layer of threads. The reduction in acid accumulation seemed to prevent the material from weakening and discoloring, which are common problems in fungal-based materials.

The study also revealed how the fungus tried to compensate for the loss of its main acid-producing factory. The fungus turned up the activity of other metabolic pathways, essentially rerouting its internal chemistry to keep producing some acid through a different route. This suggests that the fungus is highly adaptable and has backup systems to ensure it can still survive even when its primary method is blocked. The researchers noted that the presence of more available calcium in the environment, which was no longer being locked up by the missing acid, might have helped the fungus build a stronger cell wall and a more robust structure. This finding highlights a fascinating trade-off in the biology of these organisms. The same chemical process that helps the fungus survive in the wild by dissolving minerals and digesting wood appears to weaken the structural integrity of the material it creates.

By disrupting the gene responsible for oxalic acid production, the researchers demonstrated that it is possible to engineer fungi for specific purposes. While the edited fungus was less efficient at breaking down wood in a natural setting, it became a superior builder of durable materials. This suggests that for industrial applications where the goal is to create strong, sustainable building blocks rather than to decompose forests, reducing acid production is a viable strategy. The work provides a clear path forward for developing fungal materials that are not only eco-friendly but also mechanically superior, offering a new way to think about how we can harness nature's recyclers for human needs. The study confirms that the biological mechanisms fungi use to thrive in the wild can be modified to suit the demands of modern manufacturing, turning a natural weakness into a structural strength.