An l-fucose-responsive transcription factor cross-regulates the expression of a diverse array of carbohydrate-active enzymes in Trichoderma reesei

l -Fucose is a universal capping component of biomolecules found throughout all domains of life. Although fungi are renowned for their role in biomass recycling, the mechanisms by which they process l -fucose remain largely unknown. In this study, we elucidate a l -fucose-responsive system in Trichoderma reesei , a model fungus for plant cell wall degradation. Central to this system is the transcription factor FUR1, which is indispensable for growth on l -fucose. FUR1 orchestrates the expression of l -fucose catabolic enzymes, including an l -fucose dehydrogenase that exhibits distant homology to counterparts in bacteria and mammals. Through RNA sequencing and biochemical assays, we demonstrate that FUR1 also governs the enzymatic liberation of l -fucose by upregulating extracellular α- l -fucosidases. Intriguingly, FUR1 mediates l -fucose-triggered expression of a broad spectrum of enzymes that target diverse glycosidic bonds (e.g., β-glucuronidic, α-galactosidic, and β-xylosidic linkages) within complex carbohydrates. Expression of a constitutively active FUR1 mutant unlocked the production of otherwise silent glycosidases, substantially boosting the hydrolytic capacity of the fungal secretome on orange peel. These findings offer the first molecular insight into l -fucose sensing and metabolism in fungi, and advance our understanding of the fungal regulatory network for coordinated expression of biomass-degrading enzymes. l -Fucose, is a widely present constituent of polysaccharides, glycoproteins, and glycolipids in plants, animals and bacteria. l -Fucose frequently presents itself as a terminal monosaccharide, and plays critical roles in cell communication, immunity and host-microbe interactions. While fungi are well-known for breaking down various biomass molecules (e.g., lignocellulose) in nature, their metabolic and regulatory machineries for l -fucose utilization are enigmatic. Here, we examine the transcriptional response of Trichoderma reesei to l -fucose. We discovered that an orphan transcription factor activates the expression of l -fucose catabolic enzymes as well as extracellular α- l -fucosidases. Additionally, we found that this transcription factor also mediates the cross-induction by l -fucose of a wide range of enzymes that break down different types of glycosidic bonds. These findings provide the first understanding of how a fungus senses and metabolize l -fucose, and shed light on the complex regulatory systems that allow fungi to efficiently degrade biomass. This knowledge could help improve industrial fungal strains for the production of more efficient biomass-degrading enzymes.