Abstract
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.