Heat shock transcription factor splicing variant TtHSF2β-I regulates lignocellulose and lignin-related compound degradation in Trametes trogii S0301.

White-rot fungi are the primary biological decomposers of lignocellulosic biomass owing to their powerful lignin-modifying enzyme (LMEs) system. Here, we analyzed the regulatory role of TtHSF2β-I, an alternatively spliced variant of heat shock transcription factor (HSF) in Trametes trogii S0301, at three levels of substrate complexity: native poplar wood, synthetic lignocellulosic media, and specific lignin-derived monomers. Overexpression of TtHSF2β-I resulted in extensive disruption of poplar wood lignocellulose, whereas its deletion impaired fungal growth and substrate degradation. Transcriptomic analysis revealed that overexpression of TtHSF2β-I enhanced the expression of numerous genes encoding key lignocellulose-degrading enzymes. Using synthetic lignocellulose media and specific lignin-derived compounds, we further demonstrated that TtHSF2β-I controls the degradation and transformation of lignin-related compounds. Structural modeling and protein interaction assays suggested that TtHSF2β-I lacks direct DNA-binding activity, in contrast to TtHSF2α. Instead, it modulates transcription via protein-protein interactions, possibly interfering with TtHSF2α oligomerization to de-repress ligninolytic genes. In summary, our results indicate that alternative splicing plays a significant role in regulating ligninolysis in fungi. TtHSF2β-I, in particular, could be a useful target for engineering fungi with improved efficiency in biomass conversion or bioremediation.