Fungal appressoria as mechanochemical organelles for polyurethane degradation.

Polyurethane (PU) is a synthetic polymer characterized by highly stable urethane linkages that hinder biological turnover. Although fungi have been implicated in PU degradation, the molecular mechanisms that couple polymer surface sensing to enzymatic depolymerization remain largely undefined. Here we show that the marine-derived fungus Alternaria alternata FB1 employs a surface-sensing signaling pathway that drives appressorium-mediated degradation of both polyester and polyether PUs. Contact with hydrophobic polymer surfaces rapidly induces melanized appressoria that mechanically penetrate the polymer matrix and promote its oxidative and hydrolytic depolymerization. Integrative transcriptomic analysis and targeted gene disruption identify the mucin-like surface sensor Msb2 as an upstream component of the polyurethane surface-sensing machinery. Loss of Msb2 disrupts MAPK and Ca2+ signaling, impairs appressorium differentiation, and reduces expression of degradative enzymes. Biochemical profiling further reveals multiple urethane-hydrolyzing enzymes that expand the known catalytic repertoire for PU bond cleavage. Together, these findings establish a mechanistic framework linking surface recognition, appressorium development, and polymer degradation, providing insight into how fungi transform recalcitrant polyurethane materials.