Bioactive Isocoumarin Derivatives from the Endophytic Fungus <i>Paraconiothyrium bonneyae</i> NS16 of <i>Pteris multifida</i>

An endophytic fungus isolated from the roots of Pteris multifida was identified as Paraconiothyrium bonneyae NS16 based on morphology combined with multigene phylogenetic analysis (ITS, LSU, SSU, TUB2, and TEF1-α). Rice-fermented cultures of P. bonneyae NS16 yielded three new isocoumarin derivatives (1-3) together with three known compounds (4-6). Their chemical structures were elucidated by high-resolution electrospray ionization mass spectrometry, nuclear magnetic resonance, and single-crystal X-ray diffraction. In bioactivity evaluations, compounds 1-6 exhibited notable antioxidant activity, with oxygen radical absorbance capacity values ranging from 0.27 ± 0.02 to 4.94 ± 0.17 (µmol TE)/µmol. In addition, compound 1 showed inhibitory activity against α-glucosidase and protein tyrosine phosphatase 1B, with 50% inhibitory concentration values of 353.5 and 27.15 µmol/L, respectively. These results highlight P. bonneyae as a productive source of bioactive isocoumarins and support their potential as antioxidant and antidiabetic leads.