Isobenzofuranones and isocoumarins from kiwi endophytic fungus Paraphaeosphaeria sporulosa and their antibacterial activity against Pseudomonas syringae pv. actinidiae

Development of the kiwifruit industry has been severely hindered by the canker disease, which is caused by the bacterium Pseudomonas syringae pv. Actinidiae (Psa). However, endophytic fungi associated with healthy kiwi plants may protect host plants through the production of metabolites with potent anti-Psa activity. In the current study, four undescribed isobenzofuranones, namely sporulactones A‒D, two undescribed isocoumarins, namely sporulactones E and F, together with eight known analogs were isolated from the kiwi endophytic fungus Paraphaeosphaeria sporulosa. The structures with absolute configurations were established by extensive spectroscopic methods, quantum chemistry calculations, and X-ray diffraction experiments. In addition, five of the compounds exhibited anti-Psa activity, with minimum inhibitory concentration (MIC) values ranging from 25 to 100 μg/mL. These findings suggest that the small polyketide metabolites produced by P. sporulosa play an important role in the antibacterial properties of the endophyte.