Profiling the secretomes of Penicillium expansum reveals that a serine carboxypeptidase (PeSCP) is required for the fungal virulence on apple fruit

Phytopathogenic fungi often secrete effector proteins that suppress host immunity and facilitate infection. Penicillium expansum is an important postharvest fruit rot pathogen that caused blue mold. However, little is known about the function of effectors secreted by P. expansum. In this study, a LC-MS/MS based proteomic approach was used to analyze the secretomes of P. expansum grown in apple juice, a medium used to mimic natural in planta condition. A total of 103 and 148 effector candidates were identified after incubation for 24 and 72 h, respectively. As expected, the majority of the proteins identified with known function were hydrolytic enzymes, such as cell-wall degrading enzymes and peptidases/proteases. One of the identified effector candidates is a serine carboxypeptidase (PeSCP) which was detected at 24 h after incubation and significantly up-regulated at the early stage of infection on apple fruit. Deletion of PeSCP gene led to reduced conidiation, germination as well as affected fungal growth and morphology, and also resulted in altered tolerance to environmental stresses, greatly reduced extracellular carboxypeptidase activity and virulence of P. expansum on apple fruit. Scanning electron microscopy revealed that the knockout of PeSCP in P. expansum impaired its ability to penetrate host plant tissues. Together, our findings demonstrate an important role of PeSCP in the virulence of P. expansum on apple fruit.