Identification of secondary metabolite biosynthetic gene clusters associated with the infection of citrus fruit by Penicillium digitatum

Penicillium digitatum is one of the primary cause agents of postharvest loss of citrus fruits. Although a number of the secondary metabolites (SMs) that it produces have been identified, their biosynthesis pathways and biological roles in infection and decay development have not been fully characterized to date. In the present study, three SM biosynthesis pathways were identified using domain characterization, comparative genomics and co-regulation analysis. These pathways include the tryptoquialanines (TQA) biosynthesis pathway that was up-regulated during fruit infection. Subsequently, a gene knockout mutant was constructed, which revealed that the tqaA gene was involved in TQA production in P. digitatum. However, no significant variation was detected between the non-TQA mutant and the wild-type strain in fungal growth, development, stress responses or pathogenicity. In summary, this study improved the annotation of three SM biosynthesis pathways possibly involved in the infection of citrus hosts by P. digitatum. However, further investigation revealed that one of the products, TQA, was not involved in the interaction between P. digitatum and their citrus hosts.