Role of exogenous treatments of phenylalanine and tyrosine in the mechanism of yeast‐secreted protein PgASP enhancing blue mold resistance in citrus fruits

Abstract BACKGROUND Citrus fruits are highly susceptible to blue mold, a destructive disease caused by the fungus Penicillium italicum, resulting in severe postharvest losses. Studies have shown that proteins secreted by Pichia galeiformis can increase citrus fruit resistance. In particular, the protein PgASP reduces postharvest fungal infections in citrus fruits. However, the molecular mechanism by which PgASP regulates citrus fruit resistance remains unclear. The aim of this study was to investigate the mechanism of action of exogenous amino acids involved in PgASP improving citrus fruit disease resistance. RESULTS PgASP overexpression markedly improved disease resistance in citrus fruit, as shown by transient expression assays. RNA‐seq of citrus peel after 2 days of overexpression revealed that differentially expressed genes (DEGs) were predominantly mapped to amino acid metabolic pathways. Transient PgASP overexpression was associated with increased phenylalanine and tyrosine content, together with elevated phenylalanine ammonia‐lyase (PAL), cinnamate 4‐hydroxylase (C4H), 4‐coumarate‐CoA ligase (4CL), tyrosine ammonia‐lyase (TAL), and tyrosine aminotransferase (TAT) activity. Total phenol, lignin, and flavonoid content also increased. In vitro experiments showed that treatment of P. italicum mycelium with 0.1 g L −1 phenylalanine or tyrosine inhibited growth. Exogenous treatment of citrus with phenylalanine and tyrosine significantly improved citrus resistance. CONCLUSION The results demonstrate that PgASP enhanced citrus disease resistance primarily through the activation of phenylalanine and tyrosine metabolism, boosting the biosynthesis of phenolic compounds and lignin. The direct antifungal activity of these amino acids also suggests their dual role in both signaling and defense, offering new avenues for managing postharvest disease control. © 2026 Society of Chemical Industry.