Xyloglucan-specific endo-ll-1,4-glucanase (PdXEG1) gene is important for the growth, development and virulence of Penicillium digitatum

Penicillium digitatum is a necrotrophic fungus responsible for the most severe postharvest rots of citrus fruit. However, the precise mechanisms underlying the interaction between P. digitatum and citrus fruit are still unknown. In this study, the xyloglucan-specific endo-ll-1,4-glucanase gene in P. digitatum, designated as PdXEG1, was identified and characterized. Bioinformatics analysis showed that PdXEG1 is a member of the glycoside hydrolase 12 (GH12) family of the plant cell wall degrading enzymes (CWDEs) and is highly homologous with other XEG proteins. The molecular docking analysis revealed that the PdXEG1 protein can potentially recognize xyloglucan structures and exhibit hydrolytic activity. The knockout strain exhibited decreased growth rate, conidial yield, and conidial germination rate on the callus induction medium (CIM) (supplemented with citrus pomace). However, no discernible impact of PdXEG1 knockout was observed when the strain was cultured on the potato dextrose agar (PDA). The APdXEG1 strain also displayed reduced xylanase and endoglucanase activities and attenuated in vivo pathogenicity and expansion ability. Correspondingly, PdXEG1 complementation restored these effects. Additionally, transcriptome analyses revealed that PdXEG1 knockdown markedly downregulated the expression of genes encoding various CWDEs, development-related regulators and transporter proteins, in addition to affecting the basal substance metabolism of carbohydrates, amino acids and fatty acids. In conclusion, these results suggested that the PdXEG1 gene is crucial for P. digitatum growth, spore production and pathogenicity during citrus fruit infection.