Extracellular polygalacturonase, β-1,4-glucanase and β-xylosidase in Geotrichum citri-aurrantii positively progressed the sour rot incidence in satsuma mandarin fruit

Gaining an understanding of the characteristics that contribute to the infectiousness of pathogens is essential for devising new strategies to control fungal infection. This study aims to identify and clarify the role of cell wall degrading enzymes (CWDEs) secreted by the Geotrichum citri-aurantii pathogen in the development of sour rot disease in Satsuma mandarin fruit. Results from in vivo experiments showed that CWDEs were secreted during arthrospore germination. Genomic sequencing and liquid chromatography-tandem mass spectrometry results unveiled the presence of polygalacturonase (PG), β-1,4-glucanase, α-glucosidase, xyloglucan:xyloglucosyl transferase, and β-xylosidase. Enzyme activity and gene expression analyses exposed the indispensable roles of PG and β-1,4-glucanase in the initial phase, and β-xylosidase during the acute phase of infection. After infection, we observed that the quantities of cellulose, pectin, and hemicellulose declined continuously, while the soluble pectin content rose steadily. Molecular docking analysis demonstrated that the diversity of CWDE binding to substrates complicated the G. citri-aurantii infection process. Furthermore, we determined the importance of asparagine and aspartic acid in PG-substrate interaction and glycine in β-xylosidase-ligand interaction. Taken together, our findings suggest that PG is a crucial target for blocking the development of sour rot in citrus fruit.