Underlying mechanism of menthol on controlling postharvest citrus sour rot caused by Geotrichum citri-aurantii

Sour rot, caused by Geotrichum citri-aurantii, has affected fruit quality and caused severe economic losses. Menthol is widely applied to medicine and food industries due to its antiseptic and antimicrobial effects, whereas its application in postharvest fruit management is rarely reported. The present study investigated the antifungal activity of menthol and its underlying mechanism involved. The in vitro experiments showed that the minimum inhibitory concentration (MIC) of menthol was 6.4 mmol L-1. Menthol induced the formation and accumulation of reactive oxygen species (ROS) in G. citri-aurantii and triggered severe membrane lipid peroxidation, resulting in the destruction of plasma membrane integrity. The activity of extracellular alkaline phosphatase (AKP) was increased after menthol treatment. Otherwise, the decrease of blue fluorescence in the cell wall and the change of cell wall morphology were also observed, indicating that the cell wall structure was disrupted. Notably, in vivo, the disease incidence of citrus fruit decreased by 43% after the application of 128 mmol L-1 menthol. Furthermore, menthol treatment maintained the quality of citrus fruit and enhanced defense-related enzyme activities in citrus. To further explore the potential pathogenicity of G. citri-aurantii, we de novo assembled the genome of G. citri-aurantii G50 based on Illumina NovaSeq PE150 and PacBio Sequel. The revised genome size was 25.41 Mb, assembled onto 27 scaffolds with a total length of 27,548,716 bp, 38.49% GC content and 4181 protein-coding genes. Herein, we identified four CWI-MAPK-related genes (GciPKC1, GciBCK1, GciMKK1 and GciSLT2). The expressions of these genes were down-regulated after menthol treatment, influencing on downstream chitin and glucan synthesis. Collectively, these results demonstrated that menthol might be beneficial as a potential alternative for decay management of citrus sour rot. Our research provided new perspectives for postharvest disease control and management control of this pathogen and elucidated its pathogenic mechanism.