Abstract
Sour rot is a significant postharvest disease affecting citrus fruit, causing sourness and decay in various cultivars, particularly lemons. How the pathogen, Geotrichum citri-aurantii, adapts to the highly acidic environment of citrus fruit remains inadequately understood. In this study, the growth characteristics, morphological and structural changes, gene expression profiles, and adaptive mechanisms of G. citri-aurantii under highly acidic conditions were elucidated. The findings indicated that G. citri-aurantii modified the environmental pH by either alkalizing (pH < 3.00) or acidifying (pH > 3.00) the host tissue. It exhibited strong adaptability at pH 2.2, showing shortened and aggregated hyphae, delayed spore germination, and increased vacuoles. Transcriptomic analysis and qRT-PCR identified the significant regulation of key differentially expressed genes involved in cell wall remodeling, cell membrane component synthesis, carbon metabolism, and signal transduction. These regulatory changes enable the pathogen to prevent an influx of external acids and maintain the energy supply under acid stress conditions. Additionally, the Pal/Rim pH signaling pathway genes exhibit distinct response patterns in citrus cultivars with different acidities. These findings enrich the comprehension of the pathogenic process of G. citri-aurantii and offer a theoretical foundation for preventing and managing citrus sour rot.