Interactions between Endophytic Fungal Community Succession and Fruit Quality Components in Siraitia grosvenorii at Different Developmental Stages

Background Siraitia grosvenorii is a medicinal and edible plant species endemic to China. Its fruit quality is regulated jointly by developmental stage, cultivation environment, and symbiotic microorganisms, and endophytic fungi are recognized as key factors influencing the accumulation of bioactive medicinal components. However, how endophytic fungal communities succeed during fruit development and how this succession relates to fruit quality components such as mogroside V and total flavonoids remain poorly understood. Results Fruits at four developmental stages (young fruit, RG; early expansion, DG; green fruit, QG; mature fruit, CSG) were used to determine the contents of total flavonoids, water-soluble sugars, soluble proteins, and mogroside V, and high-throughput sequencing was used to profile the diversity, composition, and assembly of the endophytic fungal communities. Total flavonoid content was highest at the young fruit stage (0.72%) and declined gradually with development. Soluble protein and mogroside V contents peaked at the mature stage (5.92 mg/g and 21.85 mg/g, respectively). The fungal α-diversity indices were relatively low at the young fruit and early expansion stages but increased significantly at the mature stage, indicating higher species diversity, richness, and phylogenetic diversity at maturity. Analyses of β-diversity and community composition revealed significant differences in community structure and composition among different developmental stages, with the early expansion stage being a key turning point for community decline and subsequent reconstruction; Cladosporium delicatulum was the dominant species throughout the entire developmental period. Community assembly was governed mainly by stochastic drift (contributing > 66%), whereas the contribution of dispersal limitation rose from 3.19% at RG to 31.33% at CSG, and deterministic environmental filtering weakened markedly at maturity. Mantel test and redundancy analysis further confirmed that soluble protein and mogroside V were significant explanatory factors for community variation at the mature stage, while the correlation heatmap indicated that both were highly significantly positively correlated with the abundances of dominant fungal genera such as Cladosporium . Conclusions These results indicate that endophytic fungal community assembly in S. grosvenorii fruit shifts from early flavonoid-mediated processes to later protein- and mogroside V-mediated nutrient recruitment. The synergy between metabolites and the core microbiota established at maturity is likely a key mechanism underlying the high specialization of the mature endophytic fungal community.