Microbiota and horizontal microbial transmission dynamics associated with bramble (Rubus spp.) flowers

Microbes are integral players in plant–insect networks, and play crucial roles in host health and ecological interactions. However, the horizontal microbial transmission dynamics via flowers remain insufficiently understood, particularly in hub plants visited by diverse insect communities. Here, using 16S rRNA gene and ITS2 metabarcoding, we characterized the bacterial and fungal communities associated with open flowers, insect-excluded bagged flowers, and flower buds of bramble (Rubus spp.), a highly insect-visited hub plant. Our results show that environmental deposition and insect visitation significantly altered floral microbiota diversity and microbial load, particularly for bacterial communities. Insect visitation potentially enriched fermentative and probably also pathogenic bacteria, including Spiroplasma. While bagged flowers and open flowers both showed high within-group variation for bacterial communities, the microbial networks were more internally connected in bagged flowers, and more centralized in open flowers. The distinct network properties might be due to flower group-specific microbial hub genera introduced by different horizontal transmission routes. Moreover, cross-domain network analysis revealed hub genera unique to bacteria-fungi interactions, including Cladosporium, which was consistently detected across all flower groups. These findings highlight the dominant role of insect visitation in shaping floral microbiota and underscore the ecological significance of hub plants in horizontal microbial transmission across plant–pollinator networks.