A Seed-Derived Multi-Kingdom Synthetic Community Enhances Radish Seedling Protection Against Alternaria brassicicola Abra43

Seed-associated microorganisms represent a promising resource for developing microbiome-based strategies against seed-borne pathogens. Here, we assembled synthetic microbial communities (SynComs) from cultivable members of the natural radish seed microbiota and evaluated their capacity to protect seedlings against the seed-transmitted fungal pathogen Alternaria brassicicola Abra43. Seven SynComs representing single- or multi-kingdom combinations of bacteria (B), filamentous fungi (F), and yeasts (Y) (B, F, Y, BF, BY, FY, and BFY) were inoculated onto radish seeds. We assessed their effects on disease incidence, seedling growth, microbial community assembly (ITS1 and gyrB gene metabarcoding) and pathogen colonization (metabarcoding and specific qPCR on dhn1 gene for A. brassicicola Abra43). Protection of radish seedlings against A. brassicicola Abra43 strongly depended on SynCom composition. The yeast fraction alone provided little protection, whereas the bacterial fraction partially reduced symptoms and maintained seedling growth despite substantial pathogen colonization, suggesting increased plant tolerance. SynComs containing filamentous fungi markedly reduced A. brassicicola Abra43 abundance, and the full multi-kingdom BFY SynCom provided the most consistent protection, combining reduced disease incidence, restored seedling growth, and low pathogen relative and absolute abundance. Inoculated strains, Alternaria alternata ABAI001 and Plectosphaerella sp. FL19 were enriched in protective SynComs and displayed antagonistic activity against A. brassicicola Abra43 in vitro. Together, these results show that seed-derived multi-kingdom communities can reshape early plant microbiota and that combining bacterial and fungal strains with complementary ecological functions can improve protection against a seed-borne fungal pathogen.