Root-Exudate-Mediated Modulation of the Rhizosphere Microbiome in <i>Brassica juncea</i> var. tumida During <i>Plasmodiophora brassicae</i> Infection

Disease caused by <i>Plasmodiophora brassicae</i> severely disrupts cruciferous crops by altering root physiology and rhizosphere ecology. While pathogen-induced shifts in rhizosphere microbiomes are documented, the mechanisms linking root exudate reprogramming to microbial community remodeling remain poorly understood. Here, we integrated untargeted metabolomics and 16S rRNA sequencing to investigate how root exudates reshape the rhizosphere microbiome of tumorous stem mustard (<i>Brassica juncea</i> var. tumida) through <i>P. brassicae</i> infection. Metabolomic profiling identified 1718 root exudate metabolites, with flavones (e.g., apigenin 7-<i>O</i>-β-D-rutinoside, VIP > 1.5) and phenolic derivatives (e.g., gastrodin) being selectively enriched in infected plants. <i>P. brassicae</i> infection significantly increased rhizobacterial richness (ACE index, <i>p</i> < 0.05) and restructured the community composition, marked by enrichment of <i>Paenibacillus</i> (LDA score > 3.0). Procrustes analysis revealed tight coupling between microbial community shifts and metabolic reprogramming (M² = 0.446, <i>p</i> = 0.005), while Spearman correlations implicated pathogen-induced metabolites like geniposidic acid in recruiting beneficial <i>Paenibacillus</i>. Our results reveal that plant hosts dynamically secrete defense-related root metabolites to remodel the rhizosphere microbiome in response to <i>P. brassicae</i> infection. This dual-omics approach elucidates a chemical dialogue mediating plant–microbe–pathogen interactions, offering novel insights for engineering disease-suppressive microbiomes through root exudate manipulation.