Contrasting glucosinolate profiles in rapeseed genotypes shape the rizosphere-insect continuum and microbial detoxification potential in a root herbivore
Plant secondary metabolites are key mediators of plant–insect–microbiome interactions, yet their role in structuring functionally relevant insect-associated microbial communities remains poorly understood. Here, we combined a factorial experiment using Brassica napus genotypes differing in glucosinolates (GLS) content with distinct succession to investigate the eco-evolutionary dynamics of the microbiota of the root herbivore Delia radicum. Amplicon sequencing and microbial culturing revealed that both rhizospheric and gut microbial communities are shaped by plant genotype and soil legacy, with a subset of bacterial taxa shared across compartments. Notably, Pseudomonas brassicacearum, harboring the isothiocyanates (ITC) detoxifying gene saxA, was consistently recovered from both plant and insect habitats. Functional assays confirmed its capacity to degrade 2-phenylethyl isothiocyanate (PEITC), a major toxic GLS hydrolysis product. Other, gut derived microbial isolates exhibited heterogeneous response to PEITC, ranging from growth inhibition promotion or growth recovery after a prolonged lag phase. Despite the toxicity of ITC, insect fitness proxies were enhanced on GLS+ plants, suggesting microbiota-mediated adaptation to host chemical defenses. Our findings reveal a plant-genotype-specific filtering of environmentally acquired microbes and highlight the role of detoxifying symbionts in Delia radicum performance.