Contrasting glucosinolate profiles in rapeseed genotypes shape the rhizosphere-insect continuum and microbial detoxification potential in a root herbivore

ABSTRACT 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 glucosinolate (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 responses 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. IMPORTANCE Understanding how herbivorous insects adapt to plant chemical defenses is important in the context of new agricultural practices. This study highlights that the host plant genotype shapes not only rhizospheric and gut microbial communities but also promotes the acquisition of symbiotic bacteria capable of detoxifying harmful isothiocyanates. These findings reveal a functional microbial pathway for insect adaptation to plant defenses, with potential implications for pest management strategies. By uncovering the role of plant-associated microbiota, the acquisition of beneficial microbes, and their functional contributions to host fitness, this work provides a foundation for innovative agroecological approaches that leverage plant–microbe–insect interactions.