<i>Mesorhizobium huakuii</i> 7653 R regulates the endophytic bacterial communities and disease resistance pathways to promote clubroot resistance in <i>Brassica napus</i>

Clubroot is a global soilborne disease caused by <i>Plasmodiophora brassicae</i> that severely affects cruciferous crops. Currently, there are no effective control measures to completely eliminate pathogens in the field. Pot experiments have shown that treatment with <i>Mesorhizobium huakuii</i> 7653 R can relieve clubroot symptoms. However, whether <i>M. huakuii</i> 7653 R can be used in the field to control clubroot disease is unclear, and the mechanism of its resistance remains largely unknown. We conducted two seasons of field trials with <i>Brassica napus</i> and found that <i>M. huakuii</i> 7653 R treatment significantly reduced clubroot disease severity, and that the agronomic traits at the seedling, flowering, and harvesting stages were significantly improved compared to the control group. The average yield per hectare, thousand grain weight, and oil content increased by 83.1%, 10.4%, and 3.7%, respectively. <i>M. huakuii</i> 7653 R also enhanced the resistance and yield of the cruciferous plant <i>Brassica campestris</i> var. <i>purpuraria</i> to clubroot. We characterized the composition and structure of the endophytic microbiota using bacterial 16S sequencing, and showed that treatment with <i>M. huakuii</i> 7653 R shaped the structure of endophytic bacterial communities. The relative abundances of Actinobacteria and Bacteroidetes, which are resistant to oxidative stress, were enriched in the co-inoculation with <i>M. huakuii</i> 7653 R and <i>P. brassicae</i> on <i>B. napus</i>. Transcriptome data indicated that genes related to plant immunity, nitrogen uptake and metabolism were significantly up-regulated by co-inoculation with <i>M. huakuii</i> 7653 R and <i>P. brassicae</i>. Our findings provide a theoretical basis for the widespread application of <i>M. huakuii</i> 7653 R in clubroot biocontrol.