Identification of a Pseudomonas brassicacearum strain as a potential biocontrol agent against clubroot in cruciferous plants through endophytic bacterial community analysis and culture‐dependent isolation

Clubroot, caused by Plasmodiophora brassicae, significantly impacts cruciferous crop production worldwide. Biocontrol is an environmentally friendly and promising approach for clubroot management. Endophytic bacteria are known for their ability to promote plant growth and induce resistance against plant diseases. In the Plasmodiophora brassicae-Brassica pathosystem, microbial communities are poorly understood, and their biocontrol potential for clubroot remains unexplored. In this study, we investigated the endophytic bacterial communities in root samples with different levels of clubroot infection. Subsequently, we employed sequencing analysis and culture-dependent isolation to profile these communities. Our results indicated differences in bacterial composition between D (galled) and H (asymptomatic) roots. Notably the enrichment of Pseudomonas species in H. Culture-dependent bacterial isolation suggested that three Pseudomonas strains (H10, H13, and H15) from H had biocontrol potential, which was confirmed by their effects of 49% reductions in resting spore germination, along with abilities in environmental pH adjustment and biofilm formation. Specifically, during clubroot development, Pseudomonas brassicacearum strain H13 reduced clubroot severity by 51%, suppressed Plasmodiophora brassicae biomass by over 12% in roots and 37% in soil, effectively colonized roots, and increased soil pH. Strain H13 also influenced the expression of plant hormone-related genes, including a 3.4-fold up-regulation of NPR1 at 7 days post-inoculation (dpi), and down-regulation of CKX, AO1, and DWF4 by 84%, 88%, and 40%, respectively, at 21 dpi. These findings suggest that endophytic Pseudomonas strains, particularly strain H13, hold promise as effective biocontrol agents by reducing chemical fungicide application for sustainable clubroot management.