<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.

根肿病是由芸苔根肿菌（Plasmodiophora brassicae）引发的全球性土传病害，可严重危害十字花科作物。目前尚无有效措施可在田间完全根除该病原菌。盆栽试验结果表明，经华癸中慢生根瘤菌（Mesorhizobium huakuii）7653 R处理可缓解根肿病症状。然而，该菌株能否应用于田间防控根肿病，其抗病机制目前仍不明确。本研究针对甘蓝型油菜（Brassica napus）开展了两季田间试验，结果显示华癸中慢生根瘤菌7653 R处理可显著降低根肿病病情严重程度，且苗期、花期与收获期的农艺性状均较对照组显著改善。每公顷平均产量、千粒重及含油量分别提升83.1%、10.4%与3.7%。此外，该菌株还可提升十字花科植物紫菜薹（Brassica campestris var. purpuraria）对根肿病的抗性与产量。本研究通过细菌16S测序解析了内生微生物群的组成与结构，证实华癸中慢生根瘤菌7653 R处理可重塑内生细菌群落结构。在甘蓝型油菜上联合接种华癸中慢生根瘤菌7653 R与芸苔根肿菌后，具有抗氧化应激能力的放线菌门（Actinobacteria）与拟杆菌门（Bacteroidetes）的相对丰度显著富集。转录组数据分析显示，联合接种华癸中慢生根瘤菌7653 R与芸苔根肿菌可显著上调植物免疫、氮吸收与代谢相关基因的表达。本研究结果为华癸中慢生根瘤菌7653 R在根肿病生物防控中的规模化应用提供了理论依据。