Metabolomics and Transcriptomics Reveal the Function of Trigonelline and Its Synthesis Gene <i>BrNANMT</i> in Clubroot Susceptibility of <i>Brassica rapa</i>

Clubroot caused by Plasmodiophora brassicae, a soil-borne pathogen, threatens cruciferous plants, resulting in severe yield reductions. To identify genes and metabolites associated with clubroot resistance and susceptibility, we performed metabolome and transcriptome analyses of Brassica rapa inbred line CRBJN3-2 inoculated with resistant and susceptible P. brassicae strains. Co-expression network analysis revealed that trigonelline accumulation, linked to the nicotinic acid and nicotinamide metabolic pathways, was significantly higher in clubroot-susceptible plants. Furthermore, applying trigonelline externally aggravated clubroot in both B. rapa and Arabidopsis thaliana. Overexpression of the nicotinate N-methyltransferase gene (BrNANMT) responsible for the conversion from nicotinate to trigonelline in these plants increased disease susceptibility, while loss of this gene's function resulted in improved clubroot resistance. Our study is the first to reveal the function of trigonelline in promoting clubroot development and identify BrNANMT as a clubroot susceptibility gene and trigonelline can be used as a marker metabolite in response to P. brassicae infection. Gene editing of BrNANMT provides new insights for the development of Brassica crops with improved resistance to clubroot.