Changes in primary metabolism and associated gene expression during host-pathogen interaction in clubroot resistance of Brassica napus

The role of primary metabolism during Brassica napus-Plasmodiophora brassicae interaction leading to clubroot resistance has not yet been investigated thoroughly. In this study, we investigated some of the primary metabolites and their derivatives as well as expression of the genes involved in their biosynthesis to decipher this host-pathogen interaction. For this, two sets (clubroot resistant and susceptible) of canola lines were inoculated with P. brassicae pathotype 3A to investigate the endogenous levels of primary metabolites at 7-, 14-, and 21-days after inoculation (DAI). The associated pathways were curated, and expression of the selected genes was analyzed using qRT-PCR. Our results suggested the possible involvement of polyamines (spermidine and spermine) in clubroot susceptibility. Some of the amino acids were highly abundant at 7- or 14-DAI in both resistant and susceptible lines; however, glutamine and the amino acid derivative phenylethylamine showed higher endogenous levels in the resistant lines at later stages of infection. Organic acids such as malic, fumaric, succinic, lactic and citric acids were abundant in the susceptible lines. Conversely, the abundance of salicylic acid (SA) and the expression of benzoate/salicylate carboxyl methyltransferase (BSMT) were higher in the resistant lines at the secondary stage of infection. A reduced disease severity index and gall size were observed when exogenous SA (1.0 mM) was applied to susceptible B. napus; this further supported the role of SA in clubroot resistance. In addition, a higher accumulation of fatty acids and significant upregulation of the pathway genes, glycerol-3-phosphate dehydrogenase (GPD) and amino alcohol phosphotransferase (AAPT) were observed in the resistant lines at 14- and 21-DAI. In contrast, some of the fatty acid derivatives such as phosphatidylcholines represented a lower level in the resistant lines. In conclusion, our findings provided additional insights into the possible involvement of primary metabolites and their derivatives in clubroot resistance.