Remodelling of cell wall components potentially contributes to host resistance to Plasmodiophora brassicae in Brassica napus

Clubroot, caused by Plasmodiophora brassicae Woronin, is a major root disease in Brassica crops worldwide. Despite extensive molecular studies on the P. brassicae - B. napus pathosystem, the mechanisms underlying host resistance remain poorly understood. In this study, we used two near-isogenic lines (NILs) of canola ( B. napus ) carrying clubroot resistance derived from Chinese cabbage ( B. rapa var. pekinensis ) cv. Bilko to investigate the molecular and cellular basis of the resistance. The expression analysis of 108 genes from chromosome A03 in the resistant and susceptible NILs identified 44 genes that are important in clubroot resistance; among these, 20 were from the 21–27 Mb genomic region. These genes are associated with disease resistance-related proteins (TIR-NBS-LRR), growth regulation ( auxin - transport), defense hormones signaling ( jasmonic acid response gene), and reactive oxygen species (ROS)-mediated antioxidant response (thioredoxin). In addition, several genes from A03 involved in cell wall reinforcement and remodelling, including those encoding dirigent-like domains , pectate lyase, cellulose synthase , and pectin esterases, were strongly upregulated in response to pathogen infection. Biochemical analysis using Fourier Transform Mid-Infrared (FT-MIR) spectroscopy revealed enhanced signatures of structural polysaccharides, including cellulose, hemicellulose, and pectin, in the resistant NIL roots. Ultrastructural examination by transmission electron microscopy showed severe disruption of cell wall components in the susceptible NILs roots, whereas resistant NILs roots maintained intact cell wall boundaries and cellular organization. This study provides multi-layer evidence that the reinforcement and maintenance of cell wall integrity could be integral to clubroot resistance in canola afforded by the resistance of the Chinese cabbage cv. Bilko.