In planta storage of boron and calcium affects clubroot severity in Brassica napus

Boron (B) and calcium (Ca) are essential nutrients for plant growth that have also been reported to reduce clubroot disease caused by Plasmodiophora brassicae. This study evaluated the effects of soil-applied B (0, 8 or 16 kg B ha^-1) and Ca (0, 500 or 1000 kg Ca ha^-1) on clubroot development in Brassica napus lines differing in B sensitivity, and investigated underlying mechanisms. Ca consistently reduced clubroot severity, whereas B had a weaker, inconsistent effect and caused phytotoxicity at higher rates. Ca also alleviated B toxicity and increased shoot biomass under high B conditions. Fourier transform mid-infrared spectroscopy revealed that Ca promoted pectin demethylation and Ca-mediated crosslinking, enhancing cell wall rigidity and reducing porosity. Infection by P. brassicae induced lignification, remodeling of matrix polysaccharides, and changes in protein composition and nitrate signatures, indicating coordinated cell wall modification and metabolic reprogramming. Boron K-edge X-ray absorption near-edge structure spectra showed shifts in B speciation between healthy and infected roots, suggesting that infection alters B coordination and localization through modifications to cell wall structure and intracellular binding. Micro X-ray fluorescence mapping showed that Ca accumulation at infection sites was consistent with cell wall reinforcement, while infection also triggered zinc (Zn) accumulation, indicating altered Zn homeostasis. These results provide mechanistic insight into nutrient-mediated cell wall modification and element redistribution. These results also demonstrate that Ca is more effective and reliable than B for reducing clubroot severity, which has a practical application in nutrient strategies for clubroot management.