Loss of PMR4 callose synthase triggers jasmonic acid-dependent resistance to the clubroot disease in Arabidopsis and Brassica napus.

Clubroot caused by the protist pathogen Plasmodiophora brassicae Woronin is a major disease in Brassicaceae crops. Growing clubroot-resistant cultivars remains the most effective and practical control strategy, but diverse or emerging pathotypes can often overcome resistance genes deployed. In this study, we identified POWDERY MILDEW RESISTANT 4 (PMR4) as a potential host susceptibility factor that can be targeted as a novel genetic resource for durable clubroot resistance. Recessive PMR4 mutations in Arabidopsis thaliana conferred strong resistance to multiple P. brassicae pathotypes, independent of salicylic acid-mediated plant immunity. Using CRISPR/Cas9, we edited 2 PMR4 orthologs in the Brassica napus genome, and the resulting homozygous mutants exhibited resistance to both powdery mildew and clubroot diseases. This study reveals that P. brassicae infection induces callose deposition in Arabidopsis and B. napus roots in a PMR4-dependent manner, similar to wound- and powdery mildew-induced callose deposition in Arabidopsis leaves. The Arabidopsis pmr4-1 mutation does not affect P. brassicae primary infection but blocks the pathogen entry from epidermal cells to the stele in a jasmonic acid signaling-dependent manner, which coincides with accumulation of lignin-like and suberin compounds in periderm cell walls. Together, these findings establish PMR4-encoded callose synthase as a host susceptibility factor facilitating secondary infection by P. brassicae and demonstrate its potential as a gene-editing target for enhancing resistance to powdery mildew, clubroot, and possibly other biotic and abiotic stresses in Brassicaceae crops.