Plasmodiophora brassicae Infection Modulates Expansin Genes of Brassica rapa ssp. pekinensis

Clubroot is a soil-borne disease of cruciferous crops, including Brassica rapa ssp. pekinensis , and causes substantial yield losses. In an attempt to develop clubroot-resistant B. rapa cultivars, we investigated the role of a root-abundant expansin-like B1 (EXLB1) during Plasmodiophora brassicae inoculation. The histochemical analyses of infected transgenic reporter lines showed a role for BrEXLB1 in disease response as early as 3 dpi. The transgenic overexpression of EXLB1 in B. rapa conferred disease-sensitive phenotypes and was comparable to non-transgenic controls at 30 dpi. In contrast, the heterogeneous population of antisense BrEXLB1 -overexpressing lines conferred disease resistance against highly pathogenic P. brassicae race 2 inoculations under greenhouse conditions. Additionally, we profiled the relative expression of 32 other BrEXPs in wild-type seedlings, sampled on different days (1–10) after inoculation using qRT-PCR. The results indicate that the expression pattern of most BrEXP genes was significantly altered during different infection times, suggesting their participation in clubroot responses. In particular, the expressions of EXPA20 , EXPA21 , and EXPA34 were consistently downregulated, while the expression of EXPA5 was upregulated (log 2 FC ≥ 2) compared to controls. Altogether, our study showed that BrEXPs participate in clubroot disease response, and their genetic manipulation is likely to provide clubroot disease resistance.