Canola with Stacked Genes Shows Moderate Resistance and Resilience against a Field Population of Plasmodiophora brassicae (Clubroot) Pathotype X

Abstract

Genetic resistance is a cornerstone for managing clubroot (Plasmodiophora brassicae). However, when used repeatedly, a clubroot resistance (CR) gene can be broken rapidly. In this study, canola inbred/hybrid lines carrying one or two CR genes (Rcr1/CRaM and Crr1rutb) were assessed against P. brassicae pathotype X by repeated exposure to the same inoculum source under a controlled environment. Lines carrying two CR genes, either Rcr1 + Crr1rutb or CRaM + Crr1rutb, showed partial resistance. Selected lines were inoculated with a field pathotype X population (L-G3) at 5 × 106 resting spores/g soil, and all clubs were returned to the soil they came from six weeks after inoculation. The planting was repeated for five cycles, with diseased roots being returned to the soil after each cycle. The soil inoculum was quantified using qPCR before each planting cycle. All lines with a single CR gene were consistently susceptible, maintaining high soil inoculum levels over time. The lines carrying two CR genes showed much lower clubroot severity, resulting in a 10-fold decline in soil inoculum. These results showed that the CR-gene stacking provided moderate resistance against P. brassicae pathotype X, which may also help reduce the pathogen inoculum buildup in soil.

Comparative transcriptome analysis of canola carrying a single vs stacked resistance genes against clubroot

“…This observation aligns with previous reports on the effectiveness of certain CR genes and gene combinations in conferring resistance to specific P . brassicae pathotypes ( Strelkov et al., 2018 ; Sedaghatkish et al., 2019 ; Tonu et al., 2023 ).…”

“…It is possible that this partial resistance is derived from Crr1 rutb in line 20, as the inbred line carrying CRa M alone was susceptible. In a prior study, Tonu et al. (2023) reported that line 20 produced in different batches may vary in resistance to pathotype X, with some batches showing almost immunity.…”

“…Five remaining plants from each pot were maintained in the growth room and assessed for clubroot severity on a standard 0–3 scale at 35 dpi to confirm the success of inoculation. A disease severity index (DSI) was calculated for the five remaining plants assessed for each replicate ( Tonu et al., 2023 ).…”

“…While effective against initial five pathotypes found in Canada, this gene was observed to have lost the effectiveness in some field by 2013 ( Strelkov et al., 2018 ; Karim et al., 2020 ). Additional CR genes (A08) derived from rutabaga ( Hasan and Rahman, 2016 ) were introduced later, which showed resistance or moderate resistance to most of the novel pathotypes, including pathotype X ( Tonu et al., 2023 ). The rutabaga-derived CR gene(s) have also been pyramided with those on A03 (second-generation CR) for a broader range of efficacy.…”

“…Stacking three CR genes in Chinese cabbage ( B. rapa ) expanded the resistance against multiple P. brassicae pathotypes ( Matsumoto et al., 2012 ), although the resistance mechanisms associated with CR-gene stacking remain unexplored. In a recent study, Tonu et al. (2023) reported that canola varieties carrying stacked CR genes (A03 and A08) had greater resistance durability than those carrying either CR gene alone, when exposed repeatedly to a field P. brassicae pathotype X population.…”