Development of a Sinitic Clubroot Differential Set for the Pathotype Classification of Plasmodiophora brassicae

Abstract

Plasmodiophora brassicae , which is known for its broad genetic diversity for virulence, is the causal agent of clubroot disease of Brassica crops worldwide. Studies on pathotype characterization with four differential hosts according to Williams’ classification system showed the predominance of pathotype 4 in China. However, the genetic variability within pathotype 4 complicates the breeding of durable clubroot-resistant (CR) cultivars. Herein, a Sinitic clubroot differential (SCD) set was developed using a set of eight differential inbred lines of Chinese cabbage with known or novel CR genes. The presence of immense diversity within pathotype 4 of Williams’ system was verified, and 11 pathotypes were characterized using the developed SCD system. The scalability and practicability of the system was further confirmed with a subset of 95 field isolates from different Brassica crops and different regions of China and Korea. Sixteen pathotypes were detected from 132 field isolates, named Pb1 to Pb16, respectively. Among them, Pb1 and Pb4 were prevalent in diverse Brassica crops in the southern and northern regions of China. Pb12, Pb13, Pb14, and Pb16 showed area-specific distribution. The SCD set developed herein will provide important genetic resources for pathogenicity studies of P. brassicae and for CR breeding in Chinese cabbage and other Brassica crops.

Current and Future Pathotyping Platforms for Plasmodiophora brassicae in Canada

“…These assays aim to capture the occurrence and extent of physiologic specialization in populations of the pathogen [ 16 ]. Numerous differential sets have been proposed to study pathogenic variability in P. brassicae [ 20 , 29 , 30 , 31 , 32 ]. The differentials of Williams, one of the earliest and among the most commonly used systems, consists of four differential hosts that can distinguish a theoretical maximum of 16 pathotypes [ 29 ].…”

“…Each pathotype is assigned a number followed by a letter (for example, ‘pathotype 3A’); the numbers correspond to the Williams’ [ 29 ] pathotype designation, while the letters are unique to the CCD set and denote variants of the Williams’ pathotypes [ 20 , 23 ]. Most recently, the Sinitic Clubroot Differential (SCD) set was developed in China using differential hosts with known clubroot-resistance genes to explore the genetic variability of pathotype 4, as defined by the differentials of Williams [ 32 ]. The SCD set was essential when isolates identified as pathotype 4 on the system of Williams were found to exhibit varied virulence patterns on various Chinese cabbage ( B. rapa ) and cabbage ( B. oleracea ) cultivars.…”

Molecular Pathotyping of Plasmodiophora brassicae—Genomes, Marker Genes, and Obstacles

“…Additional adaptations were made using regional economical important hosts to fit local needs, such as the Canadian Clubroot differential set (CCD) with a focus on rapeseed resistance [41]. Other systems were focusing on Chinese cabbage resistance [42][43][44]. Those different systems make it difficult to compare pathotypes between studies, as the pathotype determined by one system cannot be translated into another system.…”

“…Other systems focusing on Chinese cabbage resistance Different pathotypes occurring dominant in different regions or areas in the world. The Williams pathotype 3 appears to be dominant in Alberta, Canada [45,46], and Korea [43], whereas Williams pathotype 4 is dominant in China [42]. Using the ECD system dominant pathotypes were also determined in Australia as 16/3/12 and 16/3/31 [47].…”

“…In Germany, the P. brassicae isolates with the Somé pathoypes 1 and 3 or ECD pathotypes 16/31/31, 16/14/30, and 16/14/31 were most frequently found [17]. The occurrence of pathotypes is somewhat fluent and new pathotypes become present in fields, and there is variation of virulence inside a pathotype, when tested on additional hosts [15,[42][43][44]46]. In addition, P. brassicae field isolates and even isolates from an individual plant root can consist out of a mixture of pathotypes and genetically different strains [48][49][50].…”