Junye Jiang | ALES Graduate Seminar

A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES PhD Final Exam Seminar by Junye Jiang. This seminar is open to the general public to attend.
Thesis Topic: Studies of pathogenicity in Plasmodiophora brassicae and segregation of clubroot resistance genes from Brassica rapa subsp. Rapifera

PhD with Drs. Stephen Strelkov and Sheau-Fang Hwang

Seminar Abstract:

The planting of clubroot resistant (CR) canola (Brassica napus) is the most effective method to manage clubroot, a soilborne disease caused by Plasmodiophora brassicae. In recent years, many P. brassicae isolates capable of overcoming resistance have been detected, often in mixtures with avirulent isolates. To improve understanding of the effect of low concentrations of virulent isolates on host resistance, three CR canola cultivars (‘45H29’, ‘L135C’ and ‘L241C’) were inoculated with pairs of isolates representing virulent/avirulent pathotypes (2*/2, 3*/3 and 5*/5) of P. brassicae, collected after or before the introduction of CR canola, respectively. Clubroot severity was significantly higher in all nine experimental treatments (low virulent + high avirulent) than in the negative control NC1 (high avirulent), and higher in seven of nine experimental treatments than in the negative control NC2 (low virulent). Disease severity was positively correlated with P. brassicae biomass in planta, as determined by quantitative PCR analysis 28 - 35 days after inoculation (dai). These results suggest that low concentrations of virulent isolates compromised the clubroot resistance in canola, facilitating infection by avirulent isolates.

In a second study, the expression of 205 P. brassicae genes encoding putative secreted proteins was compared following inoculation of the canola ‘45H29’ with pathotypes 5 (avirulent) and 5X (virulent) of the pathogen. Sixteen of these genes were differentially expressed at 14 dai, and additional monitoring at multiple timepoints (7, 14 and 21 dai) indicated that, collectively, 12 of the 16 genes were upregulated in pathotype 5X. The relative expression of the same 16 genes was also compared in the interaction between the canola ‘Westar’ and pathotype 5 (virulent on ‘Westar’), with 12 of the genes showing similar expression patterns as in the ‘45H29’/5X interaction. Given their common expression patterns in two compatible interactions, it is possible that these genes play a role in clubroot pathogenesis.

In a third and final study, clubroot resistance was introgressed from the European Clubroot Differential (ECD) 02 into the B. rapa accessions CR 2599 and CR 1505 (‘Emma’), and into the B. napus accession CR 1054 (‘Westar’). The segregation of clubroot resistance in the F₂ populations largely deviated from the expected Mendelian segregation ratios of 3:1, 9:3:3:1 and 63:1 for resistance controlled by a single, two or three dominant genes, respectively, suggesting additive effects or recessive epistasis. Quantitative trait loci (QTL) analysis of the F₂ of Popl#1 (ECD 02 × CR 2599) following inoculation with pathotype 5X revealed one genomic region between the simple sequence repeat (SSR) markers BGA06 and KB29N19 on chromosome 03 (A03), which explained 21.7 to 44.7% of the phenotypic variance. Similarly, two QTL regions, explaining 11.7-17.3% and 10.5-16.3% of the phenotypic variance, were identified in the F₂ of Popl#1 after inoculation with pathotype 5G. The SSR markers KB59N06 and B4732 delimited the first QTL, while the molecular marker CRaJY and SSR marker BGB41 flanked the second QTL. The two QTLs are located in a genomic region on the A03 chromosome of B. rapa where the CRa or CRb^Kato gene(s) are mapped. The Crr1 gene was detected on chromosome A08 in both Popl#1 and Popl#2 (ECD 02 × CR1505), indicating that ECD 02 carries this resistance gene as well. An improved understanding of pathogenicity in the clubroot pathosystem and the availability of markers for marker-assisted selection will be important in efforts to develop canola with durable clubroot resistance.