The role of auxin and indole glucosinolates in defense against clubroot infection in Brassica napus
Creators: Liu, Hui
Publisher: name: University of Alberta Library
Year: 2023

<p>Abstract</p>

<p>Clubroot, caused by the obligate parasite Plasmodiophora brassicae, is one of the most damaging diseases of the Brassicaceae. Glucosinolates (GSLs) are a group of defense-related secondary metabolites in cruciferous plants that have been associated with clubroot disease. The breakdown of GSLs results in the generation of isothiocyanates, thiocyanate, and nitriles, which are known to be involved in plant defense mechanisms against numerous herbivores and pathogens. Analysis of a database from Zhou et al. (2020) showed that the resistant rutabaga cultivar (“Whilhelmsburger”) had a different gene expression pattern in the indole GSL pathway compared with the susceptible rutabaga cultivar (“Laurentian”) in response to P. brassicae inoculation. Therefore, quantitation of selected gene targets in the GSL pathway was performed using qRT-PCR in resistant and susceptible rutabaga cultivars that were either non-inoculated or inoculated with P. brassicae. The results showed that the transcript abundance of both BnBGLU30 and BnNSP5 increased in the roots of both resistant and susceptible cvs. 7 days after inoculation (dai) with P. brassicae, suggesting that modulation of expression of specific GSL pathway genes occurs in rutabaga roots likely resulting in elevated nitrile production as a primary infection response to P. brassicae. Additionally, the GLS gene expression pattern of the resistant cv. indicated that it may have a greater capacity to produce and maintain the levels of nitriles as a primary infection response to P. brassicae than the susceptible cultivar. Future studies involving the quantification of specific GSLs-related compounds including nitriles in the pathway are required to correlate gene expression changes with changes in GSL products produced.</p>

<p>Auxin has been implicated as a key hormone for gall formation induced by P. brassicae by promoting cell hyperplasia and hypertrophy. The selective expression of BnIAGLU only in the resistant cv. (a gene that codes for an enzyme that converts the auxin indole-3-acetic acid; IAA) to an inactive glucose ester) suggests that conjugation of free IAA to an inactive IAA glucose ester may be a defense response to P. brassicae at the primary infection stage. In order to determine if modification of auxin response affects clubroot disease progression, two independent transgenic lines expressing the pea auxin receptor PsAFB6 (this auxin receptor clade does not exist in the Brassicaceae family) in the canola cv. Westar, along with their respective null lines, were phenotypically characterized for their response to clubroot infection using a hydroponic system. Implementation of a hydroponic system during the infection phase of P. brassciae with canola as the host plant allowed the characterization of the effect of modulation of auxin response on clubroot disease progression to be completed without interference with the complexities of soil- or peat-based media interactions. The canola AFB6 transgenic lines showed a notable reduction in their disease index compared to their corresponding transgenic null controls at 30 dai. When characteristic galls were present in both canola PsAFB6-expressing lines and the null lines, the PsAFB6-expressing lines exhibited significantly reduced surface area at the root-shoot transition zone compared to their corresponding null control lines, indicating that they had milder galling symptoms in response to P. brassciae inoculation. Given the mode of action for auxin receptors and the proposed role for auxin facilitating gall development, the reduction in clubroot symptoms in PsAFB6-expressing canola lines appears to be the result of reduced auxin response in these lines. Further studies are needed to confirm that a reduction in auxin response is the main mechanism involved in PsAFB6-induced reduction in clubroot galling symptoms, and to determine the exact mode of action bringing about the potential reduction in auxin response. Identifying the specific pathways involved in PsAFB6-mediated reduction in clubroot galling symptoms, and the role of GSL-related compounds in the defense against clubroot disease progression, could potentially lead to the development of novel approaches for clubroot disease management in canola.</p>