Identification and Functional Characterization of Plasmodiophora brassicae Secretory Proteins
Date
2021-06-21
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0001-6396-6214
Type
Thesis
Degree Level
Doctoral
Abstract
Plasmodiophora brassicae, a soil-borne parasitic protist, is responsible for clubroot, one of the most devastating diseases of Cruciferous plants. Due to the obligate biotrophic nature of the pathogen, investigating the pathogenesis process is difficult. Recent studies of clubroot disease have focused primarily on transcriptome analysis of the host plant responses as well as histological and cytological studies of the P. brassicae infection cycle in plants. In this thesis, I have highlighted the functional roles of P. brassicae secretory effectors/proteins in the plant-pathogen interaction during infection.
A number of P. brassicae putative effectors (PbPEs) were identified from a clubroot gall cDNA library using bioinformatics tools and techniques. From transient expression studies, a subset of PbPEs were found to localize to the host cell endomembrane system and to regulate plant immune responses, with PbPE15 showing the strongest level of immune system suppression. Two effectors targeting the chloroplast were also identified.
The P. brassicae effector PbPE23 induced light-independent necrosis via the MAP kinase pathway in both host and nonhost plants. PbPE23 also downregulated the transcript level of PATHOGENESIS RELATED 5 (PR5), integral to the plant antifungal response. A TPAP-like motif and Q-rich region at the N-terminal end of the protein, resembling that identified in many NEP1-like proteins (NLPs) of plant pathogens, was identified as required for the induction of necrosis in plants.
The two P. brassicae PbPEs, PASP1 and PASP2, localized to plasmodesmata (PD) via attachment to the PD-associated membrane system. Topological analysis together with mutation studies identified the importance of sequence motifs and the transmembrane domain (TMD) for the association with PD. Furthermore, an involvement of the actin cytoskeleton in PD-association is also described. Similarly, PbPE13, localized to plasmamembrane lipid rafts with association to PD, suggesting a role in cell-to-cell communication and possible pathogen spread through the movement of spores and/or secretory proteins to facilitate disease progression. PASP1 and PASP2 overexpressing Arabidopsis lines showed distinct root phenotypes, suggesting that these two proteins interfere with normal root cell division and growth.
Altogether, this array of PbPEs demonstrate the multi-faceted approaches taken by P. brassicae in order to redirect normal host cell functions towards the support and proliferation of the intracellular pathogen. Understanding the molecular pathology of P. brassicae and the role of effectors in disease development will provide tools and opportunities towards the design of effective control strategies for this pathogen and resulting clubroot disease.
Description
Keywords
Plasmodiophora bassicae, Effectors, Endomembrane, PCD - programmed cell death, PAMP (Pathogen-associated molecular pattern), Plasmodesmata.
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Biology
Program
Biology