Endomembrane-Targeting Plasmodiophora brassicae Effectors Modulate PAMP Triggered Immune Responses in Plants

Melatonin and copper oxide nanoparticles synergistically mitigate clubroot disease and enhance growth dynamics in Brassica rapa

Clubroot, a devastating soil borne disease affecting 30%∼50% of Brassicaceae crops worldwide, lacks effective control measures. In the present study, we explored the potential of melatonin (MT) and copper oxide nanoparticle (CuO-NPs) in mitigating clubroot severity in the Brassica rapa ssp. pekinensis. Following 18 h priming with MT, CuO-NPs, or both seeds were grown in controlled environment using synthetic potting mix. Inoculated with Plasmodiophora brassicae spores on 5th day, followed by a soil drench phyto-nano treatment with a week interval. Plants were assessed for various health and growth indices including disease, biometrics, photosynthesis, reactive oxygen species (ROS), antioxidant enzyme activity, hormones and genes expression at onset of secondary clubroot infection using established protocols. Statistical analysis employed ANOVA with Fisher's LSD for significance assessment (P < 0.05). Our results revealed that seed priming with both MT (50 μMol/L) and CuO-NPs (200 mg/L), followed by soil drenching significantly reduced clubroot incidence (38%) and disease index (57%), compared to control treatments. This synergistic effect was associated with enhanced plant growth (shoots: 48% and roots: 59%). Plants treated with both MT and CuO-NPs showed robust antioxidant defenses, significantly increased superoxide dismutase (SOD (25/29%)), catalase (CAT (83/55%)), and ascorbate peroxidase (APX (83/46%)) activity in both shoots/roots, respectively, compared to infected control. Notably, salicylic acid and jasmonic acid levels doubled in treated plants, while stress hormone abscisic acid (ABA) decreased by 80% in roots and 21% in shoots. Gene expression analysis corroborated these findings, showing that the combined treatment activated antioxidant defense genes (SOD, APX and CAT) by 1.9-7.2-fold and upregulated hormone signaling genes JAZ1 (7.8-fold), MYC2 (3.9-fold) and SABP2 (36-fold). Conversely, ABA biosynthesis genes (ABA1 and NCED1) were downregulated up to 7.2-fold, while plant resistance genes NPR1, PRB1 and PDF1.2 were dramatically increased by up to 6.3-fold compared to infected plants. Overall, our combined treatment approach significantly reduces clubroot severity in B. rapa via enhanced antioxidant defenses, improved ROS scavenging, coordinated hormonal regulation and increased pathogen response genes. This study offers promising strategy for developing effective control measures against clubroot in susceptible cruciferous crops.

Identification and Characterization of High-Molecular-Weight Proteins Secreted by Plasmodiophora brassicae That Suppress Plant Immunity

Plasmodiophora brassicae is an obligate intracellular parasitic protist that causes clubroot disease on cruciferous plants. So far, some low-molecular-weight secreted proteins from P. brassicae have been reported to play an important role in plant immunity regulation, but there are few reports on its high-molecular-weight secreted proteins. In this study, 35 putative high-molecular-weight secreted proteins (>300 amino acids) of P. brassicae (PbHMWSP) genes that are highly expressed during the infection stage were identified using transcriptome analysis and bioinformatics prediction. Then, the secretory activity of 30 putative PbHMWSPs was confirmed using the yeast signal sequence trap system. Furthermore, the genes encoding 24 PbHMWSPs were successfully cloned and their functions in plant immunity were studied. The results showed that ten PbHMWSPs could inhibit flg22-induced reactive oxygen burst, and ten PbHMWSPs significantly inhibited the expression of the SA signaling pathway marker gene PR1a. In addition, nine PbHMWSPs could inhibit the expression of a marker gene of the JA signaling pathway. Therefore, a total of 19 of the 24 tested PbHMWSPs played roles in suppressing the immune response of plants. Of these, it is worth noting that PbHMWSP34 can inhibit the expression of JA, ET, and several SA signaling pathway marker genes. The present study is the first to report the function of the high-molecular-weight secreted proteins of P. brassicae in plant immunity, which will enrich the theory of interaction mechanisms between the pathogens and plants.

The Plasmodiophora brassicae Golgi-localized UPF0016 protein PbGDT1 mediates calcium but not manganese transport in yeast and Nicotiana benthamiana

Manganese and calcium homeostasis and signalling, in eukaryotic organisms, are regulated through membrane located pumps, channels and exchangers, including the Mn2+/Ca2+ uncharacterized protein family 0016 (UPF0016). Here we show that Plasmodiophora brassicae PbGDT1 is a member of the UPF0016 and an ortholog of Saccharomyces cerevisiae Gdt1p (GCR Dependent Translation Factor 1) protein involved in manganese homeostasis as well as the calcium mediated stress response in yeast. PbGDT1 complemented the ScGdt1p and ScPMR1 (Ca2+ ATPase) double null mutant under elevated calcium stress but not under elevated manganese conditions. In both yeast and Nicotiana benthamiana, PbGDT1 localizes to the Golgi apparatus, with additional ER association in N. benthamiana. Expression of PbGDT1 in N. benthamiana, suppresses BAX-triggered cell death, further highlighting the importance of calcium homeostasis in maintaining cell physiology and integrity in a stress environment.

Decoding the Arsenal: Protist Effectors and Their Impact on Photosynthetic Hosts

Interactions between various microbial pathogens including viruses, bacteria, fungi, oomycetes, and their plant hosts have traditionally been the focus of phytopathology. In recent years, a significant and growing interest in the study of eukaryotic microorganisms not classified among fungi or oomycetes has emerged. Many of these protists establish complex interactions with photosynthetic hosts, and understanding these interactions is crucial in understanding the dynamics of these parasites within traditional and emerging types of farming, including marine aquaculture. Many phytopathogenic protists are biotrophs with complex polyphasic life cycles, which makes them difficult or impossible to culture, a fact reflected in a wide gap in the availability of comprehensive genomic data when compared to fungal and oomycete plant pathogens. Furthermore, our ability to use available genomic resources for these protists is limited by the broad taxonomic distance that these organisms span, which makes comparisons with other genomic datasets difficult. The current rapid progress in genomics and computational tools for the prediction of protein functions and interactions is revolutionizing the landscape in plant pathology. This is also opening novel possibilities, specifically for a deeper understanding of protist effectors. Tools like AlphaFold2 enable structure-based function prediction of effector candidates with divergent protein sequences. In turn, this allows us to ask better biological questions and, coupled with innovative experimental strategies, will lead into a new era of effector research, especially for protists, to expand our knowledge on these elusive pathogens and their interactions with photosynthetic hosts. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Plasmodiophora brassicae affects host gene expression by secreting the transcription factor-type effector PbZFE1

The protist pathogen Plasmodiophora brassicae hijacks the metabolism and development of host cruciferous plants and induces clubroot formation, but little is known about its regulatory mechanisms. Previously, the Pnit2int2 sequence, a sequence around the second intron of the nitrilase gene (BrNIT2) involved in auxin biosynthesis in Brassica rapa ssp. pekinensis, was identified as a specific promoter activated during clubroot formation. In this study, we hypothesized that analysis of the transcriptional regulation of Pnit2int2 could reveal how P. brassicae affects the host gene regulatory system during clubroot development. By yeast one-hybrid screening, the pathogen zinc finger protein PbZFE1 was identified to specifically bind to Pnit2int2. Specific binding of PbZFE1 to Pnit2int2 was also confirmed by electrophoretic mobility shift assay. The binding site of PbZFE1 is essential for promoter activity of Pnit2int2 in clubbed roots of transgenic Arabidopsis thaliana (Pnit2int2-2::GUS), indicating that PbZFE1 is secreted from P. brassicae and functions within plant cells. Ectopic expression of PbZEF1 in A. thaliana delayed growth and flowering time, suggesting that PbZFE1 has significant impacts on host development and metabolic systems. Thus, P. brassicae appears to secrete PbZFE1 into host cells as a transcription factor-type effector during pathogenesis.

Development and challenges of antimicrobial peptide delivery strategies in bacterial therapy: A review

The escalating global prevalence of antimicrobial resistance poses a critical threat, prompting concerns about its impact on public health. This predicament is exacerbated by the acute shortage of novel antimicrobial agents, a scarcity attributed to the rapid surge in bacterial resistance. This review delves into the realm of antimicrobial peptides, a diverse class of compounds ubiquitously present in plants and animals across various natural organisms. Renowned for their intrinsic antibacterial activity, these peptides provide a promising avenue to tackle the intricate challenge of bacterial resistance. However, the clinical utility of peptide-based drugs is hindered by limited bioavailability and susceptibility to rapid degradation, constraining efforts to enhance the efficacy of bacterial infection treatments. The emergence of nanocarriers marks a transformative approach poised to revolutionize peptide delivery strategies. This review elucidates a promising framework involving nanocarriers within the realm of antimicrobial peptides. This paradigm enables meticulous and controlled peptide release at infection sites by detecting dynamic shifts in microenvironmental factors, including pH, ROS, GSH, and reactive enzymes. Furthermore, a glimpse into the future reveals the potential of targeted delivery mechanisms, harnessing inflammatory responses and intricate signaling pathways, including adenosine triphosphate, macrophage receptors, and pathogenic nucleic acid entities. This approach holds promise in fortifying immunity, thereby amplifying the potency of peptide-based treatments. In summary, this review spotlights peptide nanosystems as prospective solutions for combating bacterial infections. By bridging antimicrobial peptides with advanced nanomedicine, a new therapeutic era emerges, poised to confront the formidable challenge of antimicrobial resistance head-on.

Testing Effects of Seed Treatments against Clubroot Disease in Various Oilseed Rape Hybrids

Clubroot disease, caused by the protist pathogen Plasmodiophora brassicae, is an emerging threat to cruciferous crops, including oilseed rape (Brassica napus L.). Most of the current commercial cultivars are highly susceptible, and efficient management tools are lacking practical implementation. Over three years and three experimental periods, we studied the effects of isotianil in comparison with Bacillus amyloliquefaciens QST713-HiCFU against clubroot disease under greenhouse experiments. Our results show control effects, which were strongly dependent on seasons, host plant genotype, and clubroot isolates: isotianil and B. amyloliquefaciens QST713-HiCFU reduced disease severity consistently at variable, but field-relevant spore concentrations of clubroot isolates; with seed treatments showing superior effects compared to drench applications. The co-application of isotianil with B. amyloliquefaciens QST713-HiCFU could, in some cases, increase the efficacy. Interestingly, all studied hybrids reacted to treatments, albeit to a somewhat different extent. When tested against a field isolate, the results obtained with the single spore isolate were partially confirmed but with greater variability. Overall, the generally positive effects of isotianil and B. amyloliquefaciens QST713-HiCFU on the reduction of clubroot were repeatedly observed. The inoculation of clubroot disease with different spore counts indicates a dose-response effect for tested products. This study highlights the importance of performing experiments holistically over multiple, consecutive seasons, with various isolates, application types, and different genetic resources of host plants.

Phagocytosis underpins the biotrophic lifestyle of intracellular parasites in the class Phytomyxea (Rhizaria)

Phytomyxea are intracellular biotrophic parasites infecting plants and stramenopiles, including the agriculturally impactful Plasmodiophora brassicae and the brown seaweed pathogen Maullinia ectocarpii. They belong to the clade Rhizaria, where phagotrophy is the main mode of nutrition. Phagocytosis is a complex trait of eukaryotes, well documented for free-living unicellular eukaryotes and specific cellular types of animals. Data on phagocytosis in intracellular, biotrophic parasites are scant. Phagocytosis, where parts of the host cell are consumed at once, is seemingly at odds with intracellular biotrophy. Here we provide evidence that phagotrophy is part of the nutritional strategy of Phytomyxea, using morphological and genetic data (including a novel transcriptome of M. ectocarpii). We document intracellular phagocytosis in P. brassicae and M. ectocarpii by transmission electron microscopy and fluorescent in situ hybridization. Our investigations confirm molecular signatures of phagocytosis in Phytomyxea and hint at a small specialized subset of genes used for intracellular phagocytosis. Microscopic evidence confirms the existence of intracellular phagocytosis, which in Phytomyxea targets primarily host organelles. Phagocytosis seems to coexist with the manipulation of host physiology typical of biotrophic interactions. Our findings resolve long debated questions on the feeding behaviour of Phytomyxea, suggesting an unrecognized role for phagocytosis in biotrophic interactions.

A tell tail sign: a conserved C-terminal tail-anchor domain targets a subset of pathogen effectors to the plant endoplasmic reticulum

The endoplasmic reticulum (ER) is the entry point to the secretory pathway and, as such, is critical for adaptive responses to biotic stress, when the demand for de novo synthesis of immunity-related proteins and signalling components increases significantly. Successful phytopathogens have evolved an arsenal of small effector proteins which collectively reconfigure multiple host components and signalling pathways to promote virulence; a small, but important, subset of which are targeted to the endomembrane system including the ER. We identified and validated a conserved C-terminal tail-anchor motif in a set of pathogen effectors known to localize to the ER from the oomycetes Hyaloperonospora arabidopsidis and Plasmopara halstedii (downy mildew of Arabidopsis and sunflower, respectively) and used this protein topology to develop a bioinformatic pipeline to identify putative ER-localized effectors within the effectorome of the related oomycete, Phytophthora infestans, the causal agent of potato late blight. Many of the identified P. infestans tail-anchor effectors converged on ER-localized NAC transcription factors, indicating that this family is a critical host target for multiple pathogens.

Getting to the root of a club - Understanding developmental manipulation by the clubroot pathogen

Plasmodiophora brassicae Wor., the clubroot pathogen, is the perfect example of an "atypical" plant pathogen. This soil-borne protist and obligate biotrophic parasite infects the roots of cruciferous crops, inducing galls or clubs that lead to wilting, loss of productivity, and plant death. Unlike many other agriculturally relevant pathosystems, research into the molecular mechanisms that underlie clubroot disease and Plasmodiophora-host interactions is limited. After release of the first P. brassicae genome sequence and subsequent availability of transcriptomic data, the clubroot research community have implicated the involvement of phytohormones during the clubroot pathogen's manipulation of host development. Herein we review the main events leading to the formation of root galls and describe how modulation of select phytohormones may be key to modulating development of the plant host to the benefit of the pathogen. Effector-host interactions are at the base of different strategies employed by pathogens to hijack plant cellular processes. This is how we suspect the clubroot pathogen hijacks host plant metabolism and development to induce nutrient-sink roots galls, emphasizing a need to deepen our understanding of this master manipulator.

The clubroot pathogen Plasmodiophora brassicae: A profile update

BACKGROUND

Plasmodiophora brassicae is the causal agent of clubroot disease of cruciferous plants and one of the biggest threats to the rapeseed (Brassica napus) and brassica vegetable industry worldwide.

DISEASE SYMPTOMS

In the advanced stages of clubroot disease wilting, stunting, yellowing, and redness are visible in the shoots. However, the typical symptoms of the disease are the presence of club-shaped galls in the roots of susceptible hosts that block the absorption of water and nutrients.

HOST RANGE

Members of the family Brassicaceae are the primary host of the pathogen, although some members of the family, such as Bunias orientalis, Coronopus squamatus, and Raphanus sativus, have been identified as being consistently resistant to P. brassicae isolates with variable virulence profile.

TAXONOMY

Class: Phytomyxea; Order: Plasmodiophorales; Family: Plasmodiophoraceae; Genus: Plasmodiophora; Species: Plasmodiophora brassicae (Woronin, 1877).

DISTRIBUTION

Clubroot disease is spread worldwide, with reports from all continents except Antarctica. To date, clubroot disease has been reported in more than 80 countries.

PATHOTYPING

Based on its virulence on different hosts, P. brassicae is classified into pathotypes or races. Five main pathotyping systems have been developed to understand the relationship between P. brassicae and its hosts. Nowadays, the Canadian clubroot differential is extensively used in Canada and has so far identified 36 different pathotypes based on the response of a set of 13 hosts.

EFFECTORS AND RESISTANCE

After the identification and characterization of the clubroot pathogen SABATH-type methyltransferase PbBSMT, several other effectors have been characterized. However, no avirulence gene is known, hindering the functional characterization of the five intercellular nucleotide-binding (NB) site leucine-rich-repeat (LRR) receptors (NLRs) clubroot resistance genes validated to date.

IMPORTANT LINK

Canola Council of Canada is constantly updating information about clubroot and P. brassicae as part of their Canola Encyclopedia: https://www.canolacouncil.org/canola-encyclopedia/diseases/clubroot/.

PHYTOSANITARY CATEGORIZATION

PLADBR: EPPO A2 list; Annex designation 9E.

Rhizomania: Hide and Seek of Polymyxa betae and the Beet Necrotic Yellow Vein Virus with Beta vulgaris

The molecular interactions between Polymyxa betae, the protist vector of sugar beet viruses, beet necrotic yellow vein virus (BNYVV), the causal agent of rhizomania, and Beta vulgaris have not been extensively studied. Here, the transmission of BNYVV to sugar beet by P. betae zoospores was optimized using genetically characterized organisms. Molecular interactions of aviruliferous and viruliferous protist infection on sugar beet were highlighted by transcriptomic analysis. P. betae alone induced limited gene expression changes in sugar beet, as a biotrophic asymptomatic parasite. Most differentially expressed plant genes were down-regulated and included resistance gene analogs and cell wall peroxidases. Several enzymes involved in stress regulation, such as the glutathione-S-transferases, were significantly induced. With BNYVV, the first stages of the P. betae life cycle on sugar beet were accelerated with a faster increase of relative protist DNA level and an earlier appearance of sporangia and sporosori in plants roots. A clear activation of plant defenses and the modulation of genes involved in plant cell wall metabolism were observed. The P. betae transcriptome in the presence of BNYVV revealed induction of genes possibly involved in the switch to the survival stage. The interactions were different depending on the presence or absence of the virus. P. betae alone alleviates plant defense response, playing hide-and-seek with sugar beet and allowing for their mutual development. Conversely, BNYVV manipulates plant defense and promotes the rapid invasion of plant roots by P. betae. This accelerated colonization is accompanied by the development of thick-walled resting spores, supporting the virus survival. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Identification and characterization of putative effectors from Plasmodiophora brassicae that suppress or induce cell death in Nicotiana benthamiana

Clubroot, caused by Plasmodiophora brassicae, is a major disease of crucifers. Effector proteins are important virulence factors in host recognition of pathogens and the interactions between pathogens and hosts. Secretory proteins, as effector candidates, have been studied in the interaction between Plasmodiophora brassicae and its hosts. In this study, 518 secretary proteins were screened from the Plasmodiophora brassicae genome. A total of 63 candidate effectors that induce or suppress cell death were identified using agroinfiltration-mediated transient expression in Nicothiana benthamiana. The candidate effectors, Pb4_102097 and Pb4_108104 showed high expressing level in the stage of rest spore maturity, could induce cell death and were associated with H₂O₂ accumulation in N. benthamiana leaves. In addition, 55 candidate effectors that could suppress BAX (Bcl-2-associated X protein) induced cell death, and 21 out of which could suppress the immunity caused by bacterial pathogen Pseudomonas syringae pv. tomato strain DC3000 expressing avrRps4 in Arabidopsis. Based on the expression pattern in different stages, 28 candidate effectors showed high expression levels during the primary and secondary infection stage. Five candidate effectors containing the RXLR motif functioned in the cytoplasm and cell membrane.

What Can We Learn from -Omics Approaches to Understand Clubroot Disease?

Clubroot is one of the most economically significant diseases worldwide. As a result, many investigations focus on both curing the disease and in-depth molecular studies. Although the first transcriptome dataset for the clubroot disease describing the clubroot disease was published in 2006, many different pathogen-host plant combinations have only recently been investigated and published. Articles presenting -omics data and the clubroot pathogen Plasmodiophora brassicae as well as different host plants were analyzed to summarize the findings in the richness of these datasets. Although genome data for the protist have only recently become available, many effector candidates have been identified, but their functional characterization is incomplete. A better understanding of the life cycle is clearly required to comprehend its function. While only a few proteome studies and metabolome analyses were performed, the majority of studies used microarrays and RNAseq approaches to study transcriptomes. Metabolites, comprising chemical groups like hormones were generally studied in a more targeted manner. Furthermore, functional approaches based on such datasets have been carried out employing mutants, transgenic lines, or ecotypes/cultivars of either Arabidopsis thaliana or other economically important host plants of the Brassica family. This has led to new discoveries of potential genes involved in disease development or in (partial) resistance or tolerance to P. brassicae. The overall contribution of individual experimental setups to a larger picture will be discussed in this review.

Arabidopsis GOLD36/MVP1/ERMO3 Is Required for Powdery Mildew Penetration Resistance and Proper Targeting of the PEN3 Transporter

The Arabidopsis PENETRATION 3 (PEN3) ATP binding cassette (ABC) transporter contributes to penetration resistance against nonadapted powdery mildew fungi and is targeted to papillae deposited at sites of interaction with the fungus. Timely recruitment of PEN3 and other components of penetration resistance to the host-pathogen interface is important for successful defense against this biotrophic pathogen. A forward genetic screen was previously carried out to identify Arabidopsis mutants that mistarget the PEN3 transporter or fail to accumulate PEN3 at sites of attempted powdery mildew penetration. This study focuses on PEN3 mistargeting in the aberrant localization of PEN3 4 (alp4) mutant and identification of the causal gene. In the alp4 mutant, PEN3 accumulates within the endomembrane system in an apparently abnormal endoplasmic reticulum and is not exported into papillae at powdery mildew penetration sites. This targeting defect compromises defenses at the host-pathogen interface, resulting in increased penetration success by a nonadapted powdery mildew. Genetic mapping identified alp4 as an allele of GOLGI DEFECTS 36 (GOLD36), a gene encoding a GDSL-lipase/esterase family protein that is involved in maintaining normal morphology and organization of multiple endomembrane compartments. Genetic complementation confirmed that mutation in GOLD36 is responsible for the PEN3 targeting and powdery mildew penetration resistance defects in alp4. These results reinforce the importance of endomembrane trafficking in resistance to haustorium-forming phytopathogens such as powdery mildew fungi.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Histopathology of the Plasmodiophora brassicae-Chinese Cabbage Interaction in Hosts Carrying Different Sources of Resistance

Clubroot is a serious soil-borne disease of crucifers caused by the obligate parasite Plasmodiophora brassicae. The genetic basis and histopathology of clubroot resistance in two Chinese cabbage (Brassica rapa ssp. pekinensis) inbred lines Bap055 and Bap246, challenged with pathotype 4 of P. brassicae, was evaluated. The Chinese cabbage cultivar "Juxin" served as a susceptible check. The resistance in Bap055 was found to be controlled by the CRa gene, while resistance in Bap246 fit a model of control by unknown recessive gene. Infection of the roots by P. brassicae was examined by inverted microscopy. Despite their resistance, primary and secondary infection were observed to occur in Bap055 and Bap246. Primary infection was detected at 2 days post-inoculation (DPI) in "Juxin," at 4 DPI in Bap055, and at 6 DPI in Bap246. Infection occurred most quickly on "Juxin," with 60% of the root hairs infected at 10 DPI, followed by Bap055 (31% of the root hairs infected at 12 DPI) and Bap246 (20% of the root hairs infected at 14 DPI). Secondary infection of "Juxin" was first observed at 8 DPI, while in Bap055 and Bap246, secondary infection was first observed at 10 DPI. At 14 DPI, the percentage of cortical infection in "Juxin," Bap055 and Bap246 was 93.3, 20.0, and 11.1%, respectively. Although cortical infection was more widespread in Bap055 than in Bap246, secondary infection in both of these hosts was restricted relative to the susceptible check, and the vascular system remained intact. A large number of binucleate secondary plasmodia were observed in "Juxin" and the vascular system was disrupted at 16 DPI; in Bap055 and Bap246, only a few secondary plasmodia were visible, with no binucleate secondary plasmodia. The defense mechanisms and expression of resistance appears to differ between Chinese cabbage cultivars carrying different sources of resistance.

A Novel Target (Oxidation Resistant 2) in Arabidopsis thaliana to Reduce Clubroot Disease Symptoms via the Salicylic Acid Pathway without Growth Penalties

The clubroot disease (Plasmodiophora brassicae) is one of the most damaging diseases worldwide among brassica crops. Its control often relies on resistant cultivars, since the manipulation of the disease hormones, such as salicylic acid (SA) alters plant growth negatively. Alternatively, the SA pathway can be increased by the addition of beneficial microorganisms for biocontrol. However, this potential has not been exhaustively used. In this study, a recently characterized protein Oxidation Resistant 2 (OXR2) from Arabidopsis thaliana is shown to increase the constitutive pathway of SA defense without decreasing plant growth. Plants overexpressing AtOXR2 (OXR2-OE) show strongly reduced clubroot symptoms with improved plant growth performance, in comparison to wild type plants during the course of infection. Consequently, oxr2 mutants are more susceptible to clubroot disease. P. brassicae itself was reduced in these galls as determined by quantitative real-time PCR. Furthermore, we provide evidence for the transcriptional downregulation of the gene encoding a SA-methyltransferase from the pathogen in OXR2-OE plants that could contribute to the phenotype.

Candidate Effectors of Plasmodiophora brassicae Pathotype 5X During Infection of Two Brassica napus Genotypes

Clubroot, caused by Plasmodiophora brassicae, is one of the most important diseases of canola (Brassica napus) in Canada. Disease management relies heavily on planting clubroot resistant (CR) cultivars, but in recent years, new resistance-breaking pathotypes of P. brassicae have emerged. Current efforts against the disease are concentrated in developing host resistance using traditional genetic breeding, omics and molecular biology. However, because of its obligate biotrophic nature, limited resources have been dedicated to investigating molecular mechanisms of pathogenic infection. We previously performed a transcriptomic study with the cultivar resistance-breaking pathotype 5X on two B. napus hosts presenting contrasting resistance/susceptibility, where we evaluated the mechanisms of host response. Since cultivar-pathotype interactions are very specific, and pathotype 5X is one of the most relevant resistance-breaking pathotypes in Canada, in this study, we analyze the expression of genes encoding putative secreted proteins from this pathotype, predicted using a bioinformatics pipeline, protein modeling and orthologous comparisons with effectors from other pathosystems. While host responses were found to differ markedly in our previous study, many common effectors are found in the pathogen while infecting both hosts, and the gene response among biological pathogen replicates seems more consistent in the effectors associated with the susceptible interaction, especially at 21 days after inoculation. The predicted effectors indicate the predominance of proteins with interacting domains (e.g., ankyrin), and genes bearing kinase and NUDIX domains, but also proteins with protective action against reactive oxygen species from the host. Many of these genes confirm previous predictions from other clubroot studies. A benzoic acid/SA methyltransferase (BSMT), which methylates SA to render it inactive, showed high levels of expression in the interactions with both hosts. Interestingly, our data indicate that E3 ubiquitin proteasome elements are also potentially involved in pathogenesis. Finally, a gene with similarity to indole-3-acetaldehyde dehydrogenase is a promising candidate effector because of its involvement in indole acetic acid synthesis, since auxin is one of the major players in clubroot development.