Transcriptional and morphological changes in the transition from mycetophagous to phytophagous phase in the plant-parasitic nematode Bursaphelenchus xylophilus

Mycobiome of Pinus pinaster trees naturally infected by the pinewood nematode Bursaphelenchus xylophilus

Fungi are important biological elements in the Pine wilt disease (PWD) complex. In the late stages of the disease, the pinewood nematode (PWN) Bursaphelenchus xylophilus feeds on the fungal flora available in the pine tree for survival and multiplication. Previous studies have confirmed a close relation between the PWN and blue-stain fungi (Ophiostomatales), which are necrotrophic pathogens associated with bark beetles (Coleoptera: Scolytidae). The PWN is able to grow densely in the presence of these fungi, which results in a higher number of nematodes transferred to the insect-vector Monochamus spp. To understand the spatial diversity and structure of Pinus pinaster mycobiome, wood samples from PWN-infected and non-infected pine trees were collected in three locations of Continental mainland Portugal with different PWD records, during the maturation phase of the insect-vector M. galloprovincialis (winter 2019-spring 2020). The PWN-mycobiome from the PWN-infected P. pinaster was also characterized. A total of 27 samples of P. pinaster and 13 samples of PWN from PWN-infected trees were characterized using ITS2 amplicon sequencing. The diversity and structure of the fungal communities in P. pinaster varied with disease status suggesting that the PWN presence affects the endophytic fungal communities. For both P. pinaster and PWN fungal communities, differences were also associated with locations (recent PWD loci Seia, and long-term PWN locus Companhia das Lezírias and Tróia). Ophiostomatales were mainly detected in PWN-infected P. pinaster. This research contributes to increase the knowledge on the ecology of the fungal communities in PWD complex.

Detection Methods for Pine Wilt Disease: A Comprehensive Review

Pine wilt disease (PWD), caused by the nematode Bursaphelenchus xylophilus, is a highly destructive forest disease that necessitates rapid and precise identification for effective management and control. This study evaluates various detection methods for PWD, including morphological diagnosis, molecular techniques, and remote sensing. While traditional methods are economical, they are limited by their inability to detect subtle or early changes and require considerable time and expertise. To overcome these challenges, this study emphasizes advanced molecular approaches such as real-time polymerase chain reaction (RT-PCR), droplet digital PCR (ddPCR), and loop-mediated isothermal amplification (LAMP) coupled with CRISPR/Cas12a, which offer fast and accurate pathogen detection. Additionally, DNA barcoding and microarrays facilitate species identification, and proteomics can provide insights into infection-specific protein signatures. The study also highlights remote sensing technologies, including satellite imagery and unmanned aerial vehicle (UAV)-based hyperspectral analysis, for their capability to monitor PWD by detecting asymptomatic diseases through changes in the spectral signatures of trees. Future research should focus on combining traditional and innovative techniques, refining visual inspection processes, developing rapid and portable diagnostic tools for field application, and exploring the potential of volatile organic compound analysis and machine learning algorithms for early disease detection. Integrating diverse methods and adopting innovative technologies are crucial to effectively control this lethal forest disease.

Bursaphelenchus xylophilus Venom Allergen-Like Protein BxVAP1, Triggering Plant Defense-Related Programmed Cell Death, Plays an Important Role in Regulating Pinus massoniana Terpene Defense Responses

Bursaphelenchus xylophilus (pine wood nematode, PWN), a migratory plant-parasitic nematode, acts as an etiological agent, inflicting considerable damage to pine forests worldwide. Plant immunity constitutes a crucial factor in resisting various pathogenic invasions. The primary defensive responses of host pines against PWN infection encompass terpene accumulation, defense response-related gene expression, and programmed cell death. Venom allergen-like proteins (VAPs), as potential effectors, are instrumental in facilitating the successful colonization of PWNs. In this study, we investigated the inhibition of B. xylophilus VAP (BxVAP1) expression by RNA interference in vitro. Following BxVAP1 silencing, the reproduction rate and migration rate of the PWN population in Pinus massoniana decreased, the expression of the α-pinene synthase gene was induced, other terpene synthase and pathogenesis-related genes were inhibited and delayed, the peak times and levels of terpene-related substances were changed, and the degree of cavitation in P. massoniana was diminished. Transient expression of BxVAP1 in Nicotiana benthamiana revealed that BxVAP1 was expressed in both the cell membrane and nucleus, inducing programmed cell death and the expression of pathogen-associated molecular pattern-triggered immunity marker genes (NbAcre31 and NbPTI5). This study is the first to demonstrate that silencing the BxVAP1 gene affects host defense responses, including terpenoid metabolism in P. massoniana, and that BxVAP1 can be recognized by N. benthamiana as an effector to trigger its innate immunity, expanding our understanding of the parasitic mechanism of B. xylophilus.

The Bursaphelenchus xylophilus Effector BxNMP1 Targets PtTLP-L2 to Mediate PtGLU Promoting Parasitism and Virulence in Pinus thunbergii

Pinus is an important economic tree species, but pine wilt disease (PWD) seriously threatens the survival of pine trees. PWD caused by Bursaphelenchus xylophilus is a major quarantine disease worldwide that causes significant economic losses. However, more information about its molecular pathogenesis is needed, resulting in a lack of effective prevention and treatment measures. In recent years, effectors have become a hot topic in exploring the molecular pathogenic mechanism of pathogens. Here, we identified a specific effector, BxNMP1, from B. xylophilus. In situ hybridization experiments revealed that BxNMP1 was specifically expressed in dorsal gland cells and intestinal cells, and RT-qPCR experiments revealed that BxNMP1 was upregulated in the early stage of infection. The sequence of BxNMP1 was different in the avirulent strain, and when BxNMP1-silenced B. xylophilus was inoculated into P. thunbergii seedlings, the disease severity significantly decreased. We demonstrated that BxNMP1 interacted with the thaumatin-like protein PtTLP-L2 in P. thunbergii. Additionally, we found that the β-1,3-glucanase PtGLU interacted with PtTLP-L2. Therefore, we hypothesized that BxNMP1 might indirectly interact with PtGLU through PtTLP-L2 as an intermediate mediator. Both targets can respond to infection, and PtTLP-L2 can enhance the resistance of pine trees. Moreover, we detected increased salicylic acid contents in P. thunbergii seedlings inoculated with B. xylophilus when BxNMP1 was silenced or when the PtTLP-L2 recombinant protein was added. In summary, we identified a key virulence effector of PWNs, BxNMP1. It positively regulates the pathogenicity of B. xylophilus and interacts directly with PtTLP-L2 and indirectly with PtGLU. It also inhibits the expression of two targets and the host salicylic acid pathway. This study provides theoretical guidance and a practical basis for controlling PWD and breeding for disease resistance.

Pine wilt disease: what do we know from proteomics?

Pine wilt disease (PWD) is a devastating forest disease caused by the pinewood nematode (PWN), Bursaphelenchus xylophilus, a migratory endoparasite that infects several coniferous species. During the last 20 years, advances have been made for understanding the molecular bases of PWN-host trees interactions. Major advances emerged from transcriptomic and genomic studies, which revealed some unique features related to PWN pathogenicity and constituted fundamental data that allowed the development of postgenomic studies. Here we review the proteomic approaches that were applied to study PWD and integrated the current knowledge on the molecular basis of the PWN pathogenicity. Proteomics has been useful for understanding cellular activities and protein functions involved in PWN-host trees interactions, shedding light into the mechanisms associated with PWN pathogenicity and being promising tools to better clarify host trees PWN resistance/susceptibility.

Functional Characterization of ShK Domain-Containing Protein in the Plant-Parasitic Nematode Bursaphelenchus xylophilus

ShK domain-containing proteins are peptides found in different parasitic and venomous organisms. From a previous transcriptomic dataset from Bursaphelenchus xylophilus, a plant-parasitic nematode that infects forest tree species, we identified 96 transcripts potentially as ShK domain-containing proteins with unknown function in the nematode genome. This study aimed to characterize and explore the functional role of genes encoding ShK domain-containing proteins in B. xylophilus biology. We selected and functionally analyzed nine candidate genes that are putatively specific to B. xylophilus. In situ hybridization revealed expression of one B. xylophilus ShK in the pharyngeal gland cells, suggesting their delivery into host cells. Most of the transcripts are highly expressed during infection and showed a significant upregulation in response to peroxide products compared to the nematode catalase enzymes. We reported, for the first time, the potential involvement of ShK domain genes in oxidative stress, suggesting that these proteins may have an important role in protecting or modulating the reactive oxygen species (ROS) activity of the host plant during parasitism.

Antioxidant processes involving epicatechin decreased symptoms of pine wilt disease

Since the pine wood nematode (PWN, Bursaphelenchus xylophilus) invasion of Northeast China, both symptomatic and asymptomatic PWN carriers have been found. Asymptomatic PWN carriers, which are more dangerous than symptomatic carriers, constitute a source of infection in the following spring. The simultaneous presence of symptomatic and asymptomatic PWN carriers indicates that Pinus koraiensis has different tolerance levels to PWN. In this study, validity of susceptibility testing discovered differential types of P. koraiensis including Latent Reservoirs, Low Susceptibles, High Susceptibles and Bell Ringers. Among those types, the Low Susceptibles and Latent Reservoirs were asymptomatic PWN carriers, and Latent Reservoirs were the most dangerous. Transcriptome and metabolomic data showed that 5 genes (3 ans and 2 anr gene) involved in the epicatechin (EC) synthesis pathway were significantly upregulated, which increased the content of EC antioxidants in Latent Reservoirs. Hydrogen peroxide (H₂O₂) staining and content determination showed that the hypersensitive response (HR) and H₂O₂, which functions as a signaling molecule in systemic acquired resistance, decreased in Latent Reservoirs. However, low contents of EC and high contents of H₂O₂ were found in the High Susceptibles of P. koraiensis. RT-PCR results showed that the expression of ans and anr was upregulated together only in Latent Reservoirs. These results show that the susceptibility of P. koraiensis to PWN differed among different individuals, although no resistant individuals were found. Latent Reservoirs, in which more PWNs resided without visible symptoms via prolonged incubation period, inhibited the symptoms caused by H₂O₂ because of increased contents of the EC antioxidants.

Comprehensive Analysis of Copy Number Variations on Glycoside Hydrolase 45 Genes among Different Bursaphelenchus xylophilus Strains

Bursaphelenchus xylophilus is considered the most dangerous quarantine pest in China. It causes enormous economic and ecological losses in many countries from Asia and Europe. The glycoside hydrolase 45 gene family has been demonstrated in early studies to contribute to the cell wall degradation ability of B. xylophilus during its infection. However, the copy number variation (CNV) of the GH45 gene and its association with B. xylophilus pathogenicity were not fully elucidated. In this study, we found that the GH45 gene with two copies is the most predominant type among 259 B. xylophilus strains collected from China and Japan. Additionally, 18 strains are identified as GH45 genes with a single copy, and only two strains are verified to have three copies. Subsequent expression analysis and inoculation test suggest that the copy numbers of the GH45 gene are correlated with gene expression as well as the B. xylophilus pathogenicity. B. xylophilus strains with more copies of the GH45 gene usually exhibit more abundant expression and cause more severe wilt symptoms on pine trees. The aforementioned results indicated the potential regulatory effects of CNV in B. xylophilus and provided novel information to better understand the molecular pathogenesis of this devastating pest.

Studies on the Requirement of Transthyretin Protein (BxTTR-52) for the Suppression of Host Innate Immunity in Bursaphelenchus xylophilus

The pinewood nematode, Bursaphelenchus xylophilus, has been determined as one of the world's top ten plant-parasitic nematodes. It causes pine wilt, a progressive disease that affects the economy and ecologically sustainable development in East Asia. B. xylophilus secretes pathogenic proteins into host plant tissues to promote infection. However, little is known about the interaction between B. xylophilus and pines. Previous studies reported transthyretin proteins in some species and their strong correlation with immune evasion, which has also been poorly studied in B. xylophilus. In this study, we cloned and functionally validated the B. xylophilus pathogenic protein BxTTR-52, containing a transthyretin domain. An in situ hybridization assay demonstrated that BxTTR-52 was expressed mainly in the esophageal glands of B. xylophilus. Confocal microscopy revealed that BxTTR-52-RFP localized to the nucleus, cytoplasm, and plasma membrane. BxTTR-52 recombinant proteins produced by Escherichia coli could be suppressed by hydrogen peroxide and antioxidant enzymes in pines. Moreover, silencing BxTTR-52 significantly attenuated the morbidity of Pinus thunbergii infected with B. xylophilus. It also suppressed the expression of pathogenesis-related genes in P. thunbergii. These results suggest that BxTTR-52 suppresses the plant immune response in the host pines and might contribute to the pathogenicity of B. xylophilus in the early infection stages.

The key molecular pattern BxCDP1 of Bursaphelenchus xylophilus induces plant immunity and enhances plant defense response via two small peptide regions

The migratory plant-parasitic nematode Bursaphelenchus xylophilus is the pathogen of the pine wilt disease (PWD), causing serious damage to pine forests in China. During the process of plant resistance to multiple pathogens, plant immunity plays a key role. In this current study, the pathogen-associated molecular pattern (PAMP) BxCDP1 in B. xylophilus has been identified, but the host target protein of BxCDP1 and its key amino acid region inducing the plant immunity have yet to be elucidated. We found that BxCDP1 could trigger superoxide production, H2O2 production, and callose deposits. A RING-H2 finger protein 1 (RHF1) of Pinus thunbergii was screened and characterized as a target protein of BxCDP1 by yeast two-hybrid and co-immunoprecipitation (Co-IP). Moreover, two peptides (namely M9 and M16) proved to be key regions of BxCDP1 to induce PAMP-triggered immunity (PTI) in Nicotiana benthamiana, which also induced the expression of pathogenesis-related (PR) genes (PtPR-3, PtPR-4, and PtPR-5) in P. thunbergii and enhanced the resistance of the host to B. xylophilus. These results indicate that BxCDP1 plays a critical role in the interaction between B. xylophilus and P. thunbergii, and both peptides M9 and M16 have the potential to be developed and utilized as immune inducers of pines against B. xylophilus in future.

A Bursaphelenchus xylophilus Effector, BxSCD3, Suppresses Plant Defense and Contributes to Virulence

Bursaphelenchus xylophilus is the most economically important species of migratory plant-parasitic nematodes (PPNs) and causes severe damage to forestry in China. The successful infection of B. xylophilus relies on the secretion of a repertoire of effector proteins. The effectors, which suppress the host pine immune response, are key to the facilitation of B. xylophilus parasitism. An exhaustive list of candidate effectors of B. xylophilus was predicted, but not all have been identified and characterized. Here, an effector, named BxSCD3, has been implicated in the suppression of host immunity. BxSCD3 could suppress pathogen-associated molecular patterns (PAMPs) PsXEG1- and INF1-triggered cell death when it was secreted into the intracellular space in Nicotiana benthamiana. BxSCD3 was highly up-regulated in the early infection stages of B. xylophilus. BxSCD3 does not affect B. xylophilus reproduction, either at the mycophagous stage or the phytophagous stage, but it contributes to the virulence of B. xylophilus. Moreover, BxSCD3 significantly influenced the relative expression levels of defense-related (PR) genes PtPR-3 and PtPR-6 in Pinus thunbergii in the early infection stage. These results suggest that BxSCD3 is an important toxic factor and plays a key role in the interaction between B. xylophilus and host pine.

A Bursaphelenchus xylophilus pathogenic protein Bx-FAR-1, as potential control target, mediates the jasmonic acid pathway in pines

BACKGROUND

The pine wilt disease (PWD) caused by Bursaphelenchus xylophilus is a devastating forest disease and its pathogenesis remains unclear. Secreted enzymes and proteins are important pathogenicity determinants and Bx-FAR-1 is an important pathogenic protein involved in the interaction between pine and B. xylophilus. However, the function of the Bx-FAR-1 protein in monitoring and prevention PWD remains unknown.

RESULTS

We found a small peptide of B. xylophilus effector Bx-FAR-1 is sufficient for immunosuppression function in Nicotiana benthamiana. Transient expression of Bx-FAR-1 in N. benthamiana revealed that nuclear localization is required for its function. The results of the ligand binding test showed that Bx-FAR-1 protein had the ability to bind fatty acid and retinol. We demonstrated that Bx-FAR-1 targeted to the nuclei of Pinus thunbergii using the polyclonal antibody by immunologic approach. The content of jasmonic acid (JA) was significantly increased in P. thunbergii infected with B. xylophilus when Bx-FAR-1 was silenced. We identified an F-box protein as the host target of Bx-FAR-1 by yeast two-hybrid and co-immunoprecipitation. Moreover, we found that Pt-F-box-1 was up-regulated during B. xylophilus infection and the expression of Pt-F-box-1 was increased in Bx-FAR-1 double-stranded RNA (dsRNA)-treated host pines.

CONCLUSION

This study illustrated that Bx-FAR-1 might mediate the JA pathway to destroy the immune system of P. thunbergii, indicating that PWN likely secretes effectors to facilitate parasitism and promote infection, which could better reveal the pathogenesis mechanisms of B. xylophilus and would be beneficial for developing disease control strategies.

The Bursaphelenchus xylophilus effector BxML1 targets the cyclophilin protein (CyP) to promote parasitism and virulence in pine

BACKGROUND

Bursaphelenchus xylophilus is the causal agent of pine wilt disease (PWD) that has caused enormous ecological and economic losses in China. The mechanism in the interaction between nematodes and pine remains unclear. Plant parasitic nematodes (PPNs) secrete effectors into host plant tissues. However, it is poorly studied that role of effector in the infection of pine wood nematode (PWN).

RESULTS

We cloned, characterized and functionally validated the B. xylophilus effector BxML1, containing an MD-2-related lipid-recognition (ML) domain. This protein inhibits immune responses triggered by the molecular pattern BxCDP1 of B. xylophilus. An insitu hybridization assay demonstrated that BxML1 was expressed mainly in the dorsal glands and intestine of B. xylophilus. Subcellular localization analysis showed the presence of BxML1 in the cytoplasm and nucleus. Furthermore, number of B. xylophilus and morbidity of pine were significantly reduced in Pinus thunbergii infected with B. xylophilus when BxML was silenced. Using yeast two-hybrid (Y2H) and coimmunoprecipitation (CoIP) assays, we found that the BxML1 interacts with cyclophilin protein PtCyP1 in P. thunbergii.

CONCLUSIONS

This study illustrated that BxML1 plays a critical role in the B. xylophilus-plant interaction and virulence of B. xylophilus.

A novel pine wood nematode effector, BxSCD1, suppresses plant immunity and interacts with an ethylene-forming enzyme in pine

The plant-parasitic nematode Bursaphelenchus xylophilus, the causal agent of pine wilt disease (PWD), causes enormous economic loss every year. Currently, little is known about the pathogenic mechanisms of PWD. Several effectors have been identified in B. xylophilus, but their functions and host targets have yet to be elucidated. Here, we demonstrated that BxSCD1 suppresses cell death and inhibits B. xylophilus PAMP BxCDP1-triggered immunity in Nicotiana benthamiana and Pinus thunbergii. BxSCD1 was transcriptionally upregulated in the early stage of B. xylophilus infection. In situ hybridization experiments showed that BxSCD1 was specifically expressed in the dorsal glands and intestine. Cysteine residues are essential for the function of BxSCD1. Transient expression of BxSCD1 in N. benthamiana revealed that it was primarily targeted to the cytoplasm and nucleus. The morbidity was significantly reduced in P. thunbergii infected with B. xylophilus when BxSCD1 was silenced. We identified 1-aminocyclopropane-1-carboxylate oxidase 1, the actual ethylene-forming enzyme, as a host target of BxSCD1 by yeast two-hybrid and coimmunoprecipitation. Overall, this study illustrated that BxSCD1 played a critical role in the B. xylophilus-plant interaction.

Comparative transcriptomic analysis of candidate effectors to explore the infection and survival strategy of Bursaphelenchus xylophilus during different interaction stages with pine trees

BACKGROUND

The pine wood nematode (PWN), Bursaphelenchus xylophilus, is a devastating pathogen of many Pinus species in China. The aim of this study was to understand the interactive molecular mechanism of PWN and its host by comparing differentially expressed genes and candidate effectors from three transcriptomes of B. xylophilus at different infection stages.

RESULTS

In total, 62, 69 and 46 candidate effectors were identified in three transcriptomes (2.5 h postinfection, 6, 12 and 24 h postinoculation and 6 and 15 d postinfection, respectively). In addition to uncharacterized pioneers, other candidate effectors were involved in the degradation of host tissues, suppression of host defenses, targeting plant signaling pathways, feeding and detoxification, which helped B. xylophilus survive successfully in the host. Seven candidate effectors were identified in both our study and the B. xylophilus transcriptome at 2.5 h postinfection, and one candidate effector was identified in all three transcriptomes. These common candidate effectors were upregulated at infection stages, and one of them suppressed pathogen-associated molecular pattern (PAMP) PsXEG1-triggered cell death in Nicotiana benthamiana.

CONCLUSIONS

The results indicated that B. xylophilus secreted various candidate effectors, and some of them continued to function throughout all infection stages. These various candidate effectors were important to B. xylophilus infection and survival, and they functioned in different ways (such as breaking down host cell walls, suppressing host defenses, promoting feeding efficiency, promoting detoxification and playing virulence functions). The present results provide valuable resources for in-depth research on the pathogenesis of B. xylophilus from the perspective of effectors.

Copy Number Variations of Glycoside Hydrolase 45 Genes in Bursaphelenchus xylophilus and Their Impact on the Pathogenesis of Pine Wilt Disease

The pine wood nematode Bursaphelenchus xylophilus parasitizes millions of pine trees worldwide each year, causing severe wilt and the death of host trees. Glycoside hydrolase 45 genes of B. xylophilus are reported to have been acquired by horizontal gene transfer from fungi and are responsible for cell wall degradation during nematode infection. Previous studies ignored the possibility of copy number variations of such genes. In this study, we determined that two of the glycoside hydrolase 45 genes evolved to maintain multiple copies with distinct expression levels, enabling the nematode to infect a variety of pine hosts. Additionally, tandem repeat variations within coding regions were also detected between different copies of glycoside hydrolase 45 genes that could result in changes in protein sequences and serve as an effective biological marker to detect copy number variations among different B. xylophilus populations. Consequently, we were able to further identify the copy number variations of glycoside hydrolase 45 genes among B. xylophilus strains with different virulence. Our results provide new insights into the pathogenicity of B. xylophilus, provide a practical marker to genotype copy number variations and may aid in population classification.

Virulence Biomarkers of Bursaphelenchus xylophilus: A Proteomic Approach

The pinewood nematode (PWN), Bursaphelenchus xylophilus, one of the most serious forest pests worldwide, is considered the causal agent of the pine wilt disease (PWD). The main host species belong to the genus Pinus, and a variation in the susceptibility of several pine species to PWN infection is well-known. It is also recognized that there is variation in the virulence among B. xylophilus isolates. In the present study, we applied a quantitative mass spectrometry-based proteomics approach to perform a deep characterization of proteomic changes across two B. xylophilus isolates with different virulence from different hosts and geographical origins. A total of 1,456 proteins were quantified and compared in the two isolates secretomes, and a total of 2,741 proteins were quantified and compared in the nematode proteomes in pine tree extract and fungus stimuli conditions. From the proteomic analyses, a group of proteins was selected and identified as potential virulence biomarkers and shed light on putative most pathogenic proteins of this plant-parasitic nematode. Proteomic data are available via ProteomeXchange with identifier PXD029377.

Comparative Analysis of Bursaphelenchus xylophilus Secretome Under Pinus pinaster and P. pinea Stimuli

The pinewood nematode (PWN), Bursaphelenchus xylophilus, the pine wilt disease's (PWD) causal agent, is a migratory endoparasitic nematode skilled to feed on pine tissues and on fungi that colonize the trees. In order to study B. xylophilus secretomes under the stimulus of pine species with different susceptibilities to disease, nematodes were exposed to aqueous pine extracts from Pinus pinaster (high-susceptible host) and P. pinea (low-susceptible host). Sequential windowed acquisition of all theoretical mass spectra (SWATH-MS) was used to determine relative changes in protein amounts between B. xylophilus secretions, and a total of 776 secreted proteins were quantified in both secretomes. From these, 22 proteins were found increased in the B. xylophilus secretome under the P. pinaster stimulus and 501 proteins increased under the P. pinea stimulus. Functional analyses of the 22 proteins found increased in the P. pinaster stimulus showed that proteins with peptidase, hydrolase, and antioxidant activities were the most represented. On the other hand, gene ontology (GO) enrichment analysis of the 501 proteins increased under the P. pinea stimulus revealed an enrichment of proteins with binding activity. The differences detected in the secretomes highlighted the diverse responses from the nematode to overcome host defenses with different susceptibilities and provide new clues on the mechanism behind the pathogenicity of this plant-parasitic nematode. Proteomic data are available via ProteomeXchange with identifier PXD024011.

Transcriptome Analysis of Bursaphelenchus xylophilus Uncovers the Impact of Stenotrophomonas maltophilia on Nematode and Pine Wilt Disease

Stenotrophomonas maltophilia influences the reproduction, pathogenicity, and gene expression of aseptic Bursaphelenchus xylophilus after inoculation of aseptic Pinus massoniana. Pine wilt disease is a destructive pine forest disease caused by B. xylophilus, and its pathogenesis is unclear. The role of bacteria associated with B. xylophilus in pine wilt disease has attracted widespread attention. S. maltophilia is one of the most dominant bacteria in B. xylophilus, and its effect is ambiguous. This study aims to explore the role of S. maltophilia in pine wilt disease. The reproduction and virulence of aseptic B. xylophilus and B. xylophilus containing S. maltophilia were examined by inoculating aseptic P. massoniana seedlings. The gene expressions of two nematode treatments were identified by transcriptome sequencing. The reproduction and virulence of B. xylophilus containing S. maltophilia were stronger than that of aseptic nematodes. There were 4240 differentially expressed genes between aseptic B. xylophilus and B. xylophilus containing S. maltophilia after inoculation of aseptic P. massoniana, including 1147 upregulated genes and 2763 downregulated genes. These differentially expressed genes were significantly enriched in some immune-related gene ontology (GO) categories, such as membrane, transporter activity, metabolic processes, and many immune-related pathways, such as the wnt, rap1, PI3K-Akt, cAMP, cGMP-PKG, MAPK, ECM-receptor interaction, and calcium signaling pathways. The polyubiquitin-rich gene, leucine-rich repeat serine/threonine-protein kinase gene, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene, acetyl-CoA carboxylase gene, and heat shock protein genes were the key genes associated with immune resistance. Moreover, there were four cell wall hydrolase genes, thirty-six detoxification- and pathogenesis-related protein genes, one effector gene and ten cathepsin L-like cysteine proteinase genes that were differentially expressed. After inoculation of the host pine, S. maltophilia could affect the virulence and reproduction of B. xylophilus by regulating the expression of parasitic, immune, and pathogenicity genes of B. xylophilus.

Ultrastructural plasticity in the plant-parasitic nematode, Bursaphelenchus xylophilus

Phenotypic plasticity is one of the most important strategies used by organisms with low mobility to survive in fluctuating environments. Phenotypic plasticity plays a vital role in nematodes because they have small bodies and lack wings or legs and thus, cannot move far by themselves. Bursaphelenchus xylophilus, the pathogenic nematode species that causes pine wilt disease, experiences fluctuating conditions throughout their life history; i.e., in both the phytophagous and mycetophagous phases. However, whether the functional morphology changes between the life phases of B. xylophilus remains unknown. Our study revealed differences in the ultrastructure of B. xylophilus between the two phases. Well-developed lateral alae and atrophied intestinal microvilli were observed in the phytophagous phase compared with the mycetophagous phase. The ultrastructure in the phytophagous phase was morphologically similar to that at the dauer stage, which enables the larvae to survive in harsh environments. It suggests that the living tree represents a harsh environment for B. xylophilus, and ultrastructural phenotypic plasticity is a key strategy for B. xylophilus to survive in a living tree. In addition, ultrastructural observations of obligate plant-parasitic species closely related to B. xylophilus revealed that B. xylophilus may be in the process of adapting to feed on plant cells.

BxCDP1 from the pine wood nematode Bursaphelenchus xylophilus is recognized as a novel molecular pattern

The migratory plant-parasitic nematode Bursaphelenchus xylophilus is the causal agent of pine wilt disease, which causes serious damage to pine forests in China. Plant immunity plays an important role in plant resistance to multiple pathogens. Activation of the plant immune system is generally determined by immune receptors, including plant pattern recognition receptors, which mediate pattern recognition. However, little is known about molecular pattern recognition in the interaction between pines and B. xylophilus. Based on the B. xylophilus transcriptome at the early stages of infection and Agrobacterium tumefaciens-mediated transient expression and infiltration of recombinant proteins produced by Pichia pastoris in many plant species, a novel molecular pattern (BxCDP1) was characterized in B. xylophilus. We found that BxCDP1 was highly up-regulated at the early infection stages of B. xylophilus, and was similar to a protein in Pararhizobium haloflavum. BxCDP1 triggered cell death in Nicotiana benthamiana when secreted into the apoplast, and this effect was dependent on brassinosteroid-insensitive 1-associated kinase 1, but independent of suppressor of BIR1-1. BxCDP1 also exhibited cell death-inducing activity in pine, Arabidopsis, tomato, pepper, and lettuce. BxCDP1 triggered reactive oxygen species production and the expression of PAMP-triggered immunity marker genes (NbAcre31, NbPTI5, and NbCyp71D20) in N. benthamiana. It also induced the expression of pathogenesis-related genes (PtPR-3, PtPR-4, and PtPR-5) in Pinus thunbergii. These results suggest that as a new B. xylophilus molecular pattern, BxCDP1 can not only be recognized by many plant species, but also triggers innate immunity in N. benthamiana and defence responses of P. thunbergii.

The anti-phytoalexin gene Bx-cathepsin W supports the survival of Bursaphelenchus xylophilus under Pinus massoniana phytoalexin stress

Background

Pine trees challenged by Bursaphelenchus xylophilus invasion produce phytoalexins to combat this nematode. Nevertheless, the phytoalexins of Asian pine trees are ineffective against B. xylophilus. The anti-phytoalexin genes of B. xylophilus disable almost all Asian pine phytoalexins, which has allowed B. xylophilus to devastate pine forests in eastern Asia over the last four decades. However, to date, the factors that stimulate anti-phytoalexin gene expression and the mechanisms by which these genes act are not well understood.

Results

Here, we described anti-phytoalexin genes in B. xylophilus using transcriptomic and bioinformatics analyses. The genes that were induced by both Pinus massoniana and carvone and had similarly elevated expression trends were considered anti-phytoalexin genes. Altogether, 187 anti-phytoalexin genes were identified, including 4 cathepsin genes. KEGG pathway enrichment indicated that those cathepsins were related to the Lysosome pathway. Since cathepsins help to maintain metabolic homeostasis by participating in the degradation of heterophagic and autophagic material, the lysosomal cathepsin gene Bx-cathepsin W was cloned and characterized. The results of the RNAi assessment indicated that the knockdown of Bx-cathepsin W reduced the survival rates of B. xylophilus under carvone or P. massoniana stress. The correlation between Bx-cathepsin W and the susceptibility of pines showed that Bx-cathepsin W might help improve the anti-phytotoxin ability of B. xylophilus.

Conclusions

The results indicated that the anti-phytoalexin gene Bx-cathepsin W supported the survival of B. xylophilus under P. massoniana phytoalexin stress. The cDNA library sequencing, differentially expressed gene identification, and WGCNA algorithm analysis provided insight at a systemic level into the gene regulation of B. xylophilus in response to the immune reaction of P. massoniana. These results will lead to a better understanding of the function of nematode defenses in host innate immunity.

Stage-specific transcriptome of Bursaphelenchus xylophilus reveals temporal regulation of effector genes and roles of the dauer-like stages in the lifecycle

The pine wood nematode Bursaphelenchus xylophilus is the causal agent of pine wilt disease, one of the most devastating forest diseases in East Asian and West European countries. The lifecycle of B. xylophilus includes four propagative larval stages and gonochoristic adults which are involved in the pathogenicity, and two stages of dispersal larvae involved in the spread of the disease. To elucidate the ecological roles of each developmental stage in the pathogenic life cycle, we performed a comprehensive transcriptome analysis using RNA-seq generated from all developmental stages of B. xylophilus and compared transcriptomes between stages. We found more than 9000 genes are differentially expressed in at least one stage of the life cycle including genes involved in general nematode biology such as reproduction and moulting but also effector genes likely to be involved in parasitism. The dispersal-stage transcriptome revealed its analogy to C. elegans dauer and the distinct roles of the two larval stages from each other regarding survival and transmission. This study provides important insights and resources to understand B. xylophilus parasitic biology.

Cathepsin L-like Cysteine Proteinase Genes Are Associated with the Development and Pathogenicity of Pine Wood Nematode, Bursaphelenchus xylophilus

The pine wood nematode (PWN), Bursaphelenchus xylophilus, is the pathogen of pine wilt disease (PWD), resulting in huge losses in pine forests. However, its pathogenic mechanism remains unclear. The cathepsin L-like cysteine proteinase (CPL) genes are multifunctional genes related to the parasitic abilities of plant-parasitic nematodes, but their functions in PWN remain unclear. We cloned three cpl genes of PWN (Bx-cpls) by rapid amplification of cDNA ends (RACE) and analyzed their characteristics using bioinformatic methods. The tissue specificity of cpl gene of PWN (Bx-cpl) was studied using in situ mRNA hybridization (ISH). The functions of Bx-cpls in development and pathogenicity were investigated using real-time quantitative PCR (qPCR) and RNA interference (RNAi). The results showed that the full-length cDNAs of Bx-cpl-1, Bx-cpl-2, and Bx-cpl-3 were 1163 bp, 1305 bp, and 1302 bp, respectively. Bx-cpls could accumulate specifically in the egg, intestine, and genital system of PWN. During different developmental stages of PWN, the expression of Bx-cpls in the egg stage was highest. After infection, the expression levels of Bx-cpls increased and reached their highest at the initial stage of PWD, then declined gradually. The silencing of Bx-cpl could reduce the feeding, reproduction, and pathogenicity of PWN. These results revealed that Bx-cpls play multiple roles in the development and pathogenic processes of PWN.

First report of Bursaphelenchus antoniae from Pinus strobus in the U.S

Juvenile, female and male nematodes were discovered in wood chips of white pine Pinus strobus from Ashley Falls, MA. Initial observations suggested these nematodes might be PWN, but closer morphological and molecular characterization proved otherwise. Comparison of measured features with those in the literature indicated this nematode population had some unique characteristics. The specimens were identified as Bursaphelenchus antoniae Penas et al., 2006 based on 18S rDNA molecular sequence vs only 95% similarity with PWN B. xylophilus. Compared to the previously described Portuguese population of B. antoniae, the sequences generated for the MA population were 98.3% similar in the ITS1, 2 rDNA and 99.9% similar for 28S rDNA. There was 99.2% similarity between the COI sequences of the US and Portuguese isolates of B. antoniae. This population has morphology consistent with that of Penas et al., 2006; however, the female tail on this MA pine population is mucronate and more attenuated than in B. antoniae from Portuguese P. pinaster found in association with Hylobius sp. Ecological associations of both populations of B. antoniae are discussed.

Juvenile, female and male nematodes were discovered in wood chips of white pine Pinus strobus from Ashley Falls, MA. Initial observations suggested these nematodes might be PWN, but closer morphological and molecular characterization proved otherwise. Comparison of measured features with those in the literature indicated this nematode population had some unique characteristics. The specimens were identified as Bursaphelenchus antoniae Penas et al., 2006 based on 18S rDNA molecular sequence vs only 95% similarity with PWN B. xylophilus. Compared to the previously described Portuguese population of B. antoniae, the sequences generated for the MA population were 98.3% similar in the ITS1, 2 rDNA and 99.9% similar for 28S rDNA. There was 99.2% similarity between the COI sequences of the US and Portuguese isolates of B. antoniae. This population has morphology consistent with that of Penas et al., 2006; however, the female tail on this MA pine population is mucronate and more attenuated than in B. antoniae from Portuguese P. pinaster found in association with Hylobius sp. Ecological associations of both populations of B. antoniae are discussed.

Genome Evolution of Plant-Parasitic Nematodes

Plant parasitism has evolved independently on at least four separate occasions in the phylum Nematoda. The application of next-generation sequencing (NGS) to plant-parasitic nematodes has allowed a wide range of genome- or transcriptome-level comparisons, and these have identified genome adaptations that enable parasitism of plants. Current genome data suggest that horizontal gene transfer, gene family expansions, evolution of new genes that mediate interactions with the host, and parasitism-specific gene regulation are important adaptations that allow nematodes to parasitize plants. Sequencing of a larger number of nematode genomes, including plant parasites that show different modes of parasitism or that have evolved in currently unsampled clades, and using free-living taxa as comparators would allow more detailed analysis and a better understanding of the organization of key genes within the genomes. This would facilitate a more complete understanding of the way in which parasitism has shaped the genomes of plant-parasitic nematodes.

An expressed, endogenous Nodavirus-like element captured by a retrotransposon in the genome of the plant parasitic nematode Bursaphelenchus xylophilus

Recently, nematode viruses infecting Caenorhabditis elegans have been reported from the family Nodaviridae, the first nematode viruses described. Here, we report the observation of a novel endogenous viral element (EVE) in the genome of Bursaphelenchus xylophilus, a plant parasitic nematode unrelated to other nematodes from which viruses have been characterised. This element derives from a different clade of nodaviruses to the previously reported nematode viruses. This represents the first endogenous nodavirus sequence, the first nematode endogenous viral element, and significantly extends our knowledge of the potential diversity of the Nodaviridae. A search for endogenous elements related to the Nodaviridae did not reveal any elements in other available nematode genomes. Further surveillance for endogenous viral elements is warranted as our knowledge of nematode genome diversity, and in particular of free-living nematodes, expands.

Bursaphelenchus xylophilus and B. mucronatus secretomes: a comparative proteomic analysis

The pinewood nematode, Bursaphelenchus xylophilus, recognized as a worldwide major forest pest, is a migratory endoparasitic nematode with capacity to feed on pine tissues and also on fungi colonizing the trees. Bursaphelenchus mucronatus, the closest related species, differs from B. xylophilus on its pathogenicity, making this nematode a good candidate for comparative analyses. Secretome profiles of B. xylophilus and B. mucronatus were obtained and proteomic differences were evaluated by quantitative SWATH-MS. From the 681 proteins initially identified, 422 were quantified and compared between B. xylophilus and B. mucronatus secretomes and from these, 243 proteins were found differentially regulated: 158 and 85 proteins were increased in B. xylophilus and B. mucronatus secretomes, respectively. While increased proteins in B. xylophilus secretome revealed a strong enrichment in proteins with peptidase activity, the increased proteins in B. mucronatus secretome were mainly related to oxidative stress responses. The changes in peptidases were evaluated at the transcription level by RT-qPCR, revealing a correlation between the mRNA levels of four cysteine peptidases with secretion levels. The analysis presented expands our knowledge about molecular basis of B. xylophilus and B. mucronatus hosts interaction and supports the hypothesis of a key role of secreted peptidases in B. xylophilus pathogenicity.