Differences Between Microbial Communities of Pinus Species Having Differing Level of Resistance to the Pine Wood Nematode

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.

Evaluation the role of Luteibacter pinisoli DP2-30 in mitigating pine wilt disease caused by Bursaphelenchus xylophilus through modulation of host microbiome

Background and aim

Pine wilt disease (PWD), caused by the pine wood nematode (PWN) Bursaphelenchus xylophilus, poses a significant threat to pine forests worldwide. This study aimed to isolate bacterial strains from the rhizosphere of healthy Pinus massoniana and elucidate their biocontrol potential in mitigating PWD through direct nematicidal activity and manipulation of host microbiome.

Results

We successfully isolated the rhizobacterium strain DP2-30 from rhizosphere of healthy pine plants, which was identified as Luteibacter pinisoli on the basis of morphological, biochemical, and molecular analyses. The fermentation filtrates of strain DP2-30 displayed direct nematicidal activity of >95% (corrected mortality rate) on PWN after 48 hours of treatment. The fermentation broth and filtrates of strain DP2-30 significantly inhibited PWN egg hatching by 49.38% and 43.05%, respectively. Additionally, root drenching of strain DP2-30 fermentation broth significantly reduced PWD severity in pine seedlings (2 years old), with a control effect of 62.50%. Microbiome analyses revealed significant variations in the diversity, structure, and relative abundance of bacterial and fungal communities of pine plants combined treated with DP2-30 and PWN (T2), solely treated with PWN (T1), and control (treated with water). Bacterial phyla, Proteobacteria, Actinobacteriota, Chloroflexi, Acidobacteriota, and Armatimonadota and fungal phyla Ascomycota, Basidiomycota and Mortierellomycota were dominant in the all root and stem samples. The application of L. pinisoli DP2-30 significantly increased the relative abundance of the family Rhodanobacteraceae in the roots and stems of pine seedlings. Additionally, intra-kingdom co-occurrence network analysis revealed reduced complexity in the bacterial networks but increased complexity in the fungal networks of treated plants, suggesting enhanced functional redundancy and ecosystem stability.

Conclusions

Overall, this study highlights the potential of L. pinisoli DP2-30 as an effective biocontrol agent against PWD by directly killing PWN and manipulating the host microbiota.

Engineered Pine Endophytic Fungus Expressing Double-Stranded RNA Targeting Lethal Genes to Control the Plant-Parasitic Nematode Bursaphelenchus xylophilus

The pine wood nematode (PWN), Bursaphelenchus xylophilus, is one of the most serious invasive forest pests, responsible for pine wilt disease. Currently, there are no effective, environmentally friendly control methods available. RNA interference technology has been extensively utilized to screen functional genes in eukaryotes and to explore sustainable pest management approaches through genetic engineering. In this study, we identified 353 predicted lethal genes in PWN by comparing its genome with those of lethal genes from Caenorhabditis elegans. We selected five predicted lethal genes (Bxy1177, Bxy1239, Bxy1104, Bxy667, and BxyAK1) with identification values exceeding 60% to evaluate their nematicidal effects on PWN. We tested the double-stranded RNA (dsRNA) of these genes using two methods: first, soaking in a synthesized dsRNA solution in vitro, and second, feeding on a dsRNA-engineered endophytic fungus, Fusarium babinda. Following dsRNA ingestion, either through soaking or fungal feeding, the expression of genes Bxy1177, Bxy667, Bxy1104, and BxyAK1 was significantly suppressed. Notably, nematode populations that consumed fungi expressing dsL1177 and dsAK1 showed substantial declines over time. These findings provide novel insights and a practical foundation for employing endophytic fungi-expressed dsRNA in sustainable pest management strategies.

Distinct biogeographic patterns for bacteria and fungi in association with Bursaphelenchus xylophilus nematodes and infested pinewood

Pinewood nematodes (PWN, Bursaphelenchus xylophilus) are destructive plant parasitic nematodes that cause pine wilt disease (PWD) by attacking the vascular systems of pine trees, resulting in widespread tree mortality. Research has shown that there are connections between nematode-associated microbes and PWD. Yet the variations in microbial communities across different geographic regions are not well-understood. In this study, we examined the bacterial and fungal communities associated with nematodes and infested wood collected from 34 sites across three vegetation zones in China, as well as samples from the United States, using 16S rRNA and internal transcribed spacer (ITS) gene amplicon sequencing. The predominant genera Pseudomonas and Rhodococcus were found in nematodes, and Acinetobacter was present in the wood of PWD-infected pine trees across China. Network analysis revealed that core bacterial taxa belonged to the Pseudomonadota and Actinomycetota phyla for the nematodes, whereas the Pseudomonadota and Bacteroidota phyla were dominant in the infested wood. Identification of enriched key microbial taxa in nematodes and infested wood across vegetation zones indicates distinct biogeographic microbial community structures and key bacterial species. Although the nematode-associated bacterial community showed consistency across geographic distances, the similarity of the PWD pine trees' bacterial community decreased with distance, suggesting a spatial correlation with environmental variables. Our findings enhance our understanding of the microbiota associated with pinewood nematode (PWN) and offer valuable insights into PWD management.

IMPORTANCE

Our research uncovered specific bacteria and fungi linked to pinewood nematode (PWN) and infested wood in three different vegetation zones in China, as well as samples from the United States. This sheds light on the critical roles of certain microbial groups, such as Pseudomonas, Acinetobacter, and Stenotrophomonas, in influencing PWN fitness. Understanding these patterns provides valuable insights into the dynamics of PWN-associated microbiomes, offering potential strategies for managing pine wilt disease (PWD). We found significant correlations between geographic distance and similarity in bacterial communities in the infested wood, indicating a spatial influence on wood-associated microbial communities due to limited dispersal and localized environmental conditions. Further investigations of these spatial patterns and driving forces are crucial for understanding the ecological processes that shape microbial communities in complex ecosystems and, ultimately, for mitigating the transmission of PWN in forests.

Engineered pine endophytic Bacillus toyonensis with nematocidal and colonization abilities for pine wilt disease control

Introduction

The pinewood nematode (PWN) is responsible for causing pine wilt disease (PWD), which has led to the significant decline of conifer species in Eurasian forests and has become a globally invasive quarantine pest. Manipulating plant-associated microbes to control nematodes is an important strategy for sustainable pest management. However, it has proven difficult to find pine-associated bacteria that possess both nematocidal activity and the ability to colonize pine tissues.

Methods

The stress experiments with turpentine and pine tissue extract were carried out to screen for the desired target strain that could adapt to the internal environment of pine trees. This strain was used to construct an engineered nematocidal strain. Additionally, a fluorescent strain was constructed to determine its dispersal ability in Pinus massoniana seedlings through plate separation, PCR detection, and fluorescence microscopy observations. The engineered nematocidal strain was tested in the greenhouse experiment to assess its ability to effectively protect P. massoniana seedlings from nematode infection.

Results

This study isolated a Bacillus toyonensis strain Bxy19 from the healthy pine stem, which showed exceptional tolerance in stress experiments. An engineered nematocidal strain Bxy19P3C6 was constructed, which expressed the Cry6Aa crystal protein and exhibited nematocidal activity. The fluorescent strain Bxy19GFP was also constructed and used to test its dispersal ability. It was observed to enter the needles of the seedlings through the stomata and colonize the vascular bundle after being sprayed on the seedlings. The strain was observed to colonize and spread in the tracheid after being injected into the stems. The strain could colonize the seedlings and persist for at least 50 days. Furthermore, the greenhouse experiments indicated that both spraying and injecting the engineered strain Bxy19P3C6 had considerable efficacy against nematode infection.

Discussion

The evidence of the colonization ability and persistence of the strain in pine advances our understanding of the control and prediction of the colonization of exogenously delivered bacteria in pines. This study provides a promising approach for manipulating plant-associated bacteria and using Bt protein to control nematodes.

Fungal Communities of the Pine Wilt Disease Complex: Studying the Interaction of Ophiostomatales With Bursaphelenchus xylophilus

Considered one of the most devastating plant-parasitic nematodes worldwide, Bursaphelenchus xylophilus (commonly known as pinewood nematode, PWN) is the causal agent of the pine wilt disease in the Eurasian coniferous forests. This migratory parasitic nematode is carried by an insect vector (Monochamus spp.) into the host tree (Pinus species), where it can feed on parenchymal cells and reproduce massively, resulting in the tree wilting. In declining trees, PWN populations are strongly dependent on fungal communities colonizing the host (predominantly ophiostomatoid fungi known to cause sapwood blue-staining, the blue-stain fungi), which not only influence their development and life cycle but also the number of individuals carried by the insect vector into a new host. Our main aim is to understand if PWN-associated mycobiota plays a key role in the development of PWD, in interaction with the PWN and the insect vector, and to what extent it can be targeted to disrupt the disease cycle. For this purpose, we characterized the fungal communities of Pinus pinaster trees infected and non-infected with PWN in three collection sites in Continental Portugal with different PWD temporal incidences. Our results showed that non-infected P. pinaster mycoflora is more diverse (in terms of abundance and fungal richness) than PWN-infected pine trees in the most recent PWD foci, as opposed to the fungal communities of long-term PWD history sites. Then, due to their ecological importance for PWN survival, representatives of the main ophiostomatoid fungi isolated (Ophiostoma, Leptographium, and Graphilbum) were characterized for their adaptative response to temperature, competition in-between taxa, and as food source for PWN. Under the conditions studied, Leptographium isolates showed promising results for PWN control. They could outcompete the other species, especially O. ips, and significantly reduce the development of PWN populations when compared to Botrytis cinerea (routinely used for PWN lab culturing), suggesting this to be a natural antagonist not only for the other blue-stain species but also for the PWN.