Isolation and characterization of extracellular vesicles from Fusarium oxysporum f. sp. cubense, a banana wilt pathogen

Establishment of an experimental system to analyse extracellular vesicles during apoplastic fungal pathogenesis

Phoma stem canker disease of oilseed rape (Brassica napus) is caused by the extracellular fungal pathogen Leptosphaeria maculans. Although this pathogen resides exclusively in apoplastic spaces surrounding plant cells, the significance of extracellular vesicles (EVs) has not been assessed. Here, we show a method to collect apoplastic fluids (AFs) from infected leaves or cotyledons for collection of EVs during the process of host colonisation. The 15,000 × g supernatants of AFs were shown to contain ribulose-bisphosphate carboxylase (RuBisCO) at 7 days post-inoculation with L. maculans, a protein that was absent from unchallenged cotyledons. RuBisCO release coincided with the switch from biotrophy to necrotrophy, suggesting the involvement of host cell death. However, RuBisCO release did not differ between compatible and incompatible interactions, suggesting necrotrophic host cell death might not be the only process involved. EVs were also collected from axenic fungal cultures and characterised for their particle size distribution using nanoparticle tracking analysis and transmission electron microscopy. The protein composition of EV-enriched fractions was analysed using SDS-PAGE and proteomics. Enrichment analysis of gene ontology terms provided evidence for involvement of glucan and chitin metabolism as well as catalase and peptidase activities. Most of the proteins identified have previously been found in EV studies and/or EV databases, and for most of the proteins evidence was found for an involvement in pathogenicity/virulence.

Core hyphosphere microbiota of Fusarium oxysporum f. sp. niveum

BACKGROUND

Bacteria and fungi are dynamically interconnected, leading to beneficial or antagonistic relationships with plants. Within this interkingdom interaction, the microbial community directly associated with the pathogen make up the pathobiome. While the overall soil bacterial community associated with Fusarium wilt diseases has been widely examined, the specific bacterial populations that directly interact with the Fusarium wilt pathogens are yet to be discovered. In this study, we define the bacterial community associated with the hyphae of Fusarium oxysporum f. sp. niveum race 2 (FON2). Using the 16S rRNA gene metabarcoding, we describe the hyphosphere pathobiome of three isolates of FON2.

RESULTS

Our results show a core microbiome that is shared among the three tested hyphospheres. The core hyphosphere community was made up of 15 OTUs (Operational Taxonomic Units) that were associated with all three FON2 isolates. This core consisted of bacterial members of the families, Oxalobacteraceae, Propionibacteriaceae, Burkholderiaceae, Micrococcaceae, Bacillaceae, Comamonadaceae, Pseudomonadaceae and unclassified bacteria. The hyphosphere of FON2 was dominated by order Burkholderiales. While all three isolate hyphospheres were dominated by these taxa, the specific OTU differed. We also note that while the dominant OTU of one hyphosphere might not be the largest OTU for other hyphospheres, they were still present across all the three isolate hyphospheres. Additionally, in the correlation and co-occurrence analysis the most abundant OTU was negatively correlated with most of the other OTU populations within the hyphosphere.

CONCLUSIONS

The study indicates a core microbiota associated with FON2. These results provide insights into the microbe-microbe dynamic of the pathogen's success and its ability to recruit a core pathobiome. Our research promotes the concept of pathogens not being lone invaders but recruits from the established host microbiome to form a pathobiome.