Multilocus analysis using putative fungal effectors to describe a population of Fusarium oxysporum from sugar beet

Genomic characterization and antifungal properties of Paenibacillus polymyxa YF, a promising biocontrol agent against Fusarium oxysporum pathogen of codonopsis root rot

Root rot, a destructive soil-borne disease, poses a significant threat to a wide range of economically important crops. Codonopsis, a high-value medicine plant, is particularly susceptible to substantial production losses caused by Fusarium oxysporum-induced root rot. In this study, we identified a promising biocontrol agent for codonopsis root rot, Paenibacillus polymyxa YF. In vitro assay demonstrated that the strain YF exhibited a 70.69% inhibition rate against F. oxysporum and broad-spectrum antifungal activities against the selected six postharvest pathogens. Additionally, the strain YF demonstrated significant plant growth-promoting properties. Subsequent in vivo inoculation assays revealed that the strain YF effectively mitigated disease symptoms of F. oxysporum-induced root rot in codonopsis, even achieving a complete disease prevention efficacy rate of 100%. Our findings further elucidated that the robust biocontrol capacity of the strain YF against F. oxysporum is mediated through multiple mechanisms, including inhibition of fusaric acid secretion, downregulation of virulence-associated genes in F. oxysporum, and the production of multiple hydrolytic enzymes. Genomic analysis showed that the strain YF has a 5.62-Mb single circular chromosome with 5,138 protein-coding genes. Comprehensive genome mining of the strain YF also identified numerous genes and gene clusters involved in bio-fertilization, resistance inducers synthesis, plant colonization, biofilm formation, and antimicrobial activity. These findings provide insights into the biocontrol mechanisms of the strain YF and offer substantial potential for its further exploration and application in crop production.

The Ubiquitous Wilt-Inducing Pathogen Fusarium oxysporum-A Review of Genes Studied with Mutant Analysis

Fusarium oxysporum is one of the most economically important plant fungal pathogens, causing devastating Fusarium wilt diseases on a diverse range of hosts, including many key crop plants. Consequently, F. oxysporum has been the subject of extensive research to help develop and improve crop protection strategies. The sequencing of the F. oxysporum genome 14 years ago has greatly accelerated the discovery and characterization of key genes contributing to F. oxysporum biology and virulence. In this review, we summarize important findings on the molecular mechanisms of F. oxysporum growth, reproduction, and virulence. In particular, we focus on genes studied through mutant analysis, covering genes involved in diverse processes such as metabolism, stress tolerance, sporulation, and pathogenicity, as well as the signaling pathways that regulate them. In doing so, we hope to present a comprehensive review of the molecular understanding of F. oxysporum that will aid the future study of this and related species.

Molecular characterization of Indian races of Fusarium oxysporum f. sp. lentis (Fol) based on secreted in Xylem (SIX) effector genes and development of a SIX11 gene-based molecular marker for specific detection of Fol

Fusarium wilt of lentil caused by Fusarium oxysporum f. sp. lentis (Fol) is a destructive pathogen limiting lentil production in India. In the present study, Secreted in Xylem (SIX) effectors genes were explored in Indian races of Fol and also a diagnostic tool for reliable detection of the disease was developed. Four SIX effectors genes, SIX11, SIX13, SIX6 and SIX2 were identified in 12 isolates of Fol belonging to seven races. SIX11 was present in all the races while SIX 13 was absent in race 6 and SIX6 was present only in race 4. The phylogenetic analysis revealed the conserved nature of the SIX genes within the forma specialis and showed sequence homology with F. oxysporum f. sp. pisi. The presence of three effectors, SIX11, SIX13 and SIX6 in race 4 correlates with high disease incidence in lentil germplasms. The in-silico characterization revealed the presence of signal peptide and localization of the effectors. Further SIX11 effector gene present in all the isolates was used to develop Fol-specific molecular marker for accurate detection. The marker developed could differentiate F. oxysporum f. sp. lycopersici, F. solani, F. oxysporum, Rhizoctonia solani and Sclerotium rolfsii and had a detection limit of 0.01ng μL- 1. The effector-based marker detection helps in the unambiguous detection of the pathogen under field conditions.

Characterization of field isolates of Fusarium spp. from eggplant in India for species complexity and virulence

Eggplant wilt, despite emerging as a severe disease in India, the etiology must be better studied for its species' complexity and variability. The identity of fungal isolates associated with eggplants of India was established morphologically followed by sequencing and phylogenetic analysis. Three species, Fusarium falciforme, Fusarium incarnatum and Fusarium proliferatum, were observed for the first time in India. The isolates were tested for pathogenicity. Though all of them were pathogenic, the isolates displayed varying degrees of virulence. In further studies, the genetic relatedness of the isolates for virulence was assessed with candidate avirulent (SIX effectors), virulent (Fow1 and Fow2) and SSR markers. The SIX effector genes could not delineate the virulent isolates and were expressed in some non-F. oxysporum isolates for the first time. Likewise, the virulent genes, Fow1 for expression across the isolates and Fow2 for random expression across the isolates, were unsuitable markers for identifying the virulent groups. Hence, the F. oxysporum and F. solani isolates were genotyped with SSR markers. Though the clustering did not correlate with their virulence levels, the dendrogram grouping revealed variability among the F. oxysporum and F. solani isolates. This study concludes that although multiple species of Fusarium are associated with eggplant wilt in India, only F. oxysporum and F. solani are widespread in the surveyed areas. Though the three markers could not delineate the race specificity of the isolates, only the SSR makers could identify the genetic variability and hence, would help screen eggplant germplasm for fusarium wilt resistance.

Historical genomics reveals the evolutionary mechanisms behind multiple outbreaks of the host-specific coffee wilt pathogen Fusarium xylarioides

BACKGROUND

Nearly 50% of crop yields are lost to pests and disease, with plants and pathogens locked in an amplified co-evolutionary process of disease outbreaks. Coffee wilt disease, caused by Fusarium xylarioides, decimated coffee production in west and central Africa following its initial outbreak in the 1920s. After successful management, it later re-emerged and by the 2000s comprised two separate epidemics on arabica coffee in Ethiopia and robusta coffee in east and central Africa.

RESULTS

Here, we use genome sequencing of six historical culture collection strains spanning 52 years to identify the evolutionary processes behind these repeated outbreaks. Phylogenomic reconstruction using 13,782 single copy orthologs shows that the robusta population arose from the initial outbreak, whilst the arabica population is a divergent sister clade to the other strains. A screen for putative effector genes involved in pathogenesis shows that the populations have diverged in gene content and sequence mainly by vertical processes within lineages. However, 15 putative effector genes show evidence of horizontal acquisition, with close homology to genes from F. oxysporum. Most occupy small regions of homology within wider scaffolds, whereas a cluster of four genes occupy a 20Kb scaffold with strong homology to a region on a mobile pathogenicity chromosome in F. oxysporum that houses known effector genes. Lacking a match to the whole mobile chromosome, we nonetheless found close associations with DNA transposons, especially the miniature impala type previously proposed to facilitate horizontal transfer of pathogenicity genes in F. oxysporum. These findings support a working hypothesis that the arabica and robusta populations partly acquired distinct effector genes via transposition-mediated horizontal transfer from F. oxysporum, which shares coffee as a host and lives on other plants intercropped with coffee.

CONCLUSION

Our results show how historical genomics can help reveal mechanisms that allow fungal pathogens to keep pace with our efforts to resist them. Our list of putative effector genes identifies possible future targets for fungal control. In turn, knowledge of horizontal transfer mechanisms and putative donor taxa might help to design future intercropping strategies that minimize the risk of transfer of effector genes between closely-related Fusarium taxa.

Identification and Expression of Secreted In Xylem Pathogenicity Genes in Fusarium oxysporum f. sp. pisi

Fusarium oxysporum is a soilborne fungal plant pathogen responsible for causing disease in many economically important crops with “special forms” (formae speciales) adapted to infect specific plant hosts. F. oxysporum f. sp. pisi (FOP) is the causal agent of Fusarium wilt disease of pea. It has been reported in every country where peas are grown commercially. Disease is generally controlled using resistant cultivars possessing single major gene resistance and therefore there is a constant risk of breakdown. The main aim of this work was to characterise F. oxysporum isolates collected from diseased peas in the United Kingdom as well as FOP isolates obtained from other researchers representing different races through sequencing of a housekeeping gene and the presence of Secreted In Xylem (SIX) genes, which have previously been associated with pathogenicity in other F. oxysporum f. spp. F. oxysporum isolates from diseased United Kingdom pea plants possessed none or just one or two known SIX genes with no consistent pattern of presence/absence, leading to the conclusion that they were foot-rot causing isolates rather than FOP. In contrast, FOP isolates had different complements of SIX genes with all those identified as race 1 containing SIX1, SIX6, SIX7, SIX9, SIX10, SIX11, SIX12, and SIX14. FOP isolates that were identified as belonging to race 2 through testing on differential pea cultivars, contained either SIX1, SIX6, SIX9, SIX13, SIX14 or SIX1, SIX6, SIX13. Significant upregulation of SIX genes was also observed in planta over the early stages of infection by different FOP races in pea roots. Race specific SIX gene profiling may therefore provide potential targets for molecular identification of FOP races but further research is needed to determine whether variation in complement of SIX genes in FOP race 2 isolates results in differences in virulence across a broader set of pea differential cultivars.

Secreted in Xylem Genes: Drivers of Host Adaptation in Fusarium oxysporum

Fusarium oxysporum (Fo) is a notorious pathogen that significantly contributes to yield losses in crops of high economic status. It is responsible for vascular wilt characterized by the browning of conductive tissue, wilting, and plant death. Individual strains of Fo are host specific (formae speciales), and approximately, 150 forms have been documented so far. The pathogen secretes small effector proteins in the xylem, termed as Secreted in Xylem (Six), that contribute to its virulence. Most of these proteins contain cysteine residues in even numbers. These proteins are encoded by SIX genes that reside on mobile pathogenicity chromosomes. So far, 14 proteins have been reported. However, formae speciales vary in SIX protein profile and their respective gene sequence. Thus, SIX genes have been employed as ideal markers for pathogen identification. Acquisition of SIX-encoding mobile pathogenicity chromosomes by non-pathogenic lines, through horizontal transfer, results in the evolution of new virulent lines. Recently, some SIX genes present on these pathogenicity chromosomes have been shown to be involved in defining variation in host specificity among formae speciales. Along these lines, the review entails the variability (formae speciales, races, and vegetative compatibility groups) and evolutionary relationships among members of F. oxysporum species complex (FOSC). It provides updated information on the diversity, structure, regulation, and (a)virulence functions of SIX genes. The improved understanding of roles of SIX in variability and virulence of Fo has significant implication in establishment of molecular framework and techniques for disease management. Finally, the review identifies the gaps in current knowledge and provides insights into potential research landscapes that can be explored to strengthen the understanding of functions of SIX genes.

Multilocus Sequence Analysis of Selected Housekeeping- and Pathogenicity-Related Genes in Venturia inaequalis

The relationship between housekeeping and pathogenicity-related genes and virulence or avirulence towards the primary Malus resistance genes (R) has not been previously studied for Venturia inaequalis fungus, the causal agent of apple scab. In this study, the sequences of two housekeeping genes encoding elongation factor alpha (EF-1α) and β-tubulin and two previously unstudied effector genes of V. inaequalis from mannosidase and glucosidase families of 100 strains collected from apple cultivars with Rvi6, Rvi1, and Rvi17 and without known scab resistance genes were submitted to the analyses. Based on the phylogenetic and diversity data, as well as recombination analyses of the sequenced regions, we assessed the phylogenetic relationships and genetic structure of the pathogen within the species and the evolutionary forces that are currently acting upon this microorganism. The topology of the obtained phylograms demonstrates the lack of a relationship between the phylogenetic position of the strain and the host cultivar and the geographical origin or race of the strain. The isolates from different hosts were differentiated but did not form diagnosable, distinct phylogenetic groups. These results suggest that the analyzed genes may be too conserved to reflect the adaptation of pathogens to apple genotypes with different R genes; thus, they do not adequately reflect race discrimination. In contrast, based on variation and gene flow estimation, genetic divergence was observed among strains virulent to apple trees containing Rvi6. The results of this study confirmed a lack of free recombination between strains and demonstrated that the analyzed regions are in linkage disequilibrium and contain non-random polymorphisms associated with the strain.

Molecular and Environmental Triggering Factors of Pathogenicity of Fusarium oxysporum and F. solani Isolates Involved in the Coffee Corky-Root Disease

Coffee corky-root disease causes serious damages to coffee crop and is linked to combined infection of Fusarium spp. and root-knot nematodes Meloidogyne spp. In this study, 70 Fusarium isolates were collected from both roots of healthy coffee plants and with corky-root disease symptoms. A phylogenetic analysis, and the detection of pathogenicity SIX genes and toxigenicity Fum genes was performed for 59 F. oxysporum and 11 F. solani isolates. Based on the molecular characterization, seven F. oxysporum and three F. solani isolates were assessed for their pathogenicity on coffee seedlings under optimal watering and water stress miming root-knot nematode effect on plants. Our results revealed that a drastic increment of plant colonization capacity and pathogenicity on coffee plants of some Fusarium isolates was caused by water stress. The pathogenicity on coffee of F. solani linked to coffee corky-root disease and the presence of SIX genes in this species were demonstrated for the first time. Our study provides evidence for understanding the pathogenic basis of F. oxysporum and F. solani isolates on coffee and revealed the presence of SIX and Fum genes as one of their pathogenicity-related mechanisms. We also highlight the relevance of chlorophyll, a fluorescence as an early and high-throughput phenotyping tool in Fusarium pathogenicity studies on coffee.

Putative Effector Genes Distinguish Two Pathogenicity Groups of Fusarium oxysporum f. sp. spinaciae

Fusarium wilt of spinach, caused by Fusarium oxysporum f. sp. spinaciae, is an important disease during warm conditions in production regions with acid soils, yet little is known about what confers pathogenicity to spinach in F. oxysporum f. sp. spinaciae genetically. To identify candidate fungal genes that contribute to spinach Fusarium wilt, each of 69 geographically diverse F. oxysporum isolates was tested for pathogenicity on each of three spinach inbreds. Thirty-nine isolates identified as F. oxysporum f. sp. spinaciae caused quantitative differences in disease severity among the inbreds that revealed two distinct pathogenicity groups of F. oxysporum f. sp. spinaciae. Putative effector gene profiles, predicted from whole-genome sequences generated for nine F. oxysporum f. sp. spinaciae isolates and five nonpathogenic, spinach-associated F. oxysporum (NPS) isolates, distinguished the F. oxysporum f. sp. spinaciae isolates from the NPS isolates, and separated the F. oxysporum f. sp. spinaciae isolates into two groups. Five of the putative effector genes appeared to be unique to F. oxysporum f. sp. spinaciae, as they were not found in 222 other publicly available genome assemblies of F. oxysporum, implicating potential involvement of these genes in pathogenicity to spinach. In addition, two combinations of the 14 known Secreted in Xylem (SIX) genes that have been affiliated with host pathogenicity in other formae speciales of F. oxysporum were identified in genome assemblies of the nine F. oxysporum f. sp. spinaciae isolates, either SIX8 and SIX9 or SIX4, SIX8, and SIX14. Characterization of these putative effector genes should aid in understanding mechanisms of pathogenicity in F. oxysporum f. sp. spinaciae, developing molecular tools for rapid detection and quantification of F. oxysporum f. sp. spinaciae, and breeding for resistance to Fusarium wilt in spinach.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Whole genome analysis of the koa wilt pathogen (Fusarium oxysporum f. sp. koae) and the development of molecular tools for early detection and monitoring

BACKGROUND

Development and application of DNA-based methods to distinguish highly virulent isolates of Fusarium oxysporum f. sp. koae [Fo koae; cause of koa wilt disease on Acacia koa (koa)] will help disease management through early detection, enhanced monitoring, and improved disease resistance-breeding programs.

RESULTS

This study presents whole genome analyses of one highly virulent Fo koae isolate and one non-pathogenic F. oxysporum (Fo) isolate. These analyses allowed for the identification of putative lineage-specific DNA and predicted genes necessary for disease development on koa. Using putative chromosomes and predicted gene comparisons, Fo koae-exclusive, virulence genes were identified. The putative lineage-specific DNA included identified genes encoding products secreted in xylem (e. g., SIX1 and SIX6) that may be necessary for disease development on koa. Unique genes from Fo koae were used to develop pathogen-specific PCR primers. These diagnostic primers allowed target amplification in the characterized highly virulent Fo koae isolates but did not allow product amplification in low-virulence or non-pathogenic isolates of Fo. Thus, primers developed in this study will be useful for early detection and monitoring of highly virulent strains of Fo koae. Isolate verification is also important for disease resistance-breeding programs that require a diverse set of highly virulent Fo koae isolates for their disease-screening assays to develop disease-resistant koa.

CONCLUSIONS

These results provide the framework for understanding the pathogen genes necessary for koa wilt disease and the genetic variation of Fo koae populations across the Hawaiian Islands.

Genomic differences between the new Fusarium oxysporum f. sp. apii (Foa) race 4 on celery, the less virulent Foa races 2 and 3, and the avirulent on celery f. sp. coriandrii

BACKGROUND

Members of the F. oxysporium species complex (FOSC) in the f. sp. apii (Foa) are pathogenic on celery and those in f. sp. coriandrii (Foci) are pathogenic on coriander (=cilantro). Foci was first reported in California in 2005; a new and highly aggressive race 4 of Foa was observed in 2013 in California. Preliminary evidence indicated that Foa can also cause disease on coriander, albeit are less virulent than Foci. Comparative genomics was used to investigate the evolutionary relationships between Foa race 4, Foa race 3, and the Foci, which are all in FOSC Clade 2, and Foa race 2, which is in FOSC Clade 3.

RESULTS

A phylogenetic analysis of 2718 single-copy conserved genes and mitochondrial DNA sequence indicated that Foa races 3 and 4 and the Foci are monophyletic within FOSC Clade 2; these strains also are in a single somatic compatibility group. However, in the accessory genomes, the Foci versus Foa races 3 and 4 differ in multiple contigs. Based on significantly increased expression of Foa race 4 genes in planta vs. in vitro, we identified 23 putative effectors and 13 possible pathogenicity factors. PCR primers for diagnosis of either Foa race 2 or 4 and the Foci were identified. Finally, mixtures of conidia that were pre-stained with different fluorochromes indicated that Foa race 4 formed conidial anastomosis tubes (CATs) with Foci. Foa race 4 and Foa race 2, which are in different somatic compatibility groups, did not form CATs with each other.

CONCLUSIONS

There was no evidence that Foa race 2 was involved in the recent evolution of Foa race 4; Foa race 2 and 4 are CAT-incompatible. Although Foa races 3 and 4 and the Foci are closely related, there is no evidence that either Foci contributed to the evolution of Foa race 4, or that Foa race 4 was the recent recipient of a multi-gene chromosomal segment from another strain. However, horizontal chromosome transfer could account for the major difference in the accessory genomes of Foa race 4 and the Foci and for their differences in host range.

Pathogenomics Characterization of an Emerging Fungal Pathogen, Fusarium oxysporum f. sp. lycopersici in Greenhouse Tomato Production Systems

In recent years, greenhouse-grown tomato (Solanum lycopersicum) plants showing vascular wilt and yellowing symptoms have been observed between 2015 and 2018 in North Carolina (NC) and considered as an emerging threat to profitability. In total, 38 putative isolates were collected from symptomatic tomatoes in 12 grower greenhouses and characterized to infer pathogenic and genomic diversity, and mating-type (MAT) idiomorphs distribution. Morphology and polymerase chain reaction (PCR) markers confirmed that all isolates were Fusarium oxysporum f. sp. lycopersici (FOL) and most of them were race 3. Virulence analysis on four different tomato cultivars revealed that virulence among isolates, resistance in tomato cultivars, and the interaction between the isolates and cultivars differed significantly (P < 0.001). Cultivar 'Happy Root' (I-1, I-2, and I-3 genes for resistance) was highly resistant to FOL isolates tested. We sequenced and examined for the presence of 15 pathogenicity genes from different classes (Fmk1, Fow1, Ftf1, Orx1, Pda1, PelA, PelD, Pep1, Pep2, eIF-3, Rho1, Scd1, Snf1, Ste12, and Sge1), and 14 Secreted In Xylem (SIX) genes to use as genetic markers to identify and differentiate pathogenic isolates of FOL. Sequence data analysis showed that five pathogenicity genes, Fmk1, PelA, Rho1, Sge1, and Ste12 were present in all isolates while Fow1, Ftf1, Orx1, Peda1, Pep1, eIF-3, Scd1, and Snf1 genes were dispersed among isolates. Two genes, Pep2 and PelD, were absent in all isolates. Of the 14 SIX genes assessed, SIX1, SIX3, SIX5, SIX6, SIX7, SIX8, SIX12, and SIX14 were identified in most isolates while the remaining SIX genes varied among isolates. All isolates harbored one of the two mating-type (MAT-1 or MAT-2) idiomorphs, but not both. The SIX4 gene was present only in race 1 isolates. Diversity assessments based on sequences of the effector SIX3- and the translation elongation factor 1-α encoding genes SIX3 and tef1-α, respectively were the most informative to differentiate pathogenic races of FOL and resulted in race 1, forming a monophyletic clade while race 3 comprised multiple clades. Furthermore, phylogeny-based on SIX3- and tef1-α gene sequences showed that the predominant race 3 from greenhouse production systems significantly overlapped with previously designated race 3 isolates from various regions of the globe.

Identification of Pathogenicity Groups and Pathogenic Molecular Characterization of Fusarium oxysporum f. sp. sesami in China

Fusarium oxysporum f. sp. sesami is an extremely destructive pathogen, causing sesame Fusarium wilt disease worldwide. To clarify the pathogenicity and the genetic characters of F. oxysporum f. sp. sesami, we systematically investigated 69 F. oxysporum isolates collected from major sesame-growing areas in China. Among these isolates, 54 isolates were pathogenic and 15 were nonpathogenic according to pathogenicity testing on sesame seedlings. For the pathogenic isolates, three F. oxysporum f. sp. sesami pathogenicity groups were defined based on the three differential sesame hosts for the first time. A translation elongation factor 1α gene tree was constructed to determine the genetic diversity of the F. oxysporum isolates but could not separate F. oxysporum f. sp. sesami isolates from the nonpathogenic isolates and other F. oxysporum formae speciales. Ten secreted-in-xylem (SIX) genes (one family of effectors) were identified in F. oxysporum f. sp. sesami isolates by a search with the genome data, and were subsequently screened in the 69 F. oxysporum isolates. Compared with the SIX gene profiles in other F. oxysporum formae speciales, the presence and sequence variations of the SIX gene homologs directly correlated with the specific pathogenicity of F. oxysporum f. sp. sesami toward sesame. Furthermore, eight of these F. oxysporum f. sp. sesami SIX genes were significantly expressed in sesame plants as infection of the F. oxysporum f. sp. sesami isolate. These findings have important significance for understanding the pathogenic basis of F. oxysporum f. sp. sesami isolates, and will contribute to improve the diagnostics to effectively control Fusarium wilt disease in sesame.

Epitypification of Fusarium oxysporum - clearing the taxonomic chaos

Fusarium oxysporum is the most economically important and commonly encountered species of Fusarium. This soil-borne fungus is known to harbour both pathogenic (plant, animal and human) and non-pathogenic strains. However, in its current concept F. oxysporum is a species complex consisting of numerous cryptic species. Identification and naming these cryptic species is complicated by multiple subspecific classification systems and the lack of living ex-type material to serve as basic reference point for phylogenetic inference. Therefore, to advance and stabilise the taxonomic position of F. oxysporum as a species and allow naming of the multiple cryptic species recognised in this species complex, an epitype is designated for F. oxysporum. Using multi-locus phylogenetic inference and subtle morphological differences with the newly established epitype of F. oxysporum as reference point, 15 cryptic taxa are resolved in this study and described as species.

Two sugar beet chitinase genes, BvSP2 and BvSE2, analysed with SNP Amplifluor-like markers, are highly expressed after Fusarium root rot inoculations and field susceptibility trial

BACKGROUND

The pathogens from Fusarium species can cause Fusarium root rot (RR) and other diseases in plant species including sugar beet (Beta vulgaris L.), and they have a strong negative impact on sugar beet yield and quality.

METHODS

A total of 22 sugar beet breeding lines were evaluated for the symptoms of RR after inoculation with Fusarium oxysporum Sch., isolate No. 5, and growth in a field trial. Two candidate genes for RR resistance, BvSP2 and BvSE2, encoding chitinases Class IV and III, respectively, were previously identified in sugar beet, and used for genotyping using modern Amplifluor-like single nucleotide polymorphism (SNP) genotyping approach. The qPCR expression analysis was used to verify responses of the candidate genes for RR infections.

RESULTS

A strong association of two SNP markers for BvSP2 and BvSE2 with resistance to RR in sugar beet was found in our study. Very high BvSP2 expression (100-fold compared to Controls) was observed in three RR resistant accessions (2182, 2236 and KWS2320) 14 days after inoculation which returned to the control level on Day 18. RR sensitive breeding line 2210 showed a delay in mRNA level, reaching maximal expression of BvSP2 18 days after inoculation. The gene BvSE2, showed a strong expression level in leaf samples from the infected field trial only in the breeding line 2236, which showed symptoms of RR, and this may be a response to other strains of F. oxysporum.

Investigation of the diversity of effector genes in the banana pathogen, Fusarium oxysporum f. sp. cubense, reveals evidence of horizontal gene transfer

It is hypothesized that the virulence of phytopathogenic fungi is mediated through the secretion of small effector proteins that interfere with the defence responses of the host plant. In Fusarium oxysporum, one family of effectors, the Secreted In Xylem (SIX) genes, has been identified. We sought to characterize the diversity and evolution of the SIX genes in the banana-infecting lineages of F. oxysporum f. sp. cubense (Foc). Whole-genome sequencing data were generated for the 23 genetic lineages of Foc, which were subsequently queried for the 14 known SIX genes (SIX1-SIX14). The sequences of the identified SIX genes were confirmed in a larger collection of Foc isolates. Genealogies were generated for each of the SIX genes identified in Foc to further investigate the evolution of the SIX genes in Foc. Within Foc, variation of the SIX gene profile, including the presence of specific SIX homologues, correlated with the pathogenic race structure of Foc. Furthermore, the topologies of the SIX gene trees were discordant with the topology of an infraspecies phylogeny inferred from EF-1α/RPB1/RPB2 (translation elongation factor-1α/RNA polymerase II subunit I/RNA polymerase II subunit II). A series of topological constraint models provided strong evidence for the horizontal transmission of SIX genes in Foc. The horizontal inheritance of pathogenicity genes in Foc counters previous assumptions that convergent evolution has driven the polyphyletic phylogeny of Foc. This work has significant implications for the management of Foc, including the improvement of diagnostics and breeding programmes.

Use of Comparative Genomics-Based Markers for Discrimination of Host Specificity in Fusarium oxysporum

The polyphyletic nature of many formae speciales of Fusarium oxysporum prevents molecular identification of newly encountered strains based on conserved, vertically inherited genes. Alternative molecular detection methods that could replace labor- and time-intensive disease assays are therefore highly desired. Effectors are functional elements in the pathogen-host interaction and have been found to show very limited sequence diversity between strains of the same forma specialis, which makes them potential markers for host-specific pathogenicity. We therefore compared candidate effector genes extracted from 60 existing and 22 newly generated genome assemblies, specifically targeting strains affecting cucurbit plant species. Based on these candidate effector genes, a total of 18 PCR primer pairs were designed to discriminate between each of the seven Cucurbitaceae-affecting formae speciales When tested on a collection of strains encompassing different clonal lineages of these formae speciales, nonpathogenic strains, and strains of other formae speciales, they allowed clear recognition of the host range of each evaluated strain. Within Fusarium oxysporum f. sp. melonis more genetic variability exists than anticipated, resulting in three F. oxysporum f. sp. melonis marker patterns that partially overlapped with the cucurbit-infecting Fusarium oxysporum f. sp. cucumerinum, Fusarium oxysporum f. sp. niveum, Fusarium oxysporum f. sp. momordicae, and/or Fusarium oxysporum f. sp. lagenariae For F. oxysporum f. sp. niveum, a multiplex TaqMan assay was evaluated and was shown to allow quantitative and specific detection of template DNA quantities as low as 2.5 pg. These results provide ready-to-use marker sequences for the mentioned F. oxysporum pathogens. Additionally, the method can be applied to find markers distinguishing other host-specific forms of F. oxysporumIMPORTANCE Pathogenic strains of Fusarium oxysporum are differentiated into formae speciales based on their host range, which is normally restricted to only one or a few plant species. However, horizontal gene transfer between strains in the species complex has resulted in a polyphyletic origin of host specificity in many of these formae speciales This hinders accurate and rapid pathogen detection through molecular methods. In our research, we compared the genomes of 88 strains of F. oxysporum with each other, specifically targeting virulence-related genes that are typically highly similar within each forma specialis Using this approach, we identified marker sequences that allow the discrimination of F. oxysporum strains affecting various cucurbit plant species through different PCR-based methods.

Fusarium Species and Their Associated Mycotoxins

The genus Fusarium includes numerous toxigenic species that are pathogenic to plants or humans, and are able to colonize a wide range of environments on earth. The genus comprises around 70 well-known species, identified by using a polyphasic approach, and as many as 300 putative species, according to phylogenetic species concepts; many putative species do not yet have formal names. Fusarium is one of the most economically important fungal genera because of yield loss due to plant pathogenic activity; mycotoxin contamination of food and feed products which often render them unaccep for marketing; and health impacts to humans and livestock, due to consumption of mycotoxins. Among the most important mycotoxins produced by species of Fusarium are the trichothecenes and the fumonisins. Fumonisins cause fatal livestock diseases and are considered potentially carcinogenic mycotoxins for humans, while trichothecenes are potent inhibitors of protein synthesis. This chapter summarizes the main aspects of morphology, pathology, and toxigenicity of the main Fusarium species that colonize different agricultural crops and environments worldwide, and cause mycotoxin contamination of food and feed.

Association of Effector Six6 with Vascular Wilt Symptoms Caused by Fusarium oxysporum on Soybean

The Fusarium oxysporum species complex (FOSC) is a widely distributed group of fungi that includes both pathogenic and nonpathogenic isolates. In a previous study, isolates within the FOSC collected primarily from soybean were assessed for the presence of 12 fungal effector genes. Although none of the assayed genes was significantly associated with wilt symptoms on soybean, the secreted in xylem 6 (Six6) gene was present only in three isolates, which all produced high levels of vascular wilt on soybean. In the current study, a collection of F. oxysporum isolates from soybean roots and F. oxysporum f. sp. phaseoli isolates from common bean was screened for the presence of the Six6 gene. Interestingly, all isolates for which the Six6 amplicon was generated caused wilt symptoms on soybean, and two-thirds of the isolates showed high levels of aggressiveness, indicating a positive association between the presence of the effector gene Six6 and induction of wilt symptoms. The expression profile of the Six6 gene analyzed by quantitative reverse-transcription polymerase chain reaction revealed an enhanced expression for the isolates that caused more severe wilt symptoms on soybean, as established by the greenhouse assay. These findings suggest the suitability of the Six6 gene as a possible locus for pathogenicity-based molecular diagnostics across the various formae speciales.

Identification of pathogenicity-related genes in Fusarium oxysporum f. sp. cepae

Pathogenic isolates of Fusarium oxysporum, distinguished as formae speciales (f. spp.) on the basis of their host specificity, cause crown rots, root rots and vascular wilts on many important crops worldwide. Fusarium oxysporum f. sp. cepae (FOC) is particularly problematic to onion growers worldwide and is increasing in prevalence in the UK. We characterized 31 F. oxysporum isolates collected from UK onions using pathogenicity tests, sequencing of housekeeping genes and identification of effectors. In onion seedling and bulb tests, 21 isolates were pathogenic and 10 were non-pathogenic. The molecular characterization of these isolates, and 21 additional isolates comprising other f. spp. and different Fusarium species, was carried out by sequencing three housekeeping genes. A concatenated tree separated the F. oxysporum isolates into six clades, but did not distinguish between pathogenic and non-pathogenic isolates. Ten putative effectors were identified within FOC, including seven Secreted In Xylem (SIX) genes first reported in F. oxysporum f. sp. lycopersici. Two highly homologous proteins with signal peptides and RxLR motifs (CRX1/CRX2) and a gene with no previously characterized domains (C5) were also identified. The presence/absence of nine of these genes was strongly related to pathogenicity against onion and all were shown to be expressed in planta. Different SIX gene complements were identified in other f. spp., but none were identified in three other Fusarium species from onion. Although the FOC SIX genes had a high level of homology with other f. spp., there were clear differences in sequences which were unique to FOC, whereas CRX1 and C5 genes appear to be largely FOC specific.

Effector profiles distinguish formae speciales of Fusarium oxysporum

Formae speciales (ff.spp.) of the fungus Fusarium oxysporum are often polyphyletic within the species complex, making it impossible to identify them on the basis of conserved genes. However, sequences that determine host-specific pathogenicity may be expected to be similar between strains within the same forma specialis. Whole genome sequencing was performed on strains from five different ff.spp. (cucumerinum, niveum, melonis, radicis-cucumerinum and lycopersici). In each genome, genes for putative effectors were identified based on small size, secretion signal, and vicinity to a "miniature impala" transposable element. The candidate effector genes of all genomes were collected and the presence/absence patterns in each individual genome were clustered. Members of the same forma specialis turned out to group together, with cucurbit-infecting strains forming a supercluster separate from other ff.spp. Moreover, strains from different clonal lineages within the same forma specialis harbour identical effector gene sequences, supporting horizontal transfer of genetic material. These data offer new insight into the genetic basis of host specificity in the F. oxysporum species complex and show that (putative) effectors can be used to predict host specificity in F. oxysporum.

Association of Putative Fungal Effectors in Fusarium oxysporum with Wilt Symptoms in Soybean

Fungi within the Fusarium oxysporum species complex can cause root rot, seedling blight, and wilt of soybean. Isolates recovered from soybean vary in aggressiveness and also the type of symptoms they produce. The aim of this study was to identify genetic markers to detect aggressive soybean wilt isolates. Eighty isolates collected primarily from soybean were tested in the greenhouse for their ability to produce wilt symptoms using susceptible 'Jack' soybean. The same 80 isolates were assessed for the presence of fungal effector genes Fmk1, Fow1, Pda1, PelA, PelD, Pep1, Prt1, Rho1, Sge1, Six1, Six6, and Snf1. All polymerase chain reaction amplicons were sequenced, phylogenies were inferred, and analysis of molecular variance (AMOVA) was performed for 10 of the 12 genes. High incidence of vascular discoloration of roots or stems was observed with 3 isolates, while moderate to low levels of incidence were observed for 25 isolates. Fungal effector genes Fmk1, Fow1, PelA, Rho1, Sge1, and Snf1 were present in all isolates screened, while Pda1, PelD, Pep1, Prt1, Six1, and Six6 were dispersed among isolates. The Bayesian and AMOVA analyses found that the genes Fmk1, Fow1, Pda1, PelA, Rho1, Sge1, and Snf1 corresponded to previously designated clades based on tef1α and mitochondrial small subunit sequences. None of the genes had a significant association with wilt symptoms on soybean. Interestingly, the Six6 gene was only present in three previously known wilt isolates from soybean, common bean, and tomato; of these, the soybean and common bean isolates produced high levels of vascular wilt in our study.

Comparative genomics and prediction of conditionally dispensable sequences in legume-infecting Fusarium oxysporum formae speciales facilitates identification of candidate effectors

BACKGROUND

Soil-borne fungi of the Fusarium oxysporum species complex cause devastating wilt disease on many crops including legumes that supply human dietary protein needs across many parts of the globe. We present and compare draft genome assemblies for three legume-infecting formae speciales (ff. spp.): F. oxysporum f. sp. ciceris (Foc-38-1) and f. sp. pisi (Fop-37622), significant pathogens of chickpea and pea respectively, the world's second and third most important grain legumes, and lastly f. sp. medicaginis (Fom-5190a) for which we developed a model legume pathosystem utilising Medicago truncatula.

RESULTS

Focusing on the identification of pathogenicity gene content, we leveraged the reference genomes of Fusarium pathogens F. oxysporum f. sp. lycopersici (tomato-infecting) and F. solani (pea-infecting) and their well-characterised core and dispensable chromosomes to predict genomic organisation in the newly sequenced legume-infecting isolates. Dispensable chromosomes are not essential for growth and in Fusarium species are known to be enriched in host-specificity and pathogenicity-associated genes. Comparative genomics of the publicly available Fusarium species revealed differential patterns of sequence conservation across F. oxysporum formae speciales, with legume-pathogenic formae speciales not exhibiting greater sequence conservation between them relative to non-legume-infecting formae speciales, possibly indicating the lack of a common ancestral source for legume pathogenicity. Combining predicted dispensable gene content with in planta expression in the model legume-infecting isolate, we identified small conserved regions and candidate effectors, four of which shared greatest similarity to proteins from another legume-infecting ff. spp.

CONCLUSIONS

We demonstrate that distinction of core and potential dispensable genomic regions of novel F. oxysporum genomes is an effective tool to facilitate effector discovery and the identification of gene content possibly linked to host specificity. While the legume-infecting isolates didn't share large genomic regions of pathogenicity-related content, smaller regions and candidate effector proteins were highly conserved, suggesting that they may play specific roles in inducing disease on legume hosts.