The multifunctional beta-oxidation enzyme is required for full symptom development by the biotrophic maize pathogen Ustilago maydis

Linking extreme seasonality and gene expression in Arctic marine protists

At high latitudes, strong seasonal differences in light availability affect marine organisms and regulate the timing of ecosystem processes. Marine protists are key players in Arctic aquatic ecosystems, yet little is known about their ecological roles over yearly cycles. This is especially true for the dark polar night period, which up until recently was assumed to be devoid of biological activity. A 12 million transcripts catalogue was built from 0.45 to 10 μm protist assemblages sampled over 13 months in a time series station in an Arctic fjord in Svalbard. Community gene expression was correlated with seasonality, with light as the main driving factor. Transcript diversity and evenness were higher during polar night compared to polar day. Light-dependent functions had higher relative expression during polar day, except phototransduction. 64% of the most expressed genes could not be functionally annotated, yet up to 78% were identified in Arctic samples from Tara Oceans, suggesting that Arctic marine assemblages are distinct from those from other oceans. Our study increases understanding of the links between extreme seasonality and biological processes in pico- and nanoplanktonic protists. Our results set the ground for future monitoring studies investigating the seasonal impact of climate change on the communities of microbial eukaryotes in the High Arctic.

Two Pex5 Proteins With Different Cargo Specificity Are Critical for Peroxisome Function in Ustilago maydis

Peroxisomes are dynamic multipurpose organelles with a major function in fatty acid oxidation and breakdown of hydrogen peroxide. Many proteins destined for the peroxisomal matrix contain a C-terminal peroxisomal targeting signal type 1 (PTS1), which is recognized by tetratricopeptide repeat (TPR) proteins of the Pex5 family. Various species express at least two different Pex5 proteins, but how this contributes to protein import and organelle function is not fully understood. Here, we analyzed truncated and chimeric variants of two Pex5 proteins, Pex5a and Pex5b, from the fungus Ustilago maydis. Both proteins are required for optimal growth on oleic acid-containing medium. The N-terminal domain (NTD) of Pex5b is critical for import of all investigated peroxisomal matrix proteins including PTS2 proteins and at least one protein without a canonical PTS. In contrast, the NTD of Pex5a is not sufficient for translocation of peroxisomal matrix proteins. In the presence of Pex5b, however, specific cargo can be imported via this domain of Pex5a. The TPR domains of Pex5a and Pex5b differ in their affinity to variations of the PTS1 motif and thus can mediate import of different subsets of matrix proteins. Together, our data reveal that U. maydis employs versatile targeting modules to control peroxisome function. These findings will promote our understanding of peroxisomal protein import also in other biological systems.

Aspergillus niger uses the peroxisomal CoA-dependent β-oxidative genes to degrade the hydroxycinnamic acids caffeic acid, ferulic acid, and p-coumaric acid

Abstract

Aromatic compounds are important molecules which are widely applied in many industries and are mainly produced from nonrenewable sources. Renewable sources such as plant biomass are interesting alternatives for the production of aromatic compounds. Ferulic acid and p-coumaric acid, a precursor for vanillin and p-vinyl phenol, respectively, can be released from plant biomass by the fungus Aspergillus niger. The degradation of hydroxycinnamic acids such as caffeic acid, ferulic acid, and p-coumaric acid has been observed in many fungi. In A. niger, multiple metabolic pathways were suggested for the degradation of hydroxycinnamic acids. However, no genes were identified for these hydroxycinnamic acid metabolic pathways. In this study, several pathway genes were identified using whole-genome transcriptomic data of A. niger grown on different hydroxycinnamic acids. The genes are involved in the CoA-dependent β-oxidative pathway in fungi. This pathway is well known for the degradation of fatty acids, but not for hydroxycinnamic acids. However, in plants, it has been shown that hydroxycinnamic acids are degraded through this pathway. We identified genes encoding hydroxycinnamate-CoA synthase (hcsA), multifunctional β-oxidation hydratase/dehydrogenase (foxA), 3-ketoacyl CoA thiolase (katA), and four thioesterases (theA-D) of A. niger, which were highly induced by all three tested hydroxycinnamic acids. Deletion mutants revealed that these genes were indeed involved in the degradation of several hydroxycinnamic acids. In addition, foxA and theB are also involved in the degradation of fatty acids. HcsA, FoxA, and KatA contained a peroxisomal targeting signal and are therefore predicted to be localized in peroxisomes.

Key points

• Metabolism of hydroxycinnamic acid was investigated in Aspergillus niger

• Using transcriptome data, multiple CoA-dependent β-oxidative genes were identified.

• Both foxA and theB are involved in hydroxycinnamate but also fatty acid metabolism.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11311-0.

Ustilago Rabenhorstiana—An Alternative Natural Itaconic Acid Producer

Itaconic acid is an industrial produced chemical by the sensitive filamentous fungus Aspergillus terreus and can replace petrochemical-based monomers for polymer industry. To produce itaconic acid with alternative renewable substrates, such as lignocellulosic based hydrolysates, a robust microorganism is needed due to varying compositions and impurities. Itaconic acid producing basidiomycetous yeasts of the family Ustilaginaceae provide this required characteristic and the species Ustilago rabenhorstiana was examined in this study. By an optimization of media components, process parameters, and a fed-batch mode with glucose the final titer increased from maximum 33.3 g·L−1 in shake flasks to 50.3 g·L−1 in a bioreactor. Moreover, itaconic acid was produced from different sugar monomers based on renewable feedstocks by U. rabenhorstiana and the robustness against weak acids as sugar degradation products was confirmed. Based on these findings, U. rabenhorstiana has a high potential as alternative natural itaconic acid producer besides the well-known U. maydis and A. terreus.

The AP-1-like transcription factor ChAP1 balances tolerance and cell death in the response of the maize pathogen Cochliobolus heterostrophus to a plant phenolic

Fungal pathogens need to contend with stresses including oxidants and antimicrobial chemicals resulting from host defenses. ChAP1 of Cochliobolus heterostrophus, agent of Southern corn leaf blight, encodes an ortholog of yeast YAP1. ChAP1 is retained in the nucleus in response to plant-derived phenolic acids, in addition to its well-studied activation by oxidants. Here, we used transcriptome profiling to ask which genes are regulated in response to ChAP1 activation by ferulic acid (FA), a phenolic abundant in the maize host. Nuclearization of ChAP1 in response to phenolics is not followed by strong expression of genes needed for oxidative stress tolerance. We, therefore, compared the transcriptomes of the wild-type pathogen and a ChAP1 deletion mutant, to study the function of ChAP1 in response to FA. We hypothesized that if ChAP1 is retained in the nucleus under plant-related stress conditions yet in the absence of obvious oxidant stress, it should have additional regulatory functions. The transcriptional signature in response to FA in the wild type compared to the mutant sheds light on the signaling mechanisms and response pathways by which ChAP1 can mediate tolerance to ferulic acid, distinct from its previously known role in the antioxidant response. The ChAP1-dependent FA regulon consists mainly of two large clusters. The enrichment of transport and metabolism-related genes in cluster 1 indicates that C. heterostrophus degrades FA and removes it from the cell. When this fails at increasing stress levels, FA provides a signal for cell death, indicated by the enrichment of cell death-related genes in cluster 2. By quantitation of survival and by TUNEL assays, we show that ChAP1 promotes survival and mitigates cell death. Growth rate data show a time window in which the mutant colony expands faster than the wild type. The results delineate a transcriptional regulatory pattern in which ChAP1 helps balance a survival response for tolerance to FA, against a pathway promoting cell death in the pathogen. A general model for the transition from a phase where the return to homeostasis dominates to a phase leading to the onset of cell death provides a context for understanding these findings.

Disruption of putative short-chain acyl-CoA dehydrogenases compromised free radical scavenging, conidiogenesis, and pathogenesis of Magnaporthe oryzae

Short-chain acyl-CoA dehydrogenase (Scad) mediated β-oxidation serves as the fastest route for generating essential energies required to support the survival of organisms under stress or starvation. In this study, we identified three putative SCAD genes in the genome of the globally destructive rice blast pathogen Magnaporthe oryzae, named as MoSCAD1, MoSCAD2, and MoSCAD3. To elucidate their function, we deployed targeted gene deletion strategy to investigate individual and the combined influence of MoSCAD genes on growth, stress tolerance, conidiation and pathogenicity of the rice blast fungus. First, localization and co-localization results obtained from this study showed that MoScad1 localizes to the endoplasmic reticulum (ER), MoScad2 localizes exclusively to the mitochondria while MoScad3 partially localizes to the mitochondria and peroxisome at all developmental stages of M. oryzae. Results obtained from this investigation showed that the deletion of MoSCAD1 and MoSCAD2 caused a minimal but significant reduction in the growth of ΔMoscad1 and ΔMoscad2 strains, while, growth characteristics exhibited by the ΔMoscad3 strain was similar to the wild-type strain. Furthermore, we observed that deletion of MoSCAD2 resulted in drastic reduction in conidiation, delayed conidia germination, triggered the development of abnormal appressorium and suppressed host penetration and colonization efficiencies of the ΔMoscad1 strain. This study provides first material evidence confirming the possible existence of ER β-oxidation pathway in M. oryzae. We also infer that mitochondria β-oxidation rather than peroxisomal and ER β-oxidation play an essential role in the vegetative growth, conidiation, appressorial morphogenesis and progression of pathogenesis in M. oryzae.

Virulence function of the Ustilago maydis sterol carrier protein 2

The peroxisomal sterol carrier protein 2 (Scp2) of the biotrophic maize pathogen Ustilago maydis was detected in apoplastic fluid, suggesting that it might function as a secreted effector protein. Here we analyze the role of the scp2 gene during plant colonization. We used reverse genetics approaches to delete the scp2 gene, determined stress sensitivity and fatty acid utilization of mutants, demonstrated secretion of Scp2, used quantitative reverse transcription polymerase chain reaction for expression analysis and expressed GFP-Scp2 fusion proteins for protein localization. scp2 mutants were strongly attenuated in virulence and this defect manifested itself during penetration. Scp2 localized to peroxisomes and peroxisomal targeting was necessary for its virulence function. Deletion of scp2 in U. maydis interfered neither with growth nor with peroxisomal β-oxidation. Conventionally secreted Scp2 protein could not rescue the virulence defect. scp2 mutants displayed an altered localization of peroxisomes. Our results show a virulence function for Scp2 during penetration that is probably carried out by Scp2 in peroxisomes. We speculate that Scp2 affects the lipid composition of membranes and in this way ensures the even cellular distribution of peroxisomes.

Acetate provokes mitochondrial stress and cell death in Ustilago maydis

The fungal pathogen Ustilago maydis causes disease on maize by mating to establish an infectious filamentous cell type that invades the host and induces tumours. We previously found that β-oxidation mutants were defective in virulence and did not grow on acetate. Here, we demonstrate that acetate inhibits filamentation during mating and in response to oleic acid. We therefore examined the influence of different carbon sources by comparing the transcriptomes of cells grown on acetate, oleic acid or glucose, with expression changes for the fungus during tumour formation in planta. Guided by the transcriptional profiling, we found that acetate negatively influenced resistance to stress, promoted the formation of reactive oxygen species, triggered cell death in stationary phase and impaired virulence on maize. We also found that acetate induced mitochondrial stress by interfering with mitochondrial functions. Notably, the disruption of oxygen perception or inhibition of the electron transport chain also influenced filamentation and mating. Finally, we made use of the connections between acetate and β-oxidation to test metabolic inhibitors for an influence on growth and virulence. These experiments identified diclofenac as a potential inhibitor of virulence. Overall, these findings support the possibility of targeting mitochondrial metabolic functions to control fungal pathogens.

The Obvious and the Hidden: Prediction and Function of Fungal Peroxisomal Matrix Proteins

Fungal peroxisomes are characterized by a number of specific biological functions. To understand the physiology and biochemistry of these organelles knowledge of the proteome content is crucial. Here, we address different strategies to predict peroxisomal proteins by bioinformatics approaches. These tools range from simple text searches to network based learning strategies. A complication of this analysis is the existence of cryptic peroxisomal proteins, which are overlooked in conventional bioinformatics queries. These include proteins where targeting information results from transcriptional and posttranscriptional alterations. But also proteins with low efficiency targeting motifs that are predominantly localized in the cytosol, and proteins lacking any canonical targeting information, can play important roles within peroxisomes. Many of these proteins are so far unpredictable. Detection and characterization of these cryptic peroxisomal proteins revealed the presence of novel peroxisomal enzymatic reaction networks in fungi.

The putative phospholipase Lip2 counteracts oxidative damage and influences the virulence of Ustilago maydis

Ustilago maydis is an obligate biotrophic fungal pathogen which causes common smut disease of corn. To proliferate in host tissue, U. maydis must gain access to nutrients and overcome plant defence responses, such as the production of reactive oxygen species. The elucidation of the mechanisms by which U. maydis meets these challenges is critical for the development of strategies to combat smut disease. In this study, we focused on the contributions of phospholipases (PLs) to the pathogenesis of corn smut disease. We identified 11 genes encoding putative PLs and characterized the transcript levels for these genes in the fungus grown in culture and during infection of corn tissue. To assess the contributions of specific PLs, we focused on two genes, lip1 and lip2, which encode putative phospholipase A₂ (PLA₂ ) enzymes with similarity to platelet-activating factor acetylhydrolases. PLA₂ enzymes are known to counteract oxidative damage to lipids in other organisms. Consistent with a role in the mitigation of oxidative damage, lip2 mutants were sensitive to oxidative stress provoked by hydrogen peroxide and by increased production of reactive oxygen species caused by inhibitors of mitochondrial functions. Importantly, mutants defective in lip2, but not lip1, were attenuated for virulence in corn seedlings. Finally, a comparative analysis of fatty acid and cardiolipin profiles in the wild-type strain and a lip2 mutant revealed differences consistent with a protective role for Lip2 in maintaining lipid homeostasis and mitochondrial health during proliferation in the hostile host environment.

Peroxisome-mitochondria interplay and disease

Peroxisomes and mitochondria are ubiquitous, highly dynamic organelles with an oxidative type of metabolism in eukaryotic cells. Over the years, substantial evidence has been provided that peroxisomes and mitochondria exhibit a close functional interplay which impacts on human health and development. The so-called "peroxisome-mitochondria connection" includes metabolic cooperation in the degradation of fatty acids, a redox-sensitive relationship, an overlap in key components of the membrane fission machineries and cooperation in anti-viral signalling and defence. Furthermore, combined peroxisome-mitochondria disorders with defects in organelle division have been revealed. In this review, we present the latest progress in the emerging field of peroxisomal and mitochondrial interplay in mammals with a particular emphasis on cooperative fatty acid β-oxidation, redox interplay, organelle dynamics, cooperation in anti-viral signalling and the resulting implications for disease.

The UmGcn5 gene encoding histone acetyltransferase from Ustilago maydis is involved in dimorphism and virulence

We isolated a gene encoding a histone acetyltransferase from Ustilago maydis (DC.) Cda., which is orthologous to the Saccharomyces cerevisiae GCN5 gene. The gene was isolated from genomic clones identified by their specific hybridization to a gene fragment obtained by the polymerase chain reaction (PCR). This gene (Umgcn5; um05168) contains an open reading frame (ORF) of 1421bp that encodes a putative protein of 473 amino acids with a Mr. of 52.6kDa. The protein exhibits a high degree of homology with histone acetyltransferases from different organisms. Null a2b2 ΔUmgcn5 mutants were constructed by substitution of the region encoding the catalytic site with a hygromycin B resistance cassette. Null a1b1 ΔUmgcn5 mutants were isolated from genetic crosses of a2b2 ΔUmgcn5 and a1b1 wild-type strains in maize. Mutants displayed a slight reduction in growth rate under different conditions, and were more sensitive than the wild type to stress conditions, but more important, they grew as long mycelial cells, and formed fuzz-like colonies under all conditions where wild-type strains grew in the yeast-like morphology and formed smooth colonies. This phenotype was not reverted by cAMP addition. Mutants were not virulent to maize plants, and were unable to form teliospores. These phenotypic alterations of the mutants were reverted by their transformation with the wild-type gene.

Peroxisomes and sexual development in fungi

Peroxisomes are versatile and dynamic organelles that are essential for the development of most eukaryotic organisms. In fungi, many developmental processes, such as sexual development, require the activity of peroxisomes. Sexual reproduction in fungi involves the formation of meiotic-derived sexual spores, often takes place inside multicellular fruiting bodies and requires precise coordination between the differentiation of multiple cell types and the progression of karyogamy and meiosis. Different peroxisomal functions contribute to the orchestration of this complex developmental process. Peroxisomes are required to sustain the formation of fruiting bodies and the maturation and germination of sexual spores. They facilitate the mobilization of reserve compounds via fatty acid β-oxidation and the glyoxylate cycle, allowing the generation of energy and biosynthetic precursors. Additionally, peroxisomes are implicated in the progression of meiotic development. During meiotic development in Podospora anserina, there is a precise modulation of peroxisome assembly and dynamics. This modulation includes changes in peroxisome size, number and localization, and involves a differential activity of the protein-machinery that drives the import of proteins into peroxisomes. Furthermore, karyogamy, entry into meiosis and sorting of meiotic-derived nuclei into sexual spores all require the activity of peroxisomes. These processes rely on different peroxisomal functions and likely depend on different pathways for peroxisome assembly. Indeed, emerging studies support the existence of distinct import channels for peroxisomal proteins that contribute to different developmental stages.

Function of peroxisomes in plant-pathogen interactions

Peroxisomes are ubiquitous organelles of eukaryotic cells that accomplish a variety of biochemical functions, including β-oxidation of fatty acids, glyoxylate cycle, etc. Many reports have been accumulating that indicate peroxisome related metabolic functions are essential for pathogenic development of plant pathogenic fungi. They include peroxisome biogenesis proteins, peroxins and preferential destruction of peroxisomes, pexophagy. Gene disrupted mutants of anthracnose disease pathogen Colletotrichum orbiculare or rice blast pathogen Magnaporthe oryzae defective in peroxins or pexophagy showed deficiency in pathogenesis. Woronin body, a peroxisome related cellular organelle that is related to endurance of fungal cells against environmental damage has essential roles in pathogenesis of M. oryzae. Also, peroxisome related metabolisms such as β-oxidation and glyoxylate cycle are essential for pathogenesis in several plant pathogenic fungi. In addition, secondary metabolisms including polyketide melanin biosynthesis of C. orbiculare and M. oryzae, and host selective toxins produced by necrotrophic pathogen Alternaria alternata have pivotal roles in fungal pathogenesis. Every such factor was listed and their functions for pathogenesis were demonstrated (Table 18.1 and Fig. 18.1).

Defects in mitochondrial and peroxisomal β-oxidation influence virulence in the maize pathogen Ustilago maydis

An understanding of metabolic adaptation during the colonization of plants by phytopathogenic fungi is critical for developing strategies to protect crops. Lipids are abundant in plant tissues, and fungal phytopathogens in the phylum basidiomycota possess both peroxisomal and mitochondrial β-oxidation pathways to utilize this potential carbon source. Previously, we demonstrated a role for the peroxisomal β-oxidation enzyme Mfe2 in the filamentous growth, virulence, and sporulation of the maize pathogen Ustilago maydis. However, mfe2 mutants still caused disease symptoms, thus prompting a more detailed investigation of β-oxidation. We now demonstrate that a defect in the had1 gene encoding hydroxyacyl coenzyme A dehydrogenase for mitochondrial β-oxidation also influences virulence, although its paralog, had2, makes only a minor contribution. Additionally, we identified a gene encoding a polypeptide with similarity to the C terminus of Mfe2 and designated it Mfe2b; this gene makes a contribution to virulence only in the background of an mfe2Δ mutant. We also show that short-chain fatty acids induce cell death in U. maydis and that a block in β-oxidation leads to toxicity, likely because of the accumulation of toxic intermediates. Overall, this study reveals that β-oxidation has a complex influence on the formation of disease symptoms by U. maydis that includes potential metabolic contributions to proliferation in planta and an effect on virulence-related morphogenesis.

Peroxisomal and mitochondrial β-oxidation pathways influence the virulence of the pathogenic fungus Cryptococcus neoformans

An understanding of the connections between metabolism and elaboration of virulence factors during host colonization by the human-pathogenic fungus Cryptococcus neoformans is important for developing antifungal therapies. Lipids are abundant in host tissues, and fungal pathogens in the phylum basidiomycota possess both peroxisomal and mitochondrial β-oxidation pathways to utilize this potential carbon source. In addition, lipids are important signaling molecules in both fungi and mammals. In this report, we demonstrate that defects in the peroxisomal and mitochondrial β-oxidation pathways influence the growth of C. neoformans on fatty acids as well as the virulence of the fungus in a mouse inhalation model of cryptococcosis. Disease attenuation may be due to the cumulative influence of altered carbon source acquisition or processing, interference with secretion, changes in cell wall integrity, and an observed defect in capsule production for the peroxisomal mutant. Altered capsule elaboration in the context of a β-oxidation defect was unexpected but is particularly important because this trait is a major virulence factor for C. neoformans. Additionally, analysis of mutants in the peroxisomal pathway revealed a growth-promoting activity for C. neoformans, and subsequent work identified oleic acid and biotin as candidates for such factors. Overall, this study reveals that β-oxidation influences virulence in C. neoformans by multiple mechanisms that likely include contributions to carbon source acquisition and virulence factor elaboration.

The cAMP/Protein Kinase A Pathway and Virulence in Cryptococcus neoformans

The basidiomycete fungus Cryptococcus neoformans is an important pathogen of immunocompromised people. The ability of the fungus to sense its environment is critical for proliferation and the generation of infectious propagules, as well as for adaptation to the mammalian host during infection. The conserved cAMP/protein kinase A pathway makes an important contribution to sensing, as demonstrated by the phenotypes of mutants with pathway defects. These phenotypes include loss of the ability to mate and to elaborate the key virulence factors capsule and melanin. This review summarizes recent work that reveals new targets of the pathway, new phenotypic consequences of signaling defects, and a more detailed understanding of connections with other aspects of cryptococcal biology including iron regulation, pH sensing, and stress.

Fatty acid metabolism in the ectomycorrhizal fungus Laccaria bicolor

Here, the genome sequence of the ectomycorrhizal basidiomycete Laccaria bicolorwas explored with the aim of constructing a genome-wide inventory of genes involved in fatty acid metabolism. Sixty-three genes of the major pathways were annotated and validated by the detection of the corresponding transcripts. Seventy-one per cent belonged to multigene families of up to five members. In the mycelium of L. bicolor, 19 different fatty acids were detected, including at low concentrations palmitvaccenic acid (16:1(11Z)), which is known to be a marker for arbuscular mycorrhizal fungi. The pathways of fatty acid biosynthesis and degradation in L. bicolor were reconstructed using lipid composition, gene annotation and transcriptional analysis. Annotation results indicated that saturated fatty acids were degraded in mitochondria, whereas degradation of modified fatty acids was confined to peroxisomes. Fatty acid synthase (FAS) was the second largest protein annotated in L. bicolor. Phylogenetic analysis indicated that L. bicolor, Ustilago maydis and Coprinopsis cinerea have a vertebrate-like type I FAS encoded as a single protein, whereas in other basidiomycetes, including the human pathogenic basidiomycete Cryptococcus neoformans, and in most ascomycetes FAS is composed of the two structurally distinct subunits α and β.

A key enzyme of the Leloir pathway is involved in pathogenicity of Leptosphaeria maculans toward oilseed rape

Agrobacterium tumefaciens-mediated random insertional mutagenesis was used to investigate pathogenicity determinants in Leptosphaeria maculans. One tagged nonpathogenic mutant, termed m186, is analyzed in detail here. Microscopic analyses of infected plant tissues revealed that m186 is specifically blocked at the invasive growth phase after an unaffected initial penetration stage and is unable to switch to the necrotrophic lifestyle. In addition, m186 exhibits an altered cell wall and seems to be affected in its ability to produce cell-wall-degrading enzymes. The T-DNA insertion occurs in the intergenic region between two head-to-tail genes, leading to a constitutive upregulation of their expression. Complementation experiments showed that only one of these two genes, Lmepi, fully accounts for the mutant phenotype. Bioinformatics and expression analyses along with functional studies suggested that the Lmepi gene encodes for the highly conserved UDP-glucose-4-epimerase, a key enzyme of the Leloir pathway involved in galactose metabolism. For the third time, this study highlights the intimate connection between primary metabolism and pathogenicity in L. maculans. This finding, along with similar data obtained from the related species Stagonospora nodorum, indicates the importance of in planta nutrition for the success of infection of plants by fungi belonging to class Dothideomycete.

Pathogen profile update: Fusarium oxysporum

TAXONOMY

Kingdom Fungi; Phylum Ascomycota; Class Sordariomycetes; Order Hypocreales; Family Nectriaceae; genus Fusarium.

HOST RANGE

Very broad at the species level. More than 120 different formae speciales have been identified based on specificity to host species belonging to a wide range of plant families.

DISEASE SYMPTOMS

Initial symptoms of vascular wilt include vein clearing and leaf epinasty, followed by stunting, yellowing of the lower leaves, progressive wilting, defoliation and, finally, death of the plant. On fungal colonization, the vascular tissue turns brown, which is clearly visible in cross-sections of the stem. Some formae speciales are not primarily vascular pathogens, but cause foot and root rot or bulb rot.

ECONOMIC IMPORTANCE

Can cause severe losses in many vegetables and flowers, field crops, such as cotton, and plantation crops, such as banana, date palm and oil palm.

CONTROL

Use of resistant varieties is the only practical measure for controlling the disease in the field. In glasshouses, soil sterilization can be performed.

USEFUL WEBSITES

http://www.broad.mit.edu/annotation/genome/fusarium_group/MultiHome.html; http://www.fgsc.net/Fusarium/fushome.htm; http://www.phi-base.org/query.php

Insight into the molecular requirements for pathogenicity of Fusarium oxysporum f. sp. lycopersici through large-scale insertional mutagenesis

BACKGROUND

Fusarium oxysporum f. sp. lycopersici is the causal agent of vascular wilt disease in tomato. In order to gain more insight into the molecular processes in F. oxysporum necessary for pathogenesis and to uncover the genes involved, we used Agrobacterium-mediated insertional mutagenesis to generate 10,290 transformants and screened the transformants for loss or reduction of pathogenicity.

RESULTS

This led to the identification of 106 pathogenicity mutants. Southern analysis revealed that the average T-DNA insertion is 1.4 and that 66% of the mutants carry a single T-DNA. Using TAIL-PCR, chromosomal T-DNA flanking regions were isolated and 111 potential pathogenicity genes were identified.

CONCLUSIONS

Functional categorization of the potential pathogenicity genes indicates that certain cellular processes, such as amino acid and lipid metabolism, cell wall remodeling, protein translocation and protein degradation, seem to be important for full pathogenicity of F. oxysporum. Several known pathogenicity genes were identified, such as those encoding chitin synthase V, developmental regulator FlbA and phosphomannose isomerase. In addition, complementation and gene knock-out experiments confirmed that a glycosylphosphatidylinositol-anchored protein, thought to be involved in cell wall integrity, a transcriptional regulator, a protein with unknown function and peroxisome biogenesis are required for full pathogenicity of F. oxysporum.