Gibberella fujikuroi mutants obtained with UV radiation and N-methyl-N'-nitro-N-nitrosoguanidine

Relation between CarS expression and activation of carotenogenesis by stress in Fusarium fujikuroi

Fusarium fujikuroi, a model organism for secondary metabolism in fungi, produces carotenoids, terpenoid pigments with antioxidant activity. Previous results indicate that carotenoid synthesis in F. fujikuroi is stimulated by light or by different stress conditions and downregulated by a RING finger protein encoded by carS gene. Here, we have analyzed the effects of three stressors, nitrogen scarcity, heat shock, and oxidative stress. We compared them with the effect of light in the wild type, a carS mutant that overproduces carotenoids, and its complemented strain. The assayed stressors increase the synthesis of carotenoids in the three strains, but mRNA levels of structural genes of carotenogenesis, carRA and carB, are only enhanced in the presence of a functional carS gene. In the wild-type strain, the four conditions affect in different manners the mRNA levels of carS: greater in the presence of light, without significant changes in nitrogen starvation, and with patent decreases after heat shock or oxidative stress, suggesting different activation mechanisms. The spores of the carS mutant are more resistant to H₂O₂ than those of the wild type; however, the mutant shows a greater H₂O₂ sensitivity at the growth level, which may be due to the participation of CarS in the regulation of genes with catalase domains, formerly described. A possible mechanism of regulation by heat stress has been found in the alternative splicing of the intron of the carS gene, located close to its 3′ end, giving rise to the formation of a shorter protein. This action could explain the inducing effect of the heat shock, but not of the other inducing conditions, which may involve other mechanisms of action on the CarS regulator, either transcriptionally or post-transcriptionally.

Controlled Transcription of Regulator Gene carS by Tet-on or by a Strong Promoter Confirms Its Role as a Repressor of Carotenoid Biosynthesis in Fusarium fujikuroi

Carotenoid biosynthesis is a frequent trait in fungi. In the ascomycete Fusarium fujikuroi, the synthesis of the carboxylic xanthophyll neurosporaxanthin (NX) is stimulated by light. However, the mutants of the carS gene, encoding a protein of the RING finger family, accumulate large NX amounts regardless of illumination, indicating the role of CarS as a negative regulator. To confirm CarS function, we used the Tet-on system to control carS expression in this fungus. The system was first set up with a reporter mluc gene, which showed a positive correlation between the inducer doxycycline and luminescence. Once the system was improved, the carS gene was expressed using Tet-on in the wild strain and in a carS mutant. In both cases, increased carS transcription provoked a downregulation of the structural genes of the pathway and albino phenotypes even under light. Similarly, when the carS gene was constitutively overexpressed under the control of a gpdA promoter, total downregulation of the NX pathway was observed. The results confirmed the role of CarS as a repressor of carotenogenesis in F. fujikuroi and revealed that its expression must be regulated in the wild strain to allow appropriate NX biosynthesis in response to illumination.

A novel lncRNA as a positive regulator of carotenoid biosynthesis in Fusarium

The fungi Fusarium oxysporum and Fusarium fujikuroi produce carotenoids, lipophilic terpenoid pigments of biotechnological interest, with xanthophyll neurosporaxanthin as the main end product. Their carotenoid biosynthesis is activated by light and negatively regulated by the RING-finger protein CarS. Global transcriptomic analysis identified in both species a putative 1-kb lncRNA that we call carP, referred to as Fo-carP and Ff-carP in each species, upstream to the gene carS and transcribed from the same DNA strand. Fo-carP and Ff-carP are poorly transcribed, but their RNA levels increase in carS mutants. The deletion of Fo-carP or Ff-carP in the respective species results in albino phenotypes, with strong reductions in mRNA levels of structural genes for carotenoid biosynthesis and higher mRNA content of the carS gene, which could explain the low accumulation of carotenoids. Upon alignment, Fo-carP and Ff-carP show 75-80% identity, with short insertions or deletions resulting in a lack of coincident ORFs. Moreover, none of the ORFs found in their sequences have indications of possible coding functions. We conclude that Fo-carP and Ff-carP are regulatory lncRNAs necessary for the active expression of the carotenoid genes in Fusarium through an unknown molecular mechanism, probably related to the control of carS function or expression.

Modulation of Activity of a Carotenoid Pathway Through the Use of the TeT-on Regulatory System: Application in the Fungus Fusarium fujikuroi

Carotenoids are widespread pigments in photosynthetic species, but they are also found in nonphotosynthetic microorganisms, such as bacteria and fungi. The amenability of fungi to genetic studies have made some fungal species advantageous models in the study of the genetics and biochemistry of carotenoid biosynthesis, while others have been used for biotechnological carotenoid production. The availability of molecular techniques that allow modulating the expression of target genes is a powerful tool in the manipulation of carotenoid synthesis. An example of an adjustable gene expression is based on the tetracycline-controlled transcriptional activation system, known as Tet-on. We describe here the material and protocols for the construction of a Tet-on regulated gene, its introduction in the filamentous fungus F. fujikuroi, and its use to modulate the expression of a negative regulator of carotenoid biosynthesis.

Comparative transcriptomic analysis unveils interactions between the regulatory CarS protein and light response in Fusarium

BACKGROUND

The orange pigmentation of the agar cultures of many Fusarium species is due to the production of carotenoids, terpenoid pigments whose synthesis is stimulated by light. The genes of the carotenoid pathway and their regulation have been investigated in detail in Fusarium fujikuroi. In this and other Fusarium species, such as F. oxysporum, deep-pigmented mutants affected in the gene carS, which encodes a protein of the RING-finger family, overproduce carotenoids irrespective of light. The induction of carotenogenesis by light and its deregulation in carS mutants are achieved on the transcription of the structural genes of the pathway. We have carried out global RNA-seq transcriptomics analyses to investigate the relationship between the regulatory role of CarS and the control by light in these fungi.

RESULTS

The absence of a functional carS gene or the illumination exert wide effects on the transcriptome of F. fujikuroi, with predominance of genes activated over repressed and a greater functional diversity in the case of genes induced by light. The number of the latter decreases drastically in a carS mutant (1.1% vs. 4.8% in the wild-type), indicating that the deregulation produced by the carS mutation affects the light response of many genes. Moreover, approximately 27% of the genes activated at least 2-fold by light or by the carS mutation are coincident, raising to 40% for an 8-fold activation threshold. As expected, the genes with the highest changes under both regulatory conditions include those involved in carotenoid metabolism. In addition, light and CarS strongly influence the expression of some genes associated with stress responses, including three genes with catalase domains, consistent with roles in the control of oxidative stress. The effects of the CarS mutation or light in the transcriptome of F. oxysporum were partially coincident with those of F. fujikuroi, indicating the conservation of the objectives of their regulatory mechanisms.

CONCLUSIONS

The CarS RING finger protein down-regulates many genes whose expression is up-regulated by light in wild strains of the two investigated Fusarium species, indicating a regulatory interplay between the mechanism of action of the CarS protein and the control by light.

HyphaTracker: An ImageJ toolbox for time-resolved analysis of spore germination in filamentous fungi

The dynamics of early fungal development and its interference with physiological signals and environmental factors is yet poorly understood. Especially computational analysis tools for the evaluation of the process of early spore germination and germ tube formation are still lacking. For the time-resolved analysis of conidia germination of the filamentous ascomycete Fusarium fujikuroi we developed a straightforward toolbox implemented in ImageJ. It allows for processing of microscopic acquisitions (movies) of conidial germination starting with drift correction and data reduction prior to germling analysis. From the image time series germling related region of interests (ROIs) are extracted, which are analysed for their area, circularity, and timing. ROIs originating from germlings crossing other hyphae or the image boundaries are omitted during analysis. Each conidium/hypha is identified and related to its origin, thus allowing subsequent categorization. The efficiency of HyphaTracker was proofed and the accuracy was tested on simulated germlings at different signal-to-noise ratios. Bright-field microscopic images of conidial germination of rhodopsin-deficient F. fujikuroi mutants and their respective control strains were analysed with HyphaTracker. Consistent with our observation in earlier studies the CarO deficient mutant germinated earlier and grew faster than other, CarO expressing strains.

Protein Activity of the Fusarium fujikuroi Rhodopsins CarO and OpsA and Their Relation to Fungus-Plant Interaction

Fungi possess diverse photosensory proteins that allow them to perceive different light wavelengths and to adapt to changing light conditions in their environment. The biological and physiological roles of the green light-sensing rhodopsins in fungi are not yet resolved. The rice plant pathogen Fusarium fujikuroi exhibits two different rhodopsins, CarO and OpsA. CarO was previously characterized as a light-driven proton pump. We further analyzed the pumping behavior of CarO by patch-clamp experiments. Our data show that CarO pumping activity is strongly augmented in the presence of the plant hormone indole-3-acetic acid and in sodium acetate, in a dose-dependent manner under slightly acidic conditions. By contrast, under these and other tested conditions, the Neurospora rhodopsin (NR)-like rhodopsin OpsA did not exhibit any pump activity. Basic local alignment search tool (BLAST) searches in the genomes of ascomycetes revealed the occurrence of rhodopsin-encoding genes mainly in phyto-associated or phytopathogenic fungi, suggesting a possible correlation of the presence of rhodopsins with fungal ecology. In accordance, rice plants infected with a CarO-deficient F. fujikuroi strain showed more severe bakanae symptoms than the reference strain, indicating a potential role of the CarO rhodopsin in the regulation of plant infection by this fungus.

HPLC Analysis of Carotenoids in Neurosporaxanthin-Producing Fungi

The ascomycetous fungi Fusarium fujikuroi and Neurospora crassa are widely used as research models in the study of secondary metabolism and photobiology, respectively. Both fungi exhibit a similar carotenoid pathway, for which all the genes and enzymes have been identified. Under standard laboratory conditions, either F. fujikuroi or N. crassa accumulate a mixture of neurosporaxanthin, a carboxylic apocarotenoid acid, and several of its carotene precursors. We formerly described methods for the identification and quantification of neurosporaxanthin. However, the differences in polarity between this acidic xanthophyll and neutral carotenes make their global analysis cumbersome. Here we propose a simple HPLC methodology for the efficient separation of neurosporaxanthin and earlier pathway intermediates in a single HPLC run. This method should be useful to check the abundance of neurosporaxanthin under different experimental conditions and to evaluate the relative proportions of their different carotene precursors. To assess the validity of the method, we have compared the carotenoid profiles in samples of mycelia of F. fujikuroi and conidia of N. crassa, in both cases obtained from surface cultures of a wild strain of each species.

Carotenoid Biosynthesis in Fusarium

Many fungi of the genus Fusarium stand out for the complexity of their secondary metabolism. Individual species may differ in their metabolic capacities, but they usually share the ability to synthesize carotenoids, a family of hydrophobic terpenoid pigments widely distributed in nature. Early studies on carotenoid biosynthesis in Fusariumaquaeductuum have been recently extended in Fusarium fujikuroi and Fusarium oxysporum, well-known biotechnological and phytopathogenic models, respectively. The major Fusarium carotenoid is neurosporaxanthin, a carboxylic xanthophyll synthesized from geranylgeranyl pyrophosphate through the activity of four enzymes, encoded by the genes carRA, carB, carT and carD. These fungi produce also minor amounts of β-carotene, which may be cleaved by the CarX oxygenase to produce retinal, the rhodopsin's chromophore. The genes needed to produce retinal are organized in a gene cluster with a rhodopsin gene, while other carotenoid genes are not linked. In the investigated Fusarium species, the synthesis of carotenoids is induced by light through the transcriptional induction of the structural genes. In some species, deep-pigmented mutants with up-regulated expression of these genes are affected in the regulatory gene carS. The molecular mechanisms underlying the control by light and by the CarS protein are currently under investigation.

A RALDH-like enzyme involved in Fusarium verticillioides development

Retinaldehyde dehydrogenases (RALDHs) convert retinal to retinoic acid, an important chordate morphogen. Retinal also occurs in some fungi, such as Fusarium and Ustilago spp., evidenced by the presence of rhodopsins and β-carotene cleaving, retinal-forming dioxygenases. Based on the assumption that retinoic acid may also be formed in fungi, we searched the Fusarium protein databases for RALDHs homologs, focusing on Fusarium verticillioides. Using crude lysates of Escherichia coli cells expressing the corresponding cDNAs, we checked the capability of best matches to convert retinal into retinoic acid in vitro. Thereby, we identified an aldehyde dehydrogenase, termed CarY, as a retinoic acid-forming enzyme, an activity that was also exerted by purified CarY. Targeted mutation of the carY gene in F. verticillioides resulted in alterations of mycelia development and conidia morphology in agar cultures, and reduced capacity to produce perithecia as a female in sexual crosses. Complementation of the mutant with a wild-type carY allele demonstrated that these alterations are caused by the lackof CarY. However, retinoic acid could not be detected by LC-MS analysis either in the wild type or the complemented carY strain in vivo, making elusive the connection between CarY enzymatic activity and retinoic acid formation in the fungus.

Biochemical Characterization of the DASH-Type Cryptochrome CryD From Fusarium fujikuroi

Proteins from the cryptochrome/photolyase family utilize UV-A, blue or even red light to achieve such diverse functions as repair of DNA lesions by photolyases and signaling by cryptochromes. DASH-type cryptochromes retained the ability to repair cyclobutane pyrimidine dimers (CPDs) in single-stranded DNA regions in vitro. However, most organisms possess conventional CPD photolyases responsible for repair of these lesions in vivo. Recent work showed that the DASH-type cryptochrome CryD plays a regulatory role in diverse light-dependent processes in Fusarium fujikuroi. Here, we report our in vitro studies on heterologously expressed FfCryD. The purified protein contains N(5) ,N(10) -methenyltetrahydrofolate and flavin adenine dinucleotide as cofactors. Photoreduction and DNA photorepair experiments confirmed that FfCryD is active in light-driven electron transfer processes. However, the protein showed comparable affinities for CPD-comprising and undamaged DNA probes. Surprisingly, after purification, full-length FfCryD as well as a truncated version containing only the PHR domain bound RNA which influenced their behavior in vitro. Moreover, binding of FfCryD to RNA indicates a putative role in RNA metabolism or in posttranscriptional control of gene expression.

The flavoproteins CryD and VvdA cooperate with the white collar protein WcoA in the control of photocarotenogenesis in Fusarium fujikuroi

Light stimulates carotenoid biosynthesis in the ascomycete fungus Fusarium fujikuroi through transcriptional activation of the structural genes of the pathway carRA, carB, and cart, but the molecular basis of this photoresponse is unknown. The F. fujikuroi genome contains genes for different predicted photoreceptors, including the WC protein WcoA, the DASH cryptochrome CryD and the Vivid-like flavoprotein VvdA. We formerly found that null mutants of wcoA, cryD or vvdA exhibit carotenoid photoinduction under continuous illumination. Here we show that the wild type exhibits a biphasic response in light induction kinetics experiments, with a rapid increase in carotenoid content in the first hours, a transient arrest and a subsequent slower increase. The mutants of the three photoreceptors show different kinetic responses: the wcoA mutants are defective in the rapid response, the cryD mutants are affected in the slower response, while the fast and slow responses were respectively enhanced and attenuated in the vvdA mutants. Transcriptional analyses of the car genes revealed a strong reduction of dark and light-induced transcript levels in the wcoA mutants, while minor or no reductions were found in the cryD mutants. Formerly, we found no change on carRA and carB photoinduction in vvdA mutants. Taken together, our data suggest a cooperative participation of WcoA and CryD in early and late stages of photoinduction of carotenoid biosynthesis in F. fujikuroi, and a possible modulation of WcoA activity by VvdA. An unexpected transcriptional induction by red light of vvdA, cryD and carRA genes suggest the participation of an additional red light-absorbing photoreceptor.

The CarO rhodopsin of the fungus Fusarium fujikuroi is a light-driven proton pump that retards spore germination

Rhodopsins are membrane-embedded photoreceptors found in all major taxonomic kingdoms using retinal as their chromophore. They play well-known functions in different biological systems, but their roles in fungi remain unknown. The filamentous fungus Fusarium fujikuroi contains two putative rhodopsins, CarO and OpsA. The gene carO is light-regulated, and the predicted polypeptide contains all conserved residues required for proton pumping. We aimed to elucidate the expression and cellular location of the fungal rhodopsin CarO, its presumed proton-pumping activity and the possible effect of such function on F. fujikuroi growth. In electrophysiology experiments we confirmed that CarO is a green-light driven proton pump. Visualization of fluorescent CarO-YFP expressed in F. fujikuroi under control of its native promoter revealed higher accumulation in spores (conidia) produced by light-exposed mycelia. Germination analyses of conidia from carO(-) mutant and carO(+) control strains showed a faster development of light-exposed carO(-) germlings. In conclusion, CarO is an active proton pump, abundant in light-formed conidia, whose activity slows down early hyphal development under light. Interestingly, CarO-related rhodopsins are typically found in plant-associated fungi, where green light dominates the phyllosphere. Our data provide the first reliable clue on a possible biological role of a fungal rhodopsin.

The gene cutA of Fusarium fujikuroi, encoding a protein of the haloacid dehalogenase family, is involved in osmotic stress and glycerol metabolism

Survival of micro-organisms in natural habitats depends on their ability to adapt to variations in osmotic conditions. We previously described the gene cut-1 of Neurospora crassa, encoding a protein of the haloacid dehalogenase family with an unknown function in the osmotic stress response. Here we report on the functional analysis of cutA, the orthologous gene in the phytopathogenic fungus Fusarium fujikuroi. cutA mRNA levels increased transiently after exposure to 0.68 M NaCl and were reduced upon return to normal osmotic conditions; deletion of the gene resulted in a partial reduction in tolerance to osmotic stress. ΔcutA mutants contained much lower intracellular levels of glycerol than the wild-type, and did not exhibit the increase following hyper-osmotic shock expected from the high osmolarity glycerol (HOG) response. cutA is linked and divergently transcribed with the putative glycerol dehydrogenase gene gldB, which showed the same regulation by osmotic shock. The intergenic cutA/gldB regulatory region contains putative stress-response elements conserved in other fungi, and both genes shared other regulatory features, such as induction by heat shock and by illumination. Photoinduction was also observed in the HOG response gene hogA, and was lost in mutants of the white collar gene wcoA. Previous data on glycerol production in Aspergillus spp. and features of the predicted CutA protein lead us to propose that F. fujikuroi produces glycerol from dihydroxyacetone, and that CutA is the enzyme involved in the synthesis of this precursor by dephosphorylation of dihydroxyacetone-3P.

Light-dependent functions of the Fusarium fujikuroi CryD DASH cryptochrome in development and secondary metabolism

DASH (Drosophila, Arabidopsis, Synechocystis, human) cryptochromes (cry-DASHs) constitute a subgroup of the photolyase cryptochrome family with diverse light-sensing roles, found in most taxonomical groups. The genome of Fusarium fujikuroi, a phytopathogenic fungus with a rich secondary metabolism, contains a gene encoding a putative cry-DASH, named CryD. The expression of the cryD gene is induced by light in the wild type, but not in mutants of the "white collar" gene wcoA. Targeted ΔcryD mutants show light-dependent phenotypic alterations, including changes in morphology and pigmentation, which disappear upon reintroduction of a wild-type cryD allele. In addition to microconidia, the colonies of the ΔcryD mutants produced under illumination and nitrogen starvation large septated spores called macroconidia, absent in wild-type colonies. The ΔcryD mutants accumulated similar amounts of carotenoids to the control strain under constant illumination, but produced much larger amounts of bikaverin under nitrogen starvation, indicating a repressing role for CryD in this biosynthetic pathway. Additionally, a moderate photoinduction of gibberellin production was exhibited by the wild type but not by the ΔcryD mutants. The phenotypic alterations of the ΔcryD mutants were only noticeable in the light, as expected from the low expression of cryD in the dark, but did not correlate with mRNA levels for structural genes of the bikaverin or gibberellin biosynthetic pathways, suggesting the participation of CryD in posttranscriptional regulatory mechanisms. This is the first report on the participation of a cry-DASH protein in the regulation of fungal secondary metabolism.

Identification and regulation of fusA, the polyketide synthase gene responsible for fusarin production in Fusarium fujikuroi

Fusarins are a class of mycotoxins of the polyketide family produced by different Fusarium species, including the gibberellin-producing fungus Fusarium fujikuroi. Based on sequence comparisons between polyketide synthase (PKS) enzymes for fusarin production in other Fusarium strains, we have identified the F. fujikuroi orthologue, called fusA. The participation of fusA in fusarin biosynthesis was demonstrated by targeted mutagenesis. Fusarin production is transiently stimulated by nitrogen availability in this fungus, a regulation paralleled by the fusA mRNA levels in the cell. Illumination of the cultures results in a reduction of the fusarin content, an effect partially explained by a high sensitivity of these compounds to light. Mutants of the fusA gene exhibit no external phenotypic alterations, including morphology and conidiation, except for a lack of the characteristic yellow and/or orange pigmentation of fusarins. Moreover, the fusA mutants are less efficient than the wild type at degrading cellophane on agar cultures, a trait associated with pathogenesis functions in Fusarium oxysporum. The fusA mutants, however, are not affected in their capacities to grow on plant tissues.

Genetic basis of carotenoid overproduction in Fusarium oxysporum

The phytopathogenic fungus Fusarium oxysporum is a model organism in the study of plant-fungus interactions. As other Fusarium species, illuminated cultures of F. oxysporum exhibit an orange pigmentation because of the synthesis of carotenoids, and its genome contains orthologous light-regulated car genes for this biosynthetic pathway. By chemical mutagenesis, we obtained carotenoid overproducing mutants of F. oxysporum, called carS, with upregulated mRNA levels of the car genes. To identify the regulatory gene responsible for this phenotype, a collection of T-DNA insertional mutants obtained by Agrobacterium mediated transformation was screened for carotenoid overproduction. Three candidate transformants exhibited a carS-like phenotype, and two of them contained T-DNA insertions in the same genomic region. The insertions did not affect the integrity of any annotated ORFs, but were linked to a gene coding for a putative RING-finger (RF) protein. Based on its similarity to the RF protein CrgA from the zygomycete Mucor circinelloides, whose mutation results in a similar carotenoid deregulation, this gene (FOXG_09307) was investigated in detail. Its expression was not affected in the transformants, but mutant alleles were found in several carS mutants. A strain carrying a partial FOXG_09307 deletion, fortuitously generated in a targeted transformation experiment, exhibited the carS phenotype. This mutant and a T-DNA insertional mutant holding a 5-bp insertion in FOXG_09307 were complemented with the wild type FOXG_09307 allele. We conclude that this gene is carS, encoding a RF protein involved in down-regulation of F. oxysporum carotenogenesis.

Adenylyl cyclase plays a regulatory role in development, stress resistance and secondary metabolism in Fusarium fujikuroi

The ascomycete fungus Fusarium fujikuroi (Gibberella fujikuroi MP-C) produces secondary metabolites of biotechnological interest, such as gibberellins, bikaverin, and carotenoids. Production of these metabolites is regulated by nitrogen availability and, in a specific manner, by other environmental signals, such as light in the case of the carotenoid pathway. A complex regulatory network controlling these processes is recently emerging from the alterations of metabolite production found through the mutation of different regulatory genes. Here we show the effect of the targeted mutation of the acyA gene of F. fujikuroi, coding for adenylyl cyclase. Mutants lacking the catalytic domain of the AcyA protein showed different phenotypic alterations, including reduced growth, enhanced production of unidentified red pigments, reduced production of gibberellins and partially derepressed carotenoid biosynthesis in the dark. The phenotype differs in some aspects from that of similar mutants of the close relatives F. proliferatum and F. verticillioides: contrary to what was observed in these species, ΔacyA mutants of F. fujikuroi showed enhanced sensitivity to oxidative stress (H₂O₂), but no change in heavy metal resistance or in the ability to colonize tomato tissue, indicating a high versatility in the regulatory roles played by cAMP in this fungal group.

The gene carD encodes the aldehyde dehydrogenase responsible for neurosporaxanthin biosynthesis in Fusarium fujikuroi

Neurosporaxanthin (β-apo-4'-carotenoic acid) biosynthesis has been studied in detail in the fungus Fusarium fujikuroi. The genes and enzymes for this biosynthetic pathway are known until the last enzymatic step, the oxidation of the aldehyde group of its precursor, β-apo-4'-carotenal. On the basis of sequence homology to Neurospora crassa YLO-1, which mediates the formation of apo-4'-lycopenoic acid from the corresponding aldehyde substrate, we cloned the carD gene of F. fujikuroi and investigated the activity of the encoded enzyme. In vitro assays performed with heterologously expressed protein showed the formation of neurosporaxanthin and other apocarotenoid acids from the corresponding apocarotenals. To confirm this function in vivo, we generated an Escherichia coli strain producing β-apo-4'-carotenal, which was converted into neurosporaxanthin upon expression of carD. Moreover, the carD function was substantiated by its targeted disruption in a F. fujikuroi carotenoid-overproducing strain, which resulted in the loss of neurosporaxanthin and the accumulation of β-apo-4'-carotenal, its derivative β-apo-4'-carotenol, and minor amounts of other carotenoids. Intermediates accumulated in the ΔcarD mutant suggest that the reactions leading to neurosporaxanthin in Neurospora and Fusarium are different in their order. In contrast to ylo-1 in N. crassa, carD mRNA content is enhanced by light, but to a lesser extent than other enzymatic genes of the F. fujikuroi carotenoid pathway. Furthermore, carD mRNA levels were higher in carotenoid-overproducing mutants, supporting a functional role for CarD in F. fujikuroi carotenogenesis. With the genetic and biochemical characterization of CarD, the whole neurosporaxanthin biosynthetic pathway of F. fujikuroi has been established.

Adhesion of Macroconidia to the Plant Surface and Virulence of Nectria haematococca

To study spore attachment of the cucurbit pathogen Nectria haematococca (anamorph, Fusarium solani f. sp. cucurbitae), mutants with adhesion-deficient macroconidia were isolated. The adhesion-deficient mutants were selected after treatment with N-methyl-N' -nitro-N-nitrosoguanidine followed by repeated enrichment for macroconidia which did not attach to polystyrene. Two independently derived mutants produced macroconidia with an approximately 50% reduction in attachment to polystyrene and to zucchini fruits. When macroconidia were inoculated into wounded zucchini fruits, the adhesion-deficient mutants were as virulent as the wild-type strain. However, in disease assays in which macroconidia were deposited onto the surface of unwounded zucchini, the mutants were less virulent than the wild type. Thus, adhesion of N. haematococca macroconidia to its host surface appears to be a virulence factor.

Regulation by light in Fusarium

The genus Fusarium stands out as research model for pathogenesis and secondary metabolism. Light stimulates the production of some Fusarium metabolites, such as the carotenoids, and in many species it influences the production of asexual spores and sexual fruiting bodies. As found in other fungi with well-known photoresponses, the Fusarium genomes contain several genes for photoreceptors, among them a set of White Collar (WC) proteins, a cryptochrome, a photolyase, a phytochrome and two presumably photoactive opsins. The mutation of the opsin genes produced no apparent phenotypic alterations, but the loss of the only WC-1 orthologous protein eliminated the photoinduced expression of the photolyase and opsin genes. In contrast to other carotenogenic species, lack of the WC photoreceptor did not impede the light-induced accumulation of carotenoids, but produced alterations in conidiation, animal pathogenicity and nitrogen-regulated secondary metabolism. The regulation and functional role of other Fusarium photoreceptors is currently under investigation.

Regulation of carotenogenesis and secondary metabolism by nitrogen in wild-type Fusarium fujikuroi and carotenoid-overproducing mutants

The fungus Fusarium fujikuroi (Gibberella fujikuroi MP-C) produces metabolites of biotechnological interest, such as gibberellins, bikaverins, and carotenoids. Gibberellin and bikaverin productions are induced upon nitrogen exhaustion, while carotenoid accumulation is stimulated by light. We evaluated the effect of nitrogen availability on carotenogenesis in comparison with bikaverin and gibberellin production in the wild type and in carotenoid-overproducing mutants (carS). Nitrogen starvation increased carotenoid accumulation in all strains tested. In carS strains, gibberellin and bikaverin biosynthesis patterns differed from those of the wild type and paralleled the expression of key genes for both pathways, coding for geranylgeranyl pyrophosphate (GGPP) and kaurene synthases for the former and a polyketide synthase for the latter. These results suggest regulatory connections between carotenoid biosynthesis and nitrogen-controlled biosynthetic pathways in this fungus. Expression of gene ggs1, which encodes a second GGPP synthase, was also derepressed in the carS mutants, suggesting the participation of Ggs1 in carotenoid biosynthesis. The carS mutations did not affect genes for earlier steps of the terpenoid pathway, such as fppS or hmgR. Light induced carotenoid biosynthesis in the wild type and carRA and carB levels in the wild-type and carS strains irrespective of nitrogen availability.

The White Collar protein WcoA of Fusarium fujikuroi is not essential for photocarotenogenesis, but is involved in the regulation of secondary metabolism and conidiation

The fungal proteins of the White Collar photoreceptor family, represented by WC-1 from Neurospora crassa, mediate the control by light of different biochemical and developmental processes, such as carotenogenesis or sporulation. Carotenoid biosynthesis is induced by light in the gibberellin-producing fungus Fusarium fujikuroi. In an attempt to identify the photoreceptor for this response, we cloned the only WC-1-like gene present in the available Fusarium genomes, that we called wcoA. The predicted WcoA polypeptide is highly similar to WC-1 and contains the relevant functional domains of this protein. In contrast to the Neurospora counterpart, wcoA expression is not affected by light. Unexpectedly, targeted wcoA disruptant strains maintain the light-induced carotenogenesis. Furthermore, the wcoA mutants show a drastic reduction of fusarin production in the light, and produce less gibberellins and more bikaverins than the parental strain under nitrogen-limiting conditions. The changes in the production of the different products indicate a key regulatory role for WcoA in secondary metabolism of this fungus. Additionally, the mutants are severely affected in conidiation rates under different culture conditions, indicating a more general regulatory role for this protein.

Identification and biochemical characterization of a novel carotenoid oxygenase: elucidation of the cleavage step in the Fusarium carotenoid pathway

The synthesis of the acidic apo-carotenoid neurosporaxanthin by the fungus Fusarium fujikuroi depends on four enzyme activities: phytoene synthase and carotene cyclase, encoded by the bifunctional gene carRA, a carotene desaturase, encoded by carB, and a postulated cleaving enzyme converting torulene (C(40)) into neurosporaxanthin (C(35)). Based on sequence homology to carotenoid oxygenases, we identified the novel fungal enzyme CarT. Sequencing of the carT allele in a torulene-accumulating mutant of F. fujikuroi revealed a mutation affecting a highly conserved amino acid, and introduction of a heterologous carT gene in this mutant restored the ability to produce neurosporaxanthin, pointing to CarT as the enzyme responsible for torulene cleavage. Expression of carT in lycopene-accumulating E. coli cells resulted in the formation of minor amounts of apo-carotenoids, but no enzymatic activity was observed in beta-carotene-accumulating cells, indicating a preference for acyclic or monocyclic carotenes. The purified CarT enzyme efficiently cleaved torulene in vitro to produce beta-apo-4'-carotenal, the aldehyde corresponding to the acidic neurosporaxanthin, and was also active on other monocyclic synthetic substrates. In agreement with its role in carotenoid biosynthesis, the carT transcript levels are induced by light and upregulated in carotenoid-overproducing mutants, as already found for other car genes.

Characterization of a gene in the car cluster of Fusarium fujikuroi that codes for a protein of the carotenoid oxygenase family

The ascomycete Fusarium fujikuroi produces carotenoids by means of the enzymes encoded by three car genes. The enzymes encoded by carRA and carB are responsible of the synthesis of beta-carotene and torulene, respectively, while the product encoded by carT cleaves torulene to produce the acidic xanthophyll neurosporaxanthin. carRA and carB are found in a cluster with a third gene, carO, which codes for an opsin-like protein. However, no information is available on the sequence or chromosomal location of carT, which has been identified only by mutant analysis. Transcription of the three clustered genes is stimulated by light and by mutations in a regulatory gene, leading to overproduction of carotenoids. We have now identified a fourth gene in the car cluster, called carX, which codes for a protein similar to known carotenoid-cleaving oxygenases. carX is transcribed divergently from carRA, and exhibits the same transcriptional pattern as carRA, carB and carO. Targeted deletion of carX resulted in a phenotype characterized by a significant increase in the overall carotenoid content. In the dark, the carX mutants accumulate at least five times more carotenoids than the wild type, and exhibit partial derepression of carRA and carB transcription. The mutants also show more intense pigmentation in the light, but the increase in the carotenoid content relative to the wild type is less than twofold. Under these conditions, the mutants also show a relative increase in the amounts of phytoene and cyclic carotenoids formed, suggesting that CarRA activity is enhanced.

Diversifying carotenoid biosynthetic pathways by directed evolution

Microorganisms and plants synthesize a diverse array of natural products, many of which have proven indispensable to human health and well-being. Although many thousands of these have been characterized, the space of possible natural products--those that could be made biosynthetically--remains largely unexplored. For decades, this space has largely been the domain of chemists, who have synthesized scores of natural product analogs and have found many with improved or novel functions. New natural products have also been made in recombinant organisms, via engineered biosynthetic pathways. Recently, methods inspired by natural evolution have begun to be applied to the search for new natural products. These methods force pathways to evolve in convenient laboratory organisms, where the products of new pathways can be identified and characterized in high-throughput screening programs. Carotenoid biosynthetic pathways have served as a convenient experimental system with which to demonstrate these ideas. Researchers have mixed, matched, and mutated carotenoid biosynthetic enzymes and screened libraries of these "evolved" pathways for the emergence of new carotenoid products. This has led to dozens of new pathway products not previously known to be made by the assembled enzymes. These new products include whole families of carotenoids built from backbones not found in nature. This review details the strategies and specific methods that have been employed to generate new carotenoid biosynthetic pathways in the laboratory. The potential application of laboratory evolution to other biosynthetic pathways is also discussed.

Fdb1 and Fdb2, Fusarium verticillioides loci necessary for detoxification of preformed antimicrobials from corn

Fusarium verticillioides is a fungus of significant economic importance because of its deleterious effects on plant and animal health and on the quality of their products. Corn (Zea mays) is the primary host for F. verticillioides, and we have investigated the impact of the plant's antimicrobial compounds (DIMBOA, DIBOA, MBOA, and BOA) on fungal virulence and systemic colonization. F. verticillioides is able to metabolize these antimicrobials, and genetic analyses indicated two loci, Fdb1 and Fdb2, were involved in detoxification. Mutation at either locus caused sensitivity and no detoxification. In vitro physiological complementation assays resulted in detoxification of BOA and suggested that an unknown intermediate compound was produced. Production of the intermediate compound involved Fdbl, and a lesion in fdb2 preventing complete metabolism of BOA resulted in transformation of the intermediate into an unidentified metabolite. Based on genetic and physiological data, a branched detoxification pathway is proposed. Use of genetically characterized detoxifying and nondetoxifying strains indicated that detoxification of the corn antimicrobials was not a major virulence factor, since detoxification was not necessary for development of severe seedling blight or for infection and endophytic colonization of seedlings. Production of the antimicrobials does not appear to be a highly effective resistance mechanism against F. verticillioides.

Lovastatin inhibits the production of gibberellins but not sterol or carotenoid biosynthesis in Gibberella fujikuroi

Sterols, carotenoids and gibberellins are synthesized after the reduction of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) to mevalonate in different subcellular compartments of the fungus Gibberella fujikuroi. Lovastatin inhibits growth in many organisms, presumably because of the inhibition of the synthesis of essential terpenoids. However, in G. fujikuroi growth of the mycelia and sterol and carotenoid content were not affected by the presence of lovastatin. Nevertheless, lovastatin did inhibit the accumulation of gibberellins in the culture medium; this inhibition, however, was counteracted by the addition of mevalonate to the medium. The conversion of HMG-CoA to mevalonate in cell-free extracts was inhibited by 10 nM lovastatin. Since G. fujikuroi apparently possesses a single gene for HMG-CoA reductase, as shown by Southern hybridization and PCR amplification, it was concluded that the biosynthesis of sterols, carotenoids and gibberellins shares a single HMG-CoA reductase, but the respective subcellular compartments are differentially accessible to lovastatin.

Inhibition of gibberellin biosynthesis by nitrate in Gibberella fujikuroi

Gibberellin production in Gibberella fujikuroi starts upon exhaustion of the nitrogen source. To determine the role of nitrate and ammonium in the regulation of gibberellin biosynthesis we have isolated mutants that cannot use nitrate as a nitrogen source. Nitrate inhibited partially the production of gibberellins in mutants devoid of nitrate reductase activity. The inhibition occurred whether nitrate was added before or after the onset of gibberellin production. Addition of tungstate to the wild type mimicked the results with nitrate reductase mutants. We conclude that nitrate inhibits gibberellin biosynthesis by itself, independently of the intracellular signal that conveys nitrogen availability.

Separate compartments for the production of sterols, carotenoids and gibberellins in Gibberella fujikuroi

Substrate flows in the sterol, carotenoid and gibberellin pathways of Gibberella fujikuroi were examined by isotope-dilution experiments. The wild type and two carotenoid mutants of this fungus were grown in minimal medium with abundant glucose, limiting ammonium nitrate and a radioactively labelled precursor (either acetate, mevalonate or leucine). The precursors did not affect growth or terpenoid production, with two exceptions; leucine allowed additional growth, as expected from the nitrogen limitation in the medium, and mevalonate inhibited the accumulation of gibberellins, but only if added before the onset of gibberellin production. The relative contributions of glucose, mevalonate, leucine and acetate as terpenoid precursors, calculated from the specific radioactivities of ergosterol, neurosporaxanthin and phytoene, were different for different products and different precursors. We conclude that the biosyntheses of sterols, gibberellins and carotenoids in Gibberella are physically separated in different subcellular compartments with independent substrate pools. The same results were obtained with the three strains, except for carotenoid production, indicating that this pathway is regulated independently from other terpenoid pathways.

The Gibberella fujikuroi niaD gene encoding nitrate reductase: isolation, sequence, homologous transformation and electrophoretic karyotype location

The Gibberella fujikuroi niaD gene, encoding nitrate reductase, has been isolated and used to develop an efficient homologous transformation system. A cosmid vector designated pGFniaD was generated based on niaD selection and shown to give comparable transformation efficiencies. Using pGFniaD, a genomic library was prepared and used for genetic transformations, giving frequencies of up to 200 transformants per microgram DNA. Of 15 transformants analysed by Southern blots, six showed homologous integration whilst the remaining nine integrated at heterologous sites, indicating that the vector may be used reliably for both types of integration. The system therefore may be used both for self-cloning of gibberellin biosynthetic genes on the basis of complementation of defective mutants, and also for gene disruption experiments. Electrophoretic karyotype determination suggested at least 11 chromosomes ranging from 2 to 6 Mb, the total genome size being at least 37 Mb. The niaD gene was assigned to chromosome V by Southern blot analysis. The niaD gene is interrupted by one intron, and remarkably the promoter sequence, but not the 3' untranslated sequence, is highly homologous to that of the corresponding Fusarium oxysporum gene. This situation appears to be unique with respect to the promoter regions of corresponding genes in related species of filamentous fungi.

Gibberellin biosynthesis in gib mutants of Gibberella fujikuroi

The Ascomycete Gibberella fujikuroi synthesizes gibberellins, fujenal, carotenoids, and other terpenoids. Twelve gib mutants, isolated through the modified gibberellin fluorescence of their culture media, were subjected to chemical and biochemical analyses. Two mutants were specifically defective in the hydroxylation of carbon 13; their total gibberellin production was normal, but their main gibberellin was GA7 instead of GA3. Four mutants were blocked in the early reactions between geranylgeranyl pyrophosphate and 7-hydroxy-kaurenoic acid; two of them could not synthesize kaurene and another one was blocked in several oxidative steps. Six mutants had partial defects in early reactions, leading to the production of one-fifth to one-third of the wild type amounts of gibberellins and fujenal. Two of these produced considerable amounts of kaurenolides due to a defect in the conversion of kaurenoic acid to 7-hydroxykaurenoic acid. Another one produced no carotenoids, but attempts to isolate mutants of reactions shared by the carotenoid and gibberellin pathways failed. The gib mutations did not modify the ability of the fungus to live as a saprophyte.

Secondary metabolite production in filamentous fungi displayed

We report the potential of differential display technology for the isolation of genes of biotechnological interest. We have assessed the usefulness of the technique for the cloning of genes involved in the production of secondary metabolites, many of which are of industrial use or interest. We have used the complex pathway for the biosynthesis of gibberellins, as well as bikaverin and carotenoids, present in the filamentous fungus Gibberella fujikuroi as a test system. From a total display of approximately 16000 PCR products for each RNA sample, 100 were derived from the derepressed but not the repressed condition. These products were analysed by Northern blotting and a subset of 16 such PCR products showed differential expression at the transcript level. A number of different mRNA species were observed on this basis which varied in their size. Hence, this approach appears suitable for the isolation of genes involved in the complex pathways often required for the synthesis of secondary metabolites in organisms which are genetically intractable. Moreover, the method has the advantage that it is quick, differential displays being obtained after 2 days and DNA clones in 6 days.

Genetics and gibberellin production in Gibberella fujikuroi

Gibberella fujikuroi (Fusarium moniliforme) is a complex group of plant pathogens. Some strains produce gibberellic acid and other gibberellins that promote growth and regulate various stages in plant development. The paper describes the research effort directed to development of genetic tools for this species. Furthermore the main features of the gibberellin biosynthetic pathway as established in Gibberella are described.

Comparison of the use of UV light and nitrosoguanidine as mutagenic treatments in Aspergillus parasiticus

In the present work we study two different mutagenic methods in Aspergillus parasiticus: UV light and Nitrosoguanidine treatments. With UV light we obtained more suitable results, with survival percentage around 1% in many experiments. With Nitrosoguanidine our results were around 10% survival rate. We suggest the use of UV light to obtain mutants from A. parasiticus.

Regulation of Gibberellin Biosynthesis in Gibberella fujikuroi

Gibberellin production by Gibberella fujikuroi started only after the nitrogen source was depleted and ceased upon its renewal. Nitrogen repression of gibberellin biosynthesis is not an indirect effect of the growth arrest that follows the depletion of an essential nutrient because gibberellins were not produced upon depletion of phosphate. Mycelia produced gibberellins when suspended in a glucose solution. Production ceased some time after depletion of glucose and resumed upon its readdition. Under certain conditions, the gibberellin production rate was inversely proportional to the glucose concentrations. The specific regulation of gibberellin biosynthesis by the nitrogen source imposes a revision of the concept that gibberellins are secondary metabolites whose production is triggered by imbalance or cessation of growth.

Heritability of fumonisin B1 production in Gibberella fujikuroi mating population A

Fumonisins are mycotoxins produced by strains belonging to several different mating populations of Gibberella fujikuroi (anamorphs, Fusarium section Liseola), a major pathogen of maize and sorghum worldwide. We studied the heritability of fumonisin production in mating population A by crossing fumonisin-producing strains collected from maize and sorghum in the United States with fumonisin-nonproducing strains collected from maize in Nepal. Random ascospore and tetrad progeny from three of these crosses were analyzed by gas chromatography-mass spectrometry and high-performance liquid chromatography for their ability to produce fumonisins on autoclaved cracked maize. In all three crosses, the ability to produce fumonisins, predominately fumonisin B1, segregated as a single gene or group of closely linked genes. Intercrosses between appropriate progeny and parents were poorly fertile, so we could not determine if the apparent single genes that were segregating in each of these crosses were allelic with one another. Mating type and spore-killer traits were scored in some crosses, and each segregated, as expected, as a single gene that was unlinked to the ability to produce fumonisins. We conclude that G. fujikuroi mating population A provides a powerful genetic system for the study of this important fungal toxin.

Obtaining mutants for protoplast fusion of gibberellin-forming Gibberella fujikuroi strains

Auxotrophic, drug-resistant, nitrate-nonutilizing, and albino mutants have been isolated in Gibberella fujikuroi following UV mutagenesis. Protoplasts of complementing auxotrophic strains, mutants with resistance markers, or mutants blocked in different steps of the nitrate assimilatory pathway have been fused to form heterokaryons, diploids, or recombinant haploids. The properties of fusant strains, including gibberellic acid productivity, have been examined and compared to parent strains.

Gibberellins and Carotenoids in the Wild Type and Mutants of Gibberella fujikuroi

A new screening procedure was used to isolate 14 gib mutants of Gibberella fujikuroi with modifications in the production of gibberellins. The production of carotenoids and gibberellins was investigated in the gib mutants and in representative car mutants with various modifications of carotenoid biosynthesis. The determinations of gibberellins were carried out with a simplified fluorescence method. One of the mutants lacked both gibberellins and carotenoids. In many mutants the two pathways compensated each other: an increase in the production of one group of compounds was accompanied by a decrease in the production of the other. Under certain conditions the compensation was quantitative when the output of the two pathways was measured in moles of the common precursor, geranylgeranyl pyrophosphate. α-Picoline, an inhibitor of lycopene cyclase in G. fujikuroi , inhibits gibberellin biosynthesis. Other agents that affect the accumulation of carotenoids have no noticeable effect on the accumulation of gibberellins; such is the case with diphenylamine and β-ionone, two inhibitors of phytoene dehydrogenation, and visible light, which stimulates carotenogenesis.

Fate of DNA encoding hygromycin resistance after meiosis in transformed strains of Gibberella fujikuroi (Fusarium moniliforme)

Stability of foreign DNA transformed into a novel host is an important parameter in decisions to permit the release of genetically engineered microorganisms into the environment. Meiotic instability of transformed DNA has been reported in fungi such as Ascobolus, Aspergillus, and Neurospora. We used strains of Gibberella fujikuroi (Fusarium moniliforme) transformed with the hygr gene from Escherichia coli to study meiotic stability of foreign DNA in this plant pathogenic fungus. Crosses with single-copy transformants segregated hygr:hygs in a 1:1 manner consistent with that expected for a Mendelian locus in a haploid organism. Multicopy transformants, however, segregated hygr:hygs in a 1:2 manner that was not consistent with Mendelian expectations for a chromosomal marker, even though two unrelated auxotrophic nuclear genes were segregating normally. Segregation ratios in crosses in which hygr was introduced via the male parent did not differ significantly from crosses in which the transformed strain served as the female parent. Some of the sensitive progeny from the crosses with the multicopy transformants carried hygr sequences. When these phenotypically sensitive progeny were crossed with a wild-type strain that carried no hygr sequences, some of the progeny were phenotypically hygr. Some progeny from some crosses were more resistant to hygromycin than were their sibs or the transformant strains that served as their parents. Transformants passaged through a maize plant only rarely segregated progeny with the high levels of resistance. The mechanism underlying these genetic instabilities is not clear but may involve unequal crossing over or methylation or both. Further work with cloned genes with homology to sequences already present in the Fusarium genome is warranted.