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

Light regulation of secondary metabolism in fungi

Fungi have evolved unique metabolic regulation mechanisms for adapting to the changing environments. One of the key features of fungal adaptation is the production of secondary metabolites (SMs), which are essential for survival and beneficial to the organism. Many of these SMs are produced in response to the environmental cues, such as light. In all fungal species studied, the Velvet complex transcription factor VeA is a central player of the light regulatory network. In addition to growth and development, the intensity and wavelength of light affects the formation of a broad range of secondary metabolites. Recent studies, mainly on species of the genus Aspergillus, revealed that the dimer of VeA-VelB and LaeA does not only regulate gene expression in response to light, but can also be involved in regulating production of SMs. Furthermore, the complexes have a wide regulatory effect on different types of secondary metabolites. In this review, we discussed the role of light in the regulation of fungal secondary metabolism. In addition, we reviewed the photoreceptors, transcription factors, and signaling pathways that are involved in light-dependent regulation of secondary metabolism. The effects of transcription factors on the production of secondary metabolites, as well as the potential applications of light regulation for the production of pharmaceuticals and other products were discussed. Finally, we provided an overview of the current research in this field and suggested potential areas for future research.

Strategies for Controlling the Sporulation in Fusarium spp

Fusarium species are the most destructive phytopathogenic and toxin-producing fungi, causing serious diseases in almost all economically important plants. Sporulation is an essential part of the life cycle of Fusarium. Fusarium most frequently produces three different types of asexual spores, i.e., macroconidia, chlamydospores, and microconidia. It also produces meiotic spores, but fewer than 20% of Fusaria have a known sexual cycle. Therefore, the asexual spores of the Fusarium species play an important role in their propagation and infection. This review places special emphasis on current developments in artificial anti-sporulation techniques as well as features of Fusarium's asexual sporulation regulation, such as temperature, light, pH, host tissue, and nutrients. This description of sporulation regulation aspects and artificial anti-sporulation strategies will help to shed light on the ways to effectively control Fusarium diseases by inhibiting the production of spores, which eventually improves the production of food plants.

Recent advances in metabolic regulation and bioengineering of gibberellic acid biosynthesis in Fusarium fujikuroi

The plant growth hormone gibberellic acid (GA₃), as one of the representative secondary metabolites, is widely used in agriculture, horticulture and brewing industry. GA₃ is detected in both plants and several fungi with the ability to stimulate plant growth. Currently, the main mode of industrial production of GA₃ is depended on the microbial fermentation via long-period submerged fermentation using Fusarium fujikuroi as the only producing strain, qualified for its natural productivity. However, the demand of large-sale industrialization of GA₃ was still restricted by the low productivity. The biosynthetic route of GA₃ in F. fujikuroi is now well-defined. Furthermore, the multi-level regulation mechanisms involved in the whole network of GA₃ production have also been gradually unveiled by the past two decades based on the identification and characterization of several global regulators and their mutual functions. Combined with the quick development of genetic manipulation techniques, the rational modification of producing strain F. fujikuroi development become practical for higher productivity achievement. Herein, we review the latest advances in the molecular regulation of GA₃ biosynthesis in F. fujikuroi and conclude a comprehensive network involving nitrogen depression, global regulator, histone modification and G protein signaling pathway. Correspondingly, the bioengineering strategies covering conventional random mutation, genetic manipulating platform development, metabolic edition and fermentation optimization were also systematically proposed.

Roles of AaVeA on Mycotoxin Production via Light in Alternaria alternata

Alternaria alternata is a principal plant pathogen responsible for the biosynthesis of mycotoxins, including tenuazonic acid (TeA), alternariol (AOH), and alternariol monomethyl ether (AME). The velvet gene VeA is involved in fungal growth, development, and secondary metabolism, including mycotoxin biosynthesis via light regulation. In this study, the detailed regulatory roles of AaVeA in A. alternata with various light sources were investigated from the comparative analyses between the wild type and the gene knockout strains. In fungal growth and conidiation, mycelial extension was independent of light regulation in A. alternata. Red light favored conidiation, but blue light repressed it. The absence of AaVeA caused the marked reduction of hyphae extension and conidiophore formation even though red light could not induce more spores in ΔAaVeA mutant. The differentially expressed genes (DEGs) enriched in hyphal growth and conidiation were drastically transcribed from the comparatively transcriptomic profile between the wild type and ΔAaVeA mutant strains with or without light. In mycotoxin production, TeA biosynthesis seems no obvious effect by light regulation, but AOH and AME formation was significantly stimulated by blue light. Nevertheless, the disruption of AaVeA resulted in a marked decrease in mycotoxin production and the action of the stimulation was lost via blue light for the abundant accumulation of AOH and AME in the ΔAaVeA strain. From DEG expression and further verification by RT-qPCR, the loss of AaVeA caused the discontinuous supply of the substrates for mycotoxin biosynthesis and the drastic decline of biosynthetic gene expression. In addition, pathogenicity depends on AaVeA regulation in tomato infected by A. alternata in vivo. These findings provide a distinct understanding of the roles of AaVeA in fungal growth, development, mycotoxin biosynthesis, and pathogenicity in response to various light sources.

Production and Activity of Cristazarin in the Lichen-Forming Fungus Cladonia metacorallifera

Lichens are a natural source of bioactive compounds. Cladonia metacorallifera var. reagens KoLRI002260 is a rare lichen known to produce phenolic compounds, such as rhodocladonic, thamnolic, and didymic acids. However, these metabolites have not been detected in isolated mycobionts. We investigated the effects of six carbon sources on metabolite biosynthesis in the C. metacorallifera mycobiont. Red pigments appeared only in Lilly and Barnett's media with fructose at 15 °C after 3 weeks of culture and decreased after 6 weeks. We purified these red pigments using preparative-scale high performance liquid chromatography and analyzed them via nuclear magnetic resonance. Results indicated that 1% fructose-induced cristazarin and 6-methylcristazarin production under light conditions. In total, 27 out of 30 putative polyketide synthase genes were differentially expressed after 3 weeks of culture, implying that these genes may be required for cristazarin production in C. metacorallifera. Moreover, the white collar genes Cmwc-1 and Cmwc-2 were highly upregulated at all times under light conditions, indicating a possible correlation between cristazarin production and gene expression. The cancer cell lines AGS, CT26, and B16F1 were sensitive to cristazarin, with IC50 values of 18.2, 26.1, and 30.9 μg/mL, respectively, which highlights the value of cristazarin. Overall, our results suggest that 1% fructose under light conditions is required for cristazarin production by C. metacorallifera mycobionts, and cristazarin could be a good bioactive compound.

Transcription factor control of virulence in phytopathogenic fungi

Plant-pathogenic fungi are a significant threat to economic and food security worldwide. Novel protection strategies are required and therefore it is critical we understand the mechanisms by which these pathogens cause disease. Virulence factors and pathogenicity genes have been identified, but in many cases their roles remain elusive. It is becoming increasingly clear that gene regulation is vital to enable plant infection and transcription factors play an essential role. Efforts to determine their regulatory functions in plant-pathogenic fungi have expanded since the annotation of fungal genomes revealed the ubiquity of transcription factors from a broad range of families. This review establishes the significance of transcription factors as regulatory elements in plant-pathogenic fungi and provides a systematic overview of those that have been functionally characterized. Detailed analysis is provided on regulators from well-characterized families controlling various aspects of fungal metabolism, development, stress tolerance, and the production of virulence factors such as effectors and secondary metabolites. This covers conserved transcription factors with either specialized or nonspecialized roles, as well as recently identified regulators targeting key virulence pathways. Fundamental knowledge of transcription factor regulation in plant-pathogenic fungi provides avenues to identify novel virulence factors and improve our understanding of the regulatory networks linked to pathogen evolution, while transcription factors can themselves be specifically targeted for disease control. Areas requiring further insight regarding the molecular mechanisms and/or specific classes of transcription factors are identified, and direction for future investigation is presented.

Analysis of the light regulatory mechanism in carotenoid production in Rhodosporidium toruloides NBRC 10032

Light stimulates carotenoid production in an oleaginous yeast Rhodosporidium toruloides NBRC 10032 by promoting carotenoid biosynthesis genes. These genes undergo two-step transcriptional activation. The potential light regulator, Cryptochrome DASH (CRY1), has been suggested to contribute to this mechanism. In this study, based on KU70 (a component of nonhomologous end joining (NHEJ)) disrupting background, CRY1 disruptant was constructed to clarify CRY1 function. From analysis of CRY1 disruptant, it was suggested that CRY1 has the activation role of the carotenogenic gene expression. To obtain further insights into the light response, mutants varying carotenoid production were generated. Through analysis of mutants, the existence of the control two-step gene activation was proposed. In addition, our data analysis showed the strong possibility that R. toruloides NBRC 10032 is a homo-diploid strain.

A Mucoralean White Collar-1 Photoreceptor Controls Virulence by Regulating an Intricate Gene Network during Host Interactions

Mucolares are an ancient group of fungi encompassing the causal agents for the lethal infection mucormycosis. The high lethality rates, the emerging character of this disease, and the broad antifungal resistance of its causal agents are mucormycosis features that are alarming clinicians and researchers. Thus, the research field around mucormycosis is currently focused on finding specific weaknesses and targets in Mucorales for developing new treatments. In this work, we tested the role of the white-collar genes family in the virulence potential of Mucor lusitanicus. Study of the three genes of this family, mcwc-1a, mcwc-1b, and mcwc-1c, resulted in a marked functional specialization, as only mcwc-1a was essential to maintain the virulence potential of M. lusitanicus. The traditional role of wc-1 genes regulating light-dependent responses is a thoroughly studied field, whereas their role in virulence remains uncharacterized. In this work, we investigated the mechanism involving mcwc-1a in virulence from an integrated transcriptomic and functional approach during the host-pathogen interaction. Our results revealed mcwc-1a as a master regulator controlling an extensive gene network. Further dissection of this gene network clustering its components by type of regulation and functional criteria disclosed a multifunctional mechanism depending on diverse pathways. In the absence of phagocytic cells, mcwc-1a controlled pathways related to cell motility and the cytoskeleton that could be associated with the essential tropism during tissue invasion. After phagocytosis, several oxidative response pathways dependent on mcwc-1a were activated during the germination of M. lusitanicus spores inside phagocytic cells, which is the first stage of the infection. The third relevant group of genes involved in virulence and regulated by mcwc-1a belonged to the "unknown function," indicating that new and hidden pathways are involved in virulence. The unknown function category is especially pertinent in the study of mucormycosis, as it is highly enriched in specific fungal genes that represent the most promising targets for developing new antifungal compounds. These results unveil a complex multifunctional mechanism used by wc-1 genes to regulate the pathogenic potential in Mucorales that could also apply to other fungal pathogens.

Impact of the White Collar Photoreceptor WcoA on the Fusarium fujikuroi Transcriptome

The proteins of the White Collar 1 family (WC) constitute a major class of flavin photoreceptors, widely distributed in fungi, that work in cooperation with a WC 2 protein forming a regulatory complex. The WC complex was investigated in great detail in Neurospora crassa, a model fungus in photobiology studies, where it controls all its major photoresponses. The fungus Fusarium fujikuroi, a model system in the production of secondary metabolites, contains a single WC-1 gene called wcoA. The best-known light response in this fungus is the photoinduction of the synthesis of carotenoids, terpenoid pigments with antioxidant properties. Loss of WcoA in F. fujikuroi results in a drastic reduction in the mRNA levels of the carotenoid genes, and a diversity of morphological and metabolic changes, including alterations in the synthesis of several secondary metabolites, suggesting a complex regulatory role. To investigate the function of WcoA, the transcriptome of F. fujikuroi was analyzed in the dark and after 15-, 60- or 240-min illumination in a wild strain and in a formerly investigated wcoA insertional mutant. Using a threshold of four-fold change in transcript levels, 298 genes were activated and 160 were repressed in the wild strain under at least one of the light exposures. Different response patterns were observed among them, with genes exhibiting either fast, intermediate, and slow photoinduction, or intermediate or slow repression. All the fast and intermediate photoresponses, and most of the slow ones, were lost in the wcoA mutant. However, the wcoA mutation altered the expression of a much larger number of genes irrespective of illumination, reaching at least 16% of the annotated genes in this fungus. Such genes include many related to secondary metabolism, as well as others related to photobiology and other cellular functions, including the production of hydrophobins. As judged by the massive transcriptomic changes exhibited by the wcoA mutant in the dark, the results point to WcoA as a master regulatory protein in F. fujikuroi, in addition to a central function as the photoreceptor responsible for most of the transcriptional responses to light in this fungus.

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.

Neurosporaxanthin Overproduction by Fusarium fujikuroi and Evaluation of Its Antioxidant Properties

Neurosporaxanthin (NX) is a carboxylic carotenoid produced by some filamentous fungi, including species of the genera Neurospora and Fusarium. NX biosynthetic genes and their regulation have been thoroughly investigated in Fusarium fujikuroi, an industrial fungus used for gibberellin production. In this species, carotenoid-overproducing mutants, affected in the regulatory gene carS, exhibit an upregulated expression of the NX pathway. Based on former data on a stimulatory effect of nitrogen starvation on carotenoid biosynthesis, we developed culture conditions with carS mutants allowing the production of deep-pigmented mycelia. With this method, we obtained samples with ca. 8 mg NX/g dry mass, in turn the highest concentration for this carotenoid described so far. NX-rich extracts obtained from these samples were used in parallel with carS-complemented NX-poor extracts obtained under the same conditions, to check the antioxidant properties of this carotenoid in in vitro assays. NX-rich extracts exhibited higher antioxidant capacity than NX-poor extracts, either when considering their quenching activity against [O2(1g)] in organic solvent (singlet oxygen absorption capacity (SOAC) assays) or their scavenging activity against different free radicals in aqueous solution and in liposomes. These results make NX a promising carotenoid as a possible feed or food additive, and encourage further studies on its chemical properties.

RNA-seq Profiling Showed Divergent Carbohydrate-Active Enzymes (CAZymes) Expression Patterns in Lentinula edodes at Brown Film Formation Stage Under Blue Light Induction

Lentinula edodes (shiitake mushroom) is one of the most important edible mushrooms worldwide. The L. edodes cultivation cycle includes a unique developing stage called brown film formation that directly affects the development of primordium and the quality of fruiting body. Brown film formation is induced by light, especially blue light. To promote our understanding of the role of blue light in brown film formation mechanisms of L. edodes, we used RNA-seq and compared the transcriptomes of L. edodes grown under blue light and in dark, and validated the expression profiles using qRT-PCR. Blue light stimulated the formation of brown film and increased the content of polysaccharides in L. edodes. Blue light also promoted L. edodes to absorb more polysaccharides by enhancing the activities of enzymes. Among the 730 differentially expressed genes (DEGs), 433 genes were up-regulated and 297 were down-regulated. Most of the DEGs were in the oxidoreductase activity group. Pentose and glucuronic acid conversion and starch and sucrose metabolism were the most important pathways in the formation of brown film. A total of 79 genes of DEGs were identified as genes encoding carbohydrate-active enzymes (CAZymes). Fifty-one of the CAZymes genes were up-regulated, suggesting that CAZymes play important roles in brown film formation to provide sufficient nutrition for L. edodes. The results will facilitate future functional investigations of the genes involved in the developmental control of L. edodes.

Blakeslea trispora Photoreceptors: Identification and Functional Analysis

Blakeslea trispora is an industrial fungal species used for large-scale production of carotenoids. However, B. trispora light-regulated physiological processes, such as carotenoid biosynthesis and phototropism, are not fully understood. In this study, we isolated and characterized three photoreceptor genes, btwc-1a, btwc-1b, and btwc-1c, in B. trispora Bioinformatics analyses of these genes and their protein sequences revealed that the functional domains (PAS/LOV [Per-ARNT-Sim/light-oxygen-voltage] domain and zinc finger structure) of the proteins have significant homology to those of other fungal blue-light regulator proteins expressed by Mucor circinelloides and Neurospora crassa The photoreceptor proteins were synthesized by heterologous expression in Escherichia coli The chromogenic groups consisting of flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) were detected to accompany BTWC-1 proteins by using high-performance liquid chromatography (HPLC) and fluorescence spectrometry, demonstrating that the proteins may be photosensitive. The absorbance changes of the purified BTWC-1 proteins seen under dark and light conditions indicated that they were light responsive and underwent a characteristic photocycle by light induction. Site-directed mutagenesis of the cysteine residual (Cys) in BTWC-1 did not affect the normal expression of the protein in E. coli but did lead to the loss of photocycle response, indicating that Cys represents a flavin-binding domain for photon detection. We then analyzed the functions of BTWC-1 proteins by complementing btwc-1a, btwc-1b, and btwc-1c into the counterpart knockout strains of M. circinelloides for each mcwc-1 gene. Transformation of the btwc-1a complement into mcwc-1a knockout strains restored the positive phototropism, while the addition of btwc-1c complement remedied the deficiency of carotene biosynthesis in the mcwc-1c knockout strains under conditions of illumination. These results indicate that btwc-1a and btwc-1c are involved in phototropism and light-inducible carotenogenesis. Thus, btwc-1 genes share a conserved flavin-binding domain and act as photoreceptors for control of different light transduction pathways in B. trispora IMPORTANCE Studies have confirmed that light-regulated carotenogenesis is prevalent in filamentous fungi, especially in mucorales. However, few investigations have been done to understand photoinduced synthesis of carotenoids and related mechanisms in B. trispora, a well-known industrial microbial strains. In the present study, three photoreceptor genes in B. trispora were cloned, expressed, and characterized by bioinformatics and photoreception analyses, and then in vivo functional analyses of these genes were constructed in M. circinelloides The results of this study will lead to a better understanding of photoreception and light-regulated carotenoid synthesis and other physiological responses in B. trispora.

Effect of light on carotenoid and lipid production in the oleaginous yeast Rhodosporidium toruloides

The oleaginous yeast Rhodosporodium toruloides is receiving widespread attention as an alternative energy source for biofuels due to its unicellular nature, high growth rate and because it can be fermented on a large-scale. In this study, R. toruloides was cultured under both light and dark conditions in order to understand the light response involved in lipid and carotenoid biosynthesis. Our results from phenotype and gene expression analysis showed that R. toruloides responded to light by producing darker pigmentation with an associated increase in carotenoid production. Whilst there was no observable difference in lipid production, slight changes in the fatty acid composition were recorded. Furthermore, a two-step response was found in three genes (GGPSI, CAR1, and CAR2) under light conditions and the expression of the gene encoding the photoreceptor CRY1 was similarly affected.

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.

Light in the Fungal World: From Photoreception to Gene Transcription and Beyond

Fungi see light of different colors by using photoreceptors such as the White Collar proteins and cryptochromes for blue light, opsins for green light, and phytochromes for red light. Light regulates fungal development, promotes the accumulation of protective pigments and proteins, and regulates tropic growth. The White Collar complex (WCC) is a photoreceptor and a transcription factor that is responsible for regulating transcription after exposure to blue light. In Neurospora crassa, light promotes the interaction of WCCs and their binding to the promoters to activate transcription. In Aspergillus nidulans, the WCC and the phytochrome interact to coordinate gene transcription and other responses, but the contribution of these photoreceptors to fungal photobiology varies across fungal species. Ultimately, the effect of light on fungal biology is the result of the coordinated transcriptional regulation and activation of signal transduction pathways.

Fungal functional ecology: bringing a trait-based approach to plant-associated fungi

Fungi play many essential roles in ecosystems. They facilitate plant access to nutrients and water, serve as decay agents that cycle carbon and nutrients through the soil, water and atmosphere, and are major regulators of macro-organismal populations. Although technological advances are improving the detection and identification of fungi, there still exist key gaps in our ecological knowledge of this kingdom, especially related to function. Trait-based approaches have been instrumental in strengthening our understanding of plant functional ecology and, as such, provide excellent models for deepening our understanding of fungal functional ecology in ways that complement insights gained from traditional and -omics-based techniques. In this review, we synthesize current knowledge of fungal functional ecology, taxonomy and systematics and introduce a novel database of fungal functional traits (Fun^Fun ). Fun^Fun is built to interface with other databases to explore and predict how fungal functional diversity varies by taxonomy, guild, and other evolutionary or ecological grouping variables. To highlight how a quantitative trait-based approach can provide new insights, we describe multiple targeted examples and end by suggesting next steps in the rapidly growing field of fungal functional ecology.

Light regulates a Phycomyces blakesleeanus gene family similar to the carotenogenic repressor gene of Mucor circinelloides

The transcription of about 5-10 % of the genes in Phycomyces blakesleeanus is regulated by light. Among the most up-regulated, we have identified four genes, crgA-D, with similarity to crgA of Mucor circinelloides, a gene encoding a repressor of light-inducible carotenogenesis. The four proteins have the same structure with two RING RING Finger domains and a LON domain, suggesting that they could act as ubiquitin ligases, as their M. circinelloides homolog. The expression of these genes is induced by light with different thresholds as in other Mucoromycotina fungi like Blakeslea trispora and M. circinelloides. Only the P. blakesleeanus crgD gene could restore the wild type phenotype in a M. circinelloides null crgA mutant suggesting that P. blakesleeanus crgD is the functional homolog of crgA in M. circinelloides. Despite their sequence similarity it is possible that the P. blakesleeanus Crg proteins do not participate in the regulation of beta-carotene biosynthesis since none of the carotene-overproducing mutants of P. blakesleeanus had mutations in any of the crg genes. Our results provide further support of the differences in the regulation of the biosynthesis of beta-carotene in these two Mucoromycotina fungi.

The Blue-Light Photoreceptor Sfwc-1 Gene Regulates the Phototropic Response and Fruiting-Body Development in the Homothallic Ascomycete Sordaria fimicola

Sordaria fimicola, a coprophilous ascomycete, is a homothallic fungus that can undergo sexual differentiation with cellular and morphological changes followed by multicellular tissue development to complete its sexual cycle. In this study, we identified and characterized the blue-light photoreceptor gene in S. fimicola The S. fimicola white collar-1 photoreceptor (SfWC-1) contains light-oxygen-voltage-sensing (LOV), Per-Arnt-Sim (PAS), and other conserved domains and is homologous to the WC-1 blue-light photoreceptor of Neurospora crassa The LOV domain of Sfwc-1 was deleted by homologous recombination using Agrobacterium-mediated protoplast transformation. The Sfwc-1(Δlov) mutant showed normal vegetative growth but produced less carotenoid pigment under illumination. The mutant showed delayed and less-pronounced fruiting-body formation, was defective in phototropism of the perithecial beaks, and lacked the fruiting-body zonation pattern compared with the wild type under the illumination condition. Gene expression analyses supported the light-induced functions of the Sfwc-1 gene in the physiology and developmental process of perithecial formation in S. fimicola Moreover, green fluorescent protein (GFP)-tagged SfWC-1 fluorescence signals were transiently strong upon light induction and prominently located inside the nuclei of living hyphae. Our studies focused on the putative blue-light photoreceptor in a model ascomycete and contribute to a better understanding of the photoregulatory functions and networks mediated by the evolutionarily conserved blue-light photoreceptors across diverse fungal phyla.IMPORTANCESordaria sp. has been a model for study of fruiting-body differentiation in fungi. Several environmental factors, including light, affect cellular and morphological changes during multicellular tissue development. Here, we created a light-oxygen-voltage-sensing (LOV) domain-deleted Sfwc-1 mutant to study blue-light photoresponses in Sordaria fimicola Phototropism and rhythmic zonation of perithecia were defective in the Sfwc-1(Δlov) mutant. Moreover, fruiting-body development in the mutant was reduced and also significantly delayed. Gene expression analysis and subcellular localization study further revealed the light-induced differential gene expression and cellular responses upon light stimulation in S. fimicola.

Deregulation of phytoene-β-carotene synthase results in derepression of astaxanthin synthesis at high glucose concentration in Phaffia rhodozyma astaxanthin-overproducing strain MK19

Background

A major obstacle to industrial-scale astaxanthin production by the yeast Phaffia rhodozyma is the strong inhibitory effect of high glucose concentration on astaxanthin synthesis. We investigated, for the first time, the mechanism of the regulatory effect of high glucose (> 100 g/L) at the metabolite and transcription levels.

Results

Total carotenoid, β-carotene, and astaxanthin contents were greatly reduced in wild-type JCM9042 at high (110 g/L) glucose; in particular, β-carotene content at 24–72 h was only 14–17% of that at low (40 g/L) glucose. The inhibitory effect of high glucose on astaxanthin synthesis appeared to be due mainly to repression of lycopene-to-β-carotene and β-carotene-to-astaxanthin steps in the pathway. Expression of carotenogenic genes crtE, pbs, and ast was also strongly inhibited by high glucose; such inhibition was mediated by creA, a global negative regulator of carotenogenic genes which is strongly induced by glucose. In contrast, astaxanthin-overproducing, glucose metabolic derepression mutant strain MK19 displayed de-inhibition of astaxanthin synthesis at 110 g/L glucose; this de-inhibition was due mainly to deregulation of pbs and ast expression, which in turn resulted from low creA expression. Failure of glucose to induce the genes reg1 and hxk2, which maintain CreA activity, also accounts for the fact that astaxanthin synthesis in MK19 was not repressed at high glucose.

Conclusion

We conclude that astaxanthin synthesis in MK19 at high glucose is enhanced primarily through derepression of carotenogenic genes (particularly pbs), and that this process is mediated by CreA, Reg1, and Hxk2 in the glucose signaling pathway.

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.

Induction of pigment production through media composition, abiotic and biotic factors in two filamentous fungi

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Pigment production and accumulation is dependent of high C:N ratios in F. oxysporum and A. chevaleri.

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Red pigment content of F. oxysporum in terms of Absorbance units per gram of biomass increased in 191% through use of blue light. Different light wavelengths stimulate synthesis of additional pigments in A. chevalieri with highest accumulation in red and UV-light.

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Stimulation of pigment production in co-culture is species – specific, being only accomplished in A. chevalieri. With a rise higher that 500% of a pigment obtained in green light.

In addition to plant-derived, fungal pigments have become an alternative in respect to synthetic ones. Besides Monascus sp., several pigment-producing fungi do not have culture conditions well-established yet. In this research, media composition, light wavelength and co-culture were evaluated, results were reported in Absorbance Units per gram of biomass (AU/Bgr). For Fusarium oxysporum a C:N ratio above 7 was advantageous, using both complex and defined media; blue LED light increased the AU/Bgr value from 18013 to 344; co-culture did not enhance pigment production. In Aspergillus chevalieri a high C:N ratio with glucose as carbon source was ideal. When exposing cultures to light, UV and red light gave the highest pigmentation; moreover, differential UV-VIS spectra in all wavelengths suggested production of additional pigments. Particularly a pigment observed when cultured in green light was also found in co-culture with yeast and there was an improvement of AU/Bgr value of 52549%. This is the first report regarding light effect and co-culture for these fungi, as well as C:N ratio for A. chevalieri.

Seeing the Light: The Roles of Red- and Blue-Light Sensing in Plant Microbes

Plants collect, concentrate, and conduct light throughout their tissues, thus enhancing light availability to their resident microbes. This review explores the role of photosensing in the biology of plant-associated bacteria and fungi, including the molecular mechanisms of red-light sensing by phytochromes and blue-light sensing by LOV (light-oxygen-voltage) domain proteins in these microbes. Bacteriophytochromes function as major drivers of the bacterial transcriptome and mediate light-regulated suppression of virulence, motility, and conjugation in some phytopathogens and light-regulated induction of the photosynthetic apparatus in a stem-nodulating symbiont. Bacterial LOV proteins also influence light-mediated changes in both symbiotic and pathogenic phenotypes. Although red-light sensing by fungal phytopathogens is poorly understood, fungal LOV proteins contribute to blue-light regulation of traits, including asexual development and virulence. Collectively, these studies highlight that plant microbes have evolved to exploit light cues and that light sensing is often coupled with sensing other environmental signals.

Expression of putative circadian clock components in the arbuscular mycorrhizal fungus Rhizoglomus irregulare

Arbuscular mycorrhizal fungi (AMF) are obligatory plant symbionts that live underground, so few studies have examined their response to light. Responses to blue light by other fungi can be mediated by White Collar-1 (WC-1) and WC-2 proteins. These wc genes, together with the frequency gene (frq), also form part of the endogenous circadian clock. The clock mechanism has never been studied in AMF, although circadian growth of their hyphae in the field has been reported. Using both genomic and transcriptomic data, we have found homologs of wc-1, wc-2, and frq and related circadian clock genes in the arbuscular mycorrhizal fungus Rhizoglomus irregulare (synonym Rhizophagus irregularis). Gene expression of wc-1, wc-2, and frq was analyzed using RT-qPCR on RNA extracted from germinating spores and from fungal material cultivated in vitro with transformed carrot roots. We found that all three core clock genes were expressed in both pre- and post-mycorrhizal stages of R. irregulare growth. Similar to the model fungus Neurospora crassa, the core circadian oscillator gene frq was induced by brief light stimulation. The presence of circadian clock and output genes in R. irregulare opens the door to the study of circadian clocks in the fungal partner of plant-AMF symbiosis. Our finding also provides new insight into the evolution of the circadian frq gene in fungi.

Comparative Transcriptome Analysis Between a Spontaneous Albino Mutant and Its Sibling Strain of Cordyceps militaris in Response to Light Stress

Albinism has been used for new variety screening in some edible mushrooms and the underlying mechanisms are fascinating. Albino fruiting body of Cordyceps militaris, a well-known edible fungus and model organism for Cordyceps, has the potential to be a nutraceutical or functional food due to its high content of metabolites and antioxidant activities. In this study, a spontaneous albino mutant strain (505) of C. militaris was obtained. In comparison to its normal sibling strain (498), the albino strain stably remained white in response to light and had significantly decreased conidia and carotenoid production but accumulated more cordycepin. Transcriptome analysis of both strains revealed that all the seven photoreceptors were expressed similarly in response to light. However, many more genes in the albino strain were differentially expressed in response to light than its sibling strain. The significantly enriched pathways in 498L vs. 505L were mainly associated with replication and repair. Some secondary metabolite backbone genes including those encoding DMAT, two NRPS-like proteins, three NPRS, and lanosterol synthase were differentially expressed in the albino when compared with that of the normal strains. Transcriptome and real-time quantitative PCR analyses indicated that some cytochrome P450s and methyltransferases might be related to the phenotypic differences observed between the two strains. This study compared the genome-wide transcriptional responses to light irradiation in a spontaneous albino mutant and its normal sibling strain of an edible fungus, and these findings potentially pave the way for further investigation of the pigment biosynthetic pathway.

The Two Cryptochrome/Photolyase Family Proteins Fulfill Distinct Roles in DNA Photorepair and Regulation of Conidiation in the Gray Mold Fungus Botrytis cinerea

The plant-pathogenic leotiomycete Botrytis cinerea is known for the strict regulation of its asexual differentiation programs by environmental light conditions. Sclerotia are formed in constant darkness; black/near-UV (NUV) light induces conidiation; and blue light represses both differentiation programs. Sensing of black/NUV light is attributed to proteins of the cryptochrome/photolyase family (CPF). To elucidate the molecular basis of the photoinduction of conidiation, we functionally characterized the two CPF proteins encoded in the genome of B. cinerea as putative positive-acting components. B. cinerea CRY1 (BcCRY1), a cyclobutane pyrimidine dimer (CPD) photolyase, acts as the major enzyme of light-driven DNA repair (photoreactivation) and has no obvious role in signaling. In contrast, BcCRY2, belonging to the cry-DASH proteins, is dispensable for photorepair but performs regulatory functions by repressing conidiation in white and especially black/NUV light. The transcription of bccry1 and bccry2 is induced by light in a White Collar complex (WCC)-dependent manner, but neither light nor the WCC is essential for the repression of conidiation through BcCRY2 when bccry2 is constitutively expressed. Further, BcCRY2 affects the transcript levels of both WCC-induced and WCC-repressed genes, suggesting a signaling function downstream of the WCC. Since both CPF proteins are dispensable for photoinduction by black/NUV light, the origin of this effect remains elusive and may be connected to a yet unknown UV-light-responsive system.IMPORTANCEBotrytis cinerea is an economically important plant pathogen that causes gray mold diseases in a wide variety of plant species, including high-value crops and ornamental flowers. The spread of disease in the field relies on the formation of conidia, a process that is regulated by different light qualities. While this feature has been known for a long time, we are just starting to understand the underlying molecular mechanisms. Conidiation in B. cinerea is induced by black/near-UV light, whose sensing is attributed to the action of cryptochrome/photolyase family (CPF) proteins. Here we report on the distinct functions of two CPF proteins in the photoresponse of B. cinerea While BcCRY1 acts as the major photolyase in photoprotection, BcCRY2 acts as a cryptochrome with a signaling function in regulating photomorphogenesis (repression of conidiation).

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.

The GATA-Type Transcription Factor Csm1 Regulates Conidiation and Secondary Metabolism in Fusarium fujikuroi

GATA-type transcription factors (TFs) such as the nitrogen regulators AreA and AreB, or the light-responsive TFs WC-1 and WC-2, play global roles in fungal growth and development. The conserved GATA TF NsdD is known as an activator of sexual development and key repressor of conidiation in Aspergillus nidulans, and as light-regulated repressor of macroconidia formation in Botrytis cinerea. In the present study, we functionally characterized the NsdD ortholog in Fusarium fujikuroi, named Csm1. Deletion of this gene resulted in elevated microconidia formation in the wild-type (WT) and restoration of conidiation in the non-sporulating velvet mutant Δvel1 demonstrating that Csm1 also plays a role as repressor of conidiation in F. fujikuroi. Furthermore, biosynthesis of the PKS-derived red pigments, bikaverin and fusarubins, is de-regulated under otherwise repressing conditions. Cross-species complementation of the Δcsm1 mutant with the B. cinerea ortholog LTF1 led to full restoration of WT-like growth, conidiation and pigment formation. In contrast, the F. fujikuroi CSM1 rescued only the defects in growth, the tolerance to H₂O₂ and virulence, but did not restore the light-dependent differentiation when expressed in the B. cinerea Δltf1 mutant. Microarray analysis comparing the expression profiles of the F. fujikuroi WT and the Δcsm1 mutant under different nitrogen conditions revealed a strong impact of this GATA TF on 19 of the 47 gene clusters in the genome of F. fujikuroi. One of the up-regulated silent gene clusters is the one containing the sesquiterpene cyclase-encoding key gene STC1. Heterologous expression of STC1 in Escherichia coli enabled us to identify the product as the volatile bioactive compound (-)-germacrene D.

Cloning and analysis of the Glwc-1 and Glwc-2 genes encoding putative blue light photoreceptor from Ganoderma lucidum

Blue light plays an important role during the growth of Ganoderma lucidum, one of the best-known medicinal macrofungi in China. In the present study, we cloned Glwc-1 and Glwc-2, the homologue of the blue light photoreceptors Ncwc-1 and Ncwc-2 of Neurospora crassa, from G. lucidum. The deduced amino acid sequence of Glwc-1 contained the similar function domains as NcWC-1 including LOV, PAS B, PAS C, and PAC domains. The deduced amino acid sequence of Glwc-2 contained PAS domain and GATA-type zinc finger (Znf) domain as well as NcWC-2. Phylogenetic analysis based on fungal WC-1 and WC-2 supported GlWC-1 and GlWC-2 were blue light receptors. The expression of Glwc-1 and Glwc-2 indicated that they might play an important role during the primordium differentiation process of G. lucidum, and the external blue light stimulation increased the expression of Glwc-1 and Glwc-2.

Filamentous ascomycetes fungi as a source of natural pigments

Filamentous fungi, including the ascomycetes Monascus, Fusarium, Penicillium and Neurospora, are being explored as novel sources of natural pigments with biological functionality for food, feed and cosmetic applications. Such edible fungi can be used in biorefineries for the production of ethanol, animal feed and pigments from waste sources. The present review gathers insights on fungal pigment production covering biosynthetic pathways and stimulatory factors (oxidative stress, light, pH, nitrogen and carbon sources, temperature, co-factors, surfactants, oxygen, tricarboxylic acid intermediates and morphology) in addition to pigment extraction, analysis and identification methods. Pigmentation is commonly regarded as the output of secondary protective mechanisms against oxidative stress and light. Although several studies have examined pigmentation in Monascus spp., research gaps exist in the investigation of interactions among factors as well as process development on larger scales under submerged and solid-state fermentation. Currently, research on pigmentation in Neurospora spp. is at its infancy, but the increasing interest for biorefineries shows potential for booming research in this area.

Light regulation of mycotoxin biosynthesis: new perspectives for food safety

Mycotoxins are secondary metabolites produced by toxigenic fungi contaminating foods and feeds in pre-, post-harvest and processing, and represent a great concern worldwide, both for the economic implications and for the health of the consumers. Many environmental conditions are involved in the regulation of mycotoxin biosynthesis. Among these, light represents one of the most important signals for fungi, influencing several physiological responses such as pigmentation, sexual development and asexual conidiation, primary and secondary metabolism, including mycotoxin biosynthesis. In this review we summarise some recent findings on the effect of specific light wavelength and intensity on mycotoxin biosynthesis in the main toxigenic fungal genera. We describe the molecular mechanism underlying light perception and its involvement in the regulation of secondary metabolism, focusing on VeA, global regulator in Aspergillus nidulans, and the White-Collar proteins, key components of light response in Neurospora crassa. Light of specific wavelength and intensity exerts different effects both on growth and on toxin production depending on the fungal genus. In Penicillium spp. red (627 nm) and blue wavelengths (455-470 nm) reduce ochratoxin A (OTA) biosynthesis by modulating the level of expression of the ochratoxin polyketide synthase. Furthermore a mutual regulation between citrinin and OTA production is reported in Penicillium toxigenic species. In Aspergillus spp. the effect of light treatment is strongly dependent on the species and culture conditions. Royal blue wavelength (455 nm) of high intensity (1,700 Lux) is capable of completely inhibit fungal growth and OTA production in Aspergillus stenyii and Penicillum verrucosum. In Fusarium spp. the effect of light exposure is less effective; mycotoxin-producing species, such as Fusarium verticillioides and Fusarium proliferatum, grow better under light conditions, and fumonisin production increased. This review provides a comprehensive picture on light regulation of mycotoxin biosynthesis and discusses possible new applications of this resource in food safety.

Seeing the world differently: variability in the photosensory mechanisms of two model fungi

Light plays an important role for most organisms on this planet, serving either as a source of energy or information for the adaptation of biological processes to specific times of day. The fungal kingdom is estimated to contain well over a million species, possibly 10-fold more, and it is estimated that a majority of the fungi respond to light, eliciting changes in several physiological characteristics including pathogenesis, development and secondary metabolism. Two model organisms for photobiological studies have taken centre-stage over the last few decades--Neurospora crassa and Aspergillus nidulans. In this review, we will first discuss our understanding of the light response in N. crassa, about which the most is known, and will then juxtapose N. crassa with A. nidulans, which, as will be described below, provides an excellent template for understanding photosensory cross-talk. Finally, we will end with a commentary on the variability of the light response among other relevant fungi, and how our molecular understanding in the aforementioned model organisms still provides a strong base for dissecting light responses in such species.

The blue-light receptor CmWC-1 mediates fruit body development and secondary metabolism in Cordyceps militaris

Light is an essential factor for pigment formation and fruit body development in Cordyceps militaris, a well-known edible and medicinal fungus. Cmwc-1, a homolog of the blue-light receptor gene white collar-1 (wc-1) in Neurospora crassa, was cloned from the C. militaris genome in our previous study. Here, Cmwc-1 gene inactivation results in thicker aerial hyphae, disordered fruit body development, a significant reduction in conidial formation, and carotenoid and cordycepin production. These characteristics were restored when the ΔCmwc-1 strains were hybridized with wild-type strains of the opposite mating type. A genome-wide expression analysis revealed that there were 1042 light-responsive genes in the wild-type strain and only 458 in the ΔCmwc-1 strain. Among five putative photoreceptors identified, Vivid, cryptochrome-1, and cyclobutane pyrimidine dimer photolyase are strongly induced by light in a Cmwc-1-dependent manner, while phytochrome and cryptochrome-2 were not induced. The transcription factors involved in the fungal light reaction were mainly of the Zn2Cys6 type. CmWC-1 regulates adenylosuccinate synthase, an important enzyme for adenosine de novo synthesis, which could explain the reduction in cordycepin production. Some G protein-coupled receptors that control fungal fruit body formation and the sexual cycle were regulated by CmWC-1, and the cAMP pathway involved in light signal transduction in N. crassa was not critical for the photoreaction in the fungus here. A transcriptional analysis indicated that steroid biosynthesis was more active in the ΔCmwc-1 strain, suggesting that CmWC-1 might switch the vegetative growth state to primordia differentiation by suppressing the expression of related genes.

The white collar complex is involved in sexual development of Fusarium graminearum

Sexual spores (ascospores) of Fusarium graminearum, a homothallic ascomycetous fungus, are believed to be the primary inocula for epidemics of the diseases caused by this species in cereal crops. Based on the light requirement for the formation of fruiting bodies (perithecia) of F. graminearum under laboratory conditions, we explored whether photoreceptors play an important role in sexual development. Here, we evaluated the roles of three genes encoding putative photoreceptors [a phytochrome gene (FgFph) and two white collar genes (FgWc-1 and FgWc-2)] during sexual development in F. graminearum. For functional analyses, we generated transgenic strains lacking one or two genes from the self-fertile Z3643 strain. Unlike the wild-type (WT) and add-back strains, the single deletion strains (ΔFgWc-1 and ΔFgWc-2) produced fertile perithecia under constant light on complete medium (CM, an unfavorable medium for sexual development) as well as on carrot agar (a perithecial induction condition). The expression of mating-type (MAT) genes increased significantly in the gene deletion strains compared to the WT under both conditions. Deletion of FgFph had no significant effect on sexual development or MAT gene expression. In contrast, all of the deletion strains examined did not show significant changes in other traits such as hyphal growth, mycotoxin production, and virulence. A split luciferase assay confirmed the in vivo protein-protein interactions among three photoreceptors along with FgLaeA, a global regulator of secondary metabolism and fungal development. Introduction of an intact copy of the A. nidulans LreA and LreB genes, which are homologs of FgWc-1 and FgWc-2, into the ΔFgWc-1 and ΔFgWc-2 strains, respectively, failed to repress perithecia formation on CM in the gene deletion strains. Taken together, these results demonstrate that FgWc-1 and FgWc-2, two central components of the blue-light sensing system, negatively regulate sexual development in F. graminearum, which differs from the regulation pattern in A. nidulans.

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.

Fungal photobiology: visible light as a signal for stress, space and time

Visible light is an important source of energy and information for much of life on this planet. Though fungi are neither photosynthetic nor capable of observing adjacent objects, it is estimated that the majority of fungal species display some form of light response, ranging from developmental decision-making to metabolic reprogramming to pathogenesis. As such, advances in our understanding of fungal photobiology will likely reach the broad fields impacted by these organisms, including agriculture, industry and medicine. In this review, we will first describe the mechanisms by which fungi sense light and then discuss the selective advantages likely imparted by their ability to do so.

Biological roles of fungal carotenoids

Carotenoids are terpenoid pigments widespread in nature, produced by bacteria, fungi, algae and plants. They are also found in animals, which usually obtain them through the diet. Carotenoids in plants provide striking yellow, orange or red colors to fruits and flowers, and play important metabolic and physiological functions, especially relevant in photosynthesis. Their functions are less clear in non-photosynthetic microorganisms. Different fungi produce diverse carotenoids, but the mutants unable to produce them do not exhibit phenotypic alterations in the laboratory, apart of lack of pigmentation. This review summarizes the current knowledge on the functional basis for carotenoid production in fungi. Different lines of evidence support a protective role of carotenoids against oxidative stress and exposure to visible light or UV irradiation. In addition, the carotenoids are intermediary products in the biosynthesis of physiologically active apocarotenoids or derived compounds. This is the case of retinal, obtained from the symmetrical oxidative cleavage of β-carotene. Retinal is the light-absorbing prosthetic group of the rhodopsins, membrane-bound photoreceptors present also in many fungal species. In Mucorales, β-carotene is an intermediary in the synthesis of trisporoids, apocarotenoid derivatives that include the sexual hormones the trisporic acids, and they are also presumably used in the synthesis of sporopollenin polymers. In conclusion, fungi have adapted their ability to produce carotenoids for different non-essential functions, related with stress tolerance or with the synthesis of physiologically active by-products.

Photo morphogenesis and photo response of the blue-light receptor gene Cmwc-1 in different strains of Cordyceps militaris

Light is a necessary environmental factor for stroma formation and development of Cordyceps militaris, a well-known edible and medicinal fungus. In this study, photo morphogenesis and the blue-light receptor gene were studied using five representative strains of C. militaris. The results suggest that light was essential for colony pigmentation and could promote conidia production. Cmwc-1, the homologe of the blue-light photoreceptor of Neurospora crassa, was cloned from the genome of C. militaris by Hi-tail PCR. The protein CmWC-1 was characterized by the presence of the LOV and PAS domains and a GATA-type Znf domain. Genetic variation analysis of Cmwc-1 in different strains showed that 15-bp deletions occurred in three strains that resulted in 5-Gln deletions in the transcription activation domain. Phylogenetic analysis based on the Sordariomycetes WC-1-like proteins suggested that the sequence of WC-1 could be used as a candidate marker for phylogenetic analysis in fungi. Cmwc-1 mRNA was light inducible and the expression level increased significantly after irradiation in all tested strains. The sequence of CmWC-1 and the relative expressions responding to irradiation in degenerate and albino strains were similar as the cultivated one. This report will help to open the still-unexplored field of stroma development for this fungus.

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 affects fumonisin production in strains of Fusarium fujikuroi, Fusarium proliferatum, and Fusarium verticillioides isolated from rice

Three Fusarium species associated with bakanae disease of rice (Fusarium fujikuroi, Fusarium proliferatum, and Fusarium verticillioides) were investigated for their ability to produce fumonisins (FB1 and FB2) under different light conditions, and for pathogenicity. Compared to darkness, the conditions that highly stimulated fumonisin production were yellow and green light in F. verticillioides strains; white and blue light, and light/dark alternation in F. fujikuroi and F. proliferatum strains. In general, all light conditions positively influenced fumonisin production with respect to the dark. Expression of the FUM1 gene, which is necessary for the initiation of fumonisin production, was in accordance with the fumonisin biosynthetic profile. High and low fumonisin-producing F. fujikuroi strains showed typical symptoms of bakanae disease, abundant fumonisin-producing F. verticillioides strains exhibited chlorosis and stunting of rice plants, while fumonisin-producing F. proliferatum strains were asymptomatic on rice. We report that F. fujikuroi might be an abundant fumonisin producer with levels comparable to that of F. verticillioides and F. proliferatum, highlighting the need of deeper mycotoxicological analyses on rice isolates of F. fujikuroi. Our results showed for the first time the influence of light on fumonisin production in isolates of F. fujikuroi, F. proliferatum, and F. verticillioides from rice.

The fungal pathogen Aspergillus fumigatus regulates growth, metabolism, and stress resistance in response to light

Light is a pervasive environmental factor that regulates development, stress resistance, and even virulence in numerous fungal species. Though much research has focused on signaling pathways in Aspergillus fumigatus, an understanding of how this pathogen responds to light is lacking. In this report, we demonstrate that the fungus does indeed respond to both blue and red portions of the visible spectrum. Included in the A. fumigatus light response is a reduction in conidial germination rates, increased hyphal pigmentation, enhanced resistance to acute ultraviolet and oxidative stresses, and an increased susceptibility to cell wall perturbation. By performing gene deletion analyses, we have found that the predicted blue light receptor LreA and red light receptor FphA play unique and overlapping roles in regulating the described photoresponsive behaviors of A. fumigatus. However, our data also indicate that the photobiology of this fungus is complex and likely involves input from additional photosensory pathways beyond those analyzed here. Finally, whole-genome microarray analysis has revealed that A. fumigatus broadly regulates a variety of metabolic genes in response to light, including those involved in respiration, amino acid metabolism, and metal homeostasis. Together, these data demonstrate the importance of the photic environment on the physiology of A. fumigatus and provide a basis for future studies into this unexplored area of its biology.