The Photoreceptor Components FaWC1 and FaWC2 of Fusarium asiaticum Cooperatively Regulate Light Responses but Play Independent Roles in Virulence Expression

The GATA transcription factor BcWCL2 regulates citric acid secretion to maintain redox homeostasis and full virulence in Botrytis cinerea

Botrytis cinerea is a typical necrotrophic plant pathogenic fungus which can deliberately acidify host tissues and trigger oxidative bursts therein to facilitate its virulence. The white collar complex (WCC), consisting of BcWCL1 and BcWCL2, is recognized as the primary light receptor in B. cinerea. Nevertheless, the specific mechanisms through which the WCC components, particularly BcWCL2 as a GATA transcription factor, control virulence are not yet fully understood. This study demonstrates that deletion of BcWCL2 results in the loss of light-sensitive phenotypic characteristics. Additionally, the Δbcwcl2 strain exhibits reduced secretion of citrate, delayed infection cushion development, weaker hyphal penetration, and decreased virulence. The application of exogenous citric acid was found to restore infection cushion formation, hyphal penetration, and virulence of the Δbcwcl2 strain. Transcriptome analysis at 48 h post-inoculation revealed that two citrate synthases, putative citrate transporters, hydrolytic enzymes, and reactive oxygen species scavenging-related genes were down-regulated in Δbcwcl2, whereas exogenous citric acid application restored the expression of the above genes involved in the early infection process of Δbcwcl2. Moreover, the expression of Bcvel1, a known regulator of citrate secretion, tissue acidification, and secondary metabolism, was down-regulated in Δbcwcl2 but not in Δbcwcl1. ChIP-qPCR and electrophoretic mobility shift assays revealed that BcWCL2 can bind to the promoter sequences of Bcvel1. Overexpressing Bcvel1 in Δbcwcl2 was found to rescue the mutant defects. Collectively, our findings indicate that BcWCL2 regulates the expression of the global regulator Bcvel1 to influence citrate secretion, tissue acidification, redox homeostasis, and virulence of B. cinerea.IMPORTANCEThis study illustrated the significance of the fungal blue light receptor component BcWCL2 protein in regulating citrate secretion in Botrytis cinerea. Unlike BcWCL1, BcWCL2 may contribute to redox homeostasis maintenance during infection cushion formation, ultimately proving to be essential for full virulence. It is also demonstrated that BcWCL2 can regulate the expression of Bcvel1 to influence host tissue acidification, citrate secretion, infection cushion development, and virulence. While the role of organic acids secreted by plant pathogenic fungi in fungus-host interactions has been recognized, this paper revealed the importance, regulatory mechanisms, and key transcription factors that control organic acid secretion. These understanding of the pathogenetic mechanism of plant pathogens can provide valuable insights for developing effective prevention and treatment strategies against fungal diseases.

The Signal Peptidase FoSpc2 Is Required for Normal Growth, Conidiation, Virulence, Stress Response, and Regulation of Light Sensitivity in Fusarium odoratissimum

Signal peptidase (SPase) is responsible for cleavage of N-terminal signal peptides in most secretory precursor proteins and many membrane proteins during maturation. In this study, we identified four components of the SPase complex (FoSec11, FoSpc1, FoSpc2, and FoSpc3) in the banana wilt fungal pathogen Fusarium odoratissimum. We proved that interactions exist among the four SPase subunits by bimolecular fluorescence complementation (BiFC) and affinity purification and mass spectrometry (AP-MS) assays. Among the four SPase genes, FoSPC2 was successfully deleted. FoSPC2 deletion caused defects in vegetative growth, conidiation, and virulence. Loss of FoSPC2 also affected the secretion of some pathogenicity-related extracellular enzymes, suggesting that SPase without FoSpc2 may have a lower efficiency in managing the maturation of the extracellular enzymes in F. odoratissimum. In addition, we found that the ΔFoSPC2 mutant had increased sensitivity to light, and the colonies of the mutant grew faster under all-dark conditions than under all-light conditions. We further observed that deletion of FoSPC2 affected expression of the blue light photoreceptor gene FoWC2, leading to cytoplasmic accumulation of FoWc2 under all-light conditions. Since FoWc2 has signal peptides, FoSpc2 may regulate the expression and subcellular localization of FoWc2 indirectly. Contrary to its response to light, the ΔFoSPC2 mutant displayed a significant decreased sensitivity to osmotic stress, and culturing the mutant under osmotic stress conditions restored both the localization of FoWc2 and light sensitivity of the ΔFoSPC2, suggesting that a cross talk between osmotic stress and light response pathways in F. odoratissimum and FoSpc2 takes part in these processes. IMPORTANCE In this study, we identified four components of SPase in the banana wilt pathogen Fusarium odoratissimum and characterized the SPase FoSpc2. Loss of FoSPC2 affected the secretion of extracellular enzymes, suggesting that SPase without FoSpc2 may have a lower efficiency in managing the maturation of the extracellular enzymes in F. odoratissimum. In addition, this is the first time that we have found a relationship between the SPase and fungal light response. Deletion of FoSPC2 resulted in decreased sensitivity to the osmotic stresses but with increased sensitivity to light. Continuous light inhibited the growth rate of the ΔFoSPC2 mutant and affected the cellular localization of the blue light photoreceptor FoWc2 in this mutant, but culturing the mutant under osmotic stress both restored the localization of FoWc2 and eliminated the light sensitivity of the ΔFoSPC2 mutant, suggesting that loss of FoSPC2 may affect a cross talk between the osmotic stress and light response pathways in F. odoratissimum.

White Collar 1 Modulates Oxidative Sensitivity and Virulence by Regulating the HOG1 Pathway in Fusarium asiaticum

Fusarium asiaticum is an epidemiologically important pathogen of cereal crops in east Asia, accounting for both yield losses and mycotoxin contamination problems in food and feed products. FaWC1, a component of the blue-light receptor White Collar complex (WCC), relies on its transcriptional regulatory zinc finger domain rather than the light-oxygen-voltage domain to regulate pathogenicity of F. asiaticum, although the downstream mechanisms remain obscure. In this study, the pathogenicity factors regulated by FaWC1 were analyzed. It was found that loss of FaWC1 resulted in higher sensitivity to reactive oxygen species (ROS) than in the wild type, while exogenous application of the ROS quencher ascorbic acid restored the pathogenicity of the ΔFawc1 strain to the level of the wild type, indicating that the reduced pathogenicity of the ΔFawc1 strain is due to a defect in ROS tolerance. Moreover, the expression levels of the high-osmolarity glycerol (HOG) mitogen-activated protein kinase (MAPK) pathway genes and their downstream genes encoding ROS scavenging enzymes were downregulated in the ΔFawc1 mutant. Upon ROS stimulation, the FaHOG1-green fluorescent protein (GFP)-expressing signal driven by the native promoter was inducible in the wild type but negligible in the ΔFawc1 strain. Overexpressing Fahog1 in the ΔFawc1 strain could recover the ROS tolerance and pathogenicity of the ΔFawc1 mutant, but it remained defective in light responsiveness. In summary, this study dissected the roles of the blue-light receptor component FaWC1 in regulating expression levels of the intracellular HOG-MAPK signaling pathway to affect ROS sensitivity and pathogenicity in F. asiaticum. IMPORTANCE The well-conserved fungal blue-light receptor White Collar complex (WCC) is known to regulate virulence of several pathogenic species for either plant or human hosts, but how WCC determines fungal pathogenicity remains largely unknown. The WCC component FaWC1 in the cereal pathogen Fusarium asiaticum was previously found to be required for full virulence. The present study dissected the roles of FaWC1 in regulating the intracellular HOG MAPK signaling pathway to affect ROS sensitivity and pathogenicity in F. asiaticum. This work thus extends knowledge of the association between fungal light receptors and the intracellular stress signaling pathway to regulate oxidative stress tolerance and pathogenicity in an epidemiologically important fungal pathogen of cereal crops.

The facilitating effect of blue light on the antifungal agent susceptibilities of passaged conidia from the ocular-derived Fusarium solani species complex

The eye is a light-receiving organ and has anatomical advantages to accept phototherapy. Fungi colonizing on the eyes, which cause ocular mycoses, are affected by daily blue light and could easily accept additional light irritation. Ocular mycoses are recalcitrant and blindness-causing eye diseases, and antifungal agent treatments are insufficient. Our team previously found that blue light could inhibit Fusarium solani hyphal growth but promote conidiation. Here, we investigated the antifungal susceptibilities and biological characteristics of the passaged conidia. Twelve Fusarium solani strains (11 ocular-derived strains and 1 standard laboratory strain) were inoculated under blue light (0.5 mW/cm2) and darkness conditions, respectively, to obtain the passaged conidia of blue light group (n = 12) and darkness group (n = 12). Two groups were tested to determine the growth abilities and in vitro antifungal susceptibilities to five antifungal drugs (voriconazole (VRC), amphotericin B (AMB), terbinafine (TRB), caspofungin (CAS), and 5-flucytosine (5FC)), which were examined by microscopy for morphological observation and spectrophotometry for turbidity analysis. The results showed that blue light group passaged conidia were more sensitive to antifungal drugs (AMB, VRC, TRB, and CAS) compared to darkness group. The MIC50 of VRC significantly decreased after blue light treatment (P < 0.05). The fungal inhibition rate significantly increased for VRC, AMB, and TRB in the low concentration range (P < 0.05 or P < 0.01). Blue light did not affect germination or hyphal extension of passaged conidia. These results suggested that blue light could facilitate fungal inhibition effect of AMB, VRC, TRB, and CAS and may improve the therapeutic efficiency in VRC and AMB clinical applications. Blue light phototherapy may provide a new adjuvant approach for the treatment of ocular mycosis.

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.

Current progress on pathogenicity-related transcription factors in Fusarium oxysporum

Fusarium oxysporum is a well-known soilborne plant pathogen that causes severe vascular wilt in economically important crops worldwide. During the infection process, F. oxysporum not only secretes various virulence factors, such as cell wall-degrading enzymes (CWDEs), effectors, and mycotoxins, that potentially play important roles in fungal pathogenicity but it must also respond to extrinsic abiotic stresses from the environment and the host. Over 700 transcription factors (TFs) have been predicted in the genome of F. oxysporum, but only 26 TFs have been functionally characterized in various formae speciales of F. oxysporum. Among these TFs, a total of 23 belonging to 10 families are required for pathogenesis through various mechanisms and pathways, and the zinc finger TF family is the largest family among these 10 families, which consists of 15 TFs that have been functionally characterized in F. oxysporum. In this review, we report current research progress on the 26 functionally analysed TFs in F. oxysporum and sort them into four groups based on their roles in F. oxysporum pathogenicity. Furthermore, we summarize and compare the biofunctions, involved pathways, putative targets, and homologs of these TFs and analyse the relationships among them. This review provides a systematic analysis of the regulation of virulence-related genes and facilitates further mechanistic analysis of TFs important in F. oxysporum virulence.

A light life together: photosensing in the plant microbiota

Bacteria and fungi of the plant microbiota can be phytopathogens, parasites or symbionts that establish mutually advantageous relationships with plants. They are often rich in photoreceptors for UVA-Visible light, and in many cases, they exhibit light regulation of growth patterns, infectivity or virulence, reproductive traits, and production of pigments and of metabolites. In addition to the light-driven effects, often demonstrated via the generation of photoreceptor gene knock-outs, microbial photoreceptors can exert effects also in the dark. Interestingly, some fungi switch their attitude towards plants in dependence of illumination or dark conditions in as much as they may be symbiotic or pathogenic. This review summarizes the current knowledge about the roles of light and photoreceptors in plant-associated bacteria and fungi aiming at the identification of common traits and general working ideas. Still, reports on light-driven infection of plants are often restricted to the description of macroscopically observable phenomena, whereas detailed information on the molecular level, e.g., protein-protein interaction during signal transduction or induction mechanisms of infectivity/virulence initiation remains sparse. As it becomes apparent from still only few molecular studies, photoreceptors, often from the red- and the blue light sensitive groups interact and mutually modulate their individual effects. The topic is of great relevance, even in economic terms, referring to plant-pathogen or plant-symbionts interactions, considering the increasing usage of artificial illumination in greenhouses, the possible light-regulation of the synthesis of plant-growth stimulating substances or herbicides by certain symbionts, and the biocontrol of pests by selected fungi and bacteria in a sustainable agriculture.

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.

The rise and shine of yeast optogenetics

Optogenetics refers to the control of biological processes with light. The activation of cellular phenomena by defined wavelengths has several advantages compared with traditional chemically inducible systems, such as spatiotemporal resolution, dose-response regulation, low cost, and moderate toxic effects. Optogenetics has been successfully implemented in yeast, a remarkable biological platform that is not only a model organism for cellular and molecular biology studies, but also a microorganism with diverse biotechnological applications. In this review, we summarize the main optogenetic systems implemented in the budding yeast Saccharomyces cerevisiae, which allow orthogonal control (by light) of gene expression, protein subcellular localization, reconstitution of protein activity, and protein sequestration by oligomerization. Furthermore, we review the application of optogenetic systems in the control of metabolic pathways, heterologous protein production and flocculation. We then revise an example of a previously described yeast optogenetic switch, named FUN-LOV, which allows precise and strong activation of the target gene. Finally, we describe optogenetic systems that have not yet been implemented in yeast, which could therefore be used to expand the panel of available tools in this biological chassis. In conclusion, a wide repertoire of optogenetic systems can be used to address fundamental biological questions and broaden the biotechnological toolkit in yeast.

The wheat pathogen Zymoseptoria tritici senses and responds to different wavelengths of light

Background

The ascomycete fungus Zymoseptoria tritici (synonyms: Mycosphaerella graminicola, Septoria tritici) is a major pathogen of wheat that causes the economically important foliar disease Septoria tritici blotch. Despite its importance as a pathogen, little is known about the reaction of this fungus to light. To test for light responses, cultures of Z. tritici were grown in vitro for 16-h days under white, blue or red light, and their transcriptomes were compared with each other and to those obtained from control cultures grown in darkness.

Results

There were major differences in gene expression with over 3400 genes upregulated in one or more of the light conditions compared to dark, and from 1909 to 2573 genes specifically upregulated in the dark compared to the individual light treatments. Differences between light treatments were lower, ranging from only 79 differentially expressed genes in the red versus blue comparison to 585 between white light and red. Many of the differentially expressed genes had no functional annotations. For those that did, analysis of the Gene Ontology (GO) terms showed that those related to metabolism were enriched in all three light treatments, while those related to growth and communication were more prevalent in the dark. Interestingly, genes for effectors that have been shown previously to be involved in pathogenicity also were upregulated in one or more of the light treatments, suggesting a possible role of light for infection.

Conclusions

This analysis shows that Z. tritici can sense and respond to light with a huge effect on transcript abundance. High proportions of differentially expressed genes with no functional annotations illuminates the huge gap in our understanding of light responses in this fungus. Differential expression of genes for effectors indicates that light could be important for pathogenicity; unknown effectors may show a similar pattern of transcription. A better understanding of the effects of light on pathogenicity and other biological processes of Z. tritici could help to manage Septoria tritici blotch in the future.