Fusarins and Fusaric Acid in Fusaria

Spectral Photoluminescent Parameters of Barley Seeds (Hordéum vulgáre) Infected with Fusarium ssp

We needed effective and sustainable technologies for better microbiological control of crops, including Fusarium. However, photoluminescent UV-Vis methods are potential for diagnosing plant diseases with Fusarium. It has not been sufficiently studied despite the application of these methods for other biological researches. The excitation spectrum of the seeds during infection shifts to the shorter wavelength and a new maximum appears in the region λ ≈ 232 nm. The photoluminescence of infected seeds increases with excitation by radiation of wavelengths λ_e,1  = 232 nm, λ_e,2  = 362 nm and λ_e,3  = 424 nm by 1.33-3.14 times, and λ_e,3  = 424 nm-decreases by 1.1 times. Statistical moments μ₃ and μ₄ , asymmetry and kurtosis change only with short-wave excitation. When analyzing the decomposition of the frequency spectrum into Gaussian curves, the most informative ratio is the ratio of right-handed and left-handed Gaussians under excitation λ_e,2  = 362 nm and λ_e,3  = 424 nm. The ratios of their maxima change during infection by 1.36-3.2 times, and for excitation by radiation λ_e,2 , the frequency boundaries of Gaussians change. The results of measurements and calculations provide a basis for the development of a method and device for photoluminescence diagnostics of fusarium seeds in UV-Vis ranges.

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.

Biological Management of Banana Fusarium Wilt Caused by Fusarium oxysporum f. sp. cubense Tropical Race 4 Using Antagonistic Fungal Isolate CSR-T-3 (Trichoderma reesei)

Fusarium wilt in bananas is one of the most devastating diseases that poses a serious threat to the banana industry globally. With no effective control measures available to date, biological control has been explored to restrict the spread and manage the outbreak. We studied the effective biological control potential of different Trichoderma spp. in the management of Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4). Expression of the defense related genes and metabolites in banana plants inoculated with Foc TR4 and treated with effective Trichoderma sp interactions were also studied. The in vitro growth inhibition of Foc TR4 by Trichoderma reesei isolate CSR-T-3 was 85.19% indicating a higher antagonistic potential than other Trichoderma isolates used in the study. Further, in in vivo assays, the banana plants treated with the isolate CSR-T-3 T. reesei had a significant reduction in the disease severity index (0.75) and also had increased phenological indices with respect to Foc TR4 treated plants. Enhanced activity of defense enzymes, such as β-1, 3-glucanase, peroxidase, chitinase, polyphenol oxidase, and phenylalanine ammonia lyase with higher phenol contents were found in the Trichoderma isolate CSR-T-3 treated banana plants challenge-inoculated with Foc TR4. Fusarium toxins, such as fusaristatin A, fusarin C, chlamydosporal, and beauveric acid were identified by LC-MS in Foc TR4-infected banana plants while high intensity production of antifungal compounds, such as ß-caryophyllene, catechin-o-gallate, soyasapogenol rhamnosyl glucoronide, peptaibols, fenigycin, iturin C19, anthocyanin, and gallocatechin-o-gallate were detected in T. reesei isolate CSR-T-3 treated plants previously inoculated with Foc TR4. Gene expression analysis indicated the upregulation of TrCBH1/TrCBH2, TrXYL1, TrEGL1, TrTMK1, TrTGA1, and TrVEL1 genes in CSR-T-3 treatment. LC-MS and gene expression analysis could ascertain the upregulation of genes involved in mycoparasitism and the signal transduction pathway leading to secondary metabolite production under CSR-T-3 treatment. The plants in the field study showed a reduced disease severity index (1.14) with high phenological growth and yield indices when treated with T. reesei isolate CSR-T-3 formulation. We report here an effective biocontrol-based management technological transformation from lab to the field for successful control of Fusarium wilt disease caused by Foc TR4 in bananas.

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.

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.

HPLC Analysis of Carotenoids in Neurosporaxanthin-Producing Fungi

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

Two separate key enzymes and two pathway-specific transcription factors are involved in fusaric acid biosynthesis in Fusarium fujikuroi

Fusaric acid (FSA) is a mycotoxin produced by several fusaria, including the rice pathogen Fusarium fujikuroi. Genes involved in FSA biosynthesis were previously identified as a cluster containing a polyketide synthase (PKS)-encoding (FUB1) and four additional genes (FUB2-FUB5). However, the biosynthetic steps leading to FSA as well as the origin of the nitrogen atom, which is incorporated into the polyketide backbone, remained unknown. In this study, seven additional cluster genes (FUB6-FUB12) were identified via manipulation of the global regulator FfSge1. The extended FUB gene cluster encodes two Zn(II)2 Cys6 transcription factors: Fub10 positively regulates expression of all FUB genes, whereas Fub12 is involved in the formation of the two FSA derivatives, i.e. dehydrofusaric acid and fusarinolic acid, serving as a detoxification mechanism. The major facilitator superfamily transporter Fub11 functions in the export of FSA out of the cell and is essential when FSA levels become critical. Next to Fub1, a second key enzyme was identified, the non-canonical non-ribosomal peptide synthetase Fub8. Chemical analyses of generated mutant strains allowed for the identification of a triketide as PKS product and the proposition of an FSA biosynthetic pathway, thereby unravelling the unique formation of a hybrid metabolite consisting of this triketide and an amino acid moiety.