Bioavailability and provitamin A activity of neurosporaxanthin in mice

Various species of ascomycete fungi synthesize the carboxylic carotenoid neurosporaxanthin. The unique chemical structure of this xanthophyll reveals that: (1) Its carboxylic end and shorter length increase the polarity of neurosporaxanthin in comparison to other carotenoids, and (2) it contains an unsubstituted β-ionone ring, conferring the potential to form vitamin A. Previously, neurosporaxanthin production was optimized in Fusarium fujikuroi, which allowed us to characterize its antioxidant properties in in vitro assays. In this study, we assessed the bioavailability of neurosporaxanthin compared to other provitamin A carotenoids in mice and examined whether it can be cleaved by the two carotenoid-cleaving enzymes: β-carotene-oxygenase 1 (BCO1) and 2 (BCO2). Using Bco1-/-Bco2-/- mice, we report that neurosporaxanthin displays greater bioavailability than β-carotene and β-cryptoxanthin, as evidenced by higher accumulation and decreased fecal elimination. Enzymatic assays with purified BCO1 and BCO2, together with feeding studies in wild-type, Bco1-/-, Bco2-/-, and Bco1-/-Bco2-/- mice, revealed that neurosporaxanthin is a substrate for either carotenoid-cleaving enzyme. Wild-type mice fed neurosporaxanthin displayed comparable amounts of vitamin A to those fed β-carotene. Together, our study unveils neurosporaxanthin as a highly bioavailable fungal carotenoid with provitamin A activity, highlighting its potential as a novel food additive.

HmbC, a Protein of the HMG Family, Participates in the Regulation of Carotenoid Biosynthesis in Fusarium fujikuroi

In the fungus Fusarium fujikuroi, carotenoid production is up-regulated by light and down-regulated by the CarS RING finger protein, which modulates the mRNA levels of carotenoid pathway genes (car genes). To identify new potential regulators of car genes, we used a biotin-mediated pull-down procedure to detect proteins capable of binding to their promoters. We focused our attention on one of the proteins found in the screening, belonging to the High-Mobility Group (HMG) family that was named HmbC. The deletion of the hmbC gene resulted in increased carotenoid production due to higher mRNA levels of car biosynthetic genes. In addition, the deletion resulted in reduced carS mRNA levels, which could also explain the partial deregulation of the carotenoid pathway. The mutants exhibited other phenotypic traits, such as alterations in development under certain stress conditions, or reduced sensitivity to cell wall degrading enzymes, revealed by less efficient protoplast formation, indicating that HmbC is also involved in other cellular processes. In conclusion, we identified a protein of the HMG family that participates in the regulation of carotenoid biosynthesis. This is probably achieved through an epigenetic mechanism related to chromatin structure, as is frequent in this class of proteins.

Fusarium Photoreceptors

Light is an important modulating signal in fungi. Fusarium species stand out as research models for their phytopathogenic activity and their complex secondary metabolism. This includes the synthesis of carotenoids, whose induction by light is their best known photoregulated process. In these fungi, light also affects other metabolic pathways and developmental stages, such as the formation of conidia. Photoreceptor proteins are essential elements in signal transduction from light. Fusarium genomes contain genes for at least eight photoreceptors: two flavoproteins, one photolyase, two cryptochromes, two rhodopsins, and one phytochrome. Mutations in five of these genes provide information about their functions in light regulation, in which the flavoprotein WcoA, belonging to the White Collar (WC) family, plays a predominant role. Global transcriptomic techniques have opened new perspectives for the study of photoreceptor functions and have recently been used in Fusarium fujikuroi on a WC protein and a cryptochrome from the DASH family. The data showed that the WC protein participates in the transcriptional control of most of the photoregulated genes, as well as of many genes not regulated by light, while the DASH cryptochrome potentially plays a supporting role in the photoinduction of many genes.

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

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

The carP lncRNA Is a carS-Related Regulatory Element with Broad Effects on the Fusarium fujikuroi Transcriptome

Carotenoid biosynthesis in the fungus Fusarium fujikuroi is regulated by environmental factors, with light being the main stimulating signal. The CarS RING-finger protein plays an important role in the downregulation of structural genes of the carotenoid pathway. A recent transcriptomic analysis on the effect of carS mutation identified a gene for a long non-coding RNA (lncRNA) upstream of carS, called carP, the deletion of which results in increased carS mRNA levels and lack of carotenoid production. We have investigated the function of carP by studying the transcriptomic effect of its deletion and the phenotypes resulting from the reintroduction of carP to a deletion strain. The RNA-seq data showed that the loss of carP affected the mRNA levels of hundreds of genes, especially after illumination. Many of these changes appeared to be cascade effects as a result of changes in carS expression, as suggested by the comparison with differentially expressed genes in a carS mutant. Carotenoid production only recovered when carP was integrated upstream of carS, but not at other genomic locations, indicating a cis-acting mechanism on carS. However, some genes hardly affected by CarS were strongly upregulated in the carP mutant, indicating that carP may have other regulatory functions as an independent regulatory element.

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.

Strain improvement of Trichoderma spp. through two-step protoplast fusion for cellulase production enhancement

Fungal protoplast fusion is an approach to introduce novel characteristics into industrially important strains. Cellulases, essential enzymes with a wide range of biotechnological applications, are produced by many species of the filamentous fungi Trichoderma. In this study, a collection of 60 natural isolates were screened for Avicel and carboxymethyl cellulose degradation, and two cellulase producers of Trichoderma virens and Trichoderma harzianum were used for protoplast fusion. One of the resulting hybrids with improved cellulase activity, C1-3, was fused with the hyperproducer Trichoderma reesei Rut-C30. A new selected hybrid, F7, was increased in cellulase activity 1.8 and 5 times in comparison with Rut-C30 and C1-3, respectively. The increases in enzyme activity correlated with an upregulation of the cellulolytic genes cbh1, cbh2, egl3, and bgl1 in the parents. The amount of mRNA of cbh1 and cbh2 in F7 resembled that of Rut-C30 while the bgl1 mRNA level was similar to that of C1-3. AFLP (amplified fragment length polymorphism) fingerprinting and GC-MS (gas chromatography - mass spectrometry) analysis represented variations in parental strains and fusants. In conclusion, the results demonstrate that a 3-interspecific hybrid strain was isolated, with improved characteristics for cellulase degradation and showing genetic polymorphisms and differences in the volatile profile, suggesting reorganizations at the genetic level.

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.

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

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

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

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

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Use of chitin and chitosan to produce new chitooligosaccharides by chitinase Chit42: enzymatic activity and structural basis of protein specificity

Background

Chitinases are ubiquitous enzymes that have gained a recent biotechnological attention due to their ability to transform biological waste from chitin into valued chito-oligomers with wide agricultural, industrial or medical applications. The biological activity of these molecules is related to their size and acetylation degree. Chitinase Chit42 from Trichoderma harzianum hydrolyses chitin oligomers with a minimal of three N-acetyl-d-glucosamine (GlcNAc) units. Gene chit42 was previously characterized, and according to its sequence, the encoded protein included in the structural Glycoside Hydrolase family GH18.

Results

Chit42 was expressed in Pichia pastoris using fed-batch fermentation to about 3 g/L. Protein heterologously expressed showed similar biochemical properties to those expressed by the natural producer (42 kDa, optima pH 5.5–6.5 and 30–40 °C). In addition to hydrolyse colloidal chitin, this enzyme released reducing sugars from commercial chitosan of different sizes and acetylation degrees. Chit42 hydrolysed colloidal chitin at least 10-times more efficiently (defined by the k_cat/K_m ratio) than any of the assayed chitosan. Production of partially acetylated chitooligosaccharides was confirmed in reaction mixtures using HPAEC-PAD chromatography and mass spectrometry. Masses corresponding to (d-glucosamine)_1–8-GlcNAc were identified from the hydrolysis of different substrates. Crystals from Chit42 were grown and the 3D structure determined at 1.8 Å resolution, showing the expected folding described for other GH18 chitinases, and a characteristic groove shaped substrate-binding site, able to accommodate at least six sugar units. Detailed structural analysis allows depicting the features of the Chit42 specificity, and explains the chemical nature of the partially acetylated molecules obtained from analysed substrates.

Conclusions

Chitinase Chit42 was expressed in a heterologous system to levels never before achieved. The enzyme produced small partially acetylated chitooligosaccharides, which have enormous biotechnological potential in medicine and food. Chit42 3D structure was characterized and analysed. Production and understanding of how the enzymes generating bioactive chito-oligomers work is essential for their biotechnological application, and paves the way for future work to take advantage of chitinolytic activities.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0895-x) contains supplementary material, which is available to authorized users.

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

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

HPLC Analysis of Carotenoids in Neurosporaxanthin-Producing Fungi

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

A RALDH-like enzyme involved in Fusarium verticillioides development

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

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.

Genetic basis of carotenoid overproduction in Fusarium oxysporum

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

Analysis of al-2 mutations in Neurospora

The orange pigmentation of the fungus Neurospora crassa is due to the accumulation of the xanthophyll neurosporaxanthin and precursor carotenoids. Two key reactions in the synthesis of these pigments, the formation of phytoene from geranylgeranyl pyrophosphate and the introduction of β cycles in desaturated carotenoid products, are catalyzed by two domains of a bifunctional protein, encoded by the gene al-2. We have determined the sequence of nine al-2 mutant alleles and analyzed the carotenoid content in the corresponding strains. One of the mutants is reddish and it is mutated in the cyclase domain of the protein, and the remaining eight mutants are albino and harbor different mutations on the phytoene synthase (PS) domain. Some of the mutations are expected to produce truncated polypeptides. A strain lacking most of the PS domain contained trace amounts of a carotenoid-like pigment, tentatively identified as the squalene desaturation product diapolycopene. In support, trace amounts of this compound were also found in a knock-out mutant for gene al-2, but not in that for gene al-1, coding for the carotene desaturase. The cyclase activity of the AL-2 enzyme from two albino mutants was investigated by heterologous expression in an appropriately engineered E. coli strain. One of the AL-2 enzymes, predictably with only 20% of the PS domain, showed full cyclase activity, suggesting functional independence of both domains. However, the second mutant showed no cyclase activity, indicating that some alterations in the phytoene synthase segment affect the cyclase domain. Expression experiments showed a diminished photoinduction of al-2 transcripts in the al-2 mutants compared to the wild type strain, suggesting a synergic effect between reduced expression and impaired enzymatic activities in the generation of their albino phenotypes.

The effects of disruption of phosphoglucose isomerase gene on carbon utilisation and cellulase production in Trichoderma reesei Rut-C30

Background

Cellulase and hemicellulase genes in the fungus Trichoderma reesei are repressed by glucose and induced by lactose. Regulation of the cellulase genes is mediated by the repressor CRE1 and the activator XYR1. T. reesei strain Rut-C30 is a hypercellulolytic mutant, obtained from the natural strain QM6a, that has a truncated version of the catabolite repressor gene, cre1. It has been previously shown that bacterial mutants lacking phosphoglucose isomerase (PGI) produce more nucleotide precursors and amino acids. PGI catalyzes the second step of glycolysis, the formation of fructose-6-P from glucose-6-P.

Results

We deleted the gene pgi1, encoding PGI, in the T. reesei strain Rut-C30 and we introduced the cre1 gene in a Δpgi1 mutant. Both Δpgi1 and cre1⁺Δpgi1 mutants showed a pellet-like and growth as well as morphological alterations compared with Rut-C30. None of the mutants grew in media with fructose, galactose, xylose, glycerol or lactose but they grew in media with glucose, with fructose and glucose, with galactose and fructose or with lactose and fructose. No growth was observed in media with xylose and glucose. On glucose, Δpgi1 and cre1⁺Δpgi1 mutants showed higher cellulase activity than Rut-C30 and QM6a, respectively. But in media with lactose, none of the mutants improved the production of the reference strains. The increase in the activity did not correlate with the expression of mRNA of the xylanase regulator gene, xyr1. Δpgi1 mutants were also affected in the extracellular β-galactosidase activity. Levels of mRNA of the glucose 6-phosphate dehydrogenase did not increase in Δpgi1 during growth on glucose.

Conclusions

The ability to grow in media with glucose as the sole carbon source indicated that Trichoderma Δpgi1 mutants were able to use the pentose phosphate pathway. But, they did not increase the expression of gpdh. Morphological characteristics were the result of the pgi1 deletion. Deletion of pgi1 in Rut-C30 increased cellulase production, but only under repressing conditions. This increase resulted partly from the deletion itself and partly from a genetic interaction with the cre1-1 mutation. The lower cellulase activity of these mutants in media with lactose could be attributed to a reduced ability to hydrolyse this sugar but not to an effect on the expression of xyr1.

Biocontrol mechanisms of Trichoderma strains

The genus Trichoderma comprises a great number of fungal strains that act as biological control agents, the antagonistic properties of which are based on the activation of multiple mechanisms. Trichoderma strains exert biocontrol against fungal phytopathogens either indirectly, by competing for nutrients and space, modifying the environmental conditions, or promoting plant growth and plant defensive mechanisms and antibiosis, or directly, by mechanisms such as mycoparasitism. These indirect and direct mechanisms may act coordinately and their importance in the biocontrol process depends on the Trichoderma strain, the antagonized fungus, the crop plant, and the environmental conditions, including nutrient availability, pH, temperature, and iron concentration. Activation of each mechanism implies the production of specific compounds and metabolites, such as plant growth factors, hydrolytic enzymes, siderophores, antibiotics, and carbon and nitrogen permeases. These metabolites can be either overproduced or combined with appropriate biocontrol strains in order to obtain new formulations for use in more efficient control of plant diseases and postharvest applications.

Increased antifungal and chitinase specific activities of Trichoderma harzianum CECT 2413 by addition of a cellulose binding domain

Trichoderma harzianum is a widely distributed soil fungus that antagonizes numerous fungal phytopathogens. The antagonism of T. harzianum usually correlates with the production of antifungal activities including the secretion of fungal cell walls that degrade enzymes such as chitinases. Chitinases Chit42 and Chit33 from T. harzianum CECT 2413, which lack a chitin-binding domain, are considered to play an important role in the biocontrol activity of this strain against plant pathogens. By adding a cellulose-binding domain (CBD) from cellobiohydrolase II of Trichoderma reesei to these enzymes, hybrid chitinases Chit33-CBD and Chit42-CBD with stronger chitin-binding capacity than the native chitinases have been engineered. Transformants that overexpressed the native chitinases displayed higher levels of chitinase specific activity and were more effective at inhibiting the growth of Rhizoctonia solani, Botrytis cinerea and Phytophthora citrophthora than the wild type. Transformants that overexpressed the chimeric chitinases possessed the highest specific chitinase and antifungal activities. The results confirm the importance of these endochitinases in the antagonistic activity of T. harzianum strains, and demonstrate the effectiveness of adding a CBD to increase hydrolytic activity towards insoluble substrates such as chitin-rich fungal cell walls.

Addition of substrate-binding domains increases substrate-binding capacity and specific activity of a chitinase from Trichoderma harzianum

Chitinase Chit42 from Trichoderma harzianum CECT 2413 is considered to play an important role in the biocontrol activity of this fungus against plant pathogens. Chit42 lacks a chitin-binding domain (ChBD). We have produced hybrid chitinases with stronger chitin-binding capacity by fusing to Chit42 a ChBD from Nicotiana tabacum ChiA chitinase and the cellulose-binding domain from cellobiohydrolase II of Trichoderma reesei. The chimeric chitinases had similar activities towards soluble substrate but higher hydrolytic activity than the native chitinase on high molecular mass insoluble substrates such as ground chitin or chitin-rich fungal cell walls.

Regulation of chitinase 33 (chit33) gene expression in Trichoderma harzianum

We investigated the regulation of chit33 expression in Trichoderma harzianum CECT 2413. This gene encodes the Chit33 endochitinase, which is a major component of the fungus' chitinolytic enzyme system and is important for biocontrol. To this end, both Northern analysis and reporter gene fusions of a 1.4-kb fragment of the 5'-upstream sequences of chit33 to the Aspergillus niger goxA gene (encoding glucose oxidase) and the Aquorea victoria green fluorescent protein were used. Northern analysis and data obtained with the reporter systems were compatible, thus showing that the 1.4-kb fragment bears all necessary information for the regulation of chit33 gene expression. chit33 is weakly expressed during growth on chitin and Rhizoctonia solani cell walls. The addition of N-acetylglucosamine transiently induced chit33 expression in resting cells of the fungus. The addition of either glucose or glycerol prevented induction of chit33 gene expression by chitin or cell walls. Incubation of T. harzianum in the presence of low concentrations (0.1%, w/v) of glucose and high concentrations (38 mM) of ammonium sulfate, or in the presence of high concentrations (1%, w/v) of glucose and low concentrations (0.38 mM) of ammonium sulfate also stimulated chit33-mRNA accumulation, although to a lower degree than induction by N-acetylglucosamine. Transfer of T. harzianum cultures to either 40 degrees C or 4 degrees C initiated a very rapid expression of chit33 in the absence of an inducer, yet only at very low levels (5%) of the induced control. Confrontation experiments, using the gfp gene as a reporter and R. solani as a host, showed that chit33 is expressed only during but not before the stage of overgrowth on R. solani. These data show that Chit33 is an enzyme involved in mycoparasitism; and its formation is controlled by induction, by either carbon or nitrogen starvation and, to a low degree, also under conditions of temperature stress.

[Improvement of Trichoderma strains for biocontrol]

The use of the fungal genus Trichoderma to control fungal plant diseases is a promising alternative to the use of chemical compounds. The aim of this work has been to obtain Trichoderma strains with improved capacity as biological control agents. To do so, the hydrolytic capacity on fungal cell walls of strains of the fungus Trichoderma harzianum has been increased. On one hand, transformation experiments with genes which coded for chitinases and glucanases have been carried out in T. harzianumstra ins. On the other hand, the medium composition has also been modified in order to eliminate proteolytic degradation of some of the overproduced enzymes. Finally, hybrid chitinolytic enzymes with substrate-binding domains have been produced as an alternative to obtain improved biocontrol strains. The transformant strains, when compared with the wild type, showed improved antifungal capacity against the phytopathogenic fungus Rhizoctonia solani, in in vitro experiments.

Increased Antifungal Activity of Trichoderma harzianum Transformants That Overexpress a 33-kDa Chitinase

ABSTRACT Transformants of the biocontrol agent Trichoderma harzianum strain CECT 2413 that overexpressed a 33-kDa chitinase (Chit33) were obtained and characterized. Strain CECT 2413 was cotransformed with the amdS gene and its own chit33 gene under the control of the pki constitutive promoter from T. reesei. Southern blotting indicated that the chit33 gene was integrated ectopically, mostly in tandem. Some transformants showed the same restriction pattern, indicating preferable sites of integration. There was no correlation between the number of integrated copies and the level of expression of the chit33 gene in the transformants. When grown in glucose, the extracellular chitinase activity of the transformants was up to 200-fold greater than that of the wild type, whereas in chitin, the activity of both the transformants and the wild type was similar. Under both conditions, the transformants were more effective in inhibiting the growth of Rhizoctonia solani as compared with the wild type. Similar results were obtained when culture supernatants from the transformants and the wild type were tested against R. solani.

Primary structure and expression pattern of the 33-kDa chitinase gene from the mycoparasitic fungus Trichoderma harzianum

A gene (chit33) from the mycoparasitic fungus Trichoderma harzianum, coding for a chitinase of 33 kDa, has been isolated and characterized. Partial amino-acid sequences from the purified 33-kDa chitinase were obtained. The amino-terminal peptide sequence was employed to design an oligonucleotide probe and was used as a primer to isolate a 1.2-kb cDNA. The cDNA codes for a protein of 321 amino acids, which includes a putative signal peptide of 19 amino acids. All microsequenced peptides found in this sequence, indicate that this cDNA codes for the 33-kDa chitinase. A high homology (approximately 43% identity) was found with fungal and plant chitinases, including yeast chitinases. However enzyme characteristics suggest a nutritional (saprophytic or mycoparasitic), rather than a morphogenetic, role for this chitinase. The chit33 gene appears as a single copy in the T. harzianum genome, is strongly suppressed by glucose, and de-repressed under starvation conditions as well as in the presence of autoclaved mycelia and/or fungal cell walls. The 33-kDa chitinase seems to be very stable except under starvation conditions. The independent regulation of each of the chitinases in T. harzianum indicates different specific roles.