Effect of solid-medium coupled with reactive oxygen species on ganoderic acid biosynthesis and MAP kinase phosphorylation in Ganoderma lucidum

MAPK CcSakA of the HOG Pathway Is Involved in Stipe Elongation during Fruiting Body Development in Coprinopsis cinerea

Mitogen-activated protein kinase (MAPK) pathways, such as the high-osmolarity glycerol mitogen-activated protein kinase (HOG) pathway, are evolutionarily conserved signaling modules responsible for transmitting environmental stress signals in eukaryotic organisms. Here, we identified the MAPK homologue in the HOG pathway of Coprinopsis cinerea, which was named CcSakA. Furthermore, during the development of the fruiting body, CcSakA was phosphorylated in the fast elongating apical part of the stipe, which meant that CcSakA was activated in the apical elongating stipe region of the fruiting body. The knockdown of CcSakA resulted in a shorter stipe of the fruiting body compared to the control strain, and the expression of phosphomimicking mutant CcSakA led to a longer stipe of the fruiting body compared to the control strain. The chitinase CcChiE1, which plays a key role during stipe elongation, was downregulated in the CcSakA knockdown strains and upregulated in the CcSakA phosphomimicking mutant strains. The results indicated that CcSakA participated in the elongation of stipes in the fruiting body development of C. cinerea by regulating the expression of CcChiE1. Analysis of the H₂O₂ concentration in different parts of the stipe showed that the oxidative stress in the elongating part of the stipe was higher than those in the non-elongating part. The results indicated that CcSakA of the HOG pathway may be activated by oxidative stress. Our results demonstrated that the HOG pathway transmits stress signals and regulates the expression of CcChiE1 during fruiting body development in C. cinerea.

Time-scale dynamics of proteome predicts the central carbon metabolism involved in triterpenoid accumulation responsive to nitrogen limitation in Ganoderma lucidum

Central carbon metabolism describes the integration of transport pathway of main carbon sources inside the cell. Nitrogen (N) limitation is a favorable approach to stimulate ganoderic triterpenoid (GT) accumulation in Ganoderma lucidum. In this study, the dynamic regulation of metabolism reassignment towards GT biosynthesis responsive to N limitation was investigated by iTRAQ-based proteome. Physiological data suggested that N limitation slightly affected cell growth but significantly enhanced GT contents in the initial 20 days. From day 10, the protein contents were halted by prolonged N limitation duration. Proteomics-based investigations revealed that the carbon skeletons integrated into GT precursors were regenerated by glycolysis and the tricarboxylic acid (TCA) cycle. Cells strategically reserved nitrogen by barely incorporating it into TCA cycle intermediates to form amino acids, and enzymes involved in protein degradation were up regulated. Furthermore, regulation of proteins in response to abiotic stress and oxidation- reduction processes played a critical role in maintaining cellular homeostasis. These findings indicated that the flux of carbon into GT following N deficiency was a consequence of the remodeling of intermediate metabolism in TCA cycle and glycolysis reactions. This study provides a rationale for genetic engineering of G. lucidum, which may enable synchronized biomass and GT synthesis.

Effects of different graphene-based nanomaterials as elicitors on growth and ganoderic acid production by Ganoderma lucidum

Graphene-based nanomaterials (GBNs) have attracted considerable interest nowadays due to their wide range of applications. However, very little attention has been paid to the application of nanomaterials as potential elicitors for production of valuable metabolites. Herein, aiming to earn insight into effects of nanomaterials on secondary metabolite biosynthesis by medicinal fungi, we evaluated the influence of GBNs on growth and production of ganoderic acid (GA) by Ganoderma lucidum in submerged culture. Graphene oxide (GO), reduced graphene oxide (rGO), and rGO/Fe₃ O₄ nanocomposite were synthesized successfully and characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy analysis. The prepared nanomaterials were added to the culture of G. lucidum at final concentrations of 50, 100, and 150 mg/L on Day 5. The results showed that the elicitation of G. lucidum with GO and rGO decreased the cell dry weight and GA production slightly, especially in higher concentrations. However, rGO/Fe₃ O₄ nanocomposite not negatively affected cell growth and improved GA production. G. lucidum growth rate responded to elicitation experiments differently and depended on the type of nanomaterials and their concentrations, but almost all GBNs caused an increase in GA content (mg/100 mg dry weight). Also, field emission scanning electron microscopy morphological study showed that under elicitation, mycelia were more condensed and tightly stacked together. The findings from this study may suggest that GBNs in low concentrations could be applied as elicitors to secondary metabolites production from higher fungus, but further environmental, physiological, and biological studies required.

High Oxygen Treatments Enhance the Contents of Phenolic Compound and Ganoderic Acid, and the Antioxidant and DNA Damage Protective Activities of Ganoderma lingzhi Fruiting Body

Ganoderma lingzhi is a famous medicinal mushroom used as Chinese medicine or functional food and has been accepted across the globe. It is important to enhance the contents of bioactive compounds, which in turn improves the quality and biological activity of G. lingzhi fruiting body. In this work, freshly harvested G. lingzhi fruiting bodies were treated continuously with air or with 60 and 80% oxygen for 6 days. Samples were collected and determined initially and at 1 day interval during treatment. A high total ganoderic acid content of 29.44 g kg^–1 was obtained in samples treated with 60% oxygen at day 3. Quantitative reverse transcriptase (qRT)-PCR and high-performance liquid chromatography (HPLC) analysis showed that the expression levels of hydroxymethylglutaryl-CoA synthase, squalene synthase, and oxidosqualene cyclase genes were substantially increased, resulting in the increase of ganoderic acids A, B, and C2 and ganoderenic acid B. The scavenging activities with 1,1-diphenyl-2-picrylhydrazyl radical, 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) radical, hydroxyl radical, and superoxide radical and the DNA damage protective activity were also enhanced by high oxygen treatment. The results of this work provided a potential method to enhance the active metabolite synthesis in the fruiting body of G. lingzhi.

Protective Effects of Lanosterol Synthase Up-Regulation in UV-B-Induced Oxidative Stress

UV-B radiation may be an important risk factor in cataract etiology. After exposure to UV-B radiation, cells show imbalances in the repair of DNA damage, which induce changes in the levels of certain proteins, including alpha-crystallin, which is the most abundant protein in the lens and crucial for the maintenance of lens transparency. Lanosterol synthase (LSS), an essential rate-limiting enzyme in cholesterol biosynthesis, might play significant roles in oxidative stress and in the maintenance of lens transparency. However, the roles of LSS in UV-B-induced apoptosis are not well understood. Therefore, we irradiated female Sprague-Dawley rats with ultraviolet radiation to establish an animal model for exploring the variations in LSS expression during the early stages of UV-B exposure. In addition, we cultured human lens epithelial (HLE) cells that overexpress LSS and exposed them to UV-B radiation to explore the function of increased LSS expression in UV-B-induced apoptosis. The data demonstrated that UV-B exposure induced oxidative stress and apoptosis in rat lens epithelial cells and that irradiance exposure increased the level of lenticular damage. Additionally, UV-B exposure decreased the alpha-crystallin content and increased the expressions of Bax and cleaved caspase-3 compared with the control levels. After exposure to UV-B, the apoptosis-related index of HLE cells overexpressing LSS was lower than that of the control cells. Furthermore, ROS overproduction might activate the sirtuin 1 (Sirt1) pathway, which induced protein expressions of sterol regulatory element-binding transcription factor 2 (SREBF2), 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR), and LSS. However, the specific mechanism of the Sirt1 pathway needed to be further studied. In summary, UV-B exposure induced oxidative injury and resulted in crystallin denaturation and apoptosis in lens epithelial cells, and LSS might play a protective role during the early stages of this process and could be an important target in the cataract prevention.

Shedding light on the mechanisms underlying the environmental regulation of secondary metabolite ganoderic acid in Ganoderma lucidum using physiological and genetic methods

The secondary metabolites of fungi are often produced at very low concentrations, and until recently the regulatory mechanisms of secondary metabolite biosynthesis have been unclear. Ganoderma lucidum is a macrofungus that is widely used as a traditional Chinese medicine or medicinal mushroom: ganoderic acid (GA) is one of the main active ingredients. Here, we review research from the last decade on which and how environmental factors regulate GA biosynthesis. These environmental factors are mainly three components: a single chemical/biological or biochemical signal, physical triggers, and nutritional conditions. Because G. lucidum is a non-model Basidiomycete, a combination of physiological and genetic research is needed to determine how those environmental factors regulate GA biosynthesis. The regulation of GA biosynthesis includes ROS, Ca²⁺, cAMP and phospholipid signaling, and cross-talk between different signaling pathways. The regulatory mechanisms for the synthesis of this secondary metabolite, from the perspective of physiology and genetics, in G. lucidum will provide ideas for studying the regulation of fungal secondary metabolism in other non-model species, especially those fungi with limitations in genetic manipulation.

Heat stress-induced reactive oxygen species participate in the regulation of HSP expression, hyphal branching and ganoderic acid biosynthesis in Ganoderma lucidum

Heat stress (HS) is an important environmental factor that affects the growth and metabolism of edible fungi, but the molecular mechanism of the heat stress response (HSR) remains unclear. We previously reported that HS treatment increased the length between two hyphal branches and induced the accumulation of ganoderic acid biosynthesis and the gene expression of heat shock proteins (HSPs) in Ganoderma lucidum. In this study, we found that HS induced a significant increase in the cytosolic ROS concentration, and exogenously added ROS scavengers NAC, VC and NADPH oxidase (Nox) inhibitor DPI reduce the cytosolic ROS accumulation in G. lucidum. In addition, the phenomena of the increased gene expression and increased length between the two hyphal branches and the accumulation of GA biosynthesis induced by HS were mitigated. Furthermore, we investigated the effects of HS on Nox-silenced strains (NoxABi-10, NoxABi-11 and NoxRi-4, NoxRi-7) and found that the level of ROS concentration was lower than that in wild-type (WT) strains treated with HS. Additionally, Nox silenced strains reduced the HS-induced increase in HSP expression, the length between two hyphal branches and GA biosynthesis compared with the WT strain. These data indicate that HS-induced ROS participate in the regulation of HSP expression, hyphal branching and ganoderic acid biosynthesis in G. lucidum. In addition, these findings identified potential pathways linking ROS networks to HSR, physiological and metabolic processes in fungi and provide a valuable reference for studying the role of ROS in HSR, mycelium growth and secondary metabolites.

Functional analysis of an APSES transcription factor (GlSwi6) involved in fungal growth, fruiting body development and ganoderic-acid biosynthesis in Ganoderma lucidum

The APSES transcription factors have been identified as key regulators of fungal development and other biological processes in fungi. In the present study, the function of Ganoderma lucidum GlSwi6, a homolog of Saccharomyces cerevisiae Swi6, was characterized. RNAi was used to examine the function of GlSwi6 in G. lucidum. Silencing GlSwi6 resulted in multiple developmental defects, including reduced fungal growth and increased hyphal branching, and the GlSwi6-silenced strains did not exhibit primordium or fruiting body formation. In addition, the H₂O₂ and ganoderic-acid (GA) levels of the GlSwi6-silenced strains decreased approximately 50% and 25%, respectively, compared with those of the WT strain. Furthermore, the addition of H₂O₂ led to the recovery of the GA levels of GlSwi6-silenced strains, implying that GlSwi6 might regulate GA biosynthesis by regulating the intracellular ROS levels. Taken together, these results indicate that GlSwi6 is involved in fungal growth, development and GA biosynthesis in G. lucidum.

Transcriptomic responses involved in enhanced production of hypocrellin A by addition of Triton X-100 in submerged cultures of Shiraia bambusicola

The addition of surfactant is a useful strategy to enhance the product yield in submerged fermentation process. In this study, we sought to explore the mechanism for the elicitation of Triton X-100 on production of hypocrellin A (HA) in cultures of Shiraia bambusicola through transcriptomic analysis. Triton X-100 at 2.5% (w/v) not only induced HA biosynthesis in mycelia, but also stimulated the release of HA into the medium. We found 23 of 2463 transcripts, possible candidate genes for HA biosynthesis under Triton X-100 induction. Gene ontology (GO) analysis showed Triton X-100 treatment changed expression of genes involved in transmembrane transport and oxidation-reduction process, indicating that enhanced HA production was mainly due to both elicited biosynthesis in mycelium and the increased membrane permeability for HA release. These data provided new insights into elicitation of surfactants in submerged cultures of fungi.

The mitogen-activated protein kinase GlSlt2 regulates fungal growth, fruiting body development, cell wall integrity, oxidative stress and ganoderic acid biosynthesis in Ganoderma lucidum

The mitogen-activated protein kinases (MAPKs) are crucial signaling instruments in eukaryotes that play key roles in regulating fungal growth, development, and secondary metabolism and in adapting to the environment. In this study, we characterized an Slt2-type MAPK in Ganoderma lucidum, GlSlt2, which was transcriptionally induced during the primordium and fruiting body stages. RNA interference was used to examine the function of GlSlt2. Knockdown of GlSlt2 caused defects in growth and increased hyphal branching as well as hypersensitivity to cell wall-disturbing substances. Consistently, the chitin and β-1,3-d-glucan contents and the expression of cell wall biosynthesis genes were decreased and down-regulated, respectively, in GlSlt2 knockdown strains compared with those in the wild type (WT). In addition, no primordium or fruiting body could be observed in GlSlt2 knockdown strains. Furthermore, the intracellular reactive oxygen species (ROS) content and ganoderic acid biosynthesis also decreased in GlSlt2 knockdown strains. Addition of H₂O₂ could recover the decreased ganoderic acid content in GlSlt2 knockdown strains, indicating that GlSlt2 might regulate ganoderic acid biosynthesis via the intracellular ROS level. Overall, GlSlt2 is involved in hyphal growth, fruiting body development, cell wall integrity, oxidative stress and ganoderic acid biosynthesis in G. lucidum.

Induction of apoptosis and ganoderic acid biosynthesis by cAMP signaling in Ganoderma lucidum

Apoptosis is an essential physiological process that controls many important biological functions. However, apoptosis signaling in relation to secondary metabolite biosynthesis in plants and fungi remains a mystery. The fungus Ganoderma lucidum is a popular herbal medicine worldwide, but the biosynthetic regulation of its active ingredients (ganoderic acids, GAs) is poorly understood. We investigated the role of 3′,5′-cyclic adenosine monophosphate (cAMP) signaling in fungal apoptosis and GA biosynthesis in G. lucidum. Two phosphodiesterase inhibitors (caffeine and 3-isobutyl-1-methylxanthine, IBMX) and an adenylate cyclase activator (sodium fluoride, NaF) were used to increase intracellular cAMP levels. Fungal apoptosis was identified by terminal deoxynucleotidyl transferase mediated dUTP nick end labeling (TUNEL) assay and a condensed nuclear morphology. Our results showed that GA production and fungal apoptosis were induced when the mycelium was treated with NaF, caffeine, or cAMP/IBMX. Downregulation of squalene synthase and lanosterol synthase gene expression by cAMP was detected in the presence of these chemicals, which indicates that these two genes are not critical for GA induction. Transcriptome analysis indicated that mitochondria might play an important role in cAMP-induced apoptosis and GA biosynthesis. To the best of our knowledge, this is the first report to reveal that cAMP signaling induces apoptosis and secondary metabolite production in fungi.

Hydrogen-rich water regulates effects of ROS balance on morphology, growth and secondary metabolism via glutathione peroxidase in Ganoderma lucidum

Ganoderma lucidum is one of the most important medicinal fungi, but the lack of basic study on the fungus has hindered the further development of its value. To investigate the roles of the redox system in G. lucidum, acetic acid (HAc) was applied as a reactive oxygen species (ROS) stress inducer, and hydrogen-rich water (HRW) was used to relieve the ROS stress in this study. Our results demonstrate that the treatment of 5% HRW significantly decreased the ROS content, maintained biomass and polar growth morphology of mycelium, and decreased secondary metabolism under HAc-induced oxidative stress. Furthermore, the roles of HRW were largely dependent on restoring the glutathione system under HAc stress in G. lucidum. To provide further evidence, we used two glutathione peroxidase (GPX)-defective strains, the gpxi strain, the mercaptosuccinic acid (MS, a GPX inhibitor)-treated wide-type (WT) strain, and gpx overexpression strains for further research. The results show that HRW was unable to relieve the HAc-induced ROS overproduction, decreased biomass, mycelium morphology change and increased secondary metabolism biosynthesis in the absence of GPX function. The gpx overexpression strains exhibited resistance to HAc-induced oxidative stress. Thus, we propose that HRW regulates morphology, growth and secondary metabolism via glutathione peroxidase under HAc stress in the fungus G. lucidum. Furthermore, our research also provides a method to study the ROS system in other fungi.

Profiling and quantifying differential gene transcription provide insights into ganoderic acid biosynthesis in Ganoderma lucidum in response to methyl jasmonate

Ganoderma lucidum is a mushroom with traditional medicinal properties that has been widely used in China and other countries in Eastern Asia. Ganoderic acids (GA) produced by G. lucidum exhibit important pharmacological activities. Previous studies have demonstrated that methyl jasmonate (MeJA) is a potent inducer of GA biosynthesis and the expression of genes involved in the GA biosynthesis pathway in G. lucidum. To further explore the mechanism of GA biosynthesis, cDNA-Amplified Fragment Length Polymorphism (cDNA-AFLP) was used to identify genes that are differentially expressed in response to MeJA. Using 64 primer combinations, over 3910 transcriptionally derived fragments (TDFs) were obtained. Reliable sequence data were obtained for 390 of 458 selected TDFs. Ninety of these TDFs were annotated with known functions through BLASTX searching the GenBank database, and 12 annotated TDFs were assigned into secondary metabolic pathways by searching the KEGGPATHWAY database. Twenty-five TDFs were selected for qRT-PCR analysis to confirm the expression patterns observed with cDNA-AFLP. The qRT-PCR results were consistent with the altered patterns of gene expression revealed by the cDNA-AFLP technique. Additionally, the transcript levels of 10 genes were measured at the mycelium, primordia, and fruiting body developmental stages of G. lucidum. The greatest expression levels were reached during primordia for all of the genes except cytochrome b2 reached its highest expression level in the mycelium stage. This study not only identifies new candidate genes involved in the regulation of GA biosynthesis but also provides further insight into MeJA-induced gene expression and secondary metabolic response in G. lucidum.