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

Heat stress-induced NO enhanced perylenequinone biosynthesis of Shiraia sp. via calcium signaling pathway

Perylenequinones (PQs) are natural photosensitizing compounds used as photodynamic therapy, and heat stress (HS) is the main limiting factor of mycelial growth and secondary metabolism of fungi. This study aimed to unravel the impact of HS-induced Ca2+ and the calcium signaling pathway on PQ biosynthesis of Shiraia sp. Slf14(w). Meanwhile, the intricate interplay between HS-induced NO and Ca2+ and the calcium signaling pathway was investigated. The outcomes disclosed that Ca2+ and the calcium signaling pathway activated by HS could effectively enhance the production of PQs in Shiraia sp. Slf14(w). Further investigations elucidated the specific mechanism through which NO signaling molecules induced by HS act upon the Ca2+/CaM (calmodulin) signaling pathway, thus propelling PQ biosynthesis in Shiraia sp. Slf14(w). This was substantiated by decoding the downstream positioning of the CaM/CaN (calcineurin) pathway in relation to NO through comprehensive analyses encompassing transcript levels, enzyme assays, and the introduction of chemical agents. Concurrently, the engagement of Ca2+ and the calcium signaling pathway in heat shock signaling was also evidenced. The implications of our study underscore the pivotal role of HS-induced Ca2+ and the calcium signaling pathway, which not only participate in heat shock signal transduction but also play an instrumental role in promoting PQ biosynthesis. Consequently, our study not only enriches our comprehension of the mechanisms driving HS signaling transduction in fungi but also offers novel insights into the PQ synthesis paradigm within Shiraia sp. Slf14(w). KEY POINTS: • The calcium signaling pathway was proposed to participate in PQ biosynthesis under HS. • HS-induced NO was revealed to act upon the calcium signaling pathway for the first time.

Glsirt1-mediated deacetylation of GlCAT regulates intracellular ROS levels, affecting ganoderic acid biosynthesis in Ganoderma lucidum

Lysine acetylation is a reversible, dynamic protein modification regulated by lysine acetyltransferases and deacetylases. However, in Basidiomycetes, the extent of lysine acetylation of nonhistone proteins remains largely unknown. Recently, we identified the deacetylase Glsirt1 as a key regulator of the biosynthesis of ganoderic acid (GA), a key secondary metabolite of Ganoderma lucidum. To gain insight into the characteristics, extent, and biological function of Glsirt1-mediated lysine acetylation in G. lucidum, we aimed to identify additional Glsirt1 substrates via comparison of acetylomes between wild-type (WT) and Glsirt1-silenced mutants. A large amount of Glsirt1-dependent lysine acetylation occurs in G. lucidum according to the results of this omics analysis, involving energy metabolism, protein synthesis, the stress response and other pathways. Our results suggest that GlCAT is a direct target of Glsirt1 and that the deacetylation of GlCAT by Glsirt1 reduces catalase activity, thereby leading to the accumulation of intracellular reactive oxygen species (ROS) and positively regulating the biosynthesis of GA. Our findings provide evidence for the involvement of nonhistone lysine acetylation in the biological processes of G. lucidum and help elucidate the involvement of important ROS signaling molecules in regulating physiological and biochemical processes in this organism. In conclusion, this proteomic analysis reveals a striking breadth of cellular processes affected by lysine acetylation and provides new nodes of intervention in the biosynthesis of secondary metabolites in G. lucidum.

Cloning and Expression Analysis of Bioluminescence Genes in Omphalotus guepiniiformis Reveal Stress-Dependent Regulation of Bioluminescence

Bioluminescence is a type of chemiluminescence that arises from a luciferase-catalyzed oxidation reaction of luciferin. Molecular biology and comparative genomics have recently elucidated the genes involved in fungal bioluminescence and the evolutionary history of their clusters. However, most studies on fungal bioluminescence have been limited to observing the changes in light intensity under various conditions. To understand the molecular basis of bioluminescent responses in Omphalotus guepiniiformis under different environmental conditions, we cloned and sequenced the genes of hispidin synthase, hispidin-3-hydroxylase, and luciferase enzymes, which are pivotal in the fungal bioluminescence pathway. Each gene showed high sequence similarity to that of other luminous fungal species. Furthermore, we investigated their transcriptional changes in response to abiotic stresses. Wound stress enhanced the bioluminescence intensity by increasing the expression of bioluminescence pathway genes, while temperature stress suppressed the bioluminescence intensity via the non-transcriptional pathway. Our data suggested that O. guepiniiformis regulates bioluminescence to respond differentially to specific environmental stresses. To our knowledge, this is the first study on fungal bioluminescence at the gene expression level. Further studies are required to address the biological and ecological meaning of different bioluminescence responses in changing environments, and O. quepiniiformis could be a potential model species.

Strategies to Increase the Production of Triterpene Acids in Ligzhi or Reishi Medicinal Mushroom (Ganoderma lucidum, Agaricomycetes): A Review

Ganoderic acids (GAs) are the main active ingredient of Ganoderma lucidum, which has been widely accepted as a medicinal mushroom. Due to the low yield of GAs produced by liquid cultured Ganoderma mycelium and solid cultured fruiting bodies, the commercial production and clinical application of GAs are limited. Therefore, it is important to increase the yield of GA in G. lucidum. A comprehensive literature search was performed with no set data range using the following keywords such as "triterpene," "ganoderic acids," "Ganoderma lucidum," and "Lingzhi" within the main databases including Web of Science, PubMed, and China National Knowledge Infrastructure (CNKI). The data were screened using titles and abstracts and those relevant to the topic were included in the paper and was not limited to studies published in English. Present review focuses on the four aspects: fermentation conditions and substrate, extrinsic elicitor, genetic engineering, and mutagenesis, which play significant roles in increasing triterpene acids production, thus providing an available reference for further research on G. lucidum fermentation.

Identification of genes associated with abiotic stress tolerance in sweetpotato using weighted gene co-expression network analysis

Sweetpotato, Ipomoea batatas (L.), a key food security crop, is negatively impacted by heat, drought, and salinity stress. The orange-fleshed sweetpotato cultivar "Beauregard" was exposed to heat, salt, and drought treatments for 24 and 48 h to identify genes responding to each stress condition in leaves. Analysis revealed both common (35 up regulated, 259 down regulated genes in the three stress conditions) and unique sets of up regulated (1337 genes by drought, 516 genes by heat, and 97 genes by salt stress) and down regulated (2445 genes by drought, 678 genes by heat, and 204 genes by salt stress) differentially expressed genes (DEGs) suggesting common, yet stress-specific transcriptional responses to these three abiotic stressors. Gene Ontology analysis of down regulated DEGs common to both heat and salt stress revealed enrichment of terms associated with "cell population proliferation" suggestive of an impact on the cell cycle by the two stress conditions. To identify shared and unique gene co-expression networks under multiple abiotic stress conditions, weighted gene co-expression network analysis was performed using gene expression profiles from heat, salt, and drought stress treated 'Beauregard' leaves yielding 18 co-expression modules. One module was enriched for "response to water deprivation," "response to abscisic acid," and "nitrate transport" indicating synergetic crosstalk between nitrogen, water, and phytohormones with genes encoding osmotin, cell expansion, and cell wall modification proteins present as key hub genes in this drought-associated module. This research lays the groundwork for exploring to a further degree, mechanisms for abiotic stress tolerance in sweetpotato.

Biotransformation of triterpenoid ganoderic acids from exogenous diterpene dihydrotanshinone I in the cultures of Ganoderma sessile

BACKGROUND

Triterpenoids have shown a wide range of biological activities including antitumor and antiviral effects. Typically, triterpenes are synthesized through the mevalonate pathway and are extracted from natural plants and fungi. In this work, triterpenoids, ganoderic acids (GAs) were discovered to be produced via biotransformation of a diterpene, 15,16-dihydrotanshinone I (DHT) in the liquid cultured Ganoderma sessile mycelium.

RESULTS

Firstly, the biotransformation products, two rare GAs were isolated and purified by column chromatography, and characterized using HR-ESI-MS spectrometry and NMR spectrometry. The two compounds were Lanosta-7,9(11),24-trien-15α,22,β-diacetoxy-3β-hydroxy-26-oic acid (LTHA) and Lanosta-7,9(11),24-trien-15α,22,β-diacetoxy-3β-carbonyl-26-oic acid (LTCA). Then, transcriptome and proteome technologies were employed to measure the expression of mRNA and protein, which further confirmed that triterpenoid GAs could be transformed from exogenous diterpenoid DHT. At the molecular level, we proposed a hypothesis of the mechanism by which DHT converted to GAs in G. sessile mycelium, and the possible genes involved in biotransformation were verified by RT-qPCR.

CONCLUSIONS

Two rare GAs were obtained and characterized. A biosynthetic pathway of GAs from DHT was proposed. Although the synthetic route was not confirmed, this study provided important insights into omics resources and candidate genes for studying the biotransformation of diterpenes into triterpenes.

Comparative proteomic analysis of two divergent strains provides insights into thermotolerance mechanisms of Ganoderma lingzhi

Heat stress (HS) is a major abiotic factor influencing fungal growth and metabolism. However, the genetic basis of thermotolerance in Ganoderma lingzhi (G. lingzhi) remains largely unknown. In this study, we investigated the thermotolerance capacities of 21 G. lingzhi strains and screened the thermo-tolerant (S566) and heat-sensitive (Z381) strains. The mycelia of S566 and Z381 were collected and subjected to a tandem mass tag (TMT)-based proteome assay. We identified 1493 differentially expressed proteins (DEPs), with 376 and 395 DEPs specific to the heat-tolerant and heat-susceptible genotypes, respectively. In the heat-tolerant genotype, upregulated proteins were linked to stimulus regulation and response. Proteins related to oxidative phosphorylation, glycosylphosphatidylinositol-anchor biosynthesis, and cell wall macromolecule metabolism were downregulated in susceptible genotypes. After HS, the mycelial growth of the heat-sensitive Z381 strain was inhibited, and mitochondrial cristae and cell wall integrity of this strain were severely impaired, suggesting that HS may inhibit mycelial growth of Z381 by damaging the cell wall and mitochondrial structure. Furthermore, thermotolerance-related regulatory pathways were explored by analyzing the protein-protein interaction network of DEPs considered to participate in the controlling the thermotolerance capacity. This study provides insights into G. lingzhi thermotolerance mechanisms and a basis for breeding a thermotolerant germplasm bank for G. lingzhi and other fungi.

Molecular regulation of fungal secondary metabolism

Many bioactive secondary metabolites synthesized by fungi have important applications in many fields, such as agriculture, food, medical and others. The biosynthesis of secondary metabolites is a complex process involving a variety of enzymes and transcription factors, which are regulated at different levels. In this review, we describe our current understanding on molecular regulation of fungal secondary metabolite biosynthesis, such as environmental signal regulation, transcriptional regulation and epigenetic regulation. The effects of transcription factors on the secondary metabolites produced by fungi were mainly introduced. It was also discussed that new secondary metabolites could be found in fungi and the production of secondary metabolites could be improved. We also highlight the importance of understanding the molecular regulation mechanisms to activate silent secondary metabolites and uncover their physiological and ecological functions. By comprehensively understanding the regulatory mechanisms involved in secondary metabolite biosynthesis, we can develop strategies to improve the production of these compounds and maximize their potential benefits.

Physicochemical Properties, Antioxidant Markers, and Meat Quality as Affected by Heat Stress: A Review

Heat stress is one of the most stressful events in livestock life, negatively impacting animal health, productivity, and product quality. Moreover, the negative impact of heat stress on animal product quality has recently attracted increasing public awareness and concern. The purpose of this review is to discuss the effects of heat stress on the quality and the physicochemical component of meat in ruminants, pigs, rabbits, and poultry. Based on PRISMA guidelines, research articles were identified, screened, and summarized based on inclusion criteria for heat stress on meat safety and quality. Data were obtained from the Web of Science. Many studies reported the increased incidences of heat stress on animal welfare and meat quality. Although heat stress impacts can be variable depending on the severity and duration, the exposure of animals to heat stress (HS) can affect meat quality. Recent studies have shown that HS not only causes physiological and metabolic disturbances in living animals but also alters the rate and extent of glycolysis in postmortem muscles, resulting in changes in pH values that affect carcasses and meat. It has been shown to have a plausible effect on quality and antioxidant activity. Acute heat stress just before slaughter stimulates muscle glycogenolysis and can result in pale, tender, and exudative (PSE) meat characterized by low water-holding capacity (WHC). The enzymatic antioxidants such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) act by scavenging both intracellular and extracellular superoxide radicals and preventing the lipid peroxidation of the plasma membrane. Therefore, understanding and controlling environmental conditions is crucial to successful animal production and product safety. The objective of this review was to investigate the effects of HS on meat quality and antioxidant status.

Cross Talk between GlAQP and NOX Modulates the Effects of ROS Balance on Ganoderic Acid Biosynthesis of Ganoderma lucidum under Water Stress

Water stress affects both the growth and development of filamentous fungi; however, the mechanisms underlying their response to water stress remain unclear. In this study, water stress was found to increase intracellular reactive oxygen species (ROS) level, ganoderic acid (GA) content, and NADPH oxidase (NOX) activity of Ganoderma lucidum by 148.45%, 75.32%, and 161.61%, respectively. Water stress induced the expression of the G. lucidum aquaporin (GlAQP) gene, which facilitated water transfer for microbial growth. Compared to wild type (WT), exposure to water stress increased growth inhibition rate, ROS level, and GA content of GlAQP-silenced strains by 37 to 41%, 36 to 38%, and 25%, respectively. Furthermore, at the early stage of fermentation in GlAQP-silenced strains, water stress resulted in 16 to 17% and 9 to 10% lower ROS level and GA content compared to WT, respectively. However, in GlAQP-overexpressing strains, ROS level and GA content were 22 to 24% and 12 to 13% higher than in WT, respectively. In GlAQP-silenced strains, water stress at the late stage resulted in 35 to 37% and 29 to 30% higher ROS level and GA content, respectively, while in GlAQP-overexpressing strains, levels were 16 to 17% and 9% lower than WT, respectively. Cross talk between GlAQP and NOX positively regulated the GA biosynthesis of G. lucidum via ROS under water stress at the early stage but this regulation became negative at the late stage. This study deepens the understanding of fungal signaling transduction under water stress and provides a reference for analyzing environmental factors that influence the regulation of the fungal secondary metabolism. IMPORTANCE Ganoderma lucidum is an advanced basidiomycete that produces medicinally active secondary metabolites (especially ganoderic acid [GA]) with high commercial value. Water stress imposes an important environmental challenge to G. lucidum. The mechanism of GA biosynthesis under water stress and the role of G. lucidum aquaporin (GlAQP) during its biosynthesis remain unclear. Moreover, the effect of the relationship between GlAQP and NADPH oxidase (NOX) on the level of reactive oxygen species and GA production under water stress is unknown. This study provides information on the biological response mechanism of G. lucidum to water stress. A new theory on the cell signaling cascade of G. lucidum tolerance to water stress is provided that also incorporates the biosynthesis of secondary metabolites involved in NOX and GlAQP.

Heme Oxygenase/Carbon Monoxide Participates in the Regulation of Ganoderma lucidum Heat-Stress Response, Ganoderic Acid Biosynthesis, and Cell-Wall Integrity

Carbon monoxide (CO), a product of organic oxidation processes, arises in vivo principally from the enzymatic reaction of heme oxygenase (HO, transcription gene named HMX1). HO/CO has been found to exert many salutary effects in multiple biological processes, including the stress response. However, whether HO/CO is involved in the regulation of the heat-stress (HS) response of Ganoderma lucidum (G. lucidum) is still poorly understood. In this paper, we reported that under heat stress, the HMX1 transcription level, HO enzyme activity, and CO content increased by 5.2-fold, 6.5-fold and 2-fold, respectively. HMX1 silenced strains showed a 12% increase in ganoderic acid (GA) content under HS as analyzed by HPLC. Furthermore, according to Western blot analysis of the protein phosphorylation levels, HMX1 attenuated the increase in phosphorylation levels of slt2, but the phosphorylation levels were prolonged over a 3 h HS time period. The chitin and glucan content in HMX1 silenced strains increased by 108% and 75%, respectively. In summary, these findings showed that the HO/CO system responds to heat stress and then regulates the HS-induced GA biosynthesis and the cell-wall integrity mediated by the Slt-MAPK phosphorylation level in G. lucidum.

Target of Rapamycin Mediated Ornithine Decarboxylase Antizyme Modulate Intracellular Putrescine and Ganoderic Acid Content in Ganoderma lucidum

Putrescine (Put) has been shown to play an important regulatory role in cell growth in organisms. As the primary center regulating the homeostasis of polyamine (PA) content, ornithine decarboxylase antizyme (AZ) can regulate PA content through feedback. Nevertheless, the regulatory mechanism of Put is poorly understood in fungi. Here, our analysis showed that GlAZ had a modulate effect on intracellular Put content by interacting with ornithine decarboxylase (ODC) proteins and reducing its intracellular protein levels. In addition, GlAZ upregulated the metabolic pathway of ganoderic acid (GA) biosynthesis in Ganoderma lucidum by modulating the intracellular Put content. However, a target of rapamycin (TOR) was found to promote the accumulation of intracellular Put after the GlTOR inhibitor Rap was added exogenously, and unbiased analyses demonstrated that GlTOR may promote Put production through its inhibitory effect on the level of GlAZ protein in GlTOR-GlAZ-cosilenced strains. The effect of TOR on fungal secondary metabolism was further explored, and the content of GA in the GlTOR-silenced strain after the exogenous addition of the inhibitor Rap was significantly increased compared with that in the untreated wild-type (WT) strain. Silencing of TOR in the GlTOR-silenced strains caused an increase in GA content, which returned to the WT state after replenishing Put. Moreover, the content of GA in GlTOR-GlAZ-cosilenced strains was also not different from that in the WT strain. Consequently, these results strongly indicate that GlTOR affects G. lucidum GA biosynthesis via GlAZ. IMPORTANCE Research on antizyme (AZ) in fungi has focused on the mechanism by which AZ inhibits ornithine decarboxylase (ODC). Moreover, there are existing reports on the regulation of AZ protein translation by TOR. However, little is known about the mechanisms that influence AZ in fungal secondary metabolism. Here, both intracellular Put content and GA biosynthesis in G. lucidum were shown to be regulated through protein interactions between GlAZ and GlODC. Furthermore, exploration of upstream regulators of GlAZ suggested that GlAZ was regulated by the upstream protein GlTOR, which affected intracellular Put levels and ganoderic acid (GA) biosynthesis. The results of our work contribute to the understanding of the upstream regulation of Put and provide new insights into PA regulatory systems and secondary metabolism in fungi.

LSD3 mediates the oxidative stress response through fine-tuning APX2 activity and the NF-YC15-GSTs module in cassava

Reactive oxygen species (ROS) overproduction leads to oxidative damage under almost all stress conditions. Lesion-Simulating Disease (LSD), a zinc finger protein, is an important negative regulator of ROS accumulation and cell death in plants. However, the in vivo role of LSD in cassava (Manihot esculenta) and the underlying molecular mechanisms remain elusive. Here, we found that MeLSD3 is essential for the oxidative stress response in cassava. MeLSD3 physically interacted with ascorbate peroxidase 2 (MeAPX2), thereby promoting its enzymatic activity. In addition, MeLSD3 also interacted with the nuclear factor YC15 (MeNF-YC15), which also interacted with nuclear factor YA2/4 (MeNF-YA2/4) and nuclear factor YB18 (MeNF-YB18) to form an MeNF-YC15-MeNF-YA2/4-MeNF-YB18 complex. Notably, MeLSD3 positively modulated the transcriptional activation of the MeNF-YC15-MeNF-YA2/4-MeNF-YB18 complex by interacting with the CCAAT boxes of the promoters of glutathione S-transferases U37/U39 (MeGST-U37/U39), activating their transcription. When one or both of MeLSD3 and the MeNF-YC15-MeNF-YA2/4-MeNF-YB18 complex were co-silenced, cassava showed decreased oxidative stress resistance, while overexpression of MeGST-U37/U39 alleviated the oxidative stress-sensitive phenotype of these silenced plants. This study illustrates the dual roles of MeLSD3 in promoting MeAPX2 activity and MeNF-YC15-MeGST-U37/U39 regulation, which underlie the oxidative stress response in cassava.

Integrated Transcriptomics and Nontargeted Metabolomics Analysis Reveal Key Metabolic Pathways in Ganoderma lucidum in Response to Ethylene

Ganoderic acid (GA) is an important secondary metabolite of Ganoderma lucidum with a diverse array of pharmacological properties. In this study, we found that exogenous ethylene increased the production of endogenous ethylene and ganoderic acid in G. lucidum. However, the mechanism by which ethylene is regulated remains unclear. As a result, we performed a combined transcriptomics and nontargeted metabolomics analysis to evaluate the regulatory mechanism of ethylene. A total of 4070 differentially expressed genes (1835 up-regulated and 2235 down-regulated) and 378 differentially accumulated metabolites (289 up-regulated and 89 down-regulated) were identified in all groups. The transcriptomics and nontargeted metabolomics data revealed that genes involved in the tricarboxylic acid (TCA) cycle, polyamine metabolic pathway, acetyl-CoA carboxylase (ACC) pathway, and triterpenoid metabolism were up-regulated, whereas the metabolic intermediates involved in these metabolic pathways were down-regulated. These findings imply that ethylene potentially accelerates normal glucose metabolism, hence increasing the number of intermediates available for downstream biological processes, including polyamine metabolism, ethylene synthesis pathway, and ganoderic acid biosynthesis. The findings will contribute significantly to our understanding of secondary metabolites biosynthesis in fungi.

Spermidine Regulates Mitochondrial Function by Enhancing eIF5A Hypusination and Contributes to Reactive Oxygen Species Production and Ganoderic Acid Biosynthesis in Ganoderma lucidum

Spermidine, a kind of polycation and one important member of the polyamine family, is essential for survival in many kinds of organisms and participates in the regulation of cell growth and metabolism. To explore the mechanism by which spermidine regulates ganoderic acid (GA) biosynthesis in Ganoderma lucidum, the effects of spermidine on GA and reactive oxygen species (ROS) contents were examined. Our data suggested that spermidine promoted the production of mitochondrial ROS and positively regulated GA biosynthesis. Further research revealed that spermidine promoted the translation of mitochondrial complexes I and II and subsequently influenced their activity. With a reduction in eukaryotic translation initiation factor 5A (eIF5A) hypusination by over 50% in spermidine synthase gene (spds) knockdown strains, the activities of mitochondrial complexes I and II were reduced by nearly 60% and 80%, respectively, and the protein contents were reduced by over 50%, suggesting that the effect of spermidine on mitochondrial complexes I and II was mediated through its influence on eIF5A hypusination. Furthermore, after knocking down eIF5A, the deoxyhypusine synthase gene (dhs), and the deoxyhypusine hydroxylase gene (dohh), the mitochondrial ROS level was reduced by nearly 50%, and the GA content was reduced by over 40%, suggesting that eIF5A hypusination contributed to mitochondrial ROS production and GA biosynthesis. In summary, spermidine maintains mitochondrial ROS homeostasis by regulating the translation and subsequent activity of complexes I and II via eIF5A hypusination and promotes GA biosynthesis via mitochondrial ROS signaling. The present findings provide new insight into the spermidine-mediated biosynthesis of secondary metabolites. IMPORTANCE Spermidine is necessary for organism survival and is involved in the regulation of various biological processes. However, the specific mechanisms underlying the various physiological functions of spermidine are poorly understood, especially in microorganisms. In this study, we found that spermidine hypusinates eIF5A to promote the production of mitochondrial ROS and subsequently regulate secondary metabolism in microorganisms. Our study provides a better understanding of the mechanism by which spermidine regulates mitochondrial function and provides new insight into the spermidine-mediated biosynthesis of secondary metabolites.

GSNOR regulates ganoderic acid content in Ganoderma lucidum under heat stress through S-nitrosylation of catalase

As a master regulator of the balance between NO signaling and protein S-nitrosylation, S-nitrosoglutathione (GSNO) reductase (GSNOR) is involved in various developmental processes and stress responses. However, the proteins and specific sites that can be S-nitrosylated, especially in microorganisms, and the physiological functions of S-nitrosylated proteins remain unclear. Herein, we show that the ganoderic acid (GA) content in GSNOR-silenced (GSNORi) strains is significantly lower (by 25%) than in wild type (WT) under heat stress (HS). Additionally, silencing GSNOR results in an 80% increase in catalase (CAT) activity, which consequently decreases GA accumulation via inhibition of ROS signaling. The mechanism of GSNOR-mediated control of CAT activity may be via protein S-nitrosylation. In support of this possibility, we show that CAT is S-nitrosylated (as shown via recombinant protein in vitro and via GSNORi strains in vivo). Additionally, Cys (cysteine) 401, Cys642 and Cys653 in CAT are S-nitrosylation sites (assayed via mass spectrometry analysis), and Cys401 may play a pivotal role in CAT activity. These findings indicate a mechanism by which GSNOR responds to stress and regulates secondary metabolite content through protein S-nitrosylation. Our results also define a new S-nitrosylation site and the function of an S-nitrosylated protein regulated by GSNOR in microorganisms.

Liu et al. identify catalase as a target of S-nitrosylation by GSNOR and the specific sites of S-nitrosylation critical for its role in regulating secondary metabolite production in Ganoderma lucidum under heat stress. This study suggests that GSNOR regulates other metabolic pathways in microorganisms through S-nitrosylation of target proteins in response to environmental changes.

Effect of ultrasonication on the metabolome and transcriptome profile changes in the fermentation of Ganoderma lucidum

Development of an efficient liquid fermentation method is helpful for food and pharmaceutical applications. This study investigated the effect of ultrasonication on the liquid fermentation of Ganoderma lucidum, a popular edible and medical fungi. Significant changes at both metabolic and transcriptional levels in mycelia were induced by ultrasound treatment. Compared with the control, 857 differential metabolites were identified (578 up- and 279 down-regulated metabolites), with more metabolites biosynthesis after sonication; 569 differentially expressed genes (DEGs) (267 up- and 302 down-) and 932 DEGs (378 up- and 554 down-) were identified in ultrasound-treated samples with recovery time of 0.5 and 3 h, respectively. Furthermore, 334 DEGs were continuously induced within the recovery time of 3 h, indicating the lasting influence of sonication on mycelia. The DEGs and differential metabolites were mainly involved in pathways of carbohydrate, energy metabolism, amino acids, terpenoids biosynthesis and metabolism and membrane transport, suggesting that ultrasound induced multifaceted effects on primary and secondary metabolism. Ultrasonication enhanced the triterpenoids production of G. lucidum (34.96 %) by up-regulating the expression of terpenoids synthase genes. This study shows that the application of ultrasound in liquid fermentation of G. lucidum is an efficient approach to produce more metabolites.

Heat stress in macrofungi: effects and response mechanisms

Temperature is one of the key factors that affects the growth and development of macrofungi. Heat stress not only negatively affects the morphology and growth rate of macrofungi, but also destroys cell structures and influences cell metabolism. Due to loosed structure of cell walls and increased membrane fluidity, which caused by heat stress, the outflow of intracellular nutrients makes macrofungi more vulnerable to invasion by pathogens. Macrofungi accumulate reactive oxygen species (ROS), Ca²⁺, and nitric oxide (NO) when heat-stressed, which transmit and amplify the heat stimulation signal through intracellular signal transduction pathways. Through regulation of some transcription factors including heat response factors (HSFs), POZCP26 and MYB, macrofungi respond to heat stress by different mechanisms. In this paper, we present mechanisms used by macrofungi to adapt and survive under heat stress conditions, including antioxidant defense systems that eliminate the excess ROS, increase in trehalose levels that prevent enzymes and proteins deformation, and stabilize cell structures and heat shock proteins (HSPs) that repair damaged proteins and synthesis of auxins, which increase the activity of antioxidant enzymes. All of these help macrofungi resist and adapt to heat stress. KEY POINTS: • The effects of heat stress on macrofungal growth and development were described. • The respond mechanisms to heat stress in macrofungi were summarized. • The further research directions of heat stress in macrofungi were discussed.

GCN4 Regulates Secondary Metabolism through Activation of Antioxidant Gene Expression under Nitrogen Limitation Conditions in Ganoderma lucidum

Nitrogen limitation has been widely reported to affect the growth and development of fungi, and the transcription factor GCN4 (general control nonderepressible 4) is involved in nitrogen restriction. Here, we found that nitrogen limitation highly induced the expression of GCN4 and promoted the synthesis of ganoderic acid (GA), an important secondary metabolite in Ganoderma lucidum. The activated GCN4 is involved in regulating GA biosynthesis. In addition, the accumulation of reactive oxygen species (ROS) also affects the synthesis of GA under nitrogen restrictions. The silencing of the gcn4 gene led to further accumulation of ROS and increased the content of GA. Further studies found that GCN4 activated the transcription of antioxidant enzyme biosynthesis genes gr, gst2, and cat3 (encoding glutathione reductase, glutathione S-transferase, and catalase, respectively) through direct binding to the promoter of these genes to reduce the ROS accumulation. In conclusion, our study found that GCN4 directly interacts with the ROS signaling pathway to negatively regulate GA biosynthesis under nitrogen-limiting conditions. This provides an essential insight into the understanding of GCN4 transcriptional regulation of the ROS signaling pathway and enriches the knowledge of nitrogen regulation mechanisms in fungal secondary metabolism of G. lucidum. IMPORTANCE Nitrogen has been widely reported to regulate secondary metabolism in fungi. Our study assessed the specific nitrogen regulatory mechanisms in Ganoderma lucidum. We found that GCN4 directly interacts with the ROS signaling pathway to negatively regulate GA biosynthesis under nitrogen-limiting conditions. Our research highlights a novel insight that GCN4, the nitrogen utilization regulator, participates in secondary metabolism through ROS signal regulation. In addition, this also provides a theoretical foundation for exploring the regulation of other physiological processes by GCN4 through ROS in fungi.

Heat stress promotes the conversion of putrescine to spermidine and plays an important role in regulating ganoderic acid biosynthesis in Ganoderma lucidum

Heat stress (HS) is inescapable environmental stress that can induce the production of ganoderic acids (GAs) in Ganoderma lucidum. Our previous studies found that putrescine (Put) played an inhibitory role in GAs biosynthesis, which appeared to be inconsistent with the upregulated transcription of the Put biosynthetic gene GlOdc under HS. To uncover the mechanism underlying this phenomenon, two spermidine (Spd) biosynthetic genes, GlSpds1 and GlSpds2, were identified and upregulated under HS. Put and Spd increased by 94% and 160% under HS, respectively, suggesting that HS induces polyamine biosynthesis and promotes the conversion of Put to Spd. By using GlSpds knockdown mutants, it is confirmed that Spd played a stimulatory role in GAs biosynthesis. In GlOdc-kd mutants, Put decreased by 62-67%, Spd decreased by approximately 34%, and GAs increased by 15-22% but sharply increased by 75-89% after supplementation with Spd. In GlSpds-kd mutants, Put increased by 31-41%, Spd decreased by approximately 63%, and GAs decreased by 24-32% and were restored to slightly higher levels than a wild type after supplementation with Spd. This result suggested that Spd, rather than Put, is a crucial factor that leads to the accumulation of GAs under HS. Spd plays a more predominant and stimulative role than Put under HS, possibly because the absolute content of Spd is 10 times greater than that of Put. GABA and H₂O₂, two major catabolites of Spd, had little effect on GAs biosynthesis. This study provides new insight into the mechanism by which environmental stimuli regulate secondary metabolism via polyamines in fungi. KEY POINTS: • HS induces polyamine biosynthesis and promotes the conversion of Put to Spd in G. lucidum. • Put and Spd played the inhibitory and stimulatory roles in regulating GAs biosynthesis, respectively. • The stimulatory role of Spd was more predominant than the inhibitory role of Put in GAs biosynthesis.

Effect of the heme oxygenase gene on mycelial growth and polysaccharide synthesis in Ganoderma lucidum

The heme oxygenase gene has antioxidant and cytoprotective effects in organisms, but no related research has been conducted in Ganoderma lucidum. For the first time, we cloned the HMX1 gene in G. lucidum. The CDS is 1092 bp in length and encodes 363 amino acids. The HMX1 protein was prokaryotically expressed and purified, and the enzyme activity of the purified protein was measured. The value of K_m was 0.699 μM, and V_m was 81.9 nmol BV h^-1  nmol^-1 protein. By constructing the silencing vector pAN7-dual-HMX1i, the transformants HMX1i1 and HMX1i2 were obtained. Compared with the wild-type (WT), the average growth rate of HMX1i1 and HMX1i2 decreased by 31% and 23%, respectively, and the mycelium biomass decreased by 53% and 48%, respectively. Compared with the WT, the extracellular polysaccharide content of HMX1i1 and HMX1i2 increased by 59% and 51%, and the intracellular polysaccharide content increased by 24% and 22%, respectively. These results indicate that the HMX1 gene affects mycelial growth and polysaccharide synthesis in G. lucidum.

Label-Free Comparative Proteomics Analysis Revealed Heat Stress Responsive Mechanism in Hypsizygus marmoreus

Heat stress is an important adverse environmental stress that influences the growth and development of Hypsizygus marmoreus (white var.). However, the molecular basis of heat stress response in H. marmoreus remains poorly understood. In this study, label-free comparative proteomic technique was applied to investigate global protein expression profile of H. marmoreus mycelia under heat stress. Confocal laser scanning microscope observation revealed that mycelia underwent autolysis and apoptosis under heat stress. Autolysis was mediated by upregulating the expression of cell wall degradation enzymes and inhibiting cell wall synthesis enzymes, and apoptosis might be induced by ROS and activation of caspases. TBARS analysis indicated that ROS was accumulated in H. marmoreus mycelia under heat stress. H. marmoreus induced antioxidant defense system by upregulating the expression of catalases, superoxide dismutases and peroxidases to prevent oxidative damage. MAPK cascade was found to be involved in heat stress signal transduction. The stress signal induced a ubiquitous defense response: inducible expression of different kinds of heat shock proteins. Trehalose synthesis enzymes were also upregulated, suggesting the accumulation of stress protector trehalose under heat stress. Besides, upregulated proteasome was identified, which could prevented the accumulation of non-functional misfolding proteins. To satisfy ATP depletion in heat response cellular processes, such as ROS scavenging, and protein folding and synthesis, enzymes involved in energy production (carbon metabolism and ATP synthesis) system were upregulated under heat stress. Taken together, these findings improve our understanding of the molecular mechanisms underlying the response of heat stress in H. marmoreus.

Heat shock increases hydrogen peroxide release from circulating hemocytes of the snail Biomphalaria glabrata

Planorbid freshwater snails are important intermediate hosts for parasitic diseases caused by parasitic worms, most notably schistosomiasis. There are numerous reports of snails, specifically Biomphalaria glabrata, having compromised defences against schistosomes after being exposed to thermal stress. Environmental modifications to the defenses of schistosome transmitting snails could have negative ramifications for human disease risk in the context of climate change. Here the effects of heat shock on the production of hydrogen peroxide, a primary anti-microbial effector in many molluscs, were examined. The present findings show that heat shock increases NADPH oxidase 2 mRNA levels and hydrogen peroxide produced by snail hemocytes, and that both of these phenotypes could be reversed by an HSP-90 inhibitor. These findings indicate that snail defense systems are altered by heat shock at a molecular level in B. glabrata, and that snail immunity to many pathogens may be altered by the rapid variations in temperature that are associated with global climate change.

Nitric oxide regulates ganoderic acid biosynthesis by the S-nitrosylation of aconitase under heat stress in Ganoderma lucidum

Nitric oxide (NO) is an important signalling molecule in stress response of organisms. We previously reported that NO decreases heat stress (HS)-induced ganoderic acid (GA) accumulation in Ganoderma lucidum. To explore the mechanisms by which NO modulates GA biosynthesis under HS, the effect of NO on the reactive oxygen species (ROS) content was examined. The results showed that NO decreased the production of mitochondrial ROS (mitROS) by 60% under HS. Further research revealed that NO reduced the mitROS content by inhibiting aconitase (Acon) activity. The GA content in Acon-silenced (Aconi) strains treated with NO donor did not differ significantly from that in untreated Aconi strains. To study the mechanism by which Acon activity is inhibited, the S-nitrosylation level of Acon was determined. Biotin-switch technology and mass spectrometry analysis were used to show that Acon is S-nitrosylated at the Cys-594 amino acid residue. Substitution of Cys-594 with a Ser, which cannot be S-nitrosylated, abolished the responsiveness of Acon to the NO-induced reduction in its enzymatic activity. These findings demonstrate that NO inhibits Acon activity through S-nitrosylation at Cys-594. In summary, these findings describe mechanism by which NO regulates GA biosynthesis via S-nitrosylation of Acon under HS condition in G. lucidum.

Putrescine regulates nitric oxide accumulation in Ganoderma lucidum partly by influencing cellular glutamine levels under heat stress

When fungi are subjected to abiotic stresses, the polyamines (PAs) level alter significantly. Here, we reveal that the polyamine putrescine (Put) could play an important role in alleviating heat stress(HS)-induced accumulation of nitric oxide (NO). Ornithine decarboxylase (ODC)-silenced mutants that were defective in Put biosynthesis exhibited significantly lower NO levels than the wild type (WT) when subjected to HS. With addition of 5 mM exogenous Put, the ODC-silenced mutant endogenous Put obviously increased under HS. At the same time, the contents of NO in the ODC-silenced mutants recovered to approximately WT levels after the administration of exogenous Put. However, the elevated NO content in the ODC-silenced mutants disappeared when exogenous Put and carboxy-PTIO (PTIO is a specific scavenger of NO) were added. Intriguingly, the content of glutamine (Gln) was significantly increased in the ODC-silenced strains. When exogenous Put was added to the WT, the Gln content was significantly decreased. The appearance of a high level of Gln was accompanied by nitrate reductase (NR) activity reduction. Further studies showed that Put influenced ganoderic acids (GAs) biosynthesis by regulating NO content, possibly through NR, under HS. Our work reported that Put regulates HS-induced NO accumulation by changing the cellular Gln level in filamentous fungi. IMPORTANCE: In our present work, it was HS as an ubiquitous environmental stress that affects the important pharmacological secondary metabolite (GAs) content in G. lucidum. Afterwards, we began to explore the network formed between multiple substances to jointly reduce the massive accumulation of GAs content caused by HS. We firstly focused on Put, a substance that enhances resistance to multiple stresses. Further, we discovered an influence on Put could changing the NO content, which has been shown to decrease the accumulation of GAs via HS. Then, we also found the change of NO content may be due to Put level that would affect intracellular Gln content. It has never been reported. And ultimately, it is Put related network that could reduce HS-inducing secondary metabolite mess in fungi.

Lactic acid accumulation under heat stress related to accelerated glycolysis and mitochondrial dysfunction inhibits the mycelial growth of Pleurotus ostreatus

High temperature is a major threat to Pleurotus ostreatus cultivation. In this study, a potential mechanism by which P. ostreatus mycelia growth is inhibited under heat stress was explored. Lactate, as a microbial fermentation product, was found unexpectedly in the mycelia of P. ostreatus under heat stress, and the time-dependent accumulation and corresponding inhibitory effect of lactate on mycelial growth was further confirmed. The addition of a glycolysis inhibitor, 2-deoxy-D-glucose (2DG), reduced the lactate content in mycelia and slightly restored mycelial growth under high-temperature conditions, which indicated the accumulation of lactate can be inhibited by glycolysis inhibition. Further data revealed mitochondrial dysfunction under high-temperature conditions, with evidence of decreased oxygen consumption and adenosine triphosphate (ATP) synthesis and increased reactive oxygen species (ROS). The removal of ROS with ascorbic acid decreased the lactate content, and mycelial growth recovered to a certain extent, indicating lactate accumulation could be affected by the mitochondrial ROS. Moreover, metabolic data showed that glycolysis and the tricarboxylic acid cycle were enhanced. This study reported the accumulation of lactate in P. ostreatus mycelia under heat stress and the inhibitory effect of lactate on the growth of mycelia, which might provide further insights into the stress response mechanism of edible fungi. Key Points • Lactate can accumulate in Pleurotus ostreatus mycelia under heat stress and inhibit its growth. • The accumulation of lactate may be due to the acceleration of glycolysis and the dysfunction of mitochondria of P. ostreatus mycelia under high-temperature stress. • The glycolysis and tricarboxylic acid cycle of P. ostreatus mycelia were accelerated under high-temperature stress.

The Role of Reactive Oxygen Species and Nitric Oxide in the Inhibition of Trichophyton rubrum Growth by HaCaT Cells

Trichophyton rubrum (T. rubrum) is one of the most important agents of dermatophyte infection in humans. The aim of this experiment was to evaluate the effect of HaCaT cells on T. rubrum, investigate the responsible mechanism of action, and explore the role of reactive oxygen species (ROS) and nitric oxide (NO) in the inhibition of T. rubrum growth by HaCaT cells. The viability of fungi treated with HaCaT cells alone and with HaCaT cells combined with pretreatment with the NADPH oxidase inhibitor (DPI) or the nitric oxide synthase (NOS) inhibitor L-NMMA was determined by enumerating the colony-forming units. NOS, ROS, and NO levels were quantified using fluorescent probes. The levels of the NOS inhibitor asymmetric dimethylarginine (ADMA) were determined by enzyme-linked immunosorbent assay (ELISA). Micromorphology was observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In addition, fungal keratinase activity was assessed by measuring dye release from keratin azure. In vitro fungal viability, keratinase activity, and ADMA content decreased after HaCaT cell intervention, whereas the levels of ROS, NO, and NOS increased. The micromorphology was abnormal. Fungi pretreated with DPI and L-NMMA exhibited opposite effects. HaCaT cells inhibited the growth and pathogenicity of T. rubrum in vitro. A suggested mechanism is that ROS and NO play an important role in the inhibition of T. rubrum growth by HaCaT cells.

Glsnf1-mediated metabolic rearrangement participates in coping with heat stress and influencing secondary metabolism in Ganoderma lucidum

The AMP-activated protein kinase (AMPK)/Sucrose-nonfermenting serine-threonine protein kinase 1 (Snf1) plays an important role in metabolic remodelling in response to energy stress. However, the role of AMPK/Snf1 in responding to other environmental stresses and metabolic remodelling in microorganisms was unclear. Heat stress (HS), which is one important environmental factor, could induce the production of reactive oxygen species and the accumulation of ganoderic acids (GAs) in Ganoderma lucidum. Here, the functions of AMPK/Snf1 were analysed under HS condition in G. lucidum. We observed that Glsnf1 was rapidly and strongly activated when G. lucidum was exposed to HS. HS significantly increased intracellular H₂O₂ levels (by approximately 1.6-fold) and decreased the dry weight of G. lucidum (by approximately 45.6%). The exogenous addition of N-acetyl-l-cysteine (NAC) and ascorbic acid (VC), which function as ROS scavengers, partially inhibited the HS-mediated reduction in biomass. Adding the AMPK/Snf1 inhibitor compound C (20 μM) under HS conditions increased the H₂O₂ content (by approximately 2.3-fold of that found in the strain without HS treatment and 1.5-fold of that found in the strain under HS treatment without compound C) and decreased the dry weight of G. lucidum (an approximately 28.5% decrease compared with that of the strain under HS conditions without compound C). Similar results were obtained by silencing the Glsnf1 gene. Further study found that Glsnf1 meditated metabolite distribution from respiration to glycolysis, which is considered a protective mechanism against oxidative stress. In addition, Glsnf1 negatively regulated the biosynthesis of GA by removing ROS. In conclusion, our results suggest that Glsnf1-mediated metabolic remodelling is involved in heat stress adaptability and the biosynthesis of secondary metabolites in G. lucidum.

Metabolic Response of Pleurotus ostreatus to Continuous Heat Stress

Heat stress seriously threatens the growth of Pleurotus ostreatus. Various studies have been performed to study the resistance of P. ostreatus to heat stress. Here, the metabolome was evaluated to determine the response of P. ostreatus mycelia to heat stress at different times (6, 12, 24, 48 h). More than 70 differential metabolites were detected and enriched in their metabolic pathways. Dynamic metabolites changes in enrichment pathways under heat stress showed that heat stress enhanced the degradation of unsaturated fatty acids and nucleotides, increased the content of amino acids and vitamins, and accelerated glycolysis and the tricarboxylic acid cycle in P. ostreatus. The time course changes of P. ostreatus metabolites under continuous heat stress demonstrated that amino acids continuously changed with heat stress, nucleotides clearly changed with heat stress at 12 and 48 h, and lipids exhibited an increasing trend with prolonged heat stress, while few types saccharides and vitamins changed under heat stress. Additionally, heat-treated P. ostreatus produced salicylic acid and other stress-resistant substances that were reported in plants. This study first reported the metabolites changes in P. ostreatus mycelia during 48 h of heat stress. The metabolic pathways and substances that changed with heat stress in this research will aid future studies on the resistance of P. ostreatus and other edible fungi to heat stress.

Heat shock protein 70 (HmHsp70) from Hypsizygus marmoreus confers thermotolerance to tobacco

The 70-kD heat shock proteins (Hsp70s) have been proved to be important for stress tolerance and protein folding and unfolding in almost all organisms. However, the functions of Hsp70s in mushroom are not well understood. In the present study, a hsp70 gene from Hypsizygus marmoreus, hmhsp70, was cloned and transferred to tobacco (Nicotiana tabacum) to evaluate its function in thermotolerance. Sequence alignments and phylogenetic analysis revealed that HmHsp70 may be located in the mitochondria region. qPCR analysis revealed that the transcription level of hmhsp70 in H. marmoreus mycelia increased after heat shock treatment in high temperature (42 °C) compared with untreated mycelia (at 25 °C). Transgenic tobaccos expressing hmhsp70 gene showed enhanced resistance to lethal temperature compared with the wild type (WT) plants. Nearly 30% of the transgenic tobaccos survived after treated at a high temperature (50 °C and 52 °C for 4 h); however, almost all the WT tobaccos died after treated at 50 °C and no WT tobacco survived after heat shock at 52 °C. This study firstly showed the function of a hsp70 gene from H. marmoreus.

Integrated Proteomics and Metabolomics Analysis Provides Insights into Ganoderic Acid Biosynthesis in Response to Methyl Jasmonate in Ganoderma Lucidum

Ganoderma lucidum is widely recognized as a medicinal basidiomycete. It was previously reported that the plant hormone methyl jasmonate (MeJA) could induce the biosynthesis of ganoderic acids (GAs), which are the main active ingredients of G. lucidum. However, the regulatory mechanism is still unclear. In this study, integrated proteomics and metabolomics were employed on G. lucidum to globally identify differences in proteins and metabolites under MeJA treatment for 15 min (M15) and 24 h (M24). Our study successfully identified 209 differential abundance proteins (DAPs) in M15 and 202 DAPs in M24. We also identified 154 metabolites by GC-MS and 70 metabolites by LC-MS in M24 that are involved in several metabolic pathways. With an in-depth analysis, we found some DAPs and metabolites that are involved in the oxidoreduction process, secondary metabolism, energy metabolism, transcriptional and translational regulation, and protein synthesis. In particular, our results reveal that MeJA treatment leads to metabolic rearrangement that inhibited the normal glucose metabolism, energy supply, and protein synthesis of cells but promoted secondary metabolites, including GAs. In conclusion, our proteomics and metabolomics data further confirm the promoting effect of MeJA on the biosynthesis of GAs in G. lucidum and will provide a valuable resource for further investigation of the molecular mechanisms of MeJA signal response and GA biosynthesis in G. lucidum and other related species.

Functions of reactive oxygen species in apoptosis and ganoderic acid biosynthesis in Ganoderma lucidum

Ganoderma lucidum is a medicinal fungus that is widely used in traditional medicine. Fungal PacC is recognized as an important transcription factor that functions during adaptation to environmental pH, fungal development and secondary metabolism. Previous studies have revealed that GlPacC plays important roles in mycelial growth, fruiting body development and ganoderic acid (GA) biosynthesis. In this study, using a terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) assay, we found that the apoptosis level was increased when PacC was silenced. The transcript and activity levels of caspase-like proteins were significantly increased in the PacC-silenced (PacCi) strains compared with the control strains. Silencing PacC also resulted in an increased reactive oxygen species (ROS) levels (∼2-fold) and decreased activity levels of enzymes involved in the antioxidant system. Further, we found that the intracellular ROS levels contributed to apoptosis and GA biosynthesis. Adding N-acetyl-cysteine and vitamin C decreased intracellular ROS and resulted in the inhibition of apoptosis in the PacCi strains. Additionally, the GA biosynthesis was different between the control strains and the PacCi strains after intracellular ROS was eliminated. Taken together, the findings showed that silencing PacC can result in an intracellular ROS burst, which increases cell apoptosis and GA biosynthesis levels. Our study provides novel insight into the functions of PacC in filamentous fungi.

Influence of PacC on the environmental stress adaptability and cell wall components of Ganoderma lucidum

The transcription factor PacC/Rim101 participates in environmental pH adaptation, development and secondary metabolism in many fungi, but whether PacC/Rim101 contributes to fungal adaptation to environmental stress remains unclear. In our previous study, a homologous gene of PacC/Rim101 was identified, and PacC-silenced strains of the agaricomycete Ganoderma lucidum were constructed. In this study, we further investigated the functions of PacC in G. lucidum and found that PacC-silenced strains were hypersensitive to environmental stresses, such as osmotic stress, oxidative stress and cell wall stress, compared with wild-type (WT) and empty-vector control (CK) strains. In addition, transmission electron microscopy images of the cell wall structure showed that the cell walls of the PacC-silenced strains were thinner (by approximately 25-30%) than those of the WT and CK strains. Further analysis of cell wall composition showed that the β-1,3-glucan content in the PacC-silenced strains was only approximately 78-80% of that in the WT strain, and the changes in β-1,3-glucan content were consistent with downregulation of glucan synthase gene expression. The ability of PacC to bind to the promoters of glucan synthase-encoding genes confirms that PacC transcriptionally regulates these genes.

Interdependent nitric oxide and hydrogen peroxide independently regulate the coix seed oil-induced triterpene acid accumulation in Ganoderma lingzhi

Recent progress has been made in adding exogenous vegetable oils in culture media to promote bioactive metabolite production in several medicinal mushrooms, but the mechanism is still unclear. In this study, we found that the vegetable oil coix seed oil (CSO) could induce the biosynthesis of triterpene acids (TAs) and also significantly increase cytoplasmic nitric oxide (NO) and hydrogen peroxide (H₂O₂) concentrations in the mycelium of Ganoderma lingzhi. The change in TA biosynthesis caused by CSO could be reversed by adding NO scavenger or H₂O₂ scavenger, and adding NO scavenger or H₂O₂ scavenger resulted in the reduction of the cytoplasmic H₂O₂ or NO concentration under CSO treatment, respectively. Moreover, adding NO scavenger or H₂O₂ scavenger reversed TA biosynthesis, which could be rescued by H₂O₂ or NO donor, respectively. Taken together, our study indicated that both NO and H₂O₂ were involved in the regulation of TA biosynthesis, and CSO-activated NO and H₂O₂ were interdependent but independently regulated the TA biosynthesis under CSO treatment in G. lingzhi.

Hydrogen sulfide, a novel small molecule signalling agent, participates in the regulation of ganoderic acids biosynthesis induced by heat stress in Ganoderma lucidum

Hydrogen sulfide (H₂S), an emerging small-molecule signalling agent, was recently shown to play a significant role in many physiological processes, but relatively few studies have been conducted on microorganisms compared with mammals and plants. By studying the pretreatment of H₂S donor sodium hydrosulfide (NaHS) and the scavenger hypotaurine (HT) and Cystathionine β-synthase silenced strains, we found that H₂S could alleviate the HS-induced ganoderic acids (GAs) biosynthesis. Our transcriptome results also showed that many signaling pathways and metabolic pathways, such as the glycolysis, TCA, oxidative phosphorylation and pentose phosphate pathway, are influenced by H₂S. Further experimental results indicated that H₂S could affect the physiological process of Ganoderma lucidum by interacting with multiple signals, including ROS, NO, AMPK, sphingolipid, mTOR, phospholipase D and MAPK, and physiological and pharmacological analyses showed that H₂S might alleviate the biosynthesis of GAs by inhibiting the intracellular calcium in G. lucidum.

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.