The signaling role of extracellular ATP in co-culture of Shiraia sp. S9 and Pseudomonas fulva SB1 for enhancing hypocrellin A production

Nitric oxide mediates red light-induced perylenequinone production in Shiraia mycelium culture

Perylenequinones (PQs) from bambusicolous Shiraia fungi serve as excellent photosensitizers for photodynamic therapy. However, the lower yield of PQ production in mycelium cultures is an important bottleneck for their clinical application. Light has long been recognized as a pivotal regulatory signal for fungal secondary metabolite biosynthesis. In this study, we explored the role of nitric oxide (NO) in the growth and PQ biosynthesis in mycelium cultures of Shiraia sp. S9 exposed to red light. The continuous irradiation with red light (627 nm, 200 lx) suppressed fungal conidiation, promoted hyphal branching, and elicited a notable increase in PQ accumulation. Red light exposure induced NO generation, peaking to 81.7 μmol/g FW on day 8 of the culture, with the involvement of nitric oxide synthase (NOS)- or nitrate reductase (NR)-dependent pathways. The application of a NO donor sodium nitroprusside (SNP) restored conidiation of Shiraia sp. S9 under red light and stimulated PQ production, which was mitigated upon the introduction of NO scavenger carboxy-PTIO or soluble guanylate cyclase inhibitor NS-2028. These results showed that red light-induced NO, as a signaling molecule, was involved in the regulation of growth and PQ production in Shiraia sp. S9 through the NO-cGMP-PKG signaling pathway. While mycelial H2O2 content exhibited no significant alternations, a transient increase of intracellular Ca2+ and extracellular ATP (eATP) content was detected upon exposure to red light. The generation of NO was found to be interdependent on cytosolic Ca2+ and eATP concentration. These signal molecules cooperated synergistically to enhance membrane permeability and elevate the transcript levels of PQ biosynthetic genes in Shiraia sp. S9. Notably, the combined treatment of red light with 5 μM SNP yielded a synergistic effect, resulting in a substantially higher level of hypocrellin A (HA, 254 mg/L), about 3.0-fold over the dark control. Our findings provide valuable insights into the regulation of NO on fungal secondary metabolite biosynthesis and present a promising strategy involving the combined elicitation with SNP for enhanced production of photoactive PQs and other valuable secondary metabolites in fungi.

Biotechnological production and potential applications of hypocrellins

Hypocrellins (HYPs), a kind of natural perylenequinones (PQs) with an oxidized pentacyclic core, are important natural compounds initially extracted from the stromata of Hypocrella bambusae and Shiraia bambusicola. They have been widely concerned for their use as anti-microbial, anti-cancers, and anti-viral photodynamic therapy agents in recent years. Considering the restrictions of natural stromal resources, submerged fermentation with Shiraia spp. has been viewed as a promising alternative biotechnology for HYP production, and great efforts have been made to improve HYP production over the past decade. This article reviews recent publications about the mycelium fermentation production of HYPs, and their bioactivities and potential applications, and especially summarizes the progresses toward manipulation of fermentation conditions. Also, their chemical structure and analytic methods are outlined. Herein, it is worth mentioning that the gene arrangement in HYP gene cluster is revised; previous unknown genes in HYP and CTB gene clusters with correct function annotation are deciphered; the homologous sequences of HYP, CTB, and elc are systematically aligned, and especially the biosynthetic pathway of HYPs is full-scale proposed. KEY POINTS: • The mycelial fermentation process and metabolic regulation of hypocrellins are reviewed. • The bioactivities and potential applications of hypocrellins are summarized. • The biosynthesis pathway and regulatory mechanisms of hypocrellins are outlined.

Insights into peculiar fungal LPMO family members holding a short C-terminal sequence reminiscent of phosphate binding motifs

Lytic polysaccharide monooxygenases (LPMOs) are taxonomically widespread copper-enzymes boosting biopolymers conversion (e.g. cellulose, chitin) in Nature. White-rot Polyporales, which are major fungal wood decayers, may possess up to 60 LPMO-encoding genes belonging to the auxiliary activities family 9 (AA9). Yet, the functional relevance of such multiplicity remains to be uncovered. Previous comparative transcriptomic studies of six Polyporales fungi grown on cellulosic substrates had shown the overexpression of numerous AA9-encoding genes, including some holding a C-terminal domain of unknown function ("X282"). Here, after carrying out structural predictions and phylogenetic analyses, we selected and characterized six AA9-X282s with different C-term modularities and atypical features hitherto unreported. Unexpectedly, after screening a large array of conditions, these AA9-X282s showed only weak binding properties to cellulose, and low to no cellulolytic oxidative activity. Strikingly, proteomic analysis revealed the presence of multiple phosphorylated residues at the surface of these AA9-X282s, including a conserved residue next to the copper site. Further analyses focusing on a 9 residues glycine-rich C-term extension suggested that it could hold phosphate-binding properties. Our results question the involvement of these AA9 proteins in the degradation of plant cell wall and open new avenues as to the divergence of function of some AA9 members.

Osmotically Activated Anion Current of Phycomyces Blakesleeanus-Filamentous Fungi Counterpart to Vertebrate Volume Regulated Anion Current

Studies of ion currents in filamentous fungi are a prerequisite for forming a complete understanding of their physiology. Cytoplasmic droplets (CDs), obtained from sporangiophores of Phycomyces blakesleeanus, are a model system that enables the characterization of ion currents in the native membrane, including the currents mediated by the channels not yet molecularly identified. Osmotically activated anionic current with outward rectification (ORIC) is a dominant current in the membrane of cytoplasmic droplets under the conditions of hypoosmotic stimulation. We have previously reported remarkable functional similarities of ORIC with the vertebrate volume regulated anionic current (VRAC), such as dose-dependent activation by osmotic difference, ion selectivity sequence, and time and voltage dependent profile of the current. Using the patch clamp method on the CD membrane, we further resolve VRAC-like ORIC characteristics in this paper. We examine the inhibition by extracellular ATP and carbenoxolone, the permeation of glutamate in presence of chloride, selectivity for nitrates, and activation by GTP, and we show its single channel behavior in excised membrane. We propose that ORIC is a functional counterpart of vertebrate VRAC in filamentous fungi, possibly with a similar essential role in anion efflux during cell volume regulation.

Heat stress enhanced perylenequinones biosynthesis of Shiraia sp. Slf14(w) through nitric oxide formation

Perylenequinones (PQs) are a class of natural polyketides used as photodynamic therapeutics. Heat stress (HS) is an important environmental factor affecting secondary metabolism of fungi. This study investigated the effects of HS treatment on PQs biosynthesis of Shiraia sp. Slf14(w) and the underlying molecular mechanism. After the optimization of HS treatment conditions, the total PQs amount reached 577 ± 34.56 mg/L, which was 20.89-fold improvement over the control. Also, HS treatment stimulated the formation of intracellular nitric oxide (NO). Genome-wide analysis of Shiraia sp. Slf14(w) revealed iNOSL and cNOSL encoding inducible and constitutive NOS-like proteins (iNOSL and cNOSL), respectively. Cloned iNOSL in Escherichia coli BL21 showed higher nitric oxide synthase (NOS) activity than cNOSL, and the expression level of iNOSL under HS treatment was observably higher than that of cNOSL, suggesting that iNOSL is more responsible for NO production in the HS-treated strain Slf14(w) and may play an important role in regulating PQs biosynthesis. Moreover, the putative biosynthetic gene clusters for PQs and genes encoding iNOSL and nitrate reductase (NR) in the HS-treated strain Slf14(w) were obviously upregulated. PQs biosynthesis and efflux stimulated by HS treatment were significantly inhibited upon the addition of NO scavenger, NOS inhibitor, and NR inhibitor, indicating that HS-induced NO, as a signaling molecule, triggered promoted PQs biosynthesis and efflux. Our results provide an effective strategy for PQs production and contribute to the understanding of heat shock signal transduction studies of other fungi.Key points• PQs titer of Shiraia sp. Slf14(w) was significantly enhanced by HS treatment.• HS-induced NO was first reported to participate in PQs biosynthetic regulation.• Novel inducible and constitutive NOS-like proteins (iNOSL and cNOSL) were obtained and their NOS activities were determined.

Advances and perspectives on perylenequinone biosynthesis

Under illumination, the fungal secondary metabolites, perylenequinones (PQs) react with molecular oxygen to generate reactive oxygen species (ROS), which, in excess can damage cellular macromolecules and trigger apoptosis. Based on this property, PQs have been widely used as photosensitizers and applied in pharmaceuticals, which has stimulated research into the discovery of new PQs and the elucidation of their biosynthetic pathways. The PQs-associated literature covering from April 1967 to September 2022 is reviewed in three sections: (1) the sources, structural diversity, and biological activities of microbial PQs; (2) elucidation of PQ biosynthetic pathways, associated genes, and mechanisms of regulation; and (3) advances in pathway engineering and future potential strategies to modify cellular metabolism and improve PQ production.

Melatonin-Induced Inhibition of Shiraia Hypocrellin A Biosynthesis Is Mediated by Hydrogen Peroxide and Nitric Oxide

Melatonin (MLT), an evolutionarily conserved pleiotropic molecule, is implicated in numerous physiological processes in plants and animals. However, the effects of MLT on microbes have seldom been reported. In this study, we examined the influence of exogenous MLT on the growth and hypocrellin biosynthesis of bambusicolous fungus Shiraia sp. S9. Hypocrellin A (HA) is a photoactivated and photoinduced perylenequinone (PQ) toxin in Shiraia. Exogenous MLT at 100.00 μM not only decreased fungal conidiation and spore germination but inhibited HA contents significantly in fungal cultures under a light/dark (24 h:24 h) shift. MLT treatment was associated with higher activity of antioxidant enzymes (superoxide dismutase, catalase and peroxidase) and a marked decline in reactive oxygen species (ROS) production in the mycelia. Moreover, MLT induced endogenous nitric oxide (NO) production during the culture. The NO donor sodium nitroprusside (SNP) potentiated MLT-induced inhibition of O₂⁻ production, but NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) enhanced O₂⁻ production, whereas MLT-induced NO level was increased by the ROS scavenger vitamin C (Vc). The changes in NO and H₂O₂ were proved to be involved in the MLT-induced downregulation of the expressions of HA biosynthetic genes, leading to the suppression of HA production. This study provides new insight into the regulatory roles of MLT on fungal secondary metabolism activities and a basis for understanding self-resistance in phototoxin-producing fungi.

Effects of Blue Light on Hypocrellin A Production in Shiraia Mycelium Cultures

Blue light is a crucial environmental cue for fungi. Hypocrellin A (HA) is a photoactive perylenequinone from Shiraia with strong antimicrobial and anticancer properties. In this study, effects of the illumination of blue light-emitting diode (LED) at 470 nm on Shiraia sp. S8 was investigated. Blue light at 50-200 lx and 4-6 h/day could enhance HA content in the mycelia, but suppress it at 300-400 lx or with longer exposure (8-24 h/day). The intermittent blue light (6 h/day) at 200 lx not only enhanced the fungal conidiation, but stimulated HA production without any growth retardation. The generation of fungal reactive oxygen species (ROS) was induced to up-regulate HA biosynthetic gene expressions. When the culture was maintained under the intermittent blue light for 8 days, HA production reached 242.76 mg/L, 2.27-fold of the dark control. On the other hand, both the degradation of HA and down-regulation of HA biosynthetic genes occurred under long exposure time (8-24 h/day), leading to the suppression of HA production. These results provide a basis for understanding the regulation of blue light on the biosynthesis of fungal photoactivated perylenequinones, and the application of a novel light elicitation to Shiraia mycelium cultures for enhanced HA production.