Fungal Guttation, a Source of Bioactive Compounds, and Its Ecological Role-A Review

Endophytic fungi from Himalayan silver birch as potential source of plant growth enhancement and secondary metabolite production

Mountain biodiversity is under unparalleled pressure due to climate change, necessitating in-depth research on high-altitude plant's microbial associations which are crucial for plant survival under stress conditions. Realizing that high-altitude tree line species of Himalaya are completely unexplored with respect to the microbial association, the present study aimed to elucidate plant growth promoting and secondary metabolite producing potential of culturable endophytic fungi of Himalayan silver birch (Betula utilis D. Don). ITS region sequencing revealed that the fungal isolates belong to Penicillium species, Pezicula radicicola, and Paraconiothyrium archidendri. These endophytes were psychrotolerant in nature with the potential to produce extracellular lytic activities. The endophytes showed plant growth promoting (PGP) traits like phosphorus solubilization and production of siderophore, indole acetic acid (IAA), and ACC deaminase. The fungal extracts also exhibited antagonistic potential against bacterial pathogens. Furthermore, the fungal extracts were found to be a potential source of bioactive compounds including the host-specific compound-betulin. Inoculation with fungal suspension improved seed germination and biomass of soybean and maize crops under net house conditions. In vitro PGP traits of the endophytes, supported by net house experiments, indicated that fungal association may support the growth and survival of the host in extreme cold conditions.

Genome sequencing and molecular networking analysis of the wild fungus Anthostomella pinea reveal its ability to produce a diverse range of secondary metabolites

BACKGROUND

Filamentous fungi are prolific producers of bioactive molecules and enzymes with important applications in industry. Yet, the vast majority of fungal species remain undiscovered or uncharacterized. Here we focus our attention to a wild fungal isolate that we identified as Anthostomella pinea. The fungus belongs to a complex polyphyletic genus in the family of Xylariaceae, which is known to comprise endophytic and pathogenic fungi that produce a plethora of interesting secondary metabolites. Despite that, Anthostomella is largely understudied and only two species have been fully sequenced and characterized at a genomic level.

RESULTS

In this work, we used long-read sequencing to obtain the complete 53.7 Mb genome sequence including the full mitochondrial DNA. We performed extensive structural and functional annotation of coding sequences, including genes encoding enzymes with potential applications in biotechnology. Among others, we found that the genome of A. pinea encodes 91 biosynthetic gene clusters, more than 600 CAZymes, and 164 P450s. Furthermore, untargeted metabolomics and molecular networking analysis of the cultivation extracts revealed a rich secondary metabolism, and in particular an abundance of sesquiterpenoids and sesquiterpene lactones. We also identified the polyketide antibiotic xanthoepocin, to which we attribute the anti-Gram-positive effect of the extracts that we observed in antibacterial plate assays.

CONCLUSIONS

Taken together, our results provide a first glimpse into the potential of Anthstomella pinea to provide new bioactive molecules and biocatalysts and will facilitate future research into these valuable metabolites.

"Fruiting Liquid" of Mushroom-Forming Fungi, A Novel Source of Bioactive Compounds - Fruiting-Body Inducer and HIF and Axl Inhibitors

In general, mushroom-forming fungi secrete liquid on the surface of mycelia just before fruiting-body formation. However, no researchers in mushroom science have paid attention to the liquid until now. We formulated a hypothesis that the liquid plays an important role(s) in the formation of the fruiting body and produces various bioactive compounds and named it the "fruiting liquid (FL)". Four novel compounds (1-4) were isolated from FL of Hypholoma lateritium and Hericium erinaceus. The structures of 1-4 except for their stereochemistry were determined by interpretation of MS and NMR data. The absolute configurations of compounds 1-4 were determined by quantum chemical calculation of the ECD spectrum, by single-crystal X-ray diffraction analyses, or by chemical syntheses. Compounds 1, 3, and 4 induced fruiting body formation of Flammulina velutipes. Compound 4 inhibited the activity of hypoxia-inducible factor, and compounds 2-4 suppressed receptor tyrosine kinase (Axl) expression.

Penicillium rhizophilum, a novel species in the section Exilicaulis isolated from the rhizosphere of sugarcane in Southwest Iran

During a survey of species diversity of Penicillium and Talaromyces in sugarcane (Saccharum officinarum) rhizosphere in the Khuzestan province of Iran [1], 195 strains were examined, from which 187 belonged to Penicillium (11 species) and eight to Talaromyces (one species). In the present study, three strains of Penicillium belonging to section Exilicaulis series Restricta, identified as P. restrictum by Ansari et al. [1], were subjected to a phylogenetic study. The multilocus phylogeny of partial β-tubulin, calmodulin and RNA polymerase II second largest subunit genes enabled the recognition of one new phylogenetic species that is here formally described as Penicillium rhizophilum sp. nov. This species is phylogenetically distinct in series Restricta, but it does not show significant morphological differences from other species previously classified in the series. Therefore, we here placed bias on the phylogenetic species concept. The holotype of Penicillium rhizophilum sp. nov. is IRAN 18169F and the ex-type culture is LA30T (=IRAN 4042CT=CBS 149737T).

Biocontrol potential of the endophytic Epicoccum nigrum HE20 against stripe rust of wheat

Biological control using endophytic microorganisms represents an eco-friendly and effective alternative to the health-hazardous chemical fungicides used to control devastating plant diseases such as stripe rust in wheat. In this study, the inhibitory potential of the endophytic Epicoccum nigrum HE20, isolated from a healthy wheat plant, was screened against uredospores germination in vitro. A high suppression (96%) in the germination of the uredospores was recorded. GC-MS analysis of the culture filtrate of E. nigrum HE20 showed a production of various secondary metabolites with an antifungal background such as butyric acid, α-linolenic acid, hexanoic acid, lactic acid, 10,12-Tricosadiynoic acid, and pentadecanoic acid. Results from the greenhouse experiment revealed that the application of E. nigrum HE20 suspension led to a reduction in the disease severity by 87.5%, compared with the untreated-infected plants. Real-time PCR results exhibited an overexpression in three defensive genes (JERF3, GLU, and PR1) in the infected wheat plants, in response to the application of E. nigrum HE20, recorded 8-, 15.8-, and 3.5-fold, respectively. In addition, an increment in the phenolic content, activity of POD, PPO, and CAT, and a reduction in the lipid peroxidation were recorded due to the endophyte application. Transmission electron microscopic observations indicated mitigation of the pathogen in wheat cells after the treatment with E. nigrum HE20 metabolite. Furthermore, a growth-promoting effect was also observed due to E. nigrum HE20 application, as well as an increment in the total photosynthetic pigments in wheat leaves. Based on these results, it can be concluded that E. nigrum HE20 is a probable efficient bioagent against stripe rust in wheat. However, its field evaluation is highly necessary in the future studies.

Auxin- and pH-induced guttation in Phycomyces sporangiophores: relation between guttation and diminished elongation growth

Guttation, the formation of exudation water, is widespread among plants and fungi, yet the underlying mechanisms remain largely unknown. We describe the conditions for inducing guttation in sporangiophores of the mucoracean fungus, Phycomyces blakesleeanus. Cultivation on peptone-enriched potato dextrose agar elicits vigorous guttation mainly below the apical growing zone, while sporangiophores raised on a glucose-mineral medium manifest only moderate guttation. Mycelia do not guttate irrespective of the employed media. The topology of guttation droplets allows identifying the non-growing part of the sporangiophore as a guttation zone, which responds to humidity and medium composition in ways that become relevant for turgor homeostasis and thus the sensor physiology of the growing zone. Apparently, the entire sporangiophore, rather than exclusively the growing zone, participates in signal reception and integration to generate a common growth output. Exogenous auxin applied to the growing zones elicits two correlated responses: (i) formation of guttation droplets in the growing and transition zones below the sporangium and (ii) a diminution of the growth rate. In sporangiophore populations, guttation-induction by exogenous control buffer occurs at low frequencies; the bias for guttation increases with increasing auxin concentration. Synthetic auxins and the transport inhibitor NPA suppress guttation completely, but leave growth rates largely unaffected. Mutants C2 carA and C148 carA madC display higher sensitivities for auxin-induced guttation compared to wild type. A working model for guttation includes aquaporins and mechanosensitive ion channels that we identified in Phycomyces by sequence domain searches.

Phenolic-Rich Extracts from Circular Economy: Chemical Profile and Activity against Filamentous Fungi and Dermatophytes

Fungal infections represent a relevant issue in agri-food and biomedical fields because they could compromise quality of food and humans' health. Natural extracts represent a safe alternative to synthetic fungicides and in the green chemistry and circular economy scenario, agro-industrial wastes and by-products offer an eco-friendly source of bioactive natural compounds. In this paper, phenolic-rich extracts from Olea europaea L. de-oiled pomace, Castanea sativa Mill. wood, Punica granatum L. peel, and Vitis vinifera L. pomace and seeds were characterized by HPLC-MS-DAD analysis. Finally, these extracts were tested as antimicrobial agents against pathogenic filamentous fungi and dermatophytes such as Aspergillus brasiliensis, Alternaria sp., Rhizopus stolonifer, and Trichophyton interdigitale. The experimental results evidenced that all extracts exhibited a significant growth inhibition for Trichophyton interdigitale. Punica granatum L., Castanea sativa Mill., and Vitis vinifera L. extracts showed a high activity against Alternaria sp. and Rhizopus stolonifer. These data are promising for the potential applications of some of these extracts as antifungal agents in the food and biomedical fields.

Parthenium hysterophorus's Endophytes: The Second Layer of Defense against Biotic and Abiotic Stresses

Parthenium hysterophorus L. is considered an obnoxious weed due to its rapid dispersal, fast multiplications, and agricultural and health hazards. In addition to its physio-molecular and phytotoxic allelochemical usage, this weed most probably uses endophytic flora as an additional line of defense to deal with stressful conditions and tolerate both biotic and abiotic stresses. The aim of this article is to report the diversity of endophytic flora (fungi and bacteria) in P. hysterophorus and their role in the stress mitigation (biotic and abiotic) of other important crops. Various endophytes were reported from P. hysterophorus and their roles in crops evaluated under biotic and abiotic stressed conditions. These endophytes have the potential to alleviate different stresses by improving crops/plants growth, development, biomass, and photosynthetic and other physiological traits. The beneficial role of the endophytes may be attributed to stress-modulating enzymes such as the antioxidants SOD, POD and APX and ACC deaminases. Additionally, the higher production of different classes of bioactive secondary metabolites, i.e., flavonoids, proline, and glutathione may also overcome tissue damage to plants under stressed conditions. Interestingly, a number of medicinally important phytochemicals such as anhydropseudo-phlegmcin-9, 10-quinone-3-amino-8-O methyl ether 'anhydropseudophlegmacin-9, 10-quinone-3-amino-8-Omethyl ether were reported from the endophytic flora of P. hysterophorus. Moreover, various reports revealed that fungal and bacterial endophytes of P. hysterophorus enhance plant growth-promoting attributes and could be added to the consortium of biofertilizers.

Salt and Metal Tolerance Involves Formation of Guttation Droplets in Species of the Aspergillus versicolor Complex

Three strains of the Aspergillus versicolor complex were isolated from a salty marsh at a former uranium mining site in Thuringia, Germany. The strains from a metal-rich environment were not only highly salt tolerant (up to 20% NaCl), but at the same time could sustain elevated Cs and Sr (both up to 100 mM) concentrations as well as other (heavy) metals present in the environment. During growth experiments when screening for differential cell morphology, the occurrence of guttation droplets was observed, specifically when elevated Sr concentrations of 25 mM were present in the media. To analyze the potential of metal tolerance being promoted by these excretions, proteomics and metabolomics of guttation droplets were performed. Indeed, proteins involved in up-regulated metabolic activities as well as in stress responses were identified. The metabolome verified the presence of amino sugars, glucose homeostasis-regulating substances, abscisic acid and bioactive alkaloids, flavones and quinones.

Size-Dependent Impact of Magnetic Nanoparticles on Growth and Sporulation of Aspergillus niger

Magnetic nanoparticles (MNPs) are becoming important DNA nanocarriers for genetic engineering of industrial fungi. However, the biological effect of MNPs on industrial fungi remains unknown. In this study, we prepared three kinds of magnetic nanoparticles with different sizes (i.e., 10 nm, 20 nm, and 200 nm) to investigate their impact on the growth and sporulation of the important industrial fungus Aspergillus niger. Transmission electron microscopy, X-ray diffraction analysis and Zeta potential analysis revealed that the three kinds of MNPs, including MNP10, MNP20 and MNP200, had uniform size distribution, regular Fe₃O₄ X-ray diffraction (XRD) patterns and similar Zeta potentials. Interestingly, although the three kinds of MNPs did not obviously inhibit growth of the fungus, the MNP20 at 500 mg/L strongly attenuated sporulation, leading to a remarkable decrease in spore numbers on culturing plates. Further investigation showed that MNP20 at the high concentration led to drastic chitin accumulation in the cell wall, indicating cell wall disruption of the MNP20-treated fungal cells. Moreover, the MNPs did not cause unusual iron dissolution and reactive oxygen species (ROS) accumulation, and the addition of ferrous ion, ferric ion or the reactive oxygen species scavenger N-acetyl-L-cysteine (NAC) had no impact on the sporulation of the fungus, suggesting that both iron dissolution and ROS accumulation did not contribute to attenuated sporulation by MNP20. This study revealed the size-dependent effect of MNPs on fungal sporulation, which was associated with MNP-induced cell wall disruption.

Nanoscale chemical mapping of exometabolites at fungal-mineral interfaces

Mineral-associated organic matter is an integral part of soil carbon pool. Biological processes contribute to the formation of such organo-mineral complexes when soil microbes, and in particular soil fungi, deposit a suite of extracellular metabolic compounds and their necromass on the mineral surfaces. While studied in bulk, micro- to nanoscale fungal-mineral interactions remain elusive. Of particular interest are the mutual effects at the interface between the fungal exometabolites and proximal mineral particles. In this work, we have grown saprotrophic and symbiotic fungi in contact with two soil minerals with contrasting properties: quartz and goethite, on top of X-ray transparent silicon nitride membrane windows and analyzed fungal hyphae by synchrotron-based scanning transmission X-ray microscopy in combination with near edge X-ray fine structure spectroscopy at C(K) and Fe(L) absorption edges. In the resultant chemical maps, we were able to visualize and differentiate organic compounds constituting the fungal cells, their extracellular metabolites, and the exometabolites adsorbing on the minerals. We found that the composition of the exometabolites differed between the fungal functional guilds, particularly, in their sugar to protein ratio and potassium concentration. In samples with quartz and goethite, we observed adsorption of the exometabolic compounds on the mineral surfaces with variations in their chemical composition around the particles. Although we did not observe clear alteration in the exometabolite chemistry upon mineral encounters, we show that fungal-mineral interaction result in reduction of Fe(III) in goethite. This process has been demonstrated for bulk systems, but, to our knowledge, this is the first observation on a single hypha scale offering insight into its underlying biological mechanisms. This demonstrates the link between processes initiated at the single-cell level to macroscale phenomena. Thus, spatially resolved chemical characterization of the microbial-mineral interfaces is crucial for an increased understanding of overall carbon cycling in soil.