1,8-dihydroxy naphthalene (DHN) - melanin confers tolerance to cadmium in isolates of melanised dark septate endophytes

The Metallotolerance and Biosorption of As(V) and Cr(VI) by Black Fungi

A collection of 34 melanized fungi isolated previously from anthropogenic contaminated sites were assessed for their tolerance to toxic concentrations of As(V) and Cr(VI) anions. Three strains of the species Cyphellophora olivacea, Rhinocladiella similis, and Exophiala mesophila (Chaetothyriales) were identified as hyper-metallotolerant, with estimated IC50 values that ranged from 11.2 to 16.9 g L-1 for As(V) and from 2.0 to 3.4 g L-1 for Cr(VI). E. mesophila and R. similis were selected for subsequent assays on their biosorption capacity and kinetics under different pH values (4.0 and 6.5) and types of biomass (active and dead cells and melanin extracts). The fungal biosorption of As(V) was relatively ineffective, but significant removal of Cr(VI) was observed from liquid cultures. The Langmuir model with second-order kinetics showed maximum sorption capacities of 39.81 mg Cr6+ g-1 for R. similis and 95.26 mg Cr6+ g-1 for E. mesophila on a dry matter basis, respectively, while the kinetic constant for these two fungi was 1.32 × 10-6 and 1.39 × 10-7 g (mg Cr6+ min)-1. Similar experiments with melanin extracts of E. mesophila showed maximum sorption capacities of 544.84 mg Cr6+ g-1 and a kinetic constant of 1.67 × 10-6 g (mg Cr6+ min)-1. These results were compared to bibliographic data, suggesting that metallotolerance in black fungi might be the result of an outer cell-wall barrier to reduce the diffusion of toxic metals into the cytoplasm, as well as the inner cell wall biosorption of leaked metals by melanin.

Physiological response mechanism of heavy metal-resistant endophytic fungi isolated from the roots of Polygonatum kingianum

This study aims to evaluate the tolerance of endophytic fungi isolated from the fibrous roots of Polygonatum kingianum to arsenic (As) and cadmium (Cd) and their physiological response mechanisms. Five isolated strains were obtained with EC50 values for As(V) ranging from 421 to 1281 mg/L, while the other three strains tolerated Cd(II) with an EC50 range of 407-1112 mg/L. Morphological and molecular identification indicated that these eight strains were Cladosporium spp. belonging to dark septate endophytes (DSEs). The contents of metal ions in mycelium sharply increased, reaching 38.87 mg/kg for strain MZ-11 under As(V) stress and 0.33 mg/kg for fungus PR-2 under Cd(II). The physiological response revealed that the biomass decreased with increasing concentrations of As(V) or Cd(II), and the activity of superoxide dismutase significantly improved under the corresponding EC50 -concentration As/Cd of the strains, as well as the contents of antioxidant substances, including metallothionein, glutathione, malondialdehyde, melanin, and proline. Taken together, the filamentous fungi of Cladosporium spp. accounted for a high proportion of fungi isolated from the fibrous roots of P. kingianum and had a strong capacity to tolerate As(V) or Cd(II) stress by improving antioxidase activities and the content of antioxidant substances, and immobilization of metal ions in hyphae.

Enhanced cadmium phytoextraction efficiency of ryegrass (Lolium perenne L.) by porous media immobilized Enterobacter sp. TY-1

Although studies on immobilized microorganisms have been conducted, their performance remains unclear for enhancing plants to remediate cadmium (Cd)-contaminated soil. In this study, a Cd-resistant strain TY-1 with good plant growth promotion traits was immobilized by biochar (BC) or oyster shell (OS) power to strengthen ryegrass to remediate Cd-contaminated soil. SEM-EDS combined with FTIR showed that TY-1 could tolerate Cd toxicity by surface precipitation, and functional groups such as hydroxyl and carbonyl groups might be involved. In the biocomposite treatments, soil pH increased, and the activity of fertility-related enzymes such as dehydrogenase increased by 109.01%-128.01%. The relative abundance of genus Saccharimonadales decreased from 7.97% to 3.35% in BS-TY and 2.61% in OS-TY, respectively. Thus, a suitable environment for ryegrass growth was created. The fresh weight, dry weight, plant height and Cd accumulation of ryegrass in TY treatment increased by 122.92%, 114.81%, 42.08% and 8.05%, respectively, compared to the control. Cd concentration in ryegrass was further increased in BC-TY and OS-TY by 24.14% and 40.23%, respectively. The improvement in soil microcosm and plant biomass forms an ongoing virtuous cycle, demonstrating that using carrier materials to improve the efficiency of microbial-assisted phytoremediation is realistic and feasible.

A Dark Septate Endophyte Improves Cadmium Tolerance of Maize by Modifying Root Morphology and Promoting Cadmium Binding to the Cell Wall and Phosphate

Dark septate endophytes (DSEs) can improve the performance of host plants grown in heavy metal-polluted soils, but the mechanism is still unclear. A sand culture experiment was performed to investigate the effects of a DSE strain (Exophiala pisciphila) on maize growth, root morphology, and cadmium (Cd) uptake under Cd stress at different concentrations (0, 5, 10, and 20 mg·kg^-1). The results indicated that the DSE significantly improved the Cd tolerance of maize, causing increases in biomass, plant height, and root morphology (length, tips, branch, and crossing number); enhancing the Cd retention in roots with a decrease in the transfer coefficient of Cd in maize plants; and increasing the Cd proportion in the cell wall by 16.0-25.6%. In addition, DSE significantly changed the chemical forms of Cd in maize roots, resulting in decreases in the proportions of pectates and protein-integrated Cd by 15.6-32.4%, but an increase in the proportion of insoluble phosphate Cd by 33.3-83.3%. The correlation analysis revealed a significantly positive relationship between the root morphology and the proportions of insoluble phosphate Cd and Cd in the cell wall. Therefore, the DSE improved the Cd tolerance of plants both by modifying root morphology, and by promoting Cd binding to the cell walls and forming an insoluble phosphate Cd of lower activity. These results of this study provide comprehensive evidence for the mechanisms by which DSE colonization enhances Cd tolerance in maize in root morphology with Cd subcellular distribution and chemical forms.

BCB1, a member of the acyl-coenzyme A synthetase family, regulates the morphogenesis and pathogenicity of Botrytis cinerea

Botrytis cinerea is a non-host-specific phytopathogenic fungus capable of infecting numerous cash crops. Here, we analyzed the functions of the Bcb1 gene in B. cinerea, which encodes a membrane protein belonging to the acyl-coenzyme A synthase family. Compared to the wild type, Bcb1-deletion mutants exhibited obvious morphological abnormalities, including slower vegetative growth and reduced melanin production. The absence of Bcb1 causes B. cinerea to form only small and incompletely developed infection cushions and fail to produce spores. The Bcb1 mutants displayed hypersensitivity to the membrane stressor SDS, the cell wall stressor Congo red, and the oxidative stressor H₂O₂ and increased resistance to intracellular osmotic stress caused by KCl compared to the wild-type strain. However, there were no differences in tolerance to extracellular osmotic stress caused by NaCl. The deletion of Bcb1 also caused a reduction in pathogenicity. The qRT‒PCR results showed that the genes Bcpks12 and Bcpks13, which are related to melanin biosynthesis, and Bcpg2, BcBOT2, and cutA, which are related to virulence, were downregulated in ∆Bcb1. These data suggest that BCB1 is important for conidial morphogenesis, and pathogenesis in B. cinerea.

The responses and detoxification mechanisms of dark septate endophytes (DSE), Exophiala salmonis, to CuO nanoparticles

To understand the tolerance mechanisms of dark septate endophytes (DSE), Exophiala salmonis, to CuO nanoparticles (CuO-NPs) with different sizes (40 and 150 nm), we investigated the morphology, antioxidant response, Cu subcellular distribution, and the melanin gene expression in the mycelia of E. salmonis. E. salmonis was cultured in liquid and solid media under the stress of increasing CuO-NP concentrations (0, 50, 100, 150, and 250 mg/L). Results showed that (1) E. salmonis showed good CuO-NP tolerance, and the tolerance to CuO-NPs at 150 nm was stronger than that at 40 nm. A large number of agglomeration structures were observed on the mycelia surface with the exception of 50 mg/L CuO-NPs with a diameter of 150 nm. (2) CuO-NP stress significantly stimulated the production of antioxidant enzymes, particularly the CuO-NPs with small particle size (40 nm). (3) Cu uptaken by E. salmonis increased proportionally with the increase of CuO-NP concentration in the medium. More than 80% Cu was absorbed in cell wall of mycelia treated with a small particle size (40 nm). (4) FTIR analysis revealed that hydroxyl, amine, carboxyl, and phosphate groups were associated with CuO-NP binding regardless of particle size. (5) Fungal melanin content increased with the addition of CuO-NPs; the increase of melanin induced by CuO-NPs with small particle size (40 nm) was more significant. (6) The expression of 1,3,6,8-tetrahydroxynaphthalene reductase (Arp2) in the melanin synthesis pathway increased under the stress of CuO-NPs, and CuO-NPs with a small particle size (40 nm) caused a significant change in the expression level of Arp2 gene than those with a large particle size (150 nm). In conclusion, E. salmonis had a strong tolerance to CuO-NPs and mitigated the toxic effects of CuO-NPs through the antioxidant system, the expression of genes related to melanin synthesis, and the synthesis of melanin.

A Melanin-Deficient Isolate of Venturia inaequalis Reveals Various Roles of Melanin in Pathogen Life Cycle and Fitness

Venturia inaequalis is the ascomycetous pathogen causing apple scabs and forms dark-pigmented spores and partially melanised infection structures. Although melanin is considered to be essential for the infection of host tissue, a spontaneously occurring melanin-deficient mutant was isolated from an abaxial side of an apple leaf and can be cultivated in vitro as well as in vivo. The morphology and development of the melanin-deficient-isolate SW01 on leaves of susceptible apple plants were compared to that of the corresponding wild-type isolate HS1. White conidia of SW01 were often wrinkled when dry and significantly increased their volume in suspension. Germination and formation of germtubes and appressoria were not impaired; however, the lack of melanisation of the appressorial ring structure at the interface with the plant cuticle significantly reduced the infection success of SW01. The colonisation of leaf tissue by non-melanised subcuticular hyphae was not affected until the initiation of conidiogenesis. Non-melanised conidiophores penetrated the plant cuticle from inside less successfully than the wild type, and the release of white conidia from less solid conidiophores above the cuticle was less frequent. Melanin in the outer cell wall of V. inaequalis was not required for the survival of conidia under ambient temperature or at -20 °C storage conditions, however, promoted the tolerance of the pathogen to copper and synthetic fungicides affecting the stability and function of the fungal cell wall, plasma membrane, respiration (QoIs) and enzyme secretion, but had no effect on the sensitivity to sulphur and SDHIs. The roles of melanin in different steps of the V. inaequalis life cycle and the epidemiology of apple scabs are discussed.

Plant association with dark septate endophytes: When the going gets tough (and stressful), the tough fungi get going

Dark septate endophytes (DSEs) comprise a diverse and ubiquitous group of fungal generalists with broad habitat niches that robustly colonize the roots of plants in stressful environments. DSEs possess adaptation strategies that determine their high tolerance to heavy metal (HM) contamination, drought, and salinity. Most DSEs developed efficient melanin-dependent and melanin-independent mechanisms of HM detoxification and osmoprotection, including intracellular immobilization and extracellular efflux of HMs and excess ions, and the scavenging of reactive oxygen species. DSEs form mutualistic relationship with plants according to the hypothesis of "habitat-adapted associations", supporting the survival of their hosts under stressful conditions. As saprophytes, DSEs mineralize a complex soil substrate improving plants' nutrition and physiological parameters. They can protect the host plant from HMs by limiting HM accumulation in plant tissues and causing their sequestration in root cell walls as insoluble compounds, preventing further HM translocation to shoots. The presence of DSE in drought-affected plants can substantially ameliorate the physiology and architecture of root systems, improving their hydraulic properties. Plant growth-promoting features, supported by the versatility and easy culturing of DSEs, determine their high potential to enhance phytoremediation and revegetation projects for HM-contaminated, saline, and desertic lands reclamation.

Insights into the beneficial roles of dark septate endophytes in plants under challenging environment: resilience to biotic and abiotic stresses

Dark septate endophytes (DSE) exert a plethora of effects in regulating plant growth, signalling and stress tolerance. The advent of metagenomics has led to the identification of various species of DSE to be associated with plant organs. They are known to modulate growth, nutrient uptake, phytohormone biosynthesis and production of active bioconstituents in several plants. The interactions between the DSE and host plants are mostly mutualistic but they can also be neutral or exhibit negative interactions. The DSE has beneficial role in removal/sequestration of toxic heavy metals from various environmental sites. Here, we discuss the beneficial role of DSE in enhancing plant tolerance to heavy metal stress, drought conditions, high salinity and protection from various plant pathogens. Furthermore, the underlying mechanism of stress resilience facilitated by DSE-plant interaction has also been discussed. The article also provides insights to some important future perspectives associated with DSE-mediated phytoremediation and reclamation of polluted land worldwide thus facilitating sustainable agriculture.

Advances in the Role of Dark Septate Endophytes in the Plant Resistance to Abiotic and Biotic Stresses

Endophytic fungi have been studied in recent decades to understand how they interact with their hosts, the types of relationships they establish, and the potential effects of this interaction. Dark septate endophytes (DSE) are isolated from healthy plants and form melanised structures in the roots, including inter- and intracellular hyphae and microsclerotia, causing low host specificity and covering a wide geographic range. Many studies have revealed beneficial relationships between DSE and their hosts, such as enhanced plant growth, nutrient uptake, and resistance to biotic and abiotic stress. Furthermore, in recent decades, studies have revealed the ability of DSE to mitigate the negative effects of crop diseases, thereby highlighting DSE as potential biocontrol agents of plant diseases (BCAs). Given the importance of these fungi in nature, this article is a review of the role of DSE as BCAs. The findings of increasing numbers of studies on these fungi and their relationships with their plant hosts are also discussed to enable their use as a tool for the integrated management of crop diseases and pests.