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Fungal endophytes can improve plant tolerance to abiotic stress conditions. Dark septate endophytes (DSEs) belong to phylogenetically non-related groups of root colonizing fungi among the Ascomycota with high melanin-producing activities. They can be isolated from roots of more than 600 plant species in diverse ecosystems. Still the knowledge about their interaction with host plants and their contribution to stress alleviation is limited. The current work aimed to test the abilities of three DSEs (Periconia macrospinosa, Cadophora sp., Leptodontidium sp.) to alleviate moderate and high salt stress in tomato plants. By including an albino mutant, the role of melanin for the interaction with plants and salt stress alleviation could also be tested. P. macrospinosa and Cadophora sp. improved shoot and root growth 6 weeks after inoculation under moderate and high salt stress conditions. No matter how much salt stress was applied, macroelement (P, N, and C) contents were unaffected by DSE inoculation. The four tested DSE strains successfully colonized the roots of tomato, but the colonization level was clearly reduced in the albino mutant of Leptodontidium sp. Any difference in the effects on plant growth between the Leptodontidium sp. wild type strain and the albino mutant could, however, not be observed. These results show that particular DSEs are able to increase salt tolerance as they promote plant growth specifically under stress condition. Increased plant biomasses combined with stable nutrient contents resulted in higher P uptake in shoots of inoculated plants at moderate and high salt conditions and higher N uptake in the absence of salt stress in all inoculated plants, in P. macrospinosa-inoculated plants at moderate salt condition and in all inoculated plants except the albino mutants at high salt condition. In summary, melanin in DSEs seems to be important for the colonization process, but does not influence growth, nutrient uptake or salt tolerance of plants. [ABSTRACT FROM AUTHOR]

Dark septate endophytes (DSEs) represent a diverse group of root-endophytic fungi that have been isolated from plant roots in many different natural and anthropogenic ecosystems. Melanin is widespread in eukaryotic organisms and possesses various functions such as protecting human skin from UV radiation, affecting the virulence of pathogens, and playing a role in development and physiology of insects. Melanin is a distinctive feature of the cell walls of DSEs and has been thought to protect these fungi from abiotic stress. Melanin in DSEs is assumed to be synthesized via the 1,8-dihydroxynaphthalene (DHN) pathway. Its function in alleviation of salt stress is not yet known. The aims of this study were: (i) investigating the growth responses of three DSEs (Periconia macrospinosa , Cadophora sp., and Leptodontidium sp.) to salt stress, (ii) analyzing melanin production under salt stress and, (iii) testing the role of melanin in salt stress tolerance of DSEs. The study shows that the three DSE species can tolerate high salt concentrations. Melanin content increased in the hyphae of all DSEs at 100 mM salt, but decreased at 500 mM. This was not reflected in the RNA accumulation of the gene encoding scytalone dehydratase which is involved in melanin biosynthesis. The application of tricyclazole, a DHN-melanin biosynthesis inhibitor, did not affect either salt stress tolerance or the accumulation of sodium in the hyphae. In addition, melanin biosynthesis mutants of Leptodontidium sp. did not show decreased growth performance compared to the wild-type, especially not at high salt concentrations. This indicates that DSEs can live under salt stress and withstand these conditions regardless of melanin accumulation. [ABSTRACT FROM AUTHOR]

In the soil-dwelling organism Pseudomonas putida KT2440, the rare earth element (REE)-utilizing, and pyrroloquinoline quinone (PQQ)-dependent ethanol dehydrogenase PedH is part of a periplasmic oxidation system that is vital for growth on various alcoholic volatiles. Production of PedH and its Ca2+-dependent counterpart PedE is inversely regulated in response to lanthanide (Ln3+) bioavailability, a mechanism termed the REE-switch. In the present study, we demonstrate that copper, zinc, and in particular, iron availability influences this regulation in a pyoverdine-independent manner by increasing the minimal Ln3+ concentration required for the REE-switch to occur by several orders of magnitude. A combined genetic and physiological approach reveals that an ABC-type transporter system encoded by the gene cluster pedA1A2BC is essential for efficient growth on 2-phenylethanol with low (nanomolar) Ln3+ concentrations. In the absence of pedA1A2BC , a ∼100-fold higher La3+-concentration is needed for PedH-dependent growth but not for the ability to repress growth based on PedE activity. From these results, we conclude that cytoplasmic uptake of lanthanides through PedA1A2BC is essential to facilitate REE-dependent growth on 2-phenylethanol under environmental conditions with poor REE bioavailability. Our data further suggest that the La3+/Fe2+/3+ ratio impacts the REE-switch through the mismetallation of putative La3+-binding proteins, such as the sensor histidine kinase PedS2, in the presence of high iron concentrations. As such, this study provides an example for the complexity of bacteria-metal interactions and highlights the importance of medium compositions when studying physiological traits in vitro in particular in regard to REE-dependent phenomena. [ABSTRACT FROM AUTHOR]

The presence of dark septate endophytes (DSEs) or arbuscular mycorrhizal fungi (AMF) in plant roots and their effects on plant fitness have been extensively described. However, little is known about their interactions when they are simultaneously colonizing a plant root, especially in trace element (TE)-polluted soils. We therefore investigated the effects of Cadophora sp. and Funneliformis mosseae on ryegrass (Lolium perenne) growth and element uptake in a Cd/Zn/Pb-polluted soil. The experiment included four treatments, i.e., inoculation with Cadophora sp., inoculation with F. mosseae, co-inoculation with Cadophora sp. and F. mosseae, and no inoculation. Ryegrass biomass and shoot Na, P, K, and Mg concentrations significantly increased following AMF inoculation as compared to non-inoculated controls. Similarly, DSE inoculation increased shoot Na concentration, whereas dual inoculation significantly decreased shoot Cd concentration. Moreover, oxidative stress determined by ryegrass leaf malondialdehyde concentration was alleviated both in the AMF and dual inoculation treatments. We used quantitative PCR and microscope observations to quantify colonization rates. They demonstrated that DSEs had no effect on AMF colonization, while AMF colonization slightly decreased DSE frequency. We also monitored fluorescein diacetate (FDA) hydrolysis and alkaline phosphatase (AP) activity in the rhizosphere soils. FDA hydrolysis remained unchanged in the three inoculated treatments, but AMF colonization increased AP activity and P mobility in the soil whereas DSE colonization did not alter AP activity. In this experiment, we unveiled the interactions between two ecologically important fungal groups likely to occur in roots which involved a decrease of oxidative stress and Cd accumulation in shoots. These results open promising perspectives on the fungal-based phytomanagement of TE-contaminated sites by the production of uncontaminated and marketable plant biomass. [ABSTRACT FROM AUTHOR]

Dark septate endophytes (DSEs) are abundant in stressful environments, including trace element (TE)-enriched soils. However, knowledge about the effects of DSEs on plant growth in such soils is poor compared to the well-known mycorrhizal fungi. The aim of this work was to evaluate the effects of three DSE strains isolated from TE-contaminated soils on the growth and mineral nutrition of Betula pendula and Populus tremula x alba grown on two contrasting TE-polluted soils. The three DSEs evenly colonized the two plant species in both soils. Nevertheless, plant responses to DSE inoculation varied from neutral to beneficial depending on soil properties. Depending on fungal strain and plant species, different factors seemed to contribute to plant growth promotion. Phialophora mustea Pr27 and Leptodontidium Pr30 decreased lipid peroxidation in birch shoots. Chlorophyll, K, and P concentrations increased in the shoots of Leptodontidium Pr30-inoculated trees, whereas Cd concentration decreased in Cadophora Fe06-inoculated birch. The absence of a general DSE-mediated plant growth–promoting behavior could represent a limiting factor for a generic use of DSEs in the tree-based phytomanagement of TE-contaminated soils. Our results suggest that the selection of strains adapted to particular edaphic conditions should not be overlooked within the framework of phytomanagement. [ABSTRACT FROM AUTHOR]

This study aimed to isolate, identify, and characterise metal-tolerant fungi colonising poplar roots at a metal-contaminated phytoremediation site. Poplar roots were colonised by arbuscular mycorrhizal, ectomycorrhizal, and endophytic fungi, and the species were determined by ITS molecular analyses. Eight different isolates were successfully isolated into pure culture. Three isolates belonging to the Helotiales (P02, P06) and the Serendipita vermifera species (P04) were highly tolerant to metals (Cd, Zn, Pb, and Cu) compared to the mycorrhizal Hebeloma isolates. The three isolates degraded complex carbohydrates, such as xylan and cellulose, indicating that they could partially degrade root cell walls and penetrate into cells. This hypothesis was confirmed by further in vitro re-synthesis experiments, which showed that the three isolates colonised root tissues of poplar plantlets whereas two of them formed microsclerotia-like structures. Taken together, these results suggest an endophytic lifestyle of these isolates. This is the first evidence of S. vermifera as a root endophyte of poplar. A new endophytic putative species belonging to the Helotiales and closely related to Leohumicola is also reported. Interestingly, and when compared to mock-inoculated plants, both P06 and P04 isolates increased the number of root tips of inoculated poplar plantlets in vitro. Moreover, the S. vermifera P04 isolate also increased the shoot biomass. The results are discussed in relation to the potential use of endophytic strains for tree-based phytoremediation of metal-contaminated sites. [ABSTRACT FROM AUTHOR]

Numerous studies address the distribution and the diversity of dark septate endophytes (DSEs) in the literature, but little is known about their ecological role and their effect on host plants, especially in metal-polluted soils. Seven DSE strains belonging to Cadophora, Leptodontidium, Phialophora and Phialocephala were isolated from roots of poplar trees from metal-polluted sites. All strains developed on a wide range of carbohydrates, including cell-wall-related compounds. The strains evenly colonized birch, eucalyptus and ryegrass roots in re-synthesis experiments. Root and shoot growth promotion was observed and was both plant and strain dependent. Two Phialophora and Leptodontidium strains particularly improved plant growth. However, there was no correlation between the level of root colonization by DSEs and the intensity of growth promotion. All strains produced auxin and six also stimulated plant growth through the release of volatile organic compounds (VOCs). SPME-GC/MS analyses revealed four major VOCs emitted by Cadophora and Leptodontidium. The strains exhibited growth at high concentrations of several metals. The ability of metal-resistant DSE strains to produce both soluble and volatile compounds for plant growth promotion indicates interesting microbial resources with high potential to support sustainable production of bioenergy crops within the context of the phytomanagement of metal-contaminated sites. [ABSTRACT FROM AUTHOR]

Dark septate endophytes (DSEs) are often trace element (TE)-tolerant fungi and are abundant in TE-polluted environments. The production of melanin, a black polymer found in cell walls, was hypothesized by several authors to play a role in the TE tolerance of DSEs. To test this hypothesis, we established a series of experiments using albino strains and melanin inhibitors and examined the responses to Cd and Zn. Six DSEs belonging to genera Cadophora sp., Leptodontidium sp. and Phialophora mustea, were evaluated. The strains mainly produced 1,8-dihydroxynaphthalene (DHN) melanin whereas 3,4-dihydroxyphenylalanin melanin was also synthetized. Cd and Zn decreased melanin synthesis in most of the strains. A reduction in melanin concentration in hyphae through the use of tricyclazole, an inhibitor of DHN-melanin synthesis, did not reduce the tolerance of the strains to Cd and Zn. Similarly, albino mutants of Leptodontidium sp. were not more sensitive to Cd and Zn than the WT strain. Moreover, tricyclazole-treated colonies accumulated less Cd but more Zn compared to untreated colonies. The Cd and Zn contents of Leptodontidium albino strains were variable and similar to that of the WT. The results suggest that melanin production is not an important functional trait that contributes to Cd and Zn tolerance, but might contribute to Cd accumulation. [ABSTRACT FROM AUTHOR]

Rare earth elements (REEs) are now considered emerging pollutants in the environment. Phytolacca americana, an REE hyperaccumulating plant, has been proposed for the remediation of REE-contaminated soils. However, there is no REE-related information for other Phytolacca species. Here, we examined five species (P. americana, P. acinosa, P. clavigera, P. bogotensis, and P. icosandra) for their response to REEs. REE accumulation and fractionation traits both occurred on the same order of magnitude among the five species. Heavy REEs were preferentially transferred to leaves relative to light REEs. Regardless of the species, lateral root length and chlorophyll content decreased under REE exposure, and lateral roots and foliar anthocyanins increased. However, plants did not experience or only slightly experienced oxidative stress. Finally, REE exposure strongly modulated the ionome of roots and, to a lesser extent, that of leaves, with a negative correlation between REE and Mn contents. In conclusion, our study provides new data on the response of several Phytolacca species to REEs. Moreover, we highlighted that the REE accumulation trait was conserved among Phytolacca species. Thus, we provide valuable information for the phytoremediation of REE-contaminated sites since the most appropriate Phytolacca species could be selected depending on the climatic/pedological area to be remediated. [ABSTRACT FROM AUTHOR]

Dark septate endophytes (DSEs) are widely distributed worldwide and can promote plant growth. Therefore, they are considered potentially important plant allies, especially in stressful environments. Previous studies have reported that DSEs cohabit roots with other microorganisms such as ectomycorrhizal (ECM), endophytic and pathogenic fungi/oomycetes. However, interactions between different DSE species have not yet been reported, and studies on the interactions between DSEs and other fungi are scarce. Using a simple and reproducible pairwise growth assay in vitro , we studied the synergistic/antagonistic interactions between eight DSEs, two ECM fungi and three root pathogens. Most of the DSE/DSE outcomes were neutral. Interestingly, we identified several DSE strains acting in synergy with other strains, as well as strains that could potentially act as biocontrol agents. Notably, three metal-tolerant DSE strains, namely, Cadophora sp. Leptodontidium sp. and Phialophora mustea , could decrease the growth of the root phytopathogens Pythium intermedium , Phytophthora citricola and Heterobasidion annosum. The present data are discussed in the general context of the use of fungal consortia as inocula in the tree-based phytomanagement of marginal lands. [ABSTRACT FROM AUTHOR]