Endophytic diazotrophic communities from rice roots are diverse and weakly associated with soil diazotrophic community composition and soil properties.

AIM: Bacteria that promote plant growth, such as diazotrophs, are valuable tools for achieving a more sustainable production of important non-legume crops like rice. Different strategies have been used to discover new bacteria capable of promoting plant growth. This work evaluated the contribution of soil diazotrophs to the endophytic communities established in the roots of rice seedlings cultivated on seven representative soils from Uruguay. METHODS AND RESULTS: The soils were classified into two groups according to the C and clay content. qPCR, terminal restriction fragment length polymorphism (T-RFLP), and 454-pyrosequencing of the nifH gene were used for analyzing diazotrophs in soil and plantlets' roots grown from seeds of the same genotype for 25 days under controlled conditions. A similar nifH abundance was found among the seven soils, roots, or leaves. The distribution of diazotrophs was more uneven in roots than in soils, with dominance indices significantly higher than in soils (nifH T-RFLP). Dominant soils' diazotrophs were mainly affiliated to Alphaproteobacteria and Planctomycetota. Conversely, Alpha, Beta, Gammaproteobacteria, and Bacillota were predominant in different roots, though undetectable in soils. Almost no nifH sequences were shared between soils and roots. CONCLUSIONS: Root endophytic diazotrophs comprised a broader taxonomic range of microorganisms than diazotrophs found in soils from which the plantlets were grown and showed strong colonization patterns.

Direct nitrogen, phosphorus and carbon exchanges between Mucoromycotina 'fine root endophyte' fungi and a flowering plant in novel monoxenic cultures.

Most plants form mycorrhizal associations with mutualistic soil fungi. Through these partnerships, resources are exchanged including photosynthetically fixed carbon for fungal-acquired nutrients. Recently, it was shown that the diversity of associated fungi is greater than previously assumed, extending to Mucoromycotina fungi. These Mucoromycotina 'fine root endophytes' (MFRE) are widespread and generally co-colonise plant roots together with Glomeromycotina 'coarse' arbuscular mycorrhizal fungi (AMF). Until now, this co-occurrence has hindered the determination of the direct function of MFRE symbiosis. To overcome this major barrier, we developed new techniques for fungal isolation and culture and established the first monoxenic in vitro cultures of MFRE colonising a flowering plant, clover. Using radio- and stable-isotope tracers in these in vitro systems, we measured the transfer of 33 P, 15 N and 14 C between MFRE hyphae and the host plant. Our results provide the first unequivocal evidence that MFRE fungi are nutritional mutualists with a flowering plant by showing that clover gained both 15 N and 33 P tracers directly from fungus in exchange for plant-fixed C in the absence of other micro-organisms. Our findings and methods pave the way for a new era in mycorrhizal research, firmly establishing MFRE as both mycorrhizal and functionally important in terrestrial ecosystems.

Deciphering the role of non-Frankia nodular endophytes in alder through in vitro and genomic characterization.

Endophytic bacterial populations are well-positioned to provide benefits to their host plants such as nutrient acquisition and plant hormone level manipulation. Actinorhizal plants such as alders are well known for their microbial symbioses that allow them to colonize harsh environments whether natural or anthropized. Although the nitrogen-fixing actinobacterium Frankia sp. is the main endophyte found in alder root nodules, other bacterial genera, whose roles remain poorly defined, inhabit this niche. In this study, we isolated a diverse panel of non-Frankia nodular endophytes (NFNE). Some NFNE were isolated from alders grown from surface-sterilized seeds and maintained in sterile conditions, suggesting these may have been seed-borne. In vitro testing of 24 NFNE revealed some possessed putative plant growth promotion traits. Their genomes were also sequenced to identify genes related to plant growth promotion traits. This study highlights the complexity of the alder nodular microbial community. It paves the way for further understanding of the biology of nodules and could help improve land reclamation practices that involve alders.

A metagenomic lens into endosphere microbial communities, promises, and discoveries.

The word endosphere represents the internal tissues of plants harboring diverse microbes capable of producing active biological products for various biotechnological and agricultural applications. The discreet standalone genes and interdependent association of microbial endophytes with plants can be an underlining factor in predicting their ecological functions. Yet-to-be-cultured endophytic microbes have geared the invention of metagenomics in various environmental studies to determine their structural diversity and functional genes with novel attributes. This review presents an overview of the general concept of metagenomics in microbial endophytic studies. First, the endosphere microbial communities were introduced, followed by metagenomic insights in endosphere biology, a promising technology. Also, the major application of metagenomics and a short brief on DNA stable isotope probing in determining functions and metabolic pathways of microbial metagenome were highlighted. Therefore, the use of metagenomics promises to provide answers to yet-to-be-cultured microbes by unraveling their diversity, functional attributes, and metabolic pathways with prospects in integrated and sustainable agriculture.

Exploring structural and molecular diversity of Ericaceae hair root mycobionts: a comparison between Northern Bohemia and Argentine Patagonia.

Core Ericaceae produce delicate hair roots with inflated rhizodermal cells that host plethora of fungal symbionts. These poorly known mycobionts include various endophytes, parasites, saprobes, and the ericoid mycorrhizal (ErM) fungi (ErMF) that form the ErM symbiosis crucial for the fitness of their hosts. Using microscopy and high-throughput sequencing, we investigated their structural and molecular diversity in 14 different host × site combinations in Northern Bohemia (Central Europe) and Argentine Patagonia (South America). While we found typical ericoid mycorrhiza in all combinations, we did not detect ectomycorrhiza and arbuscular mycorrhiza. Superficial mantles of various thickness formed by non-clamped hyphae were observed in all combinations except Calluna vulgaris from N. Bohemia. Some samples contained frequent intercellular hyphae while others possessed previously unreported intracellular haustoria-like structures linked with intracellular hyphal coils. The 711 detected fungal OTU were dominated by Ascomycota (563) and Basidiomycota (119), followed by four other phyla. Ascomycetes comprised Helotiales (255), Pleosporales (53), Chaetothyriales (42), and other 19 orders, while basidiomycetes Sebacinales (42), Agaricales (28), Auriculariales (7), and other 14 orders. While many dominant OTU from both hemispheres lacked close relatives in reference databases, many were very similar to identical to unnamed sequences from around the world. On the other hand, several significant ericaceous mycobionts were absent in our dataset, incl. Cairneyella, Gamarada, Kurtia, Lachnum, and Leohumicola. Most of the detected OTU could not be reliably linked to a particular trophic mode, and only two could be reliably assigned to the archetypal ErMF Hyaloscypha hepaticicola. Probable ErMF comprised Hyaloscypha variabilis and Oidiodendron maius, both detected only in N. Bohemia. Possible ErMF comprised sebacinoid fungi and several unnamed members of Hyaloscypha s. str. While H. hepaticicola was dominant only in C. vulgaris, this model ErM host lacked O. maius and sebacinoid mycobionts. Hyaloscypha hepaticicola was absent in two and very rare in six combinations from Patagonia. Nine OTU represented dark septate endophytes from the Phialocephala fortinii s. lat.-Acephala applanata species complex, including the most abundant OTU (the only detected in all combinations). Statistical analyses revealed marked differences between N. Bohemia and Patagonia, but also within Patagonia, due to the unique community detected in a Valdivian temperate rainforest. Our results show that the ericaceous hair roots may host diverse mycobionts with mostly unknown functions and indicate that many novel ErMF lineages await discovery. Transhemispheric differences (thousands of km) in their communities may be evenly matched by local differences (scales of km, m, and less).

Temporally Selective Modification of the Tomato Rhizosphere and Root Microbiome by Volcanic Ash Fertilizer Containing Micronutrients.

Food crops are grown with fertilizers containing nitrogen, phosphorus, and potassium (macronutrients) along with magnesium, calcium, boron, and zinc (micronutrients) at different ratios during their cultivation. Soil and plant-associated microbes have been implicated to promote plant growth, stress tolerance, and productivity. However, the high degree of variability across agricultural environments makes it difficult to assess the possible influences of nutrient fertilizers on these microbial communities. Uncovering the underlying mechanisms could lead us to achieve consistently improved food quality and productivity with minimal environmental impacts. For this purpose, we tested a commercially available fertilizer (surface-mined volcanic ash deposit Azomite) applied as a supplement to the normal fertilizer program of greenhouse-grown tomato plants. Because this treatment showed a significant increase in fruit production at measured intervals, we examined its impact on the composition of below-ground microbial communities, focusing on members identified as "core taxa" that were enriched in the rhizosphere and root endosphere compared to bulk soil and appeared above their predicted neutral distribution levels in control and treated samples. This analysis revealed that Azomite had little effect on microbial composition overall, but it had a significant, temporally selective influence on the core taxa. Changes in the composition of the core taxa were correlated with computationally inferred changes in functional pathway enrichment associated with carbohydrate metabolism, suggesting a shift in available microbial nutrients within the roots. This finding exemplifies how the nutrient environment can specifically alter the functional capacity of root-associated bacterial taxa, with the potential to improve crop productivity. IMPORTANCE Various types of soil fertilizers are used routinely to increase crop yields globally. The effects of these treatments are assessed mainly by the benefits they provide in increased crop productivity. There exists a gap in our understanding of how soil fertilizers act on the plant-associated microbial communities. The underlying mechanisms of nutrient uptake are widely complex and, thus, difficult to evaluate fully but have critical influences on both soil and plant health. Here, we presented a systematic approach to analyzing the effects of fertilizer on core microbial communities in soil and plants, leading to predictable outcomes that can be empirically tested and used to develop simple and affordable field tests. The methods described here can be used for any fertilizer and crop system. Continued effort in advancing our understanding of how fertilizers affect plant and microbe relations is needed to advance scientific understanding and help growers make better-informed decisions.

Agricultural land-use favours Mucoromycotinian, but not Glomeromycotinian, arbuscular mycorrhizal fungi across ten biomes.

Globally, agricultural land-use negatively affects soil biota that contribute to ecosystem functions such as nutrient cycling, yet arbuscular mycorrhizal fungi (AMF) are promoted as essential components of agroecosystems. Arbuscular mycorrhizal fungi include Glomeromycotinian AMF (G-AMF) and the arbuscule-producing fine root endophytes, recently re-classified into the Endogonales order within Mucoromycotina. The correct classification of Mucoromycotinian AMF (M-AMF) and the availability of new molecular tools can guide research to better the understanding of their diversity and ecology. To investigate the impact on G-AMF and M-AMF of agricultural land-use at a continental scale, we sampled DNA from paired farm and native sites across 10 Australian biomes. Glomeromycotinian AMF were present in both native and farm sites in all biomes. Putative M-AMF were favoured by farm sites, rare or absent in native sites, and almost entirely absent in tropical biomes. Temperature, rainfall, and soil pH were strong drivers of richness and community composition of both groups, and plant richness was an important mediator. Both fungal groups occupy different, but overlapping, ecological niches, with M-AMF thriving in temperate agricultural landscapes. Our findings invite exploration of the origin and spread of M-AMF and continued efforts to resolve the phylogeny of this newly reclassified group of AMF.

The endophytic bacterium Bacillus koreensis 181-22 promotes rice growth and alleviates cadmium stress under cadmium exposure.

Recently, cadmium (Cd) contamination in paddy soils has become a highly concerning pollution problem. Endophytic microbes in rice not only affect the plant growth but also contribute to ion absorption by the roots. Therefore, they are a promising, ecologically sound means of reducing the Cd transport from soils to shoots and grains of the plant. In this study, a Cd-resistant endophytic bacterium, named 181-22, with high Cd absorption capacity (90.8%) was isolated from the roots of rice planting in heavily Cd-contaminated paddy soils and was identified as Bacillus koreensis CGMCC 19,468. The strain significantly increased fresh weight of roots and shoots (44.4% and 42.7%) and dry weight of roots and shoots (71.3% and 39.9%) and decreased Cd content in the rice roots (12.8%), shoots (34.3%), and grains (39.1%) under Cd stress compared to uninoculated plant by colonizing rice roots via seed inoculation. Moreover, colonization of 181-22 reprogrammed rice physiology to alleviate Cd stress by increasing pigment and total protein content, regulating Cd-induced oxidative stress enzymes such as superoxide dismutase and catalase and reducing malondialdehyde. Thus, B. koreensis 181-22 has the potential to protect rice against Cd stress and can be used as a biofertilizer to bioremediate paddy soils contaminated with Cd. KEY POINTS: • Bacillus koreensis 181-22 colonized the inside of rice roots at high numbers via seed inoculation. • B. koreensis 181-22 promoted rice growth and decreased Cd accumulation in grains. • B. koreensis 181-22 regulated the physiological response to alleviated Cd stress in rice.

Carbon for nutrient exchange between Lycopodiella inundata and Mucoromycotina fine root endophytes is unresponsive to high atmospheric CO2.

Non-vascular plants associating with arbuscular mycorrhizal (AMF) and Mucoromycotina 'fine root endophyte' (MFRE) fungi derive greater benefits from their fungal associates under higher atmospheric [CO2] (a[CO2]) than ambient; however, nothing is known about how changes in a[CO2] affect MFRE function in vascular plants. We measured movement of phosphorus (P), nitrogen (N) and carbon (C) between the lycophyte Lycopodiella inundata and Mucoromycotina fine root endophyte fungi using 33P-orthophosphate, 15 N-ammonium chloride and 14CO2 isotope tracers under ambient and elevated a[CO2] concentrations of 440 and 800 ppm, respectively. Transfers of 33P and 15 N from MFRE to plants were unaffected by changes in a[CO2]. There was a slight increase in C transfer from plants to MFRE under elevated a[CO2]. Our results demonstrate that the exchange of C-for-nutrients between a vascular plant and Mucoromycotina FRE is largely unaffected by changes in a[CO2]. Unravelling the role of MFRE in host plant nutrition and potential C-for-N trade changes between symbionts under different abiotic conditions is imperative to further our understanding of the past, present and future roles of plant-fungal symbioses in ecosystems.

Insights into endophytic bacterial diversity of rice grown across the different agro-ecological regions of West Bengal, India.

Endophytes have recently garnered importance worldwide and multiple studies are being conducted to understand their important role and mechanism of interaction inside plants. But before we indulge in their functions it is necessary to dig into the microbiome. This will help to get a complete picture of the microbes intrinsic to their host and understand changes in community composition with respect to their habitats. To fulfil this requirement in our study we have attempted to dissect the endophytic diversity in roots of rice plant grown across the various agro-ecological zones of West Bengal by undergoing amplicon analysis of their 16S rRNA gene. Based on the measured environmental parameters agro-ecological zones can be divided into two groups: nutrient dense groups, representing zones like Gangetic, Northern hill and Terai-Teesta zone characterised by soil with higher levels of nitrogen (N) and total organic carbon and nutrient low groups representing Coastal saline, Red-laterite and Vindhyan zone mainly characterised by high electroconductivity and pH. Gammaproteobacteria, Alphaproteobacteria, Bacilli and Bacteroidetes were mostly abundant in nutrient dense sites whereas Clostridia and Planctomycetes were concentrated in nutrient low sites. Few genera (Aeromonas, Sulfurospirillum, Uliginosibacterium and Acidaminococcus) are present in samples cultivated in all the zones representing the core microbiome of rice in West Bengal, while some other genera like Lactococcus, Dickeya, Azonexus and Pectobacterium are unique to specific zone. Hence it can be concluded that this study has provided some insight in to the endophytic status of rice grown across the state of West Bengal.

Fungal Succession During the Decomposition of Ectomycorrhizal Fine Roots.

Ectomycorrhizal (ECM) fine roots account for a substantial proportion of forest production and their decomposition releases large amounts of nutrients to the soil ecosystem. However, little is known about the fungi involved in ECM decomposition, including assemblages of fungal saprotrophs, endophytes, and the ECM fungi themselves. To follow fungal succession during the degradation of senescing fine roots, understory seedlings of Abies balsamea and Picea rubens at two sites in the Acadian forest of Nova Scotia were either severed at the root collar or left as controls. Root systems were collected sequentially over two growing seasons and assessed for fine root loss and ECM mantle integrity. ECM were identified by ITS-PCR and grouped into broad morphological categories. Fungal communities colonizing the senescing fine roots were also monitored by systematically constructing clone libraries over the course of the experiment. ECM with cottony, weakly pigmented mantles (e.g., Cortinarius) degraded within the first year. Those with cottony, but intensely pigmented mantles (Piloderma), and smooth mantles with weak pigmentation (Russulaceae) degraded more slowly. Smooth, melanized ECM (Cenococcum and Tomentella) generally maintained integrity over the course of the experiment. Rates of fine root loss and changes in ECM mantle integrity were positively correlated with soil temperature. ECM DNA was detected throughout the experiment, and was not replaced by that of saprotrophic species during the two seasons sampled. However, fungal root endophytes (e.g., Helotiaceae) initially increased in abundance and then decreased as mantles degraded, suggesting a possible role in ECM decomposition.

Evolution and networks in ancient and widespread symbioses between Mucoromycotina and liverworts.

Like the majority of land plants, liverworts regularly form intimate symbioses with arbuscular mycorrhizal fungi (Glomeromycotina). Recent phylogenetic and physiological studies report that they also form intimate symbioses with Mucoromycotina fungi and that some of these, like those involving Glomeromycotina, represent nutritional mutualisms. To compare these symbioses, we carried out a global analysis of Mucoromycotina fungi in liverworts and other plants using species delimitation, ancestral reconstruction, and network analyses. We found that Mucoromycotina are more common and diverse symbionts of liverworts than previously thought, globally distributed, ancestral, and often co-occur with Glomeromycotina within plants. However, our results also suggest that the associations formed by Mucoromycotina fungi are fundamentally different because, unlike Glomeromycotina, they may have evolved multiple times and their symbiotic networks are un-nested (i.e., not forming nested subsets of species). We infer that the global Mucoromycotina symbiosis is evolutionarily and ecologically distinctive.

Facultative root-colonizing fungi dominate endophytic assemblages in roots of nonmycorrhizal Microthlaspi species.

There is increasing knowledge on the diversity of root-endophytic fungi, but limited information on their lifestyles and dependence on hosts hampers our understanding of their ecological functions. We compared diversity and biogeographical patterns of cultivable and noncultivable root endophytes to assess whether their occurrence is determined by distinct ecological factors. The endophytic diversity in roots of nonmycorrhizal Microthlaspi spp. growing across Europe was assessed using high-throughput sequencing (HTS) and compared with a previous dataset based on cultivation of endophytes from the same root samples. HTS revealed a large fungal richness undetected by cultivation, but which largely comprised taxa with restricted distributions and/or low representation of sequence reads. Both datasets coincided in a consistent high representation of widespread endophytes within orders Pleosporales, Hypocreales and Helotiales, as well as similar associations of community structure with spatial and environmental conditions. Likewise, distributions of particular endophytes inferred by HTS agreed with cultivation data in suggesting individual ecological preferences. Our findings support that Microthlaspi spp. roots are colonized mostly by saprotrophic and likely facultative endophytes, and that differential niche preferences and distribution ranges among fungi importantly drive the assembly of root-endophytic communities.

Localization of helotialean fungi on ectomycorrhizae of Castanopsis cuspidata visualized by in situ hybridization.

Non-ectomycorrhizal fungi that associate with typical ectomycorrhizae often remain hidden, and their localization inside ectomycorrhizal (ECM) roots has remained uncharacterized. In this study, the fungal community associated with the ectomycorrhizae of Castanopsis cuspidata was investigated using a culture-dependent isolation technique. Additionally, the species composition and localization were determined using molecular techniques. The results of the isolation and identification of fungal species revealed the predominance of a few species belonging to the order Helotiales. Furthermore, the fungal community structures were significantly different depending on the taxa of the ectomycorrhiza-forming fungi. A taxon-specific probe was developed to analyze the localization of one dominant Hyaloscyphaceae (Helotiales) species in ECM tissues by in situ hybridization. Hybridization signals were detected on the surface of the fungal mantle and around the ECM fungal cells within the mantle. Hyphal penetration into ECM hyphal cells of fungal mantles was also observed. Signals were not detected in the Hartig net or plant tissues inside the mantle in healthy ectomycorrhizae. These findings suggest that the analyzed species interact not only with host plant as root endophyte but also directly with the ECM fungi.

Rapid temporal changes in root colonization by arbuscular mycorrhizal fungi and fine root endophytes, not dark septate endophytes, track plant activity and environment in an alpine ecosystem.

Fungal root endophytes play an important role in plant nutrition, helping plants acquire nutrients in exchange for photosynthates. We sought to characterize the progression of root colonization by arbuscular mycorrhizal fungi (AMF), dark septate endophytes (DSE), and fine root endophytes (FRE) over an alpine growing season, and to understand the role of the host plant and environment in driving colonization levels. We sampled four forbs on a regular schedule from June 26th-September 11th from a moist meadow (3535 m a.s.l) on Niwot Ridge, Rocky Mountain Front Range, CO, USA. We quantified the degree of root colonization by storage structures, exchange structures, and hyphae of all three groups of fungi. AMF and FRE percent colonization fluctuated significantly over time, while DSE did not. All AMF structures changed over time, and the degree of change in vesicles differed by plant species. FRE hyphae, AMF arbuscules and AMF vesicles peaked late in the season as plants produced seeds. AMF hyphae levels started high, decreased, and then increased within 20 days, highlighting the dynamic nature of plant-fungal interactions. Overall, our results show that AMF and FRE, not DSE, root colonization rapidly changes over the course of a growing season and these changes are driven by plant phenology and seasonal changes in the environment.

Use of Endophytic and Rhizosphere Bacteria To Improve Phytoremediation of Arsenic-Contaminated Industrial Soils by Autochthonous Betula celtiberica.

The aim of this study was to investigate the potential of indigenous arsenic-tolerant bacteria to enhance arsenic phytoremediation by the autochthonous pseudometallophyte Betula celtiberica The first goal was to perform an initial analysis of the entire rhizosphere and endophytic bacterial communities of the above-named accumulator plant, including the cultivable bacterial species. B. celtiberica's microbiome was dominated by taxa related to Flavobacteriales, Burkholderiales, and Pseudomonadales, especially the Pseudomonas and Flavobacterium genera. A total of 54 cultivable rhizobacteria and 41 root endophytes, mainly affiliated with the phyla Proteobacteria, Bacteroidetes, Firmicutes, and Actinobacteria, were isolated and characterized with respect to several potentially useful features for metal plant accumulation, such as the ability to promote plant growth, metal chelation, and/or mitigation of heavy-metal stress. Seven bacterial isolates were further selected and tested for in vitro accumulation of arsenic in plants; four of them were finally assayed in field-scale bioaugmentation experiments. The exposure to arsenic in vitro caused an increase in the total nonprotein thiol compound content in roots, suggesting a detoxification mechanism through phytochelatin complexation. In the contaminated field, the siderophore and indole-3-acetic acid producers of the endophytic bacterial consortium enhanced arsenic accumulation in the leaves and roots of Betula celtiberica, whereas the rhizosphere isolate Ensifer adhaerens strain 91R mainly promoted plant growth. Field experimentation showed that additional factors, such as soil arsenic content and pH, influenced arsenic uptake in the plant, attesting to the relevance of field conditions in the success of phytoextraction strategies.IMPORTANCE Microorganisms and plants have developed several ways of dealing with arsenic, allowing them to resist and metabolize this metalloid. These properties form the basis of phytoremediation treatments and the understanding that the interactions of plants with soil bacteria are crucial for the optimization of arsenic uptake. To address this in our work, we initially performed a microbiome analysis of the autochthonous Betula celtiberica plants growing in arsenic-contaminated soils, including endosphere and rhizosphere bacterial communities. We then proceeded to isolate and characterize the cultivable bacteria that were potentially better suited to enhance phytoextraction efficiency. Eventually, we went to the field application stage. Our results corroborated the idea that recovery of pseudometallophyte-associated bacteria adapted to a large historically contaminated site and their use in bioaugmentation technologies are affordable experimental approaches and potentially very useful for implementing effective phytoremediation strategies with plants and their indigenous bacteria.

Genotypic diversity in root-endophytic fungi reflects efficient dispersal and environmental adaptation.

Studying community structure and dynamics of plant-associated fungi is the basis for unravelling their interactions with hosts and ecosystem functions. A recent sampling revealed that only a few fungal groups, as defined by internal transcribed spacer region (ITS) sequence similarity, dominate culturable root endophytic communities of nonmycorrhizal Microthlaspi spp. plants across Europe. Strains of these fungi display a broad phenotypic and functional diversity, which suggests a genetic variability masked by ITS clustering into operational taxonomic units (OTUs). The aims of this study were to identify how genetic similarity patterns of these fungi change across environments and to evaluate their ability to disperse and adapt to ecological conditions. A first ITS-based haplotype analysis of ten widespread OTUs mostly showed a low to moderate genotypic differentiation, with the exception of a group identified as Cadophora sp. that was highly diverse. A multilocus phylogeny based on additional genetic loci (partial translation elongation factor 1α, beta-tubulin and actin) and amplified fragment length polymorphism profiling of 185 strains representative of the five dominant OTUs revealed a weak association of genetic differences with geography and environmental conditions, including bioclimatic and soil factors. Our findings suggest that dominant culturable root endophytic fungi have efficient dispersal capabilities, and that their distribution is little affected by environmental filtering. Other processes, such as inter- and intraspecific biotic interactions, may be more important for the local assembly of their communities.

Communities of Cultivable Root Mycobionts of the Seagrass Posidonia oceanica in the Northwest Mediterranean Sea Are Dominated by a Hitherto Undescribed Pleosporalean Dark Septate Endophyte.

Seagrasses, a small group of submerged marine macrophytes, were reported to lack mycorrhizae, i.e., the root-fungus symbioses most terrestrial plants use for nutrient uptake. On the other hand, several authors detected fungal endophytes in seagrass leaves, shoots, rhizomes, and roots, and an anatomically and morphologically unique dark septate endophytic (DSE) association has been recently described in the roots of the Mediterranean seagrass Posidonia oceanica. Nevertheless, the global diversity of seagrass mycobionts is not well understood, and it remains unclear what fungus forms the DSE association in P. oceanica roots. We isolated and determined P. oceanica root mycobionts from 11 localities in the northwest Mediterranean Sea with documented presence of the DSE association and compared our results with recent literature. The mycobiont communities were low in diversity (only three species), were dominated by a single yet unreported marine fungal species (ca. 90 % of the total 177 isolates), and lacked common terrestrial and freshwater root mycobionts. Our phylogenetic analysis suggests that the dominating species represents a new monotypic lineage within the recently described Aigialaceae family (Pleosporales, Ascomycota), probably representing a new genus. Most of its examined colonies developed from intracellular microsclerotia occupying host hypodermis and resembling microsclerotia of terrestrial DSE fungi. Biological significance of this hitherto overlooked seagrass root mycobiont remains obscure, but its presence across the NW Mediterranean Sea and apparent root intracellular lifestyle indicate an intriguing symbiotic relationship with the dominant Mediterranean seagrass. Our microscopic observations suggest that it may form the DSE association recently described in P. oceanica roots.