Environmental drivers and cryptic biodiversity hotspots define endophytes in Earth's largest terrestrial biome.

Understanding how symbiotic associations differ across environmental gradients is key to predicting the fate of symbioses as environments change, and it is vital for detecting global reservoirs of symbiont biodiversity in a changing world.1,2,3 However, sampling of symbiotic partners at the full-biome scale is difficult and rare. As Earth's largest terrestrial biome, boreal forests influence carbon dynamics and climate regulation at a planetary scale. Plants and lichens in this biome host the highest known phylogenetic diversity of fungal endophytes, which occur within healthy photosynthetic tissues and can influence hosts' resilience to stress.4,5 We examined how communities of endophytes are structured across the climate gradient of the boreal biome, focusing on the dominant plant and lichen species occurring across the entire south-to-north span of the boreal zone in eastern North America. Although often invoked for understanding the distribution of biodiversity, neither a latitudinal gradient nor mid-domain effect5,6,7 can explain variation in endophyte diversity at this trans-biome scale. Instead, analyses considering shifts in forest characteristics, Picea biomass and age, and nutrients in host tissues from 46° to 58° N reveal strong and distinctive signatures of climate in defining endophyte assemblages in each host lineage. Host breadth of endophytes varies with climate factors, and biodiversity hotspots can be identified at plant-community transitions across the boreal zone at a global scale. Placed against a backdrop of global circumboreal sampling,4 our study reveals the sensitivity of endophytic fungi, their reservoirs of biodiversity, and their important symbiotic associations, to climate.

Urban biogeography of fungal endophytes across San Francisco.

In natural and agricultural systems, the plant microbiome-the microbial organisms associated with plant tissues and rhizosphere soils-has been shown to have important effects on host physiology and ecology, yet we know little about how these plant-microbe relationships play out in urban environments. Here we characterize the composition of fungal communities associated with living leaves of one of the most common sidewalk trees in the city of San Francisco, California. We focus our efforts on endophytic fungi (asymptomatic microfungi that live inside healthy leaves), which have been shown in other systems to have large ecological effects on the health of their plant hosts. Specifically, we characterized the foliar fungal microbiome of Metrosideros excelsa (Myrtaceae) trees growing in a variety of urban environmental conditions. We used high-throughput culturing, PCR, and Sanger sequencing of the internal transcribed spacer nuclear ribosomal DNA (ITS nrDNA) region to quantify the composition and structure of fungal communities growing within healthy leaves of 30 M. excelsa trees from six distinct sites, which were selected to capture the range of environmental conditions found within city limits. Sequencing resulted in 854 high-quality ITS sequences. These sequences clustered into 85 Operational Taxonomic Units (97% OTUs). We found that these communities encompass relatively high alpha (within) and beta (between-site) diversity. Because the communities are all from the same host tree species, and located in relatively close geographical proximity to one another, these analyses suggest that urban environmental factors such as heat islands or differences in vegetation or traffic density (and associated air quality) may potentially be influencing the composition of these fungal communities. These biogeographic patterns provide evidence that plant microbiomes in urban environments can be as dynamic and complex as their natural counterparts. As human populations continue to transition out of rural areas and into cities, understanding the factors that shape environmental microbial communities in urban ecosystems stands to become increasingly important.

Introducing the Microbes and Social Equity Working Group: Considering the Microbial Components of Social, Environmental, and Health Justice.

Humans are inextricably linked to each other and our natural world, and microorganisms lie at the nexus of those interactions. Microorganisms form genetically flexible, taxonomically diverse, and biochemically rich communities, i.e., microbiomes that are integral to the health and development of macroorganisms, societies, and ecosystems. Yet engagement with beneficial microbiomes is dictated by access to public resources, such as nutritious food, clean water and air, safe shelter, social interactions, and effective medicine. In this way, microbiomes have sociopolitical contexts that must be considered. The Microbes and Social Equity (MSE) Working Group connects microbiology with social equity research, education, policy, and practice to understand the interplay of microorganisms, individuals, societies, and ecosystems. Here, we outline opportunities for integrating microbiology and social equity work through broadening education and training; diversifying research topics, methods, and perspectives; and advocating for evidence-based public policy that supports sustainable, equitable, and microbial wealth for all.

Host availability drives distributions of fungal endophytes in the imperilled boreal realm.

Boreal forests represent the world's largest terrestrial biome and provide ecosystem services of global importance. Highly imperilled by climate change, these forests host Earth's greatest phylogenetic diversity of endophytes, a hyperdiverse group of symbionts that are defined by their occurrence within living, symptomless plant and lichen tissues. Endophytes shape the ecological and evolutionary trajectories of plants and are therefore key to the function and resilience of terrestrial ecosystems. A critical step in linking the ecological functions of endophytes with those of their hosts is to understand the distributions of these symbionts at the global scale; however, turnover in host taxa with geography and climate can confound insights into endophyte biogeography. As a result, global drivers of endophyte diversity and distributions are not known. Here, we leverage sampling from phylogenetically diverse boreal plants and lichens across North America and Eurasia to show that host filtering in distinctive environments, rather than turnover with geographical or environmental distance, is the main determinant of the community composition and diversity of endophytes. We reveal the distinctiveness of boreal endophytes relative to soil fungi worldwide and endophytes from diverse temperate biomes, highlighting a high degree of global endemism. Overall, the distributions of endophytes are directly linked to the availability of compatible hosts, highlighting the role of biotic interactions in shaping fungal communities across large spatial scales, and the threat that climate change poses to biological diversity and function in the imperilled boreal realm.

Observations on the Early Establishment of Foliar Endophytic Fungi in Leaf Discs and Living Leaves of a Model Woody Angiosperm, Populus trichocarpa (Salicaceae).

Fungal endophytes are diverse and widespread symbionts that occur in the living tissues of all lineages of plants without causing evidence of disease. Culture-based and culture-free studies indicate that they often are abundant in the leaves of woody angiosperms, but only a few studies have visualized endophytic fungi in leaf tissues, and the process through which most endophytes colonize leaves has not been studied thoroughly. We inoculated leaf discs and the living leaves of a model woody angiosperm, Populus trichocarpa, which has endophytes that represent three distantly-related genera (Cladosporium, Penicillium, and Trichoderma). We used scanning electron microscopy and light microscopy to evaluate the timeline and processes by which they colonize leaf tissue. Under laboratory conditions with high humidity, conidia germinated on leaf discs to yield hyphae that grew epiphytically and incidentally entered stomata, but did not grow in a directed fashion toward stomatal openings. No cuticular penetration was observed. The endophytes readily colonized the interiors of leaf discs that were detached from living leaves, and could be visualized within discs with light microscopy. Although they were difficult to visualize within the interior of living leaves following in vivo inoculations, standard methods for isolating foliar endophytes confirmed their presence.

Contributions of North American endophytes to the phylogeny, ecology, and taxonomy of Xylariaceae (Sordariomycetes, Ascomycota).

The Xylariaceae (Sordariomycetes) comprise one of the largest and most diverse families of Ascomycota, with at least 85 accepted genera and ca. 1343 accepted species. In addition to their frequent occurrence as saprotrophs, members of the family often are found as endophytes in living tissues of phylogenetically diverse plants and lichens. Many of these endophytes remain sterile in culture, precluding identification based on morphological characters. Previous studies indicate that endophytes are highly diverse and represent many xylariaceous genera; however, phylogenetic analyses at the family level generally have not included endophytes, such that their contributions to understanding phylogenetic relationships of Xylariaceae are not well known. Here we use a multi-locus, cumulative supermatrix approach to integrate 92 putative species of fungi isolated from plants and lichens into a phylogenetic framework for Xylariaceae. Our collection spans 1933 isolates from living and senescent tissues in five biomes across the continental United States, and here is analyzed in the context of previously published sequence data from described species and additional taxon sampling of type specimens from culture collections. We found that the majority of strains obtained in our surveys can be classified in the hypoxyloid and xylaroid subfamilies, although many also were found outside of these lineages (as currently circumscribed). Many endophytes were placed in lineages previously not known for endophytism. Most endophytes appear to represent novel species, but inferences are limited by potential gaps in public databases. By linking our data, publicly available sequence data, and records of ascomata, we identify many geographically widespread, host-generalist clades capable of symbiotic associations with diverse photosynthetic partners. Concomitant with such cosmopolitan host use and distributions, many xylariaceous endophytes appear to inhabit both living and non-living plant tissues, with potentially important roles as saprotrophs. Overall, our study reveals major gaps in the availability of multi-locus datasets and metadata for this iconic family, and provides new hypotheses regarding the ecology and evolution of endophytism and other trophic modes across the family Xylariaceae.

Fungal endophyte communities reflect environmental structuring across a Hawaiian landscape.

We surveyed endophytic fungal communities in leaves of a single tree species (Metrosideros polymorpha) across wide environmental gradients (500-5,500 mm of rain/y; 10-22 °C mean annual temperature) spanning short geographic distances on Mauna Loa Volcano, Hawai'i. Using barcoded amplicon pyrosequencing at 13 sites (10 trees/site; 10 leaves/tree), we found very high levels of diversity within sites (a mean of 551 ± 134 taxonomic units per site). However, among-site diversity contributed even more than did within-site diversity to the overall richness of more than 4,200 taxonomic units observed in M. polymorpha, and this among-site variation in endophyte community composition correlated strongly with temperature and rainfall. These results are consistent with suggestions that foliar endophytic fungi are hyperdiverse. They further suggest that microbial diversity may be even greater than has been assumed and that broad-scale environmental controls such as temperature and rainfall can structure eukaryotic microbial diversity. Appropriately constrained study systems across strong environmental gradients present a useful means to understand the environmental factors that structure the diversity of microbial communities.