Fungal endophytes can modulate plant invasion

Symbiotic organisms may contribute to a host plant's success or failure to grow, its ability to maintain viable populations, and potentially, its probability of establishment and spread outside its native range. Intercellular and intracellular microbial symbionts that are asymptomatic in their plant host during some or all of their life cycle - endophytes - can form mutualistic, commensal, or pathogenic relationships, and sometimes novel associations with alien plants. Fungal endophytes are likely the most common endosymbiont infecting plants, with life-history, morphological, physiological, and plant-symbiotic traits that are distinct from other endophytic guilds. Here, we review the community dynamics of fungal endophytes during the process of plant invasion, and how their functional role may shift during the different stages of invasion: transport, introduction (colonisation), establishment, and spread. Each invasion stage presents distinct ecological filters that an alien plant must overcome to advance to the subsequent stage of invasion. Endophytes can alternately aid the host in overcoming stage-specific filters, or contribute to the barriers imposed by filters (e.g. biotic resistance), thereby affecting invasion pathways. A few fungi can be transported as seed endophytes from their native range and be vertically transmitted to future generations in the non-native range, especially in graminoids. In other plant groups, alien plants mostly acquire endophytes via horizontal transmission from the invaded plant community, and the host endophyte community is shaped by host filtering and biogeographic factors (e.g. dispersal limitation, environmental filtering). Endophytes infecting alien plants (both those transported with their host and those accumulated in the non-native range) may influence invasion success by affecting plant growth, reproduction, environmental tolerance, and pathogen and herbivory defences; however, the direction and magnitude of these effects can be contingent upon the host identity, life stage, ecological conditions, and invasion stage. This context dependence may cause endophytic fungi to shift to a non-endophytic (e.g. pathogenic) functional life stage in the same or different hosts, which can modify alien-native plant community dynamics. We conclude by identifying paths in which alien hosts can exploit the context dependency of endophyte function in novel abiotic and biotic conditions and at the different stages of invasion.

Phyllosphere fungal diversity generates pervasive nonadditive effects on plant performance

Plants naturally harbor diverse microbiomes that can dramatically impact their health and productivity. However, it remains unclear how fungal microbiome diversity, especially in the phyllosphere, impacts intermicrobial interactions and consequent nonadditive effects on plant productivity. Combining manipulative experiments, field collections, culturing, microbiome sequencing, and synthetic consortia, we experimentally tested for the first time how foliar fungal community diversity impacts plant productivity. We inoculated morning glories (Ipomoea hederifolia L.) with 32 phyllosphere consortia of either low or high diversity or with single fungal taxa, and measured effects on plant productivity and allocation. We found the following: (1) nonadditive effects were pervasive with 56% of fungal consortia interacting synergistically or antagonistically to impact plant productivity, including some consortia capable of generating acute synergism (e.g. > 1000% increase in productivity above the additive expectation), (2) interactions among 'commensal' fungi were responsible for this nonadditivity in diverse consortia, (3) synergistic interactions were approximately four times stronger than antagonistic effects, (4) fungal diversity affected the magnitude but not frequency or direction of nonadditivity, and (5) diversity affected plant performance nonlinearly with the highest performance in low-diversity treatments. These findings highlight the importance of interpreting plant-microbiome interactions under a framework that incorporates intermicrobial interactions and nonadditive outcomes to understand natural complexity.

Microbial symbionts buffer hosts from the demographic costs of environmental stochasticity

Species' persistence in increasingly variable climates will depend on resilience against the fitness costs of environmental stochasticity. Most organisms host microbiota that shield against stressors. Here, we test the hypothesis that, by limiting exposure to temporally variable stressors, microbial symbionts reduce hosts' demographic variance. We parameterized stochastic population models using data from a 14-year symbiont-removal experiment including seven grass species that host Epichloë fungal endophytes. Results provide novel evidence that symbiotic benefits arise not only through improved mean fitness, but also through dampened inter-annual variance. Hosts with "fast" life-history traits benefited most from symbiont-mediated demographic buffering. Under current climate conditions, contributions of demographic buffering were modest compared to benefits to mean fitness. However, simulations of increased stochasticity amplified benefits of demographic buffering and made it the more important pathway of host-symbiont mutualism. Microbial-mediated variance buffering is likely an important, yet cryptic, mechanism of resilience in an increasingly variable world.

Age-dependent dendrobine biosynthesis in Dendrobium nobile: insights into endophytic fungal interactions

Introduction

Dendrobium nobile (D. nobile), a valued Chinese herb known for its diverse pharmacological effects, owes much of its potency to the bioactive compound dendrobine. However, dendrobine content varies significantly with plant age, and the mechanisms governing this variation remain unclear. This study delves into the potential role of endophytic fungi in shaping host-microbe interactions and influencing plant metabolism.

Methods

Using RNA-seq, we examined the transcriptomes of 1-year-old, 2-year-old, and 3-year-old D. nobile samples and through a comprehensive analysis of endophytic fungal communities and host gene expression in D. nobile stems of varying ages, we aim to identify associations between specific fungal taxa and host genes.

Results

The results revealing 192 differentially expressed host genes. These genes exhibited a gradual decrease in expression levels as the plants aged, mirroring dendrobine content changes. They were enriched in 32 biological pathways, including phagosome, fatty acid degradation, alpha-linolenic acid metabolism, and plant hormone signal transduction. Furthermore, a significant shift in the composition of the fungal community within D. nobile stems was observed along the age gradient. Olipidium, Hannaella, and Plectospherella dominated in 1-year-old plants, while Strelitziana and Trichomerium prevailed in 2-year-old plants. Conversely, 3-year-old plants exhibited additional enrichment of endophytic fungi, including the genus Rhizopus. Two gene expression modules (mediumpurple3 and darkorange) correlated significantly with dominant endophytic fungi abundance and dendrobine accumulation. Key genes involved in dendrobine synthesis were found associated with plant hormone synthesis.

Discussion

This study suggests that the interplay between different endophytic fungi and the hormone signaling system in D. nobile likely regulates dendrobine biosynthesis, with specific endophytes potentially triggering hormone signaling cascades that ultimately influence dendrobine synthesis.

Endophytic infection increases the belowground over-yielding effects of the host grass community mainly by increasing the complementary effects

Introduction

Increases in plant species diversity may increase the community diversity effect and produce community over-yielding. Epichloë endophytes, as symbiotic microorganisms, are also capable of regulating plant communities, but their effects on community diversity effects are often overlooked.

Methods

In this experiment, we investigated the effects of endophytes on the diversity effects of host plant community biomass by constructing artificial communities with 1-species monocultures and 2- and 4-species mixtures of endophyte-infected (E+) and endophyte-free (E-) Achnatherum sibiricum and three common plants in its native habitat, which were potted in live and sterilized soil.

Results and discussion

The results showed that endophyte infection significantly increased the belowground biomass and abundance of Cleistogenes squarrosa, marginally significantly increased the abundance of Stipa grandis and significantly increased the community diversity (evenness) of the 4-species mixtures. Endophyte infection also significantly increased the over-yielding effects on belowground biomass of the 4-species mixtures in the live soil, and the increase in diversity effects on belowground biomass was mainly due to the endophyte significantly increasing the complementary effects on belowground biomass. The effects of soil microorganisms on the diversity effects on belowground biomass of the 4-species mixtures were mainly derived from their influences on the complementary effects. The effects of endophytes and soil microorganisms on the diversity effects on belowground biomass of the 4-species communities were independent, and both contributed similarly to the complementary effects on belowground biomass. The finding that endophyte infection promotes belowground over-yielding in live soil at higher levels of species diversity suggests that endophytes may be one of the factors contributing to the positive relationship between species diversity and productivity and explains the stable co-existence of endophyte-infected Achnatherum sibiricum with a variety of plants in the Inner Mongolian grasslands.

A scoping review of bryophyte microbiota: diverse microbial communities in small plant packages

Plant health depends not only on the condition of the plant itself but also on its diverse community of microbes, or microbiota. Just like the better-studied angiosperms, bryophytes (mosses, liverworts, and hornworts) harbor diverse communities of bacteria, archaea, fungi, and other microbial eukaryotes. Bryophytes are increasingly recognized as important model systems for understanding plant evolution, development, physiology, and symbiotic interactions. Much of the work on bryophyte microbiota in the past focused on specific symbiont types for each bryophyte group, but more recent studies are taking a broader view acknowledging the coexistence of diverse microbial communities in bryophytes. Therefore, this review integrates studies of bryophyte microbes from both perspectives to provide a holistic view of the existing research for each bryophyte group and on key themes. The systematic search also reveals the taxonomic and geographic biases in this field, including a severe under-representation of the tropics, very few studies on viruses or eukaryotic microbes beyond fungi, and a focus on mycorrhizal fungi studies in liverworts. Such gaps may have led to errors in conclusions about evolutionary patterns in symbiosis. This analysis points to a wealth of future research directions that promise to reveal how the distinct life cycles and physiology of bryophytes interact with their microbiota.

Arbuscular Mycorrhizal Fungi Contribute to Phosphorous Uptake and Allocation Strategies of Solidago canadensis in a Phosphorous-Deficient Environment

Arbuscular mycorrhizal fungi (AMF) can facilitate the uptake of limiting or inaccessible nutrients by plants. However, the importance of AMF for invasive plants under phosphorus (P) limitation is poorly well understood because of the presence of non-focal microorganisms, such as endophytes or rhizosphere bacteria. In this study, we investigated how an invasive clonal plant Solidago canadensis benefits from the AMF Glomus intraradices by using a completely sterile culturing system, which is composed of aseptic seedlings, a pure AMF strain, and a sterile growth environment. We found that the colonization rate, abundance, and spore production of AMF in the insoluble P treatment was more than twice as much as in the available P treatment. Plant above-ground growth was enhanced almost 50% by AMF in the insoluble P treatment. Importantly, AMF were able to facilitate P acquisition by the plant in insoluble P conditions, allowing plants to have lower investment into below-ground biomass and higher benefit/return for above-ground biomass. This study demonstrated the important contribution that AMF make to plants in phosphate-deficient environments eliminating interference from non-focal microorganisms. Our results also suggest that interaction with AMF could contribute to the invasiveness of clonal plant S. canadensis in a resource-deficient environment.

Environmental stress determines the colonization and impact of an endophytic fungus on invasive knotweed

There is increasing evidence that microbes play a key role in some plant invasions. A diverse and widespread but little understood group of plant-associated microbes are the fungal root endophytes of the order Sebacinales. They are associated with exotic populations of invasive knotweed (Reynoutria ssp.) in Europe, but their effects on the invaders are unknown. We used the recently isolated Sebacinales root endophyte Serendipita herbamans to experimentally inoculate invasive knotweed and study root colonisation and effects on knotweed growth under different environmental conditions. We verified the inoculation success and fungal colonisation through immunofluorescence microscopy and qPCR. We found that S. herbamans strongly colonized invasive knotweed in low-nutrient and shade environments, but much less under drought or benign conditions. At low nutrients, the endophyte had a positive effect on plant growth, whereas the opposite was true under shaded conditions. Our study demonstrates that the root endophyte S. herbamans has the potential to colonize invasive knotweed fine roots and impact its growth, and it could thus also play a role in natural populations. Our results also show that effects of fungal endophytes on plants can be strongly environment-dependent, and may only be visible under stressful environmental conditions.

A suite of rare microbes interacts with a dominant, heritable, fungal endophyte to influence plant trait expression

Endophytes are microbes that live, for at least a portion of their life history, within plant tissues. Endophyte assemblages are often composed of a few abundant taxa and many infrequently observed, low-biomass taxa that are, in a word, rare. The ways in which most endophytes affect host phenotype are unknown; however, certain dominant endophytes can influence plants in ecologically meaningful ways-including by affecting growth and immune system functioning. In contrast, the effects of rare endophytes on their hosts have been unexplored, including how rare endophytes might interact with abundant endophytes to shape plant phenotype. Here, we manipulate both the suite of rare foliar endophytes (including both fungi and bacteria) and Alternaria fulva-a vertically transmitted and usually abundant fungus-within the fabaceous forb Astragalus lentiginosus. We report that rare, low-biomass endophytes affected host size and foliar %N, but only when the heritable fungal endophyte (A. fulva) was not present. A. fulva also reduced plant size and %N, but these deleterious effects on the host could be offset by a negative association we observed between this heritable fungus and a foliar pathogen. These results demonstrate how interactions among endophytic taxa determine the net effects on host plants and suggest that the myriad rare endophytes within plant leaves may be more than a collection of uninfluential, commensal organisms, but instead have meaningful ecological roles.

Dark septate endophyte improves salt tolerance of native and invasive lineages of Phragmites australis

Fungal endophytes can improve plant tolerance to abiotic stress. However, the role of these plant-fungal interactions in invasive species ecology and their management implications remain unclear. This study characterized the fungal endophyte communities of native and invasive lineages of Phragmites australis and assessed the role of dark septate endophytes (DSE) in salt tolerance of this species. We used Illumina sequencing to characterize root fungal endophytes of contiguous stands of native and invasive P. australis along a salinity gradient. DSE colonization was assessed throughout the growing season in the field, and effects of fungal inoculation on salinity tolerance were investigated using laboratory and greenhouse studies. Native and invasive lineages had distinct fungal endophyte communities that shifted across the salinity gradient. DSE colonization was greater in the invasive lineage and increased with salinity. Laboratory studies showed that DSE inoculation increased P. australis seedling survival under salt stress; and a greenhouse assay revealed that the invasive lineage had higher aboveground biomass under mesohaline conditions when inoculated with a DSE. We observed that P. australis can establish mutualistic associations with DSE when subjected to salt stress. This type of plant-fungal association merits further investigation in integrated management strategies of invasive species and restoration of native Phragmites.

Fungal functional ecology: bringing a trait-based approach to plant-associated fungi

Fungi play many essential roles in ecosystems. They facilitate plant access to nutrients and water, serve as decay agents that cycle carbon and nutrients through the soil, water and atmosphere, and are major regulators of macro-organismal populations. Although technological advances are improving the detection and identification of fungi, there still exist key gaps in our ecological knowledge of this kingdom, especially related to function. Trait-based approaches have been instrumental in strengthening our understanding of plant functional ecology and, as such, provide excellent models for deepening our understanding of fungal functional ecology in ways that complement insights gained from traditional and -omics-based techniques. In this review, we synthesize current knowledge of fungal functional ecology, taxonomy and systematics and introduce a novel database of fungal functional traits (Fun^Fun ). Fun^Fun is built to interface with other databases to explore and predict how fungal functional diversity varies by taxonomy, guild, and other evolutionary or ecological grouping variables. To highlight how a quantitative trait-based approach can provide new insights, we describe multiple targeted examples and end by suggesting next steps in the rapidly growing field of fungal functional ecology.

A novel experimental system using the liverwort Marchantia polymorpha and its fungal endophytes reveals diverse and context-dependent effects

Fungal symbioses are ubiquitous in plants, but their effects have mostly been studied in seed plants. This study aimed to assess the diversity of fungal endophyte effects in a bryophyte and identify factors contributing to the variability of outcomes in these interactions. Fungal endophyte cultures and axenic liverwort clones were isolated from wild populations of the liverwort, Marchantia polymorpha. These collections were combined in a gnotobiotic system to test the effects of fungal isolates on the growth rates of hosts under laboratory conditions. Under the experimental conditions, fungi isolated from M. polymorpha ranged from aggressively pathogenic to strongly growth-promoting, but the majority of isolates caused no detectable change in host growth. Growth promotion by selected fungi depended on nutrient concentrations and was inhibited by coinoculation with multiple fungi. The M. polymorpha endophyte system expands the resources for this model liverwort. The experiments presented here demonstrate a wealth of diversity in fungal interactions even in a host reported to lack standard mycorrhizal symbiosis. In addition, they show that some known pathogens of vascular plants live in M. polymorpha and can confer benefits to this nonvascular host. This highlights the importance of studying endophyte effects across the plant tree of life.