Ungulate saliva inhibits a grass-endophyte mutualism

Bridging the Gap: Plant-Endophyte Interactions as a Roadmap to Understanding Small-Molecule Communication in Marine Microbiomes

Probing the composition of the microbiome and its association with health and disease states is more accessible than ever due to the rise of affordable sequencing technology. Despite advances in our ability to identify members of symbiont communities, untangling the chemical signaling that they use to communicate with host organisms remains challenging. In order to gain a greater mechanistic understanding of how the microbiome impacts health, and how chemical ecology can be leveraged to advance small-molecule drug discovery from microorganisms, the principals governing communication between host and symbiont must be elucidated. Herein, we review common modes of interkingdom small-molecule communication in terrestrial and marine environments, describe the differences between these environments, and detail the advantages and disadvantages for studies focused on the marine environment. Finally, we propose the use of plant-endophyte interactions as a stepping stone to a greater understanding of similar interactions in marine invertebrates, and ultimately in humans.

Unpacking multi-trophic herbivore-grass-endophyte interactions: feedbacks across different scales in vegetation responses to Soay sheep herbivory

Grazing can induce changes in both plant productivity and nutritional quality, which may subsequently influence herbivore carrying capacity. While research on Soay sheep (Ovis aries L.) dynamics on Hirta Island in the St. Kilda archipelago has elucidated the complexity of population drivers, including parasites, the role of herbivore-generated feedbacks as an intrinsic regulating factor remains unclear. The sheep lack large predators and every 3-9 years undergo population crashes (overcompensatory mortality). We investigated the effects of grazing on (1) sward productivity and (2) quality (toxicity) of the primary forage species, red fescue (Festuca rubra L.), which is highly infected by an alkaloid-synthesizing fungal endophyte. Grazing had a negative impact on both forage quantity and quality. At higher sheep densities, impacts on sward growth were magnified, resulting in a nonlinear relationship with plant productivity. Simultaneously, endophyte hyphal load (and by inference, toxicity) peaked close to the time of a crash. A greenhouse experiment showed that alkaloid concentration in F. rubra increased in response to artificial defoliation. We conclude that at high sheep densities, grazing-mediated reductions in productivity, together with sustained alkaloid production, are likely to influence sheep dynamics. Future research should consider the interactive effects of forage toxicity, quantity, and nutritional content.

Complex Interactions among Sheep, Insects, Grass, and Fungi in a Simple New Zealand Grazing System

Epichloë fungi (Ascomycota) live within aboveground tissues of grasses and can have important implications for natural and managed ecosystems through production of alkaloids. Nonetheless, vertebrate herbivores may possess traits, like oral secretions, that mitigate effects of alkaloids. We tested if sheep saliva mitigates effects of Epichloë alkaloids on a beetle pest of perennial ryegrass (Lolium perenne L.) in a New Zealand pasture setting. Plants with one of several fungal isolates were clipped with scissors, grazed by sheep, or clipped with sheep saliva applied to cut ends of stems. We then assessed feeding damage by Argentine stem weevils on blade segments collected from experimental plants. We found that clipping plants induced synthesis of an alkaloid that reduces feeding by beetles and that sheep saliva mitigates this effect. Unexpectedly, the alkaloid (perloline) that explains variation in beetle feeding is one produced not by the endophyte, but rather by the plant. Yet, these effects depended upon fungal isolate. Such indirect, complex interactions may be much more common in both managed and natural grassland systems than typically thought and could have implications for managing grazing systems.

Long-term ungulate exclusion reduces fungal symbiont prevalence in native grasslands

When symbionts are inherited by offspring, they can have substantial ecological and evolutionary consequences because they occur in all host life stages. Although natural frequencies of inherited symbionts are commonly <100 %, few studies investigate the ecological drivers of variation in symbiont prevalence. In plants, inherited fungal endophytes can improve resistance to herbivory, growth under drought, and competitive ability. We evaluated whether native ungulate herbivory increased the prevalence of a fungal endophyte in the common, native bunchgrass, Festuca campestris (rough fescue, Poaceae). We used large-scale (1 ha) and long-term (7-10 year) fencing treatments to exclude native ungulates and recorded shifts in endophyte prevalence at the scale of plant populations and for individual plants. We characterized the fungal endophyte in F. campestris, Epichloë species FcaTG-1 (F. campestris taxonomic group 1) for the first time. Under ungulate exclusion, endophyte prevalence was 19 % lower in plant populations, 25 % lower within plant individuals, and 39 % lower in offspring (seeds) than in ungulate-exposed controls. Population-level endophyte frequencies were also negatively correlated with soil moisture across geographic sites. Observations of high within-plant variability in symbiont prevalence are novel for the Epichloë species, and contribute to a small, but growing, literature that documents phenotypic plasticity in plant-endophyte symbiota. Altogether, we show that native ungulates can be an important driver of symbiont prevalence in native plant populations, even in the absence of evidence for direct mechanisms of mammal deterrence. Understanding the ecological controls on symbiont prevalence could help to predict future shifts in grasslands that are dominated by Epichloë host plants.