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Virtually all plants employ direct and indirect defenses against herbivores. While it is known that plant defenses can be affected by belowground symbiotic microbes under controlled conditions, studies showing these multitrophic interactions in nature are surprisingly scarce. Here we tested for effects of rhizobia on insect attraction and direct defense (cyanogenesis) in wild lima bean (Phaseolus lunatus) plants in Costa Rica. We performed bioassays with rhizobia-inoculated (R+) and rhizobiafree (R-) potted plants distributed among native lima bean communities at two spatially separated field sites (450 km apart) and in two field seasons. Without affecting overall plant size, rhizobia altered leaf chemistry (cyanogenesis and soluble leaf nitrogen) and ultimately insect communities visiting the plants. Natural herbivorous chrysomelid beetles were strongly attracted to R+ plants, while natural enemies, ants and parasitoid wasps, preferred R- plants resulting in a particularly high herbivore:carnivore ratio on R + plants. This suggests that symbiotic microbes mediate trophic interactions by influencing both direct and indirect plant defenses against herbivores. Our results show that rhizobia affect the plant defensive phenotype and have cascading effects on plant-insect interactions in nature. 

While a considerable amount of research has explored plant community composition in primary successional systems, little is known about the microbial communities inhabiting these pioneer plant species. Fungal endophytes are ubiquitous within plants, and may play major roles in early successional ecosystems. Specifically, endophytes have been shown to affect successional processes, as well as alter host stress tolerance and litter decomposition dynamics—both of which are important components in harsh environments where soil organic matter is still scarce.

To determine possible contributions of fungal endophytes to plant colonization patterns, we surveyed six of the most common woody species on the Pumice Plain of Mount St. Helens (WA, USA; Lawetlat'la in the Cowlitz language; created during the 1980 eruption)—a model primary successional ecosystem—and found low colonization rates (< 15%), low species richness, and low diversity. Furthermore, while endophyte community composition did differ among woody species, we found only marginal evidence of temporal changes in community composition over a single field season (July–September).

Our results indicate that even after a post-eruption period of 40 years, foliar endophyte communities still seem to be in the early stages of community development, and that the dominant pioneer riparian species Sitka alder (Alnus viridisssp.sinuata) and Sitka willow (Salix sitchensis) may be serving as important microbial reservoirs for incoming plant colonizers.

 

Fungal endophytes are critical members of the plant microbiome, but their community dynamics throughout an entire growing season are underexplored. Additionally, most fungal endophyte research has centred on seed‐reproducing hosts, while spore‐reproducing plants also host endophytes and may be colonized by unique community members. In order to examine annual fungal endophyte community dynamics in a spore‐reproducing host, we explored endophytes in a single population of ferns,Polystichum munitum, in the Pacific Northwest. Through metabarcoding, we characterized the community assembly and temporal turnover of foliar endophytes throughout a growing season. From these results, we selected endophytes with outsized representations in sequence data and performedin vitrocompetition assays. Finally, we inoculated sterile fern gametophytes with dominant fungi observed in the field and determined their effects on host performance. Sequencing demonstrated that ferns were colonized by a diverse community of fungal endophytes in newly emerged tissue, but diversity decreased throughout the season leading to the preponderance of a single fungus in later sampling months. This previously undescribed endophyte appears to abundantly colonize the host to the detriment of other microfungi. Competition assays on a variety of media types failed to demonstrate that the dominant fungus was competitive against other fungi isolated from the same hosts, and inoculation onto sterile fern gametophytes did not alter growth compared to sterile controls, suggesting its effects are not antagonistic. The presence of this endophyte in the fern population probably demonstrates a case of repeated colonization driving competitive exclusion of other fungal community members.

 

Litter decomposition rates are affected by a variety of abiotic and biotic factors, including the presence of fungal endophytes in host plant tissues. This review broadly analyzes the findings of 67 studies on the roles of foliar endophytes in litter decomposition, and their effects on decomposition rates. From 29 studies and 1 review, we compiled a comprehensive table of 710 leaf-associated fungal taxa, including the type of tissue these taxa were associated with and isolated from, whether they were reported as endo- or epiphytic, and whether they had reported saprophytic abilities. Aquatic (i.e., in-stream) decomposition studies of endophyte-affected litter were significantly under-represented in the search results (p < 0.0001). Indicator species analyses revealed that different groups of fungal endophytes were significantly associated with cool or tropical climates, as well as specific plant host genera (p < 0.05). Finally, we argue that host plant and endophyte interactions can significantly influence litter decomposition rates and should be considered when interpreting results from both terrestrial and in-stream litter decomposition experiments.