Globally consistent response of plant microbiome diversity across hosts and continents to soil nutrients and herbivores.

All multicellular organisms host a diverse microbiome composed of microbial pathogens, mutualists, and commensals, and changes in microbiome diversity or composition can alter host fitness and function. Nonetheless, we lack a general understanding of the drivers of microbiome diversity, in part because it is regulated by concurrent processes spanning scales from global to local. Global-scale environmental gradients can determine variation in microbiome diversity among sites, however an individual host's microbiome also may reflect its local micro-environment. We fill this knowledge gap by experimentally manipulating two potential mediators of plant microbiome diversity (soil nutrient supply and herbivore density) at 23 grassland sites spanning global-scale gradients in soil nutrients, climate, and plant biomass. Here we show that leaf-scale microbiome diversity in unmanipulated plots depended on the total microbiome diversity at each site, which was highest at sites with high soil nutrients and plant biomass. We also found that experimentally adding soil nutrients and excluding herbivores produced concordant results across sites, increasing microbiome diversity by increasing plant biomass, which created a shaded microclimate. This demonstration of consistent responses of microbiome diversity across a wide range of host species and environmental conditions suggests the possibility of a general, predictive understanding of microbiome diversity.

A Systematic Review on the Effects of Epichloë Fungal Endophytes on Drought Tolerance in Cool-Season Grasses.

Symptomless fungal endophytes in the genus Epichloë are repeatedly mentioned to increase tolerance of cool-season grasses to a wide range of environmental stress factors, mainly drought. However, the generality of this idea is challenged because (i) most studies have been conducted on two economically important forage grasses {tall fescue [Festuca arundinacea (Schreb.) Dumort] and perennial ryegrass (Lolium perenne L.)}, (ii) endophyte-mediated mechanisms and effects on plant responses to drought have shown to be highly variable across species, and that (iii) symbiosis incidence in plant populations occurring in extremely arid environments is usually low. We question this idea by reviewing the existing information about Epichloë fungal endophyte effects on drought tolerance in cool-season grasses. We combined standard review, vote counting, and calculation of effect sizes to synthesize the literature, identify information gaps, and guide future research. The total number of studies was higher for domesticated than for wild species, a ratio that was balanced when papers with data quality for effect size calculus were considered. After the drought, endophyte-infected plants accumulated more aboveground and belowground biomass than non-infected counterparts, while no effect on tillering was observed. However, these effects remained significant for wild (even on tillering) but not for domesticated species. Interestingly, despite the continuous effort in determining physiological mechanisms behind the endophyte effects, no studies evaluated plant fecundity as a measure of ecological fitness nor vital rates (such as survival) as to escalate individual-level variables to population. Together with the high variability in results, our work shows that generalizing a positive effect of fungal endophytes in plant tolerance to drought may be misleading. Future studies combining field surveys with manipulative experiments would allow us to unravel the role of fungal endophytes in plant adaptation by considering the evolutionary history of species and populations to the different ecological contexts.

Sunlight Doubles Aboveground Carbon Loss in a Seasonally Dry Woodland in Patagonia.

Photodegradation of aboveground senescent plant material (plant litter) due to exposure to solar radiation has been identified as a dominant control on carbon (C) loss in semi-arid ecosystems [1], upturning traditional models of C cycling based only on available moisture and litter quality. In addition to the photochemical mineralization of organic matter [1, 2], sunlight alters the chemistry of cell walls in plant litter [3, 4], making them more susceptible to subsequent biotic degradation [5-7]. Nevertheless, the interactive effects of sunlight exposure, climate seasonality, and biotic decomposition on C turnover remain unresolved in terrestrial ecosystems. We show here that exposure to sunlight accelerated litter decomposition in a Patagonian woodland with a marked dry summer season. Controls on initial decomposition varied seasonally from direct photochemical mineralization in the dry summer to biotic degradation in the wet winter. By manipulating sunlight received by plant litter using spectral filters that attenuated ultraviolet and short-wave visible light, we demonstrate that direct photodegradation and its legacy, associated with increased microbial access to labile carbohydrates, are responsible for the acceleration of aboveground C turnover in this Mediterranean-type climate. Across plant species and over a 2-year period, litter exposed to the full solar spectrum decomposed twice as fast as litter that received attenuated sunlight. Changes in vegetation cover or biodiversity due to projected increased drought and dry season length [8] will likely exacerbate C losses from aboveground litter due to sunlight exposure, negatively impacting the C balance in ecosystems that are particularly vulnerable to global change [9].

There's no place like home? An exploration of the mechanisms behind plant litter-decomposer affinity in terrestrial ecosystems.

Litter decomposition in terrestrial ecosystems is an important first step for carbon and nutrient cycling, as senescent plant material is degraded and consequently incorporated, along with microbial products, into soil organic matter. The identification of litter affinity effects, whereby decomposition is accelerated in its home environment (home-field advantage, HFA), highlights the importance of plant-soil interactions that have consequences for biogeochemical cycling. While not universal, these affinity effects have been identified in a range of ecosystems, particularly in forests without disturbance. The optimization of the local decomposer community to degrade a particular combination of litter traits is the most oft-cited explanation for HFA effects, but the ways in which this specialized community can develop are only beginning to be understood. We explore ways in which HFA, or more broadly litter affinity effects, could arise in terrestrial ecosystems. Plant-herbivore interactions, microbial symbiosis, legacies from phyllosphere communities and attractors of specific soil fauna could contribute to spatially defined affinity effects for litter decomposition. Pyrosequencing soil communities and functional linkages of soil fauna provide great promise in advancing our mechanistic understanding of these interactions, and could lead to a greater appreciation of the role of litter-decomposer affinity in the maintenance of soil functional diversity.

Symbiotically modified organisms: nontoxic fungal endophytes in grasses.

We propose that symbiotically modified organisms (SMOs) should be taken into account in sustainable agriculture. In this opinion article, we present the results of a meta-analysis of the literature, with a particular focus on the potential of SMOs in forage and turf grass production, to determine the impact of endophytes in grasses on livestock, the grassland ecosystems, and associated environments. SMOs can be incorporated into breeding programs to improve grass yield, resistance to pests and weeds, and forage quality for livestock by decreasing the level of toxic alkaloids. However, the benefits of these selected grass-endophyte symbiota appear to be highly dependent on grass cultivar, fungal strain, and environmental conditions, requiring a comprehensive understanding of the genetic bases and phenotypic plasticity of the traits of the plant-microbe unit in different environments.