Simultaneous adaptation and maladaptation of tree populations to local rhizosphere microbial communities at different taxonomic scales

The soil microbiome affects patterns of local adaptation in an alpine plant under moisture stress

PREMISE

The soil microbiome plays a role in plant trait expression and fitness, and plants may be locally adapted or maladapted to their soil microbiota. However, few studies of local adaptation in plants have incorporated a microbial treatment separate from manipulations of the abiotic environment, so our understanding of microbes in plant adaptation is limited.

METHODS

Here we tested microbial effects on local adaptation in four paired populations of an abundant alpine plant from two community types, dry and moist meadow. In a 5-month greenhouse experiment, we manipulated source population, soil moisture, and soil microbiome and measured plant survival and biomass to assess treatment effects.

RESULTS

Dry meadow populations had higher biomass than moist meadow populations at low moisture, demonstrating evidence of local adaptation to soil moisture in the absence of microbes. In the presence of microbes, dry meadow populations had greater survival than moist meadow populations when grown with dry meadow microbes regardless of moisture. Moist meadow populations showed no signs of adaptation or maladaptation.

CONCLUSIONS

Our research highlights the importance of microbial mutualists in local adaptation, particularly in dry environments with higher abiotic stress. Plant populations from environments with greater abiotic stress exhibit different patterns of adaptation when grown with soil microbes versus without, while plant populations from less abiotically stressful environments do not. Improving our understanding of the role microbes play in plant adaptation will require further studies incorporating microbial manipulations.

Host population effects on ectomycorrhizal fungi vary between low and high phosphorus soils of temperate rainforests

Geographic distinctions in the affinity of tree populations for select ectomycorrhizal fungi (EMF) may occur where strong edaphic pressures act on fungal communities and their hosts. We examine this premise for Pseudotsuga menziesii var. menziesii of southwest British Columbia, using ten native seedlots collected from a range of mean annual precipitation (MAP), as a proxy for podzolization extent and phosphorus (P) deficiencies, and evaluated in contrasting low P and high P soils. After two growing seasons, seedling biomass in the high P soil dwarfed that of the low P soil, and better growth rates under high P were detected for populations from very dry and very wet origins. EMF communities on the high P soil displayed more symmetry among host populations than the low P soil (average community dissimilarity of 0.20% vs. 0.39%, respectively). Seedling foliar P% differed slightly but significantly in relation to MAP of origin. EMF species richness varied significantly among host populations but independently of climatic parameters. There were significant shifts in EMF species abundance related to seedlot MAP, particularly on the low P soil where nonlinear relationships were found for Wilcoxina mikolae, Hyaloscypha finlandica, and Rhizopogon villosulus. Despite efforts to enhance colonization by native fungi, the predominance of ruderal EMF species hindered a realistic evaluation of local adaptation among host-fungi populations. Nevertheless, the shifting affinity in taxa abundance and wider community disparity on low P soil reflected the potential for a consequential host genetic effect related to geographical patterns in P availability across temperate rainforests.

Older populations of the invader Solidago canadensis exhibit stronger positive plant-soil feedbacks and competitive ability in China

PREMISE

The enemy release hypothesis predicts that release from natural enemies, including soil-borne pathogens, liberates invasive plants from a negative regulating force. Nevertheless, invasive plants may acquire novel enemies and mutualists in the introduced range, which may cause variable effects on invader growth. However, how soil microorganisms may influence competitive ability of invasive plants along invasion chronosequences has been little explored.

METHODS

Using the invasive plant Solidago canadensis, we tested whether longer residence times are associated with stronger negative plant-soil feedbacks and thus weaker competitive abilities at the individual level. We grew S. canadensis individuals from 36 populations with different residence times across southeastern China in competition versus no competition and in three different types of soils: (1) conspecific rhizospheric soils; (2) soils from uninvaded patches; and (3) sterilized soil. For our competitor treatments, we constructed synthetic communities of four native species (Bidens parviflora, Solanum nigrum, Kalimeris indica, and Mosla scabra), which naturally co-occur with Solidago canadensis in the field.

RESULTS

Solidago canadensis populations with longer residence times experienced stronger positive plant-soil feedbacks and had greater competitive responses (i.e., produced greater above-ground biomass and grew taller) in conspecific rhizospheric soils than in sterilized or uninvaded soils. Moreover, S. canadensis from older populations significantly suppressed above-ground biomass of the native communities in rhizospheric and uninvaded soils but not in sterilized soil.

CONCLUSIONS

The present results suggest that older populations of S. canadensis experience stronger positive plant-soil feedbacks, which may enhance their competitive ability against native plant communities.

Of mutualism and migration: will interactions with novel ericoid mycorrhizal communities help or hinder northward Rhododendron range shifts?

Rapid climate change imperils many small-ranged endemic species as the climate envelopes of their native ranges shift poleward. In addition to abiotic changes, biotic interactions are expected to play a critical role in plant species' responses. Below-ground interactions are of particular interest given increasing evidence of microbial effects on plant performance and the prevalence of mycorrhizal mutualisms. We used greenhouse mesocosm experiments to investigate how natural northward migration/assisted colonization of Rhododendron catawbiense, a small-ranged endemic eastern U.S. shrub, might be influenced by novel below-ground biotic interactions from soils north of its native range, particularly with ericoid mycorrhizal fungi (ERM). We compared germination, leaf size, survival, and ERM colonization rates of endemic R. catawbiense and widespread R. maximum when sown on different soil inoculum treatments: a sterilized control; a non-ERM biotic control; ERM communities from northern R. maximum populations; and ERM communities collected from the native range of R. catawbiense. Germination rates for both species when inoculated with congeners' novel soils were significantly higher than when inoculated with conspecific soils, or non-mycorrhizal controls. Mortality rates were unaffected by treatment, suggesting that the unexpected reciprocal effect of each species' increased establishment in association with heterospecific ERM could have lasting demographic effects. Our results suggest that seedling establishment of R. catawbiense in northern regions outside its native range could be facilitated by the presence of extant congeners like R. maximum and their associated soil microbiota. These findings have direct relevance to the potential for successful poleward migration or future assisted colonization efforts.

Microbes, mutualism, and range margins: testing the fitness consequences of soil microbial communities across and beyond a native plant's range

Interactions between plants and soil fungi and bacteria are ubiquitous and have large effects on individual plant fitness. However, the degree to which spatial variation in soil microbial communities modulates plant species' distributions remains largely untested. Using the California native plant Clarkia xantiana ssp. xantiana we paired glasshouse and field reciprocal transplants of plant populations and soils to test whether plant-microbe interactions affect the plant's geographic range limit and whether there is local adaptation between plants and soil microbe communities. In the field and glasshouse, one of the two range interior inocula had a positive effect on plant fitness. In the field, this benefit was especially pronounced at the range edge and beyond, suggesting possible mutualist limitation. In the glasshouse, soil inocula from beyond-range tended to increase plant growth, suggesting microbial enemy release beyond the range margin. Amplicon sequencing revealed stark variation in microbial communities across the range boundary. Plants dispersing beyond their range limit are likely to encounter novel microbial communities. In C. x. xantiana, our results suggest that range expansion may be facilitated by fewer pathogens, but could also be hindered by a lack of mutualists. Both negative and positive plant-microbe interactions will likely affect contemporary range shifts.

Biogeographic differences in plant-soil biota relationships contribute to the exotic range expansion of Verbascum thapsus

Exotic plant species can evolve adaptations to environmental conditions in the exotic range. Furthermore, soil biota can foster exotic spread in the absence of negative soil pathogen-plant interactions or because of increased positive soil biota-plant feedbacks in the exotic range. Little is known, however, about the evolutionary dimension of plant-soil biota interactions when comparing native and introduced ranges.To assess the role of soil microbes for rapid evolution in plant invasion, we subjected Verbascum thapsus, a species native to Europe, to a reciprocal transplant experiment with soil and seed material originating from Germany (native) and New Zealand (exotic). Soil samples were treated with biocides to distinguish between effects of soil fungi and bacteria. Seedlings from each of five native and exotic populations were transplanted into soil biota communities originating from all populations and subjected to treatments of soil biota reduction: application of (a) fungicide, (b) biocide, (c) a combination of the two, and (d) control.For most of the investigated traits, native populations showed higher performance than exotic populations; there was no effect of soil biota origin. However, plants developed longer leaves and larger rosettes when treated with their respective home soil communities, indicating that native and exotic plant populations differed in their interaction with soil biota origin. The absence of fungi and bacteria resulted in a higher specific root length, suggesting that V. thapsus may compensate the absence of mutualistic microbes by increasing its root-soil surface contact. Synthesis. Introduced plants can evolve adaptations to soil biota in their new distribution range. This demonstrates the importance of biogeographic differences in plant-soil biota relationships and suggests that future studies addressing evolutionary divergence should account for differential effects of soil biota from the home and exotic range on native and exotic populations of successful plant invaders.

Connections and Feedback: Aquatic, Plant, and Soil Microbiomes in Heterogeneous and Changing Environments

Plant, soil, and aquatic microbiomes interact, but scientists often study them independently. Integrating knowledge across these traditionally separate subdisciplines will generate better understanding of microbial ecological properties. Interactions among plant, soil, and aquatic microbiomes, as well as anthropogenic factors, influence important ecosystem processes, including greenhouse gas fluxes, crop production, nonnative species control, and nutrient flux from terrestrial to aquatic habitats. Terrestrial microbiomes influence nutrient retention and particle movement, thereby influencing the composition and functioning of aquatic microbiomes, which, themselves, govern water quality, and the potential for harmful algal blooms. Understanding how microbiomes drive links among terrestrial (plant and soil) and aquatic habitats will inform management decisions influencing ecosystem services. In the present article, we synthesize knowledge of microbiomes from traditionally disparate fields and how they mediate connections across physically separated systems. We identify knowledge gaps currently limiting our abilities to actualize microbiome management approaches for addressing environmental problems and optimize ecosystem services.

Native and Invading Yellow Starthistle (Centaurea solstitialis) Microbiomes Differ in Composition and Diversity of Bacteria

Invasive species could benefit from being introduced to locations with more favorable species interactions, including the loss of enemies, the gain of mutualists, or the simplification of complex interaction networks. Microbiomes are an important source of species interactions with strong fitness effects on multicellular organisms, and these interactions are known to vary across regions. The highly invasive plant yellow starthistle (Centaurea solstitialis) has been shown to experience more favorable microbial interactions in its invasions of the Americas, but the microbiome that must contribute to this variation in interactions is unknown. We sequenced amplicons of 16S rRNA genes to characterize bacterial community compositions in the phyllosphere, ectorhizosphere, and endorhizosphere of yellow starthistle plants from seven invading populations in California, USA, and eight native populations in Europe. We tested for the differentiation of microbiomes by geography, plant compartment, and plant genotype. Bacterial communities differed significantly between native and invading plants within plant compartments, with consistently lower diversity in the microbiome of invading plants. The diversity of bacteria in roots was positively correlated with plant genotype diversity within both ranges, but this relationship did not explain microbiome differences between ranges. Our results reveal that these invading plants are experiencing either a simplified microbial environment or simplified microbial interactions as a result of the dominance of a few taxa within their microbiome. Our findings highlight several alternative hypotheses for the sources of variation that we observe in invader microbiomes and the potential for altered bacterial interactions to facilitate invasion success.IMPORTANCE Previous studies have found that introduced plants commonly experience more favorable microbial interactions in their non-native range, suggesting that changes to the microbiome could be an important contributor to invasion success. Little is known about microbiome variation across native and invading populations, however, and the potential sources of more favorable interactions are undescribed. Here, we report one of the first microbiome comparisons of plants from multiple native and invading populations, in the noxious weed yellow starthistle. We identify clear differences in composition and diversity of microbiome bacteria. Our findings raise new questions about the sources of these differences, and we outline the next generation of research that will be required to connect microbiome variation to its potential role in plant invasions.