Interactions between aboveground herbivores and the mycorrhizal mutualists of plants

Plant growth, reproduction and survival can be affected both by mycorrhizal fungi and aboveground herbivores, but few studies have examined the interactive effects of these factors on plants. Most of the available data suggest that severe herbivory reduces root colonization by vesicular-arbuscular and ectomycorrhizal fungi. However, the reverse interaction has also been documented - mycorrhizal fungi deter herbivores and interact with fungal endophytes to influence herbivory. Although consistent patterns and mechanistic explanations are yet to emerge, it is likely that aboveground herbivore-mycorrhiza interactions have important implications for plant populations and communities.     

Mycorrhizal status of epiphytes in Malaysian oil palm plantations

Epiphytic plants were collected from four oil palm plantations in Peninsular Malaysia and their mycorrhizal status determined. Conspecific plants with a terrestrial habit (16 species) and rhizosphere soils were also examined for mycorrhizal colonization and glomalean fungi, respectively. Twelve species of glomalean fungi were recovered from the four oil palm plantation soils. Of the 29 epiphytic species in 16 families belonging to the bryophytes, pteridophytes and angiosperms, only four species of angiosperms that were facultative epiphytes and a hemiepiphyte growing within 0.4 m of ground level had vesicular-arbuscular mycorrhizal (VAM) fungi. Bioassays of organic debris from oil palm trunks did not produce vesicular-arbuscular mycorrhizas on maize. Six epiphytic species grown in the greenhouse in pots containing oil palm rhizosphere soils rooted and had VAM fungi and thus may be facultative epiphytes. Five other epiphyte species failed to grow in pots and are probably obligate epiphytes. Seven epiphyte species that established themselves in pots failed to form vesicular-arbuscular mycorrhizas.     

Balancing multiple mutualists: asymmetric interactions among plants, arbuscular mycorrhizal fungi, and fungal endophytes

Most organisms engage in beneficial interactions with other species; however, little is known regarding how individuals balance the competing demands of multiple mutualisms. Here we examine three-way interactions among a widespread grass, Schedonorus phoenix , a protective fungal endophyte aboveground, Neotyphodium coenophialum , and nutritional symbionts (arbuscular mycorrhizal fungi) belowground. In a greenhouse experiment, we manipulated the presence/absence of both fungi and applied a fertilizer treatment to individual plants. Endophyte presence in host plants strongly reduced mycorrhizal colonization of roots. Additionally, for plants with the endophyte, the density of endophyte hyphae was negatively correlated with mycorrhizal colonization, suggesting a novel role for endophyte abundance in the interaction between the symbionts. Endophyte presence increased plant biomass, and there was a positive correlation between endophyte hyphal density and plant biomass. The effects of mutualists were asymmetric: mycorrhizal fungi treatments had no significant impact on the endophyte and negligible effects on plant biomass. Fertilization affected all three species – increasing plant biomass and endophyte density, but diminishing mycorrhizal colonization. Mechanisms driving negative effects of endophytes on mycorrhizae may include inhibition via endophyte alkaloids, altered nutritional requirements of the host plant, and/or temporal and spatial priority effects in the interactions among plants and multiple symbionts.     

Sex-specific variation in the interaction between Distichlis spicata (Poaceae) and mycorrhizal fungi

Associations between mycorrhizal fungi and plants can influence intraspecific competition and shape plant population structure. While variation in plant genotypes is known to affect mycorrhizal colonization in crop systems, little is known about how genotypes affect colonization in natural plant populations or how plant sex might influence colonization with mycorrhizal fungi in plant species with dimorphic sexual systems. In this study, we analyzed mycorrhizal colonization in males and females of the wetland dioecious grass Distichlis spicata, which has spatially segregated sexes. Our results suggest that D. spicata males and females interact with mycorrhizal fungi differently. We discuss the implications for the role of this sex-specific symbiotic interaction in the maintenance of the within-population sex ratio bias of D. spicata.     

Differences in AM fungal root colonization between populations of perennial Aster species have genetic reasons

We tested the hypothesis whether differences between plant populations in root colonization by arbuscular mycorrhizal (AM) fungi could be caused by genetic differentiation between populations. In addition, we investigated whether the response to AM fungi differs between plants from different populations and if it is affected by the soil in which the plants are cultivated. We used Aster amellus, which occurs in fragmented dry grasslands, as a model species and we studied six different populations from two regions, which varied in soil nutrient concentration.We found significant differences in the degree of mycorrhizal colonization of plant roots between regions in the field. To test if these differences were due to phenotypic plasticity or had a genetic basis, we performed a greenhouse experiment. The results suggested that Aster amellus is an obligate mycotrophic plant species with a high dependency upon mycorrhiza. Plant biomass was affected only by soil, and not by population or the interaction between the population and the soil. Mycorrhizal colonization was significantly affected by all three factors (soil, population, interaction of soil and population). Plants from the population originating from the soil with lower nutrient availability developed more mycorrhiza even when grown in soil with higher nutrient availability. The correspondence between mycorrhizal colonization of plants in the field and in both soils in the pot experiment suggests that the observed differences in root colonization have a genetic basis.     

Nontarget and ecological effects of transgenically altered disease resistance in crops -possible effects on the mycorrhizal symbiosis

The ability to increase crop disease resistance by using transgenic (TG) means has recently been demonstrated for several crops. The current TG procedures alter the temporal expression of transgene pathogenesis-related (PR) proteins, so that the usually inducible PR proteins are expressed constitutively in the foreign host. The constitutive expression of the transgene PR protein chitinase is believed to increase the host's nonspecific basic resistance to pathogens. A potential nontarget effect of constitutively expressing chitinase may be a decrease in the activity of beneficial microbes, especially vesicular-arbuscular mycorrhizal fungi. The decrease in activity of mycorrhizal fungi is related to reduced susceptibility of TG plant roots to colonization by these fungi, which is in turn associated with lysis of fungal cell walls by the constitutively expressed chitinase. An argument is presented that use of TG means to alter the temporal expression of PR proteins ignores a legacy of past evolutionary trade-offs in vascular plants. A major nontarget effect of expressing transgene chitinase is a reduction in the susceptibility of roots to colonization by mycorrhizal fungi. This reduction in mycorrhizal susceptibility occurs without alteration of the mycorrhizal dependence of the host on symbiont-supplied nutrients. Data are presented in support of this contention that demonstrate a strong negative association between host pathogen resistance and mycorrhizal colonization. An ecological consequence of reducing mycorrhizal colonization is a decrease in the soil's mycorrhizal propagule reserve that diminishes the next crop's production, especially under low-input cropping practices. A further consequence that has both ecological and evolutionary outcomes is the escape of the transgene for improved pathogen resistance into wild populations. By increasing a crop's disease resistance by TG means, we may inadvertently be creating a ‘super weed’ when the TG plant or the transgene escapes into wild relatives through hybridization. Hybridization of wild relatives with TG plants would be especially relevant for crops, such as sugar beet, rapeseed, and many modern cereal cultivars that have close relatives in the wild but have a relatively low requirement for symbiont supplied nutrients or are nondependent.     

Density dependent interactions between VA mycorrhizal fungi and even-aged seedlings of two perennial Fabaceae species

Summary The interaction of density and mycorrhizal effects on the growth, mineral nutrition and size distribution of seedlings of two perennial members of the Fabaceae was investigated in pot culture. Seedlings of Otholobium hirtum and Aspalathus linearis were grown at densities of 1, 4, 8 and 16 plants per 13-cm pot with or without vesicular-arbuscular (VA) mycorrhizal inoculum for 120 days. Plant mass, relative growth rates, height and leaf number all decreased with increasing plant density. This was ascribed to the decreasing availability of phosphorus per plant as density increased. O. hirtum was highly dependent on mycorrhizas for P uptake but both mycorrhizal and non-mycorrhizal A. linearis seedlings were able to extract soil P with equal ease. Plant size distribution as measured by the coefficient of variation (CV) of shoot mass was greater at higher densities. CVs of mycorrhizal O. hirtum plants were higher than those of non-mycorrhizal plants. CVs of the facultatively mycorrhizal A. linearis were similar for both mycorrhizal and non-mycorrhizal plants. Higher CVs are attributed to resource preemption by larger individuals. Individuals in populations with high CVs will probably survive stress which would result in the extinction of populations with low CVs. Mass of mycorrhizal plants of both species decreased more rapidly with increasing density than did non-mycorrhizal plant mass. It is concluded that the cost of being mycorrhizal increases as plant density increases, while the benefit decreases. The results suggest that mycorrhizas will influence density-dependent population processes of faculative and obligate mycorrhizal species.     

The influence of host plant,nitrogen fertilization and fungicide application on the abundance and seasonal dynamics of vesicular-arbuscular mycorrhizal fungi in arable soils of northern Germany

The present investigation examines whether the crop plant, increased nitrogen (N) fertilization and fungicide application influence the pattern of vesicular-arbuscular mycorrhizal fungi (VAM) populations. For this purpose, two arable field locations in Lower Saxony (Hotteln and Langreder) were chosen and the formation of mycorrhiza, spore density, number of infectious propagules (MPN) and frequency of spore types within VAM populations were investigated. The influence of crop plants was examined over two cultivation periods (1986/1987 and 1987/1988) in Hotteln, comparing winter wheat, winter barley and sugar beet. The effects of increased N fertilization and fungicide application were investigated on winter wheat in Langreder in the cultivation period 1988 only. Both the frequency of mycorrhizal infection and the spore dynamics in soil differed with the crops grown. Spore density and MPN increased until harvesting when host plants (winter wheat, winter barley) were cultivated, whereas both diminished with a non-host plant (sugar beet). Different spore types increased or decreased, according to the plant species grown, but the predominating types of the location remained constant. Increased N fertilization caused marginal inhibition of mycorrhizal infection and sporulation on winter wheat, whereas both leaf and base application of fungicides resulted in minor increases in mycorrhizal colonization of roots and sporulation in soil. Both increased N fertilization and fungicide application distinctly decreased the sporulation of one type in May, but the characteristic compositions of the VAM populations remained unchanged.     

Effects of mycorrhizal symbiosis on tallgrass prairie plant–herbivore interactions

Complex relationships occur among plants, mycorrhizal fungi, and herbivores. By altering plant nutrient status, mycorrhizas may alter herbivory or plant tolerance to herbivory via compensatory regrowth. We examined these interactions by assessing grasshopper preference and plant growth and fungal colonization responses to herbivory under mycorrhizal and non‐mycorrhizal conditions within tallgrass prairie microcosms. Mycorrhizal symbiosis increased plant regrowth following defoliation, and some strongly mycotrophic plant species showed overcompensation in response to herbivory when they were mycorrhizal. Although grasshoppers spent more time on mycorrhizal plants, herbivory intensity did not differ between mycorrhizal and non‐mycorrhizal plants. Aboveground herbivory by grasshoppers significantly increased mycorrhizal fungal colonization of plant roots. Thus mycorrhizas may greatly benefit plants subjected to herbivory by stimulating compensatory growth, and herbivores, in turn, may increase the development of the symbiosis. Our results also indicate strong interspecific differences among tallgrass prairie plant species in their responses to the interaction of aboveground herbivores and mycorrhizal symbionts.     

Mycorrhizal symbiosis increases growth,reproduction and recruitment of Abutilon theophrasti Medic. in the field

We examined in the field the effect of the vesicular-arbuscular (VA) mycorhizal symbiosis on the reproductive success of Abutilon theophrasti Medic., an early successional annual member of the Malvaceae. Mycorrhizal infection greatly enhanced vegetative growth, and flower, fruit and seed production, resulting in significantly greater recruitment the following year. In addition, the seeds produced by mycorrhizal plants were significantly larger and contained significantly more phosphorus than seeds from non-mycorrhizal plants, an effect which may improve offspring vigor. Infection by mycorrhizal fungi may thus contribute to the overall fitness of a host plant and strongly influence long-term plant population dynamics.     

Differences in above- and below-ground responses to ozone between two populations of a perennial grass

Our study examined the influence of elevated ozone levels on the growth and mycorrhizal colonization of two populations of Elymus glaucus L. (blue wildrye). We hypothesized that ozone would reduce carbon availability to the plants, particularly below ground, and would affect mycorrhizal colonization. Because of the wide geographic range of E. glaucus, two populations of plants were selected from areas of contrasting ozone histories to examine intraspecies variation in response to ozone. Two populations of E. glaucus (southern California versus northern California) exposed in a factorial experiment involving ozone, mycorrhizal inoculation with Glomus intraradices Schenck and Smith, and plant source population. Ozone had a subtle effect on leaf area and number of tillers but did not affect overall root:shoot ratio in either population. The impact of ozone on above-ground growth characteristics was most pronounced in the southern population that came from a high-ozone environment, while below-ground responses such as reduced arbuscular colonization was most pronounced in the northern population which originated in a low-ozone environment. Further analysis of soil characteristics from the northern population of plants revealed a significant reduction in active soil bacterial biomass and an increase in total fungi per gram dry weight soil, suggesting a possible role for ozone in altering soil processes. Whether or not population differences in response to ozone were due to genetic shifts resulting from prior ozone remains to be determined. However, these subtle but important differences in population response to ozone above- and below-ground have significant implications in any attempt to generalize plant response, even within a species. Future research efforts need to include better characterization of intraspecific variation in response to ozone as well as possible adaptive strategies that may result from chronic ozone exposure.     

INFLUENCE OF SUPPLEMENTAL INORGANIC NUTRIENTS ON GROWTH,SURVIVORSHIP, AND MYCORRHIZAL RELATIONSHIPS OF SCHIZACHYRIUM SCOPARIUM (POACEAE) GROWN IN FUMIGATED AND UNFUMIGATED SOIL

Little bluestem grass Schizachyrium scoparium ([Michx.] Nash) plants were grown under field conditions for 2 years in soils fumigated with methyl bromide and chloropicrin, or in unfumigated soil, and treated with supplemental inorganic nutrients (bases calcium and magnesium) phosphorus, nitrogen, and potassium. Most differences in measured plant responses were due to interactions between fumigation and nutrient treatments. These included biomass production, root mass per unit length (μg/cm), root lengths, flowering culm production, percent colonization, colonized root length, and spore production in rhizosphere soil. Plants generally responded to mycorrhizal fungal colonization by reducing total root length and producing thicker roots. Treatment of plants with bases appeared to profoundly affect the mycorrhizal association by reducing sporulation of vesicular-arbuscular mycorrhizal fungi and increasing colonization. When fumigated or unfumigated soils were considered separately, base-treated plants produced more biomass than other treatments. Base-treated plants grown on unfumigated soil had more flowering culms and longer colonized root lengths than all other plants. Percent colonization by mycorrhizal fungi and colonized root length were positively correlated with phosphorus/nitrogen ratios, but the ratio was not correlated with plant biomass production. This suggests that phosphorus is not a limiting nutrient in our soil and investment in a mycorrhizal association may not result in enhanced plant growth. The base-nutrient effects may indicate a need to reevaluate earlier studies of macro nutrient effects that did not take into account the role played by calcium and magnesium in assessing fungus-host plant interactions.     

Non-host plants: Are they mycorrhizal networks players?

Common mycorrhizal networks (CMNs) that connect individual plants of the same or different species together play important roles in nutrient and signal transportation, and plant community organization. However, about 10% of land plants are non-mycorrhizal species with roots that do not form any well-recognized types of mycorrhizas; and each mycorrhizal fungus can only colonize a limited number of plant species, resulting in numerous non-host plants that could not establish typical mycorrhizal symbiosis with a specific mycorrhizal fungus. If and how non-mycorrhizal or non-host plants are able to involve in CMNs remains unclear. Here we summarize studies focusing on mycorrhizal-mediated host and non-host plant interaction. Evidence has showed that some host-supported both arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) hyphae can access to non-host plant roots without forming typical mycorrhizal structures, while such non-typical mycorrhizal colonization often inhibits the growth but enhances the induced system resistance of non-host plants. Meanwhile, the host growth is also differentially affected, depending on plant and fungi species. Molecular analyses suggested that the AMF colonization to non-hosts is different from pathogenic and endophytic fungi colonization, and the hyphae in non-host roots may be alive and have some unknown functions. Thus we propose that non-host plants are also important CMNs players. Using non-mycorrhizal model species Arabidopsis, tripartite culture system and new technologies such as nanoscale secondary ion mass spectrometry and multi-omics, to study nutrient and signal transportation between host and non-host plants via CMNs may provide new insights into the mechanisms underlying benefits of intercropping and agro-forestry systems, as well as plant community establishment and stability.     

Grazing tolerance and mycorrhizal colonization: Effects of resource manipulation and plant size in biennial Gentianella campestris

As herbivory usually leads to loss of photosynthesizing biomass, its consequences for plants are often negative. However, in favorable conditions, effects of herbivory on plants may be neutral or even beneficial. According to the compensatory continuum hypothesis plants can tolerate herbivory best in resource-rich conditions. Besides herbivory, also primarily positive biotic interactions like mycorrhizal symbiosis, bear carbon costs. Tritrophic plant–fungus–herbivore interaction further complicates plant's cost-benefit balance, because herbivory of the host plant is expected to cause decline in mycorrhizal colonization under high availability of soil nutrients when benefits of symbiosis decline in relation to costs. To gain insight into above interactions we tested the effects of plant size and resource manipulation (simulated herbivory and fertilization) on both above-ground performance and on root fungal colonization of the biennial Gentianella campestris.Clipping caused allocation shift from height growth to branches in all groups except in large and fertilized plants. For large plants nutrient addition may have come too late, as the number of meristems was most likely determined already before the fertilization. Clipping decreased the amount of DSE (dark septate endophytic) fungi which generally are not considered to be mycorrhizal. The effect of clipping on total fungal colonization and colonization by arbuscular mycorrhizal (AM) fungal coils were found to depend on host size and resource level. Dissimilar mycorrhizal response to simulated herbivory in small vs. large plants could be due to more intensive light competition in case of small plants. Carbon limited small plants may not be able to maintain high mycorrhizal colonization, whereas large clipped plants allocate extra resources to roots and mycorrhizal fungi at the expense of above-ground parts. Our results suggest that herbivory may increase carbon limitation that leads re-growing shoots and fungal symbionts to function as competing sinks for the limited carbon reserves.     

Mycorrhiza‐mediated interference between cover crop and weed in organic winter cereal agroecosystems: The mycorrhizal colonization intensity indicator

The mycorrhizal fungi are symbiotic organisms able to provide many benefits to crop production by supplying a set of ecosystem functions. A recent ecological approach based on the ability of the fungi community to influence plant–plant interactions by extraradical mycelium development may be applied to diversified, herbaceous agroecosystems. Our hypothesis is that the introduction of a winter cereal cover crop (CC) as arbuscular mycorrhizal fungi (AMF)–host plant in an organic rotation can boosts the AMF colonization of the other plants, influencing crop–weed interference. In a 4‐years organic rotation, the effect of two winter cereal CC, rye and spelt, on weed density and AMF colonization was evaluated. The AMF extraradical mycelium on CC and weeds roots was observed by scanning electron microscopy analysis. By joining data of plant density and mycorrhization, we built the mycorrhizal colonization intensity of the Agroecosystem indicator (MA%). Both the CC were colonized by soil AMF, being the mycorrhizal colonization intensity (M%) affected by environmental conditions. Under CC, the weed density was reduced, due to the increase of the reciprocal competition in favor of CC, which benefited from mycorrhizal colonization and promoted the development of AMF extraradical mycelium. Even though non‐host plants, some weed species showed an increased mycorrhizal colonization in presence of CC respect to the control. Under intense rainfall, the MA% was less sensitive to the CC introduction. On the opposite, under highly competitive conditions, both the CC boosted significantly the mycorrhization of coexistent plants in the agroecosystem. The proposed indicator measured the agroecological service provided by the considered CCs in promoting or inhibiting the overall AMF colonization of the studied agroecosystems, as affected by weed selection and growth: It informs about agroecosystem resilience and may be profitably applied to indicate the extent of the linkage of specific crop traits to agroecosystem services, contributing to further develop the functional biodiversity theory.     

An ecological view of the formation of VA mycorrhizas

In spite of the major advances in understanding the functioning of symbioses between plants and arbuscular mycorrhizal fungi, details of the ecology of mycorrhizal fungi are not well documented. The benefits of the association are related to the timing and extent of colonization of roots, and fungi differ in their contribution to plant growth and presumably to soil aggregation. Knowledge of the processes that lead to successful colonization of roots by beneficial fungi at appropriate times for the host plants will form the basis of guidelines for soil management to maximize the benefits from the symbiosis. Fungi differ in the manner and extent to which they colonize roots. They also differ in their capacity to form propagules. The importance of hyphae, spores and propagules within living or dead mycorrhizal roots also differs among species and for the same species in different habitats. The relationships between colonization of roots and propagule formation, and between propagule distribution and abundance and subsequent mycorrhiza formation, for different fungi in field environments, are not well understood. Methods for quantifying mycorrhizal fungi are not especially suitable for distinguishing among different fungi within roots. Consequenctly, the dynamics of colonization of roots by different fungi, within and between seasons, have been little studied. Research is required that focuses on the dynamics of fungi within roots as well as on changes in the abundance of propagules of different fungi within soil. Interactions between fungi during the colonization of roots, the colonization of soil by hyphae and sporulation are all poorly understood. Without knowledge of these processes, it will by difficult to predict the likely success of inoculation with introduced fungi. Such knowledge is also required for selecting soil management procedures to enhance growth and survival of key species within the population. The relative tolerance of various fungi to perturbations in their surroundings will provide a basis for identifying those fungi that are likely to persist in specific environments. The processes that influence mycorrhizal fungi in field soils can be identified in controlled studies. However, greater emphasis is required on studying these processes with mixed populations of fungi. The role played by diversity within populations of mycorrhizal fungi is virtually unexplored.     

Exotic cheatgrass and loss of soil biota decrease the performance of a native grass

Soil disturbances can alter microbial communities including arbuscular mycorrhizal (AM) fungi, which may in turn, affect plant community structure and the abundance of exotic species. We hypothesized that altered soil microbial populations owing to disturbance would contribute to invasion by cheatgrass (Bromus tectorum), an exotic annual grass, at the expense of the native perennial grass, squirreltail (Elymus elymoides). Using a greenhouse experiment, we compared the responses of conspecific and heterospecific pairs of cheatgrass and squirreltail inoculated with soil (including live AM spores and other organisms) collected from fuel treatments with high, intermediate and no disturbance (pile burns, mastication, and intact woodlands) and a sterile control. Cheatgrass growth was unaffected by type of soil inoculum, whereas squirreltail growth, reproduction and nutrient uptake were higher in plants inoculated with soil from mastication and undisturbed treatments compared to pile burns and sterile controls. Squirreltail shoot biomass was positively correlated with AM colonization when inoculated with mastication and undisturbed soils, but not when inoculated with pile burn soils. In contrast, cheatgrass shoot biomass was negatively correlated with AM colonization, but this effect was less pronounced with pile burn inoculum. Cheatgrass had higher foliar N and P when grown with squirreltail compared to a conspecific, while squirreltail had lower foliar P, AM colonization and flower production when grown with cheatgrass. These results indicate that changes in AM communities resulting from high disturbance may favor exotic plant species that do not depend on mycorrhizal fungi, over native species that depend on particular taxa of AM fungi for growth and reproduction.     

Density-dependent response to mycorrhizal infection in Abutilon theophrasti Medic

Summary One purpose of this study was to determine whether an increase in plant density would result in a decrease in response to mycorrhizal infection (particularly as measured by phosphorus content). Increases in plant density generally result in increases in root density in the volume of soil occupied by the plants. Root density, in turn, largely determines phosphorus uptake. If mycorrhizal plants had significantly higher effective root densities than non-mycorrhizal plants due to the fungal hyphae and thus were more thorough in exploiting a given volume of soil for phosphorus, then a given increase in root density might result in a greater proportional increase in phosphorus uptake for non-mycorrhizal plants than for mycorrhizal plants. Two experiments were performed in which mycorrhizal infection and available soil volume per plant were manipulated; one in which the number of plants within a given pot size was varied (experiment 1), and another in which single plants were grown in pots of differing volume (experiment 2). The two experiments yielded similar results but for apparently different reasons. In the first experiment, for a given increase in root density, non-mycorrhizal plants had a greater proportional increase in phosphorus uptake than mycorrhizal plants. Thus, as predicted, response to mycorrhizal infection was greatest at the lowest planting density (highest available soil volume per plant, lowest root density). In experiment 2, response to infection was also greatest at the highest available soil volume per plant (largest pot), but pot size did not influence root density. These results show that the benefit from mycorrhizal infection may be partly determined by root density and they suggest that plants either occurring in patches of contrasting root density in a given community, or occurring in different communities with inherently different root densities may differ in their reliance upon mycorrhizal fungi for phosphorus uptake.     

Plant invasions facilitated by suppression of root nutrient acquisition rather than by disruption of mycorrhizal association in the native plant

Invasive species have profound negative impacts on native ranges. Unraveling the mechanisms employed by invasive plant species is crucial to controlling invasions. One important approach that invasive plants use to outcompete native plants is to disrupt mutualistic interactions between native roots and mycorrhizal fungi. However, it remains unclear how differences in the competitive ability of invasive plants affect native plant associations with mycorrhizae. Here, we examined how a native plant, Xanthium strumarium, responds to invasive plants that differed in competitive abilities (i.e., as represented by aboveground plant biomass) by measuring changes in root nitrogen concentration (root nutrient acquisition) and mycorrhizal colonization rate. We found that both root nitrogen concentration and mycorrhizal colonization rate in the native plant were reduced by invasive plants. The change in mycorrhizal colonization rate of the native plant was negatively correlated with both aboveground plant biomass of the invasive plants and the change in aboveground plant biomass of the native plant in monocultures relative to mixed plantings. In contrast, the change in root nitrogen concentration of the native plant was positively correlated with aboveground plant biomass of the invasive plants and the change in aboveground plant biomass of the native plant. When we compared the changes in mycorrhizal colonization rate and root nitrogen concentration in the native plant grown in monocultures with those of native plants grown with invasive plants, we observed a significant tradeoff. Our study shows that invasive plants can suppress native plants by reducing root nutrient acquisition rather than by disrupting symbiotic mycorrhizal associations, a novel finding likely attributable to a low dependence of the native plant on mycorrhizal fungi.     

Evidence for the evolution of native plant response to mycorrhizal fungi in post‐agricultural grasslands

Plant–microbe interactions play an important role in structuring plant communities. Arbuscular mycorrhizal fungi (AMF) are particularly important. Nonetheless, increasing anthropogenic disturbance will lead to novel plant–AMF interactions, altering longstanding co‐evolutionary trajectories between plants and their associated AMF. Although emerging work shows that plant–AMF response can evolve over relatively short time scales due to anthropogenic change, little work has evaluated how plant AMF response specificity may evolve due to novel plant–mycorrhizal interactions. Here, we examine changes in plant–AMF interactions in novel grassland systems by comparing the mycorrhizal response of plant populations from unplowed native prairies with populations from post‐agricultural grasslands to inoculation with both native prairie AMF and non‐native novel AMF. Across four plant species, we find support for evolution of differential responses to mycorrhizal inocula types, that is, mycorrhizal response specificity, consistent with expectations of local adaptation, with plants from native populations responding most to native AMF and plants from post‐agricultural populations responding most to non‐native AMF. We also find evidence of evolution of mycorrhizal response in two of the four plant species, as overall responsiveness to AMF changed from native to post‐agricultural populations. Finally, across all four plant species, roots from native prairie populations had lower levels of mycorrhizal colonization than those of post‐agricultural populations. Our results report on one of the first multispecies assessment of local adaptation to AMF. The consistency of the responses in our experiment among four species provides evidence that anthropogenic disturbance may have unintended impacts on native plant species'' association with AMF, causing evolutionary change in the benefit native plant species gain from native symbioses.