Genetic variation for sensitivity to a thyme monoterpene in associated plant species

Increased invasion of submerged macrophytes makes native species more susceptible to eutrophication in freshwater ecosystems

Invasion and eutrophication are considered to pose serious threats to freshwater biodiversity and ecosystem function. However, little is known about the synergistic effects of invasion density and nutrient concentration on native submerged macrophytes. Here, we selected a common invasive species (Elodea nuttallii) and two native plants (Hydrilla verticillata and Potamogeton maackianus) to elucidate the effects of invasion density and eutrophication on native submerged plants. We found that (1) high nutrient concentrations inhibited the growth of both invasive and native species, but E. nuttallii, with a wide ecological niche, was more tolerant to eutrophication than the two native species. (2) High invasion density had a remarkable negative effect on the growth of the two native species under the medium and high nutrient concentrations. (3) Medium and high invasion densities of E. nuttallii made native macrophytes more susceptible to eutrophication. (4) The two native macrophytes had species-specific responses to medium and high invasion densities under medium and high nutrient concentrations. Specifically, a high invasion density of E. nuttallii significantly delayed the growth of H. verticillata rather than P. maackianus. Thus, it is necessary to consider the synergistic effects of invasion with eutrophication when assessing invasion in freshwater ecosystems. And our results implied that invasion with eutrophication was a powerful factor determining the results of interspecific competition among submerged macrophytes, which could change the biodiversity, community structure and functions of freshwater ecosystems.

Species-specific interference exerted by the shrub Cistus clusii Dunal in a semi-arid Mediterranean gypsum plant community

Background

The gypsovag shrub Cistus clusii is locally dominant in semi-arid gypsum plant communities of North-Eastern Spain. This species commonly grows in species-poor patches even though it has nurse potential, suggesting interference on neighbouring species. Other Cistus species exert a chemically mediated interference on plant communities, suggesting that it might be a common phenomenon in this genus. This study aimed investigating whether C. clusii exerts chemically mediated interference on neighbouring species in gypsum plant communities. We tested in a greenhouse whether aqueous extracts from C. clusii leaves (L), roots (R) and a mixture of both (RL) affected germination, seedling survival, and growth of nine native species of gypsum communities, including C. clusii itself. We further assessed in the field richness and abundance of plants under the canopy of C. clusii compared to Gypsophila struthium (shrub with a similar architecture having a nurse role) and in open patches. Finally, we specifically assessed in the field the influence of C. clusii on the presence of the species tested in the greenhouse experiment.

Results

Aqueous extracts from C. clusii (R and RL) negatively affected either germination or survival in four of nine species. In the field, richness and abundance of plants were lower under the canopy of C. clusii than under G. struthium, but higher than in open patches. Specifically, five of nine species were less frequent than expected under the canopy of C. clusii.

Conclusions

Cistus clusii shows species-specific interference with neighbouring species in the community, which may be at least partially attributable to its phytotoxic activity. To our knowledge, this is the first report of species-specific interference by C. clusii.

Electronic supplementary material

The online version of this article (10.1186/s12898-018-0204-x) contains supplementary material, which is available to authorized users.

Coexistence via coevolution driven by reduced allelochemical effects and increased tolerance to competition between invasive and native plants

Coevolution can promote long-term coexistence of two competing species if selection acts to reduce the fitness inequality between competitors and/or strengthen negative frequency dependence within each population. However, clear coevolution between plant competitors has been rarely documented. Plant invasions offer opportunities to capture the process of coevolution. Here we investigated how the developing relationship between an invasive forb, Alliaria petiolata, and a native competitor, Pilea pumila, may affect their long-term coexistence, by testing the competitive effects of populations of varying lengths of co-occurrence on each other across a chronosequence of invasion history. Alliaria petiolata and P. pumila tended to develop greater tolerance to competition over invasion history. Their coexistence was promoted more by increases in stabilizing relative to equalizing processes. These changes likely stem in part from reductions in allelopathic traits in the invader and evolution of tolerance in the native. These results suggested that some native species can evolve tolerance against the competitive effects of strong invaders, which likely promoted their persistence in invaded communities. However, the potential for coevolutionary rescue of competing populations is likely to vary across native species, and evolutionary processes should not be expected to compensate for the ecological consequences of exotic invasions.

Intraspecific genetic variation and species coexistence in plant communities

Many studies report that intraspecific genetic variation in plants can affect community composition and coexistence. However, less is known about which traits are responsible and the mechanisms by which variation in these traits affect the associated community. Focusing on plant-plant interactions, we review empirical studies exemplifying how intraspecific genetic variation in functional traits impacts plant coexistence. Intraspecific variation in chemical and architectural traits promotes species coexistence, by both increasing habitat heterogeneity and altering competitive hierarchies. Decomposing species interactions into interactions between genotypes shows that genotype × genotype interactions are often intransitive. The outcome of plant-plant interactions varies with local adaptation to the environment and with dominant neighbour genotypes, and some plants can recognize the genetic identity of neighbour plants if they have a common history of coexistence. Taken together, this reveals a very dynamic nature of coexistence. We outline how more traits mediating plant-plant interactions may be identified, and how future studies could use population genetic surveys of genotype distribution in nature and methods from trait-based ecology to better quantify the impact of intraspecific genetic variation on plant coexistence.

Moving forward on facilitation research: response to changing environments and effects on the diversity, functioning and evolution of plant communities

Once seen as anomalous, facilitative interactions among plants and their importance for community structure and functioning are now widely recognized. The growing body of modelling, descriptive and experimental studies on facilitation covers a wide variety of terrestrial and aquatic systems throughout the globe. However, the lack of a general body of theory linking facilitation among different types of organisms and biomes and their responses to environmental changes prevents further advances in our knowledge regarding the evolutionary and ecological implications of facilitation in plant communities. Moreover, insights gathered from alternative lines of inquiry may substantially improve our understanding of facilitation, but these have been largely neglected thus far. Despite over 15 years of research and debate on this topic, there is no consensus on the degree to which plant-plant interactions change predictably along environmental gradients (i.e. the stress-gradient hypothesis), and this hinders our ability to predict how plant-plant interactions may affect the response of plant communities to ongoing global environmental change. The existing controversies regarding the response of plant-plant interactions across environmental gradients can be reconciled when clearly considering and determining the species-specificity of the response, the functional or individual stress type, and the scale of interest (pairwise interactions or community-level response). Here, we introduce a theoretical framework to do this, supported by multiple lines of empirical evidence. We also discuss current gaps in our knowledge regarding how plant-plant interactions change along environmental gradients. These include the existence of thresholds in the amount of species-specific stress that a benefactor can alleviate, the linearity or non-linearity of the response of pairwise interactions across distance from the ecological optimum of the beneficiary, and the need to explore further how frequent interactions among multiple species are and how they change across different environments. We review the latest advances in these topics and provide new approaches to fill current gaps in our knowledge. We also apply our theoretical framework to advance our knowledge on the evolutionary aspects of plant facilitation, and the relative importance of facilitation, in comparison with other ecological processes, for maintaining ecosystem structure, functioning and dynamics. We build links between these topics and related fields, such as ecological restoration, woody encroachment, invasion ecology, ecological modelling and biodiversity-ecosystem-functioning relationships. By identifying commonalities and insights from alternative lines of research, we further advance our understanding of facilitation and provide testable hypotheses regarding the role of (positive) biotic interactions in the maintenance of biodiversity and the response of ecological communities to ongoing environmental changes.

Chemistry and multibeneficial bioactivities of carvacrol (4-isopropyl-2-methylphenol), a component of essential oils produced by aromatic plants and spices

Aromatic plants produce organic compounds that may be involved in the defense of plants against phytopathogenic insects, bacteria, fungi, and viruses. One of these compounds, called carvacrol, which is found in high concentrations in essential oils such as oregano, has been reported to exhibit numerous bioactivities in cells and animals. This integrated overview surveys and interprets our present knowledge of the chemistry and analysis of carvacrol and its beneficial bioactivities. These activities include its antioxidative properties in food (e.g., lard, sunflower oil) and in vivo and the inhibition of foodborne and human antibiotic-susceptible and antibiotic-resistant pathogenic bacteria, viruses, pathogenic fungi and parasites, and insects in vitro and in human foods (e.g., apple juice, eggs, leafy greens, meat and poultry products, milk, oysters) and food animal feeds and wastes. Also covered are inhibitions of microbial and fungal toxin production and the anti-inflammatory, analgesic, antiarthritic, antiallergic, anticarcinogenic, antidiabetic, cardioprotective, gastroprotective, hepatoprotective, and neuroprotective properties of carvacrol as well as metabolic, synergistic, and mechanistic aspects. Areas for future research are also suggested. The collated information and suggested research might contribute to a better understanding of agronomical, biosynthetic, chemical, physiological, and cellular mechanisms of the described health-promoting effects of carvacrol, and facilitate and guide further studies needed to optimize the use of carvacrol as a multifunctional food in pure and encapsulated forms, in edible antimicrobial films, and in combination with plant-derived and medical antibiotics to help prevent or treat animal and human diseases.

Differences in shoot and root terpenoid profiles and plant responses to fertilisation in Tanacetum vulgare

Intraspecific chemical diversity is a common phenomenon especially found in shoots of essential oil-accumulating plant species. Abiotic factors can influence the concentration of essential oils, but the effects are inconsistent and little is known in how far these may vary within an individual and within species between chemotypes. Tanacetum vulgare L. occurs in various chemotypes that differ in the composition of mono- and sesquiterpenoids in their shoot tissues. We investigated how far shoot chemotype grouping is mirrored in root terpenoid profiles. Furthermore, we studied whether different fertilisation amounts influence the plant growth and morphological traits as well as the constitutive terpenoid concentration of leaves and roots of three chemotypes, trans-carvyl acetate, β-thujone, and camphor, to different degrees. Shoot terpenoids were dominated by monoterpenoids, while the roots contained mainly sesquiterpenoids. The clear grouping in three chemotypes based on leaf chemistry was weakly mirrored in the root terpenoid composition. Furthermore, the leaf C/N ratio and the stem height differed between chemotypes. All plants responded to increased nutrient availability with increased total biomass and specific leaf area but decreased C/N and root/shoot ratios. Leaf terpenoid concentrations decreased with increasing fertiliser supply, independent of chemotype. In contrast to the leaves, the terpenoid concentrations of the roots were unaffected by fertilisation. Our results demonstrate that aboveground and belowground organs within a species can be under different selection pressures.

Evolutionary responses of native plant species to invasive plants: a review

Strong competition from invasive plant species often leads to declines in abundances and may, in certain cases, cause localized extinctions of native plant species. Nevertheless, studies have shown that certain populations of native plant species can co-exist with invasive plant species,suggesting the possibility of adaptive evolutionary responses of those populations to the invasive plants. Empirical inference of evolutionary responses of the native plant species to invasive plants has involved experiments comparing two conspecific groups of native plants for differences in expression of growth/reproductive traits: populations that have experienced competition from the invasive plant species (i.e. experienced natives) versus populations with no known history of interactions with the invasive plant species (i.e. naıve natives). Here, I employ a meta-analysis to obtain a general pattern of inferred evolutionary responses of native plant species from 53 such studies. In general, the experienced natives had significantly higher growth/reproductive performances than naıve natives, when grown with or without competition from invasive plants.While the current results indicate that certain populations of native plant species could potentially adapt evolutionarily to invasive plant species, the ecological and evolutionary mechanisms that probably underlie such evolutionary responses remain unexplored and should be the focus of future studies.

Sensitivity of the green alga Pediastrum duplex Meyen to allelochemicals is strain-specific and not related to co-occurrence with allelopathic macrophytes

Interspecific differences in the response of microalgae to stress have numerous ecological implications. However, little is known of intraspecific sensitivities and the potential role of local genetic adaptation of populations. We compared the allelochemical sensitivity of 23 Pediastrum duplex Meyen strains, a common component of the freshwater phytoplankton. In order to test for local genetic adaptation, strains were isolated from water bodies with and without the allelopathically-active submerged macrophyte Myriophyllum. Strains were assigned to P. duplex on the basis of cell shape and colony morphology and only P. duplex strains that belonged to the same lineage in an ITS rDNA phylogeny were used. Inhibition of strain growth rates and maximum quantum yields of photosystem II were measured after exposure to tannic acid (TA) and co-culture with Myriophyllum spicatum. Growth rate inhibition varied over one order of magnitude between the P. duplex strains. There was no correlation between the presence of Myriophyllum in the source location and the sensitivity of the strains to TA or the presence of Myriophyllum, suggesting that at least strong unidirectional local adaptation to Myriophyllum had not taken place in the studied water bodies. The maximum quantum yield of photosystem II of TA exposed algae decreased, whereas the yield of algae exposed to M. spicatum was slightly higher than that of the controls. The ranking of P. duplex strain sensitivities differed between the types of exposure (single additions of TA versus co-existence with M. spicatum) and the parameter measured (growth rate versus maximum quantum yield), emphasizing the importance of measuring multiple traits when analysing strain-specific sensitivities towards allelochemicals. The observation that sensitivities to allelochemicals vary widely among strains of a single freshwater algal species should be taken into account if evaluating ecological consequences of allelopathic interactions.

Coevolution between invasive and native plants driven by chemical competition and soil biota

Although reciprocal evolutionary responses between interacting species are a driving force behind the diversity of life, pairwise coevolution between plant competitors has received less attention than other species interactions and has been considered relatively less important in explaining ecological patterns. However, the success of species transported across biogeographic boundaries suggests a stronger role for evolutionary relationships in shaping plant interactions. Alliaria petiolata is a Eurasian species that has invaded North American forest understories, where it competes with native understory species in part by producing compounds that directly and indirectly slow the growth of competing species. Here I show that populations of A. petiolata from areas with a greater density of interspecific competitors invest more in a toxic allelochemical under common conditions. Furthermore, populations of a native competitor from areas with highly toxic invaders are more tolerant to competition from the invader, suggesting coevolutionary dynamics between the species. Field reciprocal transplants confirmed that native populations more tolerant to the invader had higher fitness when the invader was common, but these traits came at a cost when the invader was rare. Exotic species are often detrimentally dominant in their new range due to their evolutionary novelty; however, the development of new coevolutionary relationships may act to integrate exotic species into native communities.

Soil microorganisms alleviate the allelochemical effects of a thyme monoterpene on the performance of an associated grass species

BACKGROUND

Plant allelochemicals released into the soil can significantly impact the performance of associated plant species thereby affecting their competitive ability. Soil microbes can potentially affect the interaction between plant and plant chemicals by degrading the allelochemicals. However, most often plant-plant chemical interactions are studied using filter paper bioassays examining the pair-wise interaction between a plant and a plant chemical, not taking into account the potential role of soil microorganisms.

METHODOLOGY/PRINCIPAL FINDINGS

To explore if the allelopathic effects on a grass by the common thyme monoterpene "carvacrol" are affected by soil microorganisms. Seedlings of the grass Agrostis capillaris originating from 3 different thyme sites were raised in the greenhouse. Seedlings were grown under four different soil treatments in a 2*2 fully factorial experiment. The monoterpene carvacrol was either added to standard greenhouse soil or left out, and soil was either sterilized (no soil microorganisms) or not (soil microorganisms present in soil). The presence of carvacrol in the soil strongly increased mortality of Agrostis plants, and this increase was highest on sterile soil. Plant biomass was reduced on soil amended with carvacrol, but only when the soil was also sterilized. Plants originating from sites where thyme produces essential oils containing mostly carvacrol had higher survival on soil treated with that monoterpene than plants originating from a site where thyme produced different types of terpenes, suggesting an adaptive response to the locally occurring terpene.

CONCLUSIONS/SIGNIFICANCE

The study shows that presence of soil microorganisms can alleviate the negative effect of a common thyme monoterpene on the performance of an associated plant species, emphasizing the role of soil microbes in modulating plant-plant chemical interactions.