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 occurrin

"Menage à trois": the presence/absence of thyme shapes the mutualistic interaction between the host plant [i]Medicago truncatula[/i] (Fabaceae) and its symbiotic bacterium [i]Sinorhizobium meliloti[/i]

UMR 1334 AGAP : Equipe DAVEM Diversité et adaptation de la vigne et des espèces méditerranéennesThe long-term maintenance of specialized mutualisms remains an evolutionary puzzle. Recent focus has been on factors governing the stability of these mutualisms, including sanctions by the host, partner choice, and coevolutionary constraint, that is, the genetic correlation (r(G)) between fitness of both partners. So far these studies have been typically carried out in a single environment. Here, we ask if the genetic correlation between fitness of the host plant Medicago truncatula (Fabaceae) and its bacterial symbiont Sinorhizobium meliloti is affected by the presence/absence of a monoterpene (carvacrol) leached into the soil by Thymus vulgaris-a common plant of the Mediterranean vegetation, often co-occuring with Medicago. We show that the presence of carvacrol in the soil dramatically affects fitness of the rhizobial partner and increases the magnitude of rG between plant and rhizobia fitness (r(G) = 0.02 +/- 0.05 vs. r(G) = 0.57 +/- 0.02). This finding emphasizes the importance of heterogeneity in the biotic environment for understanding the evolution of species interactions

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

Recent studies have shown that plant allelochemicals can have profound effects on the performance of associated species, such that plants with a history of co-existence with “chemical neighbour” plants perform better in their presence compared to naïve plants. This has cast new light on the complexity of plant–plant interactions and plant communities and has led to debates on whether plant communities are more co-evolved than traditionally thought. In order to determine whether plants may indeed evolve in response to other plants’ allelochemicals it is crucial to determine the presence of genetic variation for performance under the influence of specific allelochemicals and show that natural selection indeed operates on this variation. We studied the effect of the monoterpene carvacrol—a dominant compound in the essential oil of Thymus pulegioides—on three associated plant species originating from sites where thyme is either present or absent. We found the presence of genetic variation in both naïve and experienced populations for performance under the influence of the allelochemical but the response varied among naïve and experienced plant. Plants from experienced populations performed better than naïve plants on carvacrol soil and contained significantly more seed families with an adaptive response to carvacrol than naïve populations. This suggests that the presence of T. pulegioides can act as a selective agent on associated species, by favouring genotypes which perform best in the presence of its allelochemicals. The response to the thyme allelochemical varied from negative to neutral to positive among the species. The different responses within a species suggest that plant–plant interactions can evolve; this has implications for community dynamics and stability

Summary of the logistic regression analysis on survival of <i>Agrostis</i> plants.

<p>Significance of test deviance is indicated by:</p><p>*P<0.05;</p><p>**P<0.01;</p><p>***P<0.001.</p

Biomass of <i>Agrostis capillaris</i> plants.

<p>Biomass (+/− SE) of plants growing in pots with the thyme monterpene carvacrol added to the soil (T) or not (no T) and on soil which was either sterilized or not.</p

Summary of ANOVA on plant biomass.

<p>Significance of F ratio is indicated by:</p><p>*P<0.05;</p><p>**P<0.01;</p><p>***P<0.001.</p

Survival of <i>Agrostis capillaris</i> plants.

<p>Observed probability (+/− SE) of survival of plants growing in pots where the thyme monoterpene carvacrol is either added to the soil (T) or not added (No T) and where the soil is either sterilized or not. Grey dots: <i>Agrostis</i> originating from populations where it co-occurs with <i>Thymus pulegiodes</i> (TP), black dots: <i>Agrostis</i> originating from a single population where is co-occurs with <i>T. serpyllum</i> (TS).</p

Water availability and population origin affect the expression of the tradeoff between reproduction and growth in Plantago coronopus

[EN] Investment in reproduction and growth represent a classic tradeoff with implication for life history evolution. The local environment can play a major role in the magnitude and evolutionary consequences of such a tradeoff. Here, we examined the investment in reproductive and vegetative tissue in 40 maternal half-sib families from four different populations of the herb Plantago coronopus growing in either a dry or wet greenhouse environment. Plants originated from populations with an annual or a perennial life form, with annuals prevailing in drier habitats with greater seasonal variation in both temperature and precipitation. We found that water availability affected the expression of the tradeoff (both phenotypic and genetic) between reproduction and growth, being most accentuated under dry condition. However, populations responded very differently to water treatments. Plants from annual populations showed a similar response to drought condition with little variation among maternal families, suggesting a history of selection favouring genotypes with high allocation to reproduction when water availability is low. Plants from annual populations also expressed the highest level of plasticity. For the perennial populations, one showed a large variation among maternal families in resource allocation and expressed significant negative genetic correlations between reproductive and vegetative biomass under drought. The other perennial population showed less variation in response to treatment and had trait values similar to those of the annuals, although it was significantly less plastic. We stress the importance of considering intraspecific variation in response to environmental change such as drought, as conspecific plants exhibited very different abilities and strategies to respond to high versus low water availability even among geographically close populations.The authors are grateful to R. Nielsen for her help in the greenhouse, J. Villelas for field assistance, and to J.G. Sørensen, T. Bataillon, M. Mend ez and R. Wesselingh for comments and suggestions to improve the manuscript. MBG benefited from the support of two Spanish National projects (CGL2006-08 507 and CGL2010-21 642), BKE had support from a research project supported by the Villum Foundation.Peer reviewe