Non-symbiotic Plants Research Articles

The Impact of Aboveground Epichloë Endophytic Fungi on the Rhizosphere Microbial Functions of the Host Melica transsilvanica.

In nature, the symbiotic relationship between plants and microorganisms is crucial for ecosystem balance and plant growth. This study investigates the impact of Epichloë endophytic fungi, which are exclusively present aboveground, on the rhizosphere microbial functions of the host Melica transsilvanica. Using metagenomic methods, we analyzed the differences in microbial functional groups and functional genes in the rhizosphere soil between symbiotic (EI) and non-symbiotic (EF) plants. The results reveal that the presence of Epichloë altered the community structure of carbon and nitrogen cycling-related microbial populations in the host's rhizosphere, significantly increasing the abundance of the genes (porA, porG, IDH1) involved in the rTCA cycle of the carbon fixation pathway, as well as the abundance of nxrAB genes related to nitrification in the nitrogen-cycling pathway. Furthermore, the presence of Epichloë reduces the enrichment of virulence factors in the host rhizosphere microbiome, while significantly increasing the accumulation of resistance genes against heavy metals such as Zn, Sb, and Pb. This study provides new insights into the interactions among endophytic fungi, host plants, and rhizosphere microorganisms, and offers potential applications for utilizing endophytic fungi resources to improve plant growth and soil health.

Control of the rhizobium-legume symbiosis by the plant nitrogen demand is tightly integrated at the whole plant level and requires inter-organ systemic signaling.

Symbiotic nodules formed on legume roots with rhizobia fix atmospheric N2. Bacteria reduce N2 to NH4 + that is assimilated into amino acids by the plant. In return, the plant provides photosynthates to fuel the symbiotic nitrogen fixation. Symbiosis is tightly adjusted to the whole plant nutritional demand and to the plant photosynthetic capacities, but regulatory circuits behind this control remain poorly understood. The use of split-root systems combined with biochemical, physiological, metabolomic, transcriptomic, and genetic approaches revealed that multiple pathways are acting in parallel. Systemic signaling mechanisms of the plant N demand are required for the control of nodule organogenesis, mature nodule functioning, and nodule senescence. N-satiety/N-deficit systemic signaling correlates with rapid variations of the nodules' sugar levels, tuning symbiosis by C resources allocation. These mechanisms are responsible for the adjustment of plant symbiotic capacities to the mineral N resources. On the one hand, if mineral N can satisfy the plant N demand, nodule formation is inhibited, and nodule senescence is activated. On the other hand, local conditions (abiotic stresses) may impair symbiotic activity resulting in plant N limitation. In these conditions, systemic signaling may compensate the N deficit by stimulating symbiotic root N foraging. In the past decade, several molecular components of the systemic signaling pathways controlling nodule formation have been identified, but a major challenge remains, that is, to understand their specificity as compared to the mechanisms of non-symbiotic plants that control root development and how they contribute to the whole plant phenotypes. Less is known about the control of mature nodule development and functioning by N and C nutritional status of the plant, but a hypothetical model involving the sucrose allocation to the nodule as a systemic signaling process, the oxidative pentose phosphate pathway, and the redox status as potential effectors of this signaling is emerging. This work highlights the importance of organism integration in plant biology.

Can Aphid Herbivory Induce Intergenerational Effects of Endophyte-conferred Resistance in Grasses?

Plants have evolved mechanisms to survive herbivory. One such mechanism is the induction of defences upon attack that can operate intergenerationally. Cool-season grasses (sub-family Pooideae) obtain defences via symbiosis with vertically transmitted fungal endophytes (genus Epichloë) and can also show inducible responses. However, it is unknown whether these herbivore-induced responses can have intergenerational effects. We hypothesized that herbivory by aphids on maternal plants induces the intergenerational accumulation of endophyte-derived defensive alkaloids and resistance intensification in the progeny. We subjected mother plants symbiotic or not with Epichloë occultans, a species known for its production of anti-insect alkaloids known as lolines, to the aphid Rhopalosiphum padi. Then, we evaluated the progeny of these plants in terms of loline alkaloid concentration, resistance level (through herbivore performance), and shoot biomass. Herbivory on mother plants did not increase the concentration of lolines in seeds but it tended to affect loline concentration in progeny plants. There was an overall herbivore-induced intergenerational effect increasing the endophyte-conferred defence and resistance. Symbiotic plants were more resistant to aphids and had higher shoot biomass than their non-symbiotic counterparts. Since maternal herbivory did not affect the loline concentrations in seeds, the greater resistance of the progeny could have resulted from an inherited mechanism of epigenetic regulation. It would be interesting to elucidate the origin of this regulation since it could come from the host or the fungal symbiont. Thus, endophyte-driven differential fitness between symbiotic and non-symbiotic plants might be higher as generations pass on in presence of herbivores.

Vertically Transmitted Epichloë Systemic Endophyte Enhances Drought Tolerance of Achnatherum inebrians Host Plants through Promoting Photosynthesis and Biomass Accumulation.

Achnatherum inebrians (drunken horse grass, DHG) plants, a dominant grass species in the arid and semi-arid regions of northwest China, symbiotic with an Epichloë fungal endophyte, is well adapted to drought. However, little is known about how the presence of the foliar Epichloë endophyte enhances the tolerance of DHG to drought at the molecular level. This study explored the positive effects of the presence of the Epichloë endophyte on plant growth, biomass, and photosynthetic efficiency and processes of DHG under non-drought and two drought (moderate and severe) treatments, using RNA sequencing to compare transcriptomes. The transcriptome results showed that 32 selected unigenes involved in the photosynthesis processes within Epichloë symbiotic plants were differently expressed (DEGs) versus non-symbiotic plants. The majority of these selected DEGs were upregulated in Epichloë symbiotic plants versus non-symbiotic plants, such as upregulated unigenes (c51525.graph_c1, c47798.graph_c0 & c64087.graph_c0) under drought conditions. In line with the transcriptomes data, the presence of the Epichloë endophyte promoted the photosynthetic rate and biomass accumulation of DHG plants, and the relationship between the photosynthetic rate and biomass is linear and significant. The presence of the endophyte only increased the biomass per tiller of DHG plants under drought. This study provides further insights into the molecular mechanisms that underlie the enhanced plant growth and drought tolerance of Epichloë-symbiotic DHG plants.

Seed-borne fungal endophytes constrain reproductive success of host plants under ozone pollution

Tropospheric ozone is among the global change factors that pose a threat to plants and microorganisms. Symbiotic microorganisms can assist plants to cope with stress, but their role in the tolerance of plants to ozone is poorly understood. Here, we subjected endophyte-symbiotic and non-symbiotic plants of Lolium multiflorum, an annual species widely distributed in temperate grasslands, to high and low (i.e., charcoal-filtered air) ozone levels at vegetative and reproductive phases. Exposure to high ozone reduced leaf photochemical efficiency and greenness in both symbiotic and non-symbiotic plants. However, ozone-induced oxidative damage at biochemical level (i.e., lipid peroxidation) was mostly detected in symbiotic plants. Ozone exposure at the vegetative phase did not affect the reproductive investment in seeds, indicating full recovery from stress. Ozone exposure at the reproductive phase reduced biomass and seed production only in symbiotic plants indicating a symbiont-associated cost. At low ozone, endophyte-symbiotic plants showed a steeper slope in the relationship between seed number and seed weight (i.e., a number-weight trade-off) compared to non-symbiotic plants. However, when plants were treated at the reproductive phase, ozone increased the imbalance between seed number and seed weight in both endophyte-symbiotic and non-symbiotic plants. Plants with endophytes at the reproductive stage produced fewer seeds, which were not compensated by increased seed weight. Thus, fungal mycelium growing within ovaries or ozone-induced antioxidant systems may result in costs that finally depress the fitness of plants. Despite ozone pollution could destabilize plant-endophyte mutualisms and render them dysfunctional, other endophyte-mediated benefits (e.g., resistance to herbivory, tolerance to drought) could over-compensate these losses and explain the high incidence of the symbiosis in nature.

The Role of Fungal Microbiome Components on the Adaptation to Salinity of Festuca rubra subsp. pruinosa.

Festuca rubra subsp. pruinosa is a perennial grass that inhabits sea cliffs, a habitat where salinity and low nutrient availability occur. These plants have a rich fungal microbiome, and particularly common are their associations with Epichloë festucae in aboveground tissues and with Fusarium oxysporum and Periconia macrospinosa in roots. In this study, we hypothesized that these fungi could affect the performance of F. rubra plants under salinity, being important complements for plant habitat adaptation. Two lines of F. rubra, each one consisting of Epichloë-infected and Epichloë-free clones, were inoculated with the root endophytes (F. oxysporum and P. macrospinosa) and subjected to a salinity treatment. Under salinity, plants symbiotic with Epichloë had lower Na+ content than non-symbiotic plants, but this effect was not translated into plant growth. P. macrospinosa promoted leaf and root growth in the presence and absence of salinity, and F. oxysporum promoted leaf and root growth in the presence and absence of salinity, plus a decrease in leaf Na+ content under salinity. The growth responses could be due to functions related to improved nutrient acquisition, while the reduction of Na+ content might be associated with salinity tolerance and plant survival in the long term. Each of these three components of the F. rubra core mycobiome contributed with different functions, which are beneficial and complementary for plant adaptation to its habitat in sea cliffs. Although our results do not support an obvious role of Epichloë itself in FRP salt tolerance, there is evidence that Epichloë can interact with root endophytes, affecting host plant performance.

Root symbioses as belowground C pumps: a mitigation strategy against rising CO<sub>2</sub> levels

Mycorrhizal and N-fixing root symbioses evolved at two points in the past when global CO₂ was highest, consistent with the high demand these symbioses place on host C. Trees hosting both mycorrhiza and N-fixing bacteria are able to fix more atmospheric CO₂ and grow at faster rates than non-symbiotic plants, or plants with only mycorrhiza. We argue that on the basis of this improved C capture, N-fixing trees act as C-pumps, sequestering C and locking it in biomass, thus, if properly managed, can contribute significantly towards the mitigation of rising CO₂ levels.

Nitrogen-fixing Are polyploidy and host peptide-governed symbiont differentiation universal characteristics of endosymbiosis in the Rhizobium-legume symbiosis?

Legumes may have a symbiotic relationship with nitrogen-fixing bacteria, they can grow in nitrogen-deficient soils. There is a lot of interest in applying what we've learned about this symbiosis to non-symbiotic plants. An international study group has progressed in their understanding of this complicated biological process. Microbes play a crucial role in all creatures, including plants, whether they are useful or detrimental. Plant viability depends on the ability to monitor the bacteria in the environment.

Z-3-Hexenylacetate emissions induced by the endophyte Epichloë occultans at different levels of defoliation during the host plant's life cycle

Abstract Vertically transmitted fungal endophytes are common defensive symbionts of cool-season grasses. Protection against herbivores has been generally associated with alkaloids produced in the grass-endophyte symbiosis. However, many other changes occur in host metabolism like the release of VOCs. We aimed at characterizing the profile of volatile organic compounds (VOCs) induced by simultaneous fungal endophyte symbiosis and defoliation during the entire life cycle of the annual host grass and the asexual symbiont. We designed an outdoor factorial experiment with plots dominated by intact and damaged Lolium multiflorum plants with high and low infection levels with Epichloe occultans. After exploring the entire VOC profile, the green leaf volatile (Z)-3-hexenyl acetate (Z3-HAC) was found as the main compound emitted by the plants under field conditions. While in low-infected patches there were no differences in volatile emission during the plant life cycle, highly infected patches emitted more Z3-HAC in seedling and vegetative phases than in the reproductive phase. The role of Z3-HAC provided by symbiotic to neighbouring non-symbiotic plants in the associational protection against herbivores and fungal pathogens is discussed.

Maternal Exposure to Ozone Modulates the Endophyte-Conferred Resistance to Aphids in Lolium multiflorum Plants.

Simple SummaryGlobal change is driving the incidence of novel stress factors such as the tropospheric pollutant ozone. Plants can overcome the environmental challenges by adjusting their phenotypes that eventually, can be transmitted to the progeny. Plants also establish symbiotic interactions with beneficial fungal endophytes, some of which can be transmitted to the progeny through the seeds. We worked with the grass Lolium multiflorum and its common endophyte, Epichloë occultans that benefits the host by coffering resistance to herbivores. Specifically, we studied the effect of exposing endophyte-symbiotic and non-symbiotic plants to ozone on the level of resistance to herbivorous aphids in the progeny. The ozone history impaired the endophyte-conferred resistance to aphids in progeny. This was evident at individual weight level of aphids but not so at population level. Defensive compounds were higher in endophyte-symbiotic seeds but depressed by mother plants exposure to ozone. Despite the negative effect of maternal ozone on the resistance level of plants, symbiotic plants showed a superior biomass compared to endophyte-free plants. Our work illustrates how the environment explored by parent plants can persist transgenerationally and, depending on the symbiotic state, will affect the fate of the progeny.Plants are challenged by biotic and abiotic stress factors and the incidence of one can increase or decrease resistance to another. These relations can also occur transgenerationally. For instance, progeny plants whose mothers experienced herbivory can be more resistant to herbivores. Certain fungal endophytes that are vertically transmitted endow plants with alkaloids and resistance to herbivores. However, endophyte-symbiotic plants exposed to the oxidative agent ozone became susceptible to aphids. Here, we explored whether this effect persists transgenerationally. We exposed Lolium multiflorum plants with and without fungal endophyte Epichloë occultans to ozone (120 or 0 ppb), and then, challenged the progeny with aphids (Rhopalosiphum padi). The endophyte was the main factor determining the resistance to aphids, but its importance diminished in plants with ozone history. This negative ozone effect on the endophyte-mediated resistance was apparent on aphid individual weights. Phenolic compounds in seeds were increased by the symbiosis and diminished by the ozone. The endophyte effect on phenolics vanished in progeny plants while the negative ozone effect persisted. Independently of ozone, the symbiosis increased the plant biomass (≈24%). Although ozone can diminish the importance of endophyte symbiosis for plant resistance to herbivores, it would be compensated by host growth stimulation.

Daño, producción de semillas y persistencia del hongo endófito Epichloë occultans en plantas de Lolium multiflorum bajo un ataque de herbívoros lepidópteros y contaminación por ozono

Plants are expected to face novel challenges as consequence of human-driven global change. Outbreaks of pests and higher incidence of contaminants are increasing. Plants can improve tolerance to stress factors through associations with symbiotic microorganisms. Certain grasses establish persistent and asymptomatic symbioses with Epichloe fungal endophytes, which are known to confer protection against Herbivores and improve plant tolerance to abiotic stress factors. Nonetheless, accumulating evidence suggests the symbiosis outcome is context-dependent. We evaluated the capacity of the endophyte fungus E. occultans in protecting the annual grass Lolium multiflorum against a spontaneous larva attack of the generalist herbivore Agrotis ipsilon under episodic exposure of plants to ozone. Symbiotic and non-symbiotic plants were individually grown outdoors and exposed to ozone at different stages resulting in four treatments: control (plant never exposed to ozone), plant exposed to ozone at the vegetative stage, plant exposed to ozone at reproductive stage, and plant exposed to ozone at both stages. After the last exposure, there was an outbreak of A. ipsilon larvae. We evaluated herbivore damage, seed production per plant, and endophyte transmission to the seeds. Frequency of attacked plants was irrespective of both the endophyte and ozone exposure. However, the damage level per plant was only reduced by the endophyte. Seed production was slightly lower in endophyte-symbiotic plants and not affected by ozone. Interestingly, herbivore damaged and undamaged endophyte-symbiotic plants contributed equally to seed production. However, in plants exposed to ozone once at the vegetative or reproductive stage, endophyte-free undamaged plants had higher seed production than endophyte-free damaged plants. Ozone treatments did not affect the transmission efficiency of endophytes to the seeds. Mean endophyte transmission efficiency per plant was 95%. Medium doses of ozone seem to have undetectable effects on grass-endophyte symbiosis, not affecting the defensive mutualism nor the persistence of the symbiont across generations.

Fungal Endophytes Promote Tomato Growth and Enhance Drought and Salt Tolerance.

In a search for efficient fungal endophytes that can promote crop production and/or increase crop tolerance to abiotic stress, we isolated and tested various species harbored by wild plants. Sixty-seven endophytic fungal isolates were obtained from drought stressed, poor soil habitats, and inland high salt areas. We extensively tested the roles of Ampelomyces sp. and Penicillium sp. isolates in improving tomato growth and yield. Under greenhouse and field trails, Ampelomyces sp. and Penicillium sp. endophytes proved effective in conferring positive benefits to tomatoes placed under stress as well as under normal growing conditions. Ampelomyces sp. conferred tolerance to tomatoes placed under drought stress in addition to enhancing overall plant growth and fruit yield in comparison to non-symbiotic plants under drought stress. Penicillium sp. conferred tolerance to tomatoes placed under 300 mM salinity stress in addition to enhancing root biomass in comparison to non-symbiotic plants. Both endophytes proved efficient in enhancing plant growth, stress tolerance, recovery, and fruit yield under optimal experimental conditions in comparison to non-symbiotic plants. Field testing of tomato yield showed increased yield of symbiotic tomatoes compared to non-symbiotic ones. This data suggests that both Ampelomyces sp. and Penicillium sp. share a promising potential for improving future agricultural production, particularly with the projected changes in climate in the future.

Epichloë endophyte modifies the foliar anatomy of Lolium multiflorum Lam.

Systemic fungal endophytes may increase the adaptive potential of their hosts to different environmental conditions. However, the mechanisms involved in the adjustment are still an area of active inquiry. Considering that anatomical features in plants are the result of adaptation to different environments, this aspect of plants response to the symbionts deserves further research. In this work we investigated whether an asexual Epichloe endophyte modifies the foliar anatomy of symbiotic plants (E+) of its host grass with respect to endophyte-free plants (E-), using the Epichloe occultans–Lolium multiflorum system. The experiment was carried out in a greenhouse and plants were grown under benign, non-stressful (controlled) conditions. Symbiotic plants showed thinner leaf blades and lower mesophyll thickness. Likewise, the endophyte is associated with a greater number of vascular bundles. The stomatal density on the lower epidermis was higher in symbiotic plants than non-symbiotic plants. Our study reveals that Epichloe occultans modifies the foliar anatomy of Lolium multiflorum. Considering that leaf anatomy exhibits characters that were selected as a result of evolution in relation to specific environmental conditions, these results are promising since they can offer information about the mechanisms involved in the adaptive adjustment of symbiotic plants.

Combined Effects of Water Deficit, Exogenous Ethylene Application and Root Symbioses on Trigonelline and ABA Accumulation in Fenugreek

Secondary metabolites (SMs) have high economic impact thanks to their exploitability in chemical, pharmaceutical and cosmetic industries. Trigonella foenum-graecum has an importance due to the production of bioactive compounds with pharmaceutical values. Among them, the alkaloid trigonelline is known for its role in the treatment of different human diseases. SM accumulation is influenced by environmental factors but is modulated by the application of exogenous compounds. Ethephon, a precursor of the phytohormone ethylene, was already used to influence SM accumulation. Our work is aimed at evaluating the accumulation of trigonelline and the phytohormone abscisic acid (ABA) when three factors were combined: i) two levels of water regimes (well-watered and water deficit), ii) ethephon treatments and iii) inoculation with an arbuscular mycorrhizal (AM)-based inoculum also leading to nodulation. The content of trigonelline and ABA was significantly affected by symbioses, showing high accumulation in AM-colonized plants irrespective of the water regimes applied. In terms of trigonelline accumulation with respect to ethephon treatments, while symbiotic plants showed a dose-dependent trend, non-symbiotic plants showed a significantly difference only when 550 ppm of ethephon was applied. In conclusion, our work provides new information on the effects of both ethephon and symbioses on plant growth and accumulation of valuable compounds, such as trigonelline, in fenugreek.

Decreased Root-Knot Nematode Gall Formation in Roots of the Morning Glory Ipomoea tricolor Symbiotic with Ergot Alkaloid-Producing Fungal Periglandula Sp.

Many species of morning glories (Convolvulaceae) form symbioses with seed-transmitted Periglandula fungal endosymbionts, which produce ergot alkaloids and may contribute to defensive mutualism. Allocation of seed-borne ergot alkaloids to various tissues of several Ipomoea species has been demonstrated, including roots of I. tricolor. The goal of this study was to determine if infection of I. tricolor by the Periglandula sp. endosymbiont affects Southern root-knot nematode (Meloidogyne incognita) gall formation and host plant biomass. We hypothesized that I. tricolor plants infected by Periglandula (E+) would develop fewer nematode-induced galls compared to non-symbiotic plants (E-). E+ or E- status of plant lines was confirmed by testing methanol extracts from individual seeds for endosymbiont-produced ergot alkaloids. To test the effects of Periglandula on nematode colonization, E+ and E- I. tricolor seedlings were grown in soil infested with high densities of M. incognita nematodes (N+) or no nematodes (N-) for four weeks in the greenhouse before harvesting. After harvest, nematode colonization of roots was visualized microscopically, and total gall number and plant biomass were quantified. Four ergot alkaloids were detected in roots of E+ plants, but no alkaloids were found in E- plants. Gall formation was reduced by 50% in E+ plants compared to E- plants, independent of root biomass. Both N+ plants and E+ plants had significantly reduced biomass compared to N- and E- plants, respectively. These results demonstrate Periglandula's defensive role against biotic enemies, albeit with a potential trade-off with host plant growth.

Phylogeography of the Bradyrhizobium spp. Associated With Peanut, Arachis hypogaea: Fellow Travelers or New Associations?

Legume plants have colonized almost all terrestrial biotopes. Their ecological success is partly due to the selective advantage provided by their symbiotic association with nitrogen-fixing bacteria called rhizobia, which allow legumes to thrive on marginal lands and nitrogen depleted soils where non-symbiotic plants cannot grow. Additionally, their symbiotic capacities result in a high protein content in their aerial parts and seeds. This interesting nutritional value has led to the domestication and agricultural exploitation of several legumes grown for seeds and/or fodder for human and domestic animal consumption. Several cultivated legume species are thus grown far beyond their natural geographic range. Other legume species have become invasives, spreading into new habitats. The cultivation and establishment of legume species outside of their original range requires either that they are introduced or cultivated along with their original symbiotic partner or that they find an efficient symbiotic partner in their introduced habitat. The peanut, Arachis hypogaea, a native of South America, is now cultivated throughout the world. This species forms root nodules with Bradyrhizobium, but it is unclear whether these came with the seeds from their native range or were acquired locally. Here we propose to investigate the phylogeography of Bradyrhizobium spp. associated with a number of different wild and cultivated legume species from a range of geographical areas, including numerous strains isolated from peanut roots across the areas of peanut cultivation. This will allow us to address the question of whether introduced/cultivated peanuts associate with bacteria from their original geographic range, i.e., were introduced together with their original bacterial symbionts, or whether they acquired their current associations de novo from the bacterial community within the area of introduction. We will base the phylogenetic analysis on sequence data from both housekeeping and core genes and a symbiotic gene (nif). Differences between the phylogenetic signal of symbiotic and non-symbiotic genes could result from horizontal transfer of symbiosis capacity. Thus this study will also allow us to elucidate the processes by which this symbiotic association has evolved within this group of Bradyrhizobium spp.

Can endosymbiotic microbes modulate natural selection in plant populations? An example with Lolium perenne and its fungal endophyte

Many plant species are symbiotic with systemic microbes. For example, many grasses are inhabited by fungal endophytes that affect aspects of their host’s physiology, morphology, and reproduction. However, there have not been any analyses of the potential effect of endophytes on the strength of phenotypic selection on quantitative traits. Here, a previously published data set on several life history traits measured for two years in a field population of 12–13 Lolium perenne genotypes, each replicated as symbiotic and non-symbiotic plants, was analyzed using the standard Lande-Arnold method of selection analysis. In one year, endophytic symbionts reduced the strength of selection on the number of reproductive tillers when relative fitness was expressed as seed yield. Also, symbionts selected for reduced tiller production when fitness was expressed as mean seed mass. These changes in the strength of selection only occurred when fitness of genotypes when symbiotic was unrelated to fitness of the same genotypes when non-symbiotic. In a second year, when fitness of symbiotic and non-symbiotic groups were significantly correlated, there was no detectable selection on reproductive tiller production. Because the effects of microbial endosymbionts were only shown for one year in a single host population, additional research is needed to better assess how endosymbionts might mediate selective pressures in other natural plant populations.

Interplay between Endophyte Prevalence, Effects and Transmission: Insights from a Natural Grass Population.

Two main mechanisms are thought to affect the prevalence of endophyte-grass symbiosis in host populations: the mode of endophyte transmission, and the fitness differential between symbiotic and non-symbiotic plants. These mechanisms have mostly been studied in synthetic grass populations. If we are to improve our understanding of the ecological and evolutionary dynamics of such symbioses, we now need to determine the combinations of mechanisms actually operating in the wild, in populations shaped by evolutionary history. We used a demographic population modeling approach to identify the mechanisms operating in a natural stand of an intermediate population (i.e. 50% of plants symbiotic) of the native grass Festuca eskia. We recorded demographic data in the wild over a period of three years, with manipulation of the soil resources for half the population. We developed two stage-structured matrix population models. The first model concerned either symbiotic or non-symbiotic plants. The second model included both symbiotic and non-symbiotic plants and took endophyte transmission rates into account. According to our models, symbiotic had a significantly higher population growth rate than non-symbiotic plants, and endophyte prevalence was about 58%. Endophyte transmission rates were about 0.67 or 0.87, depending on the growth stage considered. In the presence of nutrient supplementation, population growth rates were still significantly higher for symbiotic than for non-symbiotic plants, but endophyte prevalence fell to 0%. At vertical transmission rates below 0.10–0.20, no symbiosis was observed. Our models showed that a positive benefit of the endophyte and vertical transmission rates of about 0.6 could lead to the coexistence of symbiotic and non-symbiotic F. eskia plants. The positive effect of the symbiont on host is not systematically associated with high transmission rates of the symbiont over short time scales, in particular following an environmental change.

Can the defensive mutualism between grasses and fungal endophytes protect non-symbiotic neighbours from soil pathogens?

It is proposed that Epichloe endophytes have a role protecting host grasses against pathogens. However, it is unclear whether this protection is extended to other non-symbiotic plants. Here we explored the effect of the asexual fungal symbiont, Epichloe occultans, on the interaction between Lolium multiflorum host plants and soil pathogens, and its potential positive side-effect on neighbouring plants. We conducted two microcosm experiments to assess the endophyte effect on seedling establishment of the host grass and other non-symbiotic grasses in the presence of soil pathogens. With an in-vitro experiment, we tested whether the endophyte inhibits, during seed germination, the growth of these pathogens. Independently of pathogen identity, the endophyte improved host establishment (6 %). The endophyte also enhanced the establishment of the neighbouring grass Bromus catharticus (≈20 %) only in soil with Rhizoctonia solani. The endophyte in seed reduced the growth (≈20 %) of two out of four pathogens (Fusarium acuminatum and R. solani). We conclude that asexual endophytes could protect host grasses against pathogens but most importantly, that they can have protective effects beyond their hosts. Since effects depended on pathogen and plant identity, more experiments are needed in order understand the ecological meaning of these positive side-effects.

Selenium-induced enhancement in carbohydrate metabolism in nodulated alfalfa (Medicago sativa L.) as related to the glutathione redox state

This study was undertaken to investigate the factors explaining the results of our previous study revealing that selenium (Se) may enhance the nodulation of alfalfa (Medicago sativa L.) via improved carbohydrate metabolism. Symbiotic and non-symbiotic alfalfa plants were cultivated without or with low (1 µM) Se addition at low (2 mM) and adequate (10 mM) N supply levels. The plants were analyzed for root nodulation, photosynthesis and growth parameters. The antioxidative defense and glutathione (GSH) redox state were also investigated. In the inoculated low-nitrogen (N) plants, Se markedly increased the number and fresh weight (FW) of the nodules without any effect on nitrogenase activity. Particularly in the nodulated low-N plants, it also significantly increased the net assimilation rate and soluble sugars, and enhanced protein synthesis. Though Se generally diminished the activity of antioxidative enzymes, the concentrations of hydrogen peroxide (H2O2) and superoxide radical (O2• –) remained constant or even significantly diminished. This gives evidence that Se enhanced the protection of plants against oxidative damage. Se tended to increase GSH particularly in the leaves, and significantly increased the reduced glutathione/oxidized glutathione (GSH/GSSG) particularly in the low-N plants. We suggest that Se exerts its positive effect on the carbohydrate metabolism via modulation in the glutathione redox state.