Manipulation of soil mycorrhizal fungi influences floral traits

Most plants form root hyphal relationships with mycorrhizal fungi, especially arbuscular mycorrhizal fungi (AMF). These associations are known to positively impact plant biomass and competitive ability. However, less is known about how mycorrhizas impact other ecological interactions, such as those mediated by pollinators. We performed a meta-regression of studies that manipulated AMF and measured traits related to pollination, including floral display size, rewards, visitation, and reproduction, extracting 63 studies with 423 effects. On average, the presence of mycorrhizas was associated with positive effects on floral traits. Specifically, we found impacts of AMF on floral display size, pollinator visitation and reproduction, and a positive but nonsignificant impact on rewards. Studies manipulating mycorrhizas with fungicide tended to report contrasting results, possibly because fungicide destroys both beneficial and pathogenic microbes. Our study highlights the potential for relationships with mycorrhizal fungi to play an important, yet underrecognized role in plant-pollinator interactions. With heightened awareness of the need for a more sustainable agricultural industry, mycorrhizal fungi may offer the opportunity to reduce reliance on inorganic fertilizers. At the same time, fungicides are now ubiquitous in agricultural systems. Our study demonstrates indirect ways in which plant-belowground fungal partnerships could manifest in plant-pollinator interactions.

Above- and belowground fungal biodiversity of Populus trees on a continental scale

Understanding drivers of terrestrial fungal communities over large scales is an important challenge for predicting the fate of ecosystems under climate change and providing critical ecological context for bioengineering plant-microbe interactions in model systems. We conducted an extensive molecular and microscopy field study across the contiguous United States measuring natural variation in the Populus fungal microbiome among tree species, plant niche compartments and key symbionts. Our results show clear biodiversity hotspots and regional endemism of Populus-associated fungal communities explained by a combination of climate, soil and geographic factors. Modelling climate change impacts showed a deterioration of Populus mycorrhizal associations and an increase in potentially pathogenic foliar endophyte diversity and prevalence. Geographic differences among these symbiont groups in their sensitivity to environmental change are likely to influence broader forest health and ecosystem function. This dataset provides an above- and belowground atlas of Populus fungal biodiversity at a continental scale.

Mycorrhizae influence plant vegetative and floral traits and intraspecific trait variation

PREMISE

Arbuscular mycorrhizal fungi (AMF) can strongly influence host plant vegetative growth, but less is known about AMF effects on other plant traits, the relative impacts of AMF on vegetative growth versus floral traits, or AMF-induced intraspecific variation in traits.

METHODS

In an experimental greenhouse study, we inoculated seven species of wildflowers with six species of AMF in a factorial design. We assessed how the AMF-forb combinations influenced plant survival, vegetative biomass, and floral traits and whether AMF effects on floral traits were similar in magnitude and direction to effects on vegetative biomass. For one forb species, we investigated intraspecific plant trait variation within and across AMF treatments.

RESULTS

AMF species varied from negative to positive in their effects on host plants. AMF often had inconsistent effects on vegetative biomass versus floral traits, and therefore, quantifying one or the other may provide a misleading representation of potential AMF effects. AMF treatments generated key variation in plant traits, especially floral traits, with potential consequences for plant-pollinator interactions. Given increased intraspecific trait variation in Linum lewisii plants across AMF species compared to uninoculated individuals or single AMF treatments, local AMF diversity and their host plant associations may scale up to influence community-wide patterns of trait variation and species interactions.

CONCLUSIONS

These results have implications for predicting how aboveground communities are affected by belowground communities. Including AMF effects on not just host plant biomass but also functional traits and trait variation will deepen our understanding of community structure and function, including pollination.

Novel chemicals engender myriad invasion mechanisms

Non-native invasive species (NIS) release chemicals into the environment that are unique to the invaded communities, defined as novel chemicals. Novel chemicals impact competitors, soil microbial communities, mutualists, plant enemies, and soil nutrients differently than in the species' native range. Ecological functions of novel chemicals and differences in functions between the native and non-native ranges of NIS are of immense interest to ecologists. Novel chemicals can mediate different ecological, physiological, and evolutionary mechanisms underlying invasion hypotheses. Interactions amongst the NIS and resident species including competitors, soil microbes, and plant enemies, as well as abiotic factors in the invaded community are linked to novel chemicals. However, we poorly understand how these interactions might enhance NIS performance. New empirical data and analyses of how novel chemicals act in the invaded community will fill major gaps in our understanding of the chemistry of biological invasions. A novel chemical-invasion mechanism framework shows how novel chemicals engender invasion mechanisms beyond plant-plant or plant-microorganism interactions.

Composted Municipal Green Waste Infused with Biocontrol Agents to Control Plant Parasitic Nematodes-A Review

The last few years have witnessed the emergence of alternative measures to control plant parasitic nematodes (PPNs). We briefly reviewed the potential of compost and the direct or indirect roles of soil-dwelling organisms against PPNs. We compiled and assessed the most intensively researched factors of suppressivity. Municipal green waste (MGW) was identified and profiled. We found that compost, with or without beneficial microorganisms as biocontrol agents (BCAs) against PPNs, were shown to have mechanisms for the control of plant parasitic nematodes. Compost supports a diverse microbiome, introduces and enhances populations of antagonistic microorganisms, releases nematicidal compounds, increases the tolerance and resistance of plants, and encourages the establishment of a "soil environment" that is unsuitable for PPNs. Our compilation of recent papers reveals that while the scope of research on compost and BCAs is extensive, the role of MGW-based compost (MGWC) in the control of PPNs has been given less attention. We conclude that the most environmentally friendly and long-term, sustainable form of PPN control is to encourage and enhance the soil microbiome. MGW is a valuable resource material produced in significant amounts worldwide. More studies are suggested on the use of MGWC, because it has a considerable potential to create and maintain soil suppressivity against PPNs. To expand knowledge, future research directions shall include trials investigating MGWC, inoculated with BCAs.

Holoparasitic plant-host interactions and their impact on Mediterranean ecosystems

Although photosynthesis is essential to sustain life on Earth, not all plants use sunlight to synthesize nutrients from carbon dioxide and water. Holoparasitic plants, which are important in agricultural and natural ecosystems, are dependent on other plants for nutrients. Phytohormones are crucial in holoparasitic plant-host interactions, from seed germination to senescence, not only because they act as growth and developmental regulators, but also because of their central role in the regulation of host photosynthesis and source-sink relations between the host and the holoparasitic plant. Here, we compile and discuss current knowledge on the impact and ecophysiology of holoparasitic plants (such as the broomrapes Orobanche sp. and Phelipanche sp.) that infest economically important dicotyledonous crops in Mediterranean agroecosystems (legumes [Fabaceae], sunflowers [Helianthus sp.], or tomato [Solanum lycopersicum] plants). We also highlight the role of holoparasitic plant-host interactions (such as those between Cytinus hypocistis and various shrubs of the genus Cistus) in shaping natural Mediterranean ecosystems. The roles of phytohormones in controlling plant-host interactions, abiotic factors in parasitism, and the biological significance of natural seed banks and how dormancy and germination are regulated, will all be discussed. Holoparasitic plants are unique organisms; improving our understanding of their interaction with hosts as study models will help us to better manage parasitic plants, both in agricultural and natural ecosystems.

The Influence of Arbuscular Mycorrhizal Fungi on Plant Reproduction

Arbuscular mycorrhizal (AM) fungi can influence all components of plant reproduction including pollen delivery, pollen germination, pollen tube growth, fertilization, and seed germination. AM fungi associate with plant roots, uptake nutrients, and prime plants for faster defense responses. Our literature review first identified four testable hypotheses describing how AM fungi could alter pollen delivery: (1) We hypothesize AM fungi promote floral display size. The influence of AM fungi on flower size and number is supported by literature, however there are no studies on floral color. (2) We hypothesize AM fungi promote pollen and nectar quality and quantity, and, as reported before, AM fungi promote male fitness over female fitness. (3) We hypothesize AM fungi promote both earlier and longer flowering times, but we found no consistent trend in the data for earlier or later or longer flowering times. (4) We hypothesize AM fungi alter floral secondary chemistry and VOCs, and find there is clear evidence for the alteration of floral chemistry but little data on VOCs. Second, we focus on how AM fungi could alter pollen germination, pollen tube growth, and fertilization, and present three testable hypotheses. We found evidence that AM fungi influence pollen germination and pollen tube growth, production of seeds, and seed germination. However, while most of these influences are positive they are not conclusive, because studies have been conducted in small numbers of systems and groups. Therefore, we conclude that the majority of research to date may not be measuring the influence of AM fungi on the most important components of plant reproduction: pollen germination, pollen tube growth, fertilization, and seed germination.

Genotype-specific effects of ericoid mycorrhizae on floral traits and reproduction in Vaccinium corymbosum

PREMISE

Most plants interact with mycorrhizal fungi and animal pollinators simultaneously. Yet, whether mycorrhizae affect traits important to pollination remains poorly understood and may depend on the match between host and fungal genotypes. Here, we examined how ericoid mycorrhizal fungi affected flowering phenology, floral traits, and reproductive success, among eight genotypes of highbush blueberry, Vaccinium corymbosum (Ericaceae). We asked three overarching questions: (1) Do genotypes differ in response to inoculation? (2) How does inoculation affect floral and flowering traits? (3) Are inoculated plants more attractive to pollinators and less pollen limited than non-inoculated plants of the same genotype?

METHODS

To examine these questions, we experimentally inoculated plants with ericoid mycorrhizal fungi, grew the plants in the field, and measured flowering and floral traits over 2 years. In year 2, we conducted a hand-pollination experiment to test whether plants differed in pollen limitation.

RESULTS

Inoculated plants had significantly higher levels of colonization for some genotypes, and there were significant floral trait changes in inoculated plants for some genotypes as well. On average, inoculated plants produced significantly larger floral displays, more fruits per inflorescence, and heavier fruits with lower sugar content, than non-inoculated, control plants. Hand pollination enhanced the production of fruits, and fruit mass, for non-inoculated plants but not for those that were inoculated.

CONCLUSIONS

Our results demonstrate that inoculation with ericoid mycorrhizal fungi enhanced flowering and altered investment in reproduction in genotype-specific ways. These findings underscore the importance of examining belowground symbionts and genotype-specific responses in their hosts to fully understand the drivers of aboveground interactions.

Pesticides and pollinators: A socioecological synthesis

The relationship between pesticides and pollinators, while attracting no shortage of attention from scientists, regulators, and the public, has proven resistant to scientific synthesis and fractious in matters of policy and public opinion. This is in part because the issue has been approached in a compartmentalized and intradisciplinary way, such that evaluations of organismal pesticide effects remain largely disjoint from their upstream drivers and downstream consequences. Here, we present a socioecological framework designed to synthesize the pesticide-pollinator system and inform future scholarship and action. Our framework consists of three interlocking domains-pesticide use, pesticide exposure, and pesticide effects-each consisting of causally linked patterns, processes, and states. We elaborate each of these domains and their linkages, reviewing relevant literature and providing empirical case studies. We then propose guidelines for future pesticide-pollinator scholarship and action agenda aimed at strengthening knowledge in neglected domains and integrating knowledge across domains to provide decision support for stakeholders and policymakers. Specifically, we emphasize (1) stakeholder engagement, (2) mechanistic study of pesticide exposure, (3) understanding the propagation of pesticide effects across levels of organization, and (4) full-cost accounting of the externalities of pesticide use and regulation. Addressing these items will require transdisciplinary collaborations within and beyond the scientific community, including the expertise of farmers, agrochemical developers, and policymakers in an extended peer community.

Field studies reveal functions of chemical mediators in plant interactions

Plants are at the trophic base of most ecosystems, embedded in a rich network of ecological interactions in which they evolved. While their limited range and speed of motion precludes animal-typical behavior, plants are accomplished chemists, producing thousands of specialized metabolites which may function to convey information, or even to manipulate the physiology of other organisms. Plants' complex interactions and their underlying mechanisms are typically dissected within the controlled environments of growth chambers and glasshouses, but doing so introduces conditions alien to plants evolved in natural environments, such as being pot-bound, and receiving artificial light with a spectrum very different from sunlight. The mechanistic understanding gained from a reductionist approach provides the tools required to query and manipulate plant interactions in real-world settings. The few tests conducted in natural ecosystems and agricultural fields have highlighted the limitations of studying plant interactions only in artificial environments. Here, we focus on three examples of known or hypothesized chemical mediators of plants' interactions: the volatile phytohormone ethylene (ET), more complex plant volatile blends, and as-yet-unknown mediators transferred by common mycorrhizal networks (CMNs). We highlight how mechanistic knowledge has advanced research in all three areas, and the critical importance of field work if we are to put our understanding of chemical ecology on rigorous experimental and theoretical footing, and demonstrate function.

Using niche breadth theory to explain generalization in mutualisms

For a mutualism to remain evolutionarily stable, theory predicts that mutualists should limit their associations to high-quality partners. However, most mutualists either simultaneously or sequentially associate with multiple partners that confer the same type of reward. By viewing mutualisms through the lens of niche breadth evolution, we outline how the environment shapes partner availability and relative quality, and ultimately a focal mutualist's partner breadth. We argue that mutualists that associate with multiple partners may have a selective advantage compared to specialists for many reasons, including sampling, complementarity, and portfolio effects, as well as the possibility that broad partner breadth increases breadth along other niche axes. Furthermore, selection for narrow partner breadth is unlikely to be strong when the environment erodes variation in partner quality, reduces the costs of interacting with low-quality partners, spatially structures partner communities, or decreases the strength of mutualism. Thus, we should not be surprised that most mutualists have broad partner breadth, even if it allows for ineffective partners to persist.

Reciprocal effects of pesticides and pathogens on amphibian hosts: The importance of exposure order and timing

Ecological communities are increasingly exposed to natural and anthropogenic stressors. While the effects of individual stressors have been broadly investigated, there is growing evidence that multiple stressors are frequently encountered underscoring the need to examine interactive effects. Pesticides and infectious diseases are two common stressors that regularly occur together in nature. Given the documented lethal and sublethal effects of each stressor on individuals, there is the potential for interactive effects that alter disease outcomes and pesticide toxicity. Using larval wood frogs (Lithobates sylvaticus), we examined the reciprocal interaction between insecticides (carbaryl and thiamethoxam) and the viral pathogen ranavirus by testing whether: (1) prior ranavirus infection influences pesticide toxicity and (2) sublethal pesticide exposure increases susceptibility to and transmission of ranavirus. We found that prior infection with ranavirus increased pesticide toxicity; median lethal concentration (LC50) estimates were reduced by 72 and 55% for carbaryl and thiamethoxam, respectively. Importantly, LC50 estimates were reduced to concentrations found in natural systems. This is the first demonstration that an infection can alter pesticide toxicity. We also found that prior pesticide exposure exacerbated disease-induced mortality by increasing mortality rates, but effects on infection prevalence and transmission of the pathogen were minimal. Collectively, our results underscore the importance of incorporating complexity (i.e. order and timing of exposures) into research examining the interactions between natural and anthropogenic stressors. Given the environmental heterogeneity present in nature, such research will provide a more comprehensive understanding of how stressors affect wildlife.

Stressed out symbiotes: hypotheses for the influence of abiotic stress on arbuscular mycorrhizal fungi

Abiotic stress is a widespread threat to both plant and soil communities. Arbuscular mycorrhizal (AM) fungi can alleviate effects of abiotic stress by improving host plant stress tolerance, but the direct effects of abiotic stress on AM fungi are less well understood. We propose two hypotheses predicting how AM fungi will respond to abiotic stress. The stress exclusion hypothesis predicts that AM fungal abundance and diversity will decrease with persistent abiotic stress. The mycorrhizal stress adaptation hypothesis predicts that AM fungi will evolve in response to abiotic stress to maintain their fitness. We conclude that abiotic stress can have effects on AM fungi independent of the effects on the host plant. AM fungal communities will change in composition in response to abiotic stress, which may mean the loss of important individual species. This could alter feedbacks to the plant community and beyond. AM fungi will adapt to abiotic stress independent of their host plant. The adaptation of AM fungi to abiotic stress should allow the maintenance of the plant-AM fungal mutualism in the face of changing climates.

Fungal phylogenetic diversity drives plant facilitation

Plant-plant facilitation is a crucial ecological process, as many plant species (facilitated) require the presence of an established individual (nurse) to recruit. Some plant facilitative interactions disappear during the ontogenetic development of the facilitated plant but others persist, even when the two plants are adults. We test whether the persistence of plant facilitative interactions is explained by the phylogenetic diversity of mutualistic and non-mutualistic fungi that the nurse and the facilitated species add to the shared rhizosphere. We classify plant facilitative interactions as persistent and non-persistent interactions and quantify the phylogenetic diversity of mutualistic and non-mutualistic fungi added by the plant species to the shared rhizosphere. Our results show that the facilitated species add less phylogenetic diversity of non-mutualistic fungi when plant facilitative interactions persist than when they do not persist. However, persistent and non-persistent facilitative interactions did not differ in the phylogenetic diversity of mutualistic fungi added by the facilitated species to the shared rhizosphere. Finally, the fungal phylogenetic diversity added by the nurse to the shared rhizosphere did not differ between persistent and non-persistent interactions. This study suggests that considering the fungal associates of the plant species involved in facilitative interactions can shed light on the mechanisms of persistence for plant-plant interactions.

Arbuscular mycorrhizal fungi regulate soil respiration and its response to precipitation change in a semiarid steppe

Arbuscular mycorrhizal fungi (AMF) are critical links in plant–soil continuum and play a critical role in soil carbon cycles. Soil respiration, one of the largest carbon fluxes in global carbon cycle, is sensitive to precipitation change in semiarid ecosystems. In this study, a field experiment with fungicide application and water addition was conducted during 2010–2013 in a semiarid steppe in Inner Mongolia, China, and soil respiration was continuously measured to investigate the influences of AMF on soil respiration under different precipitation regimes. Results showed that soil respiration was promoted by water addition treatment especially during drought seasons, which induced a nonlinear response of soil respiration to precipitation change. Fungicide application suppressed AMF root colonization without impacts on soil microbes. AMF suppression treatment accelerated soil respiration with 2.7, 28.5 and 37.6 g C m⁻² across three seasons, which were mainly caused by the enhanced heterotrophic component. A steeper response of soil respiration rate to precipitation was found under fungicide application treatments, suggesting a greater dampening effect of AMF on soil carbon release as water availability increased. Our study highlighted the importance of AMF on soil carbon stabilization and sequestration in semiarid steppe ecosystems especially during wet seasons.

Silencing a key gene of the common symbiosis pathway in Nicotiana attenuata specifically impairs arbuscular mycorrhizal infection without influencing the root-associated microbiome or plant growth

While the biochemical function of calcium and calmodulin-dependent protein kinase (CCaMK) is well studied, and plants impaired in the expression of CCaMK are known not to be infected by arbuscular mycorrhizal fungi (AMF) in glasshouse studies, the whole-plant and ecological consequences of CCaMK silencing are not well understood. Here we show that three independently transformed lines of Nicotiana attenuata plants silenced in CCaMK (irCCaMK) are neither infected by Rhizophagus irregularis in the glasshouse nor by native fungal inoculum in the field. The overall fungal community of field-grown roots did not differ significantly among empty vector (EV) and the transgenic lines, and the bacterial communities only showed minor differences, as revealed by the alpha-diversity parameters of bacterial OTUs, which were higher in EV plants compared with two of the three transformed lines, while beta-diversity parameters did not differ. Furthermore, growth and fitness parameters were similar in the glasshouse and field. Herbivory-inducible and basal levels of salicylic acid, jasmonic acid and abscisic acid did not differ among the genotypes, suggesting that activation of the classical defence pathways are not affected by CCaMK silencing. Based on these results, we conclude that silencing of CCaMK has few, if any, non-target effects.

Putative linkages between below- and aboveground mutualisms during alien plant invasions

Evidence of the fundamental role of below-aboveground links in controlling ecosystem processes is mostly based on studies done with soil herbivores or mutualists and aboveground herbivores. Much less is known about the links between belowground and aboveground mutualisms, which have been studied separately for decades. It has not been until recently that these mutualisms-mycorrhizas and legume-rhizobia on one hand, and pollinators and seed dispersers on the other hand-have been found to influence each other, with potential ecological and evolutionary consequences. Here we review the mechanisms that may link these two-level mutualisms, mostly reported for native plant species, and make predictions about their relevance during alien plant invasions. We propose that alien plants establishing effective mutualisms with belowground microbes might improve their reproductive success through positive interactions between those mutualists and pollinators and seed dispersers. On the other hand, changes in the abundance and diversity of soil mutualists induced by invasion can also interfere with below-aboveground links for native plant species. We conclude that further research on this topic is needed in the field of invasion ecology as it can provide interesting clues on synergistic interactions and invasional meltdowns during alien plant invasions.

Indirect effects of herbivory on plant-pollinator interactions in invasive Lythrum salicaria

PREMISE OF THE STUDY

Herbivory can affect a plant's fitness in a variety of ways, including modifying the biotic interactions of the plant. In particular, when herbivory influences floral display, we hypothesize that pollinator visitation will be altered accordingly. Here we studied the indirect effects of feeding by two beetles, Neogalerucella calmariensis and N. pusilla, released as a biological control, on plant-pollinator interactions and fitness in the invasive plant, purple loosestrife (Lythrum salicaria).

METHODS

Two herbivory treatments (ambient and simulated) were applied to plants in a naturally occurring population of purple loosestrife. During flowering, traits of plants in the treatment and control groups were recorded. Data on pollinator visitation behavior was then collected after intense larval herbivory had ended.

KEY RESULTS

Plants exposed to herbivory treatments produced more flowers and inflorescences but flowered significantly later than those in the control group. Moreover, we found a significant, positive association of herbivory with the number of flowers probed by bumblebees and with the number of times a foraging pollinator moved among inflorescences on a single plant. No differences in female fitness (fruit or seed production) were detected.

CONCLUSIONS

We conclude that herbivore-mediated differences in floral display traits impacted pollinator visitation behavior. However, as we discuss, differences in pollinator visitation did not translate into detectable differences in female reproductive success. We postulate that herbivory could influence other unmeasured aspects of fitness, such as seed quality or the number of seeds sired.

Linkages of plant-soil feedbacks and underlying invasion mechanisms

Soil microbial communities and processes have repeatedly been shown to impact plant community assembly and population growth. Soil-driven effects may be particularly pronounced with the introduction of plants to non-native ranges, as introduced plants are not typically accompanied by transference of local soil communities. Here we describe how the mechanisms by which soil community processes influence plant growth overlap with several known and well-described mechanisms of plant invasion. Critically, a given soil community process may either facilitate or limit invasion, depending upon local conditions and the specific mechanisms of soil processes involved. Additionally, as soil communities typically consist of species with short generation times, the net consequences of plant-soil feedbacks for invasion trajectories are likely to change over time, as ecological and evolutionary adjustments occur. Here we provide an overview of the ecological linkages of plant-soil feedbacks and underlying mechanisms of invasion.

Priming of anti-herbivore defense in tomato by arbuscular mycorrhizal fungus and involvement of the jasmonate pathway

Mycorrhizas play a vital role in soil fertility, plant nutrition, and resistance to environmental stresses. However, mycorrhizal effects on plant resistance to herbivorous insects and the related mechanisms are poorly understood. This study evaluated effects of root colonization of tomato (Solanum lycopersicum Mill.) by arbuscular mycorrhizal fungi (AMF) Glomus mosseae on plant defense responses against a chewing caterpillar Helicoverpa arimigera. Mycorrhizal inoculation negatively affected larval performance. Real time RT-PCR analyses showed that mycorrhizal inoculation itself did not induce transcripts of most genes tested. However, insect feeding on AMF pre-inoculated plants resulted in much stronger defense response induction of four defense-related genes LOXD, AOC, PI-I, and PI-II in the leaves of tomato plants relative to non-inoculated plants. Four tomato genotypes: a wild-type (WT) plant, a jasmonic acid (JA) biosynthesis mutant (spr2), a JA-signaling perception mutant (jai1), and a JA-overexpressing 35S::PS plant were used to determine the role of the JA pathway in AMF-primed defense. Insect feeding on mycorrhizal 35S::PS plants led to higher induction of defense-related genes relative to WT plants. However, insect feeding on mycorrhizal spr2 and jai1 mutant plants did not induce transcripts of these genes. Bioassays showed that mycorrhizal inoculation on spr2 and jai1 mutants did not change plant resistance against H. arimigera. These results indicates that mycorrhizal colonization could prime systemic defense responses in tomato upon herbivore attack, and that the JA pathway is involved in defense priming by AMF.

Wildlife ecotoxicology of pesticides: can we track effects to the population level and beyond?

During the past 50 years, the human population has more than doubled and global agricultural production has similarly risen. However, the productive arable area has increased by just 10%; thus the increased use of pesticides has been a consequence of the demands of human population growth, and its impact has reached global significance. Although we often know a pesticide's mode of action in the target species, we still largely do not understand the full impact of unintended side effects on wildlife, particularly at higher levels of biological organization: populations, communities, and ecosystems. In these times of regional and global species declines, we are challenged with the task of causally linking knowledge about the molecular actions of pesticides to their possible interference with biological processes, in order to develop reliable predictions about the consequences of pesticide use, and misuse, in a rapidly changing world.

Soil fungal effects on floral signals, rewards, and aboveground interactions in an alpine pollination web

PREMISE OF THE STUDY

Plants interact with above- and belowground organisms; the combined effects of these interactions determine plant fitness and trait evolution. To better understand the ecological and evolutionary implications of multispecies interactions, we explored linkages between soil fungi, pollinators, and floral larcenists in Polemonium viscosum (Polemoniaceae).

METHODS

Using a fungicide, we experimentally reduced fungal colonization of krummholz and tundra P. viscosum in 2008-2009. We monitored floral signals and rewards, interactions with pollinators and larcenists, and seed set for fungicide-treated and control plants.

KEY RESULTS

Fungicide effects varied among traits, between interactions, and with environmental context. Treatment effects were negligible in 2008, but stronger in 2009, especially in the less-fertile krummholz habitat. There, fungicide increased nectar sugar content and damage by larcenist ants, but did not affect pollination. Surprisingly, fungicide also enhanced seed set, suggesting that direct resource costs of soil fungi exceed indirect benefits from reduced larceny. In the tundra, fungicide effects were negligible in both years. However, pooled across treatments, colonization by mycorrhizal fungi in 2009 correlated negatively with the intensity and diversity of floral volatile organic compounds, suggesting integrated above- and belowground signaling pathways.

CONCLUSIONS

Fungicide effects on floral rewards in P. viscosum link soil fungi to ecological costs of pollinator attraction. Trait-specific linkages to soil fungi should decouple expression of sensitive and buffered floral phenotypes in P. viscosum. Overall, this study demonstrates how multitrophic linkages may lead to shifting selection pressures on interaction traits, restricting the evolution of specialization.

Indirect effects of species interactions on habitat provisioning

Species that shelter in a biogenic habitat can influence their refugia and, in turn, play an essential role in shaping local patterns of biodiversity. Here we explore a positive feedback loop between the provisioning rate of habitat-forming branching corals and their associated fishes and show how interactions between two groups of fish—the planktivorous damselfish and predatory hawkfish—altered the feedback. A field experiment confirmed that skeletal growth of branching coral (genus Pocillopora) increased substantially with increasing numbers (biomass) of resident fishes, likely because they greatly increased the interstitial concentrations of nutrients. Because there is a positive relationship between colony size and number (biomass) of associated fishes (primarily damselfishes in the Family Pomacentridae), a structure–function feedback loop exists in which increasing numbers of damselfish enhance coral growth and larger corals host greater abundances (and species richness) of fish. However, interactions between damselfishes and arc-eye hawkfish, Paracirrhites arcatus, a largely solitary resident, can disrupt this positive feedback loop. Field surveys revealed a marked pattern of fish occupancy related to coral size: Pocillopora colonies of sufficient size to host fish (>40 cm circumference) had either groups of damselfish or an arc-eye hawkfish; only larger colonies (>75 cm) were occupied by both the damselfish and hawkfish. Subsequent short- and long-term experiments revealed that on intermediate-sized Pocillopora colonies, arc-eye hawkfish prevented the establishment of damselfish by suppressing their recruitment. The demographic consequences to the host coral were substantial; in a 1-year-long experiment, intermediate-size Pocillopora occupied by hawkfish grew at half the rate of corals that hosted groups of damselfish. These findings indicate that: (1) species which occupy a biogenic habitat can enhance the provisioning rate of their habitat; (2) such positive feedbacks between community structure and ecosystem function can be disrupted by a strong interactor; (3) even substantial consequences on ecosystem processes that arise can be difficult to discern.

Evolutionary ecology of mycorrhizal functional diversity in agricultural systems

The root systems of most agronomic crops are colonized by diverse assemblages of arbuscular mycorrhizal fungi (AMF), varying in the functional benefits (e.g. nutrient transfer, pathogen protection, water uptake) provided to hosts. Little is known about the evolutionary processes that shape the composition of these fungal assemblages, nor is it known whether more diverse assemblages are beneficial to crop productivity. In this review we aim to identify the evolutionary selection pressures that shape AMF diversity in agricultural systems and explore whether promotion of AMF diversity can convincingly be linked to increases in agricultural productivity and/or sustainability. We then ask whether farmers can (and should) actively modify evolutionary selection pressures to increase AMF functioning. We focus on three agriculturally imposed selection regimes: tillage, fertilization, and continuous monoculture. We find that the uniform nature of these practices strongly selects for dominance of few AMF species. These species exhibit predictable, generally non-beneficial traits, namely heavy investment in reproduction at the expense of nutrient scavenging and transfer processes that are beneficial for hosts. A number of focus-points are given based on empirical and theoretical evidence that could be utilized to slow down negative selection pressures on AMF functioning, therein increasing crop benefit.

A case study of modified interactions with symbionts in a hybrid mediterranean orchid

PREMISE OF THE STUDY

Most studies on orchid hybrids examine separately the effects of hybridization on interactions with pollinators or with mycorrhizal fungi. Here, we simultaneously investigated both interactions in the mediterranean food-deceptive Orchis simia, O. anthropophora, and their hybrid (O. ×bergonii) and tested a possible breakdown of coevolution using a multidisciplinary approach. •

METHODS

We compared leaf growth, seed viability, emitted scent, and mycorrhizal fungi (species and rate of infection) among these three taxa. •

KEY RESULTS

We show that leaf surface is greater in adult hybrids than in the parental species, suggesting a heterosis effect for vegetative growth. We demonstrate that flowers of the two parental species emit well-differentiated bouquets of volatile organic compounds, while hybrids emit larger quantities, accumulating most compounds of the two parental species. However, hybrids fail to attract pollinators and have a 10 times lower fruit set. We determined that closely related Tulasnellales are mycorrhizal in the three taxa, suggesting that the mycorrhizal partner does not impair hybrid survival. We propose an interpretative model for O. ×bergonii compared with its parents. •

CONCLUSIONS

In hybrids, carbon resources normally devoted to reproduction may be reallocated to the mycorrhizal symbiosis as a result of the disruption of the pollination interaction in hybrids. Higher mycorrhizal infection may in turn enhance vegetative growth and scent emission. Such interplay between the two obligate biotic interactions yields new insights into hybridization among orchids.

Inherited microbial symbionts increase herbivore abundances and alter arthropod diversity on a native grass

Some microbial symbionts of plants are maternally inherited and thus functionally increase genetic and phenotypic variation within plant populations. This variation, coupled with that of the host plant and environment, may alter abundances, diversity, and trophic structure of associated plant and animal communities. Fungal endophytes in the genus Neotyphodium are vertically transmitted, asexual microbial symbionts of grasses that remain asymptomatic and rely upon their hosts for resources and transmission via seeds, often providing benefits to their hosts, including protection against herbivores. Endophyte infections may influence associated arthropod communities in agronomic grasses, but the long-term effects of endophytes and variation in host genotype and resource availability on arthropod communities in native grass populations are unknown. We conducted a long-term field experiment with four maternal genotypes of an infected (E+) native grass (Festuca arizonica) from whence the endophyte was experimentally removed (E-) and water availability was controlled, to test the effects of infection, plant genotype, and resources on abundances, biomass, diversity (richness and evenness), and trophic structure of the arthropod community. Generally, E+ grasses harbored more arthropods, including more herbivores, predators, and detritivores, suggesting that the effects of endophytes cascaded upward through trophic levels in terms of abundances, at least in early ontogeny of the host. That E+ plants harbored more herbivorous insects than E- plants suggests that infection does not increase but instead decreases resistance to herbivores, contrary to prevailing concepts of endophytes as defensive mutualists. Infection did not alter overall species richness of the arthropod community or richness of herbivores but reduced natural enemy richness, especially that of parasites, and increased richness of detritivores. Reduced richness and shifts in evenness of natural enemies on E+ plants suggest that endophytes may disproportionately affect diversity at higher trophic levels and may partially explain increases in abundances of herbivorous insects on E+ plants. Biomass of predators, detritivores, and omnivores increased on plants with supplemented water, and arthropod and herbivore biomass varied by plant genotype. Symbiont-mediated phenotypic variation interacts with variation from plant genotype and environmental factors to alter arthropod abundances and diversity, and these effects shift with ontogeny of the host.

Anatomical relations among endophytic holoparasitic angiosperms, autotrophic host plants and mycorrhizal fungi: A novel tripartite interaction

Mycorrhizae are widespread mutualistic symbioses crucial for the functioning of terrestrial ecosystems. Not all plants associate with mycorrhizae; most parasitic plants have been suggested to be nonmycorrhizal because they have developed alternative strategies to obtain nutrients. In endophytic parasitic plants, whose vegetative bodies grow completely inside their mycorrhizal host roots, the opportunity for establishing a tripartite association seems evident, but information on these systems is lacking. In studying natural associations among the endophytic holoparasite Cytinus hypocistis, their Cistaceae host species, and associated mycorrhizal fungi, we found that mycorrhizae were associated with the hosts and the parasites, reaching high frequencies of colonization. In parasitic and host root tissues, mycorrhizal fungi spread in the parenchymatic cells by intracellular growth and formed hyphal coils and vesicles, while the cambium and the vascular tissues were never colonized. This report is the first on a tripartite association of an endophytic parasitic plant, its host, and mycorrhizae in natural conditions, representing a novel trophic interaction not previously reported within the angiosperms. Additional studies on the interactions occurring among these three players are needed because they may be crucial to our understanding of how this mutualistic-antagonistic system is functioning and evolving.

Genetically based susceptibility to herbivory influences the ectomycorrhizal fungal communities of a foundation tree species

Although recent research indicates that herbivores interact with plant-associated microbes in complex ways, few studies have examined these interactions using a community approach. For example, the impact of herbivory on the community structure of ectomycorrhizal fungi (EMF) is not well known. The influence of host plant genetics on EMF community composition is also poorly understood. We used a study system in which susceptibility to herbivory has a genetic basis and a 20-yr insect removal experiment to examine the influence of chronic herbivory and plant genetics on the EMF community structure of Pinus edulis. We compared EMF communities of herbivore resistant trees, herbivore susceptible trees and herbivore susceptible trees from which herbivores were experimentally removed at two dates 10 yr apart. In both years sampled, resistant and susceptible trees differed significantly in EMF community composition. After 10 yr and 20 yr of herbivore removal, the EMF communities of removal trees were similar to those of susceptible trees, but different from resistant trees. The EMF community composition was more strongly influenced by innate genetic differences in plant traits associated with resistance and susceptibility to herbivory than by indirect effects of herbivory on host plant relationships with ectomycorrhizal fungi.

Yeasts in floral nectar: a quantitative survey

BACKGROUND AND AIMS

One peculiarity of floral nectar that remains relatively unexplored from an ecological perspective is its role as a natural habitat for micro-organisms. This study assesses the frequency of occurrence and abundance of yeast cells in floral nectar of insect-pollinated plants from three contrasting plant communities on two continents. Possible correlations between interspecific differences in yeast incidence and pollinator composition are also explored.

METHODS

The study was conducted at three widely separated areas, two in the Iberian Peninsula (Spain) and one in the Yucatán Peninsula (Mexico). Floral nectar samples from 130 species (37-63 species per region) in 44 families were examined microscopically for the presence of yeast cells. For one of the Spanish sites, the relationship across species between incidence of yeasts in nectar and the proportion of flowers visited by each of five major pollinator categories was also investigated.

KEY RESULTS

Yeasts occurred regularly in the floral nectar of many species, where they sometimes reached extraordinary densities (up to 4 x 10(5) cells mm(-3)). Depending on the region, between 32 and 44 % of all nectar samples contained yeasts. Yeast cell densities in the order of 10(4) cells mm(-3) were commonplace, and densities >10(5) cells mm(-3) were not rare. About one-fifth of species at each site had mean yeast cell densities >10(4) cells mm(-3). Across species, yeast frequency and abundance were directly correlated with the proportion of floral visits by bumble-bees, and inversely with the proportion of visits by solitary bees.

CONCLUSIONS

Incorporating nectar yeasts into the scenario of plant-pollinator interactions opens up a number of intriguing avenues for research. In addition, with yeasts being as ubiquitous and abundant in floral nectars as revealed by this study, and given their astounding metabolic versatility, studies focusing on nectar chemical features should carefully control for the presence of yeasts in nectar samples.