Reduced parasitism of a leaf-mining moth on trees with high infection frequencies of an endophytic fungus

Effects of root inoculation of entomopathogenic fungi on olfactory-mediated behavior and life-history traits of the parasitoid Aphidius ervi (Haliday) (Hymenoptera: Braconidae)

BACKGROUND

Although most biological control programs use multiple biological agents to manage pest species, to date only a few programs have combined the use of agents from different guilds. Using sweet pepper (Capsicum annuum L.), the entomopathogenic fungus Akanthomyces muscarius ARSEF 5128, the tobacco peach aphid Myzus persicae var. nicotianae and the aphid parasitoid Aphidius ervi as the experimental model, we explored whether root inoculation with an entomopathogenic fungus is compatible with parasitoid wasps for enhanced biocontrol of aphids.

RESULTS

In dual-choice behavior experiments, A. ervi was significantly attracted to the odor of M. persicae-infested C. annuum plants that had been inoculated with A. muscarius, compared to noninoculated infested plants. There was no significant difference in attraction to the odor of uninfested plants. Myzus persicae-infested plants inoculated with A. muscarius emitted significantly higher amounts of indole, (E)-nerolidol, (3E,7E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene and one unidentified terpene compared to noninoculated infested plants. Coupled gas chromatography-electroantennography, using the antennae of A. ervi, confirmed the physiological activity of these elevated compounds. Inoculation of plants with A. muscarius did not affect parasitism rate nor parasitoid longevity, but significantly increased the speed of mummy formation in parasitized aphids on fungus-inoculated plants.

CONCLUSION

Our data suggest that root inoculation of C. annuum with A. muscarius ARSEF 5128 alters the olfactory-mediated behavior of parasitoids, but has little effect on parasitism efficiency or life-history parameters. However, increased attraction of parasitoids towards M. persicae-infested plants when inoculated by entomopathogenic fungi can accelerate host localization and hence improve biocontrol efficacy. © 2023 Society of Chemical Industry.

The Occurrence of Apiognomonia hystrix and Its Pathogenicity towards Acer pseudoplatanus and Fraxinus excelsior under Field Conditions

Apiognomonia hystrix is an ascomycetous fungus within Diaporthales that is found on maples and to a lesser extent on other hardwood trees in Europe, Northern America and Asia. To date, varying opinions on the species’ status as a cause of plant diseases have been expressed. In this study, we present the results of analyses conducted from 2012–2017 at forest sites in Poland on the occurrence of A. hystrix on Acer pseudoplatanus and Fraxinus excelsior and the pathogenicity of this fungus towards both tree species. For the sycamore leaves, A. hystrix conidiomata were detected in connection with 19.2% of galls caused by Dasineura vitrina, 20.4% of galls caused by Drisina glutinosa and 67.9% of extensive vein-associated necroses. The A. hystrix colonization of galls caused by both midge species resulted in statistically significantly larger necroses. On European ash leaves, conidiomata of A. hystrix occurred in connection with 0.8% of Dasineura fraxinea galls. Perithecia of A. hystrix were detected on overwintered leaf petioles in 8.1% of A. pseudoplatanus and 1.2% of F. excelsior samples. Twelve representative cultures were characterized molecularly by barcoding three marker genes (ITS, ACT, CAL). Results of phylogenetic analyses indicate that A. hystrix isolates are genetically variable, and three lineages are distinguishable. Eight isolates, including four originating from sycamore and four from European ash, were used to determine A. hystrix pathogenicity. Among the 48 A. pseudoplatanus petioles inoculated with A. hystrix, 41 developed necrotic lesions after 8 weeks, with the average necrosis length caused by particular isolates ranging from 14.5 to 67.2 mm. None of the 48 inoculated F. excelsior petioles developed necrotic lesions. Finally, selected aspects of A. hystrix morphology on natural substrates and in vitro are discussed in this paper, as well as the species’ potential to cause disease symptoms.

Influence of the plant interacting entomopathogenic fungus Beauveria bassiana on parasitoid host choice-behavior, development, and plant defense pathways

Inoculating plants with entomopathogenic fungi may influence plant nutrient uptake and growth, and herbivore performance. Knowledge is limited concerning the effects of this symbiosis on higher trophic levels. We examined how fungal treatment of faba bean seeds with the entomopathogenic fungus Beauveria bassiana influenced the choice-behavior and development of the aphid parasitoid Aphidius colemani. We also sampled plant material for analysis of changes in expression of genes related to plant defense pathways. While parasitoids were compatible with plants inoculated with B. bassiana initially (66 vs. 65% parasitization on inoculated and control plants, respectively; similar development times of parasitoids: 9.2 days), the emergence of adult parasitoids originating from aphids on fungus treated plants was significantly lower (67 vs. 76%, respectively). We also found that the defense response changed, similar to induced systemic resistance, when plants were treated with B. bassiana, similarly to what has been found for other plant symbiotic microorganisms. These novel findings show that although the application of entomopathogenic fungi to plants can alter the plants' defense against herbivores, it may also have an impact on beneficial insects, so their function and use should be evaluated on a case-by-case basis.

Forest Tree Microbiomes and Associated Fungal Endophytes: Functional Roles and Impact on Forest Health

Terrestrial plants including forest trees are generally known to live in close association with microbial organisms. The inherent features of this close association can be commensalism, parasitism or mutualism. The term “microbiota” has been used to describe this ecological community of plant-associated pathogenic, mutualistic, endophytic and commensal microorganisms. Many of these microbiota inhabiting forest trees could have a potential impact on the health of, and disease progression in, forest biomes. Comparatively, studies on forest tree microbiomes and their roles in mutualism and disease lag far behind parallel work on crop and human microbiome projects. Very recently, our understanding of plant and tree microbiomes has been enriched due to novel technological advances using metabarcoding, metagenomics, metatranscriptomics and metaproteomics approaches. In addition, the availability of massive DNA databases (e.g., NCBI (USA), EMBL (Europe), DDBJ (Japan), UNITE (Estonia)) as well as powerful computational and bioinformatics tools has helped to facilitate data mining by researchers across diverse disciplines. Available data demonstrate that plant phyllosphere bacterial communities are dominated by members of only a few phyla (Proteobacteria, Actinobacteria, Bacteroidetes). In bulk forest soil, the dominant fungal group is Basidiomycota, whereas Ascomycota is the most prevalent group within plant tissues. The current challenge, however, is how to harness and link the acquired knowledge on microbiomes for translational forest management. Among tree-associated microorganisms, endophytic fungal biota are attracting a lot of attention for their beneficial health- and growth-promoting effects, and were preferentially discussed in this review.

Both plant genotype and herbivory shape aspen endophyte communities

Salicinoid phenolic glycosides are common defence substances in salicaceous trees and specialist leaf beetles use these compounds for their own defence against predators. Salicinoids vary qualitatively and qualitatively in aspen (Populus tremula) and this variation has a genetic basis. The foliar endophyte mycobiome is plentiful and we hypothesised that it is related to plant genotype, potentially mediated by salicinoid composition, and that interactions with the leaf beetle Chrysomela tremula may alter this relationship. We studied these three-way interactions in controlled greenhouse experiments. Endophytic fungi were isolated from sterilised leaf tissues with and without beetle damage, and from beetles. We confirmed that endophyte composition was influenced by host genotype. Beetle activity added generalist morphs to the mycobiome that overrode the initial host association. Yeast-like genera (Cryptococcus and Rhodotorula) were isolated only from beetle-damaged tissues and from beetles, whereas fast-growing filamentous fungi dominated beetle-free control plants. Competition experiments between filamentous fungi of plant origin and beetle-related yeasts suggested interaction of both stimulating and inhibiting modes of action amongst the fungi. As a result, we detected examples of amensalism, commensalism, parasitism and competition between the morphs tested, but we found no evidence of mutualism, and consequently no co-evolutionary relationship could be demonstrated, between yeasts carried by beetles, host genotype and associated filamentous morphs. Endophyte studies are method-dependent and high-throughput sequencing technology best define the fungal mycobiome, culturing however continues to be a cheap way to provide fundamental ecological insights and it is also required for experimental studies.

Plant Host and Geographic Location Drive Endophyte Community Composition in the Face of Perturbation

All plants form symbioses with endophytic fungi, which affect host plant health and function. Most endophytic fungi are horizontally transmitted, and consequently, local environment and geographic location greatly influence endophyte community composition. Growing evidence also suggests that identity of the plant host (e.g., species, genotype) can be important in shaping endophyte communities. However, little is known about how disturbances to plants affect their fungal symbiont communities. The goal of this study was to test if disturbances, from both natural and anthropogenic sources, can alter endophyte communities independent of geographic location or plant host identity. Using the plant species white snakeroot (Ageratina altissima; Asteraceae), we conducted two experiments that tested the effect of perturbation on endophyte communities. First, we examined endophyte response to leaf mining insect activity, a natural perturbation, in three replicate populations. Second, for one population, we applied fungicide to plant leaves to test endophyte community response to an anthropogenic perturbation. Using culture-based methods and Sanger sequencing of fungal isolates, we then examined abundance, diversity, and community structure of endophytic fungi in leaves subjected to perturbations by leaf mining and fungicide application. Our results show that plant host individual and geographic location are the major determinants of endophyte community composition even in the face of perturbations. Unexpectedly, we found that leaf mining did not impact endophyte communities in white snakeroot, but fungicide treatment resulted in small but significant changes in endophyte community structure. Together, our results suggest that endophyte communities are highly resistant to biotic and anthropogenic disturbances.

The defensive role of foliar endophytic fungi for a South American tree

Fungal endophytes colonize living internal plant tissues without causing any visible symptoms of disease. Endophytic fungi associated with healthy leaves may play an important role in the protection of hosts against herbivores and pathogens. In this study, the diversity of foliar endophytic fungi (FEF) of the southern temperate tree Embothrium coccineum (Proteaceae), as well as their role in plant protection in nature was determined. Fungal endophytes were isolated from 40 asymptomatic leaves by the culture method for molecular identification of the 18S rRNA gene. A relationship between FEF frequency and plant protection was evaluated in juveniles of E. coccineum Fungal endophyte frequency was estimated using real-time PCR analyses to determine endophyte DNA content per plant. A total of 178 fungal isolates were identified, with sequence data revealing 34 different operational taxonomic units (OTUs). A few common taxa dominated the fungal endophyte community, whereas most taxa qualified as rare. A significant positive correlation between plant protection (evaluated in terms of percentage of leaf damage) and FEF frequency was found. Furthermore, in vitro confrontation assays indicated that FEF were able to inhibit the growth of fungal pathogens. The data showed a relatively high diversity of fungal endophytes associated with leaves of E. coccineum, and suggest a positive relationship between fungal endophyte frequencies in leaves and host protection in nature.

Revisiting the Life Cycle of Dung Fungi, Including Sordaria fimicola

Dung fungi, such as Sordaria fimicola, generally reproduce sexually with ascospores discharged from mammalian dung after passage through herbivores. Their life cycle is thought to be obligate to dung, and thus their ascospores in Quaternary sediments have been interpreted as evidence of past mammalian herbivore activity. Reports of dung fungi as endophytes would seem to challenge the view that they are obligate to dung. However, endophyte status is controversial because surface-sterilization protocols could fail to kill dung fungus ascospores stuck to the plant surface. Thus, we first tested the ability of representative isolates of three common genera of dung fungi to affect plant growth and fecundity given that significant effects on plant fitness could not result from ascospores merely stuck to the plant surface. Isolates of S. fimicola, Preussia sp., and Sporormiella sp. reduced growth and fecundity of two of three populations of Bromus tectorum, the host from which they had been isolated. In further work with S. fimicola we showed that inoculations of roots of B. tectorum led to some colonization of aboveground tissues. The same isolate of S. fimicola reproduced sexually on inoculated host plant tissues as well as in dung after passage through sheep, thus demonstrating a facultative rather than an obligate life cycle. Finally, plants inoculated with S. fimicola were not preferred by sheep; preference had been expected if the fungus were obligate to dung. Overall, these findings make us question the assumption that these fungi are obligate to dung.

Pervasive Effects of Wildfire on Foliar Endophyte Communities in Montane Forest Trees

Plants in all terrestrial ecosystems form symbioses with endophytic fungi that inhabit their healthy tissues. How these foliar endophytes respond to wildfires has not been studied previously, but is important given the increasing frequency and intensity of severe wildfires in many ecosystems, and because endophytes can influence plant growth and responses to stress. The goal of this study was to examine effects of severe wildfires on endophyte communities in forest trees, with a focus on traditionally fire-dominated, montane ecosystems in the southwestern USA. We evaluated the abundance, diversity, and composition of endophytes in foliage of Juniperus deppeana (Cupressaceae) and Quercus spp. (Fagaceae) collected contemporaneously from areas affected by recent wildfire and paired areas not affected by recent fire. Study sites spanned four mountain ranges in central and southern Arizona. Our results revealed significant effects of fires on endophyte communities, including decreases in isolation frequency, increases in diversity, and shifts in community structure and taxonomic composition among endophytes of trees affected by recent fires. Responses to fire were similar in endophytes of each host in these fire-dominated ecosystems and reflect regional fire-return intervals, with endophytes after fire representing subsets of the regional mycoflora. Together, these findings contribute to an emerging perspective on the responses of diverse communities to severe fire, and highlight the importance of considering fire history when estimating endophyte diversity and community structure for focal biomes.

Leaf endophyte load influences fungal garden development in leaf-cutting ants

Background

Previous work has shown that leaf-cutting ants prefer to cut leaf material with relatively low fungal endophyte content. This preference suggests that fungal endophytes exact a cost on the ants or on the development of their colonies. We hypothesized that endophytes may play a role in their host plants’ defense against leaf-cutting ants. To measure the long-term cost to the ant colony of fungal endophytes in their forage material, we conducted a 20-week laboratory experiment to measure fungal garden development for colonies that foraged on leaves with low or high endophyte content.

Results

Colony mass and the fungal garden dry mass did not differ significantly between the low and high endophyte feeding treatments. There was, however, a marginally significant trend toward greater mass of fungal garden per ant worker in the low relative to the high endophyte treatment. This trend was driven by differences in the fungal garden mass per worker from the earliest samples, when leaf-cutting ants had been foraging on low or high endophyte leaf material for only 2 weeks. At two weeks of foraging, the mean fungal garden mass per worker was 77% greater for colonies foraging on leaves with low relative to high endophyte loads.

Conclusions

Our data suggest that the cost of endophyte presence in ant forage material may be greatest to fungal colony development in its earliest stages, when there are few workers available to forage and to clean leaf material. This coincides with a period of high mortality for incipient colonies in the field. We discuss how the endophyte-leaf-cutter ant interaction may parallel constitutive defenses in plants, whereby endophytes reduce the rate of colony development when its risk of mortality is greatest.

Endophytic fungus-vascular plant-insect interactions

Insect association with fungi has a long history. Theories dealing with the evolution of insect herbivory indicate that insects used microbes including fungi as their principal food materials before flowering plants evolved. Subtlety and the level of intricacy in the interactions between insects and fungi indicate symbiosis as the predominant ecological pattern. The nature of the symbiotic interaction that occurs between two organisms (the insect and the fungus), may be either mutualistic or parasitic, or between these two extremes. However, the triangular relationship involving three organisms, viz., an insect, a fungus, and a vascular plant is a relationship that is more complicated than what can be described as either mutualism or parasitism, and may represent facets of both. Recent research has revealed such a complex relationship in the vertically transmitted type-I endophytes living within agriculturally important grasses and the pestiferous insects that attack them. The intricacy of the association depends on the endophytic fungus-grass association and the insect present. Secondary compounds produced in the endophytic fungus-grass association can provide grasses with resistance to herbivores resulting in mutualistic relationship between the fungus and the plant that has negative consequences for herbivorous insects. The horizontally transmitted nongrass type-II endophytes are far less well studied and as such their ecological roles are not fully understood. This forum article explores the intricacy of dependence in such complex triangular relationships drawing from well-established examples from the fungi that live as endophytes in vascular plants and how they impact on the biology and evolution of free-living as well as concealed (e.g., gall-inducing, gall-inhabiting) insects. Recent developments with the inoculation of strains of type-I fungal endophytes into grasses and their commercialization are discussed, along with the possible roles the endophytic fungi play in the galls induced by the Cecidomyiidae (Diptera).

Fungal phyllosphere communities are altered by indirect interactions among trophic levels

Trophic interactions involving predators, herbivores, and plants have been described in terrestrial systems. However, there is almost no information on the effect of trophic interactions on microbial phyllosphere community abundance, diversity, or structure. In this study, the interaction between a parasitoid, an insect herbivore, and the fungal phyllosphere community is examined. Parasitoid wasps have an indirect negative impact on fungal community diversity. On the citrus phyllosphere, the exotic wasp species, Amitus hesperidum and Encarsia opulenta, may parasitize the citrus blackfly (Aleurocanthus woglumi). If parasitism levels are low, the blackfly may produce significant amounts of honeydew secretions on the surface of the leaf. Honeydew deposition provides a carbon-rich substrate for the development of fungal growth persisting as sooty mold on the leaves. Leaves from sooty mold-infested grapefruit (Citrus paradisi) trees were collected from multiple orchards in south Texas. The effect of different levels of exotic parasite activity, citrus blackfly, and sooty mold infestation on phyllosphere mycobiota community structure and diversity was examined. Our results suggest the presence of the parasitoid may lead to a top-down trophic cascade affecting phyllosphere fungal community diversity and structure. Additionally, persistent sooty mold deposits that have classically been referred to as Capnodium citri (and related asexual morphological forms) actually comprise a myriad of fungal species including many saprophytes and potential fruit and foliar pathogens of citrus.

Two fungal symbioses collide: endophytic fungi are not welcome in leaf-cutting ant gardens

Interactions among the component members of different symbioses are not well studied. For example, leaf-cutting ants maintain an obligate symbiosis with their fungal garden, while the leaf material they provide to their garden is usually filled with endophytic fungi. The ants and their cultivar may interact with hundreds of endophytic fungal species, yet little is known about these interactions. Experimental manipulations showed that (i) ants spend more time cutting leaves from a tropical vine, Merremia umbellata, with high versus low endophyte densities, (ii) ants reduce the amount of endophytic fungi in leaves before planting them in their gardens, (iii) the ants' fungal cultivar inhibits the growth of most endophytes tested. Moreover, the inhibition by the ants' cultivar was relatively greater for more rapidly growing endophyte strains that could potentially out-compete or overtake the garden. Our results suggest that endophytes are not welcome in the garden, and that the ants and their cultivar combine ant hygiene behaviour with fungal inhibition to reduce endophyte activity in the nest.

Defining phytochemical phenotypes: size and shape analysis of phenolic compounds in oaks (Fagaceae, Quercus) of the Chihuahuan Desert

Interspecific variation in phenolic metabolism across plant species has been correlated to numerous ecological factors, yet generalities concerning the ecological role(s) of phenolics remain elusive. Moreover, studies of allometric variation (absolute and relative concentration) in phenolic metabolism are rare despite the importance of each to ecological interactions. In this study, we characterize individual phenolics in a group of 12 oak species from the Chihuahuan Desert and examine interspecific variation in the absolute and relative concentrations of phenolics using size and shape analysis. Size and shape analysis was able to successfully identify those compounds that contribute most to the interspecific allometric variation in phenolics and classify the oak species on the basis of phenolic metabolism. White versus black oak subgenera were found to be most different in their phenolic composition, where the two black oak species contained fewer and less diverse phenolics. Within the 10 white oak species, a predominantly eastern white oak, Quercus muhlenbergii Engelmann, was found to vary significantly from the more widespread white oaks of the Chihuahuan Desert. We also report the occurrence of complex tannins in oaks of North America for the first time, update proposed pathways of ellagitannin biosynthesis in oaks, and discuss the applicability of size and shape analysis to ecological studies of phenolics.