Herbivory meets fungivory: insect herbivores feed on plant pathogenic fungi for their own benefit

Community structure, diversity and function of endophytic and soil microorganisms in boreal forest

Introduction

Despite extensive studies on soil microbial community structure and functions, the significance of plant-associated microorganisms, especially endophytes, has been overlooked. To comprehensively anticipate future changes in forest ecosystem function under future climate change scenarios, it is imperative to gain a thorough understanding of the community structure, diversity, and function of both plant-associated microorganisms and soil microorganisms.

Methods

In our study, we aimed to elucidate the structure, diversity, and function of leaf endophytes, root endophytes, rhizosphere, and soil microbial communities in boreal forest. The microbial structure and composition were determined by high-throughput sequencing. FAPROTAX and FUNGuild were used to analyze the microbial functional groups.

Results

Our findings revealed significant differences in the community structure and diversity of fungi and bacteria across leaves, roots, rhizosphere, and soil. Notably, we observed that the endophytic fungal or bacterial communities associated with plants comprised many species distinct from those found in the soil microbial communities, challenging the assumption that most of endophytic fungal or bacterial species in plants originate from the soil. Furthermore, our results indicated noteworthy differences in the composition functional groups of bacteria or fungi in leaf endophytes, root endophytes, rhizosphere, and soil, suggesting distinct roles played by microbial communities in plants and soil.

Discussion

These findings underscore the importance of recognizing the diverse functions performed by microbial communities in both plant and soil environments. In conclusion, our study emphasizes the necessity of a comprehensive understanding of the structure and function microbial communities in both plants and soil for assessing the functions of boreal forest ecosystems.

Limonene enhances rice plant resistance to a piercing-sucking herbivore and rice pathogens

Terpene synthases (TPSs) are key enzymes in terpenoids synthesis of plants and play crucial roles in regulating plant defence against pests and diseases. Here, we report the functional characterization of OsTPS19 and OsTPS20, which were upregulated by the attack of brown planthopper (BPH). BPH female adults performed concentration-dependent behavioural responses to (S)-limonene showing preference behaviour at low concentrations and avoidance behaviour at high concentrations. Overexpression lines of OsTPS19 and OsTPS20, which emitted higher amounts of the monoterpene (S)-limonene, decreased the hatching rate of BPH eggs, reduced the lesion length of sheath blight caused by Rhizoctonia solani and bacterial blight caused by Xanthomonas oryzae. While knockout lines of OsTPS19 and OsTPS20, which emitted lower amounts of (S)-limonene, were more susceptible to these pathogens. Overexpression of OsTPS19 and OsTPS20 in rice plants had adverse effects on the incidence of BPH, rice blast, and sheath blight in the field and had no significant impacts on rice yield traits. OsTPS19 and OsTPS20 were found to be involved in fine-tuning the emission of (S)-limonene in rice plants and play an important role in defence against both BPH and rice pathogens.

Penicillium-Infected Apples Benefit Larval Development of Conogethes punctiferalis via Alterations of Their Gut Bacteria Community and Gene Expression

Pathogenic microorganisms can impact the behavior and physiology of herbivores by direct or indirect means. This study demonstrated that yellow peach moth Conogethes punctiferalis larvae feeding on Penicillium-infected apples exhibited significantly longer body length and weight parameters compared to the control group. The sequencing of gut 16S rRNA showed a significant increase in the diversity and abundance of bacteria in the larvae feeding on Penicillium-infected apples. Additionally, transcriptomic sequencing of the larval gut indicated significant upregulation of genes related to digestion and cuticle formation after consuming Penicillium-infected apples. Furthermore, enzyme activity assays revealed notable changes in the trypsin and lipase activity. Consequently, these alterations in gut microbiota structure, diversity, and gene expression levels may underlie the observed growth and developmental variations in C. punctiferalis larvae mediated by pathogenic microorganisms. This study holds theoretical significance for a deeper understanding of the tripartite interaction among microorganisms, insects, and plants as well as for the development of novel pest control measures based on gut microbiota.

Editing Metabolism, Sex, and Microbiome: How Can We Help Poplar Resist Pathogens?

Poplar (Populus) is a genus of woody plants of great economic value. Due to the growing economic importance of poplar, there is a need to ensure its stable growth by increasing its resistance to pathogens. Genetic engineering can create organisms with improved traits faster than traditional methods, and with the development of CRISPR/Cas-based genome editing systems, scientists have a new highly effective tool for creating valuable genotypes. In this review, we summarize the latest research data on poplar diseases, the biology of their pathogens and how these plants resist pathogens. In the final section, we propose to plant male or mixed poplar populations; consider the genes of the MLO group, transcription factors of the WRKY and MYB families and defensive proteins BbChit1, LJAMP2, MsrA2 and PtDef as the most promising targets for genetic engineering; and also pay attention to the possibility of microbiome engineering.

VOCs from fungi-infected apples attract and increase the oviposition of yellow peach moth Conogethes punctiferalis

BACKGROUND

Plant volatile organic compounds (VOCs) modified by plant-associated microbes can attract or repel the oviposition of herbivores. Here, we explored the effects of three different fungi on apples' VOCs and the cascading impacts on the oviposition preference of yellow peach moth [YPM, Conogethes punctiferalis (Guenée)].

RESULTS

Among Penicillium crustosum-infected apples (PCA), Rhizopus oryzae-infected apples (ROA), Colletotrichum gloeosporioides-infected apples (CGA) and healthy apples (HA), mated YPM females preferred to oviposit eggs on ROA and CGA, and showed significant attractiveness to VOCs from PCA, ROA, and CGA under laboratory conditions. The VOCs analyses showed that there were significant differences between fungi-infected apples (ROA, CGA) and control treatments (mechanically damaged apples (MDA), HA) in terms of the relative contents of 13 VOCs. The relative contents of ethyl 2-methylbutyrate, ethyl caprylate, estragole, ethyl hexanoate in ROA and CGA were higher than those in MDA. The relative content of isopropyl 2-methylbutyrate in ROA was significantly higher than those in HA and CGA. The relative contents of 2-methylbutyl acetate, butyl 2-methylbutyrate, hexyl 2-methylbutyrate, amyl hexanoate, hexyl hexanoate, (E, E)-α-farnesene in ROA and CGA were lower than those in HA. The relative content of hexyl acetate in ROA and CGA was significantly higher than that in MDA, but lower than that in HA. Additionally, 10 fungi-induced VOCs were detected in ROA and/or CGA. When 20 VOCs from ROA and/or CGA were tested as individuals or mixed blends in Y-tube olfactometer assays, mated YPM females preferred amyl 2-methylbutyrate, isoamyl 2-methylbutyrate, isopropyl 2-methylbutyrate, hexyl propionate (common VOCs in ROA, CGA, and HA), and heptacosane (a fungi-induced VOC in ROA), but no significant preferences were observed between individual compounds and mixed blends, except for hexyl propionate.

CONCLUSION

Different fungi infection increased the relative contents of common VOCs from healthy and fungi-infected apples, which ultimately resulted in the significant attractiveness for the oviposition of mated YPM females. This study clarified why fungi-infected apples were more attractive to YPMs than healthy apples and screened out the crucial VOCs for YPM oviposition. © 2023 Society of Chemical Industry.

Parasites as ecosystem modulators: foliar pathogens suppress top-down effects of large herbivores

Parasites can catalyze or inhibit interactions between their hosts and other species, but the ecosystem-level effects of such interaction modifications are poorly understood. We conducted a large-scale field experiment in temperate grasslands of China to understand how foliar fungal pathogens influenced top-down effects of cattle on plant diversity and productivity. When foliar pathogens were suppressed, cattle grazing strongly reduced biomass of the dominant grass, Leymus chinensis, generating competitive release that significantly increased community-level species richness and evenness. In the absence of grazing, pathogen attack on L. chinensis had no measurable effect on host biomass. However, pathogens disrupted top-down effects of herbivory by inhibiting grazing effects on plant biomass and species richness. Mechanistically, fungal pathogens were linked to increased alkaloid and reduced nitrogen levels in leaf tissue, which appeared to deter cattle grazing on L. chinensis. In conclusion, foliar pathogens can suppress top-down effects of large herbivores on grassland community composition and ecosystem function by modifying the strength of their host's interactions with dominant consumers. Parasites may act as modulators of ecosystem function when their direct effects on host abundance are overshadowed by powerful influences on host traits that modify their interactions with competitors, herbivores, or predators.

Why can insects not biosynthesize cholesterol?

Two aspects of insect lipid biochemistry differ from the mammalian background. In one aspect, nearly a hundred years ago scientists demonstrated that the polyunsaturated fatty acid (PUFAs), linoleic acid (LA; 18:2n-6) is an essential nutrient in the diets of all mammals that have been studied in that regard. An unknown number of insect species are able to biosynthesize LA de novo. Some species take the biosynthesized LA into fatty acid elongation/desaturation pathways to produce other PUFAs, 18:3n-6, 20:3n-6 and 20:4n-6. A couple of species use the de novo produced LA to biosynthesize prostaglandins and other eicosanoids, short-lived signal moieties that mediate important physiological actions in immunity and reproduction. Insects differ from mammals, also, in their lack of genes that encode enzymes acting in biosynthesis of cholesterol. Insects require dietary cholesterol to meet their cellular, physiological, developmental, and reproductive needs. Looking at a broader view of invertebrate biochemistry, most protostomes lost all or most genes involved in cholesterol biosynthesis. The massive gene loss occurred during the Ediacaran Period, which lasted 96 million years, from the end of the Cryogenian Period (635 million years ago; MYA) to the beginning of the Cambrian Period (538.6 MYA). The key point here is that the inability to biosynthesize cholesterol is not limited to insects; it occured in most protostomes. We address the protostome need and benefits of acquiring exogenous sterols.

Bioactive volatile compounds from Penicillium digitatum-infected apples: Oviposition attractants for yellow peach moth Conogethes punctiferalis (Lepidoptera: Crambidae)

Introduction

Plant-associated microbes critically shape the dynamics of plant-and insect-associated communities. In previous studies, we reported that the yellow peach moth Conogethes punctiferalis (YPM) preferred to Penicillium digitatum-infected apples (PDA) for oviposition. However, the underlying mechanisms remains unclear.

Methods

In the present study, the behavioral and physiological experiments were conducted to determine how P. digitatum affects the oviposition selection of mated YPM females via altering host plant volatile organic compounds (VOCs).

Results

Mated YPM females were attracted to and laid more eggs on PDA than on non-infected apples (NIA), mechanically damaged apples (MDA), and P. digitatum in potato dextrose agar medium (PPD) in the oviposition selection experiments. Four-arm olfactometer assays further confirmed that odors in PDA were responsible for the attractiveness of mated YPM females. Further analyses showed that 38 VOCs were collected and identified from all treatments by GC-MS, with five specific VOCs (methyl 2-methylbutyrate, styrene, methyl caproate, butyl caprylate, and n-tetradecane) emitting from PDA. A principal component analysis (PCA) based on the absolute contents of 38 VOCs revealed a clear separation of PDA from NIA, MDA, and PPD. Moreover, when P. digitatum-induced specific VOCs were added to apples in individual or synthetic blends, there was a significantly higher percentage of mated YPM females to apples with individual or synthetic blends consisting of methyl 2-methylbutyrate, butyl caprylate, or n-tetradecane in Y-tube olfactometer experiments, suggesting that these three specific VOCs acted as predominant olfactory signals for mated YPM females to PDA.

Discussion

Taken together, the microbe P. digitatum was an important driver of the interactions between YPMs and host plants by altering plant volatiles. These findings may form the basis for developing attractant baits for field trapping YPMs in the future.

Penicillium fungi mediate behavioral responses of the yellow peach moth, Conogethes punctiferalis (Guenée) to apple fruits via altering the emissions of host plant VOCs

Plant-associated microbes have been reported as important but overlooked drivers of plant-herbivorous insect interactions. Influence of plant-associated microbes on plant-insect interactions is diverse, including beneficial, detrimental, and neutral. Here, we determined the effects of three Penicillium fungi, including Penicillium citrinum, Penicillium sumatrense, and Penicillium digitatum, on the oviposition selection and behavior of the yellow peach moth (YPM), Conogethes punctiferalis (Guenée). Compared with fungi noninfected apples (NIA), mechanically damaged apples (MDA), and P. citrinum in potato dextrose agar medium (PC), the oviposition selection and four-arm olfactometer experiments both showed that mated YPM females preferred to P. citrinum-infected apples (PCA). For P. sumatrense or P. digitatum, we also found that mated YPM females preferred to P. sumatrense-infected apples (PSA) or P. digitatum-infected apples (PDA), respectively. Among three Penicillium fungi-infected apples, the selection rates including oviposition and olfactometer behavior of mated YPM females on PDA were both higher than those on PSA and PCA. Further analyses of host plant volatile organic compounds (VOCs) by GC-MS showed that the absolute contents of ethyl hexanoate and (Z, E)-α-farnesene in PCA, PSA, and PDA were all higher than those in NIA, and a total of 16 novel VOCs were detected in fungi-infected apples (PCA, PSA, and PDA), indicating that fungi infection changed the components and proportions of apple VOCs. Taken together, three Penicillium fungi play significant roles in mediating the host selection of YPMs via altering the emissions of VOCs. These findings will be beneficial for developing formulations for field trapping of YPMs in the future.

Changes in the phyllosphere and rhizosphere microbial communities of soybean in the presence of pathogens

Soybean (Glycine max L.) is host to an array of foliar- and root-infecting pathogens that can cause significant yield losses. To provide insights into the roles of microorganisms in disease development, we evaluated the bacterial and fungal communities associated with the soybean rhizosphere and phyllosphere. For this, leaf and soil samples of healthy, Phytophthora sojae-infected and Septoria glycines-infected plants were sampled at three stages during the production cycle, and then subjected to 16S and Internal Transcribed Spacer (ITS) amplicon sequencing. The results indicated that biotic stresses did not have a significant impact on species richness and evenness regardless of growth stage. However, the structure and composition of soybean microbial communities were dramatically altered by biotic stresses, particularly for the fungal phyllosphere. Additionally, we cataloged a variety of microbial genera that were altered by biotic stresses and their associations with other genera, which could serve as biological indicators for disease development. In terms of soybean development, the rhizosphere and phyllosphere had distinct microbial communities, with the fungal phyllosphere most influenced by growth stage. Overall, this study characterized the phyllosphere and rhizosphere microbial communities of soybean, and described the impact of pathogen infection and plant development in shaping these bacterial and fungal communities.

Into the Wild: Evidence for the Enemy Release Hypothesis in the Invasive Callery Pear (Pyrus calleryana) (Rosales: Rosaceae)

Wild Callery pear (Pyrus calleryana Decne.) results from a cross between various cultivars of P. calleryana and any other Pyrus individual. While many cultivars of this species are still commercially produced and sold for horticultural purposes in the United States, Callery pear is a detrimental invasive species that encroaches on many managed and natural areas, damages equipment and injures people, pets, and livestock with its thorny branches, and likely causes detrimental ecological impacts. Despite its importance as an invasive species, the mechanisms behind Callery pear's invasion and spread are unclear. To identify potential drivers of invasion, we quantified feeding of generalist and specialist herbivores on Callery pear and four native tree species, based on insect host ranges, with choice and no-choice experiments followed by field surveys of herbivory on these same tree species. Feeding by all herbivores was lower on Callery pear than on native tree species in no-choice assays. Specifically, feeding on Callery pear was moderate by generalists and very low by specialists. Specialist feeding on Callery pear was comparable to native species in choice assays but was significantly reduced in no-choice assays. Reduced specialist feeding along with moderate generalist feeding on Callery pear in the field provides evidence for the Enemy Release Hypothesis as a potential driving mechanism behind its invasion success.

Fungi as mediators linking organisms and ecosystems

Fungi form a major and diverse component of most ecosystems on Earth. They are both micro and macroorganisms with high and varying functional diversity as well as great variation in dispersal modes. With our growing knowledge of microbial biogeography, it has become increasingly clear that fungal assembly patterns and processes differ from other microorganisms such as bacteria, but also from macroorganisms such as plants. The success of fungi as organisms and their influence on the environment lies in their ability to span multiple dimensions of time, space, and biological interactions, that is not rivalled by other organism groups. There is also growing evidence that fungi mediate links between different organisms and ecosystems, with the potential to affect the macroecology and evolution of those organisms. This suggests that fungal interactions are an ecological driving force, interconnecting different levels of biological and ecological organisation of their hosts, competitors, and antagonists with the environment and ecosystem functioning. Here we review these emerging lines of evidence by focusing on the dynamics of fungal interactions with other organism groups across various ecosystems. We conclude that the mediating role of fungi through their complex and dynamic ecological interactions underlie their importance and ubiquity across Earth's ecosystems.

Relationships between the Pathogen Erysiphe alphitoides, the Phytophagous Mite Schizotetranychus garmani (Acari: Tetranychidae) and the Predatory Mite Euseius finlandicus (Acari: Phytoseiidae) in Oak

Simple Summary

Knowledge about the relationships between plant pathogens, arthropods, and their natural enemies is scarce. We studied the relationships between the plant fungal pathogen, Erysiphe alphitoides, the phytophagous mite Schizotetranychus garmani, and the predatory mite Euseius finlandicus in leaves of pedunculate oak. In June, July and August 2016, in 30 trees located in three forests near Belgrade, Serbia, the presence of E. alphitoides, S. garmani and E. finlandicus was assessed. The occurrence of E. alphitoides was high where the population of S. garmani was high. However, the presence of the leaf pathogen E. alphitoides was not related to the amount of the predatory mite E. finlandicus. The relationships between powdery mildew and the two mite species were stable across time and space, and the presence of one mite was not influenced by the presence of the other mite.

Abstract

Food webs on forest trees include plant pathogens, arthropods, and their natural enemies. To increase the understanding of the impact of a plant pathogen on herbivore-natural enemy interactions, we studied the powdery mildew fungus Erysiphe alphitoides, the phytophagous mite Schizotetranychus garmani, and the predatory and mycophagous mite Euseius finlandicus in pedunculate oak (Quercus robur) leaves. In June, July and August of 2016, we assessed the severity of powdery mildew, mite population density and adult female mite size in 30 trees in three forests near Belgrade, Serbia. In August, the infection severity of E. alphitoides related positively to the population density of S. garmani and negatively to the body size of S. garmani females. Throughout the vegetative season, the infection severity of E. alphitoides related positively to the population density of E. finlandicus but not to its body size. The effect of E. alphitoides on the population density and adult size of S. garmani was not mediated by the population density of E. finlandicus, and vice versa. Interactions were consistent in all forests and varied with the summer month. Our findings indicate that E. alphitoides can influence the average body size and population densities of prey and predatory mites studied, irrespective of predator-prey relationships.

Defining gut mycobiota for wild animals: a need for caution in assigning authentic resident fungal taxa

Animal gut mycobiota, the community of fungi that reside within the gastrointestinal tract, make an important contribution to host health. Accordingly, there is an emerging interest to quantify the gut mycobiota of wild animals. However, many studies of wild animal gut mycobiota do not distinguish between the fungi that likely can reside within animal gastrointestinal tracts from the fungal taxa that are non-residents, such as macrofungi, lichens or plant symbionts/pathogens that can be ingested as part of the host's diet. Confounding the non-resident and resident gut fungi may obscure attempts to identify processes associated with the authentic, resident gut mycobiota per se. To redress this problem, we propose some strategies to filter the taxa identified within an apparent gut mycobiota based on an assessment of host ecology and fungal traits. Consideration of the different sources and roles of fungi present within the gastrointestinal tract should facilitate a more precise understanding of the causes and consequences of variation in wild animal gut mycobiota composition.

Vector-borne plant pathogens modify top-down and bottom-up effects on insect herbivores

Ecological theory predicts that host-plant traits affect herbivore population growth rates, which in turn modulates predator-prey interactions. However, while vector-borne plant pathogens often alter traits of both host plants and vectors, a few studies have assessed how pathogens may act as interaction modifiers within tri-trophic food webs. By applying a food web motif framework, we assessed how a vector-borne plant pathogen (Pea-enation mosaic virus, PEMV) modified both bottom-up (plant-herbivore) and top-down (predator-prey) interactions. Specifically, we assessed trophic interactions with PEMV-infectious Acyrthosiphon pisum (pea aphid) vectors compared to non-infectious aphids in a factorial experiment that manipulated predator and plant communities. We show that PEMV altered bi-trophic relationships, whereby on certain plant species, PEMV reduced vector performance but also increased their susceptibility to predators. However, on other plant species, PEMV weakened top-down control or increased vector performance. Our results suggest that vector-borne plant pathogens are important interaction modifiers for plant-herbivore-predator dynamics: host-plant response to viruses can decrease herbivore abundance by reducing herbivore performance, but also increase herbivore abundance by weakening top-down control. Broadly speaking, trophic interactions that regulate herbivore outbreaks appear to be modified for herbivores actively transmitting viruses to host plants. Consequently, management and monitoring of outbreaking herbivores should consider the infection status of focal populations.

In the tripartite combination Botrytis cinerea-Arabidopsis-Eurydema oleracea, the fungal pathogen alters the plant-insect interaction via jasmonic acid signalling activation and inducible plant-emitted volatiles

In ecosystems, plants are continuously challenged by combined stress conditions more than by a single biotic or abiotic factor. Consequently, in recent years studies on plant relationships with multiple stresses have aroused increasing interest. Here, the impact of inoculation with fungal pathogens with different lifestyles on Arabidopsis plants response to the following infestation with the invasive crop pest Eurydema oleracea was investigated. In particular, as fungal pathogens the necrotroph Botrytis cinerea and the biotroph Golovinomyces orontii were used. Plants exposed to B. cinerea, but not to G. orontii, showed reduced herbivore feeding damage. This difference was associated to different hormonal pathways triggered by the pathogens: G. orontii only induced the salicylate-mediated pathway, while B. cinerea stimulated also the jasmonate-dependent signalling, which persisted for a long time providing a long-term defence to further herbivore attack. In particular, the lower susceptibility of B. cinerea-infected Arabidopsis plants to E. oleracea was related to the stimulation of the JA-induced pathway on the production of plant volatile compounds, since treatment with VOCs emitted by B. cinerea inoculated plants inhibited both insect plant choice and feeding damage. These results indicate that necrotrophic plant pathogenic fungi modulate host volatile emission, thus affecting plant response to subsequent insect, thereby increasing the knowledge on tripartite plant-microbe-insect interactions in nature.

Interspecific interactions within a vector-borne complex are influenced by a co-occurring pathosystem

Potato virus Y (PVY) and zebra chip (ZC) disease are major threats to solanaceous crop production in North America. PVY can be spread by aphid vectors and through vegetative propagation in potatoes. ZC is associated with "Candidatus Liberibacter solanacearum" (Lso), which is transmitted by the tomato/potato psyllid, Bactericera cockerelli Šulc (Hemiptera: Triozidae). As these two pathosystems may co-occur, we studied whether the presence of one virus strain, PVY°, affected the host preference, oviposition, and egg hatch rate of Lso-free or Lso-carrying psyllids in tomato plants. We also examined whether PVY infection influenced Lso transmission success by psyllids, Lso titer and plant chemistry (amino acids, sugars, and phytohormones). Lso-carrying psyllids showed a preference toward healthy hosts, whereas the Lso-free psyllids preferentially settled on the PVY-infected tomatoes. Oviposition of the Lso-carrying psyllids was lower on PVY-infected than healthy tomatoes, but Lso transmission, titer, and psyllid egg hatch were not significantly affected by PVY. The induction of salicylic acid and its related responses, and not nutritional losses, may explain the reduced attractiveness of the PVY-infected host to the Lso-carrying psyllids. Although our study demonstrated that pre-existing PVY infection can reduce oviposition by the Lso-carrying vector, the preference of the Lso-carrying psyllids to settle on healthy hosts could contribute to Lso spread to healthy plants in the presence of PVY infection in a field.