Metarhizium: jack of all trades, master of many

Virulence and proteomic responses of Metarhizium anisopliae against Aedes albopictus larvae

The tropical climate in Malaysia provides an ideal environment for the rapid proliferation of Aedes mosquitoes, notably Aedes aegypti and Aedes albopictus, prominent vectors of dengue fever. Alarmingly, these species are increasingly developing resistance to conventional pesticides. This study aimed to evaluate the efficacy of Metarhizium anisopliae isolate HSAH5 spores, specifically on conidia (CO) and blastospores (BL), against Ae. albopictus larvae. The study centered on evaluating their pathogenic effects and the resultant changes in protein expression. Spore suspensions with varying concentrations were prepared for larvicidal bioassays, and protein expressions were analysed using liquid chromatography-mass spectrometry. Subsequently, protein annotation and network analysis were conducted to elucidate infection mechanisms and the proteomic response. Based on the lethal concentrations and time frames, CO exhibited faster larval mortality than BL at lower concentrations. Despite this, both spore types demonstrated comparable overall pathogenic effects. Results from the proteomic profiling revealed 150 proteins with varied expressions following exposure to Ae. albopictus extract, shedding light on distinct infection strategies between the spores. Gene Ontology enrichment and network analysis illustrated the diverse metabolic adaptations of M. anisopliae and interactions with mosquito larvae. This highlighted the complexity of host-pathogen dynamics and the significance of biosynthetic processes, energy storage, and cellular interaction pathways in disease progression. The BL network, consisting 80 proteins and 74 connections, demonstrates the intricate fungal mechanisms triggered by host stimuli. Conversely, the CO network, though smaller, displayed notable interconnectivity and concentrated involvement at the cell periphery, suggesting a deliberate strategy for initial host contact. This study offers valuable insights into proteome dynamics of M. anisopliae's BL and CO for managing mosquito populations and combating disease transmission, thereby significantly advancing public health and environmental conservation efforts.

Microbial agents for the control of ticks Rhipicephalus microplus

Ticks are ectoparasites responsible for the transmission of various pathogens to vertebrates. They represent one of the major threats to livestock production worldwide due to their impact on the health, production and welfare of livestock destined for human consumption. The development of resistance to the main families of ixodicides used for their control has led to the search for new alternatives, where microbial control is an option. The use of microbial control agents against the tick Rhipicephalus microplus is reviewed in this paper. Bacteria such as Bacillus thuringiensis, Serratia marcescens and Staphylococcus spp. the nematodes Steinernema spp. and Heterorhabditis spp. as well as the fungi Metarhizium anisopliae and Beauveria bassiana are the most studied organisms for use as biological control agents against ticks. Laboratory, stable and field trials with free-living and parasitised ticks have shown that microbial agents can control both susceptible and ixodicide-resistant tick populations. However, multidisciplinary studies using novel tools like genomics, transcriptomics and proteomics should be carried out to understand the virulence factors which microbial agents use to induce pathogenesis and virulence in ticks. In addition, applied research will be carried out with the aim of improving techniques for large-scale application, as well as the improvement of cultivation, storage, formulation and application methods.

What are the 100 most cited fungal genera?

The global diversity of fungi has been estimated between 2 to 11 million species, of which only about 155 000 have been named. Most fungi are invisible to the unaided eye, but they represent a major component of biodiversity on our planet, and play essential ecological roles, supporting life as we know it. Although approximately 20 000 fungal genera are presently recognised, the ecology of most remains undetermined. Despite all this diversity, the mycological community actively researches some fungal genera more commonly than others. This poses an interesting question: why have some fungal genera impacted mycology and related fields more than others? To address this issue, we conducted a bibliometric analysis to identify the top 100 most cited fungal genera. A thorough database search of the Web of Science, Google Scholar, and PubMed was performed to establish which genera are most cited. The most cited 10 genera are Saccharomyces, Candida, Aspergillus, Fusarium, Penicillium, Trichoderma, Botrytis, Pichia, Cryptococcus and Alternaria. Case studies are presented for the 100 most cited genera with general background, notes on their ecology and economic significance and important research advances. This paper provides a historic overview of scientific research of these genera and the prospect for further research. Citation: Bhunjun CS, Chen YJ, Phukhamsakda C, Boekhout T, Groenewald JZ, McKenzie EHC, Francisco EC, Frisvad JC, Groenewald M, Hurdeal VG, Luangsa-ard J, Perrone G, Visagie CM, Bai FY, Błaszkowski J, Braun U, de Souza FA, de Queiroz MB, Dutta AK, Gonkhom D, Goto BT, Guarnaccia V, Hagen F, Houbraken J, Lachance MA, Li JJ, Luo KY, Magurno F, Mongkolsamrit S, Robert V, Roy N, Tibpromma S, Wanasinghe DN, Wang DQ, Wei DP, Zhao CL, Aiphuk W, Ajayi-Oyetunde O, Arantes TD, Araujo JC, Begerow D, Bakhshi M, Barbosa RN, Behrens FH, Bensch K, Bezerra JDP, Bilański P, Bradley CA, Bubner B, Burgess TI, Buyck B, Čadež N, Cai L, Calaça FJS, Campbell LJ, Chaverri P, Chen YY, Chethana KWT, Coetzee B, Costa MM, Chen Q, Custódio FA, Dai YC, Damm U, de Azevedo Santiago ALCM, De Miccolis Angelini RM, Dijksterhuis J, Dissanayake AJ, Doilom M, Dong W, Alvarez-Duarte E, Fischer M, Gajanayake AJ, Gené J, Gomdola D, Gomes AAM, Hausner G, He MQ, Hou L, Iturrieta-González I, Jami F, Jankowiak R, Jayawardena RS, Kandemir H, Kiss L, Kobmoo N, Kowalski T, Landi L, Lin CG, Liu JK, Liu XB, Loizides M, Luangharn T, Maharachchikumbura SSN, Makhathini Mkhwanazi GJ, Manawasinghe IS, Marin-Felix Y, McTaggart AR, Moreau PA, Morozova OV, Mostert L, Osiewacz HD, Pem D, Phookamsak R, Pollastro S, Pordel A, Poyntner C, Phillips AJL, Phonemany M, Promputtha I, Rathnayaka AR, Rodrigues AM, Romanazzi G, Rothmann L, Salgado-Salazar C, Sandoval-Denis M, Saupe SJ, Scholler M, Scott P, Shivas RG, Silar P, Souza-Motta CM, Silva-Filho AGS, Spies CFJ, Stchigel AM, Sterflinger K, Summerbell RC, Svetasheva TY, Takamatsu S, Theelen B, Theodoro RC, Thines M, Thongklang N, Torres R, Turchetti B, van den Brule T, Wang XW, Wartchow F, Welti S, Wijesinghe SN, Wu F, Xu R, Yang ZL, Yilmaz N, Yurkov A, Zhao L, Zhao RL, Zhou N, Hyde KD, Crous PW (2024). What are the 100 most cited fungal genera? Studies in Mycology 108: 1-411. doi: 10.3114/sim.2024.108.01.

Distinct metabolites affect the phloem fungal communities in ash trees (Fraxinus spp.) native and nonnative to the highly invasive emerald ash borer (AGRILUS PLANIPENNIS)

Emerald ash borer (EAB, Agrilus planipennis) is an invasive killer of ash trees (Fraxinus spp.) in North America and Europe. Ash species co-evolved with EAB in their native range in Asia are mostly resistant, although the precise mechanism(s) remain unclear. Very little is also known about EAB or ash tree microbiomes. We performed the first joint comparison of phloem mycobiome and metabolites between a native and a nonnative ash species, infested and uninfested with EAB, in conjunction with investigation of larval mycobiome. Phloem mycobiome communities differed between the tree species, but both were unaffected by EAB infestation. Several indicator taxa in the larval gut shared a similarly high relative abundance only with the native host trees. Widely targeted metabolomics revealed 24 distinct metabolites in native trees and 53 metabolites in nonnative trees, respectively, that differed in relative content between infested and uninfested trees only in one species. Interestingly, four metabolites shared a strong relationship with the phloem mycobiomes, majority of which affected only the native trees. Collectively, our results demonstrate a complex interplay between host tree chemistry and mycobiome, and suggest the shared relationships between the mycobiomes of the native host tree and EAB may reflect their shared co-evolution.

Unraveling the interaction of co-encapsulated Saccharomyces cerevisiae and Metarhizium brunneum in calcium alginate-based attract-and-kill beads

BACKGROUND

Attract-and-kill (AK) beads are biological, microbial insecticides developed as an alternative to synthetic soil insecticides. For wireworm control, beads are based on calcium alginate/starch co-encapsulating the carbon dioxide (CO2) producing yeast Saccharomyces cerevisiae H205 as the attract component, and the entomopathogenic fungus Metarhizium brunneum CB15-III as the kill component. However, the physicochemical processes inside beads during co-cultivation are still unclear. Here we reveal for the first time the spatiotemporal conditions of oxygen and pH inside AK beads measured with microelectrodes and describe the impact of S. cerevisiae on CO2 and conidia formation.

RESULTS

Measurements revealed a steep oxygen gradient already 2 days after co-encapsulation, with an internal hypoxic zone. Encapsulating either S. cerevisiae or M. brunneum already decreased the average pH from 5.5 to 4.7 and 4.6, respectively. However, on day 3, co-cultivation lead to temporal strong acidification of beads down to pH 3.6 which followed the maximum CO2 productivity and coincided with the maximum conidiation rate. Decreasing the yeast load decreased the total CO2 productivity to half, and the conidial production by 93%, while specific productivities normalized to 1% yeast load increased eight-fold and three-fold, respectively, with day 3 being an exception.

CONCLUSION

Our findings indicate a general beneficial interaction between M. brunneum and S. cerevisiae, but also suggest competition for resources. These findings will contribute to develop innovative co-formulations with maximum efficiency to save application rates and costs. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Thermal ecology shapes disease outcomes of entomopathogenic fungi infecting warm-adapted insects

The thermal environment is a critical determinant of outcomes in host-pathogen interactions, yet the complexities of this relationship remain underexplored in many ecological systems. We examined the Thermal Mismatch Hypothesis (TMH) by measuring phenotypic variation in individual thermal performance profiles using a model system of two species of entomopathogenic fungi (EPF) that differ in their ecological niche, Metarhizium brunneum and M. flavoviride, and a warm-adapted model host, the mealworm Tenebrio molitor. We conducted experiments across ecologically relevant temperatures to determine the thermal performance curves for growth and virulence, measured as % survival, identify critical thresholds for these measures, and elucidate interactive host-pathogen effects. Both EPF species and the host exhibited a shared growth optima at 28 °C, while the host's growth response was moderated in sublethal pathogen infections that depended on fungus identity and temperature. However, variances in virulence patterns were different between pathogens. The fungus M. brunneum exhibited a broader optimal temperature range (23-28 °C) for virulence than M. flavoviride, which displayed a multiphasic virulence-temperature relationship with distinct peaks at 18 and 28 °C. Contrary to predictions of the TMH, both EPF displayed peak virulence at the host's optimal temperature (28 °C). The thermal profile for M. brunneum aligned more closely with that of T. molitor than that for M. flavoviride. Moreover, the individual thermal profile of M. flavoviride closely paralleled its virulence thermal profile, whereas the virulence thermal profile of M. brunneum did not track with its individual thermal performance. This suggests an indirect, midrange (23 °C) effect, where M. brunneum virulence exceeded growth. These findings suggest that the evolutionary histories and ecological adaptations of these EPF species have produced distinct thermal niches during the host interaction. This study contributes to our understanding of thermal ecology in host-pathogen interactions, underpinning the ecological and evolutionary factors that shape infection outcomes in entomopathogenic fungi. The study has ecological implications for insect population dynamics in the face of a changing climate, as well as practically for the use of these organisms in biological control.

Fungi rather than bacteria drive early mass loss from fungal necromass regardless of particle size

Microbial necromass is increasingly recognized as an important fast-cycling component of the long-term carbon present in soils. To better understand how fungi and bacteria individually contribute to the decomposition of fungal necromass, three particle sizes (>500, 250-500, and <250 μm) of Hyaloscypha bicolor necromass were incubated in laboratory microcosms inoculated with individual strains of two fungi and two bacteria. Decomposition was assessed after 15 and 28 days via necromass loss, microbial respiration, and changes in necromass pH, water content, and chemistry. To examine how fungal-bacterial interactions impact microbial growth on necromass, single and paired cultures of bacteria and fungi were grown in microplates containing necromass-infused media. Microbial growth was measured after 5 days through quantitative PCR. Regardless of particle size, necromass colonized by fungi had higher mass loss and respiration than both bacteria and uninoculated controls. Fungal colonization increased necromass pH, water content, and altered chemistry, while necromass colonized by bacteria remained mostly unaltered. Bacteria grew significantly more when co-cultured with a fungus, while fungal growth was not significantly affected by bacteria. Collectively, our results suggest that fungi act as key early decomposers of fungal necromass and that bacteria may require the presence of fungi to actively participate in necromass decomposition.

Microbial pesticides - challenges and future perspectives for testing and safety assessment with respect to human health

Plant protection measures are necessary to prevent pests and diseases from attacking and destroying crop plants and to meet consumer demands for agricultural produce. In the last decades the use of chemical pesticides has largely increased. Farmers are looking for alternatives. Biopesticides should be considered a sustainable solution. They may be less toxic than chemical pesticides, be very specific to the target pest, decompose quickly, and be less likely to cause resistance. On the other hand, lower efficacy and higher costs are two disadvantages of many biopesticides. Biopesticides include macroorganisms, natural compounds and microorganisms. Microbial pesticides are the most widely used and studied class of biopesticides. The greatest difference between microbial and chemical pesticides is the ability of the former to potentially multiply in the environment and on the crop plant after application. The data requirements for the European Union and the United States Environmental Protection Agency are highlighted, as these regulatory processes are the most followed in regions where local regulations for biopesticide products are not available or vague. New Approach Methods already proposed or harmonized for chemical pesticides are presented and discussed with respect to their use in evaluating microbial pesticide formulations. Evaluating the microbials themselves is not as simple as using the same validated New Approach Methods as for synthetic pesticides. Therefore, the authors suggest considering New Approach Method strategies specifically for microbials and global harmonization with acceptability with the advancements of such approaches. Further discussion is needed and greatly appreciated by the experts.

Zinc solubilization and organic acid production by the entomopathogenic fungus, Metarhizium pingshaense sheds light on its key ecological role in the environment

We report for the first-time higher zinc (Zn) solubilization efficiency and plant growth promotion by an entomopathogenic fungus (EPF), Metarhizium pingshaense IISR-EPF-14, which was earlier isolated from Conogethes punctiferalis, a pest of global importance. The Zn solubilizing efficiency of the fungus varied depending on the type of insoluble source of Zn used, which was observed to be 1.6 times higher in Zn3(PO4)2-amended media compared to ZnO media. In liquid media, there was a 6.2-fold increase in available Zn in ZnO-amended media, whereas a 20.2-fold increase in available Zn was recorded in Zn3(PO4)2 medium. We ascribe the production of various organic acids such as gluconic, keto-gluconic, oxalic, tartaric, malonic, succinic and formic acids, which in general, interact with insoluble Zn sources and make them soluble by forming metal cations and displacing anions as the major mechanism for Zn solubilization by M. pingshaense. However, the type and amount of organic acid produced in the media varied depending on the source of Zn used and the incubation period. Application of the fungus alone and in combination with insoluble Zn sources enhanced various plant growth parameters in rice and cardamom plants. Moreover, the uptake of Zn in rice plants was enhanced up to ~2.5-fold by fungal application. The fungus also exhibited various other plant growth-promoting traits, such as production of Indole-3-acetic acid, ammonia, siderophores, solubilization of mineral phosphate, and production of hydrolytic enzymes such as α-amylase, protease, and pectinase. Hence, apart from its use as a biological control agent, M. pingshaense has the potential to be used as a bio-fortifier to enhance the solubilization and uptake of Zn from nutrient poor soils under field conditions. Our findings shed light on the broader ecological role played by this fungus and widen its scope for utilization in sustainable agriculture.

Two Iron(II), α-Ketoglutarate-Dependent Enzymes Encoded by the PPZ Gene Cluster of Metarhizium majus Enable Production of 8-Hydroxyperamine

Entomopathogenic fungus Metarhizium majus contains the nine-gene PPZ cluster, with ppzA, encoding a peramine-producing nonribosomal peptide synthetase, as the central component. In this work, the roles of two α-ketoglutarate, iron-dependent oxygenases encoded by the PPZ genes ppzC and ppzD were elucidated. PpzD was found to produce both trans-4-hydroxy-l-proline and trans-3-hydroxy-l-proline in a 13.1:1 ratio, yielding a key precursor for peramine biosynthesis. PpzC was found to act directly on peramine, yielding the novel analogue 8-hydroxyperamine.

Soil bacteria and fungi communities are shaped by elevation influences in Colombian forest and páramo natural ecosystems

The influence of elevation on natural terrestrial ecosystems determines the arrangements of microbial communities in soils to be associated with biotic and abiotic factors. To evaluate changes of fungi and bacteria at the community level along an elevational gradient (between 1000 and 3800 m.a.s.l.), physicochemical measurements of soils, taxonomic identifications of plants, and metabarcoding sequences of the 16S rRNA gene for bacteria and the ITS1 region for fungi were obtained. The bacterial taxonomic composition showed that Acidobacteriota increased in abundance with elevation, while Actinobacteriota and Verrucomicrobiota decreased. Furthermore, Firmicutes and Proteobacteria maintained maximum levels of abundance at intermediate elevations (1200 and 2400 m.a.s.l.). In fungi, Ascomycota was more abundant at higher elevations, Basidiomycota tended to dominate at lower elevations, and Mortierellomycota had a greater presence at intermediate sites. These results correlated with the edaphic parameters of decreasing pH and increasing organic carbon and available nitrogen with elevation. In addition, the Shannon index found a greater diversity in bacteria than fungi, but both showed a unimodal pattern with maximum values in the Andean Forest at 2400 m.a.s.l. Through the microbial characterization of the ecosystems, the elevational gradient, soil properties, and vegetation were found to exert significant effects on microbial communities and alpha diversity indices. We conclude that the most abundant soil microorganisms at the sampling points differed in abundance and diversity according to the variations in factors influencing ecological communities.

Coevolution of a generalist pathogen with many hosts: the case of the amphibian chytrid Batrachochytrium dendrobatidis

Generalist pathogens maintain infectivity in numerous hosts; how this broad ecological niche impacts host-pathogen coevolution remains to be widely explored. Batrachochytrium dendrobatidis (Bd) is a highly generalist pathogenic fungus that has caused devastating declines in hundreds of amphibian species worldwide. This review examines amphibian chytridiomycosis host-pathogen interactions and available evidence for coevolution between Bd and its numerous hosts. We summarize recent evidence showing that Bd genotypes vary in geographic distribution and virulence, and that amphibian species also vary in Bd susceptibility according to their geographic distribution. How much variation can be explained by phenotypic plasticity or genetic differences remains uncertain. Recent research suggests that Bd genotypes display preferences for specific hosts and that some hosts are undergoing evolution as populations rebound from Bd outbreaks. Taken together, these findings suggest the potential for coevolution to occur and illuminate a path for addressing open questions through integrating historical and contemporary genetic data.

Frequent horizontal chromosome transfer between asexual fungal insect pathogens

Entire chromosomes are typically only transmitted vertically from one generation to the next. The horizontal transfer of such chromosomes has long been considered improbable, yet gained recent support in several pathogenic fungi where it may affect the fitness or host specificity. To date, it is unknown how these transfers occur, how common they are, and whether they can occur between different species. In this study, we show multiple independent instances of horizontal transfers of the same accessory chromosome between two distinct strains of the asexual entomopathogenic fungus Metarhizium robertsii during experimental co-infection of its insect host, the Argentine ant. Notably, only the one chromosome-but no other-was transferred from the donor to the recipient strain. The recipient strain, now harboring the accessory chromosome, exhibited a competitive advantage under certain host conditions. By phylogenetic analysis, we further demonstrate that the same accessory chromosome was horizontally transferred in a natural environment between M. robertsii and another congeneric insect pathogen, Metarhizium guizhouense. Hence, horizontal chromosome transfer is not limited to the observed frequent events within species during experimental infections but also occurs naturally across species. The accessory chromosome that was transferred contains genes that may be involved in its preferential horizontal transfer or support its establishment. These genes encode putative histones and histone-modifying enzymes, as well as putative virulence factors. Our study reveals that both intra- and interspecies horizontal transfer of entire chromosomes is more frequent than previously assumed, likely representing a not uncommon mechanism for gene exchange.

Organic farming promotes the abundance of fungi keystone taxa in bacteria-fungi interkingdom networks

Soil bacteria-fungi interactions are essential in the biogeochemical cycles of several nutrients, making these microbes major players in agroecosystems. While the impact of the farming system on microbial community composition has been extensively reported in the literature, whether sustainable farming approaches can promote associations between bacteria and fungi is still unclear. To study this, we employed 16S, ITS, and 18S DNA sequencing to uncover how microbial interactions were affected by conventional and organic farming systems on maize crops. The Bray-Curtis index revealed that bacterial, fungal, and arbuscular mycorrhizal fungi communities were significantly different between the two farming systems. Several taxa known to thrive in healthy soils, such as Nitrosophaerales, Orbiliales, and Glomus were more abundant in the organic farming system. Constrained ordination revealed that the organic farming system microbial community was significantly correlated with the β-glucosidase activity, whereas the conventional farming system microbial community significantly correlated with soil pH. Both conventional and organic co-occurrence interkingdom networks exhibited a parallel node count, however, the former had a higher number of edges, thus being denser than the latter. Despite the similar amount of fungal nodes in the co-occurrence networks, the organic farming system co-occurrence network exhibited more than 3-fold the proportion of fungal taxa as keystone nodes than the conventional co-occurrence network. The genera Bionectria, Cercophora, Geastrum, Penicillium, Preussia, Metarhizium, Myceliophthora, and Rhizophlyctis were among the fungal keystone nodes of the organic farming system network. Altogether, our results uncover that beyond differences in microbial community composition between the two farming systems, fungal keystone nodes are far more relevant in the organic farming system, thus suggesting that bacteria-fungi interactions are more frequent in organic farming systems, promoting a more functional microbial community.

Fungal infection of insects: molecular insights and prospects

Entomopathogenic fungi (EPF) distribute in different fungal phyla with variable host ranges and play essential role in regulating insect populations by infecting hosts via cuticle penetration. The representative ascomycete EPF of Metarhizium and Beauveria species have been widely used in mechanistic investigations of fungus-insect interactions and as ecofriendly mycoinsecticides. Here, we review the function of diverse genes, pathways, and secondary metabolites associated with EPF stepwise infections. In particular, emerging evidence has shown that EPF have to outcompete insect ectomicrobiotas prior to penetrating cuticles, and subvert or evade host antifungal immunity by using effector-like proteins and chemicals like plant pathogens. Future prospects are discussed for a better understanding of fungal pathobiology, which will provide novel insights into microbe-animal interactions.

Manipulating multi-level selection in a fungal entomopathogen reveals social conflicts and a method for improving biocontrol traits

Changes in parasite virulence are commonly expected to lead to trade-offs in other life history traits that can affect fitness. Understanding these trade-offs is particularly important if we want to manipulate the virulence of microbial biological control agents. Theoretically, selection across different spatial scales, i.e. between- and within-hosts, shapes these trade-offs. However, trade-offs are also dependent on parasite biology. Despite their applied importance the evolution of virulence in fungal parasites is poorly understood: virulence can be unstable in culture and commonly fails to increase in simple passage experiments. We hypothesized that manipulating selection intensity at different scales would reveal virulence trade-offs in a fungal pathogen of aphids, Akanthomyces muscarius. Starting with a genetically diverse stock we selected for speed of kill, parasite yield or infectivity by manipulating competition within and between hosts and between-populations of hosts over 7 rounds of infection. We characterized ancestral and evolved lineages by whole genome sequencing and by measuring virulence, growth rate, sporulation and fitness. While several lineages showed increases in virulence, we saw none of the trade-offs commonly found in obligately-killing parasites. Phenotypically similar lineages within treatments often shared multiple single-nucleotide variants, indicating strong convergent evolution. The most dramatic phenotypic changes were in timing of sporulation and spore production in vitro. We found that early sporulation led to reduced competitive fitness but could increase yield of spores on media, a trade-off characteristic of social conflict. Notably, the selection regime with strongest between-population competition and lowest genetic diversity produced the most consistent shift to early sporulation, as predicted by social evolution theory. Multi-level selection therefore revealed social interactions novel to fungi and showed that these biocontrol agents have the genomic flexibility to improve multiple traits-virulence and spore production-that are often in conflict in other parasites.

Plant-endophyte communication: Scaling from molecular mechanisms to ecological outcomes

Diverse communities of fungal endophytes reside in plant tissues, where they affect and are affected by plant physiology and ecology. For these intimate interactions to form and persist, endophytes and their host plants engage in intricate systems of communication. The conversation between fungal endophytes and plant hosts ultimately dictates endophyte community composition and function and has cascading effects on plant health and plant interactions. In this review, we synthesize our current knowledge on the mechanisms and strategies of communication used by endophytic fungi and their plant hosts. We discuss the molecular mechanisms of communication that lead to organ specificity of endophytic communities and distinguish endophytes, pathogens, and saprotrophs. We conclude by offering emerging perspectives on the relevance of plant-endophyte communication to microbial community ecology and plant health and function.

Fungal infection alters collective nutritional intake of ant colonies

In animals, parasitic infections impose significant fitness costs.1,2,3,4,5,6 Infected animals can alter their feeding behavior to resist infection,7,8,9,10,11,12 but parasites can manipulate animal foraging behavior to their own benefits.13,14,15,16 How nutrition influences host-parasite interactions is not well understood, as studies have mainly focused on the host and less on the parasite.9,12,17,18,19,20,21,22,23 We used the nutritional geometry framework24 to investigate the role of amino acids (AA) and carbohydrates (C) in a host-parasite system: the Argentine ant, Linepithema humile, and the entomopathogenic fungus, Metarhizium brunneum. First, using 18 diets varying in AA:C composition, we established that the fungus performed best on the high-amino-acid diet 1:4. Second, we found that the fungus reached this optimal diet when given various diet pairings, revealing its ability to cope with nutritional challenges. Third, we showed that the optimal fungal diet reduced the lifespan of healthy ants when compared with a high-carbohydrate diet but had no effect on infected ants. Fourth, we revealed that infected ant colonies, given a choice between the optimal fungal diet and a high-carbohydrate diet, chose the optimal fungal diet, whereas healthy colonies avoided it. Lastly, by disentangling fungal infection from host immune response, we demonstrated that infected ants foraged on the optimal fungal diet in response to immune activation and not as a result of parasite manipulation. Therefore, we revealed that infected ant colonies chose a diet that is costly for survival in the long term but beneficial in the short term-a form of collective self-medication.

Effect of CO2 Concentrations on Entomopathogen Fitness and Insect-Pathogen Interactions

Numerous insect species and their associated microbial pathogens are exposed to elevated CO2 concentrations in both artificial and natural environments. However, the impacts of elevated CO2 on the fitness of these pathogens and the susceptibility of insects to pathogen infections are not well understood. The yellow mealworm, Tenebrio molitor, is commonly produced for food and feed purposes in mass-rearing systems, which increases risk of pathogen infections. Additionally, entomopathogens are used to control T. molitor, which is also a pest of stored grains. It is therefore important to understand how elevated CO2 may affect both the pathogen directly and impact on host-pathogen interactions. We demonstrate that elevated CO2 concentrations reduced the viability and persistence of the spores of the bacterial pathogen Bacillus thuringiensis. In contrast, conidia of the fungal pathogen Metarhizium brunneum germinated faster under elevated CO2. Pre-exposure of the two pathogens to elevated CO2 prior to host infection did not affect the survival probability of T. molitor larvae. However, larvae reared at elevated CO2 concentrations were less susceptible to both pathogens compared to larvae reared at ambient CO2 concentrations. Our findings indicate that whilst elevated CO2 concentrations may be beneficial in reducing host susceptibility in mass-rearing systems, they may potentially reduce the efficacy of the tested entomopathogens when used as biological control agents of T. molitor larvae. We conclude that CO2 concentrations should be carefully selected and monitored as an additional environmental factor in laboratory experiments investigating insect-pathogen interactions.

Metarhizium pingshaense photolyase expression and virulence to Rhipicephalus microplus after UV-B exposure

The current study examined the impact of ultraviolet (UV)-B radiation in Metarhizium pingshaense blastospores' photolyase expression and their virulence against Rhipicephalus microplus. Blastospores were exposed to UV under laboratory and field conditions. Ticks were treated topically with fungal suspension and exposed to UV-B in the laboratory for three consecutive days. The expression of cyclobutane pyrimidine dimmers (CPDs)-photolyase gene maphr1-2 in blastospores after UV exposure followed by white light exposure was accessed after 0, 8, 12, 24, 36, and 48 h. Average relative germination of blastospores 24 h after in vitro UV exposure was 8.4% lower than 48 h. Despite this, the relative germination of blastospores exposed to UV in the field 18 h (95.7 ± 0.3%) and 28 h (97.3 ± 0.8%) after exposure were not different (p > 0.05). Ticks treated with fungus and not exposed to UV exhibited 0% survival 10 days after the treatment, while fungus-treated ticks exposed to UV exhibited 50 ± 11.2% survival. Expression levels of maphr1-2 8, 12, and 24 h after UV-B exposure were not different from time zero. Maphr1-2 expression peak in M. pingshaense blastospores occurred 36 h after UV-B exposure, in the proposed conditions and times analyzed, suggesting repair mechanisms other than CPD-mediated-photoreactivation might be leading blastospores' germination from 0 to 24 h.

Subterranean termites raise the alarm when their anti-fungal weapon falters

Termicin is an anti-fungal defensin that is disseminated from termite salivary glands. The peptide appears to be critical for the elimination with mutual grooming (allogrooming) of pathogenic spores (conidia) that have attached to the insect cuticle. There has been a recent selective sweep for an advantageous variant of this peptide in the subterranean termite Reticulitermes flavipes. We tested the anti-mycotic activity of a recombinant termicin corresponding with this variant against the conidia of different Metarhizium fungal isolates from soil close to foraging R. flavipes workers. Termicin was most effective against isolates that had previously been shown to elicit a relatively weak alarm response, as indicated by brief bouts of rapid longitudinal oscillatory movement (LOM). These isolates that elicited weak alarm were also the deadliest apparently because the survival of termites exposed to the fungus depends on a strong social immune response (LOMs and allogrooming). The selective pressure for a single termicin variant may have been driven by the most dangerous isolates that elicit a weak behavioral response. The correlation between termicin anti-fungal activity and LOM suggests that pathogen-associated molecular patterns that affect termite recognition of conidial contamination and the onset of elevated allogrooming also affect the vulnerability of conidia to the disruption of their cell membranes by termicin.

Utilization of Metarhizium as an insect biocontrol agent and a plant bioinoculant with special reference to Brazil

Brazil has a long history of using biological control and has the largest program in sugarcane agriculture to which a biocontrol program has been applied. This achievement is at least partly due to the utilization of the entomopathogenic fungus Metarhizium. This well-known fungal genus exhibits pathogenicity against a broad range of arthropod hosts and has been used globally as a biocontrol agent. This fungus is also a root symbiont, and in this capacity, it is a plant growth promoter. However, this feature (i.e., as a plant symbiont) has yet to be fully explored and implemented in Brazil, although the number of reports demonstrating Metarhizium's utility as a plant bioinoculant is increasing. The Brazilian bioproduct industry targets agricultural pests, and is limited to two Metarhizium species represented by four fungal isolates as active ingredients. Entomopathogenic fungi have also been successful in controlling arthropods of public health concern, as shown in their control of mosquitoes, which are vectors of diseases. The isolation of new indigenous Metarhizium isolates from a variety of substrates such as soil, insects, and plants shows the wide genetic diversity within this fungal genus. In this review, we emphasize the significance of Metarhizium spp. for the biological control of insects in Brazil. We also suggest that the experience and success of biological control with fungi in Brazil is an important resource for developing integrated pest management and sustainable strategies for pest control worldwide. Moreover, the future implementation prospects of species of Metarhizium being used as bioinoculants and possible new advances in the utility of this fungus are discussed.

A Drosophila melanogaster model shows that fast growing Metarhizium species are the deadliest despite eliciting a strong immune response

We used Drosophila melanogaster to investigate how differences between Metarhizium species in growth rate and mechanisms of pathogenesis influence the outcome of infection. We found that the most rapid germinators and growers in vitro and on fly cuticle were the fastest killers, suggesting that pre-penetration competence is key to Metarhizium success. Virulent strains also induced the largest immune response, which did not depend on profuse growth within hosts as virulent toxin-producing strains only proliferated post-mortem while slow-killing strains that were specialized to other insects grew profusely pre-mortem. Metarhizium strains have apparently evolved resistance to widely distributed defenses such as the defensin Toll product drosomycin, but they were inhibited by Bomanins only found in Drosophila spp. Disrupting a gene (Dif), that mediates Toll immunity has little impact on the lethality of most Metarhizium strains (an exception being the early diverged M. frigidum and another insect pathogen Beauveria bassiana). However, disrupting the sensor of fungal proteases (Persephone) allowed rapid proliferation of strains within hosts (with the exception of M. album), and flies succumbed rapidly. Persephone also mediates gender differences in immune responses that determine whether male or female flies die sooner. We conclude that some strain differences in growth within hosts depend on immune-mediated interactions but intrinsic differences in pathogenic mechanisms are more important. Thus, Drosophila varies greatly in tolerance to different Metarhizium strains, in part because some of them produce toxins. Our results further develop D. melanogaster as a tractable model system for understanding insect-Metarhizium interactions.

New mycoparasitic species in the genera Niveomyces and Pseudoniveomycesgen. nov. (Hypocreales: Cordycipitaceae), with sporothrix-like asexual morphs, from Thailand

Four new species of the genus Niveomyces are described from Thailand. They were found as mycoparasites on: Ophiocordyceps infecting flies (Diptera) for Niveomyces albus; ants (Hymenoptera) for N. formicidarum; and leafhoppers (Hemiptera) for N. hirsutellae and N. multisynnematus. A new genus, Pseudoniveomyces with two species: Pseudoniveo. blattae (type species), parasitic on Ophiocordyceps infecting cockroaches, and Pseudoniveo. arachnovorum, found on a spider egg sac, are also described. These fungi share a common feature which is a sporothrix-like asexual morph. Based on our molecular data, Sporothrix insectorum is shown to be affiliated to the genus Niveomyces, and thus a new combination N. insectorum comb. nov. is proposed. Niveomyces coronatus, N. formicidarum and N. insectorum formed the N. coronatus species complex found on ant-pathogenic Ophiocordyceps from different continents. Pseudoniveomyces species are distinguished from Niveomyces spp. based on the presence of fusoid macroconidia in culture and a red pigment diffused in the medium, resembling to Gibellula and Hevansia. The molecular phylogenetic analyses also confirmed its generic status. The host/substrates associated with the genera within Cordycipitaceae were mapped onto the phylogeny to demonstrate that mycoparasitism also evolved independently multiple times in this family. Citation: Kobmoo N, Tasanathai K, Araújo JPM, Noisripoom W, Thanakitpipattana D, Mongkolsamrit S, Himaman W, Houbraken J, Luangsa-ard JJ (2023). New mycoparasitic species in the genera Niveomyces and Pseudoniveomyces gen. nov. (Hypocreales: Cordycipitaceae), with sporothrix-like asexual morphs, from Thailand. Fungal Systematics and Evolution 12: 91-110. doi: 10.3114/fuse.2023.12.07.

Phenotypic variation and genomic variation in insect virulence traits reveal patterns of intraspecific diversity in a locust-specific fungal pathogen

Intraspecific pathogen diversity is crucial for understanding the evolution and maintenance of adaptation in host-pathogen interactions. Traits associated with virulence are often a significant source of variation directly impacted by local selection pressures. The specialist fungal entomopathogen, Metarhizium acridum, has been widely implemented as a biological control agent of locust pests in tropical regions of the world. However, few studies have accounted for natural intraspecific phenotypic and genetic variation. Here, we examine the diversity of nine isolates of M. acridum spanning the known geographic distribution, in terms of (1) virulence towards two locust species, (2) growth rates on three diverse nutrient sources, and (3) comparative genomics to uncover genomic variability. Significant variability in patterns of virulence and growth was shown among the isolates, suggesting intraspecific ecological specialization. Different patterns of virulence were shown between the two locust species, indicative of potential host preference. Additionally, a high level of diversity among M. acridum isolates was observed, revealing increased variation in subtilisin-like proteases from the Pr1 family. These results culminate in the first in-depth analysis regarding multiple facets of natural variation in M. acridum, offering opportunities to understand critical evolutionary drivers of intraspecific diversity in pathogens.

Effects of passages through an insect or a plant on virulence and physiological properties of the fungus Metarhizium robertsii

Species of the genus Metarhizium are characterized by a multitrophic lifestyle of being arthropod parasites, rhizosphere colonizers, endophytes, and saprophytes. The process of adaptation to various organisms and substrates may lead to specific physiological alterations that can be elucidated by passaging through different hosts. Changes in virulence and cultivation properties of entomopathogenic fungi subcultured on different media or passaged through a live insect host are well known. Nevertheless, comparative in-depth physiological studies on fungi after passaging through insect or plant organisms are scarce. Here, virulence, plant colonization, hydrolytic enzymatic activities, toxin production, and antimicrobial action were compared between stable (nondegenerative) parent strain Metarhizium robertsii MB-1 and its reisolates obtained after eight passages through Galleria mellonella larvae or Solanum lycopersicum or after subculturing on the Sabouraud medium. The passaging through the insect caused similar physiological alterations relative to the plant-based passaging: elevation of destruxin A, B, and E production, a decrease in protease and lipase activities, and lowering of virulence toward G. mellonella and Leptinotarsa decemlineata as compared to the parent strain. The reisolates passaged through the insect or plant showed a slight trend toward increased tomato colonization and enhanced antagonistic action on tomato-associated bacterium Bacillus pumilus as compared to the parental strain. Meanwhile, the subculturing of MB-1 on the Sabouraud medium showed stability of the studied parameters, with minimal alterations relative to the parental strain. We propose that the fungal virulence factors are reprioritized during adaptation of M. robertsii to insects, plants, and media.

Local Competition and Enhanced Defense: How Metarhizium brunneum Inhibits Verticillium longisporum in Oilseed Rape Plants

Metarhizium brunneum is a soil-borne fungal entomopathogen that can be associated with plant roots. Previous studies have demonstrated that root colonization by beneficial fungi can directly affect soil-borne pathogens through competition and antibiosis and can activate a systemic response in plants, resulting in a primed state for a faster and/or stronger response to stressors. However, the mechanisms by which Metarhizium inoculation ameliorates symptoms caused by plant pathogens are not well known. This study evaluated the ability of M. brunneum to protect oilseed rape (Brassica napus L.) plants against the soil-borne pathogen Verticillium longisporum and investigated whether the observed effects are a result of direct interaction and/or plant-mediated effects. In vitro and greenhouse experiments were conducted to measure fungal colonization of the rhizosphere and plant tissues, and targeted gene expression analysis was used to evaluate the plant response. The results show that M. brunneum delayed pathogen colonization of plant root tissues, resulting in decreased disease symptoms. Direct competition and antibiosis were found to be part of the mechanisms, as M. brunneum growth was stimulated by the pathogen and inhibited the in vitro growth of V. longisporum. Additionally, M. brunneum changed the plant response to the pathogen by locally activating key defense hormones in the salicylic acid (SA) and abscisic acid (ABA) pathways. Using a split-root setup, it was demonstrated that there is a plant-mediated effect, as improved plant growth and decreased disease symptoms were observed when M. brunneum was in the systemic compartment. Moreover, a stronger systemic induction of the gene PR1 suggested a priming effect, involving the SA pathway. Overall, this study sheds light on the mechanisms underlying the protective effects of M. brunneum against soil-borne pathogens in oilseed rape plants, highlighting the potential of this fungal entomopathogen as a biocontrol agent in sustainable agriculture.

Asphyxiation of Metarhizium robertsii during mycelial growth produces conidia with increased stress tolerance via increased expression of stress-related genes

Little is known about the impact of hypoxia and anoxia during mycelial growth on tolerance to different stress conditions of developing fungal conidia. Conidia of the insect-pathogenic fungus Metarhizium robertsii were produced on potato dextrose agar (PDA) medium under normoxia (control = normal oxygen concentrations), continuous hypoxia, and transient anoxia, as well as minimal medium under normoxia. The tolerance of the conidia produced under these different conditions was evaluated in relation to wet heat (heat stress), menadione (oxidative stress), potassium chloride (osmotic stress), UV radiation, and 4-nitroquinoline-1-oxide (=4-NQO genotoxic stress). Growth under hypoxic condition induced higher conidial tolerance of M. robertsii to menadione, KCl, and UV radiation. Transient anoxic condition induced higher conidial tolerance to KCl and UV radiation. Nutritional stress (i.e., minimal medium) induced higher conidial tolerance to heat, menadione, KCl, and UV radiation. However, neither of these treatments induced higher tolerance to 4-NQO. The gene hsp30 and hsp101 encoding a heat shock protein was upregulated under anoxic condition. In conclusion, growth under hypoxia and anoxia produced conidia with higher stress tolerances than conidia produced in normoxic condition. The nutritive stress generated by minimal medium, however, induced much higher stress tolerances. This condition also caused the highest level of gene expression in the hsp30 and hsp101 genes. Thus, the conidia produced under nutritive stress, hypoxia, and anoxia had greater adaptation to stress.

Ergot Alkaloids Contribute to the Pathogenic Potential of the Fungus Aspergillus leporis

Opportunistically pathogenic fungi have varying potential to cause disease in animals. Factors contributing to their virulence include specialized metabolites, which in some cases evolved in contexts unrelated to pathogenesis. Specialized metabolites that increase fungal virulence in the model insect Galleria mellonella include the ergot alkaloids fumigaclavine C in Aspergillus fumigatus (syn. Neosartorya fumigata) and lysergic acid α-hydroxyethylamide (LAH) in the entomopathogen Metarhizium brunneum. Three species of Aspergillus recently found to accumulate high concentrations of LAH were investigated for their pathogenic potential in G. mellonella. Aspergillus leporis was most virulent, A. hancockii was intermediate, and A. homomorphus had very little pathogenic potential. Aspergillus leporis and A. hancockii emerged from and sporulated on dead insects, thus completing their asexual life cycles. Inoculation by injection resulted in more lethal infections than did topical inoculation, indicating that A. leporis and A. hancockii were preadapted for insect pathogenesis but lacked an effective means to breach the insect's cuticle. All three species accumulated LAH in infected insects, with A. leporis accumulating the most. Concentrations of LAH in A. leporis were similar to those observed in the entomopathogen M. brunneum. LAH was eliminated from A. leporis through a CRISPR/Cas9-based gene knockout, and the resulting strain had reduced virulence to G. mellonella. The data indicate that A. leporis and A. hancockii have considerable pathogenic potential and that LAH increases the virulence of A. leporis. IMPORTANCE Certain environmental fungi infect animals occasionally or conditionally, whereas others do not. Factors that affect the virulence of these opportunistically pathogenic fungi may have originally evolved to fill some other role for the fungus in its primary environmental niche. Among the factors that may improve the virulence of opportunistic fungi are specialized metabolites--chemicals that are not essential for basic life functions but provide producers with an advantage in particular environments or under specific conditions. Ergot alkaloids are a large family of fungal specialized metabolites that contaminate crops in agriculture and serve as the foundations of numerous pharmaceuticals. Our results show that two ergot alkaloid-producing fungi that were not previously known to be opportunistic pathogens can infect a model insect and that, in at least one of the species, an ergot alkaloid increases the virulence of the fungus.

Blastospores from Metarhizium anisopliae and Metarhizium rileyi Are Not Always as Virulent as Conidia Are towards Spodoptera frugiperda Caterpillars and Use Different Infection Mechanisms

Infective conidia from entomopathogenic fungi are widely used to control insect pests. Many entomopathogenic fungi also produce yeast-like cells called blastospores under specific liquid culture conditions that can directly infect insects. However, little is known about the biological and genetic factors that allow blastospores to infect insects and make them potentially effective for biological control in the field. Here, we show that while the generalist Metarhizium anisopliae produces a higher number of and smaller blastospores, the Lepidoptera specialist M. rileyi produces fewer propagules with a higher cell volume under high-osmolarity conditions. We compared the virulence of blastospores and conidia of these two Metarhizium species towards the economically important caterpillar pest Spodoptera frugiperda. Conidia and blastospores from M. anisopliae were equally infectious, but acted slower, and killed fewer insects than M. rileyi conidia and blastospores did, where M. rielyi conidia had the highest virulence. Using comparative transcriptomics during propagule penetration of insect cuticles, we show that M. rileyi blastospores express more virulence-related genes towards S. frugiperda than do M. anisopliae blastospores. In contrast, conidia of both fungi express more virulence-related oxidative stress factors than blastospores. Our results highlight that blastospores use a different virulence mechanism than conidia use, which may be explored in new biological control strategies.

Special Issue on "Entomopathogenic Fungi: Ecology, Evolution, Adaptation": An Editorial

Entomopathogenic endophytic ascomycetes are the most widespread and commercially promising fungi and are used to solve many problems in basic and applied research in ecology, evolution, and agricultural sciences [...].

Exo- and endophytic fungi enable rapid transfer of nutrients from ant waste to orchid tissue

The epiphytic orchid Caularthron bilamellatum sacrifices its water storage tissue for nutrients from the waste of ants lodging inside its hollow pseudobulb. Here, we investigate whether fungi are involved in the rapid translocation of nutrients. Uptake was analysed with a 15 N labelling experiment, subsequent isotope ratio mass spectrometry (IRMS) and secondary ion mass spectrometry (ToF-SIMS and NanoSIMS). We encountered two hyphae types: a thick melanized type assigned to 'black fungi' (Chaetothyriales, Cladosporiales, and Mycosphaerellales) in ant waste, and a thin endophytic type belonging to Hypocreales. In few cell layers, both hyphae types co-occurred. 15 N accumulation in both hyphae types was conspicuous, while for translocation to the vessels only Hypocreales were involved. There is evidence that the occurrence of the two hyphae types results in a synergism in terms of nutrient uptake. Our study provides the first evidence that a pseudobulb (=stem)-born endophytic network of Hypocreales is involved in the rapid translocation of nitrogen from insect-derived waste to the vegetative and reproductive tissue of the host orchid. For C. bilamellatum that has no contact with the soil, ant waste in the hollow pseudobulbs serves as equivalent to soil in terms of nutrient sources.

Effects of tomato inoculation with the entomopathogenic fungus Metarhizium brunneum on spider mite resistance and the rhizosphere microbial community

Entomopathogenic fungi have been well exploited as biocontrol agents that can kill insects through direct contact. However, recent research has shown that they can also play an important role as plant endophytes, stimulating plant growth, and indirectly suppressing pest populations. In this study, we examined the indirect, plant-mediated, effects of a strain of entomopathogenic fungus, Metarhizium brunneum on plant growth and population growth of two-spotted spider mites (Tetranychus urticae) in tomato, using different inoculation methods (seed treatment, soil drenching and a combination of both). Furthermore, we investigated changes in tomato leaf metabolites (sugars and phenolics), and rhizosphere microbial communities in response to M. brunneum inoculation and spider mite feeding. A significant reduction in spider mite population growth was observed in response to M. brunneum inoculation. The reduction was strongest when the inoculum was supplied both as seed treatment and soil drench. This combination treatment also yielded the highest shoot and root biomass in both spider mite-infested and non-infested plants, while spider mite infestation increased shoot but reduced root biomass. Fungal treatments did not consistently affect leaf chlorogenic acid and rutin concentrations, but M. brunneum inoculation via a combination of seed treatment and soil drenching reinforced chlorogenic acid (CGA) induction in response to spider mites and under these conditions the strongest spider mite resistance was observed. However, it is unclear whether the M. brunneum-induced increase in CGA contributed to the observed spider mite resistance, as no general association between CGA levels and spider mite resistance was observed. Spider mite infestation resulted in up to two-fold increase in leaf sucrose concentrations and a three to five-fold increase in glucose and fructose concentrations, but these concentrations were not affected by fungal inoculation. Metarhizium, especially when applied as soil drench, impacted the fungal community composition but not the bacterial community composition which was only affected by the presence of spider mites. Our results suggest that in addition to directly killing spider mites, M. brunneum can indirectly suppress spider mite populations on tomato, although the underlying mechanism has not yet been resolved, and can also affect the composition of the soil microbial community.

Isolation of a highly virulent Metarhizium strain targeting the tea pest, Ectropis obliqua

Introduction

Tea is one of the most widely consumed beverages around the world. Larvae of the moth, Ectropis obliqua Prout (Geometridae, Lepidoptera), are one of the most destructive insect pests of tea in China. E. obliqua is a polyphagus insect that is of increasing concern due to the development of populations resistant to certain chemical insecticides. Microbial biological control agents offer an environmentally friendly and effective means for insect control that can be compatible with "green" and organic farming practices.

Methods

To identify novel E. obliqua biological control agents, soil and inset cadaver samples were collected from tea growing regions in the Fujian province, China. Isolates were analyzed morphologically and via molecular characterization to identity them at the species level. Laboratory and greenhouse insect bioassays were used to determine the effectiveness of the isolates for E. obliqua control.

Results

Eleven isolates corresponding to ten different species of Metarhizium were identified according to morphological and molecular analyses from soil and/or insect cadavers found on tea plants and/or in the surrounding soil sampled from eight different regions within the Fujian province, China. Four species of Metarhizium including M. clavatum, M. indigoticum, M. pemphigi, and M. phasmatodeae were documented for the first time in China, and the other species were identified as M. anisopliae, M. brunneum, M. lepidiotae, M. majus, M. pinghaense, and M. robertsii. Insect bioassays of the eleven isolates of Metarhizium revealed significant variation in the efficacy of each isolate to infect and kill E. obliqua. Metarhizium pingshaense (MaFZ-13) showed the highest virulence reaching a host target mortality rate of 93% in laboratory bioassays. The median lethal concentration (LC₅₀) and median lethal time (LT₅₀) values of M. pingshaense MaFZ-13 were 9.6 × 10⁴ conidia/mL and 4.8 days, respectively. Greenhouse experiments and a time-dose-mortality (TDM) models were used to further evaluate and confirm the fungal pathogenic potential of M. pingshaense MaFZ-13 against E. obliqua larvae.

Discussion

Isolation of indigenous microbial biological control agents targeting specific pests is an effective approach for collecting resources that can be exploited for pest control with lowered obstacles to approval and commercialization. Our data show the presence of four different previously unreported Metarhizium species in China. Bioassays of the eleven different Metarhizium strains isolated revealed that each could infect and kill E. obliqua to different degrees with the newly isolated M. pingshaense MaFZ-13 strain representing a particularly highly virulent isolate potentially applicable for the control of E. obliqua larvae.

Genetic variation in disease resistance in Drosophila spp. is mitigated in Drosophila sechellia by specialization to a toxic host

We found that Drosophila species vary in their susceptibility to the broad-spectrum entomopathogen, Metarhizium anisopliae (strain Ma549). Generalist species were generally more resistant than dietary specialists, with the cactophilic Drosophila buzzatii and Drosophila sechellia, a specialist of the Morinda citrifolia (Morinda) fruit, being most susceptible. Morinda fruit is reported to be toxic to most herbivores because it contains Octanoic Acid (OA). We confirmed that OA is toxic to Drosophila spp., other than D. sechellia, and we also found that OA is highly toxic to entomopathogenic fungi including Ma549 and Beauveria bassiana. Drosophila sechellia fed a diet containing OA, even at levels much less than found in Morinda fruit, had greatly reduced susceptibility to Ma549. This suggests that specializing to Morinda may have provided an enemy-free space, reducing adaptive prioritization on a strong immune response. Our results demonstrate that M. anisopliae and Drosophila species with divergent lifestyles provide a versatile model system for understanding the mechanisms of host-pathogen interactions at different scales and in environmental context.

Influence of Grafting on Rootstock Rhizosphere Microbiome Assembly in Rosa sp. 'Natal Brier'

The root microbiome is vital in plant development and health and is highly influenced by crop cultural practices. Rose (Rosa sp.) is the most popular cut flower worldwide. Grafting in rose production is a standard practice to increase yield, improve flower quality, or reduce root-associated pests and diseases. 'Natal Brier' is a standard rootstock used in most commercial operations in Ecuador and Colombia, leading countries in producing and exporting ornamentals. It is known that the rose scion genotype affects root biomass and the root exudate profile of grafted plants. However, little is known about the influence of the rose scion genotype on the rhizosphere microbiome. We examined the influence of grafting and scion genotype on the rhizosphere microbiome of the rootstock 'Natal Brier'. The microbiomes of the non-grafted rootstock and the rootstock grafted with two red rose cultivars were assessed using 16S rRNA and ITS sequencing. Grafting changed microbial community structure and function. Further, analysis of grafted plant samples revealed that the scion genotype highly influences the rootstock microbiome. Under the presented experimental conditions, the rootstock 'Natal Brier' core microbiome consisted of 16 bacterial and 40 fungal taxa. Our results highlight that the scion genotype influences root microbe's recruitment, which might also influence the functionality of assembled microbiomes.

The Dissemination of Metarhizium brunneum Conidia by Females of the Red Palm Weevil, Rhynchophorus ferrugineus, Suggests a New Mechanism for Prevention Practices

Direct contact between the conidia of entomopathogenic fungi (EPF) and their host is a prerequisite to successful infection; the host can, therefore, be infected by both direct treatment and by transmission of fungal inoculum from infested surfaces. This unique characteristic makes EPF especially relevant for the control of cryptic insects. In the case of the red palm weevil (RPW) Rhynchophorus ferrugineus, the eggs and larvae are almost inaccessible to direct-contact treatment. The objective of the present study was to investigate the mechanism of conidia transmission from a treated surface to host eggs and larvae. Foam pieces infested with Metarhizium brunneum conidial powder, conidial suspension, or distilled water were used as a laying surface for RPW females. The number of eggs laid was not affected by the EPF treatments and ranged from 2 to 14 eggs per female. However, hatching rate and larval survival were significantly reduced in the conidial powder treatment, resulted in 1.5% hatching and no live larvae. In the conidial suspension treatment, 21% of laid eggs hatched, compared to 72% in the control treatment. In both M. brunneum treatments, females' proboscis, front legs and ovipositor were covered with conidia. The females transferred conidia in both treatments to the laying holes, reaching up to 15 mm in depth. This resulted in reduced egg-hatching rate and significant larval mortality due to fungal infection. The stronger effect on egg and larval survival using dry conidia seemed to result from better conidial adhesion to the female weevil in this formulation. In future studies, this dissemination mechanism will be examined as a prevention strategy in date plantations.

Links between Soil Bacteriobiomes and Fungistasis toward Fungi Infecting the Colorado Potato Beetle

Entomopathogenic fungi can be inhibited by different soil microorganisms, but the effect of a soil microbiota on fungal growth, survival, and infectivity toward insects is insufficiently understood. We investigated the level of fungistasis toward Metarhizium robertsii and Beauveria bassiana in soils of conventional potato fields and kitchen potato gardens. Agar diffusion methods, 16S rDNA metabarcoding, bacterial DNA quantification, and assays of Leptinotarsa decemlineata survival in soils inoculated with fungal conidia were used. Soils of kitchen gardens showed stronger fungistasis toward M. robertsii and B. bassiana and at the same time the highest density of the fungi compared to soils of conventional fields. The fungistasis level depended on the quantity of bacterial DNA and relative abundance of Bacillus, Streptomyces, and some Proteobacteria, whose abundance levels were the highest in kitchen garden soils. Cultivable isolates of bacilli exhibited antagonism to both fungi in vitro. Assays involving inoculation of nonsterile soils with B. bassiana conidia showed trends toward elevated mortality of L. decemlineata in highly fungistatic soils compared to low-fungistasis ones. Introduction of antagonistic bacilli into sterile soil did not significantly change infectivity of B. bassiana toward the insect. The results support the idea that entomopathogenic fungi can infect insects within a hypogean habitat despite high abundance and diversity of soil antagonistic bacteria.

Suppression of the insect cuticular microbiomes by a fungal defensin to facilitate parasite infection

Insects can assemble defensive microbiomes on their body surfaces to defend against fungal parasitic infections. The strategies employed by fungal pathogens to combat host cuticular microbiotas remains unclear. Here, we report the identification and functional characterization of the defensin-like antimicrobial gene BbAMP1 encoded by the entomopathogenic fungus Beauveria bassiana. The mature peptide of BbAMP1 can coat fungal spores and can be secreted by the fungus to target and damage Gram-positive bacterial cells. Significant differences in insect survival were observed between the wild-type and BbAMP1 mutant strains during topical infection but not during injection assays that bypassed insect cuticles. Thus, BbAMP1 deletion considerably reduced fungal virulence while gene overexpression accelerated the fungal colonization of insects compared with the wild-type strain in natural infections. Topical infection of axenic Drosophila adults evidenced no difference in fly survivals between strains. However, the gnotobiotic infections with the addition of Gram-positive but not Gram-negative bacterial cells in fungal spore suspensions substantially increased the survival of the flies treated with ∆BbAMP1 compared to those infected by the wild-type and gene-overexpression strains. Bacterial colony counts and microbiome analysis confirmed that BbAMP1 could assist the fungus to manipulate insect surface bacterial loads. This study reveals that fungal defensin can suppress the host surface defensive microbiomes, which underscores the importance to extend the research scope of fungus-host interactions.

Effect of Probiotics on Tenebrio molitor Larval Development and Resistance against the Fungal Pathogen Metarhizium brunneum

In recent years, the yellow mealworm (Tenebrio molitor L.) has demonstrated its potential as a mass-produced edible insect for food and feed. However, challenges brought on by pathogens in intensive production systems are unavoidable and require the development of new solutions. One potential solution is the supplementation of probiotics in the insect's diet to obtain the double benefits of improved growth and enhanced immune response. The aim of this study was to evaluate the effects of diet-based probiotic supplementation on T. molitor larval survival, growth, and resistance against a fungal pathogen. Three probiotic strains, namely Pediococcus pentosacceus KVL-B19-01 isolated from T. molitor and two commercialized strains for traditional livestock, Enterococcus faecium 669 and Bacillus subtilis 597, were tested. Additionally, when larvae were 9 weeks old, a pathogen challenge experiment was conducted with the fungus Metarhizium brunneum. Results showed that both P. pentosaceus and E. faecium improved larval growth and larval survival following fungal exposure compared to the non-supplemented control diet. Since B. subtilis did not improve larval performance in terms of either development or protection against M. brunneum, this study suggests the need for further research and evaluation of probiotic strains and their modes of action when considered as a supplement in T. molitor's diet.

Bioremediation of mercury-polluted soil and water by the plant symbiotic fungus Metarhizium robertsii

Fungi are central to every terrestrial and many aquatic ecosystems, but the mechanisms underlying fungal tolerance to mercury, a global pollutant, remain unknown. Here, we show that the plant symbiotic fungus Metarhizium robertsii degrades methylmercury and reduces divalent mercury, decreasing mercury accumulation in plants and greatly increasing their growth in contaminated soils. M. robertsii does this by demethylating methylmercury via a methylmercury demethylase (MMD) and using a mercury ion reductase (MIR) to reduce divalent mercury to volatile elemental mercury. M. robertsii can also remove methylmercury and divalent mercury from fresh and sea water even in the absence of added nutrients. Overexpression of MMD and MIR significantly improved the ability of M. robertsii to bioremediate soil and water contaminated with methylmercury and divalent mercury. MIR homologs, and thereby divalent mercury tolerance, are widespread in fungi. In contrast, MMD homologs were patchily distributed among the few plant associates and soil fungi that were also able to demethylate methylmercury. Phylogenetic analysis suggests that fungi could have acquired methylmercury demethylase genes from bacteria via two independent horizontal gene transfer events. Heterologous expression of MMD in fungi that lack MMD homologs enabled them to demethylate methylmercury. Our work reveals the mechanisms underlying mercury tolerance in fungi, and may provide a cheap and environmentally friendly means of cleaning up mercury pollution.

Resporulation of Calcium Alginate Encapsulated Metarhizium anisopliae on Metham®-Fumigated Soil and Infectivity on Larvae of Tenebrio molitor

Metarhizium anisopliae infects and kills a large range of insects and is a promising biocontrol agent to manage soil insects, such as wireworm in sweetpotato. The presence of other soil microbes, which exhibit competitive fungistasis, may inhibit the establishment of M. anisopliae in soil. Microbially depleted soil, for example, sterilized soil, has been shown to improve the resporulation of the fungus from nutrient-fortified M. anisopliae. Prior to planting, sweetpotato plant beds can be disinfected with fumigants, such as Metham®, to control soil-borne pests and weeds. Metham® is a broad-spectrum soil microbial suppressant; however, its effect on Metarhizium spp. is unclear. In the research presented here, fungal resporulation was examined in Metham®-fumigated soil and the infectivity of the resulting granule sporulation was evaluated on mealworm, as a proxy for wireworm. The fungal granules grown on different soil treatments (fumigated, field and pasteurized soil) resporulated profusely (for example, 4.14 × 107 (±2.17 × 106) conidia per granule on fumigated soil), but the resporulation was not significantly different among the three soil treatments. However, the conidial germination of the resporulated granules on fumigated soil was >80%, which was significantly higher than those on pasteurized soil or field soil. The resporulated fungal granules were highly infective, causing 100% insect mortality 9 days after the inoculation, regardless of soil treatments. The results from this research show that the fungal granules applied to soils could be an infective inoculant in sweetpotato fields in conjunction with soil fumigation. Additional field studies are required to validate these results and to demonstrate integration with current farming practices.

Why Do Plant-Pathogenic Fungi Produce Mycotoxins? Potential Roles for Mycotoxins in the Plant Ecosystem

For many plant-pathogenic or endophytic fungi, production of mycotoxins, which are toxic to humans, may present a fitness gain. However, associations between mycotoxin production and plant pathogenicity or virulence is inconsistent and difficult due to the complexity of these host-pathogen interactions and the influences of environmental and insect factors. Aflatoxin receives a lot of attention due to its potent toxicity and carcinogenicity but the connection between aflatoxin production and pathogenicity is complicated by the pathogenic ability and prevalence of nonaflatoxigenic isolates in crops. Other toxins directly aid fungi in planta, trichothecenes are important virulence factors, and ergot alkaloids limit herbivory and fungal consumption due to insect toxicity. We review a panel discussion at the American Phytopathological Society's Plant Health 2021 conference, which gathered diverse experts representing different research sectors, career stages, ethnicities, and genders to discuss the diverse roles of mycotoxins in the lifestyles of filamentous fungi of the families Clavicipitaceae, Trichocomaceae (Eurotiales), and Nectriaceae (Hypocreales).

Metarhizium robertsii as a promising microbial agent for rice in situ cadmium reduction and plant growth promotion

The toxic chemical element cadmium (Cd) in paddy fields triggered increasing problems of growth inhibition and food security in rice consistently. In this study, we found Metarhizium robertsii, which is widely used as a bioinsecticide and biofertilizer in agriculture and recently found to be resistant to Cd, developed intraradical and extraradical symbiotic hyphae in rice seedlings, and successfully colonized in the rice rhizosphere soil to more than 10³ CFUs g^-1 soil at harvesting. M. robertsii colonization significantly reduced Cd accumulations in both hydroponically cultured seedlings and the matured rice cultured in Cd contaminated potting soil (2 ppm). Notably, Cd accumulation reduction of the roots, stems, leaves, husks and grains of the matured rice induced by the fungus were 44.3%, 32.1%, 35.3%, 31.9% and 24.7%, respectively. It was caused by the M. robertsii-induced suppression of Cd intake transporter gene osNramp5 in the rice roots, and the chemical stabilizing of Cd to the residual fraction in the rhizosphere soil. In addition, the colonization of M. robertsii significantly promoted the growth characters and the photosynthesis of the rice plants. This is achieved by the increase of endogenous hormone levels of indole-3-acetic, gibberellin A₃ and brassinolide induced by M. robertsii. Furthermore, the fungus enhanced the antioxidative capacities via increasing enzyme activities of catalase, peroxidase and the production of glutathione, ascorbic acid, proline in the rice plants. Our work provides theoretical basis for expanding the use of M. robertsii as in situ Cd accumulation reduction and detoxification agents for rice in contaminated paddy fields.

Silicon fertilizer mediated structural variation and niche differentiation in the rhizosphere and endosphere bacterial microbiome and metabolites of sugarcane

The microbiomes of plant are potential determinants of plant growth, productivity, and health. They provide plants with a plethora of functional capacities, namely, phytopathogens suppression, access to low-abundance nutrients, and resistance to environmental stressors. However, a comprehensive insight into the structural compositions of the bacterial abundance, diversity, richness, and function colonizing various microenvironments of plants, and specifically their association with bioactive compounds and soil edaphic factors under silicon (Si) amendment remains largely inconclusive. Here, high-throughput sequencing technology and nontargeted metabolite profiling method were adopted to test the hypotheses regarding microbiome niche abundance, diversity, richness, function, and their association with bioactive compounds and soil edaphic factors within different ecological niches (leaf, stem, root, rhizosphere, and bulk soils) under Si amendment during cane growth were we addressed. Our results demonstrated that Si correspondingly increased sugarcane theoretical production and yield, and remarkably enhanced soil nutrient status, especially Si, AP, and AK. It was also observed that bacterial diversity demonstrated tissue-dependent distribution patterns, with the bulk soil, rhizosphere soil, and root endosphere revealing the highest amount of bacterial diversity compared with the stem and leaf tissues. Moreover, Si exhibited the advantage of considerably promoting bacterial abundance in the various plant compartments. Co-occurrence interactions demonstrated that Si application has the potential to increase bacterial diversity maintenance, coexistence, and plant–soil systems bacteria connections, thereby increasing the functional diversity in the various plant tissues, which, in turn, could trigger positive growth effects in plants. Network analysis further revealed that metabolite profiles exhibited a strong association with bacterial community structures. It was also revealed that Si content had a considerable positive association with bacterial structures. Our findings suggest that the dynamic changes in microbe’s community composition in different plant and soil compartments were compartment-specific. Our study provides comprehensive empirical evidence of the significance of Si in agriculture and illuminated on differential metabolite profiles and soil microbe’s relationship.

Enhanced Cd efflux capacity and physiological stress resistance: The beneficial modulations of Metarhizium robertsii on plants under cadmium stress

Due to the high migration capacity in agricultural soil-crop systems, cadmium (Cd) is accumulated in various crops and severely inhibits plant growth. In this study, we showed that, under Cd stress, the plant-symbiotic fungus Metarhizium robertsii reduced Cd accumulation in Arabidopsis thaliana shoots and roots by 21.8 % and 23.8 %, respectively. This is achieved by M. robertsii colonization-induced elevation of Cd efflux capacity via upregulation of three PCR genes, which is confirmed by the fact that the extent to which M. robertsii reduced Cd accumulation in the WT plants was greater than the inactivating mutants of the PCR genes. M. robertsii also alleviated Cd-induced leaf etiolation in A. thaliana by increasing the chlorophyll amount and modified plant physiological status to increase Cd stress tolerance via increasing production of catalase, peroxidase and glutathione and upregulating multiple HIPP proteins involved in sequestration of Cd. Notably, consistent with that in A. thaliana, the colonization of M. robertsii also reduced the Cd accumulation in Oryza sativa seedlings by upregulating the PCR gene OsPCR1, and increased chlorophyll amount and alleviated oxidative stress. Therefore, M. robertsii colonization reduced Cd accumulation in plants, and promoted plant growth and health by elevating Cd efflux capacity and modifying physiological status.

Parametarhizium hingganense, a Novel Ectomycorrhizal Fungal Species, Promotes the Growth of Mung Beans and Enhances Resistance to Disease Induced by Rhizoctonia solani

The mutualistic interactions between mycorrhizae and plants first occurred along with the terrestrialization of plants. The majority of vascular plants are in symbiosis with mycorrhizal fungi. Due to their importance to the economy and ecology, arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi emerge as the most popular ones. However, the mechanism underlying the beneficial function of ECM fungi is not as clear as AM fungi. Here, the interaction between Parametarhizium hingganense, a novel fungal species isolated from forest litter, and mung bean (Vigna radiata) was studied. P. hingganense demonstrated P solubilization ability in vitro. Treatment of P. hingganense on the seeds resulted in promoted growth with enhanced P content. The hyphae of green fluorescence protein (GFP)-tagged P. hingganense were found to surround the roots and develop between cells, suggesting the establishment of an ectomycorrhizal symbiosis. Upon symbiosis with P. hingganense, the levels of jasmonic acid (JA) and gibberellin (GA₁) and total phenolic and flavonoid content elevated. Meanwhile, damping off caused by Rhizoctonia solani in mycorrhizal plants was alleviated. Taken together, the above findings suggested that symbiosis with P. hingganense conferred growth promotion and priming of defense responses to host plants which should be associated with facilitated P uptake and increased JA and GA₁ levels.

Metarhizium: an opportunistic middleman for multitrophic lifestyles

Metarhizium spp. mediate multiple interactions that are usually positive with respect to their long-term plant environment, and negative with respect to short-lived hosts. In particular, their ability to kill a wide range of insects maximizes protection to the plants and provides a resource of nitrogen that the fungus trades with the plant for carbon. Here, we highlight emerging concepts underlying Metarhizium-plant-insect interactions. Experiments on model systems have provided detailed mechanistic knowledge of how these fungi interact with plants and insects, and a greater understanding of the evolutionary forces driving these interactions. However, further integration of studies at the ecological and mechanistic level is needed to evaluate the importance of Metarhizium's multitrophic interactions to the structuring of natural communities.

Entomopathogenic Fungi Infecting Lepidopteran Larvae: A Case from Central Argentina

Immature stages of insects are vulnerable to various antagonists, including pathogens. While the abiotic factors affecting pathogen prevalence in insect populations are reasonably well documented, much less is known about relevant ecological interactions. We studied the probability of the larvae of three lepidopteran species to die from fungal infection as a function of insect species and food plants in central Argentina. Local free-growing food plants were used to feed the lepidopteran larvae. The prevalence of entomopathogenic fungi remained low (about 5%), which is a value well consistent with observations on similar systems in other regions. Eight fungal species recorded, primarily belonging to Fusarium and Aspergillus, add evidence to the reconsideration of the nutritional modes in these genera in distinguishing the role of some species (complexes) to cause insect infections. Food plant species were found to have a substantial effect on the prevalence of entomopathogenic fungi. This was especially clear for the most abundant fungal species, a representative of the Fusarium fujikuroi complex. Feeding on a particular plant taxon can thus have a specific fitness cost. Compared to the data collected from Northern Europe, the Argentinian assemblages from the families Aspergillaceae and Nectriaceae overlapped at the genus level but did not share species. It remains to be confirmed if this level of divergence in the composition of assemblages of entomopathogenic fungi among distant regions represents a global pattern.