Endophytic Metarhizium brunneum mitigates nutrient deficits in potato and improves plant productivity and vitality

Tripartite interactions of an endophytic entomopathogenic fungus, Asian corn borer, and host maize under elevated carbon dioxide

BACKGROUND

Biological control of insect pests is encountering an unprecedented challenge in agricultural systems due to the ongoing rise in carbon dioxide (CO2) level. The use of entomopathogenic fungi (EPF) in these systems is gaining increased attention, and EPF as crop endophytes hold the potential for combining insect pest control and yield enhancement of crops, but the effects of increased CO2 concentration on this interaction are poorly understood. Here, the introduction of endophytic EPF was explored as an alternative sustainable management strategy benefiting crops under elevated CO2, using maize (Zea mays), Asian corn borer (Ostrinia furnacalis), and EPF (Beauveria bassiana) to test changes in damage to maize plants from O. furnacalis, and the nutritional status (content of carbon, nitrogen, phosphorus, potassium), biomass, and yield of maize.

RESULTS

The results showed that endophytic B. bassiana could alleviate the damage caused by O. furnacalis larvae for maize plants under ambient CO2 concentration, and this effect was enhanced under higher CO2 concentration. Inoculation with B. bassiana effectively counteracted the adverse impact of elevated CO2 on maize plants by preserving the nitrogen content at its baseline level (comparable with ambient CO2 conditions without B. bassiana). Both simultaneous effects could explain the improvement of biomass and yield of maize under B. bassiana inoculation and elevated CO2.

CONCLUSION

This finding provides key information about the multifaceted benefits of B. bassiana as a maize endophyte. Our results highlight the promising potential of incorporating EPF as endophytes into integrated pest management strategies, particularly under elevated CO2 concentrations. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Entomopathogenic fungi in crops protection with an emphasis on bioactive metabolites and biological activities

Plant pathogens with their abundance are harmful and cause huge damage to different agricultural crops and economy of a country as well as lead towards the shortage of food for humans. For their management, the utilization of entomopathogenic fungi is an eco-friendly technique, sustainable to the environment, safe for humans and has promising effect over chemical-based pesticides. This process requires a biochemical mechanism, including the production of enzymes, toxins, and other metabolites that facilitate host infection and invasion. Essential enzymes such as chitinase, proteinase, and lipase play a direct role in breaking down the host cuticle, the primary barrier to EPF (Entomopathogenic Fungi) infection. Additionally, secondary metabolites such as destruxins in Metarhizium, beauvericin in Beauveria, hirsutellides in Hirsutella, isarolides in Isaria, cordyols in Cordyceps, and vertihemipterins in Verticillium, among others, act both directly and indirectly to disable the defense mechanisms of insect hosts, thereby accelerating the EPF infection process. The chemical composition of these secondary metabolites varies, ranging from simple non-peptide pigments such as oosporine to highly complex piperazine derivatives such as vertihemiptellides. The biocontrol efficacy of EPF is extensively studied, with numerous fungal strains commercially available on a large scale for managing arthropod pests. This review emphasizes the role of proteins and enzymes against crop pathogens, detailing their mode of action, and describing the metabolites from entomopathogenic fungi and their biological activities. In doing so, these findings contribute to establishing a symbiotic equilibrium between agricultural productivity and environmental conservation.

Ecosystem services of entomopathogenic ascomycetes

Entomopathogenic ascomycetes (EA) are an important part of the microbiota in most terrestrial ecosystems, where they can be found regulating natural populations of arthropod pests in both epigeous and hypogeous habitats while also establishing unique relationships with plants. These fungi offer direct benefits to agriculture and human welfare. In the present work, we conducted a systematic review to comprehensively assess the range of ecosystem services provided by EA, including direct and indirect pest biocontrol, plant growth promotion, plant defense against other biotic and abiotic stresses, nutrient cycling, and the production of new bioactive compounds with agricultural, pharmaceutical and medical importance. Moreover, EA are compatible with the ecosystem services provided by other microbial and macrobial biocontrol agents. This systematic review identified the need for future research to focus on evaluating the economic value of the ecological services provided by EA with a special emphasis on hypocrealean fungi. This evaluation is essential not only for the conservation but also for better regulation and exploitation of the benefits of EA in promoting agricultural sustainability, reducing the use of chemicals that enter the environment, and minimizing the negative impacts of crop protection on the carbon footprint and human health.

Endophytic Beauveria bassiana promotes plant biomass growth and suppresses pathogen damage by directional recruitment

Introduction

Entomopathogenic fungi (EPF) can colonize and establish symbiotic relationships with plants as endophytes. Recently, EPF have been reported to suppress plant pathogens and induce plant resistance to diseases. However, the potential mechanisms via which EPF as endophytes control major plant diseases in situ remain largely unknown.

Methods

Pot and field experiments were conducted to investigate the mechanisms via which an EPF, Beauveria bassiana, colonizes tomato, under Botrytis cinerea infection stress. B. bassiana blastospores were inoculated into tomato plants by root irrigation. Tomato resistance to tomato gray mold caused by B. cinerea was evaluated by artificial inoculation, and B. bassiana colonization in plants and rhizosphere soil under B. cinerea infection stress was evaluated by colony counting and quantitative PCR. Furthermore, the expression levels of three disease resistance-related genes (OXO, CHI, and atpA) in tomato leaves were determined to explore the effect of B. bassiana colonization on plant disease resistance performance in pot experiments.

Results

B. bassiana colonization could improve resistance of tomato plants to gray mold caused by B. cinerea. The incidence rate, lesion diameter, and disease index of gray mold decreased in both the pot and field experiments following B. bassiana colonization. B. bassiana was more likely to accumulate in the pathogen infected leaves, while decreasing in the rhizosphere soil, and induced the expression of plant resistance genes, which were up-regulated in leaves.

Discussion

The results indicated that plants could "recruit" B. bassiana from rhizosphere soil to diseased plants as directional effects, which then enhanced plant growth and resistance against pathogens, consequently inhibiting pathogen infection and multiplication in plants. Our findings provide novel insights that enhance our understanding of the roles of EPF during pathogen challenge.

Fungal strain and crop cultivar affect growth of sweet pepper plants after root inoculation with entomopathogenic fungi

As endophytes, entomopathogenic fungi can protect plants against biotic and abiotic stresses and at the same time promote plant growth and plant health. To date, most studies have investigated whether Beauveria bassiana can enhance plant growth and plant health, while only little is known about other entomopathogenic fungi. In this study, we evaluated whether root inoculation of the entomopathogenic fungi Akanthomyces muscarius ARSEF 5128, B. bassiana ARSEF 3097 and Cordyceps fumosorosea ARSEF 3682 can promote plant growth of sweet pepper (Capsicum annuum L.), and whether effects are cultivar-dependent. Plant height, stem diameter, number of leaves, canopy area, and plant weight were assessed four weeks following inoculation in two independent experiments using two cultivars of sweet pepper (cv. 'IDS RZ F1' and cv. 'Maduro'). Results showed that the three entomopathogenic fungi were able to enhance plant growth, particularly canopy area and plant weight. Further, results showed that effects significantly depended on cultivar and fungal strain, with the strongest fungal effects obtained for cv. 'IDS RZ F1', especially when inoculated with C. fumosorosea. We conclude that inoculation of sweet pepper roots with entomopathogenic fungi can stimulate plant growth, but effects depend on fungal strain and crop cultivar.

Endophytic fungi mediates production of bioactive secondary metabolites via modulation of genes involved in key metabolic pathways and their contribution in different biotechnological sector

Endophytic fungi stimulate the production of an enormous number of bioactive metabolites in medicinal plants and affect the different steps of biosynthetic pathways of these secondary metabolites. Endophytic fungi possess a number of biosynthetic gene clusters that possess genes for various enzymes, transcription factors, etc., in their genome responsible for the production of secondary metabolites. Additionally, endophytic fungi also modulate the expression of various genes responsible for the synthesis of key enzymes involved in metabolic pathways of such as HMGR, DXR, etc. involved in the production of a large number of phenolic compounds as well as regulate the expression of genes involved in the production of alkaloids and terpenoids in different plants. This review aims to provide a comprehensive overview of gene expression related to endophytes and their impact on metabolic pathways. Additionally, this review will emphasize the studies done to isolate these secondary metabolites from endophytic fungi in large quantities and assess their bioactivity. Due to ease in synthesis of secondary metabolites and their huge application in the medical industry, these bioactive metabolites are now being extracted from strains of these endophytic fungi commercially. Apart from their application in the pharmaceutical industry, most of these metabolites extracted from endophytic fungi also possess plant growth-promoting ability, bioremediation potential, novel bio control agents, sources of anti-oxidants, etc. The review will comprehensively shed a light on the biotechnological application of these fungal metabolites at the industrial level.

Evaluation of Metarhizium brunneum- and Metarhizium-Derived VOCs as Dual-Active Biostimulants and Pest Repellents in a Wireworm-Infested Potato Field

Wireworm, the larval stages of click beetles, are a serious pest of tubers, brassicas and other important commercial crops throughout the northern hemisphere. No effective control agent has been developed specifically for them, and many of the pesticides marketed as having secondary application against them have been withdrawn from EU and Asian markets. Metarhizium brunneum, an effective entomopathogenic fungus, and its derived volatile metabolites are known to be effective plant biostimulants and plant protectants, although field efficacy has yet to be validated. Field validation of a combined M. brunneum and derived VOC treatments was conducted in Wales, UK, to assess the effects of each as a wireworm control agent and biostimulant. Plots were treated with Tri-Soil (Trichoderma atroviridae), M. brunneum, 1-octen-3-ol or 3-octanone, or combinations thereof. Treatments were applied subsurface during potato seeding (n = 52), and potatoes were harvested at the end of the growing season. Each potato was weighed individually and scored for levels of wireworm damage. Applications of both the VOCs and the M. brunneum individually were found to significantly decrease wireworm burden (p < 0.001). Combinations of M. brunneum and 3-octanone were also found to significantly decrease wireworm damage (p < 0.001), while no effect on yield was reported, resulting in an increased saleable mass over controls (p < 0.001). Herein, we present a novel 'stimulate and deter' wireworm control strategy that can be used to significantly enhance saleable potato yields and control wireworm populations, even under high pest pressure densities.

Impact of endophytic colonization by entomopathogenic fungi on the behavior and life history of the tobacco peach aphid Myzus persicae var. nicotianae

Entomopathogenic fungi can adopt an endophytic lifestyle and provide protection against insect herbivores and plant pathogens. So far, most studies have focused on Beauveria bassiana to increase plant resistance against abiotic and biotic stresses, while only little is known for other entomopathogenic fungi. In this study, we investigated whether root inoculation of sweet pepper (Capsicum annuum L.) by the entomopathogenic fungi Akanthomyces muscarius ARSEF 5128 and B. bassiana ARSEF 3097 can improve resistance against the tobacco peach aphid Myzus persicae var. nicotianae. First, dual-choice experiments were performed to test the hypothesis that the fungi deter aphids via modifying plant volatile profiles. Next, we tested the hypothesis that endophytic colonization negatively affects aphid life history traits, such as fecundity, development and mortality rate. Aphids were significantly attracted to the odor of plants inoculated with A. muscarius over non-inoculated plants. Plants inoculated with A. muscarius emitted significantly higher amounts of β-pinene than non-inoculated plants, and significantly higher amounts of indole than B. bassiana-inoculated and non-inoculated plants. Inoculation with the fungal strains also caused significantly higher emission of terpinolene. Further, both aphid longevity and fecundity were significantly reduced by 18% and 10%, respectively, when feeding on plants inoculated with A. muscarius, although intrinsic rate of population increase did not differ between inoculated and non-inoculated plants. Sweet pepper plants inoculated with B. bassiana ARSEF 3097 did not elicit a significant behavioral response nor affected the investigated life history traits. We conclude that endophytic colonization by entomopathogenic fungi has the potential to alter olfactory behavior and performance of M. persicae var. nicotianae, but effects are small and depend on the fungal strain used.

The entomopathogenic fungus Metarhizium anisopliae enhances Arabidopsis, tomato, and maize plant growth

Species of the entomopathogenic fungi Metarhizium are used worldwide as biocontrol agents. Recently, other lifestyles have been associated with some Metarhizium species, which include their role as saprophytes, endophytes, and plant growth promoters. Herein, the effect of three Metarhizium anisopliae strains on the growth of Arabidopsis thaliana plantlets was evaluated using an in vitro split system. Arabidopsis fresh weight and total chlorophyll content significantly increased 7 days post-inoculation with the three Metarhizium anisopliae strains evaluated. The primary root length was promoted by all fungal strains without physical contact, whereas in direct contact primary root growth was inhibited. Volatile organic compounds identification revealed that during the interaction of Arabidopsis with Ma-20 and Ma-25 strains only β-caryophyllene was produced, whereas in the Arabidopsis-Ma-28 interaction o-cymene was mainly emitted. The plant growth promoting effect induced by Metarhizium anisopliae strains was also achieved in Arabidopsis, tomato and maize plants grown in soil pots. Our results showed that three Metarhizium anisopliae strains were able to increase plant fresh weight, opening promising perspectives for field production, with the advantages of insect biocontrol and plant growth promotion induced by this species of fungus.

Exploring interactions between Beauveria and Metarhizium strains through co-inoculation and responses of perennial ryegrass in a one-year trial

Perennial ryegrass (Lolium perenne L.) possesses a high level of nutritional quality and is widely used as a forage species to establish permanent pastures in southern Chile. However, the productivity of most such pastures is limited by various environmental agents, such as insect pests and drought. In this context, our work stresses the need for elucidating the ability of fungal endophytes to establish interactions with plants, and to understand how these processes contribute to plant performance and fitness. Therefore, we evaluated the colonization and impact of two native strains of the endophytic insect-pathogenic fungus (EIPF) group isolated from permanent ryegrass pastures in southern Chile. Roots and seeds of ryegrass and scarabaeid larvae were collected from nine different ryegrass pastures in the Los Ríos region of southern Chile to specifically isolate EIPFs belonging to the genera Beauveria and Metarhizium. Fungal isolations were made on 2% water agar with antibiotics, and strains were identified by analyzing the entire internal transcribed spacer (ITS) 1-5.8S-ITS2 ribosomal DNA region. Four strains of Beauveria and 33 strains of Metarhizium were isolated only in scarabaeid larvae from ryegrass pastures across four sites. Experimental mini-pastures that were either not inoculated (control) or co-inoculated with conidia of the strains Beauveria vermiconia NRRL B-67993 (P55_1) and Metarhizium aff. lepidiotae NRRL B-67994 (M25_2) under two soil humidity levels were used. Ryegrass plants were randomly collected from the mini-pastures to characterize EIPF colonization in the roots by real-time PCR and fluorescence microscopy. Aboveground biomass was measured to analyze the putative impact of colonization on the mini-pastures' aboveground phenotypic traits with R software using a linear mixed-effects model and the ANOVA statistical test. Seasonal variation in the relative abundance of EIPFs was observed, which was similar between both strains from autumn to spring, but different in summer. In summer, the relative abundance of both EIPFs decreased under normal moisture conditions, but it did not differ significantly under water stress. The aboveground biomass of ryegrass also increased from autumn to spring and decreased in summer in both the inoculated and control mini-pastures. Although differences were observed between moisture levels, they were not significant between the control and inoculated mini-pastures, except in July (fresh weight and leaf area) and October (dry weight). Our findings indicate that native strains of B. vermiconia NRRL B-67993 (P55_1) and M. aff. lepidiotae NRRL B-67994 (M25_2) colonize and co-exist in the roots of ryegrass, and these had little or no effect on the mini-pastures' aboveground biomass; however, they could have other functions, such as protection against root herbivory by insect pests.

Endophytic fungi: a tool for plant growth promotion and sustainable agriculture

Endophytic fungi are found in most, if not all, plant species on the planet. They colonise inner plant tissues without causing symptoms of disease, thus providing benefits to the host plant while also benefiting from this interaction. The global concern for the development of more sustainable agriculture has increased in recent years, and research has been performed to decipher ecology and explore the potential of endophytic interactions in plant growth. To date, many studies point to the positive aspects of endophytic colonisation, and in this review, such research is summarised based on the direct (acquisition of nutrients and phytohormone production) and indirect (induced resistance, production of antibiotics and secondary metabolites, production of siderophores and protection for abiotic and biotic stresses) benefits of endophytic colonisation. An in-depth discussion of the mechanisms is also presented.

Endophytic fungi: a tool for plant growth promotion and sustainable agriculture

Endophytic fungi are found in most, if not all, plant species on the planet. They colonise inner plant tissues without causing symptoms of disease, thus providing benefits to the host plant while also benefiting from this interaction. The global concern for the development of more sustainable agriculture has increased in recent years, and research has been performed to decipher ecology and explore the potential of endophytic interactions in plant growth. To date, many studies point to the positive aspects of endophytic colonisation, and in this review, such research is summarised based on the direct (acquisition of nutrients and phytohormone production) and indirect (induced resistance, production of antibiotics and secondary metabolites, production of siderophores and protection for abiotic and biotic stresses) benefits of endophytic colonisation. An in-depth discussion of the mechanisms is also presented.

Root-Associated Entomopathogenic Fungi Modulate Their Host Plant's Photosystem II Photochemistry and Response to Herbivorous Insects

The escalating food demand and loss to herbivores has led to increasing interest in using resistance-inducing microbes for pest control. Here, we evaluated whether root-inoculation with fungi that are otherwise known as entomopathogens improves tomato (Solanum lycopersicum) leaflets' reaction to herbivory by Spodoptera exigua (beet armyworm) larvae using chlorophyll fluorescence imaging. Plants were inoculated with Metarhizium brunneum or Beauveria bassiana, and photosystem II reactions were evaluated before and after larval feeding. Before herbivory, the fraction of absorbed light energy used for photochemistry (Φ_PSII) was lower in M. brunneum-inoculated than in control plants, but not in B. bassiana-inoculated plants. After herbivory, however, Φ_PSII increased in the fungal-inoculated plants compared with that before herbivory, similar to the reaction of control plants. At the same time, the fraction of energy dissipated as heat (Φ_NPQ) decreased in the inoculated plants, resulting in an increased fraction of nonregulated energy loss (Φ_NO) in M. brunneum. This indicates an increased singlet oxygen (¹O₂) formation not detected in B. bassiana-inoculated plants, showing that the two entomopathogenic fungi differentially modulate the leaflets' response to herbivory. Overall, our results show that M. brunneum inoculation had a negative effect on the photosynthetic efficiency before herbivory, while B. bassiana inoculation had no significant effect. However, S. exigua leaf biting activated the same compensatory PSII response mechanism in tomato plants of both fungal-inoculated treatments as in control plants.

Can Potato Plants Be Colonized with the Fungi Metarhizium and Beauveria under Their Natural Load in Agrosystems?

Beauveria and Metarhizium fungi are facultative plant endophytes that provide plant growth-stimulating, immunomodulatory, and other beneficial effects. However, little is known about the level of plant colonization by these fungi under natural conditions. We assessed the endophytic colonization of potatoes (Solanum tuberosum) with entomopathogenic fungi at their natural load in soils (102-104 colony-forming units per g). Microbiological analyses of soils and plant organs, as well as a metagenomic analysis of potato roots and leaves, were conducted in three locations in Western Siberia, consisting of conventional agrosystems and kitchen gardens. The fungi were isolated at a relatively high frequency from unsterilized roots (up to 53% of Metarhizium-positive plants). However, the fungi were sparsely isolated from the internal tissues of roots, stems, and leaves (3%). Among the genus Metarhizium, two species, M. robertsii and M. brunneum, were detected in plants as well as in soils, and the first species was predominant. A metagenomic analysis of internal potato tissues showed a low relative abundance of Beauveria and Metarhizium (<0.3%), and the communities were represented primarily by phytopathogens. We suggest that colonization of the internal tissues of potatoes occurs sporadically under a natural load of entomopathogenic fungi in soils. The lack of stable colonization of potato plants with Beauveria and Metarhizium may be due to competition with phytopathogens.

Intraspecies variation of Metarhizium brunneum against the green peach aphid, Myzus persicae, provides insight into the complexity of disease progression

BACKGROUND

Intensive application of chemical insecticides is required for aphid pest control. Among the biorational alternatives, entomopathogenic fungi are the most sustainable biocontrol agents; those of the order Hypocreales attack and cause fungal disease in arthropod hosts, with variations in host susceptibility attributed to both fungal and host characteristics. We evaluated inter- and intraspecies variations in Metarhizium spp. virulence and differences in fungal disease progression on adult and nymph stages of the green peach aphid, Myzus persicae (Sulzer), a parthenogenetically reproducing insect species.

RESULTS

Minor interspecies diversity was detected between the generalist Metarhizium species examined. Interestingly, significant intraspecies diversity was observed between Metarhizium brunneum isolates Mb7 and MbK. Infected adult aphids demonstrated similar disease progression for both isolates, mortality rates of more than 80% and fivefold reduction in fecundity. However, nymph mortality was detected only following MbK infection, with 50% mortality and significant reduction in molting rates. Confocal laser scanning microscopy demonstrated the variation in the disease stages of conidial adhesion and hemocoel colonization on each examined day post inoculation for each isolate. Significantly faster disease progression was observed in MbK-infected versus Mb7-infected nymphs, the latter demonstrating a higher percentage of uninfected nymphs accompanied with aphid molting.

CONCLUSIONS

The observed intraspecies variation suggests that altered conidial adhesion to the nymph cuticle is a major factor affecting virulence. We prove the role of nymph ecdysis as a defense mechanism disrupting fungal infection. Because significant differences were observed between closely related isolates, this study emphasizes the importance of appropriate isolate selection for biological control. © 2021 Society of Chemical Industry.

The multifunctional lifestyles of Metarhizium: evolution and applications

The genus Metarhizium is comprised of a diverse group of common soil fungi that exhibit multifunctional lifestyles with varying degrees of saprotrophic, endophytic, and insect pathogenic modes of nutrient acquisition. The transcriptome of these species is modulated to reflect immediate needs of the fungus and availability of resources-a form of transcriptional plasticity that allows for physiological adaptation to environments with diverse and dynamic exploitable nutrient sources. In this review, we discuss the endophytic, insect pathogenic lifestyles of Metarhizium spp., including their symbiotic interface, origins, and evolution, and agricultural applications. Isotope labeling experiments have demonstrated that a mutually beneficial exchange of limiting nutrients occurs between the fungus and its host plant, with nitrogen derived via insect pathogenesis being translocated from Metarhizium to host plants in exchange for fixed carbon in the form of photosynthate. Thus, the endophytic and entomopathogenic abilities of Metarhizium spp. are not exclusive of one another, but rather are interdependent and reciprocal in nature. Although endophytic, insect pathogenic fungi (EIPF) could certainly have evolved from insect pathogenic fungi, phylogenomic evidence indicates that this genus is more closely related to plant-associated fungi than animal pathogens, suggesting that Metarhizium evolved from a lineage of plant symbionts, which subsequently acquired genes for insect pathogenesis. Entomopathogenicity may have been an adaptive trait, allowing for procurement of insect-derived nitrogen that could be translocated to host plants and bartered for fixed carbon, thereby improving the stability of fungal-plant symbioses. Given their ability to simultaneously parasitize soil insects, including a number of pests of agriculturally important crops, as well as promote plant health, growth, and productivity, Metarhizium spp. are considered promising alternatives to the chemical pesticides and fertilizers that have wreaked havoc on the health and integrity of ecosystems. KEY POINTS: • Metarhizium is a fungus that is an insect pathogen as well as a plant symbiont. • The genus Metarhizium has specialist and generalist insect pathogens. • Metarhizium is phylogenetically most closely related to plant endophytes.

Entomopathogenic fungi decrease Rhizoctonia disease in potato in field conditions

Rhizoctonia potato disease is widespread in the world and causes substantial yield and quality losses in potato. This study aimed to evaluate the efficacy of entomopathogenic fungi Metarhizium robertsii and Beauveria bassiana in the inhibition of potato Rhizoctonia complex disease. The efficacy of the entomopathogenic fungi M. robertsii and B. bassiana in the defense of potato against Rhizoctonia disease (stem cancer, black scrulf and other forms of manifestation on tubers) was estimated under field conditions in Western Siberia. Preplanting treatment of the tubers with B. bassiana decreased Rhizoctonia disease in the stems and stolons. At the same time, treatment with M. robertsii did not cause a decrease in Rhizoctonia disease in these organs. However, both fungi decreased the sclerotium index on the tubers of new crops. We demonstrated two mechanisms of inhibition of Rhizoctonia solani by M. robertsii and B. bassiana, including (1) direct effect, expressed as inhibition of R. solani sclerotium formation in cocultivation assays, and (2) indirect effect, which is associated with increased peroxidase activity in potato roots under the influence of colonization by entomopathogenic fungi. We suggest that the treatment of seed tubers with B. basiana can effectively manage Rhizoctonia disease during the plant vegetative season and that both fungi significantly improve the quality of the new tuber crop.

Microbial Communities in Soils and Endosphere of Solanum tuberosum L. and their Response to Long-Term Fertilization

An understanding of how fertilization influences endophytes is crucial for sustainable agriculture, since the manipulation of the plant microbiome could affect plant fitness and productivity. This study was focused on the response of microbial communities in the soil and tubers to the regular application of manure (MF; 330 kg N/ha), sewage sludge (SF; 330 and SF3x; 990 kg N/ha), and chemical fertilizer (NPK; 330-90-300 kg N-P-K/ha). Unfertilized soil was used as a control (CF), and the experiment was set up at two distinct sites. All fertilization treatments significantly altered the prokaryotic and fungal communities in soil, whereas the influence of fertilization on the community of endophytes differed for each site. At the site with cambisol, prokaryotic and fungal endophytes were significantly shifted by MF and SF3 treatments. At the site with chernozem, neither the prokaryotic nor fungal endophytic communities were significantly associated with fertilization treatments. Fertilization significantly increased the relative abundance of the plant-beneficial bacteria Stenotrophomonas, Sphingomonas and the arbuscular mycorrhizal fungi. In tubers, the relative abundance of Fusarium was lower in MF-treated soil compared to CF. Although fertilization treatments clearly influenced the soil and endophytic community structure, we did not find any indication of human pathogens being transmitted into tubers via organic fertilizers.

Regulation by fungal endophyte of Rhodiola crenulata from enzyme genes to metabolites based on combination of transcriptome and metabolome

BACKGROUND

The contents of some its crucial metabolites tend to decrease when Rhodiola crenulata is cultured at low altitude. Interestingly, it was found that an endophyte, Phialocephala fortinii, could alleviate this problem.

RESULTS

There were 16 151 differential genes including 14 706 up-regulated and 1445 down-regulated unigenes with significant differences (P < 0.05), and a total of 1432 metabolites exhibited statistically significant (P < 0.05) metabolic differences comprising 27 different marker metabolites which showed highly significant values of VIP > 5 and P < 0.01. Results highlight differential regulation of 20 enzymatic genes that are involved in the biosynthesis of five different marker metabolites including acetaldehyde, homocysteine, cyclopropylamine, 1-pyrrolinium and halistanol sulfate.

CONCLUSIONS

The positive physiological effect of P. fortinii on R. crenulata encompasses differential regulation in carbohydrate metabolism, lipid metabolism and secondary metabolite synthesis. © 2020 Society of Chemical Industry.

Growth kinetic and nitrogen source optimization for liquid culture fermentation of Metarhizium robertsii blastospores and bioefficacy against the corn leafhopper Dalbulus maidis

The cosmopolitan entomopathogenic and root endophytic fungus Metarhizium robertsii has a versatile lifestyle and during liquid fermentation undergoes a dimorphic transformation from hyphae to conidia or microsclerotia, or from hyphae to blastospores. In all cases, these processes are mediated by environmental and nutritional cues. Blastospores could be used in spray applications to control arthropod pests above ground and may serve as an attractive alternative to the traditional solid-grown aerial conidial spores of Metarhizium spp. found in commercial products. Nitrogen is a vital nutrient in cell metabolism and growth; however, it is the expensive component in liquid cultures of entomopathogenic fungi. Our goals in this study were to optimize nitrogen sources and titers for maximum production of M. robertsii blastospores cultured in shake flasks at highly aerated conditions and to further determine their virulence against the corn leafhopper Dalbulus maidis, an important vector of serious pathogens in maize crops worldwide. Our fermentation studies revealed that the low-cost corn steep liquor (CSL) was the most suitable nitrogen source to improve blastospore growth in M. robertsii. The growth kinetic assays determined the optimal titer of 80 g L^-1 and a yield up to 4.7 × 10⁸ cells mL^-1 within 5 days of cultivation (3 days preculture and 2 days culture), at a total cost of US$0.30 L^-1. Moreover, the blastospore growth kinetic was strongly dependent on glucose and nitrogen consumptions accompanied by a slight drop in the culture pH. Insect bioassays evidenced a high virulence of these blastospores, either as dried or fresh cells, to D. maidis adults fed on maize plants. Our findings provide insights into the nutritional requirements for optimal and cost-efficient production of M. robertsii blastospores and elucidate the potential of blastospores as an ecofriendly tool against the corn leafhopper.

Bacterial decomposition of insects post-Metarhizium infection: Possible influence on plant growth

Strains of entomopathogenic fungi may have substantial differences in their final stages of mycosis. Insect cadavers are usually overgrown with mycelium after colonization of the insect body, but in many cases, bacterial decomposition of the colonized hosts occurs. We used two Metarhizium robertsii strains in the work: Mak-1 (cadavers become overgrown with mycelium and conidia) and P-72 (cadavers decay after fungal colonization). We conducted a comparative analysis of gut and cadaver microbiota in Colorado potato beetle larvae using 16S rRNA gene sequencing after infection with these strains. In addition, we estimated the content of different forms of nitrogen in cadavers and the influence of cadavers on the growth of Solanum lycopersicum on sand substrates under laboratory conditions. It was shown that infections did not lead to a significant shift in the midgut bacterial communities of infected insects compared to those of untreated insects. Importantly, bacterial communities were similar in both types of cadaver, with predominantly enterobacteria. Decomposing cadavers (P-72) were characterized by increased nitrate and ammonium, and they had a stronger growth-promoting effect on plants compared to cadavers overgrown with mycelium and conidia (Mak-1). We also estimated the colonization and growth of plants after treatment with conidia of both strains cultivated on artificial medium. Both cultures successfully colonized plants, but strain P-72 showed stronger growth promotion than Mak-1. We propose that the use of deviant strains that are unable to sporulate on cadavers leads to a faster (though only passive) flow of nitrogen from killed insects to plants.

Increased fecundity of Aphis fabae on Vicia faba plants following seed or leaf inoculation with the entomopathogenic fungus Beauveria bassiana

Since the discovery that entomopathogenic fungi can live inside plants as endophytes, researchers have been trying to understand how this affects mainly plants and herbivores. We studied how inoculation of Vicia faba L. (Fabales: Fabaceae) plants with Beauveria bassiana (Balsamo-Crivelli) Vuillemin (Ascomycota: Hypocreales) (strain GHA) either via the seeds or leaves influenced the nymph production of two successive generations of Aphis fabae Scopoli (Hemiptera: Aphididae). While we did not find any difference in nymph production for the first generation of aphids, second-generation aphids on both seed- and spray inoculated plants produced significantly higher numbers of nymphs than aphids on uninoculated plants. This emphasizes the importance of two (or multi-) generational experimentation. Beauveria bassiana was recovered from 26.0, 68.8 and 6.3% of respectively seed-, spray inoculated and control plants, thus, demonstrating its ability to live as an endophyte in V. faba. The confirmation that plants inoculated with entomopathogenic fungi can have a positive effect on pest insects makes careful consideration of these multi-trophic interactions imperative.

Non-Entomopathogenic Roles of Entomopathogenic Fungi in Promoting Plant Health and Growth

Multiple genera of hypocrealean fungi infect and kill a wide variety of arthropod pests. Several formulations based on these soilborne fungi are commercially available as biopesticides for controlling urban, garden, greenhouse, and agricultural pests. These fungi are an important part of integrated pest management strategies to maintain pest control efficacy, reduce the risk of chemical insecticide resistance, and offer environmentally sustainable pest suppression. While the entomopathogenic or pest management role of these fungi is well documented, several studies in the past decade or two have provided insights into their relationship with plants, soil, and plant pathogens, and their additional roles in promoting plant growth and health. This review highlights these endophytic, mycorrhiza-like, and disease-antagonizing roles of entomopathogenic fungi.

Seed inoculation with endophytic fungal entomopathogens promotes plant growth and reduces crown and root rot (CRR) caused by Fusarium culmorum in wheat

Fungal entomopathogens, Beauveria bassiana (NATURALIS) and Metarhizium brunneum (BIPESCO5), can promote the growth of wheat following their endophytic establishment within plants through seed treatment. Similar to endophytic B. bassiana which has already been reported as a disease antagonist by several previous studies, the present study demonstrates that M. brunneum can suppress disease pathogens following plant colonization as well. An upsurge of research hints at the ability of entomopathogenic fungi, almost exclusively considered and used as insect pathogens, to endophytically colonize the internal tissues of a wide array of host plants and subsequently confer numerous benefits including enhancement of plant growth and suppression of disease pathogens. Such an ability has mainly been investigated for Beauveria bassiana. Fewer studies have demonstrated plant growth promotion by Metarhizium brunneum colonization, whereas no studies have reported on the potential of endophytic M. brunneum as a plant disease antagonist. The present study was, therefore, conducted to investigate whether seed treatment with B. bassiana (NATURALIS) and M. brunneum (BIPESCO5) could result in their endophytic establishment in wheat and promote plant growth. The study further examines the effect of the fungal strains as endophytes against Fusarium culmorum, one of the main causal agents of crown and root rot (CRR) in wheat. Both B. bassiana and M. brunneum were able to systemically colonize roots and shoots of wheat, and promote several plant growth parameters (shoot height, root length, and fresh root and shoot weights). Moreover, endophytic colonization of wheat with either fungal entomopathogen resulted in a significant reduction in disease incidence, development and severity. These results support the notion of the multiple ecological roles that could further be played by entomopathogenic fungi. Bearing such additional roles in mind while developing these fungi as microbial agents could improve the value of many commercially available mycoinsecticides.