Stress response signaling and virulence: insights from entomopathogenic fungi

Functional analysis of diacylglycerol O-acyl transferase 2 gene to decipher its role in virulence of Botrytis cinerea

Gray mold disease inflicted by Botrytis cinerea is a serious menace responsible for significant economic loss worldwide. Due to its polyphagous nature, the pathogen has enthused inquisitiveness in researchers to unravel its complexity. Agrobacterium tumefaciens-mediated transformation was used to generate insertional mutants of Botrytis cinerea. A mutant (BCM-55) with disruption in a gene (BcDGAT2) that encodes for diacylglycerol O-acyl transferase 2 (DGAT2), showed enervated virulence on various hosts' tissues. Enzyme DGAT2 is crucial in the final step of synthesis of triacylglycerol (TAG) that plays an important role in homeostasis of membrane and cellular processes. However, the role of DGAT2 has never been reported in a phytopathogenic fungus. In this study, BCM-55 was characterized to ascertain the role of DGAT2 in virulence of B. cinerea. The insertional mutant was defective in spore production and lacked sclerotia formation as a consequence of lower accumulation of TAG. A significant delay in spore germination in BCM-55 was accompanied with a low penetration potential. Hyphae of the mutant formed swollen endings with considerable impairment in penetration. Deletion of BcDGAT2 also led to increased sensitivity towards cell wall and membrane-disturbing agents. Furthermore, BCM-55 was deficient in the production of oxalic acid and showed lower activity of a cell wall-degrading enzyme, polygalacturonase. The role of BcDGAT2 in virulence was further confirmed by targeted deletion and complementation of the gene. The results insinuate a crucial role of BcDGAT2 in penetration and consequently virulence of B. cinerea. The study provides novel insights into plant-pathogen interactions that can be exploited to develop suitable disease management strategies.

Functional analysis of the MAPK pathways in fungi

The Mitogen-Activated Protein Kinase (MAPK) signaling pathways constitute one of the most important and evolutionarily conserved mechanisms for the perception of extracellular information in all the eukaryotic organisms. The MAPK pathways are involved in the transfer to the cell of the information perceived from extracellular stimuli, with the final outcome of activation of different transcription factors that regulate gene expression in response to them. In all species of fungi, the MAPK pathways have important roles in their physiology and development; e.g. cell cycle control, mating, morphogenesis, response to different stresses, resistance to UV radiation and to temperature changes, cell wall assembly and integrity, degradation of cellular organelles, virulence, cell-cell signaling, fungus-plant interaction, and response to damage-associated molecular patterns (DAMPs). Considering the importance of the phylogenetically conserved MAPK pathways in fungi, an updated review of the knowledge on them is discussed in this article. This information reveals their importance, their distribution in fungal species evolutionarily distant and with different lifestyles, their organization and function, and the interactions occurring between different MAPK pathways, and with other signaling pathways, for the regulation of the most complex cellular processes.

Stress Adaptation

Fungal species display an extraordinarily diverse range of lifestyles. Nevertheless, the survival of each species depends on its ability to sense and respond to changes in its natural environment. Environmental changes such as fluctuations in temperature, water balance or pH, or exposure to chemical insults such as reactive oxygen and nitrogen species exert stresses that perturb cellular homeostasis and cause molecular damage to the fungal cell. Consequently, fungi have evolved mechanisms to repair this damage, detoxify chemical insults, and restore cellular homeostasis. Most stresses are fundamental in nature, and consequently, there has been significant evolutionary conservation in the nature of the resultant responses across the fungal kingdom and beyond. For example, heat shock generally induces the synthesis of chaperones that promote protein refolding, antioxidants are generally synthesized in response to an oxidative stress, and osmolyte levels are generally increased following a hyperosmotic shock. In this article we summarize the current understanding of these and other stress responses as well as the signaling pathways that regulate them in the fungi. Model yeasts such as Saccharomyces cerevisiae are compared with filamentous fungi, as well as with pathogens of plants and humans. We also discuss current challenges associated with defining the dynamics of stress responses and with the elaboration of fungal stress adaptation under conditions that reflect natural environments in which fungal cells may be exposed to different types of stresses, either sequentially or simultaneously.

Lack of resistance development in Bemisia tabaci to Isaria fumosorosea after multiple generations of selection

The emergence of insecticide resistant insect pests is of significant concern worldwide. The whitefly, Bemisia tabaci, is an important agricultural pest and has shown incredible resilience developing resistance to a number of chemical pesticides. Entomopathogenic fungi such as Isaria fumosorosea offer an attractive alternative to chemical pesticides for insect control, and this fungus has been shown to be an effective pathogen of B. tabaci. Little is known concerning the potential for the development of resistance to I. fumosorosea by B. tabaci. Five generations of successive survivors of B. tabaci infected by I. fumosorosea were assayed with I. fumosorosea. No significant differences in susceptibility to I. fumosorosea, number of ovarioles, or ovipostioning were seen between any of the generations tested. Effects of I. fumosorosea and cell-free ethyl acetate fractions derived from the fungus on the B. tabaci fat body, ovary, and vitellogenin were also investigated. These data revealed significant deformation and degradation of ovary tissues and associated vitellogenin by the fungal mycelium as well as by cell-free ethyl acetate fungal extracts. These data indicate the lack of the emergence of resistance to I. fumosorosea under the conditions tested and demonstrate invasion of the insect reproductive tissues during fungal infection.

Entomopathogenicity and Biological Attributes of Himalayan Treasured Fungus Ophiocordyceps sinensis (Yarsagumba)

Members of the entomophagous fungi are considered very crucial in the fungal domain relative to their natural phenomenon and economic perspectives; however, inadequate knowledge of their mechanisms of interaction keeps them lagging behind in parallel studies of fungi associated with agro-ecology, forest pathology and medical biology. Ophiocordyceps sinensis (syn. Cordyceps sinensis), an intricate fungus-caterpillar complex after it parasitizes the larva of the moth, is a highly prized medicinal fungus known widely for ages due to its peculiar biochemical assets. Recent technological innovations have significantly contributed a great deal to profiling the variable clinical importance of this fungus and other related fungi with similar medicinal potential. However, a detailed mechanism behind fungal pathogenicity and fungal-insect interactions seems rather ambiguous and is poorly justified, demanding special attention. The goal of the present review is to divulge an update on the published data and provides promising insights on different biological events that have remained underemphasized in previous reviews on fungal biology with relation to life-history trade-offs, host specialization and selection pressures. The infection of larvae by a fungus is not a unique event in Cordyceps; hence, other fungal species are also reviewed for effective comparison. Conceivably, the rationale and approaches behind the inheritance of pharmacological abilities acquired and stored within the insect framework at a time when they are completely hijacked and consumed by fungal parasites, and the molecular mechanisms involved therein, are clearly documented.

Correlation of cell surface proteins of distinct Beauveria bassiana cell types and adaption to varied environment and interaction with the host insect

The insect fungal pathogen Beauveria bassiana produces a number of distinct cell types that include aerial conidia, blastospores and haemolymph-derived cells, termed hyphal bodies, to adapt varied environment niches and within the host insect. These cells display distinct biochemical properties and surface structures, and a highly ordered outermost brush-like structure uniquely present on hyphal bodies, but not on any in vitro cells. Here, we found that the outermost structure on the hyphal bodies mainly consisted of proteins associated to structural wall components in that most of it could be removed by dithiothreitol (DTT) or proteinase K. DTT-treatment also caused delayed germination, decreased tolerance to ultraviolet irradiation and virulence of conidia or blastospores, with decreased adherence and alternated carbohydrate epitopes, suggesting involvement in fungal development, stress responses and virulence. To characterize these cell surface molecules, proteins were released from the living cells using DTT, and identified and quantitated using label-free quantitative mass spectrometry. Thereafter, a series of bioinformatics programs were used to predict cell surface-associated proteins (CSAPs), and 96, 166 and 54 CSAPs were predicted from the identified protein pools of conidia, blastospores and hyphal bodies, respectively, which were involved in utilization of carbohydrate, nitrogen, and lipid, detoxification, pathogen-host interaction, and likely other cellular processes. Thirteen, sixty-nine and six CSAPs were exclusive in conidia, blastospores and hyphal bodies, respectively, which were verified by eGFP-tagged proteins at their N-terminus. Our data provide a crucial cue to understand mechanism of B. bassiana to adapt to varied environment and interaction with insect host.

[Biochemical basis of tolerance to osmotic stress in phytopathogenic fungus: The case of Macrophomina phaseolina (Tassi) Goid.]

Fungus Macrophomina phaseolina (Tassi) Goid. is the causative agent of charcoal rot disease which causes significant yield losses in major crops such as maize, sorghum, soybean and common beans in Mexico. This fungus is a facultative parasite which shows broad ability to adapt itself to stressed environments where water deficits and/or high temperature stresses commonly occur. These environmental conditions are common for most cultivable lands throughout Mexico. Here we describe some basic facts related to the etiology and epidemiology of the fungus as well as to the importance of responses to stressed environments, particularly to water deficits, based on morphology and growth traits, as well as on physiology, biochemistry and pathogenicity of fungus M. phaseolina. To conclude, we show some perspectives related to future research into the genus, which emphasize the increasing need to improve the knowledge based on the application of both traditional and biotechnological tools in order to elucidate the mechanisms of resistance to environmental stress which can be extrapolated to other useful organisms to man.

Dissection of the contributions of cyclophilin genes to development and virulence in a fungal insect pathogen

Cyclophilins are ubiquitous proteins found in all domains of life, catalyzing peptidyl-prolyl cis-trans isomerization (PPIase activity) and functioning in diverse cellular processes. The filamentous insect pathogenic fungus, Beauveria bassiana, contains 11 cyclophilin genes whose roles were probed via individual gene knockouts, construction of over-expression strains, and a simultaneous gene knockdown strategy using tandem SiRNA. Mutants were examined for effects on conidiation, hyphal growth, cyclosporine and stress resistance, and insect virulence. BbCypA was found to be the most highly expressed cyclophilin during growth and purified recombinant BbCypA displayed cyclosporine sensitive PPIase activity. Except for ΔBbCypA, targeted gene knockouts or overexpression of any cyclophilin resulted in temperature sensitivity (TS). Specific cyclophilin mutants showed impaired hyphal growth and differential effects on conidiation and cyclosporine resistance. Insect bioassays revealed decreased virulence for two cyclophilins (ΔBbCypE and ΔBbCyp6) and the simultaneous gene knockdown mutant constructs (SiRNA30). The BbSiRNA30 strains were unaffected in growth, conidiation, or under osmotic or cell wall perturbing stress, but did show increased resistance to cyclosporine and a TS phenotype. These results revealed common and unique roles for cyclophilins in B. bassiana and validate a method for examining the effects of multi-gene families via simultaneous gene knockdown.

The autophagy-related genes BbATG1 and BbATG8 have different functions in differentiation, stress resistance and virulence of mycopathogen Beauveria bassiana

Autophagy-related proteins play significantly different roles in eukaryotes. In the entomopathogenic fungus Beauveria bassiana, autophagy is associated with fungal growth and development. BbATG1 (a serine/threonine protein kinase) and BbATG8 (a ubiquitin-like protein) have similar roles in autophagy, but different roles in other processes. Disruption mutants of BbATG1 and BbATG8 had impaired conidial germination under starvation stress. The mutant ΔBbATG8 exhibited enhanced sensitivity to oxidative stress, while a ΔBbATG1 mutant did not. BbATG1 and BbATG8 showed different roles in spore differentiation. The blastospore yield was reduced by 70% and 92% in ΔBbATG1 and ΔBbATG8 mutants, respectively, and the double mutant had a reduction of 95%. Conidial yield was reduced by approximately 90% and 50% in ΔBbATG1 and ΔBbATG8 mutants, respectively. A double mutant had a reduction similar to ΔBbATG1. Additionally, both BbATG1 and BbATG8 affected the levels of conidial protein BbCP15p required for conidiation. The virulence of each autophagy-deficient mutant was considerably weakened as indicated in topical and intrahemocoel injection assays, and showed a greater reduction in topical infection. However, BbATG1 and BbATG8 had different effects on fungal virulence. Our data indicate that these autophagy-related proteins have different functions in fungal stress response, asexual development and virulence.

Fungi with multifunctional lifestyles: endophytic insect pathogenic fungi

This review examines the symbiotic, evolutionary, proteomic and genetic basis for a group of fungi that occupy a specialized niche as insect pathogens as well as endophytes. We focus primarily on species in the genera Metarhizium and Beauveria, traditionally recognized as insect pathogenic fungi but are also found as plant symbionts. Phylogenetic evidence suggests that these fungi are more closely related to grass endophytes and diverged from that lineage ca. 100 MYA. We explore how the dual life cycles of these fungi as insect pathogens and endophytes are coupled. We discuss the evolution of insect pathogenesis while maintaining an endophytic lifestyle and provide examples of genes that may be involved in the transition toward insect pathogenicity. That is, some genes for insect pathogenesis may have been co-opted from genes involved in endophytic colonization. Other genes may be multifunctional and serve in both lifestyle capacities. We suggest that their evolution as insect pathogens allowed them to effectively barter a specialized nitrogen source (i.e. insects) with host plants for photosynthate. These ubiquitous fungi may play an important role as plant growth promoters and have a potential reservoir of secondary metabolites.

Advances in Genomics of Entomopathogenic Fungi

Fungi are the commonest pathogens of insects and crucial regulators of insect populations. The rapid advance of genome technologies has revolutionized our understanding of entomopathogenic fungi with multiple Metarhizium spp. sequenced, as well as Beauveria bassiana, Cordyceps militaris, and Ophiocordyceps sinensis among others. Phylogenomic analysis suggests that the ancestors of many of these fungi were plant endophytes or pathogens, with entomopathogenicity being an acquired characteristic. These fungi now occupy a wide range of habitats and hosts, and their genomes have provided a wealth of information on the evolution of virulence-related characteristics, as well as the protein families and genomic structure associated with ecological and econutritional heterogeneity, genome evolution, and host range diversification. In particular, their evolutionary transition from plant pathogens or endophytes to insect pathogens provides a novel perspective on how new functional mechanisms important for host switching and virulence are acquired. Importantly, genomic resources have helped make entomopathogenic fungi ideal model systems for answering basic questions in parasitology, entomology, and speciation. At the same time, identifying the selective forces that act upon entomopathogen fitness traits could underpin both the development of new mycoinsecticides and further our understanding of the natural roles of these fungi in nature. These roles frequently include mutualistic relationships with plants. Genomics has also facilitated the rapid identification of genes encoding biologically useful molecules, with implications for the development of pharmaceuticals and the use of these fungi as bioreactors.

Insect Pathogenic Fungi as Endophytes

In this chapter, we explore some of the evolutionary, ecological, molecular genetics, and applied aspects of a subset of insect pathogenic fungi that also have a lifestyle as endophytes and we term endophytic insect pathogenic fungi (EIPF). We focus particularly on Metarhizium spp. and Beauveria bassiana as EIPF. The discussion of the evolution of EIPF challenges a view that these fungi were first and foremost insect pathogens that eventually evolved to colonize plants. Phylogenetic evidence shows that the lineages of EIPF are most closely related to grass endophytes that diverged c. 100MYA. We discuss the relationship between genes involved in "insect pathogenesis" and those involved in "endophytism" and provide examples of genes with potential importance in lifestyle transitions toward insect pathogenicity. That is, some genes for insect pathogenesis may have been coopted from genes involved in endophytic colonization. Other genes may be multifunctional and serve in both lifestyle capacities. The interactions of EIPF with their host plants are discussed in some detail. The genetic basis for rhizospheric competence, plant communication, and nutrient exchange is examined and we highlight, with examples, the benefits of EIPF to plants, and the potential reservoir of secondary metabolites hidden within these beneficial symbioses.

MAPK cascade-mediated regulation of pathogenicity, conidiation and tolerance to abiotic stresses in the entomopathogenic fungus Metarhizium robertsii

Metarhizium robertsii has been used as a model to study fungal pathogenesis in insects, and its pathogenicity has many parallels with plant and mammal pathogenic fungi. MAPK (Mitogen-activated protein kinase) cascades play pivotal roles in cellular regulation in fungi, but their functions have not been characterized in M. robertsii. In this study, we identified the full complement of MAPK cascade components in M. robertsii and dissected their regulatory roles in pathogenesis, conidiation and stress tolerance. The nine components of the Fus3, Hog1 and Slt2-MAPK cascades are all involved in conidiation. The Fus3- and Hog1-MAPK cascades are necessary for tolerance to hyperosmotic stress, and the Slt2- and Fus3-MAPK cascades both mediate cell wall integrity. The Hog1 and Slt2-MAPK cascades contribute to pathogenicity; the Fus3-MAPK cascade is indispensable for fungal pathogenesis. During its life cycle, M. robertsii experiences multiple microenvironments as it transverses the cuticle into the haemocoel. RNA-seq analysis revealed that MAPK cascades collectively play a major role in regulating the adaptation of M. robertsii to the microenvironmental change from the cuticle to the haemolymph. The three MAPKs each regulate their own distinctive subset of genes during penetration of the cuticle and haemocoel colonization, but they function redundantly to regulate adaptation to microenvironmental change.

Interaction between TATA-Binding Protein (TBP) and Multiprotein Bridging Factor-1 (MBF1) from the Filamentous Insect Pathogenic Fungus Beauveria bassiana

TATA-binding protein (TBP) is a ubiquitous component of eukaryotic transcription factors that acts to nucleate assembly and position pre-initiation complexes. Multiprotein bridging factor 1 (MBF1) is thought to interconnect TBP with gene specific transcriptional activators, modulating transcriptional networks in response to specific signal and developmental programs. The insect pathogen, Beauveria bassiana, is a cosmopolitan fungus found in most ecosystems where it acts as an important regulator of insect populations and can form intimate associations with certain plants. In order to gain a better understanding of the function of MBF1 in filamentous fungi, its interaction with TBP was demonstrated. The MBF1 and TBP homologs in B. bassiana were cloned and purified from a heterologous E. coli expression system. Whereas purified BbTBP was shown to be able to bind oligonucleotide sequences containing the TATA-motif (Kd ≈ 1.3 nM) including sequences derived from the promoters of the B. bassiana chitinase and protease genes. In contrast, BbMBF1 was unable to bind to these same target sequences. However, the formation of a ternary complex between BbMBF1, BbTBP, and a TATA-containing target DNA sequence was seen in agarose gel electrophoretic mobility shift assays (EMSA). These data indicate that BbMBF1 forms direct interactions with BbTBP, and that the complex is capable of binding to DNA sequences containing TATA-motifs, confirming that BbTBP can link BbMBF1 to target sequences as part of the RNA transcriptional machinery in fungi.

Interplay between calcineurin and the Slt2 MAP-kinase in mediating cell wall integrity, conidiation and virulence in the insect fungal pathogen Beauveria bassiana

The entomopathogenic fungus, Beauveria bassiana, is of environmental and economic importance as an insect pathogen, currently used for the biological control of a number of pests. Cell wall integrity and conidiation are critical parameters for the ability of the fungus to infect insects and for production of the infectious propagules. The contribution of calcineurin and the Slt2 MAP kinase to cell wall integrity and development in B. bassiana was investigated. Gene knockouts of either the calcineurin CNA1 subunit or the Slt2 MAP kinase resulted in decreased tolerance to calcofluor white and high temperature. In contrast, the Δcna1 strain was more tolerant to Congo red but more sensitive to osmotic stress (NaCl, sorbitol) than the wild type, whereas the Δslt2 strain had the opposite phenotype. Changes in cell wall structure and composition were seen in the Δslt2 and Δcna1 strains during growth under cell wall stress as compared to the wild type. Both Δslt2 and Δcna1 strains showed significant alterations in growth, conidiation, and viability. Elevation of intracellular ROS levels, and decreased conidial hydrophobicity and adhesion to hydrophobic surfaces, were also seen for both mutants, as well as decreased virulence. Under cell wall stress conditions, inactivation of Slt2 significantly repressed CN-mediated phosphatase activity suggesting some level of cross talk between the two pathways. Comparative transcriptome profiling of the Δslt2 and Δcna1 strains revealed alterations in the expression of distinct gene sets, with overlap in transcripts involved in cell wall integrity, stress response, conidiation and virulence. These data illustrate convergent and divergent phenotypes and targets of the calcineurin and Slt2 pathways in B. bassiana.

Molecular and physiological effects of environmental UV radiation on fungal conidia

Conidia are specialized structures produced at the end of the asexual life cycle of most filamentous fungi. They are responsible for fungal dispersal and environmental persistence. In pathogenic species, they are also involved in host recognition and infection. Conidial production, survival, dispersal, germination, pathogenicity and virulence can be strongly influenced by exposure to solar radiation, although its effects are diverse and often species dependent. UV radiation is the most harmful and mutagenic waveband of the solar spectrum. Direct exposure to solar radiation for a few hours can kill conidia of most fungal species. Conidia are killed both by solar UV-A and UV-B radiation. In addition to killing conidia, which limits the size of the fungal population and its dispersion, exposures to sublethal doses of UV radiation can reduce conidial germination speed and virulence. The focus of this review is to provide an overview of the effects of solar radiation on conidia and on the major systems involved in protection from and repair of damage induced by solar UV radiation. The efforts that have been made to obtain strains of fungi of interest such as entomopathogens more tolerant to solar radiation will also be reviewed.

Stress tolerance and virulence of insect-pathogenic fungi are determined by environmental conditions during conidial formation

The virulence to insects and tolerance to heat and UV-B radiation of conidia of entomopathogenic fungi are greatly influenced by physical, chemical, and nutritional conditions during mycelial growth. This is evidenced, for example, by the stress phenotypes of Metarhizium robertsii produced on various substrates. Conidia from minimal medium (Czapek's medium without sucrose), complex medium, and insect (Lepidoptera and Coleoptera) cadavers had high, moderate, and poor tolerance to UV-B radiation, respectively. Furthermore, conidia from minimal medium germinated faster and had increased heat tolerance and were more virulent to insects than those from complex medium. Low water-activity or alkaline culture conditions also resulted in production of conidia with high tolerance to heat or UV-B radiation. Conidia produced on complex media exhibited lower stress tolerance, whereas those from complex media supplemented with NaCl or KCl (to reduce water activity) were more tolerant to heat and UV-B than those from the unmodified complex medium. Osmotic and nutritive stresses resulted in production of conidia with a robust stress phenotype, but also were associated with low conidial yield. Physical conditions such as growth under illumination, hypoxic conditions, and heat shock before conidial production also induced both higher UV-B and heat tolerance; but conidial production was not decreased. In conclusion, physical and chemical parameters, as well as nutrition source, can induce great variability in conidial tolerance to stress for entomopathogenic fungi. Implications are discussed in relation to the ecology of entomopathogenic fungi in the field, and to their use for biological control. This review will cover recent technologies on improving stress tolerance of entomopathogenic fungi for biological control of insects.