Agrobacterium tumefaciens-mediated genetic transformation of the entomopathogenic fungus Beauveria bassiana

Novel genomic features in entomopathogenic fungus Beauveria bassiana ILB308: accessory genomic regions and putative virulence genes involved in the infection process of soybean pest Piezodorus guildinii

BACKGROUND

Biological control methods involving entomopathogenic fungi like Beauveria bassiana have been shown to be a valuable approach in integrated pest management as an environmentally friendly alternative to control pests and pathogens. Identifying genetic determinants of pathogenicity in B. bassiana is instrumental for enhancing its virulence against insects like the resistant soybean pest Piezodorus guildinii. This study focused on comparative genomics of different B. bassiana strains and gene expression analyses to identify virulence genes in the hypervirulent strain ILB308, especially in response to infection of P. guildinii and growth on hydrocarbon HC15, a known virulence enhancer.

RESULTS

Strain ILB308 showed the highest number of virulence-related features, such as candidate virulence proteins, effectors, small secreted proteins and biosynthetic gene clusters. ILB308 also had a high percentage of unique DNA sequences, including six accessory scaffolds. Gene expression analysis at 4 days post inoculation revealed upregulation of known virulence factors, including Tudor domain proteins, LysM motif-containing proteins, subtilisin-like proteases and novel genes encoding secreted effectors and heat-labile enterotoxins. Growth on HC15 led to the upregulation of genes associated with oxidoreductase activity related to cuticular alkane degradation and fermentation metabolism/antioxidant responses in the hemolymph. The low number of known B. bassiana virulence genes points to novel or unknown mechanisms acting on the interaction between P. guildinii and strain ILB308.

CONCLUSION

The presence of accessory genomic regions and unique virulence genes in ILB308 may contribute to its higher virulence. These genes could be considered as potential targets for enhancing fungal virulence through genetic manipulation. © 2025 Society of Chemical Industry.

An efficient visual screening of gene knockout mutants in the insect pathogenic fungus Beauveria bassiana

Beauveria bassiana is an entomopathognic fungus, which is widely employed in the biological control of pests. Gene disruption is a common method for studying the functions of genes involved in fungal development or its interactions with hosts. However, generating gene deletion mutants was a time-consuming work. The transcriptional factor OpS3 has been identified as a positive regulator of a red secondary metabolite oosporein in B. bassiana. In this study, we have designed a new screening system by integrating a constitutive OpS3 expression cassette outside one of the homologous arms of target gene. Ectopic transformants predominantly exhibit a red colour with oosporein production, while knockout mutants appear as white colonies due to the loss of the OpS3 expression cassette caused by recombinant events. This screening strategy was used to obtain the deletion mutants of both tenS and NRPS genes. Correct mutants were obtained by screening fewer than 10 mutants with a positive efficiency ranging from 50% to 75%. This system significantly reduces the workload associated with DNA extraction and PCR amplification, thereby enhancing the efficiency of obtaining correct transformants in B. bassiana.

Agrobacterium tumefaciens-mediated transformation of Nigrospora sp. isolated from switchgrass leaves and antagonistic toward plant pathogens

Nigrospora is a diverse genus of fungi colonizing plants through endophytic, pathogenic, or saprobic interactions. Endophytic isolates can improve growth and development of host plants, as well as their resistance to microbial pathogens, but exactly how they do so remains poorly understood. Developing a reliable transformation method is crucial to investigate these mechanisms, in particular to identify pivotal genes for specific functions that correlate with specific traits. In this study, we identified eight isolates of Nigrospora sp. internally colonizing the leaves of switchgrass plants cultivated in North Carolina. Using an Agrobacterium tumefaciens-mediated transformation approach with control and GFP-expressing vectors, we report the first successful transformation of two Nigrospora isolates. Finally, we demonstrate that wild-type and transgenic isolates both negatively impact the growth of two plant pathogens in co-culture conditions, Bipolaris maydis and Parastagonospora nodorum, responsible for the Southern Leaf Blight and Septoria Nodorum Blotch diseases, respectively. The GFP-transformed strains developed here can therefore serve as accurate reporters of spatial interactions in future studies of Nigrospora and pathogens in the plant. Finally, the transformation method we describe lays the foundation for further genetic research on the Nigrospora genus to expand our mechanistic understanding of plant-endophyte interactions.

Expressing Parasitoid Venom Protein VRF1 in an Entomopathogen Beauveria bassiana Enhances Virulence toward Cotton Bollworm Helicoverpa armigera

Despite entomopathogenic fungi being used in various insect pest control, it is recognized that they could replace more chemical insecticides if they were more efficient. We have found that cotton bollworm Helicoverpa armigera responded to the infection of entomopathogenic fungus Beauveria bassiana by activating the Toll pathway. Koinobiont wasps also regulate host immunity and development to ensure the survival of their progeny. Previously, venom protein VRF1 was identified in Microplitis mediator. It enters H. armigera hemocytes, suppresses the expression of antimicrobial peptides (AMPs) by inhibiting the Toll pathway, and prevents parasite offspring from being encapsulated. With this in mind, we thought that it might be feasible to increase the virulence of B. bassiana by embedding VRF1 into its genome. Compared with that of wild-type (WT) B. bassiana, the median lethal dose (LD50) of the transformant expressing VRF1 (named BbVRF1) decreased approximately 2.36-fold, and the median time to lethality (LT50) was shortened to 84% when infecting H. armigera (a natural host of M. mediator). The AMP expression level of hemocytes in H. armigera infected with BbVRF1 strain was significantly downregulated compared to that in the control group infected with the WT. In addition, the LD50 of BbVRF1 against the fall armyworm Spodoptera frugiperda (an unnatural host of M. mediator) was decreased 3.45-fold and the LT50 was shortened to 73%, showing a greater virulence. Our research indicated that BbVRF1, an engineered strain of B. bassiana, has greater efficacy against pest insects both within and outside its host range (M. mediator), expanding the utilization of parasitoid wasp virulence effectors. IMPORTANCE Mycoinsecticides are essential for the development of integrated pest management as substitutes to chemical insecticides, but their usage is limited by their inferior virulence. Thus, genetically engineered bioinsecticides, including recombinant entomopathogenic fungi, have been regarded as a breakthrough to rapidly control pests. Deep knowledge of parasitoid wasps allows us to take advantage of this natural enemy of pest insects beyond raising them for field release. Our transformant BbVRF1 (Beauveria bassiana integrated with a venom protein VRF1 from Microplitis mediator) showed a higher virulence in H. armigera and S. frugiperda, demonstrating its potential for managing natural or unnatural hosts of M. mediator. This result provides a new strategy regarding which venom protein of parasitoid wasps can become part of the arsenal with which to equip entomopathogenic fungi. Utilizing parasitoid wasps with this approach could easily overcome the difficulties of artificial culture and enhance the virulence of other biocontrol agents.

A modified Agrobacterium-mediated transformation for two oomycete pathogens

Oomycetes are a group of filamentous microorganisms that include some of the biggest threats to food security and natural ecosystems. However, much of the molecular basis of the pathogenesis and the development in these organisms remains to be learned, largely due to shortage of efficient genetic manipulation methods. In this study, we developed modified transformation methods for two important oomycete species, Phytophthora infestans and Plasmopara viticola, that bring destructive damage in agricultural production. As part of the study, we established an improved Agrobacterium-mediated transformation (AMT) method by prokaryotic expression in Agrobacterium tumefaciens of AtVIP1 (VirE2-interacting protein 1), an Arabidopsis bZIP gene required for AMT but absent in oomycetes genomes. Using the new method, we achieved an increment in transformation efficiency in two P. infestans strains. We further obtained a positive GFP transformant of P. viticola using the modified AMT method. By combining this method with the CRISPR/Cas12a genome editing system, we successfully performed targeted mutagenesis and generated loss-of-function mutations in two P. infestans genes. We edited a MADS-box transcription factor-encoding gene and found that a homozygous mutation in MADS-box results in poor sporulation and significantly reduced virulence. Meanwhile, a single-copy avirulence effector-encoding gene Avr8 in P. infestans was targeted and the edited transformants were virulent on potato carrying the cognate resistance gene R8, suggesting that loss of Avr8 led to successful evasion of the host immune response by the pathogen. In summary, this study reports on a modified genetic transformation and genome editing system, providing a potential tool for accelerating molecular genetic studies not only in oomycetes, but also other microorganisms.

Tetracarboxylic acid transporter regulates growth, conidiation, and carbon utilization in Metarhizium acridum

Carbon sources and their utilization are vital for fungal growth and development. C₄-dicarboxylic acids are important carbon and energy sources that function as intermediate products of the tricarboxylic acid cycle. Transport and regulation of C₄-dicarboxylic acid uptake are mainly dependent on tetracarboxylic acid transporters (Dcts) in many microbes, although the roles of Dct genes in fungi have only been partially characterized. Here, we report on the functions of two Dct genes (Dct1 and Dct2) in the entomopathogenic fungus Metarhizium acridum. Our data showed that loss of the MaDct1 gene affected utilization of tetracarboxylic acids and other carbon sources. ΔMaDct1 mutants showed larger colony sizes with extensive mycelial growth but were delayed in conidiation with decreased conidia yield as compared to the wild-type parental strain. On the nutrient-deficient medium, SYA, the wild-type strain produced microcycle conidia, whereas the ΔMaDct1 mutant produced (normal) aerial conidia. In addition, ΔMaDct1 had decreased tolerance to cell wall perturbing agents, but increased tolerances to UV-B radiation and osmotic stress. Insect bioassays indicated that loss of MaDct1 did not affect pathogenicity. In contrast, no distinct phenotypic change was observed for the MaDct2 mutant in terms of growth and biocontrol characteristics. Transcriptomic profiling between wild type and ΔMaDct1 showed that differentially expressed genes were enriched in carbohydrate and amino acid metabolism, transport and catabolism, and signal transduction. These results demonstrate that MaDct1 regulates the conidiation pattern shift and mycelial growth by affecting utilization of carbon sources. These findings are helpful for better understanding the effect of intermediates of carbon metabolism on fungal growth and conidiation. KEY POINTS: • MaDct1 influences fungal growth and conidiation by affecting carbon source utilization. • MaDct1 regulates conidiation pattern shift under nutrient deficiency condition. • MaDct1 is involved in stress tolerance and has no effect on virulence. • MaDct2 has no effect on growth and biocontrol characteristic.

Arginine metabolism governs microcycle conidiation by changing nitric oxide content in Metarhizium acridum

Microcycle conidiation commonly exists in filamentous fungi and has great potential for mass production of mycoinsecticides. L-Arginine metabolism is essential for conidiation and conditional growth and virulence, but its role in microcycle conidiation has not been explored. Here, a unique putative arginase (MaAGA) was characterized in the entomopathogenic fungus Metarhizium acridum. Conidial germination and thermotolerance were facilitated by the disruption of MaAGA. Despite little impact on fungal growth and virulence, the disruption resulted in normal conidiation after a 60-h incubation on microcycle conidiation medium (SYA) under normal culture conditions. In the MaAGA-disruption mutant (ΔMaAGA), intracellular arginine accumulation was sharply increased. Replenishment of the direct metabolites of arginase, namely ornithine and/or urea, was unable to restore the disruption mutant's microcycle conidiation on SYA. Interestingly, nitric oxide synthase (NOS) activity and nitric oxide (NO) levels of the ΔMaAGA strain were markedly decreased in the 60-h-old SYA cultures. Finally, adding N^ω-nitro-L-arginine, an inhibitor of NOS, into the SYA converted the microcycle conidiation of the wild-type strain to normal conidiation. In contrast, adding sodium nitroprusside, an NO donor, into the SYA recovered the mutant's microcycle conidiation. The results indicate that arginine metabolism controls microcycle conidiation by changing the content of NO. KEY POINTS: • The MaAGA-disruption led to normal conidiation on microcycle conidiation medium SYA. • Nitric oxide (NO) level of the ΔMaAGA strain was markedly decreased. • Adding an NO donor into the SYA recovered the microcycle conidiation of ΔMaAGA.

The invasion process of the entomopathogenic fungus Ophiocordyceps sinensis into the larvae of ghost moths (Thitarodes xiaojinensis) using a GFP-labeled strain

Chinese cordyceps is a well-known and valuable traditional Chinese medicine that forms after Ophiocordyceps sinensis parasitizes ghost moth larvae. The low natural infection rate of O. sinensis limits large-scale artificial cultivation of Chinese cordyceps, and the invasion process is unclear. To investigate the temporal and spatial regulation when O. sinensis enters ghost moths, we constructed an O. sinensis transformant that stably expresses green fluorescent protein (GFP). Inoculating Thitarodes xiaojinensis larvae with a high concentration of GFP-labeled O. sinensis, we observed that O. sinensis conidia could adhere to the host cuticle within 2 days, germinate penetration pegs within 4 days, and produce blastospores in the host hemocoel within 6 days. The reconstructed three-dimensional (3D) structures of the invasion sites showed that penetration pegs germinated directly from O. sinensis conidia at the joining site with the larval cuticle. Differentiated appressoria or hyphae along the host epicuticle are not required for O. sinensis to invade ghost moths. Overall, the specific invasion process of O. sinensis into its host is clarified, and we provided a new perspective on the invasion process of entomopathogenic fungi.

Transcription Factor Mavib-1 Negatively Regulates Conidiation by Affecting Utilization of Carbon and Nitrogen Source in Metarhizium acridum

Conidium is the main infection unit and reproductive unit of pathogenic fungi. Exploring the mechanism of conidiation and its regulation contributes to understanding the pathogenicity of pathogenic fungi. Vib-1, a transcription factor, was reported to participate in the conidiation process. However, the regulation mechanism of Vib-1 in conidiation is still unclear. In this study, we analyzed the function of Vib-1 and its regulation mechanism in conidiation through knocking out and overexpression of Vib-1 in entomopathogenic fungus Metarhizium acridum. Results showed that the colonial growth of Mavib-1 disruption mutant (ΔMavib-1) was significantly decreased, and conidiation was earlier compared to wild type (WT), while overexpression of Mavib-1 led to a delayed conidiation especially when carbon or nitrogen sources were insufficient. Overexpression of Mavib-1 resulted in a conidiation pattern shift from microcycle conidiation to normal conidiation on nutrient-limited medium. These results indicated that Mavib-1 acted as a positive regulator in vegetative growth and a negative regulator in conidiation by affecting utilization of carbon and nitrogen sources in M. acridum. Transcription profile analysis demonstrated that many genes related to carbon and nitrogen source metabolisms were differentially expressed in ΔMavib-1 and OE strains compared to WT. Moreover, Mavib-1 affects the conidial germination, tolerance to UV-B and heat stresses, cell wall integrity, conidial surface morphology and conidial hydrophobicity in M. acridum. These findings unravel the regulatory mechanism of Mavib-1 in fungal growth and conidiation, and enrich the knowledge to conidiation pattern shift of filamentous fungi.

Enhancing the Biocontrol Potential of the Entomopathogenic Fungus in Multiple Respects via the Overexpression of a Transcription Factor Gene MaSom1

Entomopathogenic fungi play important roles in the control of populations of agricultural and disease vector pests in nature. The shortcomings of mycoinsecticides for pest management in the field cannot be completely overcome by improving single biocontrol properties of fungi. Therefore, enhancing the biocontrol potential of entomopathogenic fungi in multiple respects by genetic engineering is desirable. Transcription factors are usually involved in various important processes during fungal growth and pathogenesis via regulating a series of genes, and are important candidates for fungal improvement via genetic engineering. Herein, overexpression of MaSom1, a key transcription factor gene in the cAMP/PKA pathway, improves the biocontrol traits of Metarhizium acridum in multiple respects. When compared with WT, the MaSom1-overexpression strains exhibit enhanced tolerances to UV-B and heat shock, with increased mean 50% inhibition times by 66.9% and 155.2%, respectively. Advanced conidiation emerged accompanied by increased conidial yield up to 3.89 times after 3-day incubation for the MaSom1-overexpression strains compared to WT. Furthermore, when compared with WT, the virulence of the MaSom1-overexpression strains was also increased with the mean 50% lethality times reduced by 21.8% to 23.8%. Taken together, the MaSom1-overexpression improved the biocontrol potential of M. acridum in multiple respects. Our results provide insights into the application of key transcription factors for genetic engineering and offer a credible way to further improve the biocontrol potential of entomopathogenic fungi.

MaAreB, a GATA Transcription Factor, Is Involved in Nitrogen Source Utilization, Stress Tolerances and Virulence in Metarhizium acridum

The nitrogen catabolite repression (NCR) pathway is involved in nitrogen utilization, in which the global GATA transcription factor AreA plays an indispensable role and has been reported in many fungi. However, relatively few studies are focused on AreB, another GATA transcription factor in the NCR pathway and the functions of AreB are largely unknown in entomopathogenic fungi. Here, we characterized MaAreB in the model entomopathogenic fungus Metarhizium acridum. Sequence arrangement found that MaAreB had a conserved GATA zinc finger DNA binding domain and a leucine zipper domain. Disruption of MaAreB affected the nitrogen utilization and led to decelerated conidial germination and hyphal growth, decreased conidial yield, and lower tolerances to UV-B irradiation and heat-shock. Furthermore, the MaAreB mutant (ΔMaAreB) exhibited increased sensitivity to CFW (Calcofluor white), decreased cell wall contents (chitin and β-1,3-glucan) and reduced expression levels of some genes related to cell wall integrity, indicating that disruption of MaAreB affected the cell wall integrity. Bioassays showed that the virulence of the ΔMaAreB strain was decreased in topical inoculation but not in intra-hemocoel injection. Consistently, deletion of MaAreB severely impaired the appressorium formation and reduced the turgor pressure of appressorium. These results revealed that MaAreB regulated fungal nitrogen utilization, cell wall integrity and biological control potential, which would contribute to the functional characterization of AreB homologous proteins in other insect fungal pathogens, and even filamentous fungi.

Calcofluor white hypersensitive proteins contribute to stress tolerance and pathogenicity in entomopathogenic fungus, Metarhizium acridum

BACKGROUND

Fungal cell wall integrity is vital for fungal pathogenesis and stress tolerance. Calcofluor white (CFW), a cell wall perturbing agent, inhibits fungal growth by binding chitin in the cell wall. The roles of CFW sensitive proteins remain insufficiently understood in pathogenic fungi.

RESULTS

We investigated two calcofluor white hypersensitive proteins, MaCwh1 and MaCwh43, in the entomopathogenic fungus Metarhizium acridum. Both Green fluorescent protein (GFP)-tagged MaCwh1 and MaCwh43 localized at the endoplasmic reticulum. Our results showed that the ΔMacwh1 and ΔMacwh43 mutants were more sensitive to CFW and ultraviolet irradiation stress compared to wild-type and complement strains. ΔMacwh1 had a stronger sensitivity to these stresses than ΔMacwh43. Both ΔMacwh1 and ΔMacwh43 mutants showed smoother cell wall surface, and drastically reduced chitin and mannose glycoprotein level in the cell wall and glycerol level in conidia compared to wild type. Insect bioassay showed significantly attenuated virulence for both ΔMacwh1 and ΔMacwh43 mutants with impaired ability in penetrating the host cuticle. RNA-Seq analysis revealed that a large number of genes presumably involved in cell wall construction and modification, pathogenicity and stress response were down-regulated in both ΔMacwh1 and ΔMacwh43 mutants.

CONCLUSIONS

These findings demonstrate that both Macwh1 and Macwh43 affect the fungal cell wall ultrastructure and contribute to the stress tolerance and pest control potential in M. acrdium. © 2020 Society of Chemical Industry.

Functional and characteristic analysis of an appressorium-specific promoter PMagas1 in Metarhizium acridum

Entomopathogenic fungi have been used as important biological control agents throughout the world. To improve the biocontrol efficacy of entomopathogenic fungi, many genes have been used to improve fungal virulence or tolerance to adverse conditions via modulating their expression with strong promoters. The Magas1 gene is specifically expressed during appressorium formation and contributes to the virulence in Metarhizium acridum. In this study, we analyzed the functional region of the promoter of Magas1 gene (PMagas1) in M. acridum using 5'-deletion technique with enhanced green fluoresces protein (EGFP) as a reporter. Results showed the full length of the PMagas1 was at least 897 bp. Two regions (-897 to -611 bp and -392 to -328 bp) were essential for the activity of PMagas1. An engineered M. acridum strain was constructed with PMagas1 driving the expression of a subtilisin-like proteinase gene Pr1A (PMagas1-PR1A). Bioassay showed that the virulence was significantly increased in PMagas1-PR1A strain compared to wild type strain. Pmagas1 promoter is suitable for the overexpression of some genes during the infection of entomopathogenic fungi, which avoids the waste of nutritional resources and the influence on other fungal characteristics during the saprophytic process of constitutive promoter.

Disruption of a C69-Family Cysteine Dipeptidase Gene Enhances Heat Shock and UV-B Tolerances in Metarhizium acridum

In fungi, peptidases play a crucial role in adaptability. At present, the roles of peptidases in ultraviolet (UV) and thermal tolerance are still unclear. In this study, a C69-family cysteine dipeptidase of the entomopathogenic fungus Metarhizium acridum, MaPepDA, was expressed in Escherichia coli. The purified enzyme had a molecular mass of 56-kDa, and displayed a high activity to dipeptide substrate with an optimal Ala-Gln hydrolytic activity at about pH 6.0 and 55°C. It was demonstrated that MaPepDA is an intracellular dipeptidase localized in the cytosol, and that it is expressed during the whole fungal growth. Disruption of the MaPepDA gene increased conidial germination, growth rate, and significantly improved the tolerance to UV-B and heat stress in M. acridum. However, virulence and conidia production was largely unaffected in the ΔMaPepDA mutant. Digital gene expression data revealed that the ΔMaPepDA mutant had a higher UV-B and heat-shock tolerance compared to wild type by regulating transcription of sets of genes involved in cell surface component, cell growth, DNA repair, amino acid metabolism, sugar metabolism and some important signaling pathways of stimulation. Our results suggested that disruption of the MaPepDA could potentially improve the performance of fungal pesticides in the field application with no adverse effect on virulence and conidiation.

Colonization of Beauveria bassiana 08F04 in root-zone soil and its biocontrol of cereal cyst nematode (Heterodera filipjevi)

Cereal cyst nematodes cause serious yield losses of wheat in Hunaghuai winter wheat growing region in China. Beauveria bassiana 08F04 isolated from the surface of cysts is a promising biological control agent for cereal cyst nematodes. As the colonization capacity is a crucial criteria to assess biocontrol effectiveness for a microbial agent candidate, we aimed to label B. bassiana 08F04 for efficient monitoring of colonization in the soil. The binary pCAM-gfp plasmid containing sgfp and hph was integrated into B. bassiana 08F04 using the Agrobacterium tumefaciens-mediated transformation. The transformation caused a significant change in mycelial and conidial yields, and in extracellular chitinase activity in some transformants. The cultural filtrates of some transformants also decreased acetylcholinesterase activity and the survival of Heterodera filipjevi second-stage juveniles relative to the wild-type strain. One transformant (G10) had a growth rate and biocontrol efficacy similar to the wild-type strain, so it was used for a pilot study of B. bassiana colonization conducted over 13 weeks. Real-time PCR results and CFU counts revealed that the population of G10 increased quickly over the first 3 weeks, then decreased slowly over the following 4 weeks before stabilizing. In addition, the application of wild-type B. bassiana 08F04 and transformant G10 significantly reduced the number of H. filipjevi females in roots by 64.4% and 60.2%, respectively. The results of this study have practical applications for ecological, biological and functional studies of B. bassiana 08F04 and for bionematicide registration.

Generation of Epichloë Strains Expressing Fluorescent Proteins Suitable for Studying Host-Endophyte Interactions and Characterisation of a T-DNA Integration Event

Methods for the identification and localisation of endophytic fungi are required to study the establishment, development, and progression of host-symbiont interactions, as visible reactions or disease symptoms are generally absent from host plants. Fluorescent proteins have proved valuable as reporter gene products, allowing non-invasive detection in living cells. This study reports the introduction of genes for two fluorescent proteins, green fluorescent protein (GFP) and red fluorescent protein, DsRed, into the genomes of two distinct perennial ryegrass (Lolium perenne L.)-associated Epichloë endophyte strains using A. tumefaciens-mediated transformation. Comprehensive characterisation of reporter gene-containing endophyte strains was performed using molecular genetic, phenotypic, and bioinformatic tools. A combination of long read and short read sequencing of a selected transformant identified a single complex T-DNA insert of 35,530 bp containing multiple T-DNAs linked together. This approach allowed for comprehensive characterisation of T-DNA integration to single-base resolution, while revealing the unanticipated nature of T-DNA integration in the transformant analysed. These reporter gene endophyte strains were able to establish and maintain stable symbiotum with the host. In addition, the same endophyte strain labelled with two different fluorescent proteins were able to cohabit the same plant. This knowledge can be used to provide the basis to develop strategies to gain new insights into the host-endophyte interaction through independent and simultaneous monitoring in planta throughout its life cycle in greater detail.

Members of chitin synthase family in Metarhizium acridum differentially affect fungal growth, stress tolerances, cell wall integrity and virulence

Chitin is an important component of the fungal cell wall with a family of chitin synthases mediating its synthesis. Here, we report on the genetic characterization of the full suite of seven chitin synthases (MaChsI-VII) identified in the insect pathogenic fungus, Metarhizium acridum. Aberrant distribution of chitin was most evident in targeted gene knockouts of MaChsV and MaChsVII. Mutants of MaChsI, MaChsIII, MaChsIV showed delayed conidial germination, whereas ΔMaChsII and ΔMaChsV mutants germinated more rapidly when compared to the wild-type parent. All MaChs genes impacted conidial yield, but differentially affected stress tolerances. Inactivation of MaChsIII, MaChsV, MaChsVII resulted in cell wall fragility, and ΔMaChsV and ΔMaChsVII mutants showed high sensitivity to Congo red and calcofluor white, suggesting that the three genes are required for cell wall integrity. In addition, ΔMaChsIII and ΔMaChsVII mutants showed the highest sensitivities to heat and UV-B stress. Three of seven chitin synthase genes, MaChsIII, MaChsV, MaChsVII, were found to contribute to fungal virulence. Compared with the wild-type strain, ΔMaChsIII and ΔMaChsV mutants were reduced in virulence by topical inoculation, while the ΔMaChsVII mutant showed more severe virulence defects. Inactivation of MaChsIII, MaChsV, or MaChsVII impaired appressorium formation, affected growth of in insecta produced hyphal bodies, and altered the surface properties of conidia and hyphal bodies, resulting in defects in the ability of the mutant strains to evade insect immune responses. These data provide important links between the physiology of the cell wall and the ability of the fungus to parasitize insects and reveal differential functional consequences of the chitin synthase family in M. acridum growth, stress tolerances, cell wall integrity and virulence.

The fungal cell wall is a dynamic and flexible organelle that modulates the interaction of the pathogen with its host and acts as a critical recognition and evasion interface with host defenses. Chitin is a hallmark constituent of the fungal cell wall and all fungi contain multiple chitin synthase (Chs) genes. However, systematic characterization of chitin synthase genes has not yet been reported in entomopathogenic fungi. By using the insect pathogen Metarhizium acridum as a model, we performed a systematic genetic analysis of the seven member Chs family (ChsI-VII) in the insect pathogenic fungus. Construction of strains bearing targeted single gene mutations revealed differential contributions of specific Chs genes to growth, cell wall integrity, and stress responses. In addition, we revealed that three chitin synthase genes MaChsIII, MaChsV and MaChsVII were shown to be important for fungal appressorium formation and evasion of insect cellular and/or humoral defenses, promoting the fungal dimorphic transition to the production of hyphal bodies that occurs within hosts, and ultimately to virulence. These data provide new insights into the roles of Chs genes and chitin as critical components affecting fungal membrane structure and function.

The phosphatase gene MaCdc14 negatively regulates UV-B tolerance by mediating the transcription of melanin synthesis-related genes and contributes to conidiation in Metarhizium acridum

Reversible phosphorylation of proteins regulated by protein kinases and phosphatases mediate multiple biological events in eukaryotes. In this study, a dual-specificity cell division cycle 14 phosphatase, MaCdc14, was functionally characterized in Metarhizium acridum. Deletion of MaCdc14 decreased branch numbers, affected septum formation and resulted in multiple nuclei in each hyphal compartment, indicating nuclear division and cytokinesis defects. The spore production capacity was severely impaired with decreased conidial yield and delayed conidiation in MaCdc14-deletion mutant (ΔMaCdc14). The transcription levels of conidiation-related genes were significantly changed after MaCdc14 inactivation. The morphology of conidia was uneven in size and the germination rate of conidia was increased in ΔMaCdc14. In addition, ΔMaCdc14 displayed significantly enhanced conidial tolerance to ultraviolet (UV) irradiation but had no significant effect on the thermotolerance, the sensitivities to cell wall damage reagents, osmotic and oxidative stresses, and virulence compared to the wild-type strain and complementary transformant. Furthermore, the pigmentation of ΔMaCdc14 was increased by the upregulated expression of melanin synthesis-related genes, which may result in the enhanced UV-B tolerance of ΔMaCdc14. In summary, MaCdc14 negatively regulated UV-B tolerance by mediating the transcription of melanin synthesis-related genes, contributed to conidiation by regulating the expression levels of conidiation-related genes and also played important roles in cytokinesis and morphogenesis in Metarhizium acridum.

Truncated, strong inducible promoter Pmcl1 from Metarhizium anisopliae

In this study, Metarhizium collagen -like protein (MCL1) promoter from Metarhizium anisopliae was analysed and truncated into different sizes through series of targeted and random deletions based on the presence of various transcription factor-binding sites. Synthetic Green Fluorescent Protein (sGFP) was being utilized as a reporter gene to study the relative expression driving capability of unmodified and truncated promoters. Conserved promoter sequence analysis revealed similarity between the paralogous promoters from M. brunneum and M. acridum. sGFP expression in the haemolymph was directed with the help of mcl1 signal peptide sequence. Deleting the promoter region from - 2764 to - 1583 bp increases the promoter mcl1 (Pmcl1) activity by twofolds, while deletions of the regions upstream of - 1150 bp and - 840 bp caused a decrease of sGFP expression level (80% and 70%, respectively). Transcriptional binding sites predicted for the deleted region revealed the loss of upstream repressing sequences such as Matalpha2 along with ROX1 and Rap1 repressor-binding sites located - 2234 bp, - 1754 bp and - 1724 bp from the TSS. Compared with Pmcl1-wild type (2.7 kbp), Pmcl1-1583 bp had a shorter sequence and showed statistically significant expression in M. anisopliae. This study introduces a highly efficient strong inducible promoter for over-expression of target genes in M. anisopliae.

Mid1 affects ion transport, cell wall integrity, and host penetration of the entomopathogenic fungus Metarhizium acridum

Calcium signaling plays important roles in stress tolerance and virulence in fungi. Mid1, an accessory protein of Cch1 calcium channel, has been discussed in baker's yeast and some filamentous fungi. However, functions of the Mid1 gene in entomopathogenic fungi are not clear. In this study, the Mid1 gene was functionally characterized by deleting it in the entomopathogenic fungus Metarhizium acridum. The growth of the ΔMaMid1 mutant was similar as the wild type on normal growth medium, but inhibited by exogenous Ca²⁺, Fe²⁺, Mg²⁺, Mn²⁺, Li⁺, and calcium chelator ethylene glycol tetraacetic acid (EGTA). Cation transportation-related genes were upregulated and intracellular calcium concentration was decreased in ΔMaMid1. Deletion of the MaMid1 gene impaired the tolerance to cell wall-disrupting agents but had no impact on heat or ultraviolet irradiation tolerance compared with the wild type. Bioassays showed that ΔMaMid1 had decreased virulence, with defects in the ability to penetrate the host cuticle. Compared with the wild type, appressorium formation on locust wings and fungal growth in the insect hemocoel were significantly decreased in the ΔMaMid1 mutant in a bioassay through topical inoculation. The phenotypes of ΔMaMid1 were fully restored in a complementation strain. Taken together, our study demonstrates that the MaMid1 affects intracellular ion homeostasis and contributes to virulence by affecting the initial penetration process in M. acridum.

Genetic modification of asexual Epichloë endophytes with the perA gene for peramine biosynthesis

Development of grass-endophyte associations with minimal or no detrimental effects in combination with beneficial characteristics is important for pastoral agriculture. The feasibility of enhancing production of an endophyte-derived beneficial alkaloid through introduction of an additional gene copy was assessed in a proof-of-concept study. Sexual and asexual Epichloë species that form symbiotic associations with cool-season grasses of the Poaceae sub-family Pooideae produce bioactive alkaloids that confer resistance to herbivory by a number of organisms. Of these, peramine is thought to be crucial for protection of perennial ryegrass (Lolium perenne L.) from the Argentinian stem weevil, an economically important exotic pest in New Zealand, contributing significantly to pasture persistence. A single gene (perA) has been identified as solely responsible for peramine biosynthesis and is distributed widely across Epichloë taxa. In the present study, a functional copy of the perA gene was introduced into three recipient endophyte genomes by Agrobacterium tumefaciens-mediated transformation. The target strains included some that do not produce peramine, and others containing different perA gene copies. Mitotically stable transformants generated from all three endophyte strains were able to produce peramine in culture and in planta at variable levels. In summary, this study provides an insight into the potential for artificial combinations of alkaloid biosynthesis in a single endophyte strain through transgenesis, as well as the possibility of using novel genome editing techniques to edit the perA gene of non-peramine producing strains.

Involvement of MaSom1, a downstream transcriptional factor of cAMP/PKA pathway, in conidial yield, stress tolerances, and virulence in Metarhizium acridum

Flo8/Som1, which functions downstream from the cyclic AMP (cAMP)-dependent protein kinase A (PKA) pathway, plays important roles in hyphal development, spore formation, and virulence in yeast and several filamentous fungi. However, the functions of Som1 in entomopathogenic fungi are still a mystery. In this study, MaSom1, a Flo8/Som1 homolog, was identified and functionally characterized in a model entomopathogenic fungus Metarhizium acridum. Similar to Flo8/Som1 in other fungi, MaSom1 mainly localized to the nucleus in M. acridum. Disruption of MaSom1 reduced conidial yield, delayed conidial germination, and impaired the fungal tolerances to heat and UV-B. The expression levels of some genes involved in defenses of heat shock and UV-B radiation were significantly reduced in ΔMaSom1. MaSom1 is also important for cell wall integrity and conidial surface structures in M. acridum. Some genes related to fungal cell wall synthesis were downregulated in ΔMaSom1. Bioassays showed that ΔMaSom1 had a dramatically decreased virulence after both topical inoculation and intrahemocoel injection of the fungus in locusts. Moreover, inactivation of MaSom1 reduced appressorium formation, diminished fungal growth in locust hemolymph in vitro, and enhanced insect immune responses. Taken together, these results indicate that disruption of MaSom1 leads to a decline of fungal virulence because of impairments in conidial germination and appressorium formation, reduction of fungal growth in host hemolymph, and enhancement of insect immune responses owing to the changes in conidial surface structures.

Tri11, tri3, and tri4 genes are required for trichodermin biosynthesis of Trichoderma brevicompactum

Trichoderma brevicompactum and T. arundinaceum both can synthesize trichodermin with strong antifungal activity and high biotechnological value. The two Trichoderma species have a tri cluster, which includes seven genes (tri14, tri12, tri11, tri10, tri3, tri4, and tri6) that encode transport and regulatory enzymes required for the biosynthesis of trichodermin. Here, we isolated T. brevicompactum 0248 transformants with disrupted tri11, tri4, or tri3 gene. We also described the effect of tri11, tri3, or tri4 deletion on the expression of other genes in the tri cluster. Targeted Δtri3 knockout mutant exhibited a sharp decline in the production of trichodermin, and trichodermol, which is a substrate for trichodermin production, accumulated. Thus, the results demonstrated that tri3 was responsible for the biosynthesis of trichodermin, and the tri3 gene-encoded enzyme catalyzed the acetylation reaction of the hydroxy group at C-4 of the trichodermin skeleton. In addition, tri4 and tri11 deletion mutants were generated to evaluate the roles of tri4 and tri11 in trichodermin biosynthesis, respectively. Deletion mutant strain Δtri4 or Δtri11 did not produce trichodermin in T. brevicompactum, indicating that tri4 and tri11 are essential for trichodermin biosynthesis. This is the first to report the function of tri3, tri4 and tri11 in T. brevicompactum, although the role of tri4 and tri11 has already been described for T. arundinaceum by Cardoza et al. (Appl Environ Microbiol 77:4867-4877, 2011).

The Ste12-like transcription factor MaSte12 is involved in pathogenicity by regulating the appressorium formation in the entomopathogenic fungus, Metarhizium acridum

Homeodomain transcription factor Ste12 is a key target activated by the pathogenic mitogen-activated-protein kinase pathway, and the activated Ste12p protein regulates downstream gene expression levels to modulate phenotypes. However, the functions of Ste12-like genes in entomopathogenic fungi remain poorly understood and little is known about the downstream genes regulated by Ste12. In this study, we characterized the functions of a Ste12 orthologue in Metarhizium acridum, MaSte12, and identified its downstream target genes. The deletion mutant (ΔMaSte12) is defective in conidial germination but not in hyphal growth, conidiation, or stress tolerance. Bioassays showed that ΔMaSte12 had a dramatically decreased virulence in topical inoculations, but no significant difference was found in intrahemolymph injections when the penetration process was bypassed. The mature appressorium formation rate of ΔMaSte12 was less than 10% on locust wings, with the majority hyphae forming appressorium-like, curved but no swollen structures. Digital gene expression profiling revealed that some genes involved in cell wall synthesis and remodeling, appressorium development, and insect cuticle penetration were downregulated in ΔMaSte12. Thus, MaSte12 has critical roles in the pathogenicity of the entomopathogenic fungus M. acridum, and our study provides some explanations for the impairment of fungal virulence in ΔMaSte12. In addition, virulence is very important for fungal biocontrol agents to control insect pests effectively. This study demonstrated that MaSte12 is involved in fungal virulence but not conidial yield or fungal stress tolerance in M. acridum. Thus, MaSte12 and its downstream genes may be candidates for enhancing fungal virulence to improve mycoinsecticides.

A Botrytis cinerea KLP-7 Kinesin acts as a Virulence Determinant during Plant Infection

Botrytis cinerea is a necrotrophic pathogen that infects many important crops. In an attempt to unravel some novel factors that govern pathogenicity in B. cinerea, Agrobacterium tumefaciens mediated transformation (ATMT) was deployed, and a number of tagged transformants were generated. Among these, a mutant, BCM-29 exhibited slower growth rate, reduced conidia size, conidiation and penetration. The mutant was also defective in secretion of oxalic acid (OA) and exhibited reduced activities of polygalacturonase (PG) and pectin methyl esterases (PME). TAIL-PCR followed by BLAST search identified the tagged gene as KLP-7 that encodes for kinesin. Targeted deletion of KLP-7 resulted in several folds decrease in virulence of mutants as compared to WT, while complementation of the gene helped in rescue of virulence traits. This is the first time when a unique kinesin KLP-7 that is mainly found in the phylum Pezizomycotina has been linked to virulence in B. cinerea.

CRISPR/Cas9-mediated efficient genome editing via blastospore-based transformation in entomopathogenic fungus Beauveria bassiana

Beauveria bassiana is an environmentally friendly alternative to chemical insecticides against various agricultural insect pests and vectors of human diseases. However, its application has been limited due to slow kill and sensitivity to abiotic stresses. Understanding of the molecular pathogenesis and physiological characteristics would facilitate improvement of the fungal performance. Loss-of-function mutagenesis is the most powerful tool to characterize gene functions, but it is hampered by the low rate of homologous recombination and the limited availability of selectable markers. Here, by combining the use of uridine auxotrophy as recipient and donor DNAs harboring auxotrophic complementation gene ura5 as a selectable marker with the blastospore-based transformation system, we established a highly efficient, low false-positive background and cost-effective CRISPR/Cas9-mediated gene editing system in B. bassiana. This system has been demonstrated as a simple and powerful tool for targeted gene knock-out and/or knock-in in B. bassiana in a single gene disruption. We further demonstrated that our system allows simultaneous disruption of multiple genes via homology-directed repair in a single transformation. This technology will allow us to study functionally redundant genes and holds significant potential to greatly accelerate functional genomics studies of B. bassiana.

MaSnf1, a sucrose non-fermenting protein kinase gene, is involved in carbon source utilization, stress tolerance, and virulence in Metarhizium acridum

The protein kinase sucrose non-fermenting-1(Snf1) regulates the derepression of glucose-repressible genes and plays a major role in carbon source utilization. In this study, MaSnf1, a sucrose non-fermenting protein kinase gene, has been identified from the entomopathogenic fungus Metarhizium acridum, which has a great potential as a biocontrol agent. The functions of MaSnf1 were characterized using gene disruption and complementation strategies. Disruption of MaSnf1 reduced the conidial yield and delayed the conidial germination on potato dextrose agar (PDA) medium. MaSnf1 is also important for response to ultraviolet radiation and heat shock stress and carbon source utilization in M. acridum. Bioassays by topical inoculation and intrahemocoel injection showed that the MaSnf1 deletion mutant exhibited greatly reduced pathogenicity. The reduced expression level of chitinase gene (Chi) and protease gene (Pr1A) in MaSnf1-disruption transformant (ΔMaSnf1) most likely affects the initial penetration into its host. Additionally, the reduced expression level of acidic trehalase gene (ATM1) probably causes a decline in growth rate in insect hemolymph. Inactivation of MaSnf1 led to a significant decrease in virulence, probably owing to reduction in conidial germination, and appressorium formation, impairment in penetration, and decrease in growth rate in insect hemolymph.

Agrobacterium tumefaciens-mediated transformation in the entomopathogenic fungus Lecanicillium lecanii and development of benzimidazole fungicide resistant strains

Lecanicillium lecanii has been used in the biological control of several insects in agricultural practice. Since the gene manipulation tools for this entomopathogenic fungus have not been sufficiently developed, Agrobacterium tumefaciens-mediated transformation (ATMT) in L. lecanii was investigated in this study, using the wild-type isolate FZ9906 as a progenitor strain and the hygromycin B resistance (hph) gene as a selection marker. Furthermore, a field carbendazim-resistant (mrt) gene from Botrytis cinerea was expressed in L. lecanii FZ9906 via the ATMT system. The results revealed that the frequency of transformation surpassed 25transformants/10(6) conidia, most of the putative transformants contained a single copy of T-DNA, and the T-DNA inserts were stably inherited after five generations. All putative transformants had indistinguishable biological characteristics relative to the wild-type strain, excepting two transformants with altered growth habits or virulence. Moreover, the resistance of the putative transformants to carbendazim (MBC) was improved, and the highest one was 380-fold higher than the wild-type strain. In conclusion, ATMT is an effective and suitable system for L. lecanii transformation, and will be a useful tool for the basic and application research of gene functions and gene modifications of this strain.

MaMk1, a FUS3/KSS1-type mitogen-activated protein kinase gene, is required for appressorium formation, and insect cuticle penetration of the entomopathogenic fungus Metarhizium acridum

Entomopathogenic fungi have great potential for development as insecticides. However, large-scale use of mycoinsecticides is partially limited by poor efficiency. In many fungal pathogens, the yeast and fungal extracellular signal-regulated kinase (YERK1) subfamily is crucial to the fungal pathogenicity. In this study, a Fus3/Kss1-type mitogen-activated protein kinase (MAPK) gene MaMk1 (GenBank accession No. EFY93607) was identified in Metarhizium acridum, which encodes a member of the YERK1 subfamily. Targeted gene disruption was used to analyze the function of MaMk1 in fungal growth, conidial yield and virulence. Growth assays showed that MaMk1 disruption did not affect fungal growth and conidial yield on potato dextrose agar (PDA) plates. Bioassays by topical inoculation showed that a MaMk1-disruption mutant entirely lost its pathogenicity for the locusts, likely because of failure to penetrate the insect cuticle, which might have been caused by inability to form appressoria during infection. However, bioassays by injection showed no significant difference in virulence among the wild type (WT), ΔMaMk1 mutant and complementary transformant. ΔMaMk1 mutant failed to penetrate the cuticle outwards and sporulate on the locust cadaver. These results suggest that MaMk1 is required for penetration of the insect cuticle both into the hemocele and outside from the hemocele, but is dispensable for fungal growth in insect hemolymph. Gene expression pattern analysis showed that MaMk1 disruption downregulated expression of Mad1 and Mpl1, but did not reduce expression of Pr1 in M. acridum.

Investigating Agrobacterium-mediated transformation of Verticillium albo-atrum on plant surfaces

Background

Agrobacterium tumefaciens has long been known to transform plant tissue in nature as part of its infection process. This natural mechanism has been utilised over the last few decades in laboratories world wide to genetically manipulate many species of plants. More recently this technology has been successfully applied to non-plant organisms in the laboratory, including fungi, where the plant wound hormone acetosyringone, an inducer of transformation, is supplied exogenously. In the natural environment it is possible that Agrobacterium and fungi may encounter each other at plant wound sites, where acetosyringone would be present, raising the possibility of natural gene transfer from bacterium to fungus.

Methodology/Principal Findings

We investigate this hypothesis through the development of experiments designed to replicate such a situation at a plant wound site. A. tumefaciens harbouring the plasmid pCAMDsRed was co-cultivated with the common plant pathogenic fungus Verticillium albo-atrum on a range of wounded plant tissues. Fungal transformants were obtained from co-cultivation on a range of plant tissue types, demonstrating that plant tissue provides sufficient vir gene inducers to allow A. tumefaciens to transform fungi in planta.

Conclusions/Significance

This work raises interesting questions about whether A. tumefaciens may be able to transform organisms other than plants in nature, or indeed should be considered during GM risk assessments, with further investigations required to determine whether this phenomenon has already occurred in nature.

Knock down of chitosanase expression in phytopathogenic fungus Fusarium solani and its effect on pathogenicity

Chitosanases are lytic enzymes involved in the degradation of chitosan, a component of fungal cell walls. The phytopathogenic fungus Fusarium solani produces an extracellular chitosanase, CSN1, the role of which in the physiology and virulence of the fungus remains to be expounded. Here, we studied the expression of the CSN1 gene through gene silencing and examined its effect on fungal pathogenicity. A vector construct encoding a hairpin RNA (hpRNA) of CSN1 was constructed and introduced into the F. solani 0114 strain. The results revealed that majority of the transformants exhibited a significant reduction in chitosanase activity compared with the wild-type strain. Further, transformants with silenced CSN1 exhibited no change in mycelial growth and spore formation. However, pea pod and seedling bioassays indicated that transformants with silenced CSN1 were more virulent compared with the wild-type strain, and in sharp contrast to strains in which overexpression of the CSN1 gene resulted in virulence reduction. Although the mechanism remains unclear, our findings did suggest that F. solani chitosanase has a negative effect on fungal pathogenicity.

Agrobacterium tumefaciens-mediated transformation of Guignardia citricarpa

Guignardia citricarpa, the causal agent of Citrus Black Spot, was successfully transformed via Agrobacterium tumefaciens with cassettes for gfp and bar expression. Transformation is essential to understand the role of genes during interaction between plants and its pathogens. Using a binary plasmid vector based in the pPZP201BK, both germinated conidia and physically fragmented hyphae of G. citricarpa were transformed. Eight independent transformants of G. citricarpa resistant to ammonium glifosinate displayed GFP fluorescence. The majority (93.75%) of the G. citricarpa transformants was mitotically stable and contained a single T-DNA copy ectopically integrated to the chromosome. This is the first report of G. citricarpa transformation and will allow future work on virulence determinants of the fungus and possibly its control.