Carboxylate transporter gene JEN1 from the entomopathogenic fungus Beauveria bassiana is involved in conidiation and virulence

Genomic and Pathogenic Characterization of Akanthomyces muscarius Isolated from Living Mite Infesting Hazelnut Big Buds

The capability of entomopathogenic fungi to live as plant endophytes is well established. However, their presence in undiscovered environmental niches represents the beginning of a new challenging research journey. Recently, Akanthomyces muscarius (Ascomycota, Cordycipitaceae) (Petch) Spatafora, Kepler & B. Shrestha was isolated from hazelnut buds infested by the big bud mite pest Phytoptus avellanae Nalepa, which makes the buds swollen, reddish, and unable to further develop. Gall formation is known to be regulated by a consortium of microbes and mites, and to better understand the possible role of A. muscarius within the infested gall, its whole genome sequence was obtained using a hybrid approach of Illumina and Nanopore reads. The functional and comparative genomics analysis provided within this study may help answer questions related to the ecology and the entomopathogenicity of this fungus.

Antimicrobial Traits of Beauveria bassiana Against Rhizoctonia solani, the Causal Agent of Sheath Blight of Rice Under Field Conditions

Beauveria bassiana, an entomopathogenic fungus, has recently drawn attention worldwide not only as a potential biocontrol agent against insect pests but also for its other beneficial roles as plant disease antagonist, endophyte, plant growth promoter, and beneficial rhizosphere colonizer. In the present study, 53 native isolates of B. bassiana were screened for antifungal ability against Rhizoctonia solani, the causal agent of sheath blight of rice. Also, the mechanisms underlying such interaction and the responsible antimicrobial traits involved were studied. Following this, potential B. bassiana isolates were assayed against the reduction of sheath blight of rice under field conditions. The results showed that B. bassiana exhibited antagonistic behavior against R. solani with a percent mycelial inhibition recorded maximum of up to 71.15%. Mechanisms behind antagonism were the production of cell-wall-degrading enzymes, mycoparasitism, and the release of secondary metabolites. The study also deciphered several antimicrobial traits and the presence of virulent genes in B. bassiana as a determinant of potential plant disease antagonists. Under field conditions, combined application of the B. bassiana microbial consortium as a seed treatment, seedling root dip, and foliar sprays showed reduced sheath blight disease incidence and severity up to 69.26 and 60.50%, respectively, along with enhanced plant-growth-promoting attributes. This is one of the few studies investigating the antagonistic abilities of the entomopathogenic fungus B. bassiana against phytopathogen R. solani and the underlying mechanisms involved.

Endophytic Beauveria bassiana can protect the rice plant from sheath blight of rice caused by Rhizoctonia solani and enhance plant growth parameters

Beauveria bassiana, a potential entomopathogenic biocontrol agent, has recently drawn attention worldwide for its other additional beneficial roles such as plant disease antagonist, beneficial rhizosphere colonizer, plant growth promoter and an endophyte. In the present study, endophytic colonizing behaviour of five (5) B. bassiana isolates viz., Bb4, Bb16, Bb25, Bb44 and Bb53 were studied in rice following three (3) artificial inoculation techniques viz., seed treatment, root inoculation and foliar spray and the endophytic colonizing ability were determined by culture-based assay. After B. bassiana inoculation, rice plants were challenged with Rhizoctonia solani and disease incidence and plant growth promotion were assessed. Per cent colonization of rice stems, leaves and roots were influenced by inoculation technique, post-inoculation time (7th, 14th, 21st and 28th dpi) and plant growth medium (sterile soil, non-sterile soil), recorded maximum on 14th-day post-inoculation (dpi) i.e., 96% in stems, 92% in leaves and 28% in roots, whereas, lower colonization was recorded on 7th, 21st and 28th dpi. Whereas, the foliar spray was found best as compared to seed and root inoculation techniques, and maximum fungal recovery was observed in stems and leaves and least in roots. Upon colonization, the physical presence of B. bassiana in rice was localized by light microscopy-based studies. Potential B. bassiana strains with endophytic ability were re-isolated and their identity was determined based on morphometric and PCR-based techniques. Further, the present study also identified several virulent genes viz., BbChit1, Cdep1, Bbhog1 and Bbjen1 and extracellular hydrolytic enzymes viz., α-amylase, cellulase, lipase, pectinase and xylanase secreted by endophytic B. bassiana strains as determinants responsible for establishing the endophytic association in rice. On the other hand, a significant reduction in disease incidence was observed in the endophytic B. bassiana Bb4-, Bb16- and Bb44-inoculated plants as compared to the non-endophytic B. bassiana Bb25- and Bb53-inoculated plants along with enhanced plant growth promotion. This is one of the few studies investigating the colonization of B. bassiana in rice and its promising role as a plant disease antagonist and plant growth promoter in rice.

The Msn2 Transcription Factor Regulates Acaricidal Virulence in the Fungal Pathogen Beauveria bassiana

Beauveria bassiana holds promise as a feasible biological control agent for tick control. The B. bassiana stress–response transcription factor Msn2 is known to contribute to fungal growth, conidiogenesis, stress–response and virulence towards insects; however, little is known concerning whether Msn2 is involved in infection across Arthropoda classes. We evaluated the effects of Msn2 on B. bassiana virulence against Rhipicephalus microplus (Acari, Ixodidae) using wild-type, targeted gene knockout (ΔBbmsn2) and complemented mutant (ΔBbmsn2/Bbmsn2) strains. Reproductive parameters of R. microplus engorged females treated topically or by an intra-hemocoel injection of conidial suspensions were assessed. Treated cuticles of engorged females were analyzed by microscopy, and proteolytic activity of B. bassiana on cuticles was assessed. Topically treated engorged females showed high mean larval hatching (>84%) in control and ΔBbmsn2 treatments, whereas treatment with the wild-type or ΔBbmsn2/Bbmsn2 strains resulted in significantly decreased (lowered egg viability) larval hatching. Percent control of R. microplus topically treated with ΔBbmsn2 was lower than in the groups treated with wild-type (56.1%) or ΔBbmsn2/Bbmsn2 strains. However, no differences on reproductive parameters were detected when R. microplus were treated by intra-hemocoel injection using low (800 conidia/tick) doses for all strains tested; R. microplus injected with high doses of wild-type or mutant strains (10⁶ conidia/tick) died before laying eggs (~48 h after treatment). SEM analyses of B. bassiana infection showed similar conidial germination and formation of pseudo-appressoria on tick cuticle. Histological sections of ticks treated with the wild-type or ΔBbmsn2/Bbmsn2 strains showed fungal penetration through the cuticle, and into the tick interior. Hyphae of ΔBbmsn2, however, did not appear to penetrate or breach the tick exocuticle 120 h after treatment. Protease activity was lower on tick cuticles treated with ΔBbmsn2 than those treated with the wild-type or ΔBbmsn2/Bbmsn2 strains. These data show that loss of the Msn2 transcription factor reduced B. bassiana virulence against R. microplus, but did not interfere with conidial germination, appressoria formation or sporulation on tick cadavers, and plays only a minimal role once the cuticle is breached. Our results indicate that the BbMsn2 transcription factor acts mainly during the fungal penetration process and that decreased protease production may be one mechanism that contributes to the inability of the mutant strain to breach the tick cuticle.

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.

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.

Fungi short-chain carboxylate transporter: shift from microbe hereditary functional component to metabolic engineering target

Short-chain carboxylic acids and their derivatives are widely utilized in all aspects of our daily life. Given their specific functional groups, these molecules are also utilized in fine chemical synthesis. The traditional petroleum-based carboxylate production methods are restricted by petrol shortage and environmental pollution. Renowned for their more sustainable processes than traditional methods, biotechnological methods are preferred alternatives and have attracted increasing attention. However, the industrial application of biotechnological methods is currently limited by low factors: low productivity and low yield. Therefore, understanding the regulation of carboxylate accumulation will greatly enhance the industrial biotechnological production of short-chain carboxylate acids. The carboxylate transporter plays a crucial role in transmembrane uptake and secretion of carboxylate; therefore, regulating these transporters is of high academic and application relevance. This review concentrates on the physiological roles, regulation mechanisms, and harnessing strategies of Jen and AcpA orthologs in fungi, which provide potential clues for the biotechnological production of short-chain carboxylic acids with high efficiency.

Metabolome- and genome-scale model analyses for engineering of Aureobasidium pullulans to enhance polymalic acid and malic acid production from sugarcane molasses

BACKGROUND

Polymalic acid (PMA) is a water-soluble biopolymer with many attractive properties for food and pharmaceutical applications mainly produced by the yeast-like fungus Aureobasidium pullulans. Acid hydrolysis of PMA, resulting in release of the monomer l-malic acid (MA), which is widely used in the food and chemical industry, is a competitive process for producing bio-based platform chemicals.

RESULTS

In this study, the production of PMA and MA from sucrose and sugarcane molasses by A. pullulans was studied in shake flasks and bioreactors. Comparative metabolome analysis of sucrose- and glucose-based fermentation identified 81 intracellular metabolites and demonstrated that pyruvate from the glycolysis pathway may be a key metabolite affecting PMA synthesis. In silico simulation of a genome-scale metabolic model (iZX637) further verified that pyruvate carboxylase (pyc) via the reductive tricarboxylic acid cycle strengthened carbon flux for PMA synthesis. Therefore, an engineered strain, FJ-PYC, was constructed by overexpressing the pyc gene, which increased the PMA titer by 15.1% compared with that from the wild-type strain in a 5-L stirred-tank fermentor. Sugarcane molasses can be used as an economical substrate without any pretreatment or nutrient supplementation. Using fed-batch fermentation of FJ-PYC, we obtained the highest PMA titers (81.5, 94.2 g/L of MA after hydrolysis) in 140 h with a corresponding MA yield of 0.62 g/g and productivity of 0.67 g/L h.

CONCLUSIONS

We showed that integrated metabolome- and genome-scale model analyses were an effective approach for engineering the metabolic node for PMA synthesis, and also developed an economical and green process for PMA and MA production from renewable biomass feedstocks.

Enhanced polymalic acid production from the glyoxylate shunt pathway under exogenous alcohol stress

Polymalic acid (PMA) is a water-soluble biopolymer produced by the yeast-like fungus Aureobasidium pullulans. In this study, the physiological response of A. pullulans against exogenous alcohols stress was investigated. Interestingly, ethanol stress was an effective inducer of enhanced PMA yield, although cell growth was slightly inhibited. The stress-responsive gene malate synthase (mls), which is involved in the glyoxylate shunt, was identified and was found to be regulated by exogenous ethanol stress. Therefore, an engineered strain, YJ-MLS, was constructed by overexpressing the endogenous mls gene, which increased the PMA titer by 16.2% compared with the wild-type strain. Following addition of 1% (v/v) of ethanol, a high PMA titer of 40.0 ± 0.38 g/L was obtained using batch fermentation with the mutant YJ-MLS in a 5-L fermentor, with a strongest PMA productivity of 0.56 g/L h. This study was the interesting report to show strengthening of the carbon metabolic flow from the glyoxylate shunt for PMA synthesis, and also provided a new sight for re-recognizing the regulatory behavior of alcohol stress in eukaryotic microbes.

Identification and validation of reference genes for qRT-PCR studies of the obligate aphid pathogenic fungus Pandora neoaphidis during different developmental stages

The selection of stable reference genes is a critical step for the accurate quantification of gene expression. To identify and validate the reference genes in Pandora neoaphidis–an obligate aphid pathogenic fungus—the expression of 13classical candidate reference genes were evaluated by quantitative real-time reverse transcriptase polymerase chain reaction(qPCR) at four developmental stages (conidia, conidia with germ tubes, short hyphae and elongated hyphae). Four statistical algorithms, including geNorm, NormFinder, BestKeeper and Delta Ct method were used to rank putative reference genes according to their expression stability and indicate the best reference gene or combination of reference genes for accurate normalization. The analysis of comprehensive ranking revealed that ACT1and 18Swas the most stably expressed genes throughout the developmental stages. To further validate the suitability of the reference genes identified in this study, the expression of cell division control protein 25 (CDC25) and Chitinase 1(CHI1) genes were used to further confirm the validated candidate reference genes. Our study presented the first systematic study of reference gene(s) selection for P. neoaphidis study and provided guidelines to obtain more accurate qPCR results for future developmental efforts.

The PacC transcription factor regulates secondary metabolite production and stress response, but has only minor effects on virulence in the insect pathogenic fungus Beauveria bassiana

The PacC transcription factor is an important component of the fungal ambient pH-responsive regulatory system. Loss of pacC in the insect pathogenic fungus Beauveria bassiana resulted in an alkaline pH-dependent decrease in growth and pH-dependent increased susceptibility to osmotic (salt, sorbitol) stress and SDS. Extreme susceptibility to Congo Red was noted irrespective of pH, and ΔBbpacC conidia showed subtle increases in UV susceptibility. The ΔBbPacC mutant showed a reduced ability to acidify media during growth due to failure to produce oxalic acid. The ΔBbPacC mutant also did not produce the insecticidal compound dipicolinic acid, however, production of a yellow-colored compound was noted. The compound, named bassianolone B, was purified and its structure determined. Despite defects in growth, stress resistance, and oxalate/insecticidal compound production, only a small decrease in virulence was seen for the ΔBbpacC strain in topical insect bioassays using larvae from the greater waxmoth, Galleria mellonella or adults of the beetle, Tenebrio molitor. However, slightly more pronounced decreases were seen in virulence via intrahemcoel injection assays (G. mellonella) and in assays using T. molitor larvae. These data suggest important roles for BbpacC in mediating growth at alkaline pH, regulating secondary metabolite production, and in targeting specific insect stages.

Putative orotate transporter of Cryptococcus neoformans, Oat1, is a member of the NCS1/PRT transporter super family and its loss causes attenuation of virulence

It is well known that 5-fluoroorotic acid (5-FOA)-resistant mutants isolated from wild-type Cryptococcus neoformans are exclusively either ura3 or ura5 mutants. Unexpectedly, many of the 5-FOA-resistant mutants isolated in our selective regime were Ura⁺. We identified CNM00460 as the gene responsible for these mutations. Cnm00460 belongs to the nucleobase cation symporter 1/purine-related transporter (NCS1/PRT) super family of fungal transporters, representative members of which are uracil transporter, uridine transporter and allantoin transporter of Saccharomyces cerevisiae. Since the CNM00460 gene turned out to be involved in utilization of orotic acid, most probably as transporter, we designated this gene Orotic Acid Transporter 1 (OAT1). This is the first report of orotic acid transporter in this family. C. neoformans has four members of the NCS1/PRT family, including Cnm00460, Cnm02550, Cnj00690, and Cnn02280. Since the cnm02550∆ strain showed resistance to 5-fluorouridine, we concluded that CNM02550 encodes uridine permease and designated it URidine Permease 1 (URP1). We found that oat1 mutants were sensitive to 5-FOA in the medium containing proline as nitrogen source. A mutation in the GAT1 gene, a positive transcriptional regulator of genes under the control of nitrogen metabolite repression, in the genetic background of oat1 conferred the phenotype of weak resistance to 5-FOA even in the medium using proline as nitrogen source. Thus, we proposed the existence of another orotic acid utilization system (tentatively designated OAT2) whose expression is under the control of nitrogen metabolite repression at least in part. We found that the OAT1 gene is necessary for full pathogenic activity of C. neoformans var. neoformans.

Variation in virulence of Beauveria bassiana and B. pseudobassiana to the pine weevil Pissodes nemorensis in relation to mycelium characteristics and virulence genes

Entomopathogenic fungi such as Beauveria spp. have potential applications in the biocontrol of insect pests but little is known regarding their infectivity to the pine weevil Pissodes nemorensis. In this study, five isolates of Beauveria pseudobassiana and five isolates of Beauveria bassiana were tested for characteristics correlating with virulence on P. nemorensis. Isolate UAMH301 had the lowest mean lethal concentration value whereas the highest value was obtained with isolate LRC137. Growth rate was negatively correlated with virulence in B. bassiana, because isolate LRC137, the least virulent isolate, grew much more rapidly than the other B. bassiana isolates on SDYA. In contrast, its growth on a hyperosmotic medium was the slowest. Sporulation rate and conidial area were not correlated with virulence. Mycelial cell density was positively correlated with virulence in both species, and the four tested genes appear to be one-copy genes. Bbchit1 and Bbhog1, genes respectively encoding a chitinase and a protein kinase, induced relative expression levels were positively correlated with virulence in B. pseudobassiana. We discuss in terms of previous morphological, physiological and genetic parameters related to virulence in Beauveria and the importance of testing the expression of putative virulence genes in comparison with their basal transcript levels.

Selection of reference genes for expression analysis in the entomophthoralean fungus Pandora neoaphidis

The selection of suitable reference genes is crucial for accurate quantification of gene expression and can add to our understanding of host-pathogen interactions. To identify suitable reference genes in Pandora neoaphidis, an obligate aphid pathogenic fungus, the expression of three traditional candidate genes including 18S rRNA(18S), 28S rRNA(28S) and elongation factor 1 alpha-like protein (EF1), were measured by quantitative polymerase chain reaction at different developmental stages (conidia, conidia with germ tubes, short hyphae and elongated hyphae), and under different nutritional conditions. We calculated the expression stability of candidate reference genes using four algorithms including geNorm, NormFinder, BestKeeper and Delta Ct. The analysis results revealed that the comprehensive ranking of candidate reference genes from the most stable to the least stable was 18S (1.189), 28S (1.414) and EF1 (3). The 18S was, therefore, the most suitable reference gene for real-time RT-PCR analysis of gene expression under all conditions. These results will support further studies on gene expression in P. neoaphidis.

Genome sequence, comparative analysis, and evolutionary insights into chitinases of entomopathogenic fungus Hirsutella thompsonii

Hirsutella thompsonii (Ht) is a fungal pathogen of acarines and the primary cause of epizootics among mites. The draft genomes of two isolates of Ht (MTCC 3556: Ht3, 34.6 Mb and MTCC 6686: Ht6, 34.7 Mb) are presented and compared with the genomes of Beauveria bassiana (Bb) ARSEF 2860 and Ophiocordyceps sinensis (Os) CO18. Comparative analysis of carbohydrate active enzymes, pathogen–host interaction genes, metabolism-associated genes, and genes involved in biosynthesis of secondary metabolites in the four genomes was carried out. Reduction in gene family sizes in Ht3 and Os as compared with Ht6 and Bb is observed. Analysis of the mating type genes in Ht reveals the presence of MAT idiomorphs which is suggestive of cryptic sexual traits in Ht. We further identify and classify putative chitinases that may function as virulence factors in fungal entomopathogens due to their role in degradation of arthropod cuticle.

Evaluation of putative internal reference genes for gene expression normalization in Nannochloropsis sp. by quantitative real-time RT-PCR

Quantitative real-time reverse transcription PCR (RT-qPCR), a sensitive technique for quantifying gene expression, depends on the stability of the reference gene(s) used for data normalization. To date, few studies on reference genes have been undertaken for Nannochloropsis sp. In this study, 12 potential reference genes were evaluated for their expression stability using the geNorm and NormFinder statistical algorithms by RT-qPCR. The results showed that the best reference genes differed depending on the treatments: different light intensities (DL), the diurnal cycle (DC), high light intensity (HL) and low temperature treatments (LT). A combination of ACT1, ACT2 and TUA would be appropriate as a reference panel for normalizing gene expression data across all the treatments. ACT2 showed the most stable expression across all tested samples but was not the most stable one for individual treatments. Though 18S showed the least stable expression considering all tested samples, it is the most stable one for LT using geNorm. The expression of Lhc confirmed that the appropriate reference genes are crucial. These results provide a foundation for more accurate use of RT-qPCR under different experimental conditions in Nannochloropsis sp. gene analysis.

A fungal homologue of neuronal calcium sensor-1, Bbcsa1, regulates extracellular acidification and contributes to virulence in the entomopathogenic fungus Beauveria bassiana

Neuronal calcium sensor proteins and their homologues participate in transducing extracellular signals that affect intracellular Ca(2+) levels, which in turn regulate enzyme activities, secretion, gene expression and other biological processes. The filamentous fungus Beauveria bassiana is a broad-host-range pathogen of insects that acidifies the extracellular milieu during growth and pathogenesis towards target hosts. A collection of B. bassiana random insertion mutants were screened on pH indicator plates and one mutant was isolated that displayed reduced acidification. The random insertion site was mapped to a gene that displayed homology to the neuronal calcium sensor/frequenin protein family and was designated Bbcsa1. To validate the role of Bbcsa1 in B. bassiana, a targeted gene-knockout was constructed. Data confirmed that Bbcsa1 was not an essential gene and the ΔBbcsa1 strain displayed delayed acidification of the medium when grown in Czapek-Dox medium, as compared with the wild-type parent. HPLC profiling of secreted metabolites did not detect any major changes in the production of organic acids, although downregulation of the membrane H(+) pump/ATPase was noted in the mutant. A slight growth-deficient phenotype was observed for the ΔBbcsa1 strain on Czapek-Dox and potato dextrose media, which was accentuated at high calcium concentrations (500 mM) and 1.5 M sorbitol, but was unaffected by EDTA or SDS. Perturbations in vacuole morphology were also noted for the mutant. Insect bioassays using Galleria mellonella as the target host revealed decreased virulence in the ΔBbcsa1 mutant when applied topically, representing the natural route of infection, but no significant effect was observed when fungal cells were directly injected into target hosts. These results suggest that Bbcsa1 participates in pre-penetration or early penetration events, but is dispensable once the insect cuticle has been breached.

Selection of optimal reference genes for expression analysis in the entomopathogenic fungus Beauveria bassiana during development, under changing nutrient conditions, and after exposure to abiotic stresses

The selection of suitable reference genes is crucial for accurate quantification of gene expression. To identify suitable reference genes in Beauveria bassiana, the expression of 14 candidates (18S, 28S, β-Tub, GAPD, γ-Act, TEF, HGPT, His3, His2A, TBP, CypA, CypB, PP1, and CrzA) was measured by quantitative polymerase chain reaction at different development stages and under various nutritional and stress conditions. Expression stability, as evaluated by the geNorm and NormFinder programs, revealed that His2A/γ-Act/CrzA was the most stably expressed set of genes throughout development, while 28S/PP1/CypA and His2A/γ-Act/CypA were the most stably expressed gene sets under a variety of nutritional and stress conditions, respectively. Overall, the most stably expressed genes under all conditions examined were PP1, γ-Act, and CypA.

A conidial protein (CP15) of Beauveria bassiana contributes to the conidial tolerance of the entomopathogenic fungus to thermal and oxidative stresses

Aerial conidia are central dispersing structures for most fungi and represent the infectious propagule for entomopathogenic fungus Beauveria bassiana, thus the active ingredients of commercial mycoinsecticides. Although a number of formic-acid-extractable (FAE) cell wall proteins from conidia have been characterized, the functions of many such proteins remain obscure. We report that a conidial FAE protein, termed CP15, isolated from B. bassiana is related to fungal tolerance to thermal and oxidative stresses. The full-length genomic sequence of CP15 was shown to lack introns, encoding for a 131 amino acid protein (15.0 kDa) with no sequence identity to any known proteins in the NCBI database. The function of this new gene with two genomic copies was examined using the antisense-RNA method. Five transgenic strains displayed various degrees of silenced CP15 expression, resulting in significantly reduced conidial FAE protein profiles. The FAE protein contents of the strains were linearly correlated to the survival indices of their conidia when exposed to 30-min wet stress at 48°C (r (2) = 0.93). Under prolonged 75-min heat stress, the median lethal times (LT(50)s) of their conidia were significantly reduced by 13.6-29.5%. The CP15 silenced strains were also 20-50% less resistant to oxidative stress but were not affected with respect to UV-B or hyperosmotic stress. Our data indicate that discrete conidial proteins may mediate resistance to some abiotic stresses, and that manipulation of such proteins may be a viable approach to enhancing the environmental fitness of B. bassiana for more persisting control of insect pests in warmer climates.

Carbon catabolite repressor gene BbCre1 influences carbon source uptake but does not have a big impact on virulence in Beauveria bassiana

A gene (BbCre1, GenBank accession number EF108309) encoding a carbon catabolite repressor (CreA) with two Cys(2)His(2) zinc finger regions and a nuclear localization signal was cloned from the entomopathogenic fungus Beauveria bassiana. Overexpression and antisense strategies were used to investigate the biological functions of this gene. Compared with the wild type, the conidial yield and colony growth of BbCre1-overexpression transformants were significantly decreased on the plates with xylose or ethanol as the sole carbon source. With glucose as the sole carbon source, a significant difference was observed in the activity of Pr1A-like protease among BbCre1-overexpression transformants, antisense-BbCre1 transformants and the wild type. However, bioassays showed that knockdown or overexpression of BbCre1 did not have a significant impact on the virulence of B. bassiana to aphids. These results imply that the fungus remains virulent, even when simpler, less expensive nutrients are available, i.e. glucose.