RNA sequencing analysis identifies the metabolic and developmental genes regulated by BbSNF1 during conidiation of the entomopathogenic fungus Beauveria bassiana

Comparative Transcriptomics of the Entomopathogenic Fungus Beauveria bassiana Grown on Aerial Surface and in Liquid Environment

Beauveria bassiana, the causative agent of arthropod, proliferates in the host hemolymph (liquid environment) and shits to saprotrophic growth on the host cadaver (aerial surface). In this study, we used transcriptomic analysis to compare the gene expression modes between these two growth phases. Of 10,366 total predicted genes in B. bassiana, 10,026 and 9985 genes were expressed in aerial (AM) and submerged (SM) mycelia, respectively, with 9853 genes overlapped. Comparative analysis between two transcriptomes indicated that there were 1041 up-regulated genes in AM library when compared with SM library, and 1995 genes were down-regulated, in particular, there were 7085 genes without significant change in expression between two transcriptomes. Furthermore, of 25 amidase genes (AMD), BbAMD5 has high expression level in both transcriptomes, and its protein product was associated with cell wall in aerial and submerged mycelia. Disruption of BbAMD5 significantly reduced mycelial hydrophobicity, hydrophobin translocation, and conidiation on aerial plate. Functional analysis also indicated that BbAmd5 was involved in B. bassiana blastospore formation in broth, but dispensable for fungal virulence. This study revealed the high similarity in global expression mode between mycelia grown under two cultivation conditions.

Two aminopeptidase I homologs convergently contribute to pathobiology of fungal entomopathogen Beauveria bassiana via divergent physiology-dependent autophagy pathways for vacuolar targeting

INTRODUCTION

In yeast, the cytoplasm-to-vacuole targeting (Cvt) pathway acts as a biosynthetic autophagy-related process, in which vacuolar targeting of hydrolase is mediated by the machineries involved in the selective autophagy. However, the mechanistic insights into vacuolar targeting of hydrolases through the selective autophagy pathway still remain enigmatic in filamentous fungi.

OBJECTIVES

Our study aims to investigate the mechanisms involved in vacuolar targeting of hydrolases in filamentous fungi.

METHODS

The filamentous entomopathogenic fungus Beauveria bassiana was used as a representative of filamentous fungi. We identified the homologs of yeast aminopeptidase I (Ape1) in B. bassiana by bioinformatic analyses and characterized their physiological roles by gene function analyses. Pathways for vacuolar targeting of hydrolases were investigated via molecular trafficking analyses.

RESULTS

B. bassiana has two homologs of yeast aminopeptidase I (Ape1) which are designated as BbApe1A and BbApe1B. The two homologs of yeast Ape1 contribute to starvation tolerance, development, and virulence in B. bassiana. Significantly, BbNbr1 acts as a selective autophagy receptor to mediate the vacuolar targeting of the two Ape1 proteins, in which BbApe1B interacts with BbNbr1 also directly interacting with BbAtg8, and BbApe1A has an additional requirement of the scaffold protein BbAtg11 that interacts with BbNbr1 and BbAtg8. Protein processing occurs at both terminuses of BbApe1A and only at carboxyl terminus of BbApe1B, which is also dependent on the autophagy-related proteins. Together, the functions and translocation processes of the two Ape1 proteins are associated with autophagy in fungal lifecycle.

CONCLUSION

This study reveals the functions and translocation processes for vacuolar hydrolases in the insect-pathogenic fungi and improves our understandings of the Nbr1-mediated vacuolar targeting pathway in the filamentous fungi.

MARVEL family proteins contribute to vegetative growth, development, and virulence of the insect fungal pathogen Beauveria bassiana

Beauveria bassiana is one of the most extensively studied entomopathogenic fungi (EPF) and is widely used as a biocontrol agent against various insect pests. Proteins containing the MARVEL domain are conserved in eukaryotes, typically with four transmembrane structures. In this study, we identified the five MARVEL domain proteins in B. bassiana. Five MARVEL domain proteins were localized to cytomembrane and vacuoles in B. bassiana, but had different roles in maintaining the lipid-droplet homeostasis. These proteins were required for fungal virulence, but differentially contributed to fungal utilization of nutrients, stress tolerance, and development under aerial and submerged conditions. Notably, BbMARVEL2 was essential for conidial surface morphology. Additionally, these five MARVEL domain proteins contributed to fungal interaction with the host immune defense. This study provides new mechanistic insights into the life cycle of B. bassiana as a biocontrol agent.

Succinate Dehydrogenase Subunit C Contributes to Mycelial Growth and Development, Stress Response, and Virulence in the Insect Parasitic Fungus Beauveria bassiana

Succinate dehydrogenase (SDH), also known as respiratory chain complex II, plays a crucial role in energy production in which SdhC functions as an anchored subunit in the inner membrane of mitochondria. In this study, domain annotation analyses revealed that two SdhC domain-containing proteins were present in the filamentous insect-pathogenic fungus Beauveria bassiana, and they were named BbSdhC1 and BbSdhC2, respectively. Only BbSdhC1 localized to mitochondria; hence, this protein is considered the ortholog of SdhC in B. bassiana. Ablation of BbSdhC1 led to significantly reduced vegetative growth on various nutrients. The ΔBbsdhc1 mutant displayed the significantly reduced ATP synthesis and abnormal differentiation under aerial and submerged conditions. Notably, the BbSdhC1 loss resulted in enhanced intracellular levels of reactive oxygen species (ROS) and impaired growth of mycelia under oxidative stress. Finally, insect bioassays (via cuticle and intrahemocoel injection infection) revealed that disruption of BbSdhC1 significantly attenuated fungal virulence against the insect hosts. These findings indicate that BbSdhC1 contributes to vegetative growth, resistance to oxidative stress, differentiation, and virulence of B. bassiana due to its roles in energy generation and maintaining the homeostasis of the intracellular ROS levels. IMPORTANCE The electron transport chain (ETC) is critical for energy supply by mediating the electron flow along the mitochondrial membrane. Succinate dehydrogenase (SDH) is also known as complex II in the ETC, in which SdhC is a subunit anchored in mitochondrial membrane. However, the physiological roles of SdhC remain enigmatic in filamentous fungi. In filamentous insect-pathogenic fungus B. bassiana, SdhC is required for maintaining mitochondrial functionality, which is critical for fungal stress response, development, and pathogenicity. These findings improve our understanding of physiological mechanisms of ETC components involved in pathogenicity of the entomopathogenic fungi.

AMPK Is Involved in Regulating the Utilization of Carbon Sources, Conidiation, Pathogenicity, and Stress Response of the Nematode-Trapping Fungus Arthrobotrys oligospora

AMP-activated protein kinase (AMPK), a heterotrimeric complex, can sense energy and nutritional status in eukaryotic cells, thereby participating in the regulation of multiple cellular processes. In this study, we characterized the function of the catalytic α-subunit (SNF1) and the two regulatory β- and γ-subunits (GAL83 and SNF4) of AMPK in a representative nematode-trapping fungus, Arthrobotrys oligospora, by gene knockout, phenotypic analysis, and RNA sequencing. The ability of the AMPK complex mutants (including ΔAosnf1, ΔAogal83, and ΔAosnf4) to utilize a nonfermentable carbon source (glycerol) was reduced, and the spore yields and trap formation were remarkably decreased. Moreover, AMPK plays an important role in regulating stress response and nematode predation efficiency. Transcriptomic profiling between the wild-type strain and ΔAosnf1 showed that differentially expressed genes were enriched for peroxisome, endocytosis, fatty acid degradation, and multilipid metabolism (sphingolipid, ether lipid, glycerolipid, and glycerophospholipid). Meanwhile, a reduced lipid droplet accumulation in ΔAosnf1, ΔAogal83, and ΔAosnf4 mutants was observed, and more vacuoles appeared in the mycelia of the ΔAosnf1 mutant. These results highlight the important regulatory role of AMPK in the utilization of carbon sources and lipid metabolism, as well as providing novel insights into the regulatory mechanisms of the mycelia development, conidiation, and trap formation of nematode-trapping (NT) fungi. IMPORTANCE NT fungi are widely distributed in various ecosystems and are important factors in the control of nematode populations in nature; their trophic mycelia can form unique infectious devices (traps) for capturing nematodes. Arthrobotrys oligospora is a representative NT fungi which can develop complex three-dimensional networks (adhesive networks) for nematode predation. Here, we demonstrated that AMPK plays an important role in the glycerol utilization, conidiation, trap formation, and nematode predation of A. oligospora, which was further confirmed by transcriptomic analysis of the wild-type and mutant strains. In particular, our analysis indicated that AMPK is required for lipid metabolism, which is primarily associated with energy regulation and is essential for trap formation. Therefore, this study extends the functional study of AMPK in NT fungi and helps to elucidate the molecular mechanism of the regulation of trap development, as well as laying the foundation for the development of efficient nematode biocontrol agents.

Magnaporthe oryzae Chloroplast Targeting Endo-β-1,4-Xylanase I MoXYL1A Regulates Conidiation, Appressorium Maturation and Virulence of the Rice Blast Fungus

Endo-β-1,4-Xylanases are a group of extracellular enzymes that catalyze the hydrolysis of xylan, a principal constituent of the plant primary cell wall. The contribution of Endo-β-1,4-Xylanase I to both physiology and pathogenesis of the rice blast fungus M. oryzae is unknown. Here, we characterized the biological function of two endoxylanase I (MoXYL1A and MoXYL1B) genes in the development of M. oryzae using targeted gene deletion, biochemical analysis, and fluorescence microscopy. Phenotypic analysis of ∆Moxyl1A strains showed that MoXYL1A is required for the full virulence of M. oryzae but is dispensable for the vegetative growth of the rice blast fungus. MoXYL1B, in contrast, did not have a clear role in the infectious cycle but has a critical function in asexual reproduction of the fungus. The double deletion mutant was severely impaired in pathogenicity and virulence as well as asexual development. We found that MoXYL1A deletion compromised appressorium morphogenesis and function, leading to failure to penetrate host cells. Fluorescently tagged MoXYL1A and MoXYL1B displayed cytoplasmic localization in M. oryzae, while analysis of MoXYL1A-GFP and MoXYL1B-GFP in-planta revealed translocation and accumulation of these effector proteins into host cells. Meanwhile, sequence feature analysis showed that MoXYL1A possesses a transient chloroplast targeting signal peptide, and results from an Agrobacterium infiltration assay confirmed co-localization of MoXYL1A-GFP with ChCPN10C-RFP in the chloroplasts of host cells. MoXYL1B, accumulated to the cytoplasm of the host. Taken together, we conclude that MoXYL1A is a secreted effector protein that likely promotes the virulence of M. oryzae by interfering in the proper functioning of the host chloroplast, while the related xylanase MoXYL1B does not have a major role in virulence of M. oryzae.

Steryl Acetyl Hydrolase 1 (BbSay1) Links Lipid Homeostasis to Conidiogenesis and Virulence in the Entomopathogenic Fungus Beauveria bassiana

Beauveria bassiana, as a well-studied entomopathogenic fungus, has a great potential for the biological control of insect pests. Lipid metabolism has been linked to the life cycle of B. bassiana; however, the underlying mechanisms remain unknown. In this study, a homolog of yeast steryl acetyl hydrolase 1 (Say1) was functionally characterized. The loss of B. bassianaSAY1 (BbSAY1) impaired the lipid homeostasis in conidia, with a significant reduction in oleic acid content. The ΔBbsay1 mutant strain displayed anelevated accumulation of lipid bodies and aweakened membrane permeability. As for phenotypic aspects, gene loss resulted in significant defects in germination, conidiation, and virulence. Our findings highlight that Say1, involved in lipid homeostasis, contributes to the cytomembrane integrity, development, and virulence in B. bassiana.

Metabolite profiling reveals overexpression of the global regulator, MoLAEA leads to increased synthesis of metabolites in Magnaporthe oryzae

AIMS

To study the altered metabolic pathways and metabolites produced in overexpression and knockdown mutants of a global regulator named MoLAEA, which was recently found to regulate the expression of the genes involved in secondary metabolism in one of the most destructive plant pathogens, Magnaporthe oryzae.

METHODS AND RESULTS

Mass spectrometry-based global untargeted metabolomic profiling was used to identify altered metabolites. Metabolites were extracted from the mutant strains of MoLAEA using two extraction methods viz., aqueous and organic extraction and data acquired using liquid chromatography-tandem mass spectrometry (LC-MS/MS) in positive and negative polarities. Levels of metabolites involved in various biological pathways such as amino acid as well as polyamine biosynthesis, fatty acid and pyrimidine metabolism showed remarkable change in the mutant strains. Interestingly, metabolites involved in stress responses were produced in higher quantities in the overexpression strain whereas, certain overproduced metabolites were associated with distinctive phenotypic changes in the overexpression strain compared to the wild-type. Further, the expression of several genes involved in the stress responses was found to have higher expression in the overexpression strain.

CONCLUSIONS

The global regulator MoLAEA is involved in secondary metabolism in the plant pathogen M. oryzae such that the mutant strains showed altered level of several metabolites involved in the biosynthesis pathways compared to the wild-type. Also, metabolites involved in stress responses were overproduced in the overexpression strain and this can be seen in the higher growth in media amended with stress-inducing agents or higher expression of genes involved in stress response in the overexpression strain compared to the wild-type.

SIGNIFICANCE AND IMPACT

This is the first report of metabolite profiling relative to the global regulation of secondary metabolism in M. oryzae, where secondary metabolism is poorly understood. It opens up avenues for more relevant investigations on the genetic regulation of several of the metabolites found in the analysis, which have not been previously characterized in M. oryzae.

Snf1 Kinase Differentially Regulates Botrytis cinerea Pathogenicity according to the Plant Host

The Snf1 kinase of the glucose signaling pathway controls the response to nutritional and environmental stresses. In phytopathogenic fungi, Snf1 acts as a global activator of plant cell wall degrading enzymes that are major virulence factors for plant colonization. To characterize its role in the virulence of the necrotrophic fungus Botrytis cinerea, two independent deletion mutants of the Bcsnf1 gene were obtained and analyzed. Virulence of the Δsnf1 mutants was reduced by 59% on a host with acidic pH (apple fruit) and up to 89% on hosts with neutral pH (cucumber cotyledon and French bean leaf). In vitro, Δsnf1 mutants grew slower than the wild type strain at both pH 5 and 7, with a reduction of 20–80% in simple sugars, polysaccharides, and lipidic carbon sources, and these defects were amplified at pH 7. A two-fold reduction in secretion of xylanase activities was observed consequently to the Bcsnf1 gene deletion. Moreover, Δsnf1 mutants were altered in their ability to control ambient pH. Finally, Δsnf1 mutants were impaired in asexual sporulation and did not produce macroconidia. These results confirm the importance of BcSnf1 in pathogenicity, nutrition, and conidiation, and suggest a role in pH regulation for this global regulator in filamentous fungi.

BbWor1, a Regulator of Morphological Transition, Is Involved in Conidium-Hypha Switching, Blastospore Propagation, and Virulence in Beauveria bassiana

Morphological transition is an important adaptive mechanism in the host invasion process. Wor1 is a conserved fungal regulatory protein that controls the phenotypic switching and pathogenicity of Candida albicans. By modulating growth conditions, we simulated three models of Beauveria bassiana morphological transitions, including CTH (conidia to hyphae), HTC (hyphae to conidia), and BTB (blastospore to blastospore). Disruption of BbWor1 (an ortholog of Wor1) resulted in a distinct reduction in the time required for conidial germination (CTH), a significant increase in hyphal growth, and a decrease in the yield of conidia (HTC), indicating that BbWor1 positively controls conidium production and negatively regulates hyphal growth in conidium-hypha switching. Moreover, ΔBbWor1 prominently decreased blastospore yield, shortened the G0/G1 phase, and prolonged the G2/M phase under the BTB model. Importantly, BbWor1 contributed to conidium-hypha switching and blastospore propagation via different genetic pathways, and yeast one-hybrid testing demonstrated the necessity of BbWor1 to control the transcription of an allergen-like protein gene (BBA_02580) and a conidial wall protein gene (BBA_09998). Moreover, the dramatically weakened virulence of ΔBbWor1 was examined by immersion and injection methods. Our findings indicate that BbWor1 is a vital participant in morphological transition and pathogenicity in entomopathogenic fungi. IMPORTANCE As a well-known entomopathogenic fungus, Beauveria bassiana has a complex life cycle and involves transformations among single-cell conidia, blastospores, and filamentous hyphae. This study provides new insight into the regulation of the fungal cell morphological transitions by simulating three models. Our research identified BbWor1 as a core transcription factor of morphological differentiation that positively regulates the production of conidia and blastospores but negatively regulates hyphal growth. More importantly, BbWor1 affects fungal pathogenicity and the global transcription profiles within three models of growth stage transformation. The present study lays a foundation for the exploration of the transition mechanism of entomopathogenic fungi and provides material for the morphological study of fungi.

Transcription Activator Swi6 Interacts with Mbp1 in MluI Cell Cycle Box-Binding Complex and Regulates Hyphal Differentiation and Virulence in Beauveria bassiana

Mbp1 protein acts as a DNA-binding protein in MluI cell cycle box-binding complex (MBF) and plays an essential role in filamentous myco-pathogen Beauveria bassiana.In the current study, BbSwi6 (a homologue of yeast Swi6) was functionally characterized in B.bassiana. Both BbSwi6 and BbMbp1 localize in the nucleus and display a direct interaction relationship which is indicated by a yeast two-hybrid assay. BbSwi6 significantly contributes to hyphal growth, asexual sporulation and virulence. On the aerial surface, ΔBbSwi6 grew slower on various nutrients and displayed abnormal conidia-producing structures, which hardly produced conidia. In liquid media, BbSwi6 loss led to 90% reduction in blastospore yield. Finally, the virulence of the ΔBbSwi6 mutant was modestly weakened with a reduction of 20% in median lethal time. Comparative transcriptomics revealed that BbSwi6 mediated different transcriptomes during fungal development into conidia and blastospores. Notably, under the indicated condition, the BbSwi6-mediated transcriptome significantly differed to that mediated by BbMbp1. Our results demonstrate that, in addition to their roles as the interactive components in MBF, BbSwi6 and BbMbp1 mediate divergent genetic pathways during morphological transitions in B. bassiana.

Knock-down of glucose transporter and sucrose non-fermenting gene in the hemibiotrophic fungus Colletotrichum falcatum causing sugarcane red rot

Red rot caused by Colletotrichum falcatum, is one of the economically important disease of sugarcane and breeding for resistant varieties is considered to be the major solution to manage the disease. However, breakdown of red rot resistance become usual phenomenon due to development of newer races by culture adaptation on newly released varieties. Hence it is needed to characterize the genes responsible for pathogen virulence in order to take care of host resistance or to manage the disease by other methods. The transcript studies gave foundation to characterize the huge number of pathogenicity determinants and their role in pathogenesis. Here we studied role of two important genes viz., Glucose Transporter (GT) and Sucrose Non-Fermenting1 (SNF1) during pathogenesis of C. falcatum, which said to be involved in carbon source metabolism. Sugar metabolism has a vital role in disease progression of C. falcatum by regulating their cell growth, metabolism and development of the pathogen during various stages of infection. The present study was aimed to find out the role of GT and SNF1 genes in response to pathogenicity by RNA silencing (RNAi) approach. Knock-down of the target pathogenicity gene homologs in standard C. falcatum isolate Cf671 was carried out by amplifying sense and antisense fragments of targets individually using pSilent-1 vector. The expression cassette was cloned into the binary vector pCAMBIA1300 followed by fungal transformation through Agarobacterium mediated transformation. Resulted mutants of both the genes showed less virulence compared to wild type isolate. Simultaneously, both the mutants did not produce spores. Moreover, the molecular confirmation of the mutants displayed the expression of hygromycin gene with reduced expression of the target gene during host-pathogen interaction. Knockdown of the pathogenicity related genes (GT and SNF1) by RNAi approach corroborate the possible role of the genes in causing the disease.

HapX, an Indispensable bZIP Transcription Factor for Iron Acquisition, Regulates Infection Initiation by Orchestrating Conidial Oleic Acid Homeostasis and Cytomembrane Functionality in Mycopathogen Beauveria bassiana

Conidial maturation and germination are highly coupled physiological processes in filamentous fungi that are critical for the pathogenicity of mycopathogens. Compared to the mechanisms involved in conidial germination, those of conidial reserves during maturation are less understood. The insect-pathogenic fungus Beauveria bassiana, as a representative species of filamentous fungi, is important for applied and fundamental research. In addition to its conserved roles in fungal adaptation to iron status, the bZIP transcription factor HapX acts as a master regulator involved in conidial virulence and regulates fatty acid/lipid metabolism. Further investigation revealed that the Δ9-fatty acid desaturase gene (Ole1) is a direct downstream target of HapX. This study reveals the HapX-Ole1 pathway involved in the fatty acid/lipid accumulation associated with conidial maturation and provides new insights into the startup mechanism of infection caused by spores from pathogenic fungi.

In pathogenic filamentous fungi, conidial germination not only is fundamental for propagation in the environment but is also a critical step of infection. In the insect mycopathogen Beauveria bassiana, we genetically characterized the role of the basic leucine zipper (bZIP) transcription factor HapX (BbHapX) in conidial nutrient reserves and pathogen-host interaction. Ablation of BbHapX resulted in an almost complete loss of virulence in the topical inoculation and intrahemocoel injection assays. Comparative transcriptomic analysis revealed that BbHapX is required for fatty acid (FA)/lipid metabolism, and biochemical analyses indicated that BbHapX loss caused a significant reduction in conidial FA contents. Exogenous oleic acid could partially or completely restore the impaired phenotypes of the ΔBbHapX mutant, including germination rate, membrane integrity, vegetative growth, and virulence. BbHapX mediates fungal iron acquisition which is not required for desaturation of stearic acid. Additionally, inactivation of the Δ9-fatty acid desaturase gene (BbOle1) generated defects similar to those of the ΔBbHapX mutant; oleic acid also had significant restorative effects on the defective phenotypes of the ΔBbOle1 mutant. A gel retarding assay revealed that BbHapX directly regulated the expression of BbOle1. Lipidomic analyses indicated that both BbHapX and BbOle1 contributed to the homeostasis of phospholipids with nonpolar tails derived from oleic acid; therefore, exogenous phospholipids could significantly restore membrane integrity. These data reveal that the HapX-Ole1 pathway contributes to conidial fatty acid/lipid reserves and that there are important links between the lipid biology and membrane functionality involved in the early stages of infection caused by B.bassiana.

IMPORTANCE Conidial maturation and germination are highly coupled physiological processes in filamentous fungi that are critical for the pathogenicity of mycopathogens. Compared to the mechanisms involved in conidial germination, those of conidial reserves during maturation are less understood. The insect-pathogenic fungus Beauveria bassiana, as a representative species of filamentous fungi, is important for applied and fundamental research. In addition to its conserved roles in fungal adaptation to iron status, the bZIP transcription factor HapX acts as a master regulator involved in conidial virulence and regulates fatty acid/lipid metabolism. Further investigation revealed that the Δ9-fatty acid desaturase gene (Ole1) is a direct downstream target of HapX. This study reveals the HapX-Ole1 pathway involved in the fatty acid/lipid accumulation associated with conidial maturation and provides new insights into the startup mechanism of infection caused by spores from pathogenic fungi.

Diverse function and regulation of CmSnf1 in entomopathogenic fungus Cordyceps militaris

SNF1/AMPK protein kinases play important roles in fungal development and activation of catabolite-repressed genes. In this study, we characterized the role of SNF1 ortholog in Cordyceps militaris (CmSnf1). The vegetative growth of a CmSnf1 deletion mutant was (ΔCmSnf1) reduced by 42.2% with arabinose as a sole carbon source. Most strikingly, the ΔCmSnf1 produced only a few conidia and exhibited delayed conidial germination. We found that CmSnf1 was necessary for mycelium to penetrate the insect cuticle to form the fruiting body on silkworm pupae, consistent with the down-regulation of chitinase- and protease-encoding genes in ΔCmSnf1. However, cordycepin content increased by more than 7 times in culture supernatants. Correspondingly, the relative expression levels of cordycepin gene cluster members were also elevated. In particular, the expression of cns4 associated with cordycepin transfer was up-regulated >10-fold. Furthermore, transcriptional analysis showed that CmSnf1 regulated the expression of genes involved in cell autophagy and oxidative stress tolerance. We speculated that under environmental stress, both the ATG and SNF1 pathways might collaborate to sustain adverse environments. Our study provides an initial framework to probe the diverse function and regulation of CmSnf1 in C. militaris, which will shed more light on the direction of molecular improvement of medicinal fungi.

Autophagy-related gene BbATG11 is indispensable for pexophagy and mitophagy, and contributes to stress response, conidiation and virulence in the insect mycopathogen Beauveria bassiana

Autophagy is a conserved degradation system in eukaryotic cells that includes non-selective and selective processes. Selective autophagy functions as a selective degradation mechanism for specific substrates in which autophagy-related protein 11 (ATG11) acts as an essential scaffold protein. In B. bassiana, there is a unique ATG11 family protein, which is designated as BbATG11. Disruption of BbATG11 resulted in significantly reduced conidial germination under starvation stress. The mutant ΔBbATG11 displayed enhanced sensitivity to oxidative stress and impaired asexual reproduction. The conidial yield was reduced by approximately 75%, and this defective phenotype could be repressed by increasing exogenous nutrients. The virulence of the ΔBbATG11 mutant strain was significantly impaired as indicated in topical and intra-hemocoel injection bioassays, with a greater reduction in topical infection. Notably, BbATG11 was involved in pexophagy and mitophagy, but these two autophagic processes appeared in different fungal physiological aspects. Both pexophagy and mitophagy were associated with nutrient shift, starvation stress and growth in the host hemocoel, but only pexophagy appeared in both oxidation-stressed cells and aerial mycelia. This study highlights that BbATG11 mediates pexophagy and mitophagy in B. bassiana and links selective autophagy to the fungal stress response, conidiation and virulence.

Methylmalonate-semialdehyde dehydrogenase mediated metabolite homeostasis essentially regulate conidiation, polarized germination and pathogenesis in Magnaporthe oryzae

Plants generate multitude of aldehydes under abiotic and biotic stress conditions. Ample demonstrations have shown that rice-derived aldehydes enhance the resistance of rice against the rice-blast fungus Magnaporthe oryzae. However, how the fungal pathogen nullifies the inhibitory effects of host aldehydes to establish compatible interaction remains unknown. Here we identified and evaluated the in vivo transcriptional activities of M. oryzae aldehyde dehydrogenase (ALDH) genes. Transcriptional analysis of M. oryzae ALDH genes revealed that the acetylating enzyme Methylmalonate-Semialdehyde Dehydrogenase (MoMsdh/MoMmsdh) elevated activities during host invasion and colonization of the fungus. We further examined the pathophysiological importance of MoMSDH by deploying integrated functional genetics, and biochemical approaches. MoMSDH deletion mutant ΔMomsdh exhibited germination defect, hyper-branching of germ tube and failed to form appressoria on hydrophobic and hydrophilic surface. The MoMSDH disruption caused accumulation of small branch-chain amino acids, pyridoxine and AMP/cAMP in the ΔMomsdh mutant and altered Spitzenkörper organization in the conidia. We concluded that MoMSDH contribute significantly to the pathogenesis of M. oryzae by regulating the mobilization of Spitzenkörper during germ tube morphogenesis, appressoria formation by acting as metabolic switch regulating small branch-chain amino acids, inositol, pyridoxine and AMP/cAMP homeostasis.

FvSNF1, the sucrose non-fermenting protein kinase gene of Fusarium virguliforme, is required for cell-wall-degrading enzymes expression and sudden death syndrome development in soybean

Fusarium virguliforme is a soil-borne pathogenic fungus that causes sudden death syndrome (SDS) in soybean. Its pathogenicity is believed to require the activity of cell-wall-degrading enzymes (CWDEs). The sucrose non-fermenting protein kinase 1 gene (SNF1) is a key component of the glucose de-repression pathway in yeast, and a regulator of gene expression for CWDEs in some plant pathogenic fungi. To elucidate the functional role of the SNF1 homolog in F. virguliforme, FvSNF1 was disrupted using a split-marker strategy. Disruption of FvSNF1 in F. virguliforme abolishes galactose utilization and causes poor growth on xylose, arabinose and sucrose. However, the resulting Fvsnf1 mutant grew similar to wild-type and ectopic transformants on glucose, fructose, maltose, or pectin as the main source of carbon. The Fvsnf1 mutant displayed no expression of the gene-encoding galactose oxidase (GAO), a secretory enzyme that catalyzes oxidation of D-galactose. It also exhibited a significant reduction in the expression of several CWDE-coding genes in contrast to the wild-type strain. Greenhouse pathogenicity assays revealed that the Fvsnf1 mutant was severely impaired in its ability to cause SDS on challenged soybean plants. Microscopy and microtome studies on infected roots showed that the Fvsnf1 mutant was defective in colonizing vascular tissue of infected plants. Cross and longitudinal sections of infected roots stained with fluorescein-labeled wheat germ agglutinin and Congo red showed that the Fvsnf1 mutant failed to colonize the xylem vessels and phloem tissue at later stages of infection. Quantification of the fungal biomass in inoculated roots further confirmed a reduced colonization of roots by the Fvsnf1 mutant when compared to the wild type. These findings suggest that FvSNF1 regulates the expression of CWDEs in F. virguliforme, thus affecting the virulence of the fungus on soybean.

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.