Role of the Botrytis cinerea FKBP12 ortholog in pathogenic development and in sulfur regulation

The target of rapamycin signaling pathway regulates vegetative development, aflatoxin biosynthesis, and pathogenicity in Aspergillus flavus

The target of rapamycin (TOR) signaling pathway is highly conserved and plays a crucial role in diverse biological processes in eukaryotes. Despite its significance, the underlying mechanism of the TOR pathway in Aspergillus flavus remains elusive. In this study, we comprehensively analyzed the TOR signaling pathway in A. flavus by identifying and characterizing nine genes that encode distinct components of this pathway. The FK506-binding protein Fkbp3 and its lysine succinylation are important for aflatoxin production and rapamycin resistance. The TorA kinase plays a pivotal role in the regulation of growth, spore production, aflatoxin biosynthesis, and responses to rapamycin and cell membrane stress. As a significant downstream effector molecule of the TorA kinase, the Sch9 kinase regulates aflatoxin B1 (AFB1) synthesis, osmotic and calcium stress response in A. flavus, and this regulation is mediated through its S_TKc, S_TK_X domains, and the ATP-binding site at K340. We also showed that the Sch9 kinase may have a regulatory impact on the high osmolarity glycerol (HOG) signaling pathway. TapA and TipA, the other downstream components of the TorA kinase, play a significant role in regulating cell wall stress response in A. flavus. Moreover, the members of the TapA-phosphatase complexes, SitA and Ppg1, are important for various biological processes in A. flavus, including vegetative growth, sclerotia formation, AFB1 biosynthesis, and pathogenicity. We also demonstrated that SitA and Ppg1 are involved in regulating lipid droplets (LDs) biogenesis and cell wall integrity (CWI) signaling pathways. In addition, another phosphatase complex, Nem1/Spo7, plays critical roles in hyphal development, conidiation, aflatoxin production, and LDs biogenesis. Collectively, our study has provided important insight into the regulatory network of the TOR signaling pathway and has elucidated the underlying molecular mechanisms of aflatoxin biosynthesis in A. flavus.

Cross-Talk and Multiple Control of Target of Rapamycin (TOR) in Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a necrotrophic phytopathogenic fungus that cross-talks with its hosts for control of cell-death pathways for colonization. Target of rapamycin (TOR) is a central regulator that controls cell growth, intracellular metabolism, and stress responses in a variety of eukaryotes, but little is known about TOR signaling in S. sclerotiorum. In this study, we identified a conserved TOR signaling pathway and characterized SsTOR as a critical component of this pathway. Hyphal growth of S. sclerotiorum was retarded by silencing SsTOR, moreover, sclerotia and compound appressoria formation were severely disrupted. Notably, pathogenicity assays of strains shows that the virulence of the SsTOR-silenced strains were dramatically decreased. SsTOR was determined to participate in cell wall integrity (CWI) by regulating the phosphorylation level of SsSmk3, a core MAP kinase in the CWI pathway. Importantly, the inactivation of SsTOR induced autophagy in S. sclerotiorum potentially through SsAtg1 and SsAtg13. Taken together, our results suggest that SsTOR is a global regulator controlling cell growth, stress responses, cell wall integrity, autophagy, and virulence of S. sclerotiorum. IMPORTANCE TOR is a conserved protein kinase that regulates cell growth and metabolism in response to growth factors and nutrient abundance. Here, we used gene silencing to characterize SsTOR, which is a critical component of TOR signaling pathway. SsTOR-silenced strains have limited mycelium growth, and the virulence of the SsTOR-silenced strains was decreased. Phosphorylation analysis indicated that SsTOR influenced CWI by regulating the phosphorylation level of SsSmk3. Autophagy is essential to preserve cellular homeostasis in response to cellular and environmental stresses. Inactivation of SsTOR induced autophagy in S. sclerotiorum potentially through SsAtg1 and SsAtg13. These findings further indicated that SsTOR is a global regulator of the growth, development, and pathogenicity of S. sclerotiorum in multiple ways.

TOR inhibition primes immunity and pathogen resistance in tomato in a salicylic acid-dependent manner

All organisms need to sense and process information about the availability of nutrients, energy status, and environmental cues to determine the best time for growth and development. The conserved target of rapamycin (TOR) protein kinase has a central role in sensing and perceiving nutritional information. TOR connects environmental information about nutrient availability to developmental and metabolic processes to maintain cellular homeostasis. Under favourable energy conditions, TOR is activated and promotes anabolic processes such as cell division, while suppressing catabolic processes. Conversely, when nutrients are limited or environmental stresses are present, TOR is inactivated, and catabolic processes are promoted. Given the central role of TOR in regulating metabolism, several previous works have examined whether TOR is wired to plant defence. To date, the mechanisms by which TOR influences plant defence are not entirely clear. Here, we addressed this question by testing the effect of inhibiting TOR on immunity and pathogen resistance in tomato. Examining which hormonal defence pathways are influenced by TOR, we show that tomato immune responses and disease resistance to several pathogens increase on TOR inhibition, and that TOR inhibition-mediated resistance probably requires a functional salicylic acid, but not jasmonic acid, pathway. Our results support the notion that TOR is a master regulator of the development-defence switch in plants.

Target of rapamycin controls hyphal growth and pathogenicity through FoTIP4 in Fusarium oxysporum

Fusarium oxysporum is the causal agent of the devastating Fusarium wilt by invading and colonizing the vascular system in various plants, resulting in substantial economic losses worldwide. Target of rapamycin (TOR) is a central regulator that controls intracellular metabolism, cell growth, and stress responses in eukaryotes, but little is known about TOR signalling in F. oxysporum. In this study, we identified conserved FoTOR signalling pathway components including FoTORC1 and FoTORC2. Pharmacological assays showed that F. oxysporum is hypersensitive to rapamycin in the presence of FoFKBP12 while the deletion mutant strain ΔFofkbp12 is insensitive to rapamycin. Transcriptomic data indicated that FoTOR signalling controls multiple metabolic processes including ribosome biogenesis and cell wall-degrading enzymes (CWDEs). Genetic analysis revealed that FoTOR1 interacting protein 4 (FoTIP4) acts as a new component of FoTOR signalling to regulate hyphal growth and pathogenicity of F. oxysporum. Importantly, transcript levels of genes associated with ribosome biogenesis and CWDEs were dramatically downregulated in the ΔFotip4 mutant strain. Electrophoretic mobility shift assays showed that FoTIP4 can bind to the promoters of ribosome biogenesis- and CWDE-related genes to positively regulate the expression of these genes. These results suggest that FoTOR signalling plays central roles in regulating hyphal growth and pathogenicity of F. oxysporum and provide new insights into FoTOR1 as a target for controlling and preventing Fusarium wilt in plants.

High-dose rapamycin exerts a temporary impact on T. reesei RUT-C30 through gene trFKBP12

BACKGROUND

Knowledge with respect to regulatory systems for cellulase production is prerequisite for exploitation of such regulatory networks to increase cellulase production, improve fermentation efficiency and reduce the relevant production cost. The target of rapamycin (TOR) signaling pathway is considered as a central signaling hub coordinating eukaryotic cell growth and metabolism with environmental inputs. However, how and to what extent the TOR signaling pathway and rapamycin are involved in cellulase production remain elusive.

RESULT

At the early fermentation stage, high-dose rapamycin (100 μM) caused a temporary inhibition effect on cellulase production, cell growth and sporulation of Trichoderma reesei RUT-C30 independently of the carbon sources, and specifically caused a tentative morphology defect in RUT-C30 grown on cellulose. On the contrary, the lipid content of T. reesei RUT-C30 was not affected by rapamycin. Accordingly, the transcriptional levels of genes involved in the cellulase production were downregulated notably with the addition of rapamycin. Although the mRNA levels of the putative rapamycin receptor trFKBP12 was upregulated significantly by rapamycin, gene trTOR (the downstream effector of the rapamycin-FKBP12 complex) and genes associated with the TOR signaling pathways were not changed markedly. With the deletion of gene trFKBP12, there is no impact of rapamycin on cellulase production, indicating that trFKBP12 mediates the observed temporary inhibition effect of rapamycin.

CONCLUSION

Our study shows for the first time that only high-concentration rapamycin induced a transient impact on T. reesei RUT-C30 at its early cultivation stage, demonstrating T. reesei RUT-C30 is highly resistant to rapamycin, probably due to that trTOR and its related signaling pathways were not that sensitive to rapamycin. This temporary influence of rapamycin was facilitated by gene trFKBP12. These findings add to our knowledge on the roles of rapamycin and the TOR signaling pathways play in T. reesei.

The Destructive Fungal Pathogen Botrytis cinerea-Insights from Genes Studied with Mutant Analysis

Botrytis cinerea is one of the most destructive fungal pathogens affecting numerous plant hosts, including many important crop species. As a molecularly under-studied organism, its genome was only sequenced at the beginning of this century and it was recently updated with improved gene annotation and completeness. In this review, we summarize key molecular studies on B. cinerea developmental and pathogenesis processes, specifically on genes studied comprehensively with mutant analysis. Analyses of these studies have unveiled key genes in the biological processes of this pathogen, including hyphal growth, sclerotial formation, conidiation, pathogenicity and melanization. In addition, our synthesis has uncovered gaps in the present knowledge regarding development and virulence mechanisms. We hope this review will serve to enhance the knowledge of the biological mechanisms behind this notorious fungal pathogen.

Defects in the Ferroxidase That Participates in the Reductive Iron Assimilation System Results in Hypervirulence in Botrytis Cinerea

The plant-pathogenic fungus B. cinerea causes enormous economic losses, estimated at anywhere between $10 billion and $100 billion worldwide, under both pre- and postharvest conditions. Here, we present the characterization of a loss-of-function mutant in a component involved in iron acquisition that displays hypervirulence. While in different microbial systems iron uptake mechanisms appear to be critical to achieve full pathogenic potential, we found that the absence of the ferroxidase that is part of the reductive iron assimilation system leads to hypervirulence in this fungus. This is an unusual and rather underrepresented phenotype, which can be modulated by iron levels in the plant and provides an unexpected link between iron acquisition, reactive oxygen species (ROS) production, and pathogenesis in the Botrytis-plant interaction.

The plant pathogen Botrytis cinerea is responsible for gray-mold disease, which infects a wide variety of species. The outcome of this host-pathogen interaction, a result of the interplay between plant defense and fungal virulence pathways, can be modulated by various environmental factors. Among these, iron availability and acquisition play a crucial role in diverse biological functions. How B. cinerea obtains iron, an essential micronutrient, during infection is unknown. We set out to determine the role of the reductive iron assimilation (RIA) system during B. cinerea infection. This system comprises the BcFET1 ferroxidase, which belongs to the multicopper oxidase (MCO) family of proteins, and the BcFTR1 membrane-bound iron permease. Gene knockout and complementation studies revealed that, compared to the wild type, the bcfet1 mutant displays delayed conidiation, iron-dependent sclerotium production, and significantly reduced whole-cell iron content. Remarkably, this mutant exhibited a hypervirulence phenotype, whereas the bcftr1 mutant presents normal virulence and unaffected whole-cell iron levels and developmental programs. Interestingly, while in iron-starved plants wild-type B. cinerea produced slightly reduced necrotic lesions, the hypervirulence phenotype of the bcfet1 mutant is no longer observed in iron-deprived plants. This suggests that B. cinerea bcfet1 knockout mutants require plant-derived iron to achieve larger necrotic lesions, whereas in planta analyses of reactive oxygen species (ROS) revealed increased ROS levels only for infections caused by the bcfet1 mutant. These results suggest that increased ROS production, under an iron sufficiency environment, at least partly underlie the observed infection phenotype in this mutant.

Three proline rotamases involved in calcium homeostasis play differential roles in stress tolerance, virulence and calcineurin regulation of Beauveria bassiana

FK506-sensitive proline rotamases (FPRs), also known as FK506-binding proteins (FKBPs), can mediate immunosuppressive drug resistance in budding yeast but their physiological roles in filamentous fungi remain opaque. Here, we report that three FPRs (cytosolic/nuclear 12.15-kD Fpr1, membrane-associated 14.78-kD Fpr2 and nuclear 50.43-kD Fpr3) are all equally essential for cellular Ca²⁺ homeostasis and contribute significantly to calcineurin activity at different levels in the insect-pathogenic fungus Beauveria bassiana although the deletion of fpr1 alone conferred resistance to FK506. Radial growth, conidiation, conidial viability and virulence were less compromised in the absence of fpr1 or fpr2 than in the absence of fpr3, which abolished almost all growth on scant media and reduced growth moderately on rich media. The Δfpr3 mutant was more sensitive to Na⁺ , K⁺ , Mn²⁺ , Ca²⁺ , Cu²⁺ , metal chelate, heat shock and UVB irradiation than was Δfpr2 while both mutants were equally sensitive to Zn²⁺ , Mg²⁺ , Fe²⁺ , H₂ O₂ and cell wall-perturbing agents. In contrast, the Δfpr1 mutant was less sensitive to fewer stress cues. Most of 32 examined genes involved in DNA damage repair, Na⁺ /K⁺ detoxification or osmotolerance and Ca²⁺ homeostasis were downregulated sharply in Δfpr2 and Δfpr3 but rarely so affected in Δfpr1, coinciding well with their phenotypic changes. These findings uncover important, but differential, roles of three FPRs in the fungal adaptation to insect host and environment and provide novel insight into their essential roles in calcium signalling pathway.

Genome-wide transcriptomic analysis of the response of Botrytis cinerea to wuyiencin

Grey mould is caused by the ascomycetes Botrytis cinerea in a range of crop hosts. As a biological control agent, the nucleoside antibiotic wuyiencin has been industrially produced and widely used as an effective fungicide. To elucidate the effects of wuyiencin on the transcriptional regulation in B. cinerea, we, for the first time, report a genome-wide transcriptomic analysis of B. cinerea treated with wuyiencin. 2067 genes were differentially expressed, of them, 886 and 1181 genes were significantly upregulated and downregulated, respectively. Functional categorization indicated that transcript levels of genes involved in amino acid metabolism and those encoding putative secreted proteins were altered in response to wuyiencin treatment. Moreover, the expression of genes involved in protein synthesis and energy metabolism (oxidative phosphorylation) and of those encoding ATP-binding cassette transporters was markedly upregulated, whereas that of genes participating in DNA replication, cell cycle, and stress response was downregulated. Furthermore, wuyiencin resulted in mycelial malformation and negatively influenced cell growth rate and conidial yield in B. cinerea. Our results suggest that this nucleoside antibiotic regulates all aspects of cell growth and differentiation in B. cinerea. To summarize, some new candidate pathways and target genes that may related to the protective and antagonistic mechanisms in B. cinerea were identified underlying the action of biological control agents.

Target Of Rapamycin pathway in the white-rot fungus Phanerochaete chrysosporium

The Target Of Rapamycin (TOR) signaling pathway is known to regulate growth in response to nutrient availability and stress in eukaryotic cells. In the present study, we have investigated the TOR pathway in the white-rot fungus Phanerochaete chrysosporium. Inhibition of TOR activity by rapamycin affects conidia germination and hyphal growth highlighting the conserved mechanism of susceptibility to rapamycin. Interestingly, the secreted protein content is also affected by the rapamycin treatment. Finally, homologs of the components of TOR pathway can be identified in P. chrysosporium. Altogether, those results indicate that the TOR pathway of P. chrysosporium plays a central role in this fungus.

Characterization of three FK506-binding proteins in the entomopathogenic fungus Beauveria bassiana

FK506 binding proteins (FKBPs) participate in regulation of diverse biological processes. However, the role of these proteins in insect-pathogenic fungi is far from well understood. To investigate the functions of FKBPs in Beauveria bassiana, a widely used entomopathogenic fungus for control of insect pests, we identify three putative FKBP genes, Bbfkbp12, Bbfkbp15, and Bbfkbp50, in the fungus. Gene-disruption experiments show that loss of Bbfkbp12 results in a significant increase of resistance of B. bassiana against the immunosuppressive compounds FK506 and rapamycin, while loss of Bbfkbp50 leads to the resistance to the ergosterol synthesis inhibitor lovastatin. Transcription assays of calcineurin (CaN)- and mTOR (mammalian target of rapamycin)-downstream target genes confirm that BbFKBP12 is the target of both FK506 and rapamycin, associated with CaN- and mTOR-signal pathways in B. bassiana. GFP-tagging of the proteins shows that BbFKBP12 and BbFKBP15 localize in cytoplasm while BbFKBP50 in nucleus. Our results provide useful information for the study of functions of CaN- and mTOR-mediated signaling, and ergosterol synthesis in the entomopathogenic fungi.

Host-induced gene silencing of BcTOR in Botrytis cinerea enhances plant resistance to grey mould

Botrytis cinerea is the causal agent of grey mould for more than 200 plant species, including economically important vegetables, fruits and crops, which leads to economic losses worldwide. Target of rapamycin (TOR) acts a master regulator to control cell growth and proliferation by integrating nutrient, energy and growth factors in eukaryotic species, but little is known about whether TOR can function as a practicable target in the control of plant fungal pathogens. Here, we characterize TOR signalling of B. cinerea in the regulation of growth and pathogenicity as well as its potential value in genetic engineering for crop protection by bioinformatics analysis, pharmacological assays, biochemistry and genetics approaches. The results show that conserved TOR signalling occurs, and a functional FK506-binding protein 12 kD (FKBP12) mediates the interaction between rapamycin and B. cinerea TOR (BcTOR). RNA sequencing (RNA-Seq) analysis revealed that BcTOR displayed conserved functions, particularly in controlling growth and metabolism. Furthermore, pathogenicity assay showed that BcTOR inhibition efficiently reduces the infection of B. cinerea in plant leaves of Arabidopsis and potato or tomato fruits. Additionally, transgenic plants expressing double-stranded RNA of BcTOR through the host-induced gene silencing method could produce abundant small RNAs targeting BcTOR, and significantly block the occurrence of grey mould in potato and tomato. Taken together, our results suggest that BcTOR is an efficient target for genetic engineering in control of grey mould, and also a potential and promising target applied in the biocontrol of plant fungal pathogens.

Cys2His2 Zinc Finger Transcription Factor BcabaR1 Positively Regulates Abscisic Acid Production in Botrytis cinerea

Abscisic acid (ABA) is one of the five classical phytohormones involved in increasing the tolerance of plants for various kinds of stresses caused by abiotic or biotic factors, and it also plays important roles in regulating the activation of innate immune cells and glucose homeostasis in mammals. For these reasons, as a "stress hormone," ABA has recently received attention as a candidate drug for agriculture and biomedical applications, prompting significant development of ABA synthesis. Some plant-pathogenic fungi can synthesize natural ABA. The fungus Botrytis cinerea has been used for biotechnological production of ABA. Identification of the transcription factors (TFs) involved in regulation of ABA biosynthesis in B. cinerea would provide new clues to understand how ABA is synthesized and regulated. In this study, we defined a novel Cys₂His₂ TF, BcabaR1, that regulates the transcriptional levels of ABA synthase genes (bcaba1, bcaba2, bcaba3, and bcaba4) in an ABA-overproducing mutant, B. cinerea TBC-A. Electrophoretic mobility shift assays revealed that recombinant BcabaR1 can bind specifically to both a 14-nucleotide sequence motif and a 39-nucleotide sequence motif in the promoter region of bcaba1 to -4 genes in vitro A decreased transcriptional level of the bcabaR1 gene in B. cinerea led to significantly decreased ABA production and downregulated transcription of bcaba1 to -4 When bcabaR1 was overexpressed in B. cinerea, ABA production was significantly increased, with upregulated transcription of bcaba1 to -4 Thus, in this study, we found that BcabaR1 acts as a positive regulator of ABA biosynthesis in B. cinereaIMPORTANCE Abscisic acid (ABA) could make a potentially important contribution to theoretical research and applications in agriculture and medicine. Botrytis cinerea is a plant-pathogenic fungus that was found to produce ABA. There has been a view that ABA is related to the interaction between pathogenic fungi and plants. Identification of regulatory genes involved in ABA biosynthesis may facilitate an understanding of the underlying molecular mechanisms of ABA biosynthesis and the pathogenesis of B. cinerea Here, we present a positive regulator, BcabaR1, of ABA biosynthesis in B. cinerea that can affect the transcriptional level of the ABA biosynthesis gene cluster, bcaba1 to -4, by directly binding to the conserved sequence elements in the promoter of the bcaba1 to -4 genes. This TF was found to be specifically involved in regulation of ABA biosynthesis. This work provides new clues for finding other ABA biosynthesis genes and improving ABA yield in B. cinerea.

Secretome Characterization and Correlation Analysis Reveal Putative Pathogenicity Mechanisms and Identify Candidate Avirulence Genes in the Wheat Stripe Rust Fungus Puccinia striiformis f. sp. tritici

Stripe (yellow) rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most destructive diseases of wheat worldwide. Planting resistant cultivars is an effective way to control this disease, but race-specific resistance can be overcome quickly due to the rapid evolving Pst population. Studying the pathogenicity mechanisms is critical for understanding how Pst virulence changes and how to develop wheat cultivars with durable resistance to stripe rust. We re-sequenced 7 Pst isolates and included additional 7 previously sequenced isolates to represent balanced virulence/avirulence profiles for several avirulence loci in seretome analyses. We observed an uneven distribution of heterozygosity among the isolates. Secretome comparison of Pst with other rust fungi identified a large portion of species-specific secreted proteins, suggesting that they may have specific roles when interacting with the wheat host. Thirty-two effectors of Pst were identified from its secretome. We identified candidates for Avr genes corresponding to six Yr genes by correlating polymorphisms for effector genes to the virulence/avirulence profiles of the 14 Pst isolates. The putative AvYr76 was present in the avirulent isolates, but absent in the virulent isolates, suggesting that deleting the coding region of the candidate avirulence gene has produced races virulent to resistance gene Yr76. We conclude that incorporating avirulence/virulence phenotypes into correlation analysis with variations in genomic structure and secretome, particularly presence/absence polymorphisms of effectors, is an efficient way to identify candidate Avr genes in Pst. The candidate effector genes provide a rich resource for further studies to determine the evolutionary history of Pst populations and the co-evolutionary arms race between Pst and wheat. The Avr candidates identified in this study will lead to cloning avirulence genes in Pst, which will enable us to understand molecular mechanisms underlying Pst-wheat interactions, to determine the effectiveness of resistance genes and further to develop durable resistance to stripe rust.

Characterization of the FKBP12-Encoding Genes in Aspergillus fumigatus

Invasive aspergillosis, largely caused by Aspergillus fumigatus, is responsible for a growing number of deaths among immunosuppressed patients. Immunosuppressants such as FK506 (tacrolimus) that target calcineurin have shown promise for antifungal drug development. FK506-binding proteins (FKBPs) form a complex with calcineurin in the presence of FK506 (FKBP12-FK506) and inhibit calcineurin activity. Research on FKBPs in fungi is limited, and none of the FKBPs have been previously characterized in A. fumigatus. We identified four orthologous genes of FKBP12, the human FK506 binding partner, in A. fumigatus and designated them fkbp12-1, fkbp12-2, fkbp12-3, and fkbp12-4. Deletional analysis of the four genes revealed that the Δfkbp12-1 strain was resistant to FK506, indicating FKBP12-1 as the key mediator of FK506-binding to calcineurin. The endogenously expressed FKBP12-1-EGFP fusion protein localized to the cytoplasm and nuclei under normal growth conditions but also to the hyphal septa following FK506 treatment, revealing its interaction with calcineurin. The FKBP12-1-EGFP fusion protein didn’t localize at the septa in the presence of FK506 in the cnaA deletion background, confirming its interaction with calcineurin. Testing of all deletion strains in the Galleria mellonella model of aspergillosis suggested that these proteins don’t play an important role in virulence. While the Δfkbp12-2 and Δfkbp12-3 strains didn’t show any discernable phenotype, the Δfkbp12-4 strain displayed slight growth defect under normal growth conditions and inhibition of the caspofungin-mediated “paradoxical growth effect” at higher concentrations of the antifungal caspofungin. Together, these results indicate that while only FKBP12-1 is the bona fide binding partner of FK506, leading to the inhibition of calcineurin in A. fumigatus, FKBP12-4 may play a role in basal growth and the caspofungin-mediated paradoxical growth response. Exploitation of differences between A. fumigatus FKBP12-1 and human FKBP12 will be critical for the generation of fungal-specific FK506 analogs to inhibit fungal calcineurin and treat invasive fungal disease.

GATA-Dependent Glutaminolysis Drives Appressorium Formation in Magnaporthe oryzae by Suppressing TOR Inhibition of cAMP/PKA Signaling

Fungal plant pathogens are persistent and global food security threats. To invade their hosts they often form highly specialized infection structures, known as appressoria. The cAMP/ PKA- and MAP kinase-signaling cascades have been functionally delineated as positive-acting pathways required for appressorium development. Negative-acting regulatory pathways that block appressorial development are not known. Here, we present the first detailed evidence that the conserved Target of Rapamycin (TOR) signaling pathway is a powerful inhibitor of appressorium formation by the rice blast fungus Magnaporthe oryzae. We determined TOR signaling was activated in an M. oryzae mutant strain lacking a functional copy of the GATA transcription factor-encoding gene ASD4. Δasd4 mutant strains could not form appressoria and expressed GLN1, a glutamine synthetase-encoding orthologue silenced in wild type. Inappropriate expression of GLN1 increased the intracellular steady-state levels of glutamine in Δasd4 mutant strains during axenic growth when compared to wild type. Deleting GLN1 lowered glutamine levels and promoted appressorium formation by Δasd4 strains. Furthermore, glutamine is an agonist of TOR. Treating Δasd4 mutant strains with the specific TOR kinase inhibitor rapamycin restored appressorium development. Rapamycin was also shown to induce appressorium formation by wild type and Δcpka mutant strains on non-inductive hydrophilic surfaces but had no effect on the MAP kinase mutant Δpmk1. When taken together, we implicate Asd4 in regulating intracellular glutamine levels in order to modulate TOR inhibition of appressorium formation downstream of cPKA. This study thus provides novel insight into the metabolic mechanisms that underpin the highly regulated process of appressorium development.

Many fungal pathogens destroy important crops by first gaining entrance to the host using specialized appressorial cells. Understanding the molecular mechanisms that control appressorium formation could provide new routes for managing severe plant diseases. Here, we describe a previously unknown regulatory pathway that suppresses appressorium formation by the rice pathogen Magnaporthe oryzae. We provide evidence that a mutant M. oryzae strain, unable to form appressoria, accumulates intracellular glutamine that, in turn, inappropriately activates a conserved signaling pathway called TOR. Reducing intracellular glutamine levels, or inactivating TOR, restored appressorium formation to the mutant strain. TOR activation is thus a powerful inhibitor of appressorium formation and could be leveraged to develop sustainable mitigation practices against recalcitrant fungal pathogens.

The TOR signaling pathway regulates vegetative development and virulence in Fusarium graminearum

The target of rapamycin (TOR) signaling pathway plays critical roles in controlling cell growth in a variety of eukaryotes. However, the contribution of this pathway in regulating virulence of plant pathogenic fungi is unknown. We identified and characterized nine genes encoding components of the TOR pathway in Fusarium graminearum. Biological, genetic and biochemical functions of each component were investigated. The FgFkbp12-rapamycin complex binds to the FgTor kinase. The type 2A phosphatases FgPp2A, FgSit4 and FgPpg1 were found to interact with FgTap42, a downstream component of FgTor. Among these, we determined that FgPp2A is likely to be essential for F. graminearum survival, and FgSit4 and FgPpg1 play important roles in cell wall integrity by positively regulating the phosphorylation of FgMgv1, a key MAP kinase in the cell wall integrity pathway. In addition, the FgPpg1 interacting protein, FgTip41, is involved in regulating mycelial growth and virulence. Notably, FgTip41 does not interact with FgTap42 but with FgPpg1, suggesting the existence of FgTap42:FgPpg1:FgTip41 heterotrimer in F. graminearum, a complex not observed in the yeast model. Collectively, we defined a genetic regulatory framework that elucidates how the TOR pathway regulates virulence and vegetative development in F. graminearum.

Genome update of Botrytis cinerea strains B05.10 and T4

We report here an update of the Botrytis cinerea strains B05.10 and T4 genomes, as well as an automated preliminary gene structure annotation. High-coverage de novo assemblies and reference-based alignments led to a correction of wrong base calls, elimination of sequence gaps, and improved joining of contigs. The new assemblies have substantially lower numbers of scaffolds and a concomitant increase in the N(50).The list of protein-coding genes was generated using the evidence-driven gene predictor Augustus, with expressed sequence tag evidence and RNA-Seq data as input.

pH modulation differs during sunflower cotyledon colonization by the two closely related necrotrophic fungi Botrytis cinerea and Sclerotinia sclerotiorum

During pathogenesis on sunflower cotyledons, Botrytis cinerea and Sclerotinia sclerotiorum show a striking resemblance in symptom development. Based on pH change profiles, the colonization process of both fungi can be divided into two stages. The first stage is associated with a pH decrease, resulting from an accumulation of citric and succinic acids. The second stage is correlated with a pH increase, resulting from an accumulation of ammonia. In this article, we also report that oxalic acid is produced at the late stage of the colonization process and that ammonia accumulation is concomitant with a decrease in free amino acids in decaying tissues. Sclerotinia sclerotiorum produces eight-fold more oxalic acid and two-fold less ammonia than B. cinerea. Consequently, during sunflower cotyledon colonization by B. cinerea, pH dynamics differ significantly from those of S. sclerotiorum. In vitro assays support the in planta results and show that decreases in pH are linked to glucose consumption. At different stages of the colonization process, expression profiles of genes encoding secreted proteases were investigated. This analysis highlights that the expression levels of the B. cinerea protease genes are higher than those of S. sclerotiorum. This work suggests that the overt similarities of S. sclerotiorum and B. cinerea symptom development have probably masked our recognition of the dynamic and potentially different metabolic pathways active during host colonization by these two necrotrophic fungi.