A novel gene, CBP1, encoding a putative extracellular chitin-binding protein, may play an important role in the hydrophobic surface sensing of Magnaporthe grisea during appressorium differentiation

Chitin Deacetylase Homologous Gene cda Contributes to Development and Aflatoxin Synthesis in Aspergillus flavus

The fungal cell wall serves as the primary interface between fungi and their external environment, providing protection and facilitating interactions with the surroundings. Chitin is a vital structural element in fungal cell wall. Chitin deacetylase (CDA) can transform chitin into chitosan through deacetylation, providing various biological functions across fungal species. Although this modification is widespread in fungi, the biological functions of CDA enzymes in Aspergillus flavus remain largely unexplored. In this study, we aimed to investigate the biofunctions of the CDA family in A. flavus. The A. flavus genome contains six annotated putative chitin deacetylases. We constructed knockout strains targeting each member of the CDA family, including Δcda1, Δcda2, Δcda3, Δcda4, Δcda5, and Δcda6. Functional analyses revealed that the deletion of CDA family members neither significantly affects the chitin content nor exhibits the expected chitin deacetylation function in A. flavus. However, the Δcda6 strain displayed distinct phenotypic characteristics compared to the wild-type (WT), including an increased conidia count, decreased mycelium production, heightened aflatoxin production, and impaired seed colonization. Subcellular localization experiments indicated the cellular localization of CDA6 protein within the cell wall of A. flavus filaments. Moreover, our findings highlight the significance of the CBD1 and CBD2 structural domains in mediating the functional role of the CDA6 protein. Overall, we analyzed the gene functions of CDA family in A. flavus, which contribute to a deeper understanding of the mechanisms underlying aflatoxin contamination and lay the groundwork for potential biocontrol strategies targeting A. flavus.

In silico approaches for the identification of potential allergens among hypothetical proteins from Alternaria alternata and its functional annotation

Direct exposure to the fungal species Alternaria alternata is a major risk factor for the development of asthma, allergic rhinitis, and inflammation. As of November 23rd 2020, the NCBI protein database showed 11,227 proteins from A. alternata genome as hypothetical proteins (HPs). Allergens are the main causative of several life-threatening diseases, especially in fungal infections. Therefore, the main aim of the study is to identify the potentially allergenic inducible proteins from the HPs in A. alternata and their associated functional assignment for the complete understanding of the complex biological systems at the molecular level. AlgPred and Structural Database of Allergenic Proteins (SDAP) were used for the prediction of potential allergens from the HPs of A. alternata. While analyzing the proteome data, 29 potential allergens were predicted by AlgPred and further screening in SDAP confirmed the allergic response of 10 proteins. Extensive bioinformatics tools including protein family classification, sequence-function relationship, protein motif discovery, pathway interactions, and intrinsic features from the amino acid sequence were used to successfully predict the probable functions of the 10 HPs. The functions of the HPs are characterized as chitin-binding, ribosomal protein P1, thaumatin, glycosyl hydrolase, and NOB1 proteins. The subcellular localization and signal peptide prediction of these 10 proteins has further provided additional information on localization and function. The allergens prediction and functional annotation of the 10 proteins may facilitate a better understanding of the allergenic mechanism of A. alternata in asthma and other diseases. The functional domain level insights and predicted structural features of the allergenic proteins help to understand the pathogenesis and host immune tolerance. The outcomes of the study would aid in the development of specific drugs to combat A. alternata infections.

Cell Wall Carbohydrate Dynamics during the Differentiation of Infection Structures by the Apple Scab Fungus, Venturia inaequalis

Scab, caused by the biotrophic fungal pathogen Venturia inaequalis, is the most economically important disease of apples. During infection, V. inaequalis colonizes the subcuticular host environment, where it develops specialized infection structures called runner hyphae and stromata. These structures are thought to be involved in nutrient acquisition and effector (virulence factor) delivery, but also give rise to conidia that further the infection cycle. Despite their importance, very little is known about how these structures are differentiated. Likewise, nothing is known about how these structures are protected from host defenses or recognition by the host immune system. To better understand these processes, we first performed a glycosidic linkage analysis of sporulating tubular hyphae from V. inaequalis developed in culture. This analysis revealed that the V. inaequalis cell wall is mostly composed of glucans (44%) and mannans (37%), whereas chitin represents a much smaller proportion (4%). Next, we used transcriptomics and confocal laser scanning microscopy to provide insights into the cell wall carbohydrate composition of runner hyphae and stromata. These analyses revealed that, during subcuticular host colonization, genes of V. inaequalis putatively associated with the biosynthesis of immunogenic carbohydrates, such as chitin and β-1,6-glucan, are downregulated relative to growth in culture, while on the surface of runner hyphae and stromata, chitin is deacetylated to the less-immunogenic carbohydrate chitosan. These changes are anticipated to enable the subcuticular differentiation of runner hyphae and stromata by V. inaequalis, as well as to protect these structures from host defenses and recognition by the host immune system. IMPORTANCE Plant-pathogenic fungi are a major threat to food security. Among these are subcuticular pathogens, which often cause latent asymptomatic infections, making them difficult to control. A key feature of these pathogens is their ability to differentiate specialized subcuticular infection structures that, to date, remain largely understudied. This is typified by Venturia inaequalis, which causes scab, the most economically important disease of apples. In this study, we show that, during subcuticular host colonization, V. inaequalis downregulates genes associated with the biosynthesis of two immunogenic cell wall carbohydrates, chitin and β-1,6-glucan, and coats its subcuticular infection structures with a less-immunogenic carbohydrate, chitosan. These changes are anticipated to enable host colonization by V. inaequalis and provide a foundation for understanding subcuticular host colonization by other plant-pathogenic fungi. Such an understanding is important, as it may inform the development of novel control strategies against subcuticular plant-pathogenic fungi.

Combating powdery mildew: Advances in molecular interactions between Blumeria graminis f. sp. tritici and wheat

Wheat powdery mildew caused by a biotrophic fungus Blumeria graminis f. sp. tritici (Bgt), is a widespread airborne disease which continues to threaten global wheat production. One of the most chemical-free and cost-effective approaches for the management of wheat powdery mildew is the exploitation of resistant cultivars. Accumulating evidence has reported that more than 100 powdery mildew resistance genes or alleles mapping to 63 different loci (Pm1-Pm68) have been identified from common wheat and its wild relatives, and only a few of them have been cloned so far. However, continuous emergence of new pathogen races with novel degrees of virulence renders wheat resistance genes ineffective. An essential breeding strategy for achieving more durable resistance is the pyramiding of resistance genes into a single genotype. The genetics of host-pathogen interactions integrated with temperature conditions and the interaction between resistance genes and their corresponding pathogen a virulence genes or other resistance genes within the wheat genome determine the expression of resistance genes. Considerable progress has been made in revealing Bgt pathogenesis mechanisms, identification of resistance genes and breeding of wheat powdery mildew resistant cultivars. A detailed understanding of the molecular interactions between wheat and Bgt will facilitate the development of novel and effective approaches for controlling powdery mildew. This review gives a succinct overview of the molecular basis of interactions between wheat and Bgt, and wheat defense mechanisms against Bgt infection. It will also unleash the unsung roles of epigenetic processes, autophagy and silicon in wheat resistance to Bgt.

The CfMcm1 Regulates Pathogenicity, Conidium Germination, and Sexual Development in Colletotrichum fructicola

Glomerella leaf spot (GLS), caused by Colletotrichum fructicola, is a severe disease worldwide on apple, causing defoliation, leaf and fruit spot, and substantial yield loss. However, little is known about its molecular mechanisms of pathogenesis. Previous transcriptome analysis revealed that a transcription factor, CfMcm1, was induced during leaf infection. In the present work, expression pattern analysis verified that the CfMcm1 gene was strongly expressed in conidia and early infection. Phenotypic analysis revealed that the gene deletion mutant ΔCfMcm1 lost pathogenicity to apple leaves by inhibiting conidial germination and appressorium formation. In addition to appressorium-mediated pathogenicity, ΔCfMcm1 colonization and hyphal extension in wounded apple fruit was also reduced, and conidial germination mode and conidial color were altered. ΔCfMcm1 displayed impairment of cell wall integrity and response to stress caused by oxidation, osmosis, and an acid environment. Furthermore, the deletion mutant produced fewer and smaller perithecia and no ascospores. In contrast, melanin deposition in mycelia of ΔCfMcm1 was strengthened. Further comparative transcriptome and quantitative PCR analysis revealed that CfMcm1 modulated expression of genes related to conidial development (CfERG5A, CfERG5B, CfHik5, and CfAbaA), appressorium formation (CfCBP1 and CfCHS7), pectin degradation (CfPelA and CfPelB), sexual development (CfMYB, CfFork, CfHMG, and CfMAT1-2-1), and melanin biosynthesis (CfCmr1, CfPKS1, CfT4HR1, CfTHR1, and CfSCD1). Our results demonstrated that CfMcm1 is a pivotal regulator possessing multiple functions in pathogenicity, asexual and sexual reproduction, and melanin biosynthesis.

Infection Strategies and Pathogenicity of Biotrophic Plant Fungal Pathogens

Biotrophic plant pathogenic fungi are widely distributed and are among the most damaging pathogenic organisms of agriculturally important crops responsible for significant losses in quality and yield. However, the pathogenesis of obligate parasitic pathogenic microorganisms is still under investigation because they cannot reproduce and complete their life cycle on an artificial medium. The successful lifestyle of biotrophic fungal pathogens depends on their ability to secrete effector proteins to manipulate or evade plant defense response. By integrating genomics, transcriptomics, and effectoromics, insights into how the adaptation of biotrophic plant fungal pathogens adapt to their host populations can be gained. Efficient tools to decipher the precise molecular mechanisms of rust–plant interactions, and standardized routines in genomics and functional pipelines have been established and will pave the way for comparative studies. Deciphering fungal pathogenesis not only allows us to better understand how fungal pathogens infect host plants but also provides valuable information for plant diseases control, including new strategies to prevent, delay, or inhibit fungal development. Our review provides a comprehensive overview of the efforts that have been made to decipher the effector proteins of biotrophic fungal pathogens and demonstrates how rapidly research in the field of obligate biotrophy has progressed.

Cell Wall Integrity and Its Industrial Applications in Filamentous Fungi

Signal transduction pathways regulating cell wall integrity (CWI) in filamentous fungi have been studied taking into account findings in budding yeast, and much knowledge has been accumulated in recent years. Given that the cell wall is essential for viability in fungi, its architecture has been analyzed in relation to virulence, especially in filamentous fungal pathogens of plants and humans. Although research on CWI signaling in individual fungal species has progressed, an integrated understanding of CWI signaling in diverse fungi has not yet been achieved. For example, the variety of sensor proteins and their functional differences among different fungal species have been described, but the understanding of their general and species-specific biological functions is limited. Our long-term research interest is CWI signaling in filamentous fungi. Here, we outline CWI signaling in these fungi, from sensor proteins required for the recognition of environmental changes to the regulation of cell wall polysaccharide synthesis genes. We discuss the similarities and differences between the functions of CWI signaling factors in filamentous fungi and in budding yeast. We also describe the latest findings on industrial applications, including those derived from studies on CWI signaling: the development of antifungal agents and the development of highly productive strains of filamentous fungi with modified cell surface characteristics by controlling cell wall biogenesis.

Thinopyrum intermedium TiAP1 interacts with a chitin deacetylase from Blumeria graminis f. sp. tritici and increases the resistance to Bgt in wheat

The biotrophic fungal pathogen Blumeria graminis f. sp. tritici (Bgt) is a crucial factor causing reduction in global wheat production. Wild wheat relatives, for example Thinopyrum intermedium, is one of the wild-used parents in wheat disease-resistant breeding. From T. intermedium line, we identified the aspartic protease gene, TiAP1, which is involved in resistance against Bgt. TiAP1 is a secreted protein that accumulates in large amounts at the infection sites of Bgt and extends to the intercellular space. Yeast two-hybrid, luciferase complementation imaging and bimolecular florescent complimentary analysis showed that TiAP1 interacted with the chitin deacetylase (BgtCDA1) of Bgt. The yeast expression, purification and in vitro test confirmed the chitin deacetylase activity of BgtCDA1. The bombardment and VIGS-mediated host-induced gene silencing showed that BgtCDA1 promotes the invasion of Bgt. Transcriptome analysis showed the cell wall xylan metabolism, lignin biosynthesis-related and defence genes involved in the signal transduction were up-regulated in the transgenic TiAP1 wheat induced by Bgt. The TiAP1 in wheat may inactivate the deacetylation function of BgtCDA1, cause chitin oligomers expose to wheat chitin receptor, then trigger the wheat immune response to inhibit the growth and penetration of Bgt, and thereby enhance the resistance of wheat to pathogens.

Host-Induced Gene Silencing of a Multifunction Gene Sscnd1 Enhances Plant Resistance Against Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a devastating necrotrophic fungal pathogen and has a substantial economic impact on crop production worldwide. Magnaporthe appressoria-specific (MAS) proteins have been suggested to be involved in the appressorium formation in Magnaporthe oryzae. Sscnd1, an MAS homolog gene, is highly induced at the early infection stage of S. sclerotiorum. Knock-down the expression of Sscnd1 gene severely reduced the virulence of S. sclerotiorum on intact rapeseed leaves, and their virulence was partially restored on wounded leaves. The Sscnd1 gene-silenced strains exhibited a defect in compound appressorium formation and cell integrity. The instantaneous silencing of Sscnd1 by tobacco rattle virus (TRV)-mediated host-induced gene silencing (HIGS) resulted in a significant reduction in disease development in tobacco. Three transgenic HIGS Arabidopsis lines displayed high levels of resistance to S. sclerotiorum and decreased Sscnd1 expression. Production of specific Sscnd1 siRNA in transgenic HIGS Arabidopsis lines was confirmed by stem-loop qRT-PCR. This study revealed that the compound appressorium-related gene Sscnd1 is required for cell integrity and full virulence in S. sclerotiorum and that Sclerotinia stem rot can be controlled by expressing the silencing constructs of Sscnd1 in host plants.

A virulence-related lectin traffics into eisosome and contributes to functionality of cytomembrane and cell-wall in the insect-pathogenic fungus Beauveria bassiana

Lectins are characterized of the carbohydrate-binding ability and play comprehensive roles in fungal physiology (e.g., defense response, development and host-pathogen interaction). Beauveria bassiana, a filamentous entomopathogenic fungus, has a lectin-like protein containing a Fruit Body_domain (BbLec1). BbLec1 could bind to chitobiose and chitin in fungal cell wall. BbLec1 proteins interacted with each other to form multimers, and translocated into eisosomes. Further, the interdependence between BbLec1 and the eisosome protein PliA was essential for stabilizing the eisosome architecture. To test the BbLec1 roles in B. bassiana, we constructed the gene disruption and complementation mutants. Notably, the BbLec1 loss resulted in the impaired cell wall in mycelia and conidia as well as conidial formation capacity. In addition, disruption of BbLec1 led to the reduced cytomembrane integrity and the enhanced sensitivity to osmotic stress. Finally, ΔBbLec1 mutant strain displayed the weakened virulence when compared with the wild-type strain. Taken together, BbLec1 traffics into eisosome and links the functionality of eisosome to development and virulence of B. bassiana.

Comparative transcriptomic analysis of races 1, 2, 5 and 6 of Fusarium oxysporum f.sp. pisi in a susceptible pea host identifies differential pathogenicity profiles

BACKGROUND

The fungal pathogen Fusarium oxysporum f.sp. pisi (Fop) causes Fusarium wilt in peas. There are four races globally: 1, 2, 5 and 6 and all of these races are present in Australia. Molecular infection mechanisms have been studied in a few other F. oxysporum formae speciales; however, there has been no transcriptomic Fop-pea pathosystem study.

RESULTS

A transcriptomic study was carried out to understand the molecular pathogenicity differences between the races. Transcriptome analysis at 20 days post-inoculation revealed differences in the differentially expressed genes (DEGs) in the Fop races potentially involved in fungal pathogenicity variations. Most of the DEGs in all the races were engaged in transportation, metabolism, oxidation-reduction, translation, biosynthetic processes, signal transduction, proteolysis, among others. Race 5 expressed the most virulence-associated genes. Most genes encoding for plant cell wall degrading enzymes, CAZymes and effector-like proteins were expressed in race 2. Race 6 expressed the least number of genes at this time point.

CONCLUSION

Fop races deploy various factors and complex strategies to mitigate host defences to facilitate colonisation. This investigation provides an overview of the putative pathogenicity genes in different Fop races during the necrotrophic stage of infection. These genes need to be functionally characterised to confirm their pathogenicity/virulence roles and the race-specific genes can be further explored for molecular characterisation.

Molecular virulence determinants of Magnaporthe oryzae: disease pathogenesis and recent interventions for disease management in rice plant

Magnaporthe oryzae, causative agent of the rice blast disease, is a major concern for the loss in yield of rice crop across the globe. It is known for its characteristic melanised dome-shaped appressorium containing a dense melanin layer. The melanised layer is of considerable importance as it is required to generate turgor pressure for initiating peg formation, consequently rupturing the plant cuticle. Various virulence factors play an important role in the disease progression as well as pathogenesis of the fungus. Some of the proteins encoded by virulence genes are associated with signalling, secondary metabolism, protein deprivation, defence responses and conidiation. The purpose of this review is to describe various fungal virulence determinants and provide insights into the molecular mechanisms that are involved in progression of the disease. Besides, the recent molecular approaches being employed to combat the rice blast have also been elaborated.

Regulation of biotic interactions and responses to abiotic stresses by MAP kinase pathways in plant pathogenic fungi

Like other eukaryotes, fungi use MAP kinase (MAPK) pathways to mediate cellular changes responding to external stimuli. In the past two decades, three well-conserved MAP kinase pathways have been characterized in various plant pathogenic fungi for regulating responses and adaptations to a variety of biotic and abiotic stresses encountered during plant infection or survival in nature. The invasive growth (IG) pathway is homologous to the yeast pheromone response and filamentation pathways. In plant pathogens, the IG pathway often is essential for pathogenesis by regulating infection-related morphogenesis, such as appressorium formation, penetration, and invasive growth. The cell wall integrity (CWI) pathway also is important for plant infection although the infection processes it regulates vary among fungal pathogens. Besides its universal function in cell wall integrity, it often plays a minor role in responses to oxidative and cell wall stresses. Both the IG and CWI pathways are involved in regulating known virulence factors as well as effector genes during plant infection and mediating defenses against mycoviruses, bacteria, and other fungi. In contrast, the high osmolarity growth (HOG) pathway is dispensable for virulence in some fungi although it is essential for plant infection in others. It regulates osmoregulation in hyphae and is dispensable for appressorium turgor generation. The HOG pathway also plays a major role for responding to oxidative, heat, and other environmental stresses and is overstimulated by phenylpyrrole fungicides. Moreover, these three MAPK pathways crosstalk and coordinately regulate responses to various biotic and abiotic stresses. The IG and CWI pathways, particularly the latter, also are involved in responding to abiotic stresses to various degrees in different fungal pathogens, and the HOG pathway also plays a role in interactions with other microbes or fungi. Furthermore, some infection processes or stress responses are co-regulated by MAPK pathways with cAMP or Ca2+/CaM signaling. Overall, functions of individual MAP kinase pathways in pathogenesis and stress responses have been well characterized in a number of fungal pathogens, showing the conserved genetic elements with diverged functions, likely by rewiring transcriptional regulatory networks. In the near future, applications of genomics and proteomics approaches will likely lead to better understanding of crosstalk among the MAPKs and with other signaling pathways as well as roles of MAPKs in defense against other microbes (biotic interactions).

Diversity and Function of Appressoria

Endophytic, saprobic, and pathogenic fungi have evolved elaborate strategies to obtain nutrients from plants. Among the diverse plant-fungi interactions, the most crucial event is the attachment and penetration of the plant surface. Appressoria, specialized infection structures, have been evolved to facilitate this purpose. In this review, we describe the diversity of these appressoria and classify them into two main groups: single-celled appressoria (proto-appressoria, hyaline appressoria, melanized (dark) appressoria) and compound appressoria. The ultrastructure of appressoria, their initiation, their formation, and their function in fungi are discussed. We reviewed the molecular mechanisms regulating the formation and function of appressoria, their strategies to evade host defenses, and the related genomics and transcriptomics. The current review provides a foundation for comprehensive studies regarding their evolution and diversity in different fungal groups.

The Membrane-Bound Protein, MoAfo1, Is Involved in Sensing Diverse Signals from Different Surfaces in the Rice Blast Fungus

To establish an infection, fungal pathogens must recognize diverse signals from host surfaces. The rice blast fungus, Magnaporthe oryzae, is one of the best models studying host-pathogen interactions. This fungus recognizes physical or chemical signals from the host surfaces and initiates the development of an infection structure called appressorium. Here, we found that protein MoAfo1(appressorium formation, MGG_10422) was involved in sensing signal molecules such as cutin monomers and long chain primary alcohols required for appressorium formation. The knockout mutant (ΔMoafo1) formed a few abnormal appressoria on the onion and rice sheath surfaces. However, it produced normal appressoria on the surface of rice leaves. MoAfo1 localized to the membranes of the cytoplasm and vacuole-like organelles in conidia and appressoria. Additionally, the ΔMoafo1 mutant showed defects in appressorium morphology, appressorium penetration, invasive growth, and pathogenicity. These multiple defects might be partially due to failure to respond properly to oxidative stress. These findings broaden our understanding of the fungal mechanisms at play in the recognition of the host surface during rice blast infection.

Gene expression in Rhizoglomus irregulare at two different time points of mycorrhiza establishment in Helianthus annuus roots, as revealed by RNA-seq analysis

Arbuscular mycorrhizal fungi (AMF) play a fundamental role in plant growth and nutrition in natural and agricultural ecosystems. Despite the importance of such symbionts, the different developmental changes occurring during the AMF life cycle have not been fully elucidated at the molecular level. Here, the RNA-seq approach was used to investigate Rhizoglomus irregulare specific and common transcripts at two different time points of mycorrhizal establishment in Helianthus annuus in vivo. Four days after inoculation, transcripts related to cellular remodeling (actin and tubulin), cellular signaling (calmodulin, serine/threonine protein kinase, 14-3-3 protein, and calcium transporting ATPase), lipid metabolism (fatty acid desaturation, steroid hormone, and glycerophospholipid biosynthesis), and biosynthetic processes were detected. In addition to such transcripts, 16 days after inoculation, expressed genes linked to binding and catalytic activities; ion (K⁺, Ca²⁺, Fe²⁺, Zn²⁺, Mn²⁺, Pi, ammonia), sugar, and lipid transport; and those involved in vacuolar polyphosphate accumulation were found. Knowledge of transcriptomic changes required for symbiosis establishment and performance is of great importance to understand the functional role of AMF symbionts in food crop nutrition and health, and in plant diversity in natural ecosystems.

A Bacillus thuringiensis Chitin-Binding Protein is Involved in Insect Peritrophic Matrix Adhesion and Takes Part in the Infection Process

Bacillus thuringiensis (Bt) is used for insect pest control, and its larvicidal activity is primarily attributed to Cry toxins. Other factors participate in infection, and limited information is available regarding factors acting on the peritrophic matrix (PM). This study aimed to investigate the role of a Bt chitin-binding protein (CBPA) that had been previously shown to be expressed at pH 9 in vitro and could therefore be expressed in the alkaline gut of lepidopteron larvae. A ∆cbpA mutant was generated that was 10-fold less virulent than wild-type Bt HD73 towards Ostrinia furnacalis neonate larvae, indicating its important role in infection. Purified recombinant Escherichia coli CBPA was shown to have a chitin affinity, thus indicating a possible interaction with the chitin-rich PM. A translational GFP-CBPA fusion elucidated the localization of CBPA on the bacterial surface, and the transcriptional activity of the promoter PcbpA was immediately induced and confirmed at pH 9. Next, in order to connect surface expression and possible in vivo gut activity, last instar Galleria mellonella (Gm) larvae (not susceptible to Bt HD-73) were used as a model to follow CBPA in gut expression, bacterial transit, and PM adhesion. CBPA-GFP was quickly expressed in the Gm gut lumen, and more Bt HD73 strain bacteria adhered to the PM than those of the ∆cbpA mutant strain. Therefore, CBPA may help to retain the bacteria, via the PM binding, close to the gut surface and thus takes part in the early steps of Bt gut interactions.

Magnaporthe oryzae Auxiliary Activity Protein MoAa91 Functions as Chitin-Binding Protein To Induce Appressorium Formation on Artificial Inductive Surfaces and Suppress Plant Immunity

The appressoria that are generated by the rice blast fungus Magnaporthe oryzae in response to surface cues are important for successful colonization. Previous work showed that regulators of G-protein signaling (RGS) and RGS-like proteins play critical roles in appressorium formation. However, the mechanisms by which these proteins orchestrate surface recognition for appressorium induction remain unclear. Here, we performed comparative transcriptomic studies of ΔMorgs mutant and wild-type strains and found that M. oryzae Aa91 (MoAa91), a homolog of the auxiliary activity family 9 protein (Aa9), was required for surface recognition of M. oryzae We found that MoAA91 was regulated by the MoMsn2 transcription factor and that its disruption resulted in defects in both appressorium formation on the artificial inductive surface and full virulence of the pathogen. We further showed that MoAa91 was secreted into the apoplast space and was capable of competing with the immune receptor chitin elicitor-binding protein precursor (CEBiP) for chitin binding, thereby suppressing chitin-induced plant immune responses. In summary, we have found that MoAa91 is a novel signaling molecule regulated by RGS and RGS-like proteins and that MoAa91 not only governs appressorium development and virulence but also functions as an effector to suppress host immunity.IMPORTANCE The rice blast fungus Magnaporthe oryzae generates infection structure appressoria in response to surface cues largely due to functions of signaling molecules, including G-proteins, regulators of G-protein signaling (RGS), mitogen-activated protein (MAP) kinase pathways, cAMP signaling, and TOR signaling pathways. M. oryzae encodes eight RGS and RGS-like proteins (MoRgs1 to MoRgs8), and MoRgs1, MoRgs3, MoRgs4, and MoRgs7 were found to be particularly important in appressorium development. To explore the mechanisms by which these proteins regulate appressorium development, we have performed a comparative in planta transcriptomic study and identified an auxiliary activity family 9 protein (Aa9) homolog that we named MoAa91. We showed that MoAa91 was secreted from appressoria and that the recombinant MoAa91 could compete with a chitin elicitor-binding protein precursor (CEBiP) for chitin binding, thereby suppressing chitin-induced plant immunity. By identifying MoAa91 as a novel signaling molecule functioning in appressorium development and an effector in suppressing host immunity, our studies revealed a novel mechanism by which RGS and RGS-like proteins regulate pathogen-host interactions.

Quantitative proteomics analysis reveals important roles of N-glycosylation on ER quality control system for development and pathogenesis in Magnaporthe oryzae

Genetic studies have shown essential functions of N-glycosylation during infection of the plant pathogenic fungi, however, systematic roles of N-glycosylation in fungi is still largely unknown. Biological analysis demonstrated N-glycosylated proteins were widely present at different development stages of Magnaporthe oryzae and especially increased in the appressorium and invasive hyphae. A large-scale quantitative proteomics analysis was then performed to explore the roles of N-glycosylation in M. oryzae. A total of 559 N-glycosites from 355 proteins were identified and quantified at different developmental stages. Functional classification to the N-glycosylated proteins revealed N-glycosylation can coordinate different cellular processes for mycelial growth, conidium formation, and appressorium formation. N-glycosylation can also modify key components in N-glycosylation, O-glycosylation and GPI anchor pathways, indicating intimate crosstalk between these pathways. Interestingly, we found nearly all key components of the endoplasmic reticulum quality control (ERQC) system were highly N-glycosylated in conidium and appressorium. Phenotypic analyses to the gene deletion mutants revealed four ERQC components, Gls1, Gls2, GTB1 and Cnx1, are important for mycelial growth, conidiation, and invasive hyphal growth in host cells. Subsequently, we identified the Gls1 N-glycosite N497 was important for invasive hyphal growth and partially required for conidiation, but didn’t affect colony growth. Mutation of N497 resulted in reduction of Gls1 in protein level, and localization from ER into the vacuole, suggesting N497 is important for protein stability of Gls1. Our study showed a snapshot of the N-glycosylation landscape in plant pathogenic fungi, indicating functions of this modification in cellular processes, developments and pathogenesis.

The fungal pathogen Magnaporthe oryzae can cause rice blast and wheat blast diseases, which threatens worldwide food production. During infection, M. oryzae follows a sequence of distinct developmental stages adapted to survival and invasion of the host environment. M. oryzae attaches onto the host by the conidium, and then develops an appressorium to breach the host cuticle. After penetrating, it forms invasive hyphae to quickly spread in the host cells. Numerous genetic studies have focused on the mechanisms underlying each step in the infection process, but systemic approaches are needed for a broader, integrated understanding of regulatory events during M. oryzae pathogenesis. Many infection-related signaling events are regulated through post-translational protein modifications within the pathogen. N-linked glycosylation, in which a glycan moiety is added to the amide group of an asparagine residue, is an abundant modification known to be essential for M. oryzae infection. In this study, we employed a quantitative proteomics analysis to unravel the overall regulatory mechanisms of N-glycosylation at different developmental stages of M. oryzae. We detected changes in N-glycosylation levels at 559 glycosylated residues (N-glycosites) in 355 proteins during different stages, and determined that the ER quality control system is elaborately regulated by N-glycosylation. The insights gained will help us to better understand the regulatory mechanisms of infection in pathogenic fungi. These findings may be also important for developing novel strategies for fungal disease control.

Extremely Low Concentrations of Acetic Acid Stimulate Cell Differentiation in Rice Blast Fungus

Metabolic switching and rewiring play a dynamic role in programmed cell differentiation. Many pathogenic microbes need to survive in nutrient-deficient conditions and use the glyoxylate cycle, an anaplerotic pathway of the tricarboxylic acid cycle, to produce carbohydrates. The plant pathogenic fungus Magnaporthe oryzae (Pyricularia oryzae) has a unique chitin deacetylase, Cbp1. The spatiotemporal activity of this protein is required for modification of the M. oryzae wall and for cell differentiation into the specialized infection structure (appressorium). Here we show that acetic acid, another product released by the Cbp1-catalyzed conversion of chitin into chitosan, induces appressorium formation. An extremely low concentration (fM) of acetic acid restored cell differentiation in a Δcbp1 mutant possibly through the glyoxylate cycle.

•

Acidification occurred by chitin deacetylase activity during cell differentiation

•

Extremely low concentrations of exogenous acetic acid stimulated cell differentiation

•

Exogenous acetic acid induced ICL1 expression, a member of the glyoxylate cycle

•

Deletion of ICL1 inhibited acetic acid-mediated cell differentiation

In Silico Identification of Potential Inhibitor Against a Fungal Histone Deacetylase, RPD3 from Magnaporthe Oryzae

Histone acetylation and deacetylation play an essential role in the epigenetic regulation of gene expression. Histone deacetylases (HDAC) are a group of zinc-binding metalloenzymes that catalyze the removal of acetyl moieties from lysine residues from histone tails. These enzymes are well known for their wide spread biological effects in eukaryotes. In rice blast fungus, Magnaporthe oryzae, MoRPD3 (an ortholog of Saccharomyces cerevisiae Rpd3) was shown to be required for growth and development. Thus in this study, the class I HDAC, MoRpd3 is considered as a potential drug target, and its 3D structure was modelled and validated. Based on the model, a total of 1880 compounds were virtually screened (molecular docking) against MoRpd3 and the activities of the compounds were assessed by docking scores. The in silico screening suggested that [2-[[4-(2-methoxyethyl) phenoxy] methyl] phenyl] boronic acid (−8.7 kcal/mol) and [4-[[4-(2-methoxyethyl) phenoxy] methyl] phenyl] boronic acid (−8.5 kcal/mol) are effective in comparison to trichostatin A (−7.9 kcal/mol), a well-known general HDAC inhibitor. The in vitro studies for inhibition of appressorium formation by [2-[[4-(2-methoxyethyl) phenoxy] methyl] phenyl] boronic acid has resulted in the maximum inhibition at lower concentrations (1 μM), while the trichostatin A exhibited similar levels of inhibition at 1.5 μM. These findings thus suggest that 3D quantitative structure activity relationship studies on [2-[[4-(2-methoxyethyl) phenoxy] methyl] phenyl] boronic acid compound can further guide the design of more potential and specific HDAC inhibitors.

Cellophane surface-induced gene, VdCSIN1, regulates hyphopodium formation and pathogenesis via cAMP-mediated signalling in Verticillium dahliae

The soil-borne vascular pathogen Verticillium dahliae infects many dicotyledonous plants to cause devastating wilt diseases. During colonization, V. dahliae spores develop hyphae surrounding the roots. Only a few hyphae that adhere tightly to the root surface form hyphopodia at the infection site, which further differentiate into penetration pegs to facilitate infection. The molecular mechanisms controlling hyphopodium formation in V. dahliae remain unclear. Here, we uncovered a cellophane surface-induced gene (VdCSIN1) as a regulator of V. dahliae hyphopodium formation and pathogenesis. Deletion of VdCSIN1 compromises hyphopodium formation, hyphal development and pathogenesis. Exogenous application of cyclic adenosine monophosphate (cAMP) degradation inhibitor or disruption of the cAMP phosphodiesterase gene (VdPDEH) partially restores hyphopodium formation in the VdΔcsin1 mutant. Moreover, deletion of VdPDEH partially restores the pathogenesis of the VdΔcsin1 mutant. These findings indicate that VdCSIN1 regulates hyphopodium formation via cAMP-mediated signalling to promote host colonization by V. dahliae.

Visualizing fungicide action: an in vivo tool for rapid validation of fungicides with target location HOG pathway

BACKGROUND

The mitogen-activated protein kinase MoHog1p was fused with a green fluorescent protein (GFP) in the filamentous fungus Magnaporthe oryzae. The MoHOG1::GFP mutant was found to be an excellent tool visualizing in vivo fungicide-dependent translocation of MoHog1p into the nucleus. Validation of pathway specificity was achieved by generating fluorescence-labelled MoHog1p in the ΔMohik1 'loss of function' mutant strain.

RESULTS

GFP-labelled MoHog1p expressed in the wildtype and in ΔMohik1 demonstrates that fludioxonil is acting on the HOG pathway and even more precisely that fungicide action is dependent on the group III histidine kinase MoHik1p. GFP-tagged MoHog1p translocated into the nucleus upon fungicide treatment in the MoHOG1::GFP mutant within seconds, but did not do so in the ΔMohik1/HOG1::GFP mutant.

CONCLUSION

Here, we developed a rapid in vivo tool for fluorescent-based validation of fungicides targeting the HOG-signaling pathway. Furthermore, using the fluorescent mutants generated in this study, we are able to visualize that fungicide action is dependent on the histidine kinase MoHik1p but operates in a different mechanism of pathway activation compared to osmotic stress. © 2018 Society of Chemical Industry.

Regulatory network of genes associated with stimuli sensing, signal transduction and physiological transformation of appressorium in Magnaporthe oryzae

Rice blast caused by Magnaporthe oryzae is the most destructive disease affecting the rice production (Oryza sativa), with an average global loss of 10-30% per annum. Recent reports have indicated that the fungus also inflicts blast disease on wheat (Triticum aestivum) posing a serious threat to the wheat production. Due to its easily detected infectious process and manoeuvrable genetic manipulation, M. oryzae is considered a model organism for exploring the molecular mechanism underlying fungal pathogenicity during the pathogen-host interaction. M. oryzae utilises an infectious structure called appressorium to breach the host surface by generating high turgor pressure. The appressorium development is induced by physical and chemical cues which are coordinated by the highly conserved cAMP/PKA, MAPK and calcium signalling cascades. Genes involved in the appressorium development have been identified and well studied in M. oryzae, a summary of the working gene network linking stimuli sensing and physiological transformation of appressorium is needed. This review provides a comprehensive discussion regarding the regulatory networks underlying appressorium development with particular emphasis on sensing of appressorium inducing stimuli, signal transduction, transcriptional regulation and the corresponding developmental and physiological responses. We also discussed the crosstalk and interaction of various pathways during the appressorium development.

Chitin-deacetylase activity induces appressorium differentiation in the rice blast fungus Magnaporthe oryzae

The rice blast fungus Magnaporthe oryzae differentiates a specialized infection structure called an appressorium to invade rice cells. In this report, we show that CBP1, which encodes a chitin-deacetylase, is involved in the induction phase of appressorium differentiation. We demonstrate that the enzymatic activity of Cbp1 is critical for appressorium formation. M. oryzae has six CDA homologues in addition to Cbp1, but none of these are indispensable for appressorium formation. We observed chitosan localization at the fungal cell wall using OGA⁴⁸⁸. This observation suggests that Cbp1-catalysed conversion of chitin into chitosan occurs at the cell wall of germ tubes during appressorium differentiation by M. oryzae. Taken together, our results provide evidence that the chitin deacetylase activity of Cbp1 is necessary for appressorium formation.

Nutrient Sensing at the Plasma Membrane of Fungal Cells

To respond to the changing environment, cells must be able to sense external conditions. This is important for many processes including growth, mating, the expression of virulence factors, and several other regulatory effects. Nutrient sensing at the plasma membrane is mediated by different classes of membrane proteins that activate downstream signaling pathways: nontransporting receptors, transceptors, classical and nonclassical G-protein-coupled receptors, and the newly defined extracellular mucin receptors. Nontransporting receptors have the same structure as transport proteins, but have lost the capacity to transport while gaining a receptor function. Transceptors are transporters that also function as a receptor, because they can rapidly activate downstream signaling pathways. In this review, we focus on these four types of fungal membrane proteins. We mainly discuss the sensing mechanisms relating to sugars, ammonium, and amino acids. Mechanisms for other nutrients, such as phosphate and sulfate, are discussed briefly. Because the model yeast Saccharomyces cerevisiae has been the most studied, especially regarding these nutrient-sensing systems, each subsection will commence with what is known in this species.

WISH, a novel CFEM GPCR is indispensable for surface sensing, asexual and pathogenic differentiation in rice blast fungus

We have selected and characterized a unique Conserved Fungal-specific Extra-cellular Membrane-spanning (CFEM) domain containing PTH11 like G-protein coupled receptor (GPCR), which is responsible for Water wettability, Infection, Surface sensing and Hyper-conidiation (WISH). The pathogenicity gene WISH is predicted to encode a novel seven transmembrane protein in the rice blast fungus, Magnaporthe oryzae, one of the deadliest pathogens of rice. We generated knockout mutants through a homologous recombination-based method to understand the function of the gene. These mutants are nonpathogenic due to a defect in sensing hydrophobic surface and appressorium differentiation. The mutant failed to undergo early events of pathogenesis, and appressorium development is diminished on inductive hydrophobic surface and was unable to penetrate susceptible rice leaves. The Δwish mutant did not develop any appressorium, suggesting that WISH protein is required for appressorium morphogenesis and is also involved in host surface recognition. We examined various aspects of pathogenesis and the results indicated involvement of WISH in preventing autolysis of vegetative hyphae, determining surface hydrophobicity and maintenance of cell-wall integrity. WISH gene from M. oryzae strain B157 complemented the Δwish mutant, indicating functional authenticity. Exogenous activation of cellular signaling failed to suppress the defects in Δwish mutants. These findings suggest that WISH GPCR senses diverse extracellular signals to play multiple roles and might have effects on PTH11 and MPG1 genes especially as an upstream effector of appressorium differentiation. It is for the first time that a typical GPCR containing seven transmembrane helices involved in the early events of plant pathogenesis of M. oryzae has been functionally characterized.

The prediction of a pathogenesis-related secretome of Puccinia helianthi through high-throughput transcriptome analysis

Background

Many plant pathogen secretory proteins are known to be elicitors or pathogenic factors,which play an important role in the host-pathogen interaction process. Bioinformatics approaches make possible the large scale prediction and analysis of secretory proteins from the Puccinia helianthi transcriptome. The internet-based software SignalP v4.1, TargetP v1.01, Big-PI predictor, TMHMM v2.0 and ProtComp v9.0 were utilized to predict the signal peptides and the signal peptide-dependent secreted proteins among the 35,286 ORFs of the P. helianthi transcriptome.

Results

908 ORFs (accounting for 2.6% of the total proteins) were identified as putative secretory proteins containing signal peptides. The length of the majority of proteins ranged from 51 to 300 amino acids (aa), while the signal peptides were from 18 to 20 aa long. Signal peptidase I (SpI) cleavage sites were found in 463 of these putative secretory signal peptides. 55 proteins contained the lipoprotein signal peptide recognition site of signal peptidase II (SpII). Out of 908 secretory proteins, 581 (63.8%) have functions related to signal recognition and transduction, metabolism, transport and catabolism. Additionally, 143 putative secretory proteins were categorized into 27 functional groups based on Gene Ontology terms, including 14 groups in biological process, seven in cellular component, and six in molecular function. Gene ontology analysis of the secretory proteins revealed an enrichment of hydrolase activity. Pathway associations were established for 82 (9.0%) secretory proteins. A number of cell wall degrading enzymes and three homologous proteins specific to Phytophthora sojae effectors were also identified, which may be involved in the pathogenicity of the sunflower rust pathogen.

Conclusions

This investigation proposes a new approach for identifying elicitors and pathogenic factors. The eventual identification and characterization of 908 extracellularly secreted proteins will advance our understanding of the molecular mechanisms of interactions between sunflower and rust pathogen and will enhance our ability to intervene in disease states.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-017-1577-0) contains supplementary material, which is available to authorized users.

The Colletotrichum gloeosporioides RhoB regulates cAMP and stress response pathways and is required for pathogenesis

Rho GTPases regulate morphology and multiple cellular functions such as asexual development, polarity establishment, and differentiation in fungi. To determine the roles of CgRhoB, a Rho GTPase protein, here we characterized CgRhoB in the poplar anthracnose fungus Colletotrichum gloeosporioides. First of all, we determined that conidial germination was inhibited and intracellular cyclic AMP (cAMP) level was increased in the CgRhoB deletion mutants. Loss of CgRhoB resulted in shorter germ tubes and enhanced appressoria formation after germination on the hydrophobic surface. Exogenous addition of cAMP to the wild type generated the similar phenotypes of ΔCgRhoB inoculated in CM liquid. Furthermore, deletion of CgRhoB had discernible effect upon the sensitivity of C. gloeosporioides to cell wall perturbing agents and altered the distribution of chitin on the cell wall. H₂O₂ sensitivity assay showed the hypersensitive effect on the oxidative stress, and transcriptional analysis revealed that transcription of genes involved in peroxidase activities was altered in the mutants. Finally, virulence assay revealed that CgRhoB was required for pathogenicity. Taken together, our results showed that CgRhoB was associated with appressoria formation and pathogenicity, and affected cAMP level and stress pathways.

Secretome analysis of virulent Pyrenophora teres f. teres isolates

Pyrenophora teres f. teres (Ptt) causes net form net blotch disease of barley, partially by producing necrosis-inducing proteins. The protein profiles of the culture filtrates of 28 virulent isolates were compared by a combination of 2DE and 1D-PAGE with 105 spots and 51 bands chosen for analysis by liquid chromatography electrospray ionization tandem mass spectrometry. A total of 259 individual proteins were identified with 63 of these proteins being common to the selected virulent isolates. Ptt secretes a broad spectrum of proteins including cell wall degrading enzymes; virulence factors and effectors; proteins associated with fungal pathogenesis and development; and proteins related to oxidation-reduction processes. Potential virulence factors and effectors identified included proteins with glucosidase activity, ricin B and concanavalin A-like lectins, glucanases, spherulin, cutinase, pectin lyase, leucine-rich repeat protein, and ceratoplatanin. Small proteins with unknown function but cysteine-rich, common to effectors, were also identified. Differences in the secretion profile of the Ptt isolates have also provided important insight into the different mechanisms contributing to virulence and the development of net form net blotch symptoms.

Chitosan Mediates Germling Adhesion in Magnaporthe oryzae and Is Required for Surface Sensing and Germling Morphogenesis

The fungal cell wall not only plays a critical role in maintaining cellular integrity, but also forms the interface between fungi and their environment. The composition of the cell wall can therefore influence the interactions of fungi with their physical and biological environments. Chitin, one of the main polysaccharide components of the wall, can be chemically modified by deacetylation. This reaction is catalyzed by a family of enzymes known as chitin deacetylases (CDAs), and results in the formation of chitosan, a polymer of β1,4-glucosamine. Chitosan has previously been shown to accumulate in the cell wall of infection structures in phytopathogenic fungi. Here, it has long been hypothesized to act as a 'stealth' molecule, necessary for full pathogenesis. In this study, we used the crop pathogen and model organism Magnaporthe oryzae to test this hypothesis. We first confirmed that chitosan localizes to the germ tube and appressorium, then deleted CDA genes on the basis of their elevated transcript levels during appressorium differentiation. Germlings of the deletion strains showed loss of chitin deacetylation, and were compromised in their ability to adhere and form appressoria on artificial hydrophobic surfaces. Surprisingly, the addition of exogenous chitosan fully restored germling adhesion and appressorium development. Despite the lack of appressorium development on artificial surfaces, pathogenicity was unaffected in the mutant strains. Further analyses demonstrated that cuticular waxes are sufficient to over-ride the requirement for chitosan during appressorium development on the plant surface. Thus, chitosan does not have a role as a 'stealth' molecule, but instead mediates the adhesion of germlings to surfaces, thereby allowing the perception of the physical stimuli necessary to promote appressorium development. This study thus reveals a novel role for chitosan in phytopathogenic fungi, and gives further insight into the mechanisms governing appressorium development in M.oryzae.

Magnaporthe oryzae is a filamentous fungal pathogen which causes devastating crop losses in rice. Successful invasion of the host is dependent upon the ability of the fungus to remain undetected by the innate immune system of the plant, which recognizes conserved components of the fungal cell wall, such as chitin. Previous studies have demonstrated that infection-related changes in cell wall composition are necessary to allow the fungus to remain undetected during infection. One such change that has long been hypothesized to have a role as a 'stealth mechanism' is the deacetylation of the polysaccharide chitin by enzymes known as chitin deacetylases. The deacetylation of chitin produces a polysaccharide known as chitosan, which has previously been shown to accumulate specifically on infection structures in plant pathogenic fungi. However, in this study, we show that germling-localized chitosan is not required for pathogenicity, arguing against a role as a 'stealth mechanism' at this stage. Instead, chitosan is required for the development of the appressorium, a critical fungal infection structure required for the penetration of plant cells. This requirement can be attributed to chitosan mediating the adhesion of germlings to surfaces, which is required for the perception of physical stimuli.

Autophagy in plant pathogenic fungi

Autophagy is a conserved cellular process that degrades cytoplasmic constituents in vacuoles. Plant pathogenic fungi develop special infection structures and/or secrete a range of enzymes to invade their plant hosts. It has been demonstrated that monitoring autophagy processes can be extremely useful in visualizing the sequence of events leading to pathogenicity of plant pathogenic fungi. In this review, we introduce the molecular mechanisms involved in autophagy. In addition, we explore the relationship between autophagy and pathogenicity in plant pathogenic fungi. Finally, we discuss the various experimental strategies available for use in the study of autophagy in plant pathogenic fungi.

A Venom Gland Extracellular Chitin-Binding-Like Protein from Pupal Endoparasitoid Wasps, Pteromalus Puparum, Selectively Binds Chitin

Chitin-binding proteins (CBPs) are present in many species and they act in a variety of biological processes. We analyzed a Pteromalus puparum venom apparatus proteome and transcriptome and identified a partial gene encoding a possible CBP. Here, we report cloning a full-length cDNA of a sequence encoding a chitin-binding-like protein (PpCBP) from P. puparum, a pupal endoparasitoid of Pieris rapae. The cDNA encoded a 96-amino-acid protein, including a secretory signal peptide and a chitin-binding peritrophin-A domain. Phylogenetic analysis of chitin binding domains (CBDs) of cuticle proteins and peritrophic matrix proteins in selected insects revealed that the CBD of PpCBP clustered with the CBD of Nasonia vitripennis. The PpCBP is specifically expressed in the venom apparatus of P. puparum, mostly in the venom gland. PpCBP expression was highest at day one after adult eclosion and much lower for the following five days. We produced a recombinant PpCBP and binding assays showed the recombinant protein selectively binds chitin but not cellulose in vitro. We infer that PpCBP serves a structural role in the venom reservoir, or may be injected into the host to help wound healing of the host exoskeleton.

Activation of the signalling mucin MoMsb2 and its functional relationship with Cbp1 in Magnaporthe oryzae

Various surface signals are recognized by Magnaporthe oryzae to activate the Pmk1 MAP kinase that is essential for appressorium formation and invasive growth. One of upstream sensors of the Pmk1 pathway is the MoMsb2 signalling mucin. However, the activation of MoMsb2 and its relationship with other sensors is not clear. In this study, we showed that the cleavage and transmembrane domains are essential for MoMsb2 functions. Cleavage of MoMsb2 was further confirmed by western blot analysis, and five putative cleavage sites were functionally characterized. Expression of the extracellular region alone partially rescued the defects of Momsb2 in appressorium formation and virulence. The cytoplasmic region of MoMsb2, although dispensable for appressorium formation, was more important for penetration and invasive growth. Interestingly, the Momsb2 cbp1 double mutant deleted of both mucin genes was blocked in Pmk1 activation. It failed to form appressoria on artificial surfaces and was non-pathogenic. In addition, we showed that MoMsb2 interacts with Ras2 but not with MoCdc42 in co-immunoprecipitation assays. Overall, results from this study indicated that the extracellular and cytoplasmic regions of MoMsb2 have distinct functions in appressorium formation, penetration and invasive growth, and MoMsb2 has overlapping functions with Cbp1 in recognizing environmental signals for Pmk1 activation.

Identification and characterization of a novel gene encoding the NBS1 protein in Pyricularia oryzae

The ascomycete Pyricularia oryzae (teleomorph: Magnaporthe oryzae) causes one of the most serious diseases known as rice blast. The Nijmegen breakage syndrome protein (NBS1) is essential for DNA repair; thus, we studied the P. oryzae NBS1 homolog (PoNBS1). A PoNBS1 null mutant exhibited high sensitivity to DNA damage-inducing agents. The mutant also exhibited the retarded hyphal growth, and induced abnormal conidial germination and shape, but showed normal appressorium formation. The phenotypes of the null mutant were complemented by introducing the cDNA of PoNBS1 driven by a TrpC promoter of Aspergillus nidulans. In addition, the null mutant similarly complemented with the PoNBS1 cDNA lacking the FHA domain that had a normal phenotype except for hyphal growth. These results suggest that PoNBS1 is involved in DNA repair and normal development in P. oryzae. Moreover, the FHA domain of PoNBS1 participates in normal hyphal growth.

Combining metabolomics and gene expression analysis reveals that propionyl- and butyryl-carnitines are involved in late stages of arbuscular mycorrhizal symbiosis

The arbuscular mycorrhizal (AM) symbiosis is a widespread mutualistic association between soil fungi (Glomeromycota) and the roots of most plant species. AM fungi are obligate biotrophs whose development is partially under the control of their plant host. We explored the possibility to combine metabolomic and transcriptomic approaches to find putative mycorrhiza-associated metabolites regulating AM fungal development. Methanol extracts of Medicago truncatula roots colonized or not with the AM fungus Rhizophagus irregularis were analyzed and compared by ultra-high-performance liquid chromatography (UHPLC), high-resolution mass spectrometry (Q-TOF), and multivariate statistical discrimination. We detected 71 mycorrhiza-associated analytes exclusively present or at least 10-fold more abundant in mycorrhizal roots. To identify among these analytes those that could regulate AM fungal development, we fractionated by preparative and semi-preparative HPLC the mycorrhizal and non-mycorrhizal root extracts and established how the 71 analytes were distributed among the fractions. Then we tested the activity of the fractions on germinating spores of R. irregularis by quantifying the expression of 96 genes known for their diverse in planta expression patterns. These investigations reveal that propionyl- and butyryl-carnitines accumulated in mycorrhizal roots. The results suggest that these two molecules regulate fungal gene expression in planta and represent interesting candidates for further biological characterization.

A eukaryotic molecular target candidate of roxithromycin: fungal differentiation as a sensitive drug target analysis system

Roxithromycin (RXM), active against prokaryotes, has beneficial side effects such as anti-cancer activities on mammalian cells, but the mechanisms underlying these effects remain unclear. We found that RXM inhibited the cellular differentiation of the rice blast fungus Magnaporthe oryzae. Hence, we screened the targets of RXM by the T7 phage display method with fungal genomic DNA, and identified MoCDC27 (M. oryzae Cell Division Cycle 27) as a candidate. We generated mocdc27 knockdown mutants that the appressoria formation was less affected by RXM. A complemented mutant restored sensitivity against RXM to the level of the wild type. These results suggest that MoCDC27 was involved in the inhibition of appressorium formation by RXM, and that the complex of RXM-MoCDC27 affected another molecule involved in appressorium formation. The T7 phage display method with fungal genomic DNA can be a useful tool in the quest for drug target.

Differences between appressoria formed by germ tubes and appressorium-like structures developed by hyphal tips in Magnaporthe oryzae

Melanized appressoria are highly specialized infection structures formed by germ tubes of the rice blast fungus Magnaporthe oryzae for plant infection. M. oryzae also forms appressorium-like structures on hyphal tips. Whereas appressorium formation by conidial germ tubes has been well characterized, formation of appressorium-like structures by hyphal tips is under-investigated. In a previous study, we found that the chs7 deletion mutant failed to form appressoria on germ tubes but were normal in the development of appressorium-like structures on artificial hydrophobic surfaces. In this study, we compared the differences between the formation of appressoria by germ tubes and appressorium-like structures by hyphal tips in M. oryzae. Structurally, both appressoria and appressorium-like structures had a melanin layer that was absent in the pore region. In general, the latters were 1.4-fold larger in size but had lower turgor pressure than appressoria, which is consistent with its lower efficiency in plant penetration. Treatments with cAMP, IBMX, or a cutin monomer efficiently induced appressorium formation but not the development of appressorium-like structures. In contrast, coating surfaces with waxes stimulated the formation of both infection structures. Studies with various signaling mutants indicate that Osm1 and Mps1 are dispensable but Pmk1 is essential for both appressorium formation and development of appressorium-like structures on hyphal tips. Interestingly, the cpkA mutant was reduced in the differentiation of appressorium-like structures but not appressorium formation. We also observed that the con7 mutant generated in our lab failed to form appressorium-like structures on hyphal tips but still produced appressoria by germ tubes on hydrophobic surfaces. Con7 is a transcription factor regulating the expression of CHS7. Overall, these results indicate that the development of appressorium-like structures by hyphal tips and formation of appressoria by germ tubes are not identical differentiation processes in M. oryzae and may involve different molecular mechanisms.

In silico identification of novel hevein-like peptide precursors

Lectins are proteins with ability to bind reversibly and non-enzymatically to a specific carbohydrate. They are involved in numerous biological processes and show enormous biotechnological potential. Among plant lectins, the hevein domain is extremely common, being observed in several kinds of lectins. Moreover, this domain is also observed in an important class of antimicrobial peptides named hevein-like peptides. Due to higher cysteine residues conservation, hevein-like peptides could be mined among the sequence databases. By using the pattern CX(4,5)CC[GS]X(2)GXCGX[GST]X(2,3)[FWY]C[GS]X[AGS] novel hevein-like peptide precursors were found from three different plants: Oryza sativa, Vitis vinifera and Selaginella moellendorffii. In addition, an hevein-like peptide precursor from the phytopathogenic fungus Phaeosphaeria nodorum was also identified. The molecular models indicate that they have the same scaffold as others, composed of an antiparallel β-sheet and short helices. Nonetheless, the fungal hevein-like peptide probably has a different disulfide bond pattern. Despite this difference, the complexes between peptide and N,N,N-triacetylglucosamine are stable, according to molecular dynamics simulations. This is the first report of an hevein-like peptide from an organism outside the plant kingdom. The exact role of an hevein-like peptide in the fungal biology must be clarified, while in plants they are clearly involved in plant defense. In summary, data here reported clear shows that an in silico strategy could lead to the identification of novel hevein-like peptides that could be used as biotechnological tools in the fields of health and agribusiness.

The Cyclase-associated protein Cap1 is important for proper regulation of infection-related morphogenesis in Magnaporthe oryzae

Surface recognition and penetration are critical steps in the infection cycle of many plant pathogenic fungi. In Magnaporthe oryzae, cAMP signaling is involved in surface recognition and pathogenesis. Deletion of the MAC1 adenylate cyclase gene affected appressorium formation and plant infection. In this study, we used the affinity purification approach to identify proteins that are associated with Mac1 in vivo. One of the Mac1-interacting proteins is the adenylate cyclase-associated protein named Cap1. CAP genes are well-conserved in phytopathogenic fungi but none of them have been functionally characterized. Deletion of CAP1 blocked the effects of a dominant RAS2 allele and resulted in defects in invasive growth and a reduced intracellular cAMP level. The Δcap1 mutant was defective in germ tube growth, appressorium formation, and formation of typical blast lesions. Cap1-GFP had an actin-like localization pattern, localizing to the apical regions in vegetative hyphae, at the periphery of developing appressoria, and in circular structures at the base of mature appressoria. Interestingly, Cap1, similar to LifeAct, did not localize to the apical regions in invasive hyphae, suggesting that the apical actin cytoskeleton differs between vegetative and invasive hyphae. Domain deletion analysis indicated that the proline-rich region P2 but not the actin-binding domain (AB) of Cap1 was responsible for its subcellular localization. Nevertheless, the AB domain of Cap1 must be important for its function because CAP1^ΔAB only partially rescued the Δcap1 mutant. Furthermore, exogenous cAMP induced the formation of appressorium-like structures in non-germinated conidia in CAP1^ΔAB transformants. This novel observation suggested that AB domain deletion may result in overstimulation of appressorium formation by cAMP treatment. Overall, our results indicated that CAP1 is important for the activation of adenylate cyclase, appressorium morphogenesis, and plant infection in M. oryzae. CAP1 may also play a role in feedback inhibition of Ras2 signaling when Pmk1 is activated.

In Magnaporthe oryzae, cAMP signaling is known to play an important role in surface recognition and plant penetration. The Mac1 adenylate cyclase is essential for plant infection. To better understand Mac1 activation mechanisms, in this study we used the affinity purification approach to identify proteins that are associated with Mac1 in vivo. One of the Mac1-interacting protein is the adenylate cyclase associated protein (CAP) encoded by the CAP1 gene. Results from our study indicated that Cap1 is important for Mac1 activation and plant infection in M. oryzae. The Δcap1 mutant was defective in germ tube growth and appressorium formation and failed to cause typical blast lesions. Like LifeAct, Cap1 localized to apical patches in vegetative hyphae but not in invasive hyphae. The P2 proline-rich region was important for Cap1 localization but the actin-binding domain played a role in feedback inhibition of Ras signaling. To our knowledge, functional characterization of CAP genes has not been reported in filamentous fungi. Our results indicate that CAP1 is important for regulating adenylate cyclase activities, appressorium morphogenesis, and plant infection. Further characterization of CAP1 will be important to better understand the interaction between cAMP signaling and the PMK1 pathway in M. oryzae.

Gene expression profiling during conidiation in the rice blast pathogen Magnaporthe oryzae

Conidiation of phytopathogenic fungi is a key developmental process that plays a central role in their life cycles and in epidemics. However, there is little information on conidiation-induced molecular changes in the rice blast fungus Magnaporthe oryzae. As a first step to understand conidiogenesis in this fungus, we measured genome-wide gene expression profiles during conidiation using a whole genome oligonucleotide microarray. At a two-fold expression difference, approximately 4.42% and 4.08% of genes were upregulated and downregulated, respectively, during conidiation. The differentially expressed genes were functionally categorized by gene ontology (GO) term analysis, which demonstrated that the gene set encoded proteins that function in metabolism, cell wall biosynthesis, transcription, and molecule transport. To define the events of the complicated process of conidiogenesis, another set of microarray experiments was performed using a deletion mutant for MoHOX2, a stage-specific transcriptional regulator essential for conidial formation, which was expressed de novo in a conidiation-specific manner in M. oryzae. Gene expression profiles were compared between the wild-type and the ΔMohox2 mutant during conidiation. This analysis defined a common gene set that was upregulated in the wild-type and downregulated in the ΔMohox2 mutant during conidiation; this gene set is expected to include conidiation-related downstream genes of MoHOX2. We identified several hundred genes that are differentially-expressed during conidiation; our results serve as an important resource for understanding the conidiation, a process in M. oryzae, which is critical for disease development.

Expression of a bacterial chitosanase in rice plants improves disease resistance to the rice blast fungus Magnaporthe oryzae

Plant fungal pathogens change their cell wall components during the infection process to avoid degradation by host lytic enzymes, and conversion of the cell wall chitin to chitosan is likely to be one infection strategy of pathogens. Thus, introduction of chitosan-degradation activity into plants is expected to improve fungal disease resistance. Chitosanase has been found in bacteria and fungi, but not in higher plants. Here, we demonstrate that chitosanase, Cho1, from Bacillus circulans MH-K1 has antifungal activity against the rice blast fungus Magnaporthe oryzae. Introduction of the cho1 gene conferred chitosanase activity to rice cells. Transgenic rice plants expressing Cho1 designed to be localized in the apoplast showed increased resistance to M. oryzae accompanied by increased generation of hydrogen peroxide in the infected epidermal cells. These results strongly suggest that chitosan exists in the enzyme-accessible surface of M. oryzae during the infection process and that the enhancement of disease resistance is attributable to the antifungal activity of the secreted Cho1 and to increased elicitation of the host defense response.

Expression specificity of CBP1 is regulated by transcriptional repression during vegetative growth of Magnaporthe oryzae

The rice blast fungus Magnaporthe oryzae produces appressoria during the infection of a host. In M. oryzae, the appressorium formation-related gene CBP1 (Chitin Binding Protein 1) is specifically expressed during the early stage of appressorium differentiation. The transcription factor CON7 activates CBP1 expression. However, many aspects of the regulation of CBP1 expression are still unknown. In this report, the CBP1 5' upstream region was analyzed using an egfp reporter. Deletion of the CBP1 5 ' upstream region caused derepression of reporter gene activity during vegetative growth. This result suggests that CBP1 expression is repressed during vegetative growth. The key 5 ' upstream sequences for CBP1 repression were examined. Furthermore, cis- and trans-acting elements of the negative regulatory region were investigated. Here, we discuss the transcriptional regulatory mechanism of CBP1.

Multiple plant surface signals are sensed by different mechanisms in the rice blast fungus for appressorium formation

Surface recognition and penetration are among the most critical plant infection processes in foliar pathogens. In Magnaporthe oryzae, the Pmk1 MAP kinase regulates appressorium formation and penetration. Its orthologs also are known to be required for various plant infection processes in other phytopathogenic fungi. Although a number of upstream components of this important pathway have been characterized, the upstream sensors for surface signals have not been well characterized. Pmk1 is orthologous to Kss1 in yeast that functions downstream from Msb2 and Sho1 for filamentous growth. Because of the conserved nature of the Pmk1 and Kss1 pathways and reduced expression of MoMSB2 in the pmk1 mutant, in this study we functionally characterized the MoMSB2 and MoSHO1 genes. Whereas the Momsb2 mutant was significantly reduced in appressorium formation and virulence, the Mosho1 mutant was only slightly reduced. The Mosho1 Momsb2 double mutant rarely formed appressoria on artificial hydrophobic surfaces, had a reduced Pmk1 phosphorylation level, and was nonresponsive to cutin monomers. However, it still formed appressoria and caused rare, restricted lesions on rice leaves. On artificial hydrophilic surfaces, leaf surface waxes and primary alcohols-but not paraffin waxes and alkanes- stimulated appressorium formation in the Mosho1 Momsb2 mutant, but more efficiently in the Momsb2 mutant. Furthermore, expression of a dominant active MST7 allele partially suppressed the defects of the Momsb2 mutant. These results indicate that, besides surface hydrophobicity and cutin monomers, primary alcohols, a major component of epicuticular leaf waxes in grasses, are recognized by M. oryzae as signals for appressorium formation. Our data also suggest that MoMsb2 and MoSho1 may have overlapping functions in recognizing various surface signals for Pmk1 activation and appressorium formation. While MoMsb2 is critical for sensing surface hydrophobicity and cutin monomers, MoSho1 may play a more important role in recognizing rice leaf waxes.

The rice blast fungus is a major pathogen of rice and a model for studying fungal-plant interactions. Like many other fungal pathogens, it can recognize physical and chemical signals present on the rice leaf surface and form a highly specialized infection structure known as appressorium. A well conserved signal transduction pathway involving the protein kinase gene PMK1 is known to regulate appressorium formation and plant penetration in this pathogen. However, it is not clear about the sensor genes that are involved in recognizing various plant surface signals. In this study we functionally characterize two putative sensor genes called MoMSB2 and MoSHO1. Genetic and biochemical analyses indicated that these two genes have overlapping functions in recognizing different physical and chemical signals present on the rice leaf surface for the activation of the Pmk1 pathway and appressorium formation. We found that primary alcohols, a major component of leaf waxes in grasses, can be recognized by the rice blast fungus as chemical cues. While MoMSB2 is critical for sensing hydrophobicity and precursors of cutin molecules of rice leaves, MoSHO1 appears to be more important than MoMSB2 for recognizing wax components.

Common genetic pathways regulate organ-specific infection-related development in the rice blast fungus

Magnaporthe oryzae is the most important fungal pathogen of rice (Oryza sativa). Under laboratory conditions, it is able to colonize both aerial and underground plant organs using different mechanisms. Here, we characterize an infection-related development in M. oryzae produced on hydrophilic polystyrene (PHIL-PS) and on roots. We show that fungal spores develop preinvasive hyphae (pre-IH) from hyphopodia (root penetration structures) or germ tubes and that pre-IH also enter root cells. Changes in fungal cell wall structure accompanying pre-IH are seen on both artificial and root surfaces. Using characterized mutants, we show that the PMK1 (for pathogenicity mitogen-activated protein kinase 1) pathway is required for pre-IH development. Twenty mutants with altered pre-IH differentiation on PHIL-PS identified from an insertional library of 2885 M. oryzae T-DNA transformants were found to be defective in pathogenicity. The phenotypic analysis of these mutants revealed that appressorium, hyphopodium, and pre-IH formation are genetically linked fungal developmental processes. We further characterized one of these mutants, M1373, which lacked the M. oryzae ortholog of exportin-5/Msn5p (EXP5). Mutants lacking EXP5 were much less virulent on roots, suggesting an important involvement of proteins and/or RNAs transported by EXP5 during M. oryzae root infection.