A Pmk1-interacting gene is involved in appressorium differentiation and plant infection in Magnaporthe oryzae

Novel cellular functions of Cys2-His2 zinc finger proteins in anthracnose development and dissemination on pepper fruits by Colletotrichum scovillei

Colletotrichum species are notorious for causing anthracnose on many fruits, leading to significant economic losses worldwide. As a model, we functionally characterized cys2-his2 (C2H2) zinc finger proteins (CsCZFs) in Colletotrichum scovillei, a major causal agent of pepper fruit anthracnose in many countries. In all, 62 CsCZFs were identified by in silico genomic analysis. Twelve were selected based on their expression profiles to generate targeted deletion mutants for functional investigation. ΔCsczf1 markedly reduced conidiation and constitutive expression of CsCZF1 partially recovered conidiation in an asexual reproduction-defective mutant, ΔCshox2. Deletion of CsCZF12, orthologous to the calcineurin-responsive transcription factor Crz1, impaired autophagy in C. scovillei. ΔCsczf9 was defective in surface recognition, appressorium formation, and suppression of host defenses. CsCZF9 was identified as an essential and novel regulator under the control of the mitogen-activated protein kinase (CsPMK1) in an early step of appressorium development in C. scovillei. This study provides novel insights into CsCZF-mediated regulation of differentiation and pathogenicity in C. scovillei, contributing to understanding the regulatory mechanisms governing fruit anthracnose epidemics.IMPORTANCEThe phytopathogenic fungus Colletotrichum scovillei is known to cause serious anthracnose on chili pepper. However, the molecular mechanism underlying anthracnose caused by this fungus remains largely unknown. Here, we systematically analyzed the functional roles of cys2-his2 zinc finger proteins (CsCZFs) in the dissemination and pathogenic development of this fungus. Our results showed that CsCZF1 plays an important role in conidiation and constitutive expression of CsCZF1 restored conidiation in an asexual reproduction-defective mutant, ΔCshox2. The CsCZF9, a novel target of the mitogen-activated protein kinase (CsPMK1), is essential for surface recognition to allow appressorium formation and suppression of host defenses in C. scovillei. The CsCZF12, orthologous to the calcineurin-responsive transcription factor Crz1, is involved in the autophagy of C. scovillei. Our findings reveal a comprehensive mechanism underlying CsCZF-mediated regulation of differentiation and pathogenicity of C. scovillei, which contributes to the understanding of fruit anthracnose epidemics and the development of novel strategies for disease management.

The phosphorylation landscape of infection-related development by the rice blast fungus

Many of the world's most devastating crop diseases are caused by fungal pathogens that elaborate specialized infection structures to invade plant tissue. Here, we present a quantitative mass-spectrometry-based phosphoproteomic analysis of infection-related development by the rice blast fungus Magnaporthe oryzae, which threatens global food security. We mapped 8,005 phosphosites on 2,062 fungal proteins following germination on a hydrophobic surface, revealing major re-wiring of phosphorylation-based signaling cascades during appressorium development. Comparing phosphosite conservation across 41 fungal species reveals phosphorylation signatures specifically associated with biotrophic and hemibiotrophic fungal infection. We then used parallel reaction monitoring (PRM) to identify phosphoproteins regulated by the fungal Pmk1 MAPK that controls plant infection by M. oryzae. We define 32 substrates of Pmk1 and show that Pmk1-dependent phosphorylation of regulator Vts1 is required for rice blast disease. Defining the phosphorylation landscape of infection therefore identifies potential therapeutic interventions for the control of plant diseases.

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.

A double-edged sword: reactive oxygen species (ROS) during the rice blast fungus and host interaction

Magnaporthe oryzae is a hemibiotrophic fungus that also needs host nutrients for propagation during infection. During its interaction with rice, reactive oxygen species (ROS) mediate important signaling reactions impacting both the pathogen and the host. In M. oryzae, the accumulation of ROS is important for the formation and maturation of the infectious structure appressorium. On the other hand, upon M. oryzae infection, rice generates further ROS to restrict invasive hyphae (IH) spreading. Despite ROS receptors remaining to be identified, M. oryzae recruits several strategies to respond and suppress ROS accumulation through the secretion of various effector molecules. These findings suggest that the balance between the generation and scavenging of ROS is sophisticatedly controlled during M. oryzae-rice interaction. In this review, we discuss advances to understand the regulation mechanisms for the generation, accumulation, and transduction of ROS.

The Biology of Invasive Growth by the Rice Blast Fungus Magnaporthe oryzae

This introductory chapter describes the life cycle of Magnaporthe oryzae, the causal agent of rice blast disease. During plant infection, M. oryzae forms a specialized infection structure called an appressorium, which generates enormous turgor, applied as a mechanical force to breach the rice cuticle. Appressoria form in response to physical cues from the hydrophobic rice leaf cuticle and nutrient availability. The signaling pathways involved in perception of surface signals are described and the mechanism by which appressoria function is also introduced. Re-polarization of the appressorium requires a septin complex to organize a toroidal F-actin network at the base of the cell. Septin aggregation requires a turgor-dependent sensor kinase, Sln1, necessary for re-polarization of the appressorium and development of a rigid penetration hypha to rupture the leaf cuticle. Once inside the plant, the fungus undergoes secretion of a large set of effector proteins, many of which are directed into plant cells using a specific secretory pathway. Here they suppress plant immunity, but can also be perceived by rice immune receptors, triggering resistances. M. oryzae then manipulates pit field sites, containing plasmodesmata, to facilitate rapid spread from cell to cell in plant tissue, leading to disease symptom development.

Transcription factor MoMsn2 targets the putative 3-methylglutaconyl-CoA hydratase-encoding gene MoAUH1 to govern infectious growth via mitochondrial fusion/fission balance in Magnaporthe oryzae

Mitochondrial quality and quantity are essential for a cell to maintain normal cellular functions. Our previous study revealed that the transcription factor MoMsn2 plays important roles in the development and virulence of Magnaporthe oryzae. However, to date, no study has reported its underlying regulatory mechanism in phytopathogens. Here, we explored the downstream target genes of MoMsn2 using a chromatin immunoprecipitation sequencing (ChIP-Seq) approach. In total, 332 target genes and five putative MoMsn2-binding sites were identified. The 332 genes exhibited a diverse array of functions and the highly represented were genes involved in metabolic and catalytic processes. Based on the ChIP-Seq data, we found that MoMsn2 plays a role in maintaining mitochondrial morphology, likely by targeting a number of mitochondria-related genes. Further investigation revealed that MoMsn2 targets the putative 3-methylglutaconyl-CoA hydratase-encoding gene (MoAUH1) to control mitochondrial morphology and mitophagy, which are critical for the infectious growth of the pathogen. Meanwhile, the deletion of MoAUH1 resulted in phenotypes similar to the ΔMomsn2 mutant in mitochondrial morphology, mitophagy and virulence. Overall, our results provide evidence for the regulatory mechanisms of MoMsn2, which targets MoAUH1 to modulate its transcript levels, thereby disturbing the mitochondrial fusion/fission balance. This ultimately affects the development and virulence of M. oryzae.

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.

Magnaporthe oryzae Abp1, a MoArk1 Kinase-Interacting Actin Binding Protein, Links Actin Cytoskeleton Regulation to Growth, Endocytosis, and Pathogenesis

The actin cytoskeleton and actin-coupled endocytosis are conserved cellular processes required for the normal growth and pathogenesis of the rice blast fungus Magnaporthe oryzae. We have previously shown that actin regulating kinase MoArk1 regulates actin dynamics and endocytosis to play a key role in virulence of the fungus. To understand the underlying mechanism, we have characterized the actin-binding protein MoAbp1 that interacts with MoArk1 from M. oryzae. The ΔMoabp1 mutant exhibited delayed endocytosis and defects in growth, host penetration, and invasive growth. Consistent with its putative function associated with actin-binding, MoAbp1 regulates the localization of actin patches and plays a role in MoArk1 phosphorylation. In addition, MoAbp1 interacts with MoCap (adenylyl cyclase-associated protein) affecting its normal patch localization pattern and the actin protein MoAct1 through its conserved domains. Taken together, our results support a notion that MoAbp1 functions as a protein scaffold linking MoArk1, MoCap1, and MoAct1 to regulate actin cytoskeleton dynamics critical in growth and pathogenicity of the blast fungus.

Heat-Shock Proteins MoSsb1, MoSsz1, and MoZuo1 Attenuate MoMkk1-Mediated Cell-Wall Integrity Signaling and Are Important for Growth and Pathogenicity of Magnaporthe oryzae

The mitogen-activated protein kinase (MAPK) MoMkk1 governs the cell-wall integrity (CWI) pathway in rice blast fungus Magnaporthe oryzae. To understand the underlying mechanism, we have identified MoSsb1 as one of the MoMkk1-interacting proteins. MoSsb1 is a stress-seventy subfamily B (Ssb) protein homolog, sharing high amino acid sequence homology with the 70-kDa heat shock proteins (Hsp70s). Hsp70 are a family of conserved and ubiquitously expressed chaperones that regulate protein biogenesis by promoting protein folding, preventing protein aggregation, and controlling protein degradation. We found that MoSsb1 regulates the synthesis of nascent polypeptide chains and this regulation is achieved by being in complex with other members of Hsp70s MoSsz1 and 40-kDa Hsp40 MoZuo1. MoSsb1 is important for the growth, conidiation, and full virulence of the blast fungus and this role is also shared by MoSsz1 and MoZuo1. Importantly, MoSsb1, MoSsz1, and MoZuo1 are all involved in the regulation of the CWI MAPK pathway by modulating MoMkk1 biosynthesis. Our studies reveal novel insights into how MoSsb1, MoSsz1, and MoZuo1 affect CWI signaling that is involved in regulating growth, differentiation, and virulence of M. oryzae and highlight the conserved functional mechanisms of heat-shock proteins in pathogenic fungi.

MoMip11, a MoRgs7-interacting protein, functions as a scaffolding protein to regulate cAMP signaling and pathogenicity in the rice blast fungus Magnaporthe oryzae

The rice blast fungus Magnaporthe oryzae has eight regulators of G-protein signaling (RGS) and RGS-like proteins (MoRgs1 to MoRgs8) that exhibit both distinct and shared regulatory functions in the growth, differentiation and pathogenicity of the fungus. We found MoRgs7 with a unique RGS-seven transmembrane (7-TM) domain motif is localized to the highly dynamic tubule-vesicular compartments during early appressorium differentiation followed by gradually degradation. To explore whether this involves an active signal perception of MoRgs7, we identified a Gbeta-like/RACK1 protein homolog in M. oryzae MoMip11 that interacts with MoRgs7. Interestingly, MoMip11 selectively interacted with several components of the cAMP regulatory pathway, including Gα MoMagA and the high-affinity phosphodiesterase MoPdeH. We further showed that MoMip11 promotes MoMagA activation and suppresses MoPdeH activity thereby upregulating intracellular cAMP levels. Moreover, MoMip11 is required for the response to multiple stresses, a role also shared by Gbeta-like/RACK1 adaptor proteins. In summary, we revealed a unique mechanism by which MoMip11 links MoRgs7 and G-proteins to reugulate cAMP signaling, stress responses and pathogenicity of M. oryzae. Our studies revealed the multitude of regulatory networks that govern growth, development and pathogenicity in this important causal agent of rice blast.

New findings on phosphodiesterases, MoPdeH and MoPdeL, in Magnaporthe oryzae revealed by structural analysis

The cyclic adenosine monophosphate (cAMP) signalling pathway mediates signal communication and sensing during infection-related morphogenesis in eukaryotes. Many studies have implicated cAMP as a critical mediator of appressorium development in the rice blast fungus, Magnaporthe oryzae. The cAMP phosphodiesterases, MoPdeH and MoPdeL, as key regulators of intracellular cAMP levels, play pleiotropic roles in cell wall integrity, cellular morphology, appressorium formation and infectious growth in M. oryzae. Here, we analysed the roles of domains of MoPdeH and MoPdeL separately or in chimeras. The results indicated that the HD and EAL domains of MoPdeH are indispensable for its phosphodiesterase activity and function. Replacement of the MoPdeH HD domain with the L1 and L2 domains of MoPdeL, either singly or together, resulted in decreased cAMP hydrolysis activity of MoPdeH. All of the transformants exhibited phenotypes similar to that of the ΔMopdeH mutant, but also revealed that EAL and L1 play additional roles in conidiation, and that L1 is involved in infectious growth. We further found that the intracellular cAMP level is important for surface signal recognition and hyphal autolysis. The intracellular cAMP level negatively regulates Mps1-MAPK and positively regulates Pmk1-MAPK in the rice blast fungus. Our results provide new information to better understand the cAMP signalling pathway in the development, differentiation and plant infection of the fungus.

MoDnm1 Dynamin Mediating Peroxisomal and Mitochondrial Fission in Complex with MoFis1 and MoMdv1 Is Important for Development of Functional Appressorium in Magnaporthe oryzae

Dynamins are large superfamily GTPase proteins that are involved in various cellular processes including budding of transport vesicles, division of organelles, cytokinesis, and pathogen resistance. Here, we characterized several dynamin-related proteins from the rice blast fungus Magnaporthe oryzae and found that MoDnm1 is required for normal functions, including vegetative growth, conidiogenesis, and full pathogenicity. In addition, we found that MoDnm1 co-localizes with peroxisomes and mitochondria, which is consistent with the conserved role of dynamin proteins. Importantly, MoDnm1-dependent peroxisomal and mitochondrial fission involves functions of mitochondrial fission protein MoFis1 and WD-40 repeat protein MoMdv1. These two proteins display similar cellular functions and subcellular localizations as MoDnm1, and are also required for full pathogenicity. Further studies showed that MoDnm1, MoFis1 and MoMdv1 are in complex to regulate not only peroxisomal and mitochondrial fission, pexophagy and mitophagy progression, but also appressorium function and host penetration. In summary, our studies provide new insights into how MoDnm1 interacts with its partner proteins to mediate peroxisomal and mitochondrial functions and how such regulatory events may link to differentiation and pathogenicity in the rice blast fungus.

Dynamin superfamily members are involved in budding of transport vesicles and division of organelles in eukaryotic cells. To further understand how dynamins function in phytopathogenic fungi, we characterized several dynamin-related proteins from the rice blast fungus M. oryzae. In addition to revealing major conserved dynamin functions, we described how MoDnm1 interacts with mitochondrial fission protein MoFis1 and WD repeat adaptor protein MoMdv1 to mediate peroxisomal and mitochondrial fission, pexophagy and mitophagy. Importantly, we provided evidence to demonstrate that MoDnm1-, MoFis1- and MoMdv1-dependent peroxisomal and mitochondrial functions are linked to differentiation and pathogenicity of the rice blast fungus.

Phosphodiesterase MoPdeH targets MoMck1 of the conserved mitogen-activated protein (MAP) kinase signalling pathway to regulate cell wall integrity in rice blast fungus Magnaporthe oryzae

In the rice blast fungus Magnaporthe oryzae, the high-affinity cyclic adenosine monophosphate (cAMP) phosphodiesterase MoPdeH is important not only for cAMP signalling and pathogenicity, but also for cell wall integrity (CWI) maintenance through an unknown mechanism. By utilizing affinity purification, we found that MoPdeH interacts with MoMck1, one of the components of the mitogen-activated protein (MAP) kinase cascade that regulates CWI. Overexpression of MoMCK1 suppressed defects in autolysis and pathogenicity of the ΔMopdeH mutant, although partially, suggesting that MoPdeH plays a critical role in CWI maintenance mediated by the MAP kinase pathway. We found that MoMck1 and two other MAP kinase cascade components, MoMkk1 and MoMps1, modulate intracellular cAMP levels by regulating the expression of MoPDEH through a feedback loop. In addition, disruption of MoMKK1 resulted in less aerial hyphal formation, defective asexual development and attenuated pathogenicity. Moreover, MoMkk1 plays a role in the response to osmotic stress via regulation of MoOsm1 phosphorylation levels, whereas endoplasmic reticulum (ER) stress enhances MoMps1 phosphorylation and loss of the MAP kinase cascade component affects the unfolded protein response (UPR) pathway. Taken together, our findings demonstrate that MoPdeH functions upstream of the MoMck1-MoMkk1-MoMps1 MAP kinase pathway to regulate CWI, and that MoPdeH also mediates crosstalk between the cAMP signalling pathway, the osmotic sensing high osmolarity glycerol (HOG) pathway and the dithiothreitol (DTT)-induced UPR pathway in M. oryzae.

Global genome and transcriptome analyses of Magnaporthe oryzae epidemic isolate 98-06 uncover novel effectors and pathogenicity-related genes, revealing gene gain and lose dynamics in genome evolution

Genome dynamics of pathogenic organisms are driven by pathogen and host co-evolution, in which pathogen genomes are shaped to overcome stresses imposed by hosts with various genetic backgrounds through generation of a variety of isolates. This same principle applies to the rice blast pathogen Magnaporthe oryzae and the rice host; however, genetic variations among different isolates of M. oryzae remain largely unknown, particularly at genome and transcriptome levels. Here, we applied genomic and transcriptomic analytical tools to investigate M. oryzae isolate 98-06 that is the most aggressive in infection of susceptible rice cultivars. A unique 1.4 Mb of genomic sequences was found in isolate 98-06 in comparison to reference strain 70-15. Genome-wide expression profiling revealed the presence of two critical expression patterns of M. oryzae based on 64 known pathogenicity-related (PaR) genes. In addition, 134 candidate effectors with various segregation patterns were identified. Five tested proteins could suppress BAX-mediated programmed cell death in Nicotiana benthamiana leaves. Characterization of isolate-specific effector candidates Iug6 and Iug9 and PaR candidate Iug18 revealed that they have a role in fungal propagation and pathogenicity. Moreover, Iug6 and Iug9 are located exclusively in the biotrophic interfacial complex (BIC) and their overexpression leads to suppression of defense-related gene expression in rice, suggesting that they might participate in biotrophy by inhibiting the SA and ET pathways within the host. Thus, our studies identify novel effector and PaR proteins involved in pathogenicity of the highly aggressive M. oryzae field isolate 98-06, and reveal molecular and genomic dynamics in the evolution of M. oryzae and rice host interactions.

Genetic variations in pathogens, such as the causal agent of rice blast Magnaporthe oryzae, often lead to circumvention of disease-resistance cultivars. Previous genome-wide analyses of model organisms suggest that pathogen effectors are also rapidly evolving, especially in regions with high genome plasticity. However, genetic variations among different isolates remain largely unknown in M. oryzae, particularly at the genome and transcriptome levels. In this study, we provided a systematic genomic and interaction transcriptome profile for a dominant rice blast field isolate, resulting in identification of 134 candidate effectors. Two effectors, Iug6 and Iug9, and one pathogenicity-related (PaR) gene product, Iug18, were subjected to functional characterization. We found that Iug6 and Iug9 are located in the biotrophic interfacial complex (BIC) and their overexpression leads to suppression of defense-related gene expression in rice, while Iug18 appears to be a novel PaR protein. Our studies support the hypothesis that isolate-unique genes may serve as a source of genetic variability in the M. oryzae population encountering different environments. Our studies also facilitate further understanding of effectors and genomic variations in pathogenicity of M. oryzae.

System-wide characterization of bZIP transcription factor proteins involved in infection-related morphogenesis of Magnaporthe oryzae

The basic leucine zipper (bZIP) domain-containing transcription factors (TFs) function as key regulators of cellular growth and differentiation in eukaryotic organisms including fungi. We have previously identified MoAp1 and MoAtf1 as bZIP TFs in Magnaporthe oryzae and demonstrated that they regulate the oxidative stress response and are critical in conidiogenesis and pathogenicity. Studies of bZIP proteins could provide a novel strategy for controlling rice blast, but a systematic examination of the bZIP proteins has not been carried out. Here, we identified 19 additional bZIP TFs and characterized their functions. We found that the majority of these TFs exhibit active functions, most notably, in conidiogenesis. We showed that MoHac1 regulates the endoplasmic reticulum stress response through a conserved unfolded protein response pathway, MoMetR controls amino acid metabolism to govern growth and differentiation, and MoBzip10 governs appressorium function and invasive hyphal growth. Moreover, MoBzip5 participates in appressorium formation through a pathway distinct from that MoBzip10, and MoMeaB appears to exert a regulatory role through nutrient uptake and nitrogen utilization. Collectively, our results provide insights into shared and specific functions associated with each of these TFs and link the regulatory roles to the fungal growth, conidiation, appressorium formation, host penetration and pathogenicity.

Protein kinases in plant-pathogenic fungi: conserved regulators of infection

Phytopathogenic fungi have evolved an amazing diversity of infection modes and nutritional strategies, yet the signaling pathways that govern pathogenicity are remarkably conserved. Protein kinases (PKs) catalyze the reversible phosphorylation of proteins, regulating a variety of cellular processes. Here, we present an overview of our current understanding of the different classes of PKs that contribute to fungal pathogenicity on plants and of the mechanisms that regulate and coordinate PK activity during infection-related development. In addition to the well-studied PK modules, such as MAPK (mitogen-activated protein kinase) and cAMP (cyclic adenosine monophosphate)-PKA (protein kinase A) cascades, we also discuss new PK pathways that have emerged in recent years as key players of pathogenic development and disease. Understanding how conserved PK signaling networks have been recruited during the evolution of fungal pathogenicity not only advances our knowledge of the highly elaborate infection process but may also lead to the development of novel strategies for the control of plant disease.

Shared and distinct functions of two Gti1/Pac2 family proteins in growth, morphogenesis and pathogenicity of Magnaporthe oryzae

Gti1/Pac2 are conserved family proteins that regulate morphogenic transition in yeasts such as Schizosaccharomyces pombe and Candida albicans, and they also control toxin production and pathogenicity in filamentous fungus Fusarium graminearum. To test the functions of Gti1/Pac2 paralogues MoGti1 and MoPac2 in the rice blast fungus Magnaporthe oryzae, we generated respective ΔMogti1 and ΔMopac2 mutant strains. We found that MoGti1 and MoPac2 exhibit shared and distinct roles in hyphal growth, conidiation, sexual reproduction, stress responses, surface hydrophobility, invasive hyphal growth and pathogenicity. Consistent with the putative conserved function of MoGti1, we showed that MoGti1-GFP is localized to the nucleus, whereas MoPac2-GFP is mainly found in the cytoplasm. In addition, we provided evidence that the nuclear localization of MoGti1 could be subject to regulation by MoPmk1 mitogen-activated protein kinase. Moreover, we found that the reduced pathogenicity in the ΔMopac2 mutant corresponds with an increased expression of plant defence genes, including PR1a, AOS2, LOX1, PAD4, and CHT1. Taken together, our studies provide a comprehensive analysis of two similar but distinct Gti1/Pac2 family proteins in M. oryzae, which underlines the important yet conserved functions of these family proteins in plant pathogenic fungi.

Mitogen-activated protein kinase signaling in plant-interacting fungi: distinct messages from conserved messengers

Mitogen-activated protein kinases (MAPKs) are evolutionarily conserved proteins that function as key signal transduction components in fungi, plants, and mammals. During interaction between phytopathogenic fungi and plants, fungal MAPKs help to promote mechanical and/or enzymatic penetration of host tissues, while plant MAPKs are required for activation of plant immunity. However, new insights suggest that MAPK cascades in both organisms do not operate independently but that they mutually contribute to a highly interconnected molecular dialogue between the plant and the fungus. As a result, some pathogenesis-related processes controlled by fungal MAPKs lead to the activation of plant signaling, including the recruitment of plant MAPK cascades. Conversely, plant MAPKs promote defense mechanisms that threaten the survival of fungal cells, leading to a stress response mediated in part by fungal MAPK cascades. In this review, we make use of the genomic data available following completion of whole-genome sequencing projects to analyze the structure of MAPK protein families in 24 fungal taxa, including both plant pathogens and mycorrhizal symbionts. Based on conserved patterns of sequence diversification, we also propose the adoption of a unified fungal MAPK nomenclature derived from that established for the model species Saccharomyces cerevisiae. Finally, we summarize current knowledge of the functions of MAPK cascades in phytopathogenic fungi and highlight the central role played by MAPK signaling during the molecular dialogue between plants and invading fungal pathogens.

Different chitin synthase genes are required for various developmental and plant infection processes in the rice blast fungus Magnaporthe oryzae

Chitin is a major component of fungal cell wall and is synthesized by chitin synthases (Chs). Plant pathogenic fungi normally have multiple chitin synthase genes. To determine their roles in development and pathogenesis, we functionally characterized all seven CHS genes in Magnaporthe oryzae. Three of them, CHS1, CHS6, and CHS7, were found to be important for plant infection. While the chs6 mutant was non-pathogenic, the chs1 and chs7 mutants were significantly reduced in virulence. CHS1 plays a specific role in conidiogenesis, an essential step for natural infection cycle. Most of chs1 conidia had no septum and spore tip mucilage. The chs6 mutant was reduced in hyphal growth and conidiation. It failed to penetrate and grow invasively in plant cells. The two MMD-containing chitin synthase genes, CHS5 and CHS6, have a similar expression pattern. Although deletion of CHS5 had no detectable phenotype, the chs5 chs6 double mutant had more severe defects than the chs6 mutant, indicating that they may have overlapping functions in maintaining polarized growth in vegetative and invasive hyphae. Unlike the other CHS genes, CHS7 has a unique function in appressorium formation. Although it was blocked in appressorium formation by germ tubes on artificial hydrophobic surfaces, the chs7 mutant still produced melanized appressoria by hyphal tips or on plant surfaces, indicating that chitin synthase genes have distinct impacts on appressorium formation by hyphal tip and germ tube. The chs7 mutant also was defective in appressorium penetration and invasive growth. Overall, our results indicate that individual CHS genes play diverse roles in hyphal growth, conidiogenesis, appressorium development, and pathogenesis in M. oryzae, and provided potential new leads in the control of this devastating pathogen by targeting specific chitin synthases.