Proper regulation of cyclic AMP-dependent protein kinase is required for growth, conidiation, and appressorium function in the anthracnose fungus Colletotrichum lagenarium

VmSom1 is essential for growth, development, maintenance of cell wall integrity and virulence in Valsa mali

Apple Valsa canker disease, caused by Valsa mali Miyabe et Yamada, severely endangers the healthy growth of apple trees. The Som1, located downstream of the cyclic AMP-dependent protein kinase A (cAMP-PKA) pathway, plays crucial roles in the growth, development, morphological differentiation, and virulence of filamentous fungi. In this study, we identify and functionally characterize VmSom1, a homolog of Som1, in Valsa mali. The VmSom1 gene is located on chromosome 12, encoding an 824 amino acid protein. Phylogenetic analysis reveals VmSom1 as a fungal Som1 homolog. The VmSom1 deletion mutants exhibit slower growth rates and fail to produce pycnidia. Additionally, their hyphal growth is significantly inhibited on media containing Calcofluor White, Congo Red, NaCl, and sorbitol. The growth rate of VmSom1 deletion mutants is reduced on maltose, lactose, sucrose and fructose media but increases on glucose medium. Moreover, the mycelial growth rate of the VmSom1 deletion mutant is significantly lower than that of the wild-type strain in peptone, NH4SO4, NaNO3, and no nitrogen. Notably, the distances between the septa increase, and chitin concentration shifts to the hyphal tip in the VmSom1 deletion mutant. Furthermore, compared with the wild-type strain, the VmSom1 deletion mutant exhibits fewer diseased spots on apple fruit and branches. Overall, our findings demonstrate that VmSom1 is involved in regulating the growth and development, colony surface hydrophobicity, osmotic stress, cell wall integrity maintenance, carbon and nitrogen source utilization, septa formation, and virulence of V. mali.

Determination of cyclic adenosine phosphate and protein content in dormant chlamydospore and nondormant chlamydospore of Arthrobotrys flagrans

Arthrobotrys flagrans, a nematode-eating fungus, is an effective component of animal parasitic nematode biocontrol agents. In the dried formulation, the majority of spores are in an endogenous dormant state. This study focuses on dormant chlamydospore and nondormant chlamydospore of A. flagrans to investigate the differences in cyclic adenosine monophosphate (cAMP) and protein content between the two types of spores. cAMP and soluble proteins were extracted from the nondormant chlamydospore and dormant chlamydospore of two isolates of A. flagrans. The cAMP Direct Immunoassay Kit and Bradford protein concentration assay kit (Coomassie brilliant blue method) were used to detect the cAMP and protein content in two types of spores. Results showed that the content of cAMP in dormant spores of both isolates was significantly higher than that in nondormant spores (p < 0.05). The protein content of dormant spores in DH055 bacteria was significantly higher than that of nondormant spores (p < 0.05). In addition, the protein content of dormant spores of the SDH035 strain was slightly higher than that of nondormant spores, but the difference was not significant (p > 0.05). The results obtained in this study provide evidence for the biochemical mechanism of chlamydospore dormancy or the germination of the nematophagous fungus A. flagrans.

Protein kinase A regulatory subunit is required for normal growth, zoosporogenesis, and pathogenicity in Phytophthora sojae

Phytophthora sojae, one of the most devastating Oomycete pathogens, causes severe diseases that lead to economic loss in the soybean industry. The production of zoospores play a crucial role during the development of Phytophthora disease. In this work, CRISPR/Cas9 genome editing technology were used to obtain protein kinase A regulatory subunit (PsPkaR) knockout mutants. The role of PsPkaR in the production of zoospores and pathogenicity of P. sojae was analyzed. The overall findings indicate that PsPkaR is involved in regulating the growth process of P. sojae, primarily affecting the hyphal morphology and growth rate. Additionally, PsPkaR participates in the regulation of the release process of zoospores. Specifically, knocking-out PsPkaR resulted in incomplete cytoplasmic differentiation and uneven protoplast division, leading to abnormal release of zoospores. Furthermore, when the PsPkaR knockout mutants were inoculated on soybean leaves, the pathogenicity was significantly reduced compared to that of the wild-type and control strains. These findings of this study provide important clues and evidence regarding the role of the cAMP-PKA signaling pathway in the interaction between P. sojae and its host. This work contributes to a better understanding of the pathogenic mechanism of P. sojae and the development of corresponding prevention and control strategies.

Selective deployment of virulence effectors correlates with host specificity in a fungal plant pathogen

The hemibiotrophic fungal plant pathogen Colletotrichum orbiculare is predicted to secrete hundreds of effector proteins when the pathogen infects cucurbit crops, such as cucumber and melon, and tobacco (Nicotiana benthamiana), a distantly related Solanaceae species. Here, we report the identification of sets of C. orbiculare effector genes that are differentially required for fungal virulence to two phylogenetically distant host species. Through targeted gene knockout screening of C. orbiculare 'core' effector candidates defined based on in planta gene expression, we identified: four host-specific virulence effectors (named effector proteins for cucurbit infection, or EPCs) that are required for full virulence of C. orbiculare to cucurbit hosts, but not to the Solanaceae host N. benthamiana; and five host-nonspecific virulence effectors, which collectively contribute to fungal virulence to both hosts. During host infection, only a small subset of genes, including the host-specific EPC effector genes, showed preferential expression on one of the hosts, while gene expression profiles of the majority of other genes, including the five host-nonspecific effector genes, were common to both hosts. This work suggests that C. orbiculare adopts a host-specific effector deployment strategy, in addition to general host-blind virulence mechanisms, for adaptation to cucurbit hosts.

Protein kinase A participates in hyphal and appressorial development by targeting Efg1-mediated transcription of a Rab GTPase in Setosphaeria turcica

The cyclic adenosine monophosphate (cAMP) signalling pathway plays an important role in the regulation of the development and pathogenicity of filamentous fungi. cAMP-dependent protein kinase A (PKA) is the conserved element downstream of cAMP, and its diverse mechanisms in multiple filamentous fungi are not well known yet. In the present study, gene knockout mutants of two catalytic subunits of PKA (PKA-C) in Setosphaeria turcica were created to illustrate the regulatory mechanisms of PKA-Cs on the development and pathogenicity of S. turcica. As a result, StPkaC2 was proved to be the main contributor of PKA activity in S. turcica. In addition, it was found that both StPkaC1 and StPkaC2 were necessary for conidiation and invasive growth, while only StPkaC2 played a negative role in the regulation of filamentous growth. We reveal that only StPkaC2 could interact with the transcription factor StEfg1, and it inhibited the transcription of StRAB1, a Rab GTPase homologue coding gene in S. turcica, whereas StPkaC1 could specifically interact with a transcriptional regulator StFlo8, which could rescue the transcriptional inhibition of StEfg1 on StRAB1. We also demonstrated that StRAB1 could positively influence the biosynthesis of chitin in hyphae, thus changing the filamentous growth. Our findings clarify that StPkaC2 participates in chitin biosynthesis to modulate mycelium development by targeting the Efg1-mediated transcription of StRAB1, while StFlo8, interacting with StPkaC1, acts as a negative regulator during this process.

PKR Protects the Major Catalytic Subunit of PKA Cpk1 from FgBlm10-Mediated Proteasome Degradation in Fusarium graminearum

For optimal proteolytic function, the proteasome core (CP or 20S) must associate with activators. The cAMP-PKA pathway is reported to affect the activity of the proteasome in humans. However, the relationship between the proteasome and PKA is not well characterized. Our results showed that the major catalytic subunit Cpk1 was degraded without the protection of Pkr. Eleven (out of 67) pkr suppressors had FgBlm10 C-terminal truncation, one suppressor had an amino acid change mutation in the PRE6 ortholog (FGRRES_07282), and one in the PRE5 ortholog (FGRRES_05222). These mutations rescued the defects in growth and conidial morphology, Cpk1 stability, and PKA activities in the pkr mutant. The interaction of FgBlm10 with FgPre5 and FgPre6 were detected by co-immunoprecipitation, and the essential elements for their interaction were characterized, including the FgBlm10 C-terminus, amino acid D82 of FgPre6 and K62 of FgPre5. Additional FgBlm10-interacting proteins were identified in the wild type and pkr mutant, suggesting that PKA regulates the preference of FgBlm10-mediated proteasome assembly. In addition, PKA indirectly affected the phosphorylation of FgBlm10, and its localization in the nucleus. The truncation of the FgBlm10 C terminus also enhanced nuclear import and bleomycin resistance, suggesting its role in proteasome assembly at DNA damage sites. Collectively, our data demonstrated that regulation between PKA and proteasome degradation is critical for the vegetative growth of F. graminearum.

Protein Kinase Signaling Pathways in Plant-Colletotrichum Interaction

Anthracnose is a fungal disease caused by members of Colletotrichum that affect a wide range of crop plants. Strategies to improve crop resistance are needed to reduce the yield losses; and one strategy is to manipulate protein kinases that catalyze reversible phosphorylation of proteins regulating both plant immune responses and fungal pathogenesis. Hence, in this review, we present a summary of the current knowledge of protein kinase signaling pathways in plant-Colletotrichum interaction as well as the relation to a more general understanding of protein kinases that contribute to plant immunity and pathogen virulence. We highlight the potential of combining genomic resources and phosphoproteomics research to unravel the key molecular components of plant-Colletotrichum interactions. Understanding the molecular interactions between plants and Colletotrichum would not only facilitate molecular breeding of resistant cultivars but also help the development of novel strategies for controlling the anthracnose disease.

Fungal effector SIB1 of Colletotrichum orbiculare has unique structural features and can suppress plant immunity in Nicotiana benthamiana

Fungal plant pathogens secrete virulence-related proteins, called effectors, to establish host infection; however, the details are not fully understood yet. Functional screening of effector candidates using Agrobacterium-mediated transient expression assay in Nicotiana benthamiana identified two virulence-related effectors, named SIB1 and SIB2 (Suppression of Immunity in N. benthamiana), of an anthracnose fungus Colletotrichum orbiculare, which infects both cucurbits and N. benthamiana. The Agrobacterium-mediated transient expression of SIB1 or SIB2 increased the susceptibility of N. benthamiana to C. orbiculare, which suggested these effectors can suppress immune responses in N. benthamiana. The presence of SIB1 and SIB2 homologs was found to be limited to the genus Colletotrichum. SIB1 suppressed both (i) the generation of reactive oxygen species triggered by two different pathogen-associated molecular patterns, chitin and flg22, and (ii) the cell death response triggered by the Phytophthora infestans INF1 elicitin in N. benthamiana. We determined the NMR-based structure of SIB1 to obtain its structural insights. The three-dimensional structure of SIB1 comprises five β-strands, each containing three disulfide bonds. The overall conformation was found to be a cylindrical shape, such as the well-known antiparallel β-barrel structure. However, the β-strands were found to display a unique topology, one pair of these β-strands formed a parallel β-sheet. These results suggest that the effector SIB1 present in Colletotrichum fungi has unique structural features and can suppress pathogen-associated molecular pattern-triggered immunity in N. benthamiana.

RNA-Seq of in planta-expressed Magnaporthe oryzae genes identifies MoSVP as a highly expressed gene required for pathogenicity at the initial stage of infection

The ascomycete fungus Magnaporthe oryzae is a hemibiotrophic pathogen that causes rice blast disease. Magnaporthe oryzae infects rice leaves, stems and panicles, and induces severe reductions in yield. Effector proteins secreted by M. oryzae in planta are thought to be involved its virulence activity. Here, using RNA-sequencing (RNA-Seq), we generated transcriptome data for M. oryzae isolate Ina168 during the initial stages of infection. We prepared samples from conidia (the inoculum) and from peeled epidermal cotyledon tissue of susceptible barley Hordeum vulgare 'Nigrate' at 12, 24, 36 and 48 hours post-inoculation (hpi). We also generated a draft genome sequence of M. oryzae isolate Ina168 and used it as a reference for mapping the RNA-Seq reads. Gene expression profiling across all stages of M. oryzae infection revealed 1728 putative secreted effector protein genes. We selected seven such genes that were strongly up-regulated at 12 hpi and down-regulated at 24 or 36 hpi and performed gene knockout analysis to determine their roles in pathogenicity. Knockout of MoSVP, encoding a small putative secreted protein with a hydrophobic surface binding protein A domain, resulted in a reduction in pathogenicity, suggesting that MoSVP is a novel virulence effector of M. oryzae.

Cross-reactivity of a rice NLR immune receptor to distinct effectors from the rice blast pathogen Magnaporthe oryzae provides partial disease resistance

Unconventional integrated domains in plant intracellular immune receptors of the nucleotide-binding leucine-rich repeat (NLRs) type can directly bind translocated effector proteins from pathogens and thereby initiate an immune response. The rice (Oryza sativa) immune receptor pairs Pik-1/Pik-2 and RGA5/RGA4 both use integrated heavy metal-associated (HMA) domains to bind the effectors AVR–Pik and AVR–Pia, respectively, from the rice blast fungal pathogen Magnaporthe oryzae. These effectors both belong to the MAX effector family and share a core structural fold, despite being divergent in sequence. How integrated domains in NLRs maintain specificity of effector recognition, even of structurally similar effectors, has implications for understanding plant immune receptor evolution and function. Here, using plant cell death and pathogenicity assays and protein–protein interaction analyses, we show that the rice NLR pair Pikp-1/Pikp-2 triggers an immune response leading to partial disease resistance toward the “mis-matched” effector AVR–Pia in planta and that the Pikp–HMA domain binds AVR–Pia in vitro. We observed that the HMA domain from another Pik-1 allele, Pikm, cannot bind AVR–Pia, and it does not trigger a plant response. The crystal structure of Pikp–HMA bound to AVR–Pia at 1.9 Å resolution revealed a binding interface different from those formed with AVR–Pik effectors, suggesting plasticity in integrated domain-effector interactions. The results of our work indicate that a single NLR immune receptor can bait multiple pathogen effectors via an integrated domain, insights that may enable engineering plant immune receptors with extended disease resistance profiles.

Conserved fungal effector suppresses PAMP-triggered immunity by targeting plant immune kinases

Plant pathogens have optimized their own effector sets to adapt to their hosts. However, certain effectors, regarded as core effectors, are conserved among various pathogens, and may therefore play an important and common role in pathogen virulence. We report here that the widely distributed fungal effector NIS1 targets host immune components that transmit signaling from pattern recognition receptors (PRRs) in plants. NIS1 from two Colletotrichum spp. suppressed the hypersensitive response and oxidative burst, both of which are induced by pathogen-derived molecules, in Nicotiana benthamianaMagnaporthe oryzae NIS1 also suppressed the two defense responses, although this pathogen likely acquired the NIS1 gene via horizontal transfer from Basidiomycota. Interestingly, the root endophyte Colletotrichum tofieldiae also possesses a NIS1 homolog that can suppress the oxidative burst in N. benthamiana We show that NIS1 of multiple pathogens commonly interacts with the PRR-associated kinases BAK1 and BIK1, thereby inhibiting their kinase activities and the BIK1-NADPH oxidase interaction. Furthermore, mutations in the NIS1-targeting proteins, i.e., BAK1 and BIK1, in Arabidopsis thaliana also resulted in reduced immunity to Colletotrichum fungi. Finally, M. oryzae lacking NIS1 displayed significantly reduced virulence on rice and barley, its hosts. Our study therefore reveals that a broad range of filamentous fungi maintain and utilize the core effector NIS1 to establish infection in their host plants and perhaps also beneficial interactions, by targeting conserved and central PRR-associated kinases that are also known to be targeted by bacterial effectors.

Involvement of MaSom1, a downstream transcriptional factor of cAMP/PKA pathway, in conidial yield, stress tolerances, and virulence in Metarhizium acridum

Flo8/Som1, which functions downstream from the cyclic AMP (cAMP)-dependent protein kinase A (PKA) pathway, plays important roles in hyphal development, spore formation, and virulence in yeast and several filamentous fungi. However, the functions of Som1 in entomopathogenic fungi are still a mystery. In this study, MaSom1, a Flo8/Som1 homolog, was identified and functionally characterized in a model entomopathogenic fungus Metarhizium acridum. Similar to Flo8/Som1 in other fungi, MaSom1 mainly localized to the nucleus in M. acridum. Disruption of MaSom1 reduced conidial yield, delayed conidial germination, and impaired the fungal tolerances to heat and UV-B. The expression levels of some genes involved in defenses of heat shock and UV-B radiation were significantly reduced in ΔMaSom1. MaSom1 is also important for cell wall integrity and conidial surface structures in M. acridum. Some genes related to fungal cell wall synthesis were downregulated in ΔMaSom1. Bioassays showed that ΔMaSom1 had a dramatically decreased virulence after both topical inoculation and intrahemocoel injection of the fungus in locusts. Moreover, inactivation of MaSom1 reduced appressorium formation, diminished fungal growth in locust hemolymph in vitro, and enhanced insect immune responses. Taken together, these results indicate that disruption of MaSom1 leads to a decline of fungal virulence because of impairments in conidial germination and appressorium formation, reduction of fungal growth in host hemolymph, and enhancement of insect immune responses owing to the changes in conidial surface structures.

The PKR regulatory subunit of protein kinase A (PKA) is involved in the regulation of growth, sexual and asexual development, and pathogenesis in Fusarium graminearum

Fusarium graminearum is a causal agent of wheat scab disease and a producer of deoxynivalenol (DON) mycotoxins. Treatment with exogenous cyclic adenosine monophosphate (cAMP) increases its DON production. In this study, to better understand the role of the cAMP-protein kinase A (PKA) pathway in F. graminearum, we functionally characterized the PKR gene encoding the regulatory subunit of PKA. Mutants deleted of PKR were viable, but showed severe defects in growth, conidiation and plant infection. The pkr mutant produced compact colonies with shorter aerial hyphae with an increased number of nuclei in hyphal compartments. Mutant conidia were morphologically abnormal and appeared to undergo rapid autophagy-related cell death. The pkr mutant showed blocked perithecium development, but increased DON production. It had a disease index of less than unity and failed to spread to neighbouring spikelets. The mutant was unstable and spontaneous suppressors with a faster growth rate were often produced on older cultures. A total of 67 suppressor strains that grew faster than the original mutant were isolated. Three showed a similar growth rate and colony morphology to the wild-type, but were still defective in conidiation. Sequencing analysis with 18 candidate PKA-related genes in three representative suppressor strains identified mutations only in the CPK1 catalytic subunit gene. Further characterization showed that 10 of the other 64 suppressor strains also had mutations in CPK1. Overall, these results showed that PKR is important for the regulation of hyphal growth, reproduction, pathogenesis and DON production, and mutations in CPK1 are partially suppressive to the deletion of PKR in F. graminearum.

Cpk2, a Catalytic Subunit of Cyclic AMP-PKA, Regulates Growth and Pathogenesis in Rice Blast

The cAMP-Protein Kinase A signaling, anchored on CpkA, is necessary for appressorium development and host penetration, but indispensable for infectious growth in Magnaporthe oryzae. In this study, we identified and characterized the gene encoding the second catalytic subunit, CPK2, whose expression was found to be lower compared to CPKA at various stages of pathogenic growth in M. oryzae. Deletion of CPK2 caused no alterations in vegetative growth, conidiation, appressorium formation, or pathogenicity. Surprisingly, the cpkAΔcpk2Δ double deletion strain displayed significant reduction in growth rate and conidiation compared to the single deletion mutants. Interestingly, loss of CPKA and CPK2 resulted in morphogenetic defects in germ tubes (with curled/wavy and serpentine growth pattern) on hydrophobic surfaces, and a complete failure to produce appressoria therein, thus suggesting an important role for CPK2-mediated cAMP-PKA in surface sensing and response pathway. CPKA promoter-driven expression of CPK2 partially suppressed the defects in host penetration and pathogenicity in the cpkAΔ. Such ectopic CPK2 expressing strain successfully penetrated the rice leaves, but was unable to produce proper secondary invasive hyphae, thus underscoring the importance of CpkA in growth and differentiation in planta. The Cpk2-GFP localized to the nuclei and cytoplasmic vesicles in conidia and germ tubes. The Cpk2-GFP colocalized with CpkA-mCherry on vesicles in the cytosol, but such overlap was not evident in the nuclei. Our studies indicate that CpkA and Cpk2 share overlapping functions, but also play distinct roles during pathogenesis-associated signaling and morphogenesis in the rice blast fungus.

Subcellular compartmentation, interdependency and dynamics of the cyclic AMP-dependent PKA subunits during pathogenic differentiation in rice blast

The cAMP-dependent PKA signalling plays a central role in growth, asexual development and pathogenesis in fungal pathogens. Here, we functionally characterised RPKA, the regulatory subunit of cAMP/PKA and studied the dynamics and organisation of the PKA subunits in the rice blast pathogen Magnaporthe oryzae. The RPKA subunit was essential for proper vegetative growth, asexual sporulation and surface hydrophobicity in M. oryzae. A spontaneous suppressor mutation, SMR19, that restored growth and conidiation in the RPKA deletion mutant was isolated and characterised. SMR19 enhanced conidiation and appressorium formation but failed to suppress the pathogenesis defects in rpkAΔ. The PKA activity was undetectable in the mycelial extracts of SMR19, which showed a single mutation (val242leu) in the highly conserved active site of the catalytic subunit (CPKA) of cAMP/PKA. The two subunits of cAMP/PKA showed different subcellular localisation patterns with RpkA being predominantly nucleocytoplasmic in conidia, while CpkA was largely cytosolic and/or vesicular. The CpkA anchored RpkA in cytoplasmic vesicles, and localisation of PKA in the cytoplasm was governed by CpkA in a cAMP-dependant or independent manner. We show that there exists a tight regulation of PKA subunits at the level of transcription, and the cAMP signalling is differentially compartmentalised in a stage-specific manner in rice blast.

Diacylglycerol acyl transferase: A pathogenicity related gene in Colletotrichum gloeosporioides

To gain more insight into the molecular mechanisms of Colletotrichum gloeosporioides pathogenesis, restriction enzyme-mediated integration (REMI) mutagenesis identified the mutants of C. gloeosporioides impaired in pathogenicity. Transformants screened for defects in pathogenicity using detached leaves and fruits. Of the 20 REMI transformants tested, two mutants (H4 and H7) showed reduced pathogenicity on leaves of apple, kiwi, mango, peach, and fruits of guava, apple, and capsicum. One tagged gene from the genome sequence of mutant H4 was recovered by inverse PCR. Sequence analysis of the tagged site in mutant H4 revealed insertion in diacylglycerol acyltransferase gene which encodes diacylglycerol acyltransferase enzyme, catalyzing the steps involved in the biosynthesis of triacylglycerol, an important component of biological membranes and source of energy. Therefore, tagging of diacylglycerol acyltransferase gene in mutant H4 resulted in reduced pathogenicity, indicating possible role of this gene in pathogenicity of C. gloeosporioides.

Volatiles from Plants Induced by Multiple Aphid Attacks Promote Conidial Performance of Lecanicillium lecanii

Herbivore-induced plant volatiles (HIPVs) are clues that help predatory insects search for food. The hypothesis that entomopathogenic fungi, which protect plants, benefit from the release of HIPVs was tested. The plant Arabidopsis thaliana was used as the source of HIPVs. The insect herbivore Lipaphis erysimi (Kaltenbach) was used as the inducer, and the fungal pathogen of the aphid Lecanicillium lecanii was exposed to HIPVs to test our hypothesis. When exposed to aphid-induced A. thaliana volatiles, the mortality of aphids pre-treated with a conidial suspension of L. lecanii, the conidial germination and the appressorial formation were significantly increased compared with the control. The decan-3-ol and 4-methylpentyl isothiocyanate that were detected in the headspace seemed to have positive and negative affection, respectively. Moreover, HIPVs generated from groups of eight aphids per plant promoted significantly increased conidial germination and appressorial formation compared with HIPVs from groups of one, two and four aphids per plant. Our results demonstrated that the pathogenicity of the entomopathogenic fungus L. lecanii was enhanced when exposed to HIPVs and that the HIPVs were affected by the number of insect herbivores that induced them.

Caffeine fostering of mycoparasitic fungi against phytopathogens

Caffeine (1,3,7-trimethixanthine) is a typical purine alkaloid produced in more than 80 plant species. Its biological role is considered to strengthen plant's defense capabilities, directly as a toxicant to biotic attackers (allelopathy) and indirectly as an activator of defense system (priming). Caffeine is actively secreted into rhizosphere through primary root, and possibly affects the structure of microbe community nearby. The fungal community in coffee plant rhizosphere is enriched with particular species, including Trichoderma family, a mycoparasite that attacks and kills phytopathogens by coiling and destroying their hyphae. In the present study, the caffeine response of 8 filamentous fungi, 4 mycoparasitic Trichoderma, and 4 prey phytopathogens, was examined. Results showed that allelopathic effect of caffeine on fungal growth and development was differential, being stronger on pathogens than on Trichoderma species. Upon confronting, the prey immediately ceased the growth, whereas the predator continued to grow, indicating active mycoparasitism to have occurred. Caffeine enhanced mycoparasitism up to 1.7-fold. Caffeine thus functions in a double-track manner against fungal pathogens: first by direct suppression of growth and development, and second by assisting their natural enemy. These observations suggest that caffeine is a powerful weapon in the arms race between plants and pathogens by fostering enemy's enemy, and we propose the idea of "caffeine fostering" as the third role of caffeine.

Role of the MoYAK1 protein kinase gene in Magnaporthe oryzae development and pathogenicity

Conidiation and appressorium differentiation are key processes for polycyclic dissemination and infection in many pathogens. Our previous study using DNA microarray led to the discovery of the MoYAK1 gene in Magnaporthe oryzae that is orthologous to YAK1 in Saccharomyces cerevisiae. Although the mechanistic roles of YAK1 in S. cerevisiae have been described, roles of MoYAK1 in M. oryzae, a phytopathogenic fungus responsible for rice blast, remain uncharacterized. Targeted disruption of MoYAK1 results in pleiotropic defects in M. oryzae development and pathogenicity. The ΔMoyak1 mutant exhibits a severe reduction in aerial hyphal formation and conidiation. Conidia in the ΔMoyak1 are delayed in germination and demonstrate decreased glycogen content in a conidial age-dependent manner. The expression of hydrophobin-coding genes is dramatically changed in the ΔMoyak1 mutant, leading to a loss of surface hydrophobicity. Unlike the complete inability of the ΔMoyak1 mutant to develop appressoria on an inductive surface, the mutant forms appressoria of abnormal morphology in response to exogenous cyclic adenosine-5'-monophosphate and host-driven signals, which are all defective in penetrating host tissues due to abnormalities in glycogen and lipid metabolism, turgor generation and cell wall integrity. These data indicate that MoYAK1 is a protein kinase important for the development and pathogenicity of M. oryzae.

Structural basis of pathogen recognition by an integrated HMA domain in a plant NLR immune receptor

Plants have evolved intracellular immune receptors to detect pathogen proteins known as effectors. How these immune receptors detect effectors remains poorly understood. Here we describe the structural basis for direct recognition of AVR-Pik, an effector from the rice blast pathogen, by the rice intracellular NLR immune receptor Pik. AVR-PikD binds a dimer of the Pikp-1 HMA integrated domain with nanomolar affinity. The crystal structure of the Pikp-HMA/AVR-PikD complex enabled design of mutations to alter protein interaction in yeast and in vitro, and perturb effector-mediated response both in a rice cultivar containing Pikp and upon expression of AVR-PikD and Pikp in the model plant Nicotiana benthamiana. These data reveal the molecular details of a recognition event, mediated by a novel integrated domain in an NLR, which initiates a plant immune response and resistance to rice blast disease. Such studies underpin novel opportunities for engineering disease resistance to plant pathogens in staple food crops.

Ras GTPase activating protein CoIra1 is involved in infection-related morphogenesis by regulating cAMP and MAPK signaling pathways through CoRas2 in Colletotrichum orbiculare

Colletotrichum orbiculare is the causative agent of anthracnose disease on cucurbitaceous plants. Several signaling pathways, including cAMP–PKA and mitogen-activating protein kinase (MAPK) pathways are involved in the infection-related morphogenesis and pathogenicity of C. orbiculare. However, upstream regulators of these pathways for this species remain unidentified. In this study, CoIRA1, encoding RAS GTPase activating protein, was identified by screening the Agrobacterium tumefaciens-mediated transformation (AtMT) mutant, which was defective in the pathogenesis of C. orbiculare. The coira1 disrupted mutant showed an abnormal infection-related morphogenesis and attenuated pathogenesis. In Saccharomyces cerevisiae, Ira1/2 inactivates Ras1/2, which activates adenylate cyclase, leading to the synthesis of cAMP. Increase in the intracellular cAMP levels in coira1 mutants and dominant active forms of CoRAS2 introduced transformants indicated that CoIra1 regulates intracellular cAMP levels through CoRas2. Moreover, the phenotypic analysis of transformants that express dominant active form CoRAS2 in the comekk1 mutant or a dominant active form CoMEKK1 in the coras2 mutant indicated that CoRas2 regulates the MAPK CoMekk1–Cmk1 signaling pathway. The CoRas2 localization pattern in vegetative hyphae of the coira1 mutant was similar to that of the wild-type, expressing a dominant active form of RFP–CoRAS2. Moreover, we demonstrated that bimolecular fluorescence complementation (BiFC) signals between CoIra1 and CoRas2 were detected in the plasma membrane of vegetative hyphae. Therefore, it is likely that CoIra1 negatively regulates CoRas2 in vegetative hyphae. Furthermore, cytological analysis of the localization of CoIraI and CoRas2 revealed the dynamic cellular localization of the proteins that leads to proper assembly of F-actin at appressorial pore required for successful penetration peg formation through the pore. Thus, our results indicated that CoIra1 is involved in infection-related morphogenesis and pathogenicity by proper regulation of cAMP and MAPK signaling pathways through CoRas2.

MoST1 encoding a hexose transporter-like protein is involved in both conidiation and mycelial melanization of Magnaporthe oryzae

In a large-scale gene disruption screen of Magnaporthe oryzae, a gene MoST1 encoding a protein belonging to the hexose transporter family was identified as a gene required for conidiation and culture pigmentation. The gene MoST1 located on chromosome V of the M. oryzae genome was predicted to be 1892 bp in length with two introns encoding a 547-amino-acid protein with 12 putative transmembrane domains. Targeted gene disruption of MoST1 resulted in a mutant (most1) with extremely poor conidiation and defects in colony melanization. These phenotypes were complemented by re-introduction of an intact copy of MoST1. We generated a transgenic line harboring a vector containing the MoST1 promoter fused with a reporter protein gene mCherry. The mCherry fluorescence was observed in mycelia, conidia, germ tubes, and appressoria in M. oryzae. There are 66 other hexose transporter-like genes in M. oryzae, and we performed complementation assay with three genes most closely related to MoST1. However, none of them complemented the most1 mutant in conidiation and melanization, indicating that the homologs do not complement the function of MoST1. These results suggest that MoST1 has a specific role for conidiation and mycelial melanization, which is not shared by other hexose transporter family of M. oryzae.

The catalytic subunit of cAMP-dependent protein kinase A StPKA-c contributes to conidiation and early invasion in the phytopathogenic fungus Setosphaeria turcica

Cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) is an important mediator of signal transduction in eukaryotic cells. Thus, identifying its function is necessary to understand the cAMP signaling network. StPKA-c, the PKA catalytic subunit gene in Setosphaeria turcica, was investigated by RNA interference technology. Transformant strains M3, M5, and M9 with diverse StPKA-c silencing efficiency were confirmed by reverse transcription polymerase chain reaction and Northern blot. Compared with the wild-type strain 01-23, the transformant strains exhibited increased growth rate and significantly decreased conidium production. In addition, the ratios of spore germination and appressorium formation and penetration were slightly reduced. Relative to the wild-type strain, the transformants demonstrated different colony color, greatly reduced pathogenicity, and similar HT-toxin activity. Further studies showed that the content of intracellular melanin in the transformants significantly decreased, and the transcription of transcriptional factor StMR was down-regulated correspondingly. The transcription and enzyme activity of xylanase was also impaired. Thus, we proposed that StPKA-c was mainly involved in the mycelium growth, conidiation, and pathogenesis of S. turcica. Furthermore, it was positively correlated with the biosyntheses of melanin and xylanase but dispensable for the activity of HT-toxin.

Deployment of the Burkholderia glumae type III secretion system as an efficient tool for translocating pathogen effectors to monocot cells

Genome sequences of plant fungal pathogens have enabled the identification of effectors that cooperatively modulate the cellular environment for successful fungal growth and suppress host defense. Identification and characterization of novel effector proteins are crucial for understanding pathogen virulence and host-plant defense mechanisms. Previous reports indicate that the Pseudomonas syringae pv. tomato DC3000 type III secretion system (T3SS) can be used to study how non-bacterial effectors manipulate dicot plant cell function using the effector detector vector (pEDV) system. Here we report a pEDV-based effector delivery system in which the T3SS of Burkholderia glumae, an emerging rice pathogen, is used to translocate the AVR-Pik and AVR-Pii effectors of the fungal pathogen Magnaporthe oryzae to rice cytoplasm. The translocated AVR-Pik and AVR-Pii showed avirulence activity when tested in rice cultivars containing the cognate R genes. AVR-Pik reduced and delayed the hypersensitive response triggered by B. glumae in the non-host plant Nicotiana benthamiana, indicative of an immunosuppressive virulence activity. AVR proteins fused with fluorescent protein and nuclear localization signal were delivered by B. glumae T3SS and observed in the nuclei of infected cells in rice, wheat, barley and N. benthamiana. Our bacterial T3SS-enabled eukaryotic effector delivery and subcellular localization assays provide a useful method for identifying and studying effector functions in monocot plants.

Identifying pathogenicity genes in the rubber tree anthracnose fungus Colletotrichum gloeosporioides through random insertional mutagenesis

To gain more insight into the molecular mechanisms of Colletotrichum gloeosporioides pathogenesis, Agrobacterium tumefaciens-mediated transformation (ATMT) was used to identify mutants of C. gloeosporioides impaired in pathogenicity. An ATMT library of 4128 C. gloeosporioides transformants was generated. Transformants were screened for defects in pathogenicity with a detached copper brown leaf assay. 32 mutants showing reproducible pathogenicity defects were obtained. Southern blot analysis showed 60.4% of the transformants had single-site T-DNA integrations. 16 Genomic sequences flanking T-DNA were recovered from mutants by thermal asymmetric interlaced PCR, and were used to isolate the tagged genes from the genome sequence of wild-type C. gloeosporioides by Basic Local Alignment Search Tool searches against the local genome database of the wild-type C. gloeosporioides. One potential pathogenicity genes encoded calcium-translocating P-type ATPase. Six potential pathogenicity genes had no known homologs in filamentous fungi and were likely to be novel fungal virulence factors. Two putative genes encoded Glycosyltransferase family 28 domain-containing protein and Mov34/MPN/PAD-1 family protein, respectively. Five potential pathogenicity genes had putative function matched with putative protein of other Colletotrichum species. Two known C. gloeosporioides pathogenicity genes were also identified, the encoding Glomerella cingulata hard-surface induced protein and C. gloeosporioides regulatory subunit of protein kinase A gene involved in cAMP-dependent PKA signal transduction pathway.

Cyclic AMP-dependent protein kinase A negatively regulates conidia formation by the tangerine pathotype of Alternaria alternata

The necrotrophic fungal pathogen Alternaria alternata causes brown spot diseases in many citrus cultivars. The FUS3 and SLT2 mitogen-activated protein kinases (MAPK)-mediated signaling pathways have been shown to be required for conidiation. Exogenous application of cAMP to this fungal pathogen decreased conidia formation considerably. This study determined whether a cAMP-activated protein kinase A (PKA) is required for conidiation. Using loss-of-function mutations in PKA catalytic and regulatory subunit-coding genes, we demonstrated that PKA negatively regulates conidiation. Fungal mutants lacking PKA catalytic subunit gene (PKA ( cat )) reduced growth, lacked detectable PKA activity, and produced higher amounts of conidia compared to wild-type. Introduction of a functional copy of PKA ( cat ) into a null mutant partially restored PKA activity and produced wild-type level of conidia. In contrast, fungi lacking PKA regulatory subunit gene (PKA ( reg )) produced detectable PKA activity, exhibited severe growth reduction, formed swelling hyphal segments, and produced no mature conidia. Introduction of the PKA ( reg ) gene to a regulatory subunit mutant restored all phenotypes to wild type. PKA ( reg )-null mutants induced fewer necrotic lesions on citrus compared to wild-type, whereas PKA ( cat ) mutant displayed wild-type virulence. Overall, our studies indicate that PKA and FUS3-mediated signaling pathways apparently have very different roles in the regulation of conidia production and A. alternata pathogenesis in citrus.

Protein kinase A regulatory subunit isoforms regulate growth and differentiation in Mucor circinelloides: essential role of PKAR4

The protein kinase A (PKA) signaling pathway plays a role in regulating growth and differentiation in the dimorphic fungus Mucor circinelloides. PKA holoenzyme is comprised of two catalytic (C) and two regulatory (R) subunits. In M. circinelloides, four genes encode the PKAR1, PKAR2, PKAR3, and PKAR4 isoforms of R subunits. We have constructed null mutants and demonstrate that each isoform has a different role in growth and differentiation. The most striking finding is that pkaR4 is an essential gene, because only heterokaryons were obtained in knockout experiments. Heterokaryons with low levels of wild-type nuclei showed an impediment in the emission of the germ tube, suggesting a pivotal role of this gene in germ tube emergence. The remaining null strains showed different alterations in germ tube emergence, sporulation, and volume of the mother cell. The pkaR2 null mutant showed an accelerated germ tube emission and was the only mutant that germinated under anaerobic conditions when glycine was used as a nitrogen source, suggesting that pkaR2 participates in germ tube emergence by repressing it. From the measurement of the mRNA and protein levels of each isoform in the wild-type and knockout strains, it can be concluded that the expression of each subunit has its own mechanism of differential regulation. The PKAR1 and PKAR2 isoforms are posttranslationally modified by ubiquitylation, suggesting another regulation point in the specificity of the signal transduction. The results indicate that each R isoform has a different role in M. circinelloides physiology, controlling the dimorphism and contributing to the specificity of cyclic AMP (cAMP)-PKA pathway.

Large-scale gene disruption in Magnaporthe oryzae identifies MC69, a secreted protein required for infection by monocot and dicot fungal pathogens

To search for virulence effector genes of the rice blast fungus, Magnaporthe oryzae, we carried out a large-scale targeted disruption of genes for 78 putative secreted proteins that are expressed during the early stages of infection of M. oryzae. Disruption of the majority of genes did not affect growth, conidiation, or pathogenicity of M. oryzae. One exception was the gene MC69. The mc69 mutant showed a severe reduction in blast symptoms on rice and barley, indicating the importance of MC69 for pathogenicity of M. oryzae. The mc69 mutant did not exhibit changes in saprophytic growth and conidiation. Microscopic analysis of infection behavior in the mc69 mutant revealed that MC69 is dispensable for appressorium formation. However, mc69 mutant failed to develop invasive hyphae after appressorium formation in rice leaf sheath, indicating a critical role of MC69 in interaction with host plants. MC69 encodes a hypothetical 54 amino acids protein with a signal peptide. Live-cell imaging suggested that fluorescently labeled MC69 was not translocated into rice cytoplasm. Site-directed mutagenesis of two conserved cysteine residues (Cys36 and Cys46) in the mature MC69 impaired function of MC69 without affecting its secretion, suggesting the importance of the disulfide bond in MC69 pathogenicity function. Furthermore, deletion of the MC69 orthologous gene reduced pathogenicity of the cucumber anthracnose fungus Colletotrichum orbiculare on both cucumber and Nicotiana benthamiana leaves. We conclude that MC69 is a secreted pathogenicity protein commonly required for infection of two different plant pathogenic fungi, M. oryzae and C. orbiculare pathogenic on monocot and dicot plants, respectively.

Protein kinase A and fungal virulence: a sinister side to a conserved nutrient sensing pathway

Diverse fungal species are the cause of devastating agricultural and human diseases. As successful pathogenesis is dependent upon the ability of the fungus to adapt to the nutritional and chemical environment of the host, the understanding of signaling pathways required for such adaptation will provide insights into the virulence of these pathogens and the potential identification of novel targets for antifungal intervention. The cAMP-PKA signaling pathway is well conserved across eukaryotes. In the nonpathogenic yeast, S. cerevisiae, PKA is activated in response to extracellular nutrients and subsequently regulates metabolism and growth. Importantly, this pathway is also a regulator of pathogenesis, as defects in PKA signaling lead to an attenuation of virulence in diverse plant and human pathogenic fungi. This review will compare and contrast PKA signaling in S. cerevisiae vs. various pathogenic species and provide a framework for the role of this pathway in regulating fungal virulence.

A calmodulin inhibitor, W-7 influences the effect of cyclic adenosine 3', 5'-monophosphate signaling on ligninolytic enzyme gene expression in Phanerochaete chrysosporium

The capacity of white-rot fungi to degrade wood lignin may be highly applicable to the development of novel bioreactor systems, but the mechanisms underlying this function are not yet fully understood. Lignin peroxidase (LiP) and manganese peroxidase (MnP), which are thought to be very important for the ligninolytic property, demonstrated increased activity in Phanerochaete chrysosporium RP-78 (FGSC #9002, ATCC MYA-4764™) cultures following exposure to 5 mM cyclic adenosine 3', 5'-monophosphate (cAMP) and 500 μM 3'-isobutyl-1-methylxanthine (IBMX), a phosphodiesterase inhibitor. Real-time reverse transcription polymerase chain reaction (RT-PCR) analysis revealed that transcription of most LiP and MnP isozyme genes was statistically significantly upregulated in the presence of the cAMP and IBMX compared to the untreated condition. However, 100 μM calmodulin (CaM) inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), which had insignificant effects on fungal growth and intracellular cAMP concentration, not only offset the increased activity and transcription induced by the drugs, but also decreased them to below basal levels. Like the isozyme genes, transcription of the CaM gene (cam) was also upregulated by cAMP and IBMX. These results suggest that cAMP signaling functions to increase the transcription of LiP and MnP through the induction of cam transcription.

Two novel transcriptional regulators are essential for infection-related morphogenesis and pathogenicity of the rice blast fungus Magnaporthe oryzae

The cyclic AMP-dependent protein kinase A signaling pathway plays a major role in regulating plant infection by the rice blast fungus Magnaporthe oryzae. Here, we report the identification of two novel genes, MoSOM1 and MoCDTF1, which were discovered in an insertional mutagenesis screen for non-pathogenic mutants of M. oryzae. MoSOM1 or MoCDTF1 are both necessary for development of spores and appressoria by M. oryzae and play roles in cell wall differentiation, regulating melanin pigmentation and cell surface hydrophobicity during spore formation. MoSom1 strongly interacts with MoStu1 (Mstu1), an APSES transcription factor protein, and with MoCdtf1, while also interacting more weakly with the catalytic subunit of protein kinase A (CpkA) in yeast two hybrid assays. Furthermore, the expression levels of MoSOM1 and MoCDTF1 were significantly reduced in both Δmac1 and ΔcpkA mutants, consistent with regulation by the cAMP/PKA signaling pathway. MoSom1-GFP and MoCdtf1-GFP fusion proteins localized to the nucleus of fungal cells. Site-directed mutagenesis confirmed that nuclear localization signal sequences in MoSom1 and MoCdtf1 are essential for their sub-cellular localization and biological functions. Transcriptional profiling revealed major changes in gene expression associated with loss of MoSOM1 during infection-related development. We conclude that MoSom1 and MoCdtf1 functions downstream of the cAMP/PKA signaling pathway and are novel transcriptional regulators associated with cellular differentiation during plant infection by the rice blast fungus.

Magnaporthe oryzae, the causal agent of rice blast disease, is an important model fungal pathogen for understanding the molecular basis of plant-fungus interactions. In M. oryzae, the conserved cAMP/PKA signaling pathway has been demonstrated to be crucial for regulating infection-related morphogenesis and pathogenicity, including the control of sporulation and appressorium formation. In this study, we report the identification of two novel pathogenicity-related genes, MoSOM1 and MoCDTF1, by T-DNA insertional mutagenesis. Our results show that MoSOM1 or MoCDTF1 are essential for sporulation, appressorium formatiom and pathogenicity, and also play a key role in hyphal growth, melanin pigmentation and cell surface hydrophobicity. Nuclear localization sequences and conserved domains of the MoSom1 and MoCdtf1 proteins are crucial for their biological function. MoSom1 interacts physically with the transcription factors MoCdtf1 and MoStu1. We also show evidence that MoSom1 has the capacity to interact with CpkA, suggesting that MoSom1 may act downstream of the cAMP/PKA signaling pathway to regulate infection-related morphogenesis and pathogenicity in M. oryzae. Our studies extend the current understanding of downstream components of the conserved cAMP/PKA pathway and its precise role in regulating infection-related development and cellular differentiation by M. oryzae.

Loss of cAMP-dependent protein kinase A affects multiple traits important for root pathogenesis by Fusarium oxysporum

The soilborne fungal pathogen Fusarium oxysporum causes vascular wilt and root rot diseases in many plant species. We investigated the role of cyclic AMP-dependent protein kinase A of F. oxysporum (FoCPKA) in growth, morphology, and root attachment, penetration, and pathogenesis in Arabidopsis thaliana. Affinity of spore attachment to root surfaces of A. thaliana, observed microscopically and measured by atomic force microscopy, was reduced by a loss-of-function mutation in the gene encoding the catalytic subunit of FoCPKA. The resulting mutants also failed to penetrate into the vascular system of A. thaliana roots and lost virulence. Even when the mutants managed to enter the vascular system via physically wounded roots, the degree of vascular colonization was significantly lower than that of the corresponding wild-type strain O-685 and no noticeable disease symptoms were observed. The mutants also had reduced vegetative growth and spore production, and their hyphal growth patterns were distinct from those of O-685. Coinoculation of O-685 with an focpkA mutant or a strain nonpathogenic to A. thaliana significantly reduced disease severity and the degree of root colonization by O-685. Several experimental tools useful for studying mechanisms of fungal root pathogenesis are also introduced.

A yeast STE11 homologue CoMEKK1 is essential for pathogenesis-related morphogenesis in Colletotrichum orbiculare

Several signal transduction pathways, including mitogen-activated protein kinase (MAPK) pathways, are involved in appressorium development in Colletotrichum orbiculare, the causal agent of cucumber anthracnose disease. In this study, CoMEKK1, a yeast MAPK kinases (MAPKK) kinase STE11 homolog, was identified as a disrupted gene in an Agrobacterium tumefaciens-mediated transformation mutant. The phenotype of comekk1 disruptant was similar to that of cmk1, a Saccharomyces cerevisiae Fus3/Kss1 MAPK homolog mutant. Moreover, comekk1 and cmk1 mutants were sensitive to high osmotic and salinity stresses, indicating that Comekk1p/Cmk1p signal transduction is involved in stress tolerance. The transformants of the wild type and the comekk1 mutant expressing a constitutively active form of the CoMEKK1 showed slower hyphal growth and abnormal appressorium formation, whereas those of the cmk1 disruptant did not. A Cmk1p-green fluorescent protein (GFP) intracellular localization experiment indicated that nuclear localization of the Cmk1p-GFP fusion protein induced by salt stress was diminished in comekk1 mutants. These results indicate that Comekk1p functions upstream of Cmk1p.

A subunit of protein kinase a regulates growth and differentiation in the fungus Mucor circinelloides

The cyclic AMP (cAMP)-dependent protein kinase A (PKA) signaling pathway plays a role in regulating development, growth, and virulence in a number of fungi. To determine whether PKA plays a similar function in zygomycete fungi, a mutant of Mucor circinelloides was generated that lacks pkaR1, one of the regulatory subunits of PKA. The mutant showed a reduction in growth and alterations in germination rates, cell volume, germ tube length, and asexual sporulation. The lack of pkaR1 gene resulted in a highly decreased, but not null, cAMP binding activity and in a protein kinase activity that was still dependent on cAMP, although with a higher -/+ cAMP activity ratio, suggesting the existence of other cAMP binding activities. Consequently, three proteins analogous to pkaR1 were predicted from the recently sequenced genome of M. circinelloides and were named pkaR2, pkaR3, and pkaR4. Two of the proteins, pkaR2 and pkaR3, with cAMP binding activity were isolated from the wild-type strain and identified by mass spectrometry. The expression of all genes was detected at the mRNA level by semiquantitative reverse transcription-PCR, and they showed a differential expression at different developmental stages. This is the first time that a fungus is reported to have more than one gene encoding the regulatory subunit of PKA.

The cAMP-dependent signaling pathway and its role in conidial germination, growth, and virulence of the gray mold Botrytis cinerea

In Botrytis cinerea, some components of the cAMP-dependent pathway, such as alpha subunits of heterotrimeric G proteins and the adenylate cyclase BAC, have been characterized and their impact on growth, conidiation, germination, and virulence has been demonstrated. Here, we describe the functions of more components of the cAMP cascade: the catalytic subunits BcPKA1 and BcPKA2 and the regulatory subunit BcPKAR of the cAMP-dependent protein kinase (PKA). Although Deltabcpka2 mutants showed no obvious phenotypes, growth and virulence were severely affected by deletion of both bcpka1 and bcpkaR. Similar to Deltabac, lesion development of Deltabcpka1 and DeltabcpkaR was slower than in controls and soft rot of leaves never occurred. In contrast to Deltabac, Deltabcpka1 and DeltabcpkaR mutants sporulated in planta, and growth rate, conidiation, and conidial germination were not impaired, indicating PKA-independent functions of cAMP. Unexpectedly, Deltabcpka1 and DeltabcpkaR showed identical phenotypes, suggesting the total loss of PKA activity in both mutants. The deletion of bcras2 encoding the fungal-specific Ras GTPase resulted in significantly delayed germination and decreased growth rates. Both effects could be partially restored by exogenous cAMP, suggesting that BcRAS2 activates the adenylate cyclase in addition to the Galpha subunits BCG1 and BCG3, thus influencing cAMP-dependent signal transduction.

Protein kinase A regulates growth, sporulation, and pigment formation in Aspergillus fumigatus

Aspergillus fumigatus is an opportunistic human pathogenic fungus causing severe infections in immunocompromised patients. Cyclic AMP (cAMP) signal transduction plays an important role in virulence. A central component of this signaling cascade is protein kinase A (PKA), which regulates cellular processes by phosphorylation of specific target proteins. Here we describe the generation and analysis of A. fumigatus mutants expressing the gene encoding the catalytic subunit of PKA, pkaC1, under control of an inducible promoter. Strains overexpressing pkaC1 showed high PKA activity, reduced growth, sporulation deficiency, and formation of a dark pigment in the mycelium. These data indicate that cAMP-PKA signaling is involved in the regulation of important processes, such as growth, asexual reproduction, and biosynthesis of secondary metabolites. Furthermore, elevated PKA activity led to increased expression of the pksP gene. The polyketide synthase PksP is an essential enzyme for production of dihydroxynaphthalene-melanin in A. fumigatus and contributes to virulence. Our results suggest that increased pksP expression is responsible for pigment formation in the mycelium. Comparative proteome analysis of the pkaC1-overexpressing strain and the wild-type strain led to the identification of proteins regulated by the cAMP-PKA signal transduction pathway. We showed that elevated PKA activity resulted in activation of stress-associated proteins and of enzymes involved in protein biosynthesis and glucose catabolism. In contrast, proteins which were involved in nucleotide and amino acid biosynthesis were downregulated, as were enzymes involved in catabolism of carbon sources other than glucose.

Transcriptome analysis reveals new insight into appressorium formation and function in the rice blast fungus Magnaporthe oryzae

BACKGROUND

Rice blast disease is caused by the filamentous Ascomycetous fungus Magnaporthe oryzae and results in significant annual rice yield losses worldwide. Infection by this and many other fungal plant pathogens requires the development of a specialized infection cell called an appressorium. The molecular processes regulating appressorium formation are incompletely understood.

RESULTS

We analyzed genome-wide gene expression changes during spore germination and appressorium formation on a hydrophobic surface compared to induction by cAMP. During spore germination, 2,154 (approximately 21%) genes showed differential expression, with the majority being up-regulated. During appressorium formation, 357 genes were differentially expressed in response to both stimuli. These genes, which we refer to as appressorium consensus genes, were functionally grouped into Gene Ontology categories. Overall, we found a significant decrease in expression of genes involved in protein synthesis. Conversely, expression of genes associated with protein and amino acid degradation, lipid metabolism, secondary metabolism and cellular transportation exhibited a dramatic increase. We functionally characterized several differentially regulated genes, including a subtilisin protease (SPM1) and a NAD specific glutamate dehydrogenase (Mgd1), by targeted gene disruption. These studies revealed hitherto unknown findings that protein degradation and amino acid metabolism are essential for appressorium formation and subsequent infection.

CONCLUSION

We present the first comprehensive genome-wide transcript profile study and functional analysis of infection structure formation by a fungal plant pathogen. Our data provide novel insight into the underlying molecular mechanisms that will directly benefit efforts to identify fungal pathogenicity factors and aid the development of new disease management strategies.

Carbon source affects PKA-dependent polarity of Neurospora crassa in a CRE-1-dependent and independent manner

A defect in mcb, encoding the cAMP-dependent protein kinase A (PKA) regulatory subunit in Neurospora crassa, which confers an apolar growth phenotype, is accompanied by an increase in PKA activity levels. Both PKA and CRE-1 [a key carbon catabolite repression (CCR) regulator] mediate the cellular response to carbon-source availability. Inactivation of the cre-1 gene resulted in reduced growth rate, abnormal hyphal morphology and altered CCR. Both PKA and CRE-1 affected morphology in a carbon-dependent manner, as fructose suppressed the apolar morphology of the mcb strain and enabled faster growth of the Deltacre-1 mutant. An increase in cre-1 transcript abundance was observed in mcb and a reduction in PKA activity levels was measured in Deltacre-1. CRE-1 is involved in determining PKA-dependent polarity, as an mcb;Deltacre-1 strain displayed partial reestablishment of hyphal polarity. Taken together, our results demonstrate regulatory interactions between PKA and CRE-1 that affect cell polarity in a filamentous fungus.

The Slt2-type MAP kinase Bmp3 of Botrytis cinerea is required for normal saprotrophic growth, conidiation, plant surface sensing and host tissue colonization

SUMMARY Mitogen-activated protein kinases (MAPKs) play important roles in signal transduction and regulation of various aspects of saprotrophic and pathogenic growth in plant pathogenic fungi. We have generated a Botrytis cinerea knock-out mutant in the bmp3 gene encoding a homologue of the yeast Slt2 cell wall integrity MAPK. The Deltabmp3 mutant showed reduced vegetative growth on various media, strongly impaired conidiation and loss of sclerotia formation. Growth retardation of the mutant was enhanced in media with low osmolarity, whereas nearly wild-type growth rates were observed under high osmolarity conditions. The Deltabmp3 mutant did not show increased susceptibility to cell wall damage induced by glucanase, Calcofluor White or Nikkomycin Z, but was more susceptible to the oxidizing agent paraquat and the phenylpyrrole fungicide fludioxonil. Deltabmp3 conidia showed normal germination and germ tube growth on agar, but excessive germ tube elongation on hard surfaces and reduced penetration efficiency, indicating a defect in surface sensing. After penetration, development of necrotic lesions induced by the Deltabmp3 mutant was retarded. All these defects were restored by genetic complementation of the mutant with the wild-type bmp3 gene.

Protein kinase A subunits of the ascomycete pathogen Mycosphaerella graminicola regulate asexual fructification, filamentation, melanization and osmosensing

SUMMARY As in many fungi, asexual reproduction of Mycosphaerella graminicola in planta is a complex process that requires proper differentiation of the infectious hyphae in the substomatal cavities of foliar tissue before pycnidia with conidia can be formed. In this study, we have investigated the role of the cAMP signalling pathway in development and pathogenicity of this pathogen by disruption of the genes encoding the catalytic (designated MgTpk2) and regulatory subunit (designated MgBcy1) of protein kinase A. The MgTpk2 and MgBcy1 mutants showed altered phenotypes in vitro when grown under different growth conditions. On potato dextrose agar (PDA), MgBcy1 mutants showed altered osmosensitivity and reduced melanization, whereas the MgTpk2 mutants showed accelerated melanization when compared with the M. graminicola IPO323 wild-type strain and ectopic transformants. MgTpk2 mutants also secreted a dark-brown pigment into yeast glucose broth medium. In germination and microconidiation assays, both mutants showed a germination pattern similar to that of the controls on water agar, whereas on PDA filamentous growth of MgTpk2 mutants was impaired. Pathogenicity assays showed that the MgTpk2 and MgBcy1 mutants were less virulent as they caused only limited chlorotic and necrotic symptoms at the tips of the inoculated leaves. Further analyses of the infection process showed that MgTpk2 and MgBcy1 mutants were able to germinate, penetrate and colonize mesophyll tissue, but were unable to produce the asexual fructifications, which was particularly due to inappropriate differentiation during the late stage of this morphogenesis-related process.

Multiple contributions of peroxisomal metabolic function to fungal pathogenicity in Colletotrichum lagenarium

In Colletotrichum lagenarium, which is the causal agent of cucumber anthracnose, PEX6 is required for peroxisome biogenesis and appressorium-mediated infection. To verify the roles of peroxisome-associated metabolism in fungal pathogenicity, we isolated and functionally characterized ICL1 of C. lagenarium, which encodes isocitrate lyase involved in the glyoxylate cycle in peroxisomes. The icl1 mutants failed to utilize fatty acids and acetate for growth. Although Icl1 has no typical peroxisomal targeting signals, expression analysis of the GFP-Icl1 fusion protein indicated that Icl1 localizes in peroxisomes. These results indicate that the glyoxylate cycle that occurs inside the peroxisome is required for fatty acid and acetate metabolism for growth. Importantly, in contrast with the pex6 mutants that form nonmelanized appressoria, the icl1 mutants formed appressoria that were highly pigmented with melanin, suggesting that the glyoxylate cycle is not essential for melanin biosynthesis in appressoria. However, the icl1 mutants exhibited a severe reduction in virulence. Appressoria of the icl1 mutants failed to develop penetration hyphae in the host plant, suggesting that ICL1 is involved in host invasion. The addition of glucose partially restored virulence of the icl1 mutant. Heat shock treatment of the host plant also enabled the icl1 mutants to develop lesions, implying that the infection defect of the icl1 mutant is associated with plant defense. Together with the requirement of PEX6 for appressorial melanization, our findings suggest that peroxisomal metabolic pathways play functional roles in appressorial melanization and subsequent host invasion steps, and the latter step requires the glyoxylate cycle.

Deletion of the regulatory subunit of protein kinase A in Aspergillus fumigatus alters morphology, sensitivity to oxidative damage, and virulence

Aspergillus fumigatus is an important opportunistic fungal pathogen. The cAMP-dependent protein kinase (PKA) signaling pathway plays an important role in regulating morphology, growth, and virulence in a number of fungal pathogens of plants and animals. We have constructed a mutant of A. fumigatus that lacks the regulatory subunit of PKA, pkaR, and analyzed the growth and development, sensitivity to oxidative damage, and virulence of the mutant, along with those of the wild type and a complemented mutant. Both growth and germination rates of the mutant are reduced, and there are morphological abnormalities in conidiophores, leading to reduced conidiation. Conidia from the DeltapkaR mutant are more sensitive to killing by hydrogen peroxide, menadione, paraquat, and diamide. However, the hyphae of the mutant are killed to a greater extent only by paraquat and diamide, whereas they are less susceptible to the effects of hydrogen peroxide. In an immunosuppressed mouse model, intranasally administered conidia of the mutant are significantly less virulent than those of the wild type or a complemented mutant. Unregulated PKA signaling is detrimental to the virulence of A. fumigatus, perhaps through the reduced susceptibility of the mutant to damage by oxidizing agents and reduced growth kinetics.

A gene involved in modifying transfer RNA is required for fungal pathogenicity and stress tolerance of Colletotrichum lagenarium

7-Methylguanosine (m7G) modification of tRNA occurs widely in prokaryotes and eukaryotes, although information about its biological roles is limited. Here, we report that a gene involved in m7G modification of tRNA is required for infection by the phytopathogenic fungus Colletotrichum lagenarium. Analysis of the infection-deficient mutant of C. lagenarium, produced by plasmid insertional mutagenesis, identified a tagged gene that is designated APH1. The aph1 mutants, generated by targeted gene disruption, exhibit significant reduction in pathogenicity on the host plants. We conclude that APH1 is required for fungal infection in C. lagenarium. Aph1 showed a strong similarity to Saccharomyces cerevisiae Trm8 involved in m7G modification of tRNA. The m7G content of tRNA from the aph1 deletion mutant was severely reduced compared with that from the wild type, indicating that APH1 is required for m7G methyltransferase activity. Appressoria formed by the aph1 mutants developed penetration hyphae into cellophane, suggesting that appressoria of the mutants retain basic function for penetration. However, the aph1 mutants failed to develop intracellular penetration hyphae into epidermis of the host plants, suggesting a specific requirement of APH1 for appressorium-mediated host invasion. The mutants also had increased sensitivity to salinity and H2O2 stresses. Interestingly, a heat shock treatment on the host plants enabled the aph1 mutant to penetrate them. These data suggest that the APH1 is required for the plant invasion, probably to overcome environmental stresses derived from basal preinvasion (penetration) defence of the host plants.

Nonpathogenic strains of Colletotrichum lindemuthianum trigger progressive bean defense responses during appressorium-mediated penetration

The fungal bean pathogen Colletotrichum lindemuthianum differentiates appressoria in order to penetrate bean tissues. We showed that appressorium development in C. lindemuthianum can be divided into three stages, and we obtained three nonpathogenic strains, including one strain blocked at each developmental stage. H18 was blocked at the appressorium differentiation stage; i.e., no genuine appressoria were formed. H191 was blocked at the appressorium maturation stage; i.e., appressoria exhibited a pigmentation defect and developed only partial internal turgor pressure. H290 was impaired in appressorium function; i.e., appressoria failed to penetrate into bean tissues. Furthermore, these strains could be further discriminated according to the bean defense responses that they induced. Surprisingly, appressorium maturation, but not appressorium function, was sufficient to induce most plant defense responses tested (superoxide ion production and strong induction of pathogenesis-related proteins). However, appressorium function (i.e., entry into the first host cell) was necessary for avirulence-mediated recognition of the fungus.