Fungal Histidine Phosphotransferase Plays a Crucial Role in Photomorphogenesis and Pathogenesis in Magnaporthe oryzae

An array of signal-specific MoYpd1 isoforms determines full virulence in the pathogenic fungus Magnaporthe oryzae

Magnaporthe oryzae is placed first on a list of the world's top ten plant pathogens with the highest scientific and economic importance. The locus MGG_07173 occurs only once in the genome of M. oryzae and encodes the phosphotransfer protein MoYpd1p, which plays an important role in the high osmolarity glycerol (HOG) signaling pathway for osmoregulation. Originating from this locus, at least three MoYPD1 isoforms are produced in a signal-specific manner. The transcript levels of these MoYPD1-isoforms were individually affected by external stress. Salt (KCI) stress raised MoYPD1_T0 abundance, whereas osmotic stress by sorbitol elevates MoYPD1_T1 levels. In line with this, signal-specific nuclear translocation of green fluorescent protein-fused MoYpd1p isoforms in response to stress was observed. Mutant strains that produce only one of the MoYpd1p isoforms are less virulent, suggesting a combination thereof is required to invade the host successfully. In summary, we demonstrate signal-specific production of MoYpd1p isoforms that individually increase signal diversity and orchestrate virulence in M. oryzae.

Osmotic Stress Responses, Cell Wall Integrity, and Conidiation Are Regulated by a Histidine Kinase Sensor in Trichoderma atroviride

Trichoderma atroviride responds to various environmental stressors through the mitogen-activated protein kinase (MAPK) Tmk3 and MAPK-kinase Pbs2 signaling pathways. In fungi, orthologues to Tmk3 are regulated by a histidine kinase (HK) sensor. However, the role of T. atroviride HKs remains unknown. In this regard, the function of the T. atroviride HK Nik1 was analyzed in response to stressors regulated by Tmk3. The growth of the Δnik1 mutant strains was compromised under hyperosmotic stress; mycelia were less resistant to lysing enzymes than the WT strain, while conidia of Δnik1 were more sensitive to Congo red; however, ∆pbs2 and ∆tmk3 strains showed a more drastic defect in cell wall stability. Light-regulated blu1 and grg2 gene expression was induced upon an osmotic shock through Pbs2-Tmk3 but was independent of Nik1. The encoding chitin synthases chs1 and chs2 genes were downregulated after an osmotic shock in the WT, but chs1 and chs3 expression were enhanced in ∆nik1, ∆pbs2, and ∆tmk3. The vegetative growth and conidiation by light decreased in ∆nik1, although Nik1 was unrequired to activate the light-responsive genes by Tmk3. Altogether, Nik1 regulates responses related to the Pbs2-Tmk3 pathway and suggests the participation of additional HKs to respond to stress.

A frameshift in Yersinia pestis rcsD alters canonical Rcs signalling to preserve flea-mammal plague transmission cycles

Multiple genetic changes in the enteric pathogen Yersinia pseudotuberculosis have driven the emergence of Yesinia pestis, the arthropod-borne, etiological agent of plague. These include developing the capacity for biofilm-dependent blockage of the flea foregut to enable transmission by flea bite. Previously, we showed that pseudogenization of rcsA, encoding a component of the Rcs signalling pathway, is an important evolutionary step facilitating Y. pestis flea-borne transmission. Additionally, rcsD, another important gene in the Rcs system, harbours a frameshift mutation. Here, we demonstrated that this rcsD mutation resulted in production of a small protein composing the C-terminal RcsD histidine-phosphotransferase domain (designated RcsD-Hpt) and full-length RcsD. Genetic analysis revealed that the rcsD frameshift mutation followed the emergence of rcsA pseudogenization. It further altered the canonical Rcs phosphorylation signal cascade, fine-tuning biofilm production to be conducive with retention of the pgm locus in modern lineages of Y. pestis. Taken together, our findings suggest that a frameshift mutation in rcsD is an important evolutionary step that fine-tuned biofilm production to ensure perpetuation of flea-mammal plague transmission cycles.

Identification of Mycoparasitism-Related Genes against the Phytopathogen Botrytis cinerea via Transcriptome Analysis of Trichoderma harzianum T4

Trichoderma harzianum is a well-known biological control agent (BCA) that is effective against a variety of plant pathogens. In previous studies, we found that T. harzianum T4 could effectively control the gray mold in tomatoes caused by Botrytis cinerea. However, the research on its biocontrol mechanism is not comprehensive, particularly regarding the mechanism of mycoparasitism. In this study, in order to further investigate the mycoparasitism mechanism of T. harzianum T4, transcriptomic sequencing and real-time fluorescence quantitative PCR (RT-qPCR) were used to identify the differentially expressed genes (DEGs) of T. harzianum T4 at 12, 24, 48 and 72 h of growth in the cell wall of B. cinerea (BCCW) or a sucrose medium. A total of 2871 DEGs and 2148 novel genes were detected using transcriptome sequencing. Through GO and KEGG enrichment analysis, we identified genes associated with mycoparasitism at specific time periods, such as encoding kinases, signal transduction proteins, carbohydrate active enzymes, hydrolytic enzymes, transporters, antioxidant enzymes, secondary metabolite synthesis, resistance proteins, detoxification genes and genes associated with extended hyphal longevity. To validate the transcriptome data, RT-qCPR was performed on the transcriptome samples. The RT-qPCR results show that the expression trend of the genes was consistent with the RNA-Seq data. In order to validate the screened genes associated with mycoparasitism, we performed a dual-culture antagonism test on T. harzianum and B. cinerea. The results of the dual-culture RT-qPCR showed that 15 of the 24 genes were upregulated during and after contact between T. harzianum T4 and B. cinerea (the same as BCCW), which further confirmed that these genes were involved in the mycoparasitism of T. harzianum T4. In conclusion, the transcriptome data provided in this study will not only improve the annotation information of gene models in T. harzianum T4 genome, but also provide important transcriptome information regarding the process of mycoparasitism at specific time periods, which can help us to further understand the mechanism of mycoparasitism, thus providing a potential molecular target for T. harzianum T4 as a biological control agent.

Role of the blue light receptor gene Icwc-1 in mycelium growth and fruiting body formation of Isaria cicadae

The Isaria cicadae, is well known highly prized medicinal mushroom with great demand in food and pharmaceutical industry. Due to its economic value and therapeutic uses, natural sources of wild I. cicadae are over-exploited and reducing continuously. Therefore, commercial cultivation in controlled environment is an utmost requirement to fulfill the consumer's demand. Due to the lack of knowledge on fruiting body (synnemata) development and regulation, commercial cultivation is currently in a difficult situation. In the growth cycle of macrofungi, such as mushrooms, light is the main factor affecting growth and development, but so far, specific effects of light on the growth and development of I. cicadae is unknown. In this study, we identified a blue light receptor white-collar-1 (Icwc-1) gene homologue with well-defined functions in morphological development in I. cicadae based on gene knockout technology and transcriptomic analysis. It was found that the Icwc-1 gene significantly affected hyphal growth and fruiting body development. This study confirms that Icwc-1 acts as an upstream regulatory gene that regulates genes associated with fruiting body formation, pigment-forming genes, and related genes for enzyme synthesis. Transcriptome data analysis also found that Icwc-1 affects many important metabolic pathways of I. cicadae, i.e., amino acid metabolism and fatty acid metabolism. The above findings will not only provide a comprehensive understanding about the molecular mechanism of light regulation in I. cicadae, but also provide new insights for future breeding program and improving this functional food production.

Diverse Effects of Amino Acids on Monascus Pigments Biosynthesis in Monascus purpureus

Amino acids could act as nitrogen sources, amido group donors, or bioactive molecules in fungi fermentation, and consequently, play important roles in Monascus pigments (MPs) biosynthesis. But the understanding of the effects of various amino acids on MPs biosynthesis is still incomprehensive. In this work, 20 free amino acids were added to the fermentation medium to evaluate their effects on MPs biosynthesis in Monascus purpureus RP2. Six amino acids, namely, histidine (HIS), lysine (LYS), tyrosine (TYR), phenylalanine (PHE), methionine (MET), and cysteine (CYS), were selected as the valuable ones as they exerted significant effects on the production yield and even on the biosynthesis metabolic curves of MPs. Moreover, the dose-dependent and synergistic effects of valuable amino acids on MPs biosynthesis were observed by tests of serial concentrations and different combinations. In addition, it revealed that HIS and MET were the prominent amino acids with dominant and universal influences on MPs biosynthesis. The analog compounds of HIS (amitrole) and MET [calcium 2-hydroxy-4-(methylthio)] were added to the fermentation medium, and the results further confirmed the extraordinary effects of HIS and MET and their analogs on MPs biosynthesis. Furthermore, the gene transcription profile indicated that a differential expression pattern was observed in the polyketide synthase (PKS) cluster responsible for MPs biosynthesis in response to HIS and MET, revealing that they could oppositely regulate MPs biosynthesis in different ways. These findings would benefit the understanding of MPs biosynthesis regulation mechanism in M. purpureus and contribute to the industrial production of MPs by fermentation.

Alternative splicing diversifies the transcriptome and proteome of the rice blast fungus during host infection

Alternative splicing (AS) contributes to diversifying and regulating cellular responses to environmental conditions and developmental cues by differentially producing multiple mRNA and protein isoforms from a single gene. Previous studies on AS in pathogenic fungi focused on profiling AS isoforms under a limited number of conditions. We analysed AS profiles in the rice blast fungus Magnaporthe oryzae, a global threat to rice production, using high-quality transcriptome data representing its vegetative growth (mycelia) and multiple host infection stages. We identified 4,270 AS isoforms derived from 2,413 genes, including 499 genes presumably regulated by infection-specific AS. AS appears to increase during infection, with 32.7% of the AS isoforms being produced during infection but absent in mycelia. Analysis of the isoforms observed at each infection stage showed that 636 AS isoforms were more abundant than corresponding annotated mRNAs, especially after initial hyphal penetration into host cell. Many such dominant isoforms were predicted to encode regulatory proteins such as transcription factors and phospho-transferases. We also identified the genes encoding distinct proteins via AS and confirmed the translation of some isoforms via a proteomic analysis, suggesting potential AS-mediated neo-functionalization of some genes during infection. Comprehensive profiling of the pattern of genome-wide AS during multiple stages of rice-M. oryzae interaction established a foundational resource that will help investigate the role and regulation of AS during rice infection.

A Radical Reimagining of Fungal Two-Component Regulatory Systems

Bacterial two-component regulatory systems (TCSs) mediate signal transduction by transferring phosphoryl groups between sensor kinase and response regulator proteins, sometimes using intermediary histidine-phosphotransferase (Hpt) domains to form multistep phosphorelays. Because (i) almost all known fungal sensor kinases exhibit a domain architecture characteristic of bacterial TCS phosphorelays, (ii) all known fungal Hpts are stand-alone proteins suited to shuttle between cytoplasm and nucleus, and (iii) the best-characterized fungal TCS is a canonical phosphorelay, it is widely assumed that most or all fungal TCSs function via phosphorelays. However, fungi generally encode more sensor kinases than Hpts or response regulators, leading to a disparity between putative phosphorelay inputs and outputs. The simplest resolution of this paradox is to hypothesize that most fungal sensor kinases do not participate in phosphorelays. Reimagining how fungal TCSs might function leads to multiple testable predictions.

Metabolic Basis of Pathogenesis and Host Adaptation in Rice Blast

The blast disease, caused by the ascomycete Magnaporthe oryzae, poses a great threat to rice production worldwide. Increasing use of fungicides and/or blast-resistant varieties of rice (Oryza sativa) has proved to be ineffective in long-term control of blast disease under field conditions. To develop effective and durable resistance to blast, it is important to understand the cellular mechanisms underlying pathogenic development in M. oryzae. In this review, we summarize the latest research in phototropism, autophagy, nutrient and redox signaling, and intrinsic phytohormone mimics in M. oryzae for cellular and metabolic adaptation(s) during its interactions with the host plants.

PlMAPK10, a Mitogen-Activated Protein Kinase (MAPK) in Peronophythora litchii, Is Required for Mycelial Growth, Sporulation, Laccase Activity, and Plant Infection

Mitogen-activated protein kinase (MAPK) pathways are ubiquitous and evolutionarily conserved signal transduction modules directing cellular respond to a diverse array of stimuli, in the eukaryotic organisms. In this study, PlMAPK10 was identified to encode a MAPK in Peronophythora litchii, the oomycete pathogen causing litchi downy blight disease. PlMAPK10, containing a specific and highly conserved dual phosphorylation lip sequence SEY (Serine-Glutamic-Tyrosine), represents a novel group of MAPKs as previously reported. Transcriptional profiling showed that PlMAPK10 expression was up-regulated in zoospore and cyst stages. To elucidate its function, the PlMAPK10 gene was silenced by stable transformation. PlMAPK10 silence did not impair oospore production, sporangium germination, zoospore encyst, or cyst germination but hindered hyphal growth, sporulation, pathogenicity, likely due to altering laccase activity. Over all, our results indicated that a MAPK encoded by PlMAPK10 gene in P. litchii is important for pathogenic development.

Glycerol-3-Phosphate Shuttle Is Involved in Development and Virulence in the Rice Blast Fungus Pyricularia oryzae

The glycerol-3-phosphate (G-3-P) shuttle is an important pathway for delivery of cytosolic reducing equivalents into mitochondrial oxidative phosphorylation, and plays essential physiological roles in yeast, plants, and animals. However, its role has been unclear in filamentous and pathogenic fungi. Here, we characterize the function of the G-3-P shuttle in Pyricularia oryzae by genetic and molecular analyses. In P. oryzae, a glycerol-3-phosphate dehydrogenase 1 (PoGpd1) is involved in NO production, conidiation, and utilization of several carbon sources (pyruvate, sodium acetate, glutamate, and glutamine). A glycerol-3-phosphate dehydrogenase 2 (PoGpd2) is essential for glycerol utilization and fungal development. Deletion of PoGPD2 led to delayed aerial hyphal formation, accelerated aerial hyphal collapse, and reduced conidiation on complete medium (CM) under a light-dark cycle. Aerial mycelial surface hydrophobicity to water and Tween 20 was decreased in ΔPogpd2. Melanin synthesis genes required for cell wall construction and two transcription factor genes (COS1 and CONx2) required for conidiation and/or aerial hyphal differentiation were down-regulated in the aerial mycelia of ΔPogpd2 and ΔPogpd1. Culturing under continuous dark could complement the defects of aerial hyphal differentiation of ΔPogpd2 observed in a light-dark cycle. Two light-sensitive protein genes (PoSIR2 encoding an NAD⁺-dependent deacetylase and TRX2 encoding a thioredoxin 2) were up-regulated in ΔPogpd2 cultured on CM medium in a light-dark cycle. ΔPogpd2 showed an increased intracellular NAD⁺/NADH ratio and total NAD content, and alteration of intracellular ATP production. Culturing on minimal medium also could restore aerial hyphal differentiation of ΔPogpd2, which is deficient on CM medium in a light-dark cycle. Two glutamate synthesis genes, GDH1 and PoGLT1, which synthesize glutamate coupled with oxidation of NADH to NAD⁺, were significantly up-regulated in ΔPogpd2 in a light-dark cycle. Moreover, deletion of PoGpd1 or PoGpd2 led to reduced virulence of conidia or hyphae on rice. The glycerol-3-phosphate shuttle is involved in cellular redox, fungal development, and virulence in P. oryzae.