The Magnaporthe grisea species complex and plant pathogenesis

The dual-specificity kinase MoLKH1-mediated cell cycle, autophagy, and suppression of plant immunity is critical for development and pathogenicity of Magnaporthe oryzae

Cell cycle progression, autophagic cell death during appressorium development, and ROS degradation at the infection site are important for the development of rice blast disease. However, the association of cell cycle, autophagy and ROS detoxification remains largely unknown in M. oryzae. Here, we identify the dual-specificity kinase MoLKH1, which serves as an important cell cycle regulator required for appressorium formation by regulating cytokinesis and cytoskeleton in M. oryzae. MoLKH1 is transcriptionally activated by H2O2 and required for H2O2-induced autophagic cell death and suppression of ROS-activated plant defense during plant invasion of M. oryzae. In addition, the Molkh1 mutant also showed several phenotypic defects, including delayed growth, abnormal conidiation, damaged cell wall integrity, impaired glycogen and lipid transport, reduced secretion of extracellular enzymes and effectors, and attenuated virulence of M. oryzae. Nuclear localization of MoLKH1 requires the nuclear localization sequence, Lammer motif, as well as the kinase active site and ATP-binding site in this protein. Site-directed mutagenesis showed that each of them plays crucial roles in fungal growth and pathogenicity of M. oryzae. In conclusion, our results demonstrate that MoLKH1-mediated cell cycle, autophagy, and suppression of plant immunity play crucial roles in development and pathogenicity of M. oryzae.

Ammonium enhances rice resistance to Magnaporthe oryzae through H2O2 accumulation

Nitrogen (N) is essential for the physiological processes of plants. However, the specific mechanisms by which different nitrogen forms influence rice blast pathogenesis remain poorly understood. This study used hydroponic assays to explore how ammonium (NH4+) and nitrate (NO3-) affect rice after inoculation with Magnaporthe oryzae (M. oryzae). The results showed that NH4+, compared to NO3-, significantly reduced disease severity, fungal growth, fungal hyphae number, the expansion capacity of infectious hyphae, and disease-related loss of photosynthesis. Additionally, NH4+ enhanced the expression of defense-related genes, including OsPBZ1, OsCHT1, OsPR1a, and OsPR10. NH4+-treated rice also exhibited higher hydrogen peroxide (H2O2) accumulation and increased antioxidant enzyme activities. Moreover, susceptibility to rice blast disease increased when H2O2 was scavenged, while a reduction in susceptibility was observed with the application of exogenous H2O2. These results suggest that ammonium enhances rice resistance to M. oryzae, potentially through H2O2 accumulation. The findings provide valuable insights into how different nitrogen forms affect plant immunity in rice, which is crucial for controlling rice blast and ensuring stable food production.

MoRgs3 functions in intracellular reactive oxygen species perception-integrated cAMP signaling to promote appressorium formation in Magnaporthe oryzae

Regulator of G-protein signaling (RGS) proteins exhibit GTPase-accelerating protein activities to govern G-protein function. In the rice blast fungus Magnaporthe oryzae, there is a family of at least eight RGS and RGS-like proteins (MoRgs1 to MoRgs8), each exhibiting distinct or shared functions in the growth, appressorium formation, and pathogenicity. MoRgs3 recently emerged as one of the crucial regulators that senses intracellular oxidation during appressorium formation. To explore this unique regulatory mechanism of MoRgs3, we identified the nucleoside diphosphate kinase MoNdk1 that interacts with MoRgs3. MoNdk1 phosphorylates MoRgs3 under induced intracellular reactive oxygen species levels, and MoRgs3 phosphorylation is required for appressorium formation and pathogenicity. In addition, we showed that MoRgs3 phosphorylation determines its interaction with MoCrn1, a coronin-like actin-binding protein homolog, which regulates MoRgs3 internalization. Finally, we provided evidence demonstrating that MoRgs3 functions in MoMagA-mediated cAMP signaling to regulate normal appressorium induction. By revealing a novel signal perception mechanism, our studies highlighted the complexity of regulation during the appressorium function and pathogenicity of the blast fungus.

IMPORTANCE

We report that MoRgs3 becomes phosphorylated in an oxidative intracellular environment during the appressorium formation stage. We found that this phosphorylation is carried out by MoNdk1, a nucleoside diphosphate kinase. In addition, this phosphorylation leads to a higher binding affinity between MoRgs3 and MoCrn1, a coronin-like actin-binding protein that was implicated in the endocytic transport of several other RGS proteins of Magnaporthe oryzae. We further found that the internalization of MoRgs3 is indispensable for its GTPase-activating protein function toward the Gα subunit MoMagA. Importantly, we characterized how such cellular regulatory events coincide with cAMP signaling-regulated appressorium formation and pathogenicity in the blast fungus. Our studies uncovered a novel intracellular reactive oxygen species signal-transducing mechanism in a model pathogenic fungus with important basic and applied implications.

A Rapid, Equipment-Free Method for Detecting Avirulence Genes of Pyricularia oryzae Using a Lateral Flow Strip-Based RPA Assay

Rice blast, caused by Pyricularia oryzae, is one of the most destructive rice diseases worldwide. Using resistant rice varieties is the most cost-effective way to control rice blast. Consequently, it is critical to monitor the distribution frequency of avirulence (Avr) genes in rice planting fields to facilitate the breeding of resistant rice varieties. In this study, we established a rapid recombinase polymerase amplification-lateral flow dipstick (RPA-LFD) detection system for the identification of AvrPik, Avr-Piz-t, and Avr-Pi9. The optimized reaction temperature and duration were 37°C and 20 min, indicating that the reaction system could be initiated by body temperature without relying on any precision instruments. Specificity analysis showed that the primer and probe combinations targeting the three Avr genes exhibited a remarkable specificity at genus-level detection. Under the optimized condition, the lower detected thresholds of AvrPik, Avr-Piz-t, and Avr-Pi9 were 10 fg/μl, 100 fg/μl, and 10 pg/μl, respectively. Notably, the detection sensitivity of the three Avr genes was much higher than that of PCR. In addition, we also successfully detected the presence of AvrPik, Avr-Piz-t, and Avr-Pi9 in the leaf and panicle blast lesions with the RPA-LFD detection system. In particular, the genomic DNA was extracted using the simpler PEG-NaOH rapid extraction method. In summary, we developed an RPA detection system for AvrPik, Avr-Pi9, and Avr-Piz-t, combined with the PEG-NaOH rapid DNA extraction method. The innovative approach achieved rapid, real-time, and accurate detection of the three Avr genes in the field, which is helpful to understand the distribution frequency of the three Avr genes in the field and provide theoretical reference for the scientific layout of resistant rice varieties.

Genome-wide association analysis uncovers rice blast resistance alleles of Ptr and Pia

A critical step to maximize the usefulness of genome-wide association studies (GWAS) in plant breeding is the identification and validation of candidate genes underlying genetic associations. This is of particular importance in disease resistance breeding where allelic variants of resistance genes often confer resistance to distinct populations, or races, of a pathogen. Here, we perform a genome-wide association analysis of rice blast resistance in 500 genetically diverse rice accessions. To facilitate candidate gene identification, we produce de-novo genome assemblies of ten rice accessions with various rice blast resistance associations. These genome assemblies facilitate the identification and functional validation of novel alleles of the rice blast resistance genes Ptr and Pia. We uncover an allelic series for the unusual Ptr rice blast resistance gene, and additional alleles of the Pia resistance genes RGA4 and RGA5. By linking these associations to three thousand rice genomes we provide a useful tool to inform future rice blast breeding efforts. Our work shows that GWAS in combination with whole-genome sequencing is a powerful tool for gene cloning and to facilitate selection of specific resistance alleles for plant breeding.

The CHY-type zinc finger protein MoChy1 is involved in polarized growth, conidiation, autophagy and pathogenicity of Magnaporthe oryzae

Polarized growth is critical for the development of filamentous phytopathogens, and the CHY-type zinc finger protein Chy1 regulates microtubule assembly to influence polarized growth and thereby affect plant infections. However, the biological role of a Chy1 homolog MoChy1 remains unknown in Magnaporthe oryzae. We found here that the MoChy1-GFP was distributed in the cytoplasm outside the vacuole in hyphae and localized mainly to the vacuole compartments as the appressorium matured. The Mochy1 mutants showed an extremely slow growth rate, curved and branched mycelium, reduced conidiation, and a smaller size in the appressorium. Meanwhile, the Mochy1 mutants showed increased sensitivity to benomyl, damaged microtubule cytoskeleton, and mislocalized polarisome protein MoSpa2 and chitin synthase MoChs6 in hyphae. Compared to Guy11, the Mochy1 mutants exhibited increased sensitivity to H2O2, impaired ability to eliminate host-derived ROS and reduced penetration into host plants, resulting in a strong reduction in pathogenicity of Mochy1 mutants. Furthermore, the Mochy1 mutants also exhibited defects in chitin distribution, osmotic stress tolerance, and septin ring organization during appressorium differentiation and fungal development. Nonselective autophagy was negatively regulated in Mochy1 mutants compared to Guy11. In summary, MoChy1 plays multiple roles in fungal polar growth and full virulence of M. oryzae.

First telomere-to-telomere gapless assembly of the rice blast fungus Pyricularia oryzae

Rice blast caused by Pyricularia oryzae (syn., Magnaporthe oryzae) was one of the most destructive diseases of rice throughout the world. Genome assembly was fundamental to genetic variation identification and critically impacted the understanding of its ability to overcome host resistance. Here, we report a gapless genome assembly of rice blast fungus P. oryzae strain P131 using PacBio, Illumina and high throughput chromatin conformation capture (Hi-C) sequencing data. This assembly contained seven complete chromosomes (43,237,743 bp) and a circular mitochondrial genome (34,866 bp). Approximately 14.31% of this assembly carried repeat sequences, significantly greater than its previous assembled version. This assembly had a 99.9% complement in BUSCO evaluation. A total of 14,982 genes protein-coding genes were predicted. In summary, we assembled the first telomere-to-telomere gapless genome of P. oryzae, which would be a valuable genome resource for future research on the genome evolution and host adaptation.

MoMkk1 and MoAtg1 dichotomously regulating autophagy and pathogenicity through MoAtg9 phosphorylation in Magnaporthe oryzae

Autophagy is a central biodegradation pathway critical in eliminating intracellular cargo to maintain cellular homeostasis and improve stress resistance. At the same time, the key component of the mitogen-activated protein kinase cascade regulating cell wall integrity signaling MoMkk1 has an essential role in the autophagy of the rice blast fungus Magnaporthe oryzae. Still, the mechanism of how MoMkk1 regulates autophagy is unclear. Interestingly, we found that MoMkk1 regulates the autophagy protein MoAtg9 through phosphorylation. MoAtg9 is a transmembrane protein subjected to phosphorylation by autophagy-related protein kinase MoAtg1. Here, we provide evidence demonstrating that MoMkk1-dependent MoAtg9 phosphorylation is required for phospholipid translocation during isolation membrane stages of autophagosome formation, an autophagic process essential for the development and pathogenicity of the fungus. In contrast, MoAtg1-dependent phosphorylation of MoAtg9 negatively regulates this process, also impacting growth and pathogenicity. Our studies are the first to demonstrate that MoAtg9 is subject to MoMkk1 regulation through protein phosphorylation and that MoMkk1 and MoAtg1 dichotomously regulate autophagy to underlie the growth and pathogenicity of M. oryzae.IMPORTANCEMagnaporthe oryzae utilizes multiple signaling pathways to promote colonization of host plants. MoMkk1, a cell wall integrity signaling kinase, plays an essential role in autophagy governed by a highly conserved autophagy kinase MoAtg1-mediated pathway. How MoMkk1 regulates autophagy in coordination with MoAtg1 remains elusive. Here, we provide evidence that MoMkk1 phosphorylates MoAtg9 to positively regulate phospholipid translocation during the isolation membrane or smaller membrane structures stage of autophagosome formation. This is in contrast to the negative regulation of MoAtg9 by MoAtg1 for the same process. Intriguingly, MoMkk1-mediated MoAtg9 phosphorylation enhances the fungal infection of rice, whereas MoAtg1-dependant MoAtg9 phosphorylation significantly attenuates it. Taken together, we revealed a novel mechanism of autophagy and virulence regulation by demonstrating the dichotomous functions of MoMkk1 and MoAtg1 in the regulation of fungal autophagy and pathogenicity.

Csn5 inhibits autophagy by regulating the ubiquitination of Atg6 and Tor to mediate the pathogenicity of Magnaporthe oryzae

Csn5 is subunit 5 of the COP9 signalosome (CSN), but the mechanism by which it strictly controls the pathogenicity of pathogenic fungi through autophagy remains unclear. Here, we found that Csn5 deficiency attenuated pathogenicity and enhanced autophagy in Magnaporthe oryzae. MoCSN5 knockout led to overubiquitination and overdegradation of MoTor (the core protein of the TORC1 complex [target of rapamycin]) thereby promoted autophagy. In addition, we identified MoCsn5 as a new interactor of MoAtg6. Atg6 was found to be ubiquitinated through linkage with lysine 48 (K48) in cells, which is necessary for infection-associated autophagy in pathogenic fungi. K48-ubiquitination of Atg6 enhanced its degradation and thereby inhibited autophagic activity. Our experimental results indicated that MoCsn5 promoted K48-ubiquitination of MoAtg6, which reduced the MoAtg6 protein content and thus inhibited autophagy. Aberrant ubiquitination and autophagy in ΔMocsn5 led to pleiotropic defects in the growth, development, stress resistance, and pathogenicity of M. oryzae. In summary, our study revealed a novel mechanism by which Csn5 regulates autophagy and pathogenicity in rice blast fungus through ubiquitination.

Targeting Magnaporthe oryzae effector MoErs1 and host papain-like protease OsRD21 interaction to combat rice blast

Effector proteins secreted by plant pathogenic fungi are important artilleries against host immunity, but there is no precedent of such effectors being explored as antifungal targets. Here we demonstrate that MoErs1, a species-specific effector protein secreted by the rice blast fungus Magnaporthe oryzae, inhibits the function of rice papain-like cysteine protease OsRD21 involved in rice immunity. Disrupting MoErs1-OsRD21 interaction effectively controls rice blast. In addition, we show that FY21001, a structure-function-based designer compound, specifically binds to and inhibits MoErs1 function. FY21001 significantly and effectively controls rice blast in field tests. Our study revealed a novel concept of targeting pathogen-specific effector proteins to prevent and manage crop diseases.

Pyricularia oryzae: Lab star and field scourge

Pyricularia oryzae (syn. Magnaporthe oryzae), is a filamentous ascomycete that causes a major disease called blast on cereal crops, as well as on a wide variety of wild and cultivated grasses. Blast diseases have a tremendous impact worldwide particularly on rice and on wheat, where the disease emerged in South America in the 1980s, before spreading to Asia and Africa. Its economic importance, coupled with its amenability to molecular and genetic manipulation, have inspired extensive research efforts aiming at understanding its biology and evolution. In the past 40 years, this plant-pathogenic fungus has emerged as a major model in molecular plant-microbe interactions. In this review, we focus on the clarification of the taxonomy and genetic structure of the species and its host range determinants. We also discuss recent molecular studies deciphering its lifecycle.

TAXONOMY

Kingdom: Fungi, phylum: Ascomycota, sub-phylum: Pezizomycotina, class: Sordariomycetes, order: Magnaporthales, family: Pyriculariaceae, genus: Pyricularia.

HOST RANGE

P. oryzae has the ability to infect a wide range of Poaceae. It is structured into different host-specialized lineages that are each associated with a few host plant genera. The fungus is best known to cause tremendous damage to rice crops, but it can also attack other economically important crops such as wheat, maize, barley, and finger millet.

DISEASE SYMPTOMS

P. oryzae can cause necrotic lesions or bleaching on all aerial parts of its host plants, including leaf blades, sheaths, and inflorescences (panicles, spikes, and seeds). Characteristic symptoms on leaves are diamond-shaped silver lesions that often have a brown margin and whose appearance is influenced by numerous factors such as the plant genotype and environmental conditions. USEFUL WEBSITES Resources URL Genomic data repositories http://genome.jouy.inra.fr/gemo/ Genomic data repositories http://openriceblast.org/ Genomic data repositories http://openwheatblast.net/ Genome browser for fungi (including P. oryzae) http://fungi.ensembl.org/index.html Comparative genomics database https://mycocosm.jgi.doe.gov/mycocosm/home T-DNA mutant database http://atmt.snu.kr/ T-DNA mutant database http://www.phi-base.org/ SNP and expression data https://fungidb.org/fungidb/app/.

Phenazine biosynthesis protein MoPhzF regulates appressorium formation and host infection through canonical metabolic and noncanonical signaling function in Magnaporthe oryzae

Microbe-produced secondary metabolite phenazine-1-carboxylic acid (PCA) facilitates pathogen virulence and defense mechanisms against competitors. Magnaporthe oryzae, a causal agent of the devastating rice blast disease, needs to compete with other phyllosphere microbes and overcome host immunity for successful colonization and infection. However, whether M. oryzae produces PCA or it has any other functions remains unknown. Here, we found that the MoPHZF gene encodes the phenazine biosynthesis protein MoPhzF, synthesizes PCA in M. oryzae, and regulates appressorium formation and host virulence. MoPhzF is likely acquired through an ancient horizontal gene transfer event and has a canonical function in PCA synthesis. In addition, we found that PCA has a role in suppressing the accumulation of host-derived reactive oxygen species (ROS) during infection. Further examination indicated that MoPhzF recruits both the endoplasmic reticulum membrane protein MoEmc2 and the regulator of G-protein signaling MoRgs1 to the plasma membrane (PM) for MoRgs1 phosphorylation, which is a critical regulatory mechanism in appressorium formation and pathogenicity. Collectively, our studies unveiled a canonical function of MoPhzF in PCA synthesis and a noncanonical signaling function in promoting appressorium formation and host infection.

Curd, seed yield and disease resistance of cauliflower are enhanced by oligosaccharides

Background

Oligosaccharides have been demonstrated as promoters for enhancing plant growth across several crops by elevating their secondary metabolites. However, the exploration of employing diverse oligosaccharides for qualitative trait improvements in cauliflower largely unknown. This study was intended to uncover the unexplored potential, evaluating the stimulatory effects of three oligosaccharides on cauliflower's curd and seed production.

Methods

Two experiments were initiated in the early (15 September) and mid-season (15 October). Four treatments were implemented, encompassing a control (water) alongside chitosan oligosaccharide (COS 50 mg.L-1) with a degree of polymerization (DP) 2-10, oligo galacturonic acid (OGA 50 mg.L-1) with DP 2-10 and alginate oligosaccharide (AOS 50 mg.L-1) with DP 2-7.

Results

Oligosaccharides accelerated plant height (4-17.6%), leaf number (17-43%), curd (5-14.55%), and seed yield (17.8-64.5%) in both early and mid-season compared to control. These enhancements were even more pronounced in the mid-season (7.6-17.6%, 21.37-43%, 7.27-14.55%, 25.89-64.5%) than in the early season. Additionally, three oligosaccharides demonstrated significant disease resistance against black rot in both seasons, outperforming the control. As a surprise, the early season experienced better growth parameters than the mid-season. However, performance patterns remained more or less consistent in both seasons under the same treatments. COS and OGA promoted plant biomass and curd yield by promoting Soil Plant Analysis Development (SPAD) value and phenol content. Meanwhile, AOS increased seed yield (56.8-64.5%) and elevated levels of chlorophyll, ascorbic acid, flavonoids, while decreasing levels of hydrogen per oxide (H2O2), malondialdehyde (MDA), half maximal inhibitory concentration (IC50), and disease index. The correlation matrix and principal component analysis (PCA) supported these relations and findings. Therefore, COS and OGA could be suggested for curd production and AOS for seed production in the early season, offering resistance to both biotic and abiotic stresses for cauliflower cultivation under field conditions.

The complete mitochondrial genome of the rice blast fungus Pyricularia oryzae Cavara 1892 strain Guy11 and phylogenetic analysis

The complete mitochondrial genome of Pyricularia oryzae Cavara 1892 strain Guy11 is 34,865 bp in length (GenBank accession number OP095391), containing 29 tRNA genes, 2 rRNA genes, and 15 protein-coding genes (PCGs). The gene order and orientation are novel compared to other Sordariomycetes species with sequenced mitogenomes in the GenBank database. Phylogenetic analysis suggests that P. oryzae Guy11 and 19 other Sordariomycetes species form a monophyletic group. The complete mitochondrial sequence of P. oryzae Guy11 will be a valuable resource for species identification, population genetics, phylogenetics, and comparative genomics studies in Sordariomycetes and Magnaporthales.

Oligosaccharides increased both leaf biomass and steviol glycosides content of Stevia rebaudiana

Steviol glycosides (SGs) are a variety of important natural sweeteners. They are 200-350 times sweeter than sucrose without calories. Currently, their production is still mainly dependent on extraction from Stevia rebaudiana Bertoni (stevia). Oligosaccharides are environmentally friendly elicitors that promote plant growth and accumulation of secondary metabolites. In the present study, different concentrations of chitosan oligosaccharides (COS) and alginate oligosaccharides (AOS) were applied to stevia to explore their effect on growth and SGs biosynthesis. It was found that both COS and AOS promoted biomass production by increasing the leaf number and photosynthetic efficiency, which may be related to the decreased content of abscisic acid. The content of SGs was significantly increased after 50 mg/L AOS treatment, which not only increased the contents of stevioside (STV) and rebaudioside A (Reb A) significantly, but some important minority glucosides, like Reb E, Reb D, and Reb M. The increased SGs contents were the combined effect of the higher expression of SGs biosynthesis related genes, including KAH, UGT74G1, UGT85C2, and UGT91D2. The geometry changes of stem induced by COS and AOS may help to increase the lodging resistance of stevia. Thus, COS and AOS can be used in the field planting of stevia to increase the yield of SGs for industrial purposes.

Methionine biosynthesis enzyme MoMet2 is required for rice blast fungus pathogenicity by promoting virulence gene expression via reducing 5mC modification

The emergence of fungicide resistance severely threatens crop production by limiting the availability and application of established fungicides. Therefore, it is urgent to identify new fungicidal targets for controlling plant diseases. Here, we characterized the function of a conserved homoserine O-acetyltransferase (HOA) from the rice blast fungus Magnaporthe oryzae that could serve as the candidate antifungal target. Deletion of the MoMET2 and MoCYS2 genes encoding HOAs perturbed the biosynthesis of methionine and S-adenyl methionine, a methyl group donor for epigenetic modifications, and severely attenuated the development and virulence of M. oryzae. The ∆Momet2 mutant is significantly increased in 5-methylcytosine (5mC) modification that represses the expression of genes required for pathogenicity, including MoGLIK and MoCDH-CYT. We further showed that host-induced gene silencing (HIGS) targeting MoMET2 and MoCYS2 effectively controls rice blasts. Our studies revealed the importance of HOA in the development and virulence of M. oryzae, which suggests the potential feasibility of HOA as new targets for novel anti-rice blast measurements.

A nonclassically secreted effector of Magnaporthe oryzae targets host nuclei and plays important roles in fungal growth and plant infection

Rice blast caused by Magnaporthe oryzae is one of the most destructive diseases and poses a growing threat to food security worldwide. Like many other filamentous pathogens, rice blast fungus releases multiple types of effector proteins to facilitate fungal infection and modulate host defence responses. However, most of the characterized effectors contain an N-terminal signal peptide. Here, we report the results of the functional characterization of a nonclassically secreted nuclear targeting effector in M. oryzae (MoNte1). MoNte1 has no signal peptide, but can be secreted and translocated into plant nuclei driven by a nuclear targeting peptide. It could also induce hypersensitive cell death when transiently expressed in Nicotiana benthamiana. Deletion of the MoNTE1 gene caused a significant reduction of fungal growth and conidiogenesis, partially impaired appressorium formation and host colonization, and also dramatically attenuated the pathogenicity. Taken together, these findings reveal a novel effector secretion pathway and deepen our understanding of rice-M. oryzae interactions.

Interaction of calcium responsive proteins and transcriptional factors with the PHO regulon in yeasts and fungi

Phosphate and calcium ions are nutrients that play key roles in growth, differentiation and the production of bioactive secondary metabolites in filamentous fungi. Phosphate concentration regulates the biosynthesis of hundreds of fungal metabolites. The central mechanisms of phosphate transport and regulation, mediated by the master Pho4 transcriptional factor are known, but many aspects of the control of gene expression need further research. High ATP concentration in the cells leads to inositol pyrophosphate molecules formation, such as IP3 and IP7, that act as phosphorylation status reporters. Calcium ions are intracellular messengers in eukaryotic organisms and calcium homeostasis follows elaborated patterns in response to different nutritional and environmental factors, including cross-talking with phosphate concentrations. A large part of the intracellular calcium is stored in vacuoles and other organelles forming complexes with polyphosphate. The free cytosolic calcium concentration is maintained by transport from the external medium or by release from the store organelles through calcium permeable transient receptor potential (TRP) ion channels. Calcium ions, particularly the free cytosolic calcium levels, control the biosynthesis of fungal metabolites by two mechanisms, 1) direct interaction of calcium-bound calmodulin with antibiotic synthesizing enzymes, and 2) by the calmodulin-calcineurin signaling cascade. Control of very different secondary metabolites, including pathogenicity determinants, are mediated by calcium through the Crz1 factor. Several interactions between calcium homeostasis and phosphate have been demonstrated in the last decade: 1) The inositol pyrophosphate IP3 triggers the release of calcium ions from internal stores into the cytosol, 2) Expression of the high affinity phosphate transporter Pho89, a Na+/phosphate symporter, is controlled by Crz1. Also, mutants defective in the calcium permeable TRPCa7-like of Saccharomyces cerevisiae shown impaired expression of Pho89. This information suggests that CrzA and Pho89 play key roles in the interaction of phosphate and calcium regulatory pathways, 3) Finally, acidocalcisomes organelles have been found in mycorrhiza and in some melanin producing fungi that show similar characteristics as protozoa calcisomes. In these organelles there is a close interaction between orthophosphate, pyrophosphate and polyphosphate and calcium ions that are absorbed in the polyanionic polyphosphate matrix. These advances open new perspectives for the control of fungal metabolism.

Characteristics of the Fecal Microbiome of Piglets with Diarrhea Identified Using Shotgun Metagenomics Sequencing

Diarrhea in piglets is one of the most common diseases leading to high mortality and, as a result, to economic losses. Shotgun metagenomic sequencing was performed on the DNBSEQ-G50, MGI system to study the role of the fecal microbiome in the development of diarrhea in newborn piglets. Analysis of the study data showed that the composition of the fecal microbiome at the level of bacteria and fungi did not differ in piglets with diarrhea from the healthy group. Bacteria belonging to the phyla Firmicutes, Bacteroidetes, Proteobacteria, Actinobacteria, and Fusobacteria were the most abundant. However, a higher level of bacterial alpha diversity was observed in the group of piglets with diarrhea, which may be due to dysbacteriosis and inflammation. The study of the virome showed the difference between the two types of phages: Bacteroides B40-8 prevailed in diseased piglets, while Escherichia virus BP4 was found in greater numbers in healthy piglets. The results of our study suggest that the association between the fecal microbiome and susceptibility to diarrhea in suckling piglets may have been previously overestimated.

Overexpression of a beta-1,6-glucanase gene GluM in transgenic rice confers high resistance to rice blast, sheath blight and false smut

BACKGROUND

Frequent fungal diseases tend to lead to severe losses in rice production. As a main component of the fungal cell wall, glucan plays an important role in the growth and development of fungi. Glucanase can inhibit the growth of fungi by breaking glycosidic bonds, and may be a promising target for developing rice varieties with broad-spectrum disease resistance.

RESULTS

We transferred a codon-optimized β-1,6-glucanase gene (GluM) from myxobacteria into the japonica rice variety Zhonghua11 (ZH11), and obtained a large number of individual transgenic plants with GluM overexpression. Based on molecular analysis, three single-copy homozygous lines with GluM overexpression were selected for assessment of fungal disease resistance at the T₃ generation. Compared with that of the recipient cultivar ZH11, the area of rice blast lesion in transgenic rice was reduced by 82.71%; that of sheath blight lesion was decreased by 35.76%-43.67%; the sheath blight resistance in the field was enhanced by an average of 0.75 grade over 3 years; and the incidence of diseased panicles due to rice false smut was decreased by 65.79%. More importantly, there was no obvious loss of yield (without a significant effect on agronomic traits). Furthermore, plants overexpressing a β-1,6-glucanase gene showed higher disease resistance than rice plants overexpressing a β-1,3-glucanase gene derived from tobacco.

CONCLUSION

The β-1,6-glucanase gene GluM can confer broad-spectrum disease resistance to rice, providing an environmentally friendly alternative way to effectively manage fungal pathogens in rice production. © 2023 Society of Chemical Industry.

Antifungal mechanisms of the antagonistic bacterium Bacillus mojavensis UTF-33 and its potential as a new biopesticide

Biological control has gradually become the dominant means of controlling fungal disease over recent years. In this study, an endophytic strain of UTF-33 was isolated from acid mold (Rumex acetosa L.) leaves. Based on 16S rDNA gene sequence comparison, and biochemical and physiological characteristics, this strain was formally identified as Bacillus mojavensis. Bacillus mojavensis UTF-33 was sensitive to most of the antibiotics tested except neomycin. Moreover, the filtrate fermentation solution of Bacillus mojavensis UTF-33 had a significant inhibitory effect on the growth of rice blast and was used in field evaluation tests, which reduced the infestation of rice blast effectively. Rice treated with filtrate fermentation broth exhibited multiple defense mechanisms in response, including the enhanced expression of disease process-related genes and transcription factor genes, and significantly upregulated the gene expression of titin, salicylic acid pathway-related genes, and H₂O₂ accumulation, in plants; this may directly or indirectly act as an antagonist to pathogenic infestation. Further analysis revealed that the n-butanol crude extract of Bacillus mojavensis UTF-33 could retard or even inhibit conidial germination and prevent the formation of adherent cells both in vitro and in vivo. In addition, the amplification of functional genes for biocontrol using specific primers showed that Bacillus mojavensis UTF-33 expresses genes that can direct the synthesis of bioA, bmyB, fenB, ituD, srfAA and other substances; this information can help us to determine the extraction direction and purification method for inhibitory substances at a later stage. In conclusion, this is the first study to identify Bacillus mojavensis as a potential agent for the control of rice diseases; this strain, and its bioactive substances, have the potential to be developed as biopesticides.

Hydrophobic cue-induced appressorium formation depends on MoSep1-mediated MoRgs7 phosphorylation and internalization in Magnaporthe oryzae

The rice blast fungus Magnaporthe oryzae forms specialized infectious structures called appressoria that breach host cells to initiate infection. Previous studies demonstrated that the regulator of G-protein signaling (RGS)-like protein MoRgs7 undergoes endocytosis upon fungal sensing of hydrophobic environmental cues to activate cAMP signaling required for appressorium formation. However, the mechanism by which MoRgs7 internalizes and its fate remains undetermined. We here show that MoSep1, a conserved protein kinase of Mitotic Exit Network (MEN), phosphorylates MoRgs7 to regulate its function. MoRgs7 phosphorylation determines its interaction with MoCrn1, a coronin-like actin-binding protein homolog that also modulates the internalization of MoRgs7. Importantly, the endocytic transport of MoRgs7 is critical for its GTPase-activating protein (GAP) function important in cAMP signaling. Together, our findings revealed a novel mechanism by which M. oryzae activates MoRgs7-mediated hydrophobic cue-sensing signal transduction involving protein phosphorylation and endocytic transport to govern appressorium formation and fungal pathogenicity.

A high-quality reference genome for the fission yeast Schizosaccharomyces osmophilus

Fission yeasts are an ancient group of fungal species that diverged from each other from tens to hundreds of million years ago. Among them is the preeminent model organism Schizosaccharomyces pombe, which has significantly contributed to our understandings of molecular mechanisms underlying fundamental cellular processes. The availability of the genomes of S. pombe and 3 other fission yeast species S. japonicus, S. octosporus, and S. cryophilus has enabled cross-species comparisons that provide insights into the evolution of genes, pathways, and genomes. Here, we performed genome sequencing on the type strain of the recently identified fission yeast species S. osmophilus and obtained a complete mitochondrial genome and a nuclear genome assembly with gaps only at rRNA gene arrays. A total of 5,098 protein-coding nuclear genes were annotated and orthologs for more than 95% of them were identified. Genome-based phylogenetic analysis showed that S. osmophilus is most closely related to S. octosporus and these 2 species diverged around 16 million years ago. To demonstrate the utility of this S. osmophilus reference genome, we conducted cross-species comparative analyses of centromeres, telomeres, transposons, the mating-type region, Cbp1 family proteins, and mitochondrial genomes. These analyses revealed conservation of repeat arrangements and sequence motifs in centromere cores, identified telomeric sequences composed of 2 types of repeats, delineated relationships among Tf1/sushi group retrotransposons, characterized the evolutionary origins and trajectories of Cbp1 family domesticated transposases, and discovered signs of interspecific transfer of 2 types of mitochondrial selfish elements.

MoErv14 mediates the intracellular transport of cell membrane receptors to govern the appressorial formation and pathogenicity of Magnaporthe oryzae

Magnaporthe oryzae causes rice blasts posing serious threats to food security worldwide. During infection, M. oryzae utilizes several transmembrane receptor proteins that sense cell surface cues to induce highly specialized infectious structures called appressoria. However, little is known about the mechanisms of intracellular receptor tracking and their function. Here, we described that disrupting the coat protein complex II (COPII) cargo protein MoErv14 severely affects appressorium formation and pathogenicity as the ΔMoerv14 mutant is defective not only in cAMP production but also in the phosphorylation of the mitogen-activated protein kinase (MAPK) MoPmk1. Studies also showed that either externally supplementing cAMP or maintaining MoPmk1 phosphorylation suppresses the observed defects in the ΔMoerv14 strain. Importantly, MoErv14 is found to regulate the transport of MoPth11, a membrane receptor functioning upstream of G-protein/cAMP signaling, and MoWish and MoSho1 function upstream of the Pmk1-MAPK pathway. In summary, our studies elucidate the mechanism by which the COPII protein MoErv14 plays an important function in regulating the transport of receptors involved in the appressorium formation and virulence of the blast fungus.

Knockout of amino acid transporter gene OsLHT1 accelerates leaf senescence and enhances resistance to rice blast fungus

Plant amino acid transporters (AATs) regulate not only long-distance transport and reallocation of nitrogen (N) from source to sink organs, but also amount of amino acids in leaves hijacked by invaded pathogens. However, the function of AATs in plant defense responses to pathogen infection remains unknown. In this study, we found that rice amino acid transporter gene OsLHT1 was expressed in leaves and up-regulated by maturing, N starvation and inoculation of blast fungus Magnaporthe oryzae. Knockout of OsLHT1 resulted in development stage- and N supply-dependent premature senescence of leaves at vegetative growth stage. In comparison to wild type, Oslht1 mutant lines showed sustained rusty red spots on fully mature leaf blades irrespective of N supply levels. Notably, no relationship between the severity of leaf rusty red spots and concentration of total N or amino acids was found in Oslht1 mutants at different developmental stages. Disruption of OsLHT1 altered transport and metabolism of amino acids and biosynthesis of flavones and flavonoids, enhanced expression of jasmonic acid- and salicylic acid-related defense genes and production of jasmonic acid and salicylic acid, accumulation of reactive oxygen species. OsLHT1 inactivation dramatically prevented the leaf invasion of M. oryzae, the hemi-biotrophic ascomycete fungus. Overall, these results establish a module connecting the activity of amino acid transporter with leaf metabolism and defense to rice blast fungus.

In-silico prediction of highly promising natural fungicides against the destructive blast fungus Magnaporthe oryzae

Magnaporthe oryzae causes destructive blast disease in more than 50 species of the major cereal crops rice, wheat and maize and destroys food of millions of people worldwide. Application of synthetic chemical fungicides are environmentally hazardous and unreliable in controlling M. oryzae. Conversely, naturally occurring biofungicides with multiple modes of actions are needed to be discovered for combatting the blast fungus. To find the effective biofungicides, we performed molecular docking study of some potential antifungal natural compounds targeting two proteins including a single-stranded DNA binding protein MoSub1 (4AGH), and an effector protein AVR-Pik (5E9G) of M. oryzae that regulates transcription in fungus and/or suppresses the host cell immunity. The thirty-nine natural compounds previously shown to inhibit M. oryzae growth and reproduction were put under molecular docking against these two proteins followed by simulation, free energy, and interaction analysis of protein-ligand complexes. The virtual screening revealed that two alkaloidal metabolites, camptothecin and GKK1032A2 showed excellent binding energy with any of these target proteins compared to reference commercial fungicides, azoxystrobin and strobilurin. Of the detected compounds, GKK1032A2 bound to both target proteins of M. oryzae. Both compounds showed excellent bioactivity scores as compared to the reference fungicides. Results of our computational biological study suggest that both camptothecin and GKK1032A2 are potential fungicides that could also be considered as lead compounds to design novel fungicides against the blast fungus. Furthermore, the GKK1032A2 acted as a multi-site mode of action fungicide against M. oryzae.

MoMih1 is indispensable for asexual development, cell wall integrity, and pathogenicity of Magnaporthe oryzae

Asexual spore serves as essential inoculum of rice blast during the disease cycle, and differentiation of young conidium from conidiophore is intimately regulated by cell cycle. Mih1 encodes a dual-specificity phosphatase that involved in the G2/M transition of the mitotic cell cycle by regulating the Cdk1 activity in eukaryotes. Till now, the roles of Mih1 homologue, however, remain unclear in Magnaporthe oryzae. We here functionally characterized the Mih1 homologue MoMih1 in M. oryzae. MoMih1 is localized to both the cytoplasm and nucleus and can physically interact with the CDK protein MoCdc28 in vivo. Loss of MoMih1 led to delayed nucleus division and a high level of Tyr15 phosphorylation of MoCdc28. The MoMih1 mutants showed retarded mycelial growth with a defective polar growth, less fungal biomass, and shorter distance between diaphragms, compared with the KU80. Asexual reproduction altered in MoMih1 mutants, with both abnormal conidial morphogenesis and decreased conidiation. The MoMih1 mutants severely attenuated the virulence to host plants due to the impaired ability of penetration and biotrophic growth. The incapability of scavenging of host-derived reactive oxygen species, which was possibly ascribed to the severely decreased extracellular enzymes activities, were partially associated with deficiency of pathogenicity. Besides, the MoMih1 mutants displayed also improper localization of retromer protein MoVps26 and polarisome component MoSpa2, and defects of cell wall integrity (CWI), melanin pigmentation, chitin synthesis, and hydrophobicity. In conclusion, our results demonstrate that MoMih1 plays pleiotropic roles during fungal development and plant infection of M. oryzae.

Screening of Candidate Effectors from Magnaporthe oryzae by In Vitro Secretomic Analysis

Magnaporthe oryzae is the causal agent of rice blast, one of the most serious diseases of rice worldwide. Secreted proteins play essential roles during a M. oryzae-rice interaction. Although much progress has been made in recent decades, it is still necessary to systematically explore M. oryzae-secreted proteins and to analyze their functions. This study employs a shotgun-based proteomic analysis to investigate the in vitro secretome of M. oryzae by spraying fungus conidia onto the PVDF membrane to mimic the early stages of infection, during which 3315 non-redundant secreted proteins were identified. Among these proteins, 9.6% (319) and 24.7% (818) are classified as classically or non-classically secreted proteins, while the remaining 1988 proteins (60.0%) are secreted through currently unknown secretory pathway. Functional characteristics analysis show that 257 (7.8%) and 90 (2.7%) secreted proteins are annotated as CAZymes and candidate effectors, respectively. Eighteen candidate effectors are selected for further experimental validation. All 18 genes encoding candidate effectors are significantly up- or down-regulated during the early infection process. Sixteen of the eighteen candidate effectors cause the suppression of BAX-mediated cell death in Nicotiana benthamiana by using an Agrobacterium-mediated transient expression assay, suggesting their involvement in pathogenicity related to secretion effectors. Our results provide high-quality experimental secretome data of M. oryzae and will expand our knowledge on the molecular mechanisms of M. oryzae pathogenesis.

MoMaf1 Mediates Vegetative Growth, Conidiogenesis, and Pathogenicity in the Rice Blast Fungus Magnaporthe oryzae

In eukaryotes, Maf1 is an essential and specific negative regulator of RNA polymerase (Pol) III. Pol III, which synthesizes 5S RNA and transfer RNAs (tRNAs), is suppressed by Maf1 under the conditions of nutrient starvation or environmental stress. Here, we identified M. oryzae MoMaf1, a homolog of ScMaf1 in budding yeast. A heterogeneous complementation assay revealed that MoMaf1 restored growth defects in the ΔScmaf1 mutant under SDS stress. Destruction of MoMAF1 elevated 5S rRNA content and increased sensitivity to cell wall agents. Moreover, the ΔMomaf1 mutant exhibited reduced vegetative growth, conidiogenesis, and pathogenicity. Interestingly, we found that MoMaf1 underwent nuclear-cytoplasmic shuffling, through which MoMaf1 accumulated in nuclei under nutrient deficiency or upon the interaction of M. oryzae with rice. Therefore, this study can help to elucidate the pathogenic molecular mechanism of M. oryzae.

A Subunit of the COP9 Signalosome, MoCsn6, Is Involved in Fungal Development, Pathogenicity, and Autophagy in Rice Blast Fungus

The COP9 signalosome (CSN) is a highly conserved protein complex in eukaryotes, affecting various development and signaling processes. To date, the biological functions of the COP9 signalosome and its subunits have not been determined in Magnaporthe oryzae. In this study, we characterized the CSN in M. oryzae (which we named MoCsn6) and analyzed its biological functions. MoCsn6 is involved in fungal development, autophagy, and plant pathogenicity. Compared with the wild-type strain 70-15, ΔMocsn6 mutants showed a significantly reduced growth rate, sporulation rate, and germ tube germination rate. Pathogenicity assays showed that the ΔMocsn6 mutants did not cause or significantly reduced the number of disease spots on isolated barley leaves. After the MoCSN6 gene was complemented into the ΔMocsn6 mutant, vegetative growth, sporulation, and pathogenicity were restored. The Osm1 and Pmk1 phosphorylation pathways were also disrupted in the ΔMocsn6 mutants. Furthermore, we found that MoCsn6 participates in the autophagy pathway by interacting with the autophagy core protein MoAtg6 and regulating its ubiquitination level. Deletion of MoCSN6 resulted in rapid lipidation of MoAtg8 and degradation of the autophagic marker protein green fluorescent protein-tagged MoAtg8 under nutrient and starvation conditions, suggesting that MoCsn6 negatively regulates autophagic activity. Taken together, our results demonstrate that MoCsn6 plays a crucial role in regulating fungal development, pathogenicity, and autophagy in M. oryzae. IMPORTANCE Magnaporthe oryzae, a filamentous fungus, is the cause of many cereal diseases. Autophagy is involved in fungal development and pathogenicity. The COP9 signalosome (CSN) has been extensively studied in ubiquitin pathways, but its regulation of autophagy has rarely been reported in plant-pathogenic fungi. Investigations on the relationship between CSN and autophagy will deepen our understanding of the pathogenic mechanism of M. oryzae and provide new insights into the development of new drug targets to control fungal diseases. In this study, the important function of Csn6 in the autophagy regulation pathway and its impact on the pathogenicity of M. oryzae were determined. We showed that Csn6 manages autophagy by interacting with the autophagy core protein Atg6 and regulating its ubiquitination level. Furthermore, future investigations that explore the function of CSN will deepen our understanding of autophagy mechanisms in rice blast fungus.

Understanding the Rice Fungal Pathogen Tilletia horrida from Multiple Perspectives

Rice kernel smut (RKS), caused by the fungus Tilletia horrida, has become a major disease in rice-growing areas worldwide, especially since the widespread cultivation of high-yielding hybrid rice varieties. The disease causes a significant yield loss during the production of rice male sterile lines by producing masses of dark powdery teliospores. This review mainly summarizes the pathogenic differentiation, disease cycle, and infection process of the T. horrida, as well as the decoding of the T. horrida genome, functional genomics, and effector identification. We highlight the identification and characterization of virulence-related pathways and effectors of T. horrida, which could foster a better understanding of the rice-T. horrida interaction and help to elucidate its pathogenicity molecular mechanisms. The multiple effective disease control methods for RKS are also discussed, included chemical fungicides, the mining of resistant rice germplasms/genes, and the monitoring and early warning signs of this disease in field settings.

Surveillance of Rice Blast Resistance Effectiveness and Emerging Virulent Isolates in Taiwan

Rice blast caused by Magnaporthe oryzae is a dangerous threat to rice production and food security worldwide. Breeding and proper deployment of resistant varieties are effective and environmentally friendly strategies to manage this notorious disease. However, a highly dynamic and quickly evolved rice blast pathogen population in the field has made disease control with resistance germplasms more challenging. Therefore, continued monitoring of pathogen dynamics and application of effective resistance varieties are critical tasks to prolong or sustain field resistance. Here, we report a team project that involved evaluation of rice blast resistance genes and surveillance of M. oryzae field populations in Taiwan. A set of International Rice Research Institute-bred blast-resistant lines (IRBLs) carrying single blast resistance genes was utilized to monitor the field effectiveness of rice blast resistance. Resistance genes such as Ptr (formerly Pita2) and Pi9 exhibited the best and most durable resistance against the rice blast fungus population in Taiwan. Interestingly, line IRBLb-B harboring the Pib gene with good field protection has recently shown susceptible lesions in some locations. To dissect the genotypic features of virulent isolates against the Pib resistance gene, M. oryzae isolates were collected and analyzed. Screening of the AvrPib locus revealed that the majority of field isolates still maintained the wild-type AvrPib status but eight virulent genotypes were found. Pot3 insertion appeared to be a major way to disrupt the AvrPib avirulence function. Interestingly, a novel AvrPib double-allele genotype among virulent isolates was first identified. Pot2 repetitive element-based polymerase chain reaction (rep-PCR) fingerprinting analysis indicated that mutation events may occur independently among different lineages in different geographic locations of Taiwan. This study provides our surveillance experience of rice blast disease and serves as the foundation to sustain rice production.

Highly potent natural fungicides identified in silico against the cereal killer fungus Magnaporthe oryzae

Magnaporthe oryzae is one of the most notorious fungal pathogens that causes blast disease in cereals, and results in enormous loss of grain production. Many chemical fungicides are being used to control the pathogen but none of them are fully effective in controlling blast disease. Therefore, there is a demand for the discovery of a new natural biofungicide to manage the blast disease efficiently. A large number of new natural products showed inhibitory activities against M. oryzae in vitro. To find out effective biofungicides, we performed in silico molecular docking analysis of some of the potent natural compounds targeting four enzymes namely, scytalone dehydratase, SDH1 (PDB ID:1STD), trihydroxynaphthalene reductase, 3HNR (PDB ID:1YBV), trehalose-6-phosphate synthase, Tps1 (PDB ID:6JBI) and isocitrate lyase, ICL1 (PDB ID:5E9G) of M. oryzae fungus that regulate melanin biosynthesis and/or appresorium formation. Thirty-nine natural compounds that were previously reported to inhibit the growth of M. oryzae were subjected to rigid and flexible molecular docking against aforementioned enzymes followed by molecular dynamic simulation. The results of virtual screening showed that out of 39, eight compounds showed good binding energy with any one of the target enzymes as compared to reference commercial fungicides, azoxystrobin and strobilurin. Among the compounds, camptothecin, GKK1032A2 and chaetoviridin-A bind with more than one target enzymes of M. oryzae. All of the compounds except tricyclazole showed good bioactivity score. Taken together, our results suggest that all of the eight compounds have the potential to develop new fungicides, and remarkably, camptothecin, GKK1032A2 and chaetoviridin-A could act as multi-site mode of action fungicides against the blast fungus M. oryzae.

Functional characterization of MoSdhB in conferring resistance to pydiflumetofen in blast fungus Magnaporthe oryzae

BACKGROUND

Rice (Oryza sativa) is an important cereal crop around the world, and has constantly been threaten by the most destructive fungus Magnaporthe oryzae. Pydiflumetofen, a novel succinate dehydrogenase inhibitor (SDHI), is currently being used for controlling various fungal diseases. However, the potential resistance risk of M. oryzae to pydiflumetofen has remained unclear to date, and finding the resistance mechanism is critical for the usage of this fungicide.

RESULTS

The M. oryzae strain Guy11 is sensitive to pydiflumetofen, with EC₅₀ value of 1.24 μg mL^-1 . 58 pydiflumetofen-resistant (PR) mutants were obtained through pydiflumetofen-induced spontaneous mutation, with a mean EC₅₀ value >500 μg mL ^-1 , and the resistance factor (RF) >400. The PR mutants displayed positive cross-resistance to carboxin, but were more sensitive to fluopyram. Sequencing analysis showed that all PR mutants presented a cytosine-to-thymine transition at nucleotide position +1218, resulting in a replacement of histidine 245 by tyrosine (H245Y) on MoSdhB. The mutation of MoSdhB exhibited strong resistant phenotype, but no detectable growth deficits in fungal development, including vegetative growth and pathogenicity of M. oryzae. An allele-specific PCR for rapid detection of the H245Y mutants was established in M. oryzae.

CONCLUSION

The M. oryzae is sensitive to pydiflumetofen, and shows a medium to high resistance risk to pydiflumetofen. A point mutation of MoSdhB (H245Y) is responsible for the fungal resistance to pydiflumetofen in M. oryzae. © 2022 Society of Chemical Industry.

Distinctive phosphorylation pattern during mitotic exit network (MEN) regulation is important for the development and pathogenicity of Magnaporthe oryzae

The mitotic exit network (MEN) pathway is a vital kinase cascade regulating the timely and correct progress of cell division. In the rice blast fungus Magnaporthe oryzae, the MEN pathway, consisting of conserved protein kinases MoSep1 and MoMob1-MoDbf2, is important in the development and pathogenicity of the fungus. We found that deletion of MoSEP1 affects the phosphorylation of MoMob1, but not MoDbf2, in contrast to what was found in the buddy yeast Saccharomyces cerevisiae, and verified this finding by in vitro phosphorylation assay and mass spectrometry (MS) analysis. We also found that S43 residue is the critical phosphor-site of MoMob1 by MoSep1, and proved that MoSep1-dependent MoMob1 phosphorylation is essential for cell division during the development of M. oryzae. We further provided evidence demonstrating that MoSep1 phosphorylates MoMob1 to maintain the cell cycle during vegetative growth and infection. Taken together, our results revealed that the MEN pathway has both distinct and conservative functions in regulating the cell cycle during the development and pathogenesis of M. oryzae.

Wheat blast: A review from a genetic and genomic perspective

The newly emerged wheat blast fungus Magnaporthe oryzae Triticum (MoT) is a severe threat to global wheat production. The fungus is a distinct, exceptionally diverse lineage of the M. oryzae, causing rice blast disease. Genome-based approaches employing MoT-specific markers are used to detect MoT field isolates. Sequencing the whole genome indicates the presence of core chromosome and mini-chromosome sequences that harbor effector genes and undergo divergent evolutionary routes. Significant genetic and pathotype diversity within the fungus population gives ample potential for evolutionary change. Identifying and refining genetic markers allows for tracking genomic regions with stable blast resistance. Introgression of quantitative and R gene resistance into popular cultivars is crucial to controlling disease in areas where the pathogen population is diverse and well established. Novel approaches such as CRISPR/Cas-9 genome editing could generate resistant varieties in wheat within a short time. This chapter provides an extensive summary of the genetic and genomic aspects of the wheat blast fungus MoT and offers an essential resource for wheat blast research in the affected areas.

De novo purine nucleotide biosynthesis mediated by MoAde4 is required for conidiation, host colonization and pathogenicity in Magnaporthe oryzae

Amidophosphoribosyltransferase catalyzes the conversion of 5-phosphoribosyl-1-pyrophosphate into 5-phosphoribosyl-1-amine in the de novo purine biosynthetic pathway. Herein, we identified and characterized the functions of MoAde4, an orthologue of yeast Ade4 in Magnaporthe oryzae. MoAde4 is a 537-amino acid protein containing GATase_6 and pribosyltran domains. MoADE4 transcripts were highly expressed during the conidiation, early-infection, and late-infection stages of the fungus. Disruption of the MoADE4 gene resulted in ΔMoade4 exhibiting adenine, adenosine, and hypoxanthine auxotrophy on minimal medium. Conidia quantification assays showed that sporulation was significantly reduced in the ΔMoade4 mutant. The conidia of ΔMoade4 could still form appressoria but mostly failed to penetrate the rice cuticle. Pathogenicity tests showed that ΔMoade4 was completely nonpathogenic on rice and barley leaves, which was attributed to restricted infectious hyphal growth within the primary cells. The ΔMoade4 mutant was defective in the induction of strong host immunity. Exogenous adenine partially rescued conidiation, infectious hyphal growth, and the pathogenicity defects of the ΔMoade4 mutant on barley and rice leaves. Taken together, our results demonstrated that purine nucleotide biosynthesis orchestrated by MoAde4 is required for fungal development and pathogenicity in M. oryzae. These findings therefore act as a suitable target for antifungal development against recalcitrant plant fungal pathogens. KEY POINTS: • MoAde4 is crucial for de novo purine nucleotide biosynthesis. • MoAde4 is pivotal for conidiogenesis and appressorium development of M. oryzae. • MoAde4 is involoved in the pathogenicity of M. oryzae.

Homeostasis of cell wall integrity pathway phosphorylation is required for the growth and pathogenicity of Magnaporthe oryzae

The cell wall provides a crucial barrier to stress imposed by the external environment. In the rice blast fungus Magnaporthe oryzae, this stress response is mediated by the cell wall integrity (CWI) pathway, consisting of a well-characterized protein phosphorylation cascade. However, other regulators that maintain CWI phosphorylation homeostasis, such as protein phosphatases (PPases), remain unclear. Here, we identified two PPases, MoPtc1 and MoPtc2, that function as negative regulators of the CWI pathway. MoPtc1 and MoPtc2 interact with MoMkk1, one of the key components of the CWI pathway, and are crucial for the vegetative growth, conidial formation, and virulence of M. oryzae. We also demonstrate that both MoPtc1 and MoPtc2 dephosphorylate MoMkk1 in vivo and in vitro, and that CWI stress leads to enhanced interaction between MoPtc1 and MoMkk1. CWI stress abolishes the interaction between MoPtc2 and MoMkk1, providing a means of deactivation for CWI signalling. Our studies reveal that CWI signalling in M. oryzae is a highly coordinated regulatory mechanism vital for stress response and pathogenicity.

MoIug4 is a novel secreted effector promoting rice blast by counteracting host OsAHL1-regulated ethylene gene transcription

Magnaporthe oryzae secretes several effectors that modulate and hijack rice processes to colonize host cells, but the underlying mechanisms remain unclear. We report on a novel cytoplasmic effector MoIug4 that targets the rice ethylene pathway as a transcription repressor to subvert host immunity. We found that MoIug4 binds to the promoter of the host OsEIN2 gene that encodes a central signal transducer in the ethylene-signaling pathway. We also identified a MoIug4 interacting protein, OsAHL1, which acts as an AT-hook motif-containing protein binding to the A/T-rich promoter regions. Our knockout and overexpression studies showed that OsAHL1 positively regulates plant immunity in response to M. oryzae infection. OsAHL1 exhibits transcriptional regulatory activities by binding the OsEIN2 promoter region, similar to MoIug4. Intriguingly, we found that MoIug4 exhibits a higher binding affinity than OsAHL1 to the OsEIN2 promoter, suggesting differential regulatory specificities. These results revealed a counter-defense strategy by which the pathogen effector suppresses the activation of host defense genes by interfering with host transcription activator functions.

Regulation of photosynthetic light reaction proteins via reversible phosphorylation

The regulation of photosynthesis occurs at different levels including the control of nuclear and plastid genes transcription, RNA processing and translation, protein translocation, assemblies and their post translational modifications. Out of all these, post translational modification enables rapid response of plants towards changing environmental conditions. Among all post-translational modifications, reversible phosphorylation is known to play a crucial role in the regulation of light reaction of photosynthesis. Although, phosphorylation of PS II subunits has been extensively studied but not much attention is given to other photosynthetic complexes such as PS I, Cytochrome b6f complex and ATP synthase. Phosphorylation reaction is known to protect photosynthetic apparatus in challenging environment conditions such as high light, elevated temperature, high salinity and drought. Recent studies have explored the role of photosynthetic protein phosphorylation in conferring plant immunity against the rice blast disease. The evolution of phosphorylation of different subunits of photosynthetic proteins occurred along with the evolution of plant lineage for their better adaptation to the changing environment conditions. In this review, we summarize the progress made in the research field of phosphorylation of photosynthetic proteins and highlights the missing links that need immediate attention.

Proteomic Analysis Reveals Salicylic Acid as a Pivotal Signal Molecule in Rice Response to Blast Disease Infection

Rice blast disease caused by a fungus, Magnaporthe grisea, is one of the most destructive diseases in rice production worldwide, and salicylic acid (SA) can efficiently decrease the damage of M. grisea. Here, we combined the 2-Dimensional-Liquid Chromatography and the Matrix-assisted laser desorption/ionization time-of-flight mass spectrometer (2D-LC-MALDI-TOF-TOF MS) techniques to compare and identify differentially expressed labelled proteins by the isobaric tags for relative and absolute quantitation (iTRAQ) between the blast-resistant cultivar Minghui and the susceptible rice cultivar Nipponbare in response to blast fungus infection. The group samples were treated with salicylic acid and compared to control samples. A total of 139 DEPs from the two cultivars showed either more than a two-fold change or alternating regulation patterns. Protein functionality analysis also exhibited that these proteins are involved in a wide range of molecular functions including: energy-related activity (30%), signal transduction (11%), redox homeostasis (15%), amino acid and nitrogen metabolism (4%), carbohydrate metabolism (5%), protein folding and assembly (10%), protein hydrolysis (9%), protein synthesis (12%), and other unknown functions (4%). Specifically, we demonstrated that exogenous treatment with salicylic acid promoted recovery in both rice cultivars from Magnaporthe grisea infection by enhancing: the regulation of signal transduction, increasing energy conversion and production through the regulation of the glycolytic pathway, and other various biochemical processes. These findings may facilitate future studies of the molecular mechanisms of rice blast resistance.

A Calcineurin Regulator MoRCN1 Is Important for Asexual Development, Stress Response, and Plant Infection of Magnaporthe oryzae

The calcium/calcineurin signaling pathway plays a key role in the development and virulence of plant pathogenic fungi, but the regulation of this signaling pathway is still not clear. In this study, we identified a calcineurin regulator MoRCN1 in the plant pathogenic fungus Magnaporthe oryzae and found it is important for virulence by regulating the calcineurin pathway. MoRCN1 deletion mutants were severely decreased in colony growth and conidia formation. More importantly, the deletion of MoRCN1 led to a significant reduction in virulence due to defects in appressorium formation and invasive growth. The ΔMorcn1 mutants were more sensitive to different stresses and induced host ROS accumulation, suggesting a role of MoRCN1 in stress adaptation. We found that MoRCN1 directly interacted with the calcineurin catalytic subunit MoCNA and affected its protein stability, which was therefore important for regulating the calcineurin pathway. Transcriptome analysis showed that MoRCN1 significantly activated 491 genes and suppressed 337 genes in response to calcium ion, partially overlapped with the MoCRZ1-bound genes. Gene Ontology and KEGG pathway analyses indicated that MoRCN1-regulated genes were enriched in stress adaptation, lipid metabolism, and secondary metabolite biosynthesis, reflecting a function of MoRCN1 in host cell adaptation. Altogether, these results suggest MoRCN1 functions as a regulator of the calcium/calcineurin signaling pathway for fungal development and infection of host cells.

Structured Framework and Genome Analysis of Magnaporthe grisea Inciting Pearl Millet Blast Disease Reveals Versatile Metabolic Pathways, Protein Families, and Virulence Factors

Magnaporthe grisea (T.T. Herbert) M.E. Barr is a major fungal phytopathogen that causes blast disease in cereals, resulting in economic losses worldwide. An in-depth understanding of the basis of virulence and ecological adaptation of M. grisea is vital for devising effective disease management strategies. Here, we aimed to determine the genomic basis of the pathogenicity and underlying biochemical pathways in Magnaporthe using the genome sequence of a pearl millet-infecting M. grisea PMg_Dl generated by dual NGS techniques, Illumina NextSeq 500 and PacBio RS II. The short and long nucleotide reads could be draft assembled in 341 contigs and showed a genome size of 47.89 Mb with the N50 value of 765.4 Kb. Magnaporthe grisea PMg_Dl showed an average nucleotide identity (ANI) of 86% and 98% with M. oryzae and Pyricularia pennisetigena, respectively. The gene-calling method revealed a total of 10,218 genes and 10,184 protein-coding sequences in the genome of PMg_Dl. InterProScan of predicted protein showed a distinct 3637 protein families and 695 superfamilies in the PMg_Dl genome. In silico virulence analysis revealed the presence of 51VFs and 539 CAZymes in the genome. The genomic regions for the biosynthesis of cellulolytic endo-glucanase and beta-glucosidase, as well as pectinolytic endo-polygalacturonase, pectin-esterase, and pectate-lyases (pectinolytic) were detected. Signaling pathways modulated by MAPK, PI3K-Akt, AMPK, and mTOR were also deciphered. Multicopy sequences suggestive of transposable elements such as Type LTR, LTR/Copia, LTR/Gypsy, DNA/TcMar-Fot1, and Type LINE were recorded. The genomic resource presented here will be of use in the development of molecular marker and diagnosis, population genetics, disease management, and molecular taxonomy, and also provide a genomic reference for ascomycetous genome investigations in the future.

MoErv29 promotes apoplastic effector secretion contributing to virulence of the rice blast fungus Magnaporthe oryzae

During plant-pathogenic fungi and host plants interactions, numerous pathogen-derived proteins are secreted resulting in the activation of the unfolded protein response (UPR) pathway. For efficient trafficking of secretory proteins, including those important in disease progression, the cytoplasmic coat protein complex II (COPII) exhibits a multifunctional role whose elucidation remains limited. Here, we discovered that the COPII cargo receptor MoErv29 functions as a target of MoHac1, a previously identified transcription factor of the UPR pathway. In Magnaporthe oryzae, deletion of MoERV29 severely affected the vegetative growth, conidiation and biotrophic invasion of the fungus in susceptible rice hosts. We demonstrated that MoErv29 is required for the delivery of secreted proteins through recognition and binding of the amino-terminal tripeptide motifs following the signal peptide. By using bioinformatics analysis, we predicted a cargo spectrum of MoErv29 and found that MoErv29 is required for the secretion of many proteins, including extracellular laccases and apoplastic effectors. This secretion is mediated through the conventional endoplasmic reticulum-Golgi secretion pathway and is important for conferring host recognition and disease resistance. Taken together, our results revealed how MoErv29 operates on effector secretion, and our findings provided a critical link between COPII vesicle trafficking and the UPR pathway.

Hybrid de novo genome-reassembly reveals new insights on pathways and pathogenicity determinants in rice blast pathogen Magnaporthe oryzae RMg_Dl

Blast disease incited by Magnaporthe oryzae is a major threat to sustain rice production in all rice growing nations. The pathogen is widely distributed in all rice paddies and displays rapid aerial transmissions, and seed-borne latent infection. In order to understand the genetic variability, host specificity, and molecular basis of the pathogenicity-associated traits, the whole genome of rice infecting Magnaporthe oryzae (Strain RMg_Dl) was sequenced using the Illumina and PacBio (RSII compatible) platforms. The high-throughput hybrid assembly of short and long reads resulted in a total of 375 scaffolds with a genome size of 42.43 Mb. Furthermore, comparative genome analysis revealed 99% average nucleotide identity (ANI) with other oryzae genomes and 83% against M. grisea, and 73% against M. poe genomes. The gene calling identified 10,553 genes with 10,539 protein-coding sequences. Among the detected transposable elements, the LTR/Gypsy and Type LINE showed high occurrence. The InterProScan of predicted protein sequences revealed that 97% protein family (PFAM), 98% superfamily, and 95% CDD were shared among RMg_Dl and reference 70-15 genome, respectively. Additionally, 550 CAZymes with high GH family content/distribution and cell wall degrading enzymes (CWDE) such endoglucanase, beta-glucosidase, and pectate lyase were also deciphered in RMg_Dl. The prevalence of virulence factors determination revealed that 51 different VFs were found in the genome. The biochemical pathway such as starch and sucrose metabolism, mTOR signaling, cAMP signaling, MAPK signaling pathways related genes were identified in the genome. The 49,065 SNPs, 3267 insertions and 3611 deletions were detected, and majority of these varinats were located on downstream and upstream region. Taken together, the generated information will be useful to develop a specific marker for diagnosis, pathogen surveillance and tracking, molecular taxonomy, and species delineation which ultimately leads to device improved management strategies for blast disease.

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

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

Class I myosin mediated endocytosis and polarization growth is essential for pathogenicity of Magnaporthe oryzae

In eukaryotes, myosin provides the necessary impetus for a series of physiological processes, including organelle movement, cytoplasmic flow, cell division, and mitosis. Previously, three members of myosin were identified in Magnaporthe oryzae, with class II and class V myosins playing important roles in intracellular transport, fungal growth, and pathogenicity. However, limited is known about the biological function of the class I myosin protein in the rice blast fungus. Here, we found that Momyo1 is highly expressed during conidiation and infection. Functional characterization of this gene via RNA interference (RNAi) revealed that Momyo1 is required for vegetative growth, conidiation, melanin pigmentation, and pathogenicity of M. oryzae. The Momyo1 knockdown mutant is defective in formation of appressorium-like structures (ALS) at the hyphal tips. In addition, Momyo1 also displays defects on cell wall integrity, hyphal hydrophobicity, extracellular enzyme activities, endocytosis, and formation of the Spitzenkörper. Furthermore, Momyo1 was identified to physically interact with the MoShe4, a She4p/Dim1p orthologue potentially involved in endocytosis, polarization of the actin cytoskeleton. Overall, our findings provide a novel insight into the regulatory mechanism of Momyo1 that is involved in fungal growth, cell wall integrity, endocytosis, and virulence of M. oryzae. KEY POINTS: • Momyo1 is required for vegetative growth and pigmentation of M. oryzae. • Momyo1 is essential for cell wall integrity and endocytosis of M. oryzae. • Momyo1 is involved in hyphal surface hydrophobicity of M. oryzae.

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.

Aspergillus fumigatus biofilms: Toward understanding how growth as a multicellular network increases antifungal resistance and disease progression

Aspergillus fumigatus is a saprophytic, filamentous fungus found in soils and compost and the causative agent of several pulmonary diseases in humans, birds, and other mammals. A. fumigatus and other filamentous fungi grow as networks of filamentous hyphae that have characteristics of a classic microbial biofilm. These characteristics include production of an extracellular matrix (ECM), surface adhesion, multicellularity, and increased antimicrobial drug resistance. A. fumigatus biofilm growth occurs in vivo at sites of infection, highlighting the importance of defining mechanisms underlying biofilm development and associated emergent properties. We propose that there are 3 distinct phases in the development of A. fumigatus biofilms: biofilm initiation, immature biofilm, and mature biofilm. These stages are defined both temporally and by unique genetic and structural changes over the course of development. Here, we review known mechanisms within each of these stages that contribute to biofilm structure, ECM production, and increased resistance to contemporary antifungal drugs. We highlight gaps in our understanding of biofilm development and function that when addressed are expected to aid in the development of novel antifungal therapies capable of killing filamentous fungal biofilms.