The transcriptional landscape of plant infection by the rice blast fungus Magnaporthe oryzae reveals distinct families of temporally co-regulated and structurally conserved effectors

Live cell imaging of plant infection provides new insight into the biology of pathogenesis by the rice blast fungus Magnaporthe oryzae

Magnaporthe oryzae is the causal agent of rice blast, one of the most serious diseases affecting rice cultivation around the world. During plant infection, M. oryzae forms a specialised infection structure called an appressorium. The appressorium forms in response to the hydrophobic leaf surface and relies on multiple signalling pathways, including a MAP kinase phosphorelay and cAMP-dependent signalling, integrated with cell cycle control and autophagic cell death of the conidium. Together, these pathways regulate appressorium morphogenesis.The appressorium generates enormous turgor, applied as mechanical force to breach the rice cuticle. Re-polarisation of the appressorium requires a turgor-dependent sensor kinase which senses when a critical threshold of turgor has been reached to initiate septin-dependent re-polarisation of the appressorium and plant infection. Invasive growth then requires differential expression and secretion of a large repertoire of effector proteins secreted by distinct secretory pathways depending on their destination, which is also governed by codon usage and tRNA thiolation. Cytoplasmic effectors require an unconventional Golgi-independent secretory pathway and evidence suggests that clathrin-mediated endocytosis is necessary for their delivery into plant cells. The blast fungus then develops a transpressorium, a specific invasion structure used to move from cell-to-cell using pit field sites containing plasmodesmata, to facilitate its spread in plant tissue. This is controlled by the same MAP kinase signalling pathway as appressorium development and requires septin-dependent hyphal constriction. Recent progress in understanding the mechanisms of rice infection by this devastating pathogen using live cell imaging procedures are presented.

Mechanisms of regulated cell death during plant infection by the rice blast fungus Magnaporthe oryzae

Fungi are the most important group of plant pathogens, responsible for many of the world's most devastating crop diseases. One of the reasons they are such successful pathogens is because several fungi have evolved the capacity to breach the tough outer cuticle of plants using specialized infection structures called appressoria. This is exemplified by the filamentous ascomycete fungus Magnaporthe oryzae, causal agent of rice blast, one of the most serious diseases affecting rice cultivation globally. M. oryzae develops a pressurized dome-shaped appressorium that uses mechanical force to rupture the rice leaf cuticle. Appressoria form in response to the hydrophobic leaf surface, which requires the Pmk1 MAP kinase signalling pathway, coupled to a series of cell-cycle checkpoints that are necessary for regulated cell death of the fungal conidium and development of a functionally competent appressorium. Conidial cell death requires autophagy, which occurs within each cell of the spore, and is regulated by components of the cargo-independent autophagy pathway. This results in trafficking of the contents of all three cells to the incipient appressorium, which develops enormous turgor of up to 8.0 MPa, due to glycerol accumulation, and differentiates a thickened, melanin-lined cell wall. The appressorium then re-polarizes, re-orienting the actin and microtubule cytoskeleton to enable development of a penetration peg in a perpendicular orientation, that ruptures the leaf surface using mechanical force. Re-polarization requires septin GTPases which form a ring structure at the base of the appressorium, which delineates the point of plant infection, and acts as a scaffold for actin re-localization, enhances cortical rigidity, and forms a lateral diffusion barrier to focus polarity determinants that regulate penetration peg formation. Here we review the mechanism of regulated cell death in M. oryzae, which requires autophagy but may also involve ferroptosis. We critically evaluate the role of regulated cell death in appressorium morphogenesis and examine how it is initiated and regulated, both temporally and spatially, during plant infection. We then use this synopsis to present a testable model for control of regulated cell death during appressorium-dependent plant infection by the blast fungus.

A conserved fungal Knr4/Smi1 protein is crucial for maintaining cell wall stress tolerance and host plant pathogenesis

Filamentous plant pathogenic fungi pose significant threats to global food security, particularly through diseases like Fusarium Head Blight (FHB) and Septoria Tritici Blotch (STB) which affects cereals. With mounting challenges in fungal control and increasing restrictions on fungicide use due to environmental concerns, there is an urgent need for innovative control strategies. Here, we present a comprehensive analysis of the stage-specific infection process of Fusarium graminearum in wheat spikes by generating a dual weighted gene co-expression network (WGCN). Notably, the network contained a mycotoxin-enriched fungal module (F12) that exhibited a significant correlation with a detoxification gene-enriched wheat module (W12). This correlation in gene expression was validated through quantitative PCR. By examining a fungal module with genes highly expressed during early symptomless infection that was correlated to a wheat module enriched in oxidative stress genes, we identified a gene encoding FgKnr4, a protein containing a Knr4/Smi1 disordered domain. Through comprehensive analysis, we confirmed the pivotal role of FgKnr4 in various biological processes, including oxidative stress tolerance, cell cycle stress tolerance, morphogenesis, growth, and pathogenicity. Further studies confirmed the observed phenotypes are partially due to the involvement of FgKnr4 in regulating the fungal cell wall integrity pathway by modulating the phosphorylation of the MAP-kinase MGV1. Orthologues of the FgKnr4 gene are widespread across the fungal kingdom but are absent in other Eukaryotes, suggesting the protein has potential as a promising intervention target. Encouragingly, the restricted growth and highly reduced virulence phenotypes observed for ΔFgknr4 were replicated upon deletion of the orthologous gene in the wheat fungal pathogen Zymoseptoria tritici. Overall, this study demonstrates the utility of an integrated network-level analytical approach to pinpoint genes of high interest to pathogenesis and disease control.

Comparative Genomics and Pathogenicity Analysis of Three Fungal Isolates Causing Barnyard Grass Blast

Barnyard grass is one of the most serious rice weeds, often growing near paddy fields and therefore potentially serving as a bridging host for the rice blast fungus. In this study, we isolated three fungal strains from diseased barnyard grass leaves in a rice field. Using a pathogenicity assay, we confirmed that they were capable of causing blast symptoms on barnyard grass and rice leaves to various extents. Based on morphology characterization and genome sequence analyses, we confirmed that these three strains were Epicoccum sorghinum (SCAU-1), Pyricularia grisea (SCAU-2), and Exserohilum rostratum (SCAU-6). The established Avirulence (Avr) genes Avr-Pia, Avr-Pita2, and ACE1 were detected by PCR amplification in SCAU-2, but not in SCAU-1 or SCAU-6. Furthermore, the whole-genome sequence analysis helped to reveal the genetic variations and potential virulence factors relating to the host specificity of these three fungal pathogens. Based on the evolutionary analysis of single-copy orthologous proteins, we found that the genes encoding glycoside hydrolases, carbohydrate esterases, oxidoreductase, and multidrug transporters in SCAU-1 and SCAU-6 were expanded, while expansion in SCAU-2 was mainly related to carbohydrate esterases. In summary, our study provides clues to understand the pathogenic mechanisms of fungal isolates from barnyard grass with the potential to cause rice blast.

FolSas2 is a regulator of early effector gene expression during Fusarium oxysporum infection

Fusarium oxysporum f. sp. lycopersici (Fol) that causes a globally devastating wilt disease on tomato relies on the secretion of numerous effectors to mount an infection, but how the pathogenic fungus precisely regulates expression of effector genes during plant invasion remains elusive. Here, using molecular and cellular approaches, we show that the histone H4K8 acetyltransferase FolSas2 is a transcriptional regulator of early effector gene expression in Fol. Autoacetylation of FolSas2 on K269 represses K335 ubiquitination, preventing its degradation by the 26S proteasome. During the early infection process, Fol elevates FolSas2 acetylation by differentially changing transcription of itself and the FolSir1 deacetylase, leading to specific accumulation of the enzyme at this stage. FolSas2 subsequently activates the expression of an array of effectors genes, and as a consequence, Fol invades tomato successfully. These findings reveal a regulatory mechanism of effector gene expression via autoacetylation of a histone modifier during plant fungal invasion.

tRNA-m1A methylation controls the infection of Magnaporthe oryzae by supporting ergosterol biosynthesis

Ergosterols are essential components of fungal plasma membranes. Inhibitors targeting ergosterol biosynthesis (ERG) genes are critical for controlling fungal pathogens, including Magnaporthe oryzae, the fungus that causes rice blast. However, the translational mechanisms governing ERG gene expression remain largely unexplored. Here, we show that the Trm6/Trm61 complex catalyzes dynamic N1-methyladenosine at position 58 (m1A58) in 51 transfer RNAs (tRNAs) of M. oryzae, significantly influencing translation at both the initiation and elongation stages. Notably, tRNA m1A58 promotes elongation speed at most cognate codons mainly by enhancing eEF1-tRNA binding rather than affecting tRNA abundance or charging. The absence of m1A58 leads to substantial decreases in the translation of ERG genes, ergosterol production, and, consequently, fungal virulence. Simultaneously targeting the Trm6/Trm61 complex and the ergosterol biosynthesis pathway markedly improves rice blast control. Our findings demonstrate an important role of m1A58-mediated translational regulation in ergosterol production and fungal infection, offering a potential strategy for fungicide development.

Dual RNA-seq reveals distinct families of co-regulated and structurally conserved effectors in Botrytis cinerea infection of Arabidopsis thaliana

BACKGROUND

Botrytis cinerea is a broad-host-range pathogen causing gray mold disease and significant yield losses of numerous crops. However, the mechanisms underlying its rapid invasion and efficient killing of plant cells remain unclear.

RESULTS

In this study, we elucidated the dynamics of B. cinerea infection in Arabidopsis thaliana by live cell imaging and dual RNA sequencing. We found extensive transcriptional reprogramming events in both the pathogen and the host, which involved metabolic pathways, signaling cascades, and transcriptional regulation. For the pathogen, we identified 591 candidate effector proteins (CEPs) and comprehensively analyzed their co-expression, sequence similarity, and structural conservation. The results revealed temporal co-regulation patterns of these CEPs, indicating coordinated deployment of effectors during B. cinerea infection. Through functional screening of 48 selected CEPs in Nicotiana benthamiana, we identified 11 cell death-inducing proteins (CDIPs) in B. cinerea.

CONCLUSIONS

The findings provide important insights into the transcriptional dynamics and effector biology driving B. cinerea pathogenesis. The rapid infection of this pathogen involves the temporal co-regulation of CEPs and the prominent role of CDIPs in host cell death. This work highlights significant changes in gene expression associated with gray mold disease, underscoring the importance of a diverse repertoire of effectors crucial for successful infection.

The roles of Magnaporthe oryzae avirulence effectors involved in blast resistance/susceptibility

Phytopathogens represent an ongoing threat to crop production and a significant impediment to global food security. During the infection process, these pathogens spatiotemporally deploy a large array of effectors to sabotage host defense machinery and/or manipulate cellular pathways, thereby facilitating colonization and infection. However, besides their pivotal roles in pathogenesis, certain effectors, known as avirulence (AVR) effectors, can be directly or indirectly perceived by plant resistance (R) proteins, leading to race-specific resistance. An in-depth understanding of the intricate AVR-R interactions is instrumental for genetic improvement of crops and safeguarding them from diseases. Magnaporthe oryzae (M. oryzae), the causative agent of rice blast disease, is an exceptionally virulent and devastating fungal pathogen that induces blast disease on over 50 monocot plant species, including economically important crops. Rice-M. oryzae pathosystem serves as a prime model for functional dissection of AVR effectors and their interactions with R proteins and other target proteins in rice due to its scientific advantages and economic importance. Significant progress has been made in elucidating the potential roles of AVR effectors in the interaction between rice and M. oryzae over the past two decades. This review comprehensively discusses recent advancements in the field of M. oryzae AVR effectors, with a specific focus on their multifaceted roles through interactions with corresponding R/target proteins in rice during infection. Furthermore, we deliberated on the emerging strategies for engineering R proteins by leveraging the structural insights gained from M. oryzae AVR effectors.

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.

Paths of Least Resistance: Unconventional Effector Secretion by Fungal and Oomycete Plant Pathogens

Effector secretion by different routes mediates the molecular interplay between host plant and pathogen, but mechanistic details in eukaryotes are sparse. This may limit the discovery of new effectors that could be utilized for improving host plant disease resistance. In fungi and oomycetes, apoplastic effectors are secreted via the conventional endoplasmic reticulum (ER)-Golgi pathway, while cytoplasmic effectors are packaged into vesicles that bypass Golgi in an unconventional protein secretion (UPS) pathway. In Magnaporthe oryzae, the Golgi bypass UPS pathway incorporates components of the exocyst complex and a t-SNARE, presumably to fuse Golgi bypass vesicles to the fungal plasma membrane. Upstream, cytoplasmic effector mRNA translation in M. oryzae requires the efficient decoding of AA-ending codons. This involves the modification of wobble uridines in the anticodon loop of cognate tRNAs and fine-tunes cytoplasmic effector translation and secretion rates to maintain biotrophic interfacial complex integrity and permit host infection. Thus, plant-fungal interface integrity is intimately tied to effector codon usage, which is a surprising constraint on pathogenicity. Here, we discuss these findings within the context of fungal and oomycete effector discovery, delivery, and function in host cells. We show how cracking the codon code for unconventional cytoplasmic effector secretion in M. oryzae has revealed AA-ending codon usage bias in cytoplasmic effector mRNAs across kingdoms, including within the RxLR-dEER motif-encoding sequence of a bona fide Phytophthora infestans cytoplasmic effector, suggesting its subjection to translational speed control. By focusing on recent developments in understanding unconventional effector secretion, we draw attention to this important but understudied area of host-pathogen interactions. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Emerging Roles of Exocyst Complex in Fungi: A Review

The exocyst complex, an evolutionarily conserved octameric protein assembly, plays a central role in the targeted binding and fusion of vesicles at the plasma membrane. In fungal cells, this transport system is essential for polarized growth, morphogenesis, cell wall maintenance and virulence. Recent advances have greatly improved our understanding of the role and regulation of the exocyst complex in fungi. This review synthesizes these developments and focuses on the intricate interplay between the exocyst complex, specific fungal cargos and regulatory proteins. Insights into thestructure of the exocyst and its functional dynamics have revealed new dimensions of its architecture and its interactions with the cellular environment. Furthermore, the regulation of exocyst activity involves complex signaling pathways and interactions with cytoskeletal elements that are crucial for its role in vesicle trafficking. By exploring these emerging themes, this review provides a comprehensive overview of the multifaceted functions of the exocyst complex in fungal biology. Understanding these mechanisms offers potential avenues for novel therapeutic strategies against fungal pathogens and insights into the general principles of vesicle trafficking in eukaryotic cells. The review therefore highlights the importance of the exocyst complex in maintaining cellular functions and its broader implications in fungal pathogenicity and cell biology.

Multiple Horizontal Mini-chromosome Transfers Drive Genome Evolution of Clonal Blast Fungus Lineages

Crop disease pandemics are often driven by asexually reproducing clonal lineages of plant pathogens that reproduce asexually. How these clonal pathogens continuously adapt to their hosts despite harboring limited genetic variation, and in absence of sexual recombination remains elusive. Here, we reveal multiple instances of horizontal chromosome transfer within pandemic clonal lineages of the blast fungus Magnaporthe (Syn. Pyricularia) oryzae. We identified a horizontally transferred 1.2Mb accessory mini-chromosome which is remarkably conserved between M. oryzae isolates from both the rice blast fungus lineage and the lineage infecting Indian goosegrass (Eleusine indica), a wild grass that often grows in the proximity of cultivated cereal crops. Furthermore, we show that this mini-chromosome was horizontally acquired by clonal rice blast isolates through at least nine distinct transfer events over the past three centuries. These findings establish horizontal mini-chromosome transfer as a mechanism facilitating genetic exchange among different host-associated blast fungus lineages. We propose that blast fungus populations infecting wild grasses act as genetic reservoirs that drive genome evolution of pandemic clonal lineages that afflict cereal crops.

Mapping the molecular landscape of Lotus japonicus nodule organogenesis through spatiotemporal transcriptomics

Legumes acquire nitrogen-fixing ability by forming root nodules. Transferring this capability to more crops could reduce our reliance on nitrogen fertilizers, thereby decreasing environmental pollution and agricultural production costs. Nodule organogenesis is complex, and a comprehensive transcriptomic atlas is crucial for understanding the underlying molecular events. Here, we utilized spatial transcriptomics to investigate the development of nodules in the model legume, Lotus japonicus. Our investigation has identified the developmental trajectories of two critical regions within the nodule: the infection zone and peripheral tissues. We reveal the underlying biological processes and provide gene sets to achieve symbiosis and material exchange, two essential aspects of nodulation. Among the candidate regulatory genes, we illustrate that LjNLP3, a transcription factor belonging to the NIN-LIKE PROTEIN family, orchestrates the transition of nodules from the differentiation to maturation. In summary, our research advances our understanding of nodule organogenesis and provides valuable data for developing symbiotic nitrogen-fixing crops.

The cysteine-rich virulence factor NipA of Arthrobotrys flagrans interferes with cuticle integrity of Caenorhabditis elegans

Animals protect themself from microbial attacks by robust skins or a cuticle as in Caenorhabditis elegans. Nematode-trapping fungi, like Arthrobotrys flagrans, overcome the cuticle barrier and colonize the nematode body. While lytic enzymes are important for infection, small-secreted proteins (SSPs) without enzymatic activity, emerge as crucial virulence factors. Here, we characterized NipA (nematode induced protein) which A. flagrans secretes at the penetration site. In the absence of NipA, A. flagrans required more time to penetrate C. elegans. Heterologous expression of the fungal protein in the epidermis of C. elegans led to blister formation. NipA contains 13 cysteines, 12 of which are likely to form disulfide bridges, and the remaining cysteine was crucial for blister formation. We hypothesize that NipA interferes with cuticle integrity to facilitate fungal entry. Genome-wide expression analyses of C. elegans expressing NipA revealed mis-regulation of genes associated with extracellular matrix (ECM) maintenance and innate immunity.

AvrSr27 is a zinc-bound effector with a modular structure important for immune recognition

Effector proteins are central to the success of plant pathogens, while immunity in host plants is driven by receptor-mediated recognition of these effectors. Understanding the molecular details of effector-receptor interactions is key for the engineering of novel immune receptors. Here, we experimentally determined the crystal structure of the Puccinia graminis f. sp. tritici (Pgt) effector AvrSr27, which was not accurately predicted using AlphaFold2. We characterised the role of the conserved cysteine residues in AvrSr27 using in vitro biochemical assays and examined Sr27-mediated recognition using transient expression in Nicotiana spp. and wheat protoplasts. The AvrSr27 structure contains a novel β-strand rich modular fold consisting of two structurally similar domains that bind to Zn2+ ions. The N-terminal domain of AvrSr27 is sufficient for interaction with Sr27 and triggering cell death. We identified two Pgt proteins structurally related to AvrSr27 but with low sequence identity that can also associate with Sr27, albeit more weakly. Though only the full-length proteins, trigger Sr27-dependent cell death in transient expression systems. Collectively, our findings have important implications for utilising protein prediction platforms for effector proteins, and those embarking on bespoke engineering of immunity receptors as solutions to plant disease.

A genome-wide association study of panicle blast resistance to Magnaporthe oryzae in rice

Rice blast, caused by Magnaporthe oryzae (M. oryzae), is one of the most serious diseases worldwide. Developing blast-resistant rice varieties is an effective strategy to control the spread of rice blast and reduce the reliance on chemical pesticides. In this study, 477 sequenced rice germplasms from 48 countries were inoculated and assessed at the booting stage. We found that 23 germplasms exhibited high panicle blast resistance against M. oryzae. Genome-wide association analysis (GWAS) identified 43 quantitative trait loci (QTLs) significantly associated (P < 1.0 × 10-4) with resistance to rice panicle blast. These QTL intervals encompass four genes (OsAKT1, OsRACK1A, Bsr-k1 and Pi25/Pid3) previously reported to contribute to rice blast resistance. We selected QTLs with -Log10 (P-value) greater than 6.0 or those detected in two-year replicates, amounting to 12 QTLs, for further candidate gene analysis. Three blast resistance candidate genes (Os06g0316800, Os06g0320000, Pi25/Pid3) were identified based on significant single nucleotide polymorphisms (SNP) distributions within annotated gene sequences across these 12 QTLs and the differential expression levels among blast-resistant varieties after 72 h of inoculation. Os06g0316800 encodes a glycine-rich protein, OsGrp6, an important component of plant cell walls involved in cellular stress responses and signaling. Os06g0320000 encodes a protein with unknown function (DUF953), part of the thioredoxin-like family, which is crucial for maintaining reactive oxygen species (ROS) homeostasis in vivo, named as OsTrxl1. Lastly, Pi25/Pid3 encodes a disease resistance protein, underscoring its potential importance in plant biology. By analyzing the haplotypes of these three genes, we identified favorable haplotypes for blast resistance, providing valuable genetic resources for future rice blast resistance breeding programs.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-024-01486-5.

Zinc-finger (ZiF) fold secreted effectors form a functionally diverse family across lineages of the blast fungus Magnaporthe oryzae

Filamentous plant pathogens deliver effector proteins into host cells to suppress host defence responses and manipulate metabolic processes to support colonization. Understanding the evolution and molecular function of these effectors provides knowledge about pathogenesis and can suggest novel strategies to reduce damage caused by pathogens. However, effector proteins are highly variable, share weak sequence similarity and, although they can be grouped according to their structure, only a few structurally conserved effector families have been functionally characterized to date. Here, we demonstrate that Zinc-finger fold (ZiF) secreted proteins form a functionally diverse effector family in the blast fungus Magnaporthe oryzae. This family relies on the Zinc-finger motif for protein stability and is ubiquitously present in blast fungus lineages infecting 13 different host species, forming different effector tribes. Homologs of the canonical ZiF effector, AVR-Pii, from rice infecting isolates are present in multiple M. oryzae lineages. Wheat infecting strains of the fungus also possess an AVR-Pii like allele that binds host Exo70 proteins and activates the immune receptor Pii. Furthermore, ZiF tribes may vary in the proteins they bind to, indicating functional diversification and an intricate effector/host interactome. Altogether, we uncovered a new effector family with a common protein fold that has functionally diversified in lineages of M. oryzae. This work expands our understanding of the diversity of M. oryzae effectors, the molecular basis of plant pathogenesis and may ultimately facilitate the development of new sources for pathogen resistance.

Multi-omics analysis revealed that the protein kinase MoKin1 affected the cellular response to endoplasmic reticulum stress in the rice blast fungus, Magnaporthe oryzae

BACKGROUND

Previous studies have shown that protein kinase MoKin1 played an important role in the growth, conidiation, germination and pathogenicity in rice blast fungus, Magnaporthe oryzae. ΔMokin1 mutant showed significant phenotypic defects and significantly reduced pathogenicity. However, the internal mechanism of how MoKin1 affected the development of physiology and biochemistry remained unclear in M. oryzae.

RESULT

This study adopted a multi-omics approach to comprehensively analyze MoKin1 function, and the results showed that MoKin1 affected the cellular response to endoplasmic reticulum stress (ER stress). Proteomic analysis revealed that the downregulated proteins in ΔMokin1 mutant were enriched mainly in the response to ER stress triggered by the unfolded protein. Loss of MoKin1 prevented the ER stress signal from reaching the nucleus. Therefore, the phosphorylation of various proteins regulating the transcription of ER stress-related genes and mRNA translation was significantly downregulated. The insensitivity to ER stress led to metabolic disorders, resulting in a significant shortage of carbohydrates and a low energy supply, which also resulted in severe phenotypic defects in ΔMokin1 mutant. Analysis of MoKin1-interacting proteins indicated that MoKin1 really took participate in the response to ER stress.

CONCLUSION

Our results showed the important role of protein kinase MoKin1 in regulating cellular response to ER stress, providing a new research direction to reveal the mechanism of MoKin1 affecting pathogenic formation, and to provide theoretical support for the new biological target sites searching and bio-pesticides developing.

The structural landscape and diversity of Pyricularia oryzae MAX effectors revisited

Magnaporthe AVRs and ToxB-like (MAX) effectors constitute a family of secreted virulence proteins in the fungus Pyricularia oryzae (syn. Magnaporthe oryzae), which causes blast disease on numerous cereals and grasses. In spite of high sequence divergence, MAX effectors share a common fold characterized by a ß-sandwich core stabilized by a conserved disulfide bond. In this study, we investigated the structural landscape and diversity within the MAX effector repertoire of P. oryzae. Combining experimental protein structure determination and in silico structure modeling we validated the presence of the conserved MAX effector core domain in 77 out of 94 groups of orthologs (OG) identified in a previous population genomic study. Four novel MAX effector structures determined by NMR were in remarkably good agreement with AlphaFold2 (AF2) predictions. Based on the comparison of the AF2-generated 3D models we propose a classification of the MAX effectors superfamily in 20 structural groups that vary in the canonical MAX fold, disulfide bond patterns, and additional secondary structures in N- and C-terminal extensions. About one-third of the MAX family members remain singletons, without strong structural relationship to other MAX effectors. Analysis of the surface properties of the AF2 MAX models also highlights the high variability within the MAX family at the structural level, potentially reflecting the wide diversity of their virulence functions and host targets.

Impact of bacterial volatiles on the plant growth attributes and defense mechanism of rice seedling

Rice is a major dietary element for about two billion people worldwide and it faces numerous biotic and abiotic stress for its cultivation. Rice blast disease caused by Magnaporthe oryzae reduce up to 30 % rice yield. Overuse of synthetic chemicals raises concerns about health and environment; so, there is an urgent need to explore innovative sustainable strategies for crop productivity. The main aim of this study is to explore the impact of bacterial volatiles (BVCs) on seedling growth and defense mechanisms of rice under in-vitro condition. On the basis of plant growth promoting properties, six bacterial strains were selected out of ninety-one isolated strains for this study; Pantoea dispersa BHUJPVR01, Enterobacter cloacae BHUJPVR02, Enterobacter sp. BHUJPVR12, Priestia aryabhattai BHUJPVR13, Pseudomonas sp. BHUJPVWRO5 and Staphylococcus sp. BHUJPVWLE7. Through the emission of bacterial volatiles compounds (BVCs), Enterobacter sp., P. dispersa and P. aryabhattai significantly reduces the growth of rice blast fungus Magnaporthe oryzae by 69.20 %, 66.15 % and 62.31 % respectively. Treatment of rice seedlings with BVCs exhibited significant enhancement in defence enzyme levels, including guaiacol peroxidase, polyphenol oxidase, total polyphenols, and total flavonoids by a maximum of up to 24 %, 48 %, 116 % and 80 %, respectively. Furthermore, BVCs effectively promote shoot height, root height, and root counts of rice. All BVCs treated plant showed a significant increase in shoot height. P. dispersa treated plants showed the highest increase of 60 % shoot and 110 % root length, respectively. Root counts increased up to 30% in plants treated with E. cloacae and Staphylococcus sp. The BVCs can be used as a sustainable approach for enhancing plant growth attributes, productivity and defence mechanism of rice plant under biotic and abiotic stresses.

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/.

Septin-dependent invasive growth by the rice blast fungus Magnaporthe oryzae

Septin GTPases are morphogenetic proteins that are widely conserved in eukaryotic organisms fulfilling diverse roles in cell division, differentiation and development. In the filamentous fungal pathogen Magnaporthe oryzae, the causal agent of the devastating blast diseases of rice and wheat, septins have been shown to be essential for plant infection. The blast fungus elaborates a specialised infection structure called an appressorium with which it mechanically ruptures the plant cuticle. Septin aggregation and generation of a hetero-oligomeric ring structure at the base of the infection cell is indispensable for plant infection. Furthermore, once the fungus enters host tissue it develops another infection structure, the transpressorium, enabling it to move between living host plant cells, which also requires septins for its function. Specific inhibition of septin aggregation-either genetically or with chemical inhibitors-prevents plant infection. Significantly, by screening for inhibitors of septin aggregation, broad spectrum anti-fungal compounds have been identified that prevent rice blast and a number of other cereal diseases in field trials. We review the recent advances in our understanding of septin biology and their potential as targets for crop disease control.

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.

Novel Secreted Effectors Conserved Among Smut Fungi Contribute to the Virulence of Ustilago maydis

Fungal pathogens deploy a set of molecules (proteins, specialized metabolites, and sRNAs), so-called effectors, to aid the infection process. In comparison to other plant pathogens, smut fungi have small genomes and secretomes of 20 Mb and around 500 proteins, respectively. Previous comparative genomic studies have shown that many secreted effector proteins without known domains, i.e., novel, are conserved only in the Ustilaginaceae family. By analyzing the secretomes of 11 species within Ustilaginaceae, we identified 53 core homologous groups commonly present in this lineage. By collecting existing mutants and generating additional ones, we gathered 44 Ustilago maydis strains lacking single core effectors as well as 9 strains containing multiple deletions of core effector gene families. Pathogenicity assays revealed that 20 of these 53 mutant strains were affected in virulence. Among the 33 mutants that had no obvious phenotypic changes, 13 carried additional, sequence-divergent, structurally similar paralogs. We report a virulence contribution of seven previously uncharacterized single core effectors and of one effector family. Our results help to prioritize effectors for understanding U. maydis virulence and provide genetic resources for further characterization. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

The structural repertoire of Fusarium oxysporum f. sp. lycopersici effectors revealed by experimental and computational studies

Plant pathogens secrete proteins, known as effectors, that function in the apoplast or inside plant cells to promote virulence. Effector recognition by cell-surface or cytosolic receptors results in the activation of defence pathways and plant immunity. Despite their importance, our general understanding of fungal effector function and recognition by immunity receptors remains poor. One complication often associated with effectors is their high sequence diversity and lack of identifiable sequence motifs precluding prediction of structure or function. In recent years, several studies have demonstrated that fungal effectors can be grouped into structural classes, despite significant sequence variation and existence across taxonomic groups. Using protein X-ray crystallography, we identify a new structural class of effectors hidden within the secreted in xylem (SIX) effectors from Fusarium oxysporum f. sp. lycopersici (Fol). The recognised effectors Avr1 (SIX4) and Avr3 (SIX1) represent the founding members of the Fol dual-domain (FOLD) effector class, with members containing two distinct domains. Using AlphaFold2, we predicted the full SIX effector repertoire of Fol and show that SIX6 and SIX13 are also FOLD effectors, which we validated experimentally for SIX6. Based on structural prediction and comparisons, we show that FOLD effectors are present within three divisions of fungi and are expanded in pathogens and symbionts. Further structural comparisons demonstrate that Fol secretes effectors that adopt a limited number of structural folds during infection of tomato. This analysis also revealed a structural relationship between transcriptionally co-regulated effector pairs. We make use of the Avr1 structure to understand its recognition by the I receptor, which leads to disease resistance in tomato. This study represents an important advance in our understanding of Fol-tomato, and by extension plant-fungal interactions, which will assist in the development of novel control and engineering strategies to combat plant pathogens.

Identification and functional characterisation of a locus for target site integration in Fusarium graminearum

BACKGROUND

Fusarium Head Blight (FHB) is a destructive floral disease of different cereal crops. The Ascomycete fungus Fusarium graminearum (Fg) is one of the main causal agents of FHB in wheat and barley. The role(s) in virulence of Fg genes include genetic studies that involve the transformation of the fungus with different expression cassettes. We have observed in several studies where Fg genes functions were characterised that integration of expression cassettes occurred randomly. Random insertion of a cassette may disrupt gene expression and/or protein functions and hence the overall conclusion of the study. Target site integration (TSI) is an approach that consists of identifying a chromosomal region where the cassette can be inserted. The identification of a suitable locus for TSI in Fg would avert the potential risks of ectopic integration.

RESULTS

Here, we identified a highly conserved intergenic region on chromosome 1 suitable for TSI. We named this intergenic region TSI locus 1. We developed an efficient cloning vector system based on the Golden Gate method to clone different expression cassettes for use in combination with TSI locus 1. We present evidence that integrations in the TSI locus 1 affects neither fungal virulence nor fungal growth under different stress conditions. Integrations at the TSI locus 1 resulted in the expression of different gene fusions. In addition, the activities of Fg native promoters were not altered by integration into the TSI locus 1. We have developed a bespoke bioinformatic pipeline to analyse the existence of ectopic integrations, cassette truncations and tandem insertions of the cassette that may occurred during the transformation process. Finally, we established a protocol to study protein secretion in wheat coleoptiles using confocal microscopy and the TSI locus 1.

CONCLUSION

The TSI locus 1 can be used in Fg and potentially other cereal infecting Fusarium species for diverse studies including promoter activity analysis, protein secretion, protein localisation studies and gene complementation. The bespoke bioinformatic pipeline developed in this work together with PCR amplification of the insert could be an alternative to Southern blotting, the gold standard technique used to identify ectopic integrations, cassette truncations and tandem insertions in fungal transformation.

Effector-triggered susceptibility by the rice blast fungus Magnaporthe oryzae

Rice blast, the most destructive disease of cultivated rice world-wide, is caused by the filamentous fungus Magnaporthe oryzae. To cause disease in plants, M. oryzae secretes a diverse range of effector proteins to suppress plant defense responses, modulate cellular processes, and support pathogen growth. Some effectors can be secreted by appressoria even before host penetration, while others accumulate in the apoplast, or enter living plant cells where they target specific plant subcellular compartments. During plant infection, the blast fungus induces the formation of a specialized plant structure known as the biotrophic interfacial complex (BIC), which appears to be crucial for effector delivery into plant cells. Here, we review recent advances in the cell biology of M. oryzae-host interactions and show how new breakthroughs in disease control have stemmed from an increased understanding of effector proteins of M. oryzae are deployed and delivered into plant cells to enable pathogen invasion and host susceptibility.

Phosphate accumulation in rice leaves promotes fungal pathogenicity and represses host immune responses during pathogen infection

Rice is one of the most important crops in the world and a staple food for more than half of the world's population. At present, the blast disease caused by the fungus Magnaporthe oryzae poses a severe threat to food security through reduction of rice yields worldwide. High phosphate fertilization has previously been shown to increase blast susceptibility. At present, however, our knowledge on the mechanisms underpinning phosphate-induced susceptibility to M. oryzae infection in rice is limited. In this work, we conducted live cell imaging on rice sheaths inoculated with a M. oryzae strain expressing two fluorescently-tagged M. oryzae effectors. We show that growing rice under high phosphate fertilization, and subsequent accumulation of phosphate in leaf sheaths, promotes invasive growth of M. oryzae. Consistent with this, stronger expression of M. oryzae effectors and Pathogenicity Mitogen-activated Protein Kinase (PMK1) occurs in leaf sheaths of rice plants grown under high a phosphate regime. Down-regulation of fungal genes encoding suppressors of plant cell death and up-regulation of plant cell death-inducing effectors also occurs in sheaths of phosphate over-accumulating rice plants. Treatment with high Pi causes alterations in the expression of fungal phosphate transporter genes potentially contributing to pathogen virulence. From the perspective of the plant, Pi accumulation in leaf sheaths prevents H2O2 accumulation early during M. oryzae infection which was associated to a weaker activation of Respiratory Burst Oxidase Homologs (RBOHs) genes involved in reactive oxygen species (ROS) production. Further, a weaker activation of defense-related genes occurs during infection in rice plants over-accumulating phosphate. From these results, it can be concluded that phosphate fertilization has an effect on the two interacting partners, pathogen and host. Phosphate-mediated stimulation of fungal effector genes (e.g., potentiation of fungal pathogenicity) in combination with repression of pathogen-inducible immune responses (e.g., ROS accumulation, defense gene expression) explains higher colonization by M. oryzae in rice tissues accumulating phosphate. Phosphate content can therefore be considered as an important factor in determining the outcome of the rice/M. oryzae interaction. As fertilizers and pesticides are commonly used in rice cultivation to maintain optimal yield and to prevent losses caused by pathogens, a better understanding of how phosphate impacts blast susceptibility is crucial for developing strategies to rationally optimize fertilizer and pesticide use in rice production.

Infection-specific transcriptional patterns of the maize pathogen Cochliobolus heterostrophus unravel genes involved in asexual development and virulence

Southern corn leaf blight (SCLB) caused by Cochliobolus heterostrophus is a destructive disease that threatens global maize (Zea mays) production. Despite many studies being conducted, very little is known about molecular processes employed by the pathogen during infection. There is a need to understand the fungal arms strategy and identify novel functional genes as targets for fungicide development. Transcriptome analysis based on RNA sequencing was carried out across conidia germination and host infection by C. heterostrophus. The present study revealed major changes in C. heterostrophus gene expression during host infection. Several differentially expressed genes (DEGs) induced during C. heterostrophus infection could be involved in the biosynthesis of secondary metabolites, peroxisome, energy metabolism, amino acid degradation and oxidative phosphorylation. In addition, histone acetyltransferase, secreted proteins, peroxisomal proteins, NADPH oxidase and transcription factors were selected for further functional validation. Here, we demonstrated that histone acetyltransferases (Hat2 and Rtt109), secreted proteins (Cel61A and Mep1), peroxisomal proteins (Pex11A and Pex14), NADPH oxidases (NoxA, NoxD and NoxR) and transcription factors (Crz1 and MtfA) play essential roles in C. heterostrophus conidiation, stress adaption and virulence. Taken together, our study revealed major changes in gene expression associated with C. heterostrophus infection and identified a diverse repertoire of genes critical for successful infection.

Till death do us pair: Co-evolution of plant-necrotroph interactions

Plants use programmed cell death as a potent defense response against biotrophic pathogens that require living host cells to thrive. However, cell death can promote infection by necrotrophic pathogens. This discrepancy creates specific co-evolutionary dynamics in the interaction between plants and necrotrophs. Necrotrophic pathogens produce diverse cell death-inducing effectors that act redundantly on several plant targets and sometimes suppress plant immune responses as an additional function. Plants use surface receptors that recognize necrotrophic effectors to increase quantitative disease resistance, some of which evolved independently in several plant lineages. Co-evolution has shaped molecular mechanisms involved in plant-necrotroph interactions into robust systems, relying on degenerate and multifunctional modules, general-purpose components, and compartmentalized functioning.

Pseudocercospora fijiensis Conidial Germination Is Dominated by Pathogenicity Factors and Effectors

Conidia play a vital role in the survival and rapid spread of fungi. Many biological processes of conidia, such as adhesion, signal transduction, the regulation of oxidative stress, and autophagy, have been well studied. In contrast, the contribution of pathogenicity factors during the development of conidia in fungal phytopathogens has been poorly investigated. To date, few reports have centered on the pathogenicity functions of fungal phytopathogen conidia. Pseudocercospora fijiensis is a hemibiotrophic fungus and the causal agent of the black Sigatoka disease in bananas and plantains. Here, a conidial transcriptome of P. fijiensis was characterized computationally. Carbohydrates, amino acids, and lipid metabolisms presented the highest number of annotations in Gene Ontology. Common conidial functions were found, but interestingly, pathogenicity factors and effectors were also identified. Upon analysis of the resulting proteins against the Pathogen-Host Interaction (PHI) database, 754 hits were identified. WideEffHunter and EffHunter effector predictors identified 618 effectors, 265 of them were shared with the PHI database. A total of 1107 conidial functions devoted to pathogenesis were found after our analysis. Regarding the conidial effectorome, it was found to comprise 40 canonical and 578 non-canonical effectors. Effectorome characterization revealed that RXLR, LysM, and Y/F/WxC are the largest effector families in the P. fijiensis conidial effectorome. Gene Ontology classification suggests that they are involved in many biological processes and metabolisms, expanding our current knowledge of fungal effectors.

Unconventional secretion of Magnaporthe oryzae effectors in rice cells is regulated by tRNA modification and codon usage control

Microbial pathogens deploy effector proteins to manipulate host cell innate immunity, often using poorly understood unconventional secretion routes. Transfer RNA (tRNA) anticodon modifications are universal, but few biological functions are known. Here, in the rice blast fungus Magnaporthe oryzae, we show how unconventional effector secretion depends on tRNA modification and codon usage. We characterized the M. oryzae Uba4-Urm1 sulfur relay system mediating tRNA anticodon wobble uridine 2-thiolation (s2U34), a conserved modification required for efficient decoding of AA-ending cognate codons. Loss of s2U34 abolished the translation of AA-ending codon-rich messenger RNAs encoding unconventionally secreted cytoplasmic effectors, but mRNAs encoding endoplasmic reticulum-Golgi-secreted apoplastic effectors were unaffected. Increasing near-cognate tRNA acceptance, or synonymous AA- to AG-ending codon changes in PWL2, remediated cytoplasmic effector production in Δuba4. In UBA4+, expressing recoded PWL2 caused Pwl2 super-secretion that destabilized the host-fungus interface. Thus, U34 thiolation and codon usage tune pathogen unconventional effector secretion in host rice cells.

Adaptive evolution in virulence effectors of the rice blast fungus Pyricularia oryzae

Plant pathogens secrete proteins called effectors that target host cellular processes to promote disease. Recently, structural genomics has identified several families of fungal effectors that share a similar three-dimensional structure despite remarkably variable amino-acid sequences and surface properties. To explore the selective forces that underlie the sequence variability of structurally-analogous effectors, we focused on MAX effectors, a structural family of effectors that are major determinants of virulence in the rice blast fungus Pyricularia oryzae. Using structure-informed gene annotation, we identified 58 to 78 MAX effector genes per genome in a set of 120 isolates representing seven host-associated lineages. The expression of MAX effector genes was primarily restricted to the early biotrophic phase of infection and strongly influenced by the host plant. Pangenome analyses of MAX effectors demonstrated extensive presence/absence polymorphism and identified gene loss events possibly involved in host range adaptation. However, gene knock-in experiments did not reveal a strong effect on virulence phenotypes suggesting that other evolutionary mechanisms are the main drivers of MAX effector losses. MAX effectors displayed high levels of standing variation and high rates of non-synonymous substitutions, pointing to widespread positive selection shaping the molecular diversity of MAX effectors. The combination of these analyses with structural data revealed that positive selection acts mostly on residues located in particular structural elements and at specific positions. By providing a comprehensive catalog of amino acid polymorphism, and by identifying the structural determinants of the sequence diversity, our work will inform future studies aimed at elucidating the function and mode of action of MAX effectors.

Does a Similar 3D Structure Mean a Similar Folding Pathway? The Presence of a C-Terminal α-Helical Extension in the 3D Structure of MAX60 Drastically Changes the Folding Pathway Described for Other MAX-Effectors from Magnaporthe oryzae

Does a similar 3D structure mean a similar folding pathway? This question is particularly meaningful when it concerns proteins sharing a similar 3D structure, but low sequence identity or homology. MAX effectors secreted by the phytopathogenic fungus Magnaporthe oryzae present such characteristics. They share a common 3D structure, a ß-sandwich with the same topology for all the family members, but an extremely low sequence identity/homology. In a previous study, we have investigated the folding of two MAX effectors, AVR-Pia and AVR-Pib, using High-Hydrostatic-Pressure NMR and found that they display a similar folding pathway, with a common folding intermediate. In the present work, we used a similar strategy to investigate the folding conformational landscape of another MAX effector, MAX60, and found a very different folding intermediate. Our analysis strongly supports that the presence of a C-terminal α-helical extension in the 3D structure of MAX60 could be responsible for its different folding pathway.

The emerging role of septins in fungal pathogenesis

Fungal pathogens undergo specific morphogenetic transitions in order to breach the outer surfaces of plants and invade the underlying host tissue. The ability to change cell shape and switch between non-polarised and polarised growth habits is therefore critical to the lifestyle of plant pathogens. Infection-related development involves remodelling of the cytoskeleton, plasma membrane and cell wall at specific points during fungal pathogenesis. Septin GTPases are components of the cytoskeleton that play pivotal roles in actin remodelling, micron-scale plasma membrane curvature sensing and cell polarity. Septin assemblages, such as rings, collars and gauzes, are known to have important roles in cell shape changes and are implicated in formation of specialised infection structures to enter plant cells. Here, we review and compare the reported functions of septins of plant pathogenic fungi, with a special focus on invasive growth. Finally, we discuss septins as potential targets for broad-spectrum antifungal plant protection strategies.

The Phantom Menace: latest findings on effector biology in the rice blast fungus

Magnaporthe oryzae is a hemibiotrophic fungus responsible for the economically devastating and recalcitrant rice blast disease. However, the blast fungus is not only restricted to rice plants as it can also infect wheat, millet, and other crops. Despite previous outstanding discoveries aimed to understand and control the disease, the fungus remains one of the most important pathogens that threatens global food security. To cause disease, M. oryzae initiates morphological changes to attach, penetrate, and colonize rice cells, all while suppressing plant immune defenses that would otherwise hinder its proliferation. As such, M. oryzae actively secretes a battery of small proteins called "effectors" to manipulate host machinery. In this review, we summarize the latest findings in effector identification, expression, regulation, and functionality. We review the most studied effectors and their roles in pathogenesis. Additionally, we discern the current methodologies to structurally catalog effectors, and we highlight the importance of climate change and its impact on the future of rice blast disease.

Metabolic efficiency reshapes the seminal relationship between pathogen growth rate and virulence

A cornerstone of classical virulence evolution theories is the assumption that pathogen growth rate is positively correlated with virulence, the amount of damage pathogens inflict on their hosts. Such theories are key for incorporating evolutionary principles into sustainable disease management strategies. Yet, empirical evidence raises doubts over this central assumption underpinning classical theories, thus undermining their generality and predictive power. In this paper, we identify a key component missing from current theories which redefines the growth-virulence relationship in a way that is consistent with data. By modifying the activity of a single metabolic gene, we engineered strains of Magnaporthe oryzae with different nutrient acquisition and growth rates. We conducted in planta infection studies and uncovered an unexpected non-monotonic relationship between growth rate and virulence that is jointly shaped by how growth rate and metabolic efficiency interact. This novel mechanistic framework paves the way for a much-needed new suite of virulence evolution theories.

Identification and Characterization of Novel Candidate Effector Proteins from Magnaporthe oryzae

The fungal pathogen Magnaporthe oryzae secretes a large number of effector proteins to facilitate infection, most of which are not functionally characterized. We selected potential candidate effector genes from the genome of M. oryzae, field isolate P131, and cloned 69 putative effector genes for functional screening. Utilizing a rice protoplast transient expression system, we identified that four candidate effector genes, GAS1, BAS2, MoCEP1 and MoCEP2 induced cell death in rice. In particular, MoCEP2 also induced cell death in Nicotiana benthamiana leaves through Agrobacteria-mediated transient gene expression. We further identified that six candidate effector genes, MoCEP3 to MoCEP8, suppress flg22-induced ROS burst in N. benthamiana leaves upon transient expression. These effector genes were highly expressed at a different stage after M. oryzae infection. We successfully knocked out five genes in M. oryzae, MoCEP1, MoCEP2, MoCEP3, MoCEP5 and MoCEP7. The virulence tests suggested that the deletion mutants of MoCEP2, MoCEP3 and MoCEP5 showed reduced virulence on rice and barley plants. Therefore, those genes play an important role in pathogenicity.

Clathrin-mediated endocytosis facilitates the internalization of Magnaporthe oryzae effectors into rice cells

Fungi and oomycetes deliver effectors into living plant cells to suppress defenses and control plant processes needed for infection. Little is known about the mechanism by which these pathogens translocate effector proteins across the plasma membrane into the plant cytoplasm. The blast fungus Magnaporthe oryzae secretes cytoplasmic effectors into a specialized biotrophic interfacial complex (BIC) before translocation. Here we show that cytoplasmic effectors within BICs are packaged into punctate membranous effector compartments that are occasionally observed in the host cytoplasm. Live cell imaging with fluorescently labeled proteins in rice (Oryza sativa) showed that these effector puncta colocalize with the plant plasma membrane and with CLATHRIN LIGHT CHAIN 1, a component of clathrin-mediated endocytosis (CME). Inhibiting CME using virus-induced gene silencing and chemical treatments resulted in cytoplasmic effectors in swollen BICs lacking effector puncta. By contrast, fluorescent marker co-localization, gene silencing and chemical inhibitor studies failed to support a major role for clathrin-independent endocytosis in effector translocation. Effector localization patterns indicated that cytoplasmic effector translocation occurs underneath appressoria before invasive hyphal growth. Taken together, this study provides evidence that cytoplasmic effector translocation is mediated by clathrin-mediated endocytosis in BICs and suggests a role for M. oryzae effectors in co-opting plant endocytosis.