An extracellular Zn-only superoxide dismutase from Puccinia striiformis confers enhanced resistance to host-derived oxidative stress

Antioxidant Systems of Plant Pathogenic Fungi: Functions in Oxidative Stress Response and Their Regulatory Mechanisms

During the infection process, plant pathogenic fungi encounter plant-derived oxidative stress, and an appropriate response to this stress is crucial to their survival and establishment of the disease. Plant pathogenic fungi have evolved several mechanisms to eliminate oxidants from the external environment and maintain cellular redox homeostasis. When oxidative stress is perceived, various signaling transduction pathways are triggered and activate the downstream genes responsible for the oxidative stress response. Despite extensive research on antioxidant systems and their regulatory mechanisms in plant pathogenic fungi, the specific functions of individual antioxidants and their impacts on pathogenicity have not recently been systematically summarized. Therefore, our objective is to consolidate previous research on the antioxidant systems of plant pathogenic fungi. In this review, we explore the plant immune responses during fungal infection, with a focus on the generation and function of reactive oxygen species. Furthermore, we delve into the three antioxidant systems, summarizing their functions and regulatory mechanisms involved in oxidative stress response. This comprehensive review provides an integrated overview of the antioxidant mechanisms within plant pathogenic fungi, revealing how the oxidative stress response contributes to their pathogenicity.

Stripe rust effector Pst21674 compromises wheat resistance by targeting transcription factor TaASR3

Pathogens compromise host defense responses by strategically secreting effector proteins. However, the molecular mechanisms by which effectors manipulate disease-resistance factors to evade host surveillance remain poorly understood. In this study, we characterized a Puccinia striiformis f. sp. tritici (Pst) effector Pst21674 with a signal peptide. Pst21674 was significantly upregulated during Pst infections in wheat (Triticum aestivum L.) and knocking down Pst21674 by host-induced gene silencing led to reduced Pst pathogenicity and restricted hyphal spread in wheat. Pst21674 interaction with the abscisic acid-, stress-, and ripening-induced protein TaASR3 was validated mainly in the nucleus. Size exclusion chromatography, bimolecular fluorescence complementation, and luciferase complementation imaging assays confirmed that TaASR3 could form a functional tetramer. Virus-induced gene silencing and overexpression demonstrated that TaASR3 contributes to wheat resistance to stripe rust by promoting accumulation of reactive oxygen species and cell death. Additionally, transcriptome analysis revealed that the expression of defense-related genes was regulated in transgenic wheat plants overexpressing TaASR3. Interaction between Pst21674 and TaASR3 interfered with the polymerization of TaASR3 and suppressed TaASR3-mediated transcriptional activation of defense-related genes. These results indicate that Pst21674 serves as an important virulence factor secreted into the host nucleus to impede wheat resistance to Pst, possibly by targeting and preventing polymerization of TaASR3.

RNAi Technology: A New Path for the Research and Management of Obligate Biotrophic Phytopathogenic Fungi

Powdery mildew and rust fungi are major agricultural problems affecting many economically important crops and causing significant yield losses. These fungi are obligate biotrophic parasites that are completely dependent on their hosts for growth and reproduction. Biotrophy in these fungi is determined by the presence of haustoria, specialized fungal cells that are responsible for nutrient uptake and molecular dialogue with the host, a fact that undoubtedly complicates their study under laboratory conditions, especially in terms of genetic manipulation. RNA interference (RNAi) is the biological process of suppressing the expression of a target gene through double-stranded RNA that induces mRNA degradation. RNAi technology has revolutionized the study of these obligate biotrophic fungi by enabling the analysis of gene function in these fungal. More importantly, RNAi technology has opened new perspectives for the management of powdery mildew and rust diseases, first through the stable expression of RNAi constructs in transgenic plants and, more recently, through the non-transgenic approach called spray-induced gene silencing (SIGS). In this review, the impact of RNAi technology on the research and management of powdery mildew and rust fungi will be addressed.

A Chloroplast-Localized Glucose-6-Phosphate Dehydrogenase Positively Regulates Stripe Rust Resistance in Wheat

Glucose-6-phosphate dehydrogenase (G6PDH), the rate-limiting enzyme of the pentose phosphate pathway (PPP), plays a pivotal role in plant stress responses. However, the function and mechanism of G6PDHs in crop plants challenged by fungal pathogens remain poorly understood. In this study, a wheat G6DPH gene responding to infection by Puccinia striiformis f. sp. tritici (Pst), designated TaG6PDH2, was cloned and functionally identified. TaG6PDH2 expression was significantly upregulated in wheat leaves inoculated with Pst or treated with abiotic stress factors. Heterologous mutant complementation and enzymatic properties indicate that TaG6PDH2 encodes a G6PDH protein. The transient expression of TaG6PDH2 in Nicotiana benthamiana leaves and wheat protoplasts revealed that TaG6PDH2 is a chloroplast-targeting protein. Silencing TaG6PDH2 via the barley stripe mosaic virus (BSMV)-induced gene silencing (VIGS) system led to compromised wheat resistance to the Pst avirulent pathotype CYR23, which is implicated in weakened H₂O₂ accumulation and cell death. In addition, TaG6PDH2 was confirmed to interact with the wheat glutaredoxin TaGrxS4. These results demonstrate that TaG6PDH2 endows wheat with increased resistance to stripe rust by regulating reactive oxygen species (ROS) production.

A stripe rust fungal effector PstSIE1 targets TaSGT1 to facilitate pathogen infection

Puccinia striiformis f. sp. tritici (Pst), the causal agent of stripe rust, is a destructive pathogen of Triticum aestivum (wheat), threatening wheat production worldwide. Pst delivers hundreds of effectors to manipulate processes in its hosts during infection. The SGT1 (suppressor of the G2 allele of skp1), RAR1 (required for Mla12 resistance) and HSP90 (heat-shock protein 90) proteins form a chaperone complex that acts as a core modulator in plant immunity. However, little is known about how Pst effectors target this immune component to suppress plant immunity. Here, we identified a Pst effector PstSIE1 that interacts with TaSGT1 in wheat and is upregulated during the early infection stage. Transient expression of PstSIE1 suppressed cell death in Nicotiana benthamiana induced by VmE02 and PcNLP2. Transgenic expression of PstSIE1-RNAi constructs in wheat significantly reduced the virulence of Pst. Overexpression of PstSIE1 in wheat increased the number of rust pustules and reduced the accumulation of reactive oxygen species (ROS), indicating that PstSIE1 functions as an important pathogenicity factor in Pst. PstSIE1 was found to compete with TaRAR1 to bind TaSGT1, thus disrupting the formation of the TaRAR1-TaSGT1 subcomplex. Taken together, PstSIE1 is an important Pst effector targeting the immune component TaSGT1 and involved in suppressing wheat defense.

Glycine-serine-rich effector PstGSRE4 in Puccinia striiformis f. sp. tritici inhibits the activity of copper zinc superoxide dismutase to modulate immunity in wheat

Puccinia striiformis f. sp. tritici (Pst) secretes an array of specific effector proteins to manipulate host immunity and promote pathogen colonization. In a previous study, we functionally characterized a glycine-serine-rich effector PstGSRE1 with a glycine-serine-rich motif (m9). However, the mechanisms of glycine-serine-rich effectors (GSREs) remain obscure. Here we report a new glycine-serine-rich effector, PstGSRE4, which has no m9-like motif but inhibits the enzyme activity of wheat copper zinc superoxide dismutase TaCZSOD2, which acts as a positive regulator of wheat resistance to Pst. By inhibiting the enzyme activity of TaCZSOD2, PstGSRE4 reduces H₂O₂ accumulation and HR areas to facilitate Pst infection. These findings provide new insights into the molecular mechanisms of GSREs of rust fungi in regulating plant immunity.

Pst secretes numerous effectors to modulate host defense systems. However, the mechanisms of these effectors, especially for glycine-rich or serine-rich effectors, remain obscure. In this study, we identified a new glycine-serine-rich effector, PstGSRE4, which exhibits unusual biochemical properties and is highly induced during early stages of infection. Transgenic expression of PstGSRE4-RNAi constructs in wheat significantly reduced virulence of Pst and increased H₂O₂ accumulation in wheat. Overexpression of PstGSRE4 in wheat significantly increased virulence of Pst and reduced H₂O₂ accumulation in wheat. PstGSRE4 was shown to target the ROS-associated regulatory factor TaCZSOD2, which was proved as a positive regulator of wheat immunity in this study. Further study revealed that PstGSRE4 inhibited the enzyme activity of TaCZSOD2 and thus compromises the host immune systems. This work reveals a novel strategy that rust fungi exploit to modulate host defense and facilitate pathogen infection.

Action Mechanisms of Effectors in Plant-Pathogen Interaction

Plant pathogens are one of the main factors hindering the breeding of cash crops. Pathogens, including oomycetes, fungus, and bacteria, secrete effectors as invasion weapons to successfully invade and propagate in host plants. Here, we review recent advances made in the field of plant-pathogen interaction models and the action mechanisms of phytopathogenic effectors. The review illustrates how effectors from different species use similar and distinct strategies to infect host plants. We classify the main action mechanisms of effectors in plant-pathogen interactions according to the infestation process: targeting physical barriers for disruption, creating conditions conducive to infestation, protecting or masking themselves, interfering with host cell physiological activity, and manipulating plant downstream immune responses. The investigation of the functioning of plant pathogen effectors contributes to improved understanding of the molecular mechanisms of plant-pathogen interactions. This understanding has important theoretical value and is of practical significance in plant pathology and disease resistance genetics and breeding.

Basidiomycota Fungi and ROS: Genomic Perspective on Key Enzymes Involved in Generation and Mitigation of Reactive Oxygen Species

Our review includes a genomic survey of a multitude of reactive oxygen species (ROS) related intra- and extracellular enzymes and proteins among fungi of Basidiomycota, following their taxonomic classification within the systematic classes and orders, and focusing on different fungal lifestyles (saprobic, symbiotic, pathogenic). Intra- and extracellular ROS metabolism-involved enzymes (49 different protein families, summing 4170 protein models) were searched as protein encoding genes among 63 genomes selected according to current taxonomy. Extracellular and intracellular ROS metabolism and mechanisms in Basidiomycota are illustrated in detail. In brief, it may be concluded that differences between the set of extracellular enzymes activated by ROS, especially by H2O2, and involved in generation of H2O2, follow the differences in fungal lifestyles. The wood and plant biomass degrading white-rot fungi and the litter-decomposing species of Agaricomycetes contain the highest counts for genes encoding various extracellular peroxidases, mono- and peroxygenases, and oxidases. These findings further confirm the necessity of the multigene families of various extracellular oxidoreductases for efficient and complete degradation of wood lignocelluloses by fungi. High variations in the sizes of the extracellular ROS-involved gene families were found, however, among species with mycorrhizal symbiotic lifestyle. In addition, there are some differences among the sets of intracellular thiol-mediation involving proteins, and existence of enzyme mechanisms for quenching of intracellular H2O2 and ROS. In animal- and plant-pathogenic species, extracellular ROS enzymes are absent or rare. In these fungi, intracellular peroxidases are seemingly in minor role than in the independent saprobic, filamentous species of Basidiomycota. Noteworthy is that our genomic survey and review of the literature point to that there are differences both in generation of extracellular ROS as well as in mechanisms of response to oxidative stress and mitigation of ROS between fungi of Basidiomycota and Ascomycota.

A secreted catalase contributes to Puccinia striiformis resistance to host-derived oxidative stress

Plants can produce reactive oxygen species (ROS) to counteract pathogen invasion, and pathogens have also evolved corresponding ROS scavenging strategies to promote infection and pathogenicity. Catalases (CATs) have been found to play pivotal roles in detoxifying H2O2 formed by superoxide anion catalyzed by superoxide dismutases (SODs). However, few studies have addressed H2O2 removing during rust fungi infection of wheat. In this study, we cloned a CAT gene PsCAT1 from Puccinia striiformis f. sp. tritici (Pst), which encodes a monofunctional heme-containing catalase. PsCAT1 exhibited a high degree of tolerance to pH and temperature, and forms high homopolymers.Heterologous complementation assays in Saccharomyces cerevisiae reveal that the signal peptide of PsCAT1 is functional. Overexpression of PsCAT1 enhanced S. cerevisiae resistance to H2O2. Transient expression of PsCAT1 in Nicotiana benthamiana suppressed Bax-induced cell death. Knockdown of PsCAT1 using a host-induced gene silencing (HIGS) system led to the reduced virulence of Pst, which was correlated to H2O2 accumulation in HIGS plants. These results indicate that PsCAT1 acts as an important pathogenicity factor that facilitates Pst infection by scavenging host-derived H2O2.

Versatile effectors of phytopathogenic fungi target host immunity

Phytopathogenic fungi secrete a large arsenal of effector molecules, including proteinaceous effectors, small RNAs, phytohormones and derivatives thereof. The pathogenicity of fungal pathogens is primarily determined by these effectors that are secreted into host cells to undermine innate immunity, as well as to facilitate the acquisition of nutrients for their in planta growth and proliferation. After conventional and non-conventional secretion, fungal effectors are translocated into different subcellular compartments of the host cells to interfere with various biological processes. In extracellular spaces, apoplastic effectors cope with physical and chemical barriers to break the first line of plant defenses. Intracellular effectors target essential immune components on the plasma membrane, in the cytosol, including cytosolic organelles, and in the nucleus to suppress host immunity and reprogram host physiology, favoring pathogen colonization. In this review, we comprehensively summarize the recent advances in fungal effector biology, with a focus on the versatile virulence functions of fungal effectors in promoting pathogen infection and colonization. A perspective of future research on fungal effector biology is also discussed.

The Extracellular Superoxide Dismutase Sod5 From Fusarium oxysporum Is Localized in Response to External Stimuli and Contributes to Fungal Pathogenicity

Reactive oxygen species (ROS) produced by hosts serve as a general defense mechanism against various pathogens. At the interaction site between the host and pathogen, host cells rapidly accumulate high concentrations of ROS, called the oxidative burst, that damage and kill the invading microbes. However, successful pathogens usually survive in a high ROS environment and have evolved strategies to overcome these detrimental effects. Here we characterized the biological function of the extracellular superoxide dismutase (SOD) FoSod5 from Fusarium oxysporum f. sp. vasinfectum. FoSOD5 is strongly up-regulated during infection of cotton, and a ΔFoSOD5 mutant was significantly reduced in virulence on cotton. Purified 6 × His-FoSod5 could significantly inhibit the reduction of NBT and WST-1, indicating that FoSod5 was a functional SOD protein. Based on CRISPR/Cas9 technology, several different FoSod5 variants were generated and used to assess the secretion, expression, and subcellular localization of FoSod5 in F. oxysporum. The subcellular localization of FoSod5 is altered under different environmental conditions. During normal growth conditions, FoSod5 was primarily localized to the phialides; however, in a nutrient-limited environment, FoSod5 was localized to a wide array of fungal structures including the septum and cell wall. FoSod5 is an alkaline-induced glycosylphosphatidylinositol (GPI) protein and the GPI anchor was required for proper protein subcellular localization. The multiple mechanisms fungi utilize to tolerate the oxidative burst is indicative of the importance of this plant defense response; however, the presence of a conserved extracellular SOD in many phytopathogenic fungi suggests tolerance to ROS is initiated prior to the ROS entering the fungal cell.

Convergent Adaptation to Quantitative Host Resistance in a Major Plant Pathogen

Plant pathogens can adapt to quantitative resistance, eroding its effectiveness. The aim of this work was to reveal the genomic basis of adaptation to such a resistance in populations of the fungus Pseudocercospora fijiensis, a major devastating pathogen of banana, by studying convergent adaptation on different cultivars. Samples from P. fijiensis populations showing a local adaptation pattern on new banana hybrids with quantitative resistance were compared, based on a genome scan approach, with samples from traditional and more susceptible cultivars in Cuba and the Dominican Republic. Whole-genome sequencing of pools of P. fijiensis isolates (pool-seq) sampled from three locations per country was conducted according to a paired population design. The findings of different combined analyses highly supported the existence of convergent adaptation on the study cultivars between locations within but not between countries. Five to six genomic regions involved in this adaptation were detected in each country. An annotation analysis and available biological data supported the hypothesis that some genes within the detected genomic regions may play a role in quantitative pathogenicity, including gene regulation. The results suggested that the genetic basis of fungal adaptation to quantitative plant resistance is at least oligogenic, while highlighting the existence of specific host-pathogen interactions for this kind of resistance.IMPORTANCE Understanding the genetic basis of pathogen adaptation to quantitative resistance in plants has a key role to play in establishing durable strategies for resistance deployment. In this context, a population genomic approach was developed for a major plant pathogen (the fungus Pseudocercospora fijiensis causing black leaf streak disease of banana) whereby samples from new resistant banana hybrids were compared with samples from more susceptible conventional cultivars in two countries. A total of 11 genomic regions for which there was strong evidence of selection by quantitative resistance were detected. An annotation analysis and available biological data supported the hypothesis that some of the genes within these regions may play a role in quantitative pathogenicity. These results suggested a polygenic basis of quantitative pathogenicity in this fungal pathogen and complex molecular plant-pathogen interactions in quantitative disease development involving several genes on both sides.

Distinct Transcriptomic Reprogramming in the Wheat Stripe Rust Fungus During the Initial Infection of Wheat and Barberry

Puccinia striiformis f. sp. tritici is the causal agent of wheat stripe rust that causes severe yield losses all over the world. As a macrocyclic heteroecious rust fungus, it is able to infect two unrelated host plants, wheat and barberry. Its urediniospores infect wheat and cause disease epidemic, while its basidiospores parasitize barberry to fulfill the sexual reproduction. This complex life cycle poses interesting questions on the different mechanisms of pathogenesis underlying the infection of the two different hosts. In the present study, transcriptomes of P. striiformis f. sp. tritici during the initial infection of wheat and barberry leaves were qualitatively and quantitatively compared. As a result, 142 wheat-specifically expressed genes (WEGs) were identified, which was far less than the 2,677 barberry-specifically expressed genes (BEGs). A larger proportion of evolutionarily conserved genes were observed in BEGs than that in WEGs, implying a longer history of the interaction between P. striiformis f. sp. tritici and barberry. Additionally, P. striiformis f. sp. tritici differentially expressed genes (DEGs) between wheat at 1 and 2 days postinoculation (dpi) and barberry at 3 and 4 dpi were identified by quantitative analysis. Gene Ontology analysis of these DEGs and expression patterns of P. striiformis f. sp. tritici pathogenic genes, including those encoding candidate secreted effectors, cell wall-degrading enzymes, and nutrient transporters, demonstrated that urediniospores and basidiospores exploited distinct strategies to overcome host defense systems. These results represent the first analysis of the P. striiformis f. sp. tritici transcriptome in barberry and contribute to a better understanding of the evolutionary processes and strategies of different types of rust spores during the infection process on different hosts.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

In-depth secretome analysis of Puccinia striiformis f. sp. tritici in infected wheat uncovers effector functions

The importance of wheat yellow rust disease, caused by Puccinia striiformis f. sp. tritici (Pst), has increased substantially due to the emergence of aggressive new Pst races in the last couple of decades. In an era of escalating human populations and climate change, it is vital to understand the infection mechanism of Pst in order to develop better strategies to combat wheat yellow disease. The present study focuses on the identification of small secreted proteins (SSPs) and candidate-secreted effector proteins (CSEPs) that are used by the pathogen to support infection and control disease development. We generated de novo assembled transcriptomes of Pst collected from wheat fields in central Anatolia. We inoculated both susceptible and resistant seedlings with Pst and analyzed haustoria formation. At 10 days post-inoculation (dpi), we analyzed the transcriptomes and identified 10550 Differentially Expressed Unigenes (DEGs), of which 6072 were Pst-mapped. Among those Pst-related genes, 227 were predicted as PstSSPs. In silico characterization was performed using an approach combining the transcriptomic data and data mining results to provide a reliable list to narrow down the ever-expanding repertoire of predicted effectorome. The comprehensive analysis detected 14 Differentially Expressed Small-Secreted Proteins (DESSPs) that overlapped with the genes in available literature data to serve as the best CSEPs for experimental validation. One of the CSEPs was cloned and studied to test the reliability of the presented data. Biological assays show that the randomly selected CSEP, Unigene17495 (PSTG_10917), localizes in the chloroplast and is able to suppress cell death induced by INF1 in a Nicotiana benthamiana heterologous expression system.

A Cu-only superoxide dismutase from stripe rust fungi functions as a virulence factor deployed for counter defense against host-derived oxidative stress

Plants quickly accumulate reactive oxygen species (ROS) to resist against pathogen invasion, while pathogens strive to escape host immune surveillance by degrading ROS. However, the nature of the strategies that fungal pathogens adopt to counteract host-derived oxidative stress is manifold and requires deep investigation. In this study, a superoxide dismutase (SOD) from Puccinia striiformis f. sp. tritici (Pst) PsSOD2 with a signal peptide (SP) and the glycophosphatidyl inositol (GPI) anchor, strongly induced during infection, was analysed for its biological characteristics and potential role in wheat-Pst interactions. The results showed that PsSOD2 encodes a Cu-only SOD and responded to ROS treatment. Heterologous complementation assays in Saccharomyces cerevisiae suggest that the SP of PsSOD2 is functional for its secretion. Transient expression in Nicotiana benthamiana leaves revealed that PsSOD2 is localized to the plasma membrane. In addition, knockdown of PsSOD2 by host-induced gene silencing reduced Pst virulence and resulted in restricted hyphal development and increased ROS accumulation. In contrast, heterologous transient assays of PsSOD2 suppressed flg22-elicited ROS production. Taken together, our data indicate that PsSOD2, as a virulence factor, was induced and localized to the plasma membrane where it may function to scavenge host-derived ROS for promoting fungal infection.

TaRac6 Is a Potential Susceptibility Factor by Regulating the ROS Burst Negatively in the Wheat-Puccinia striiformis f. sp. tritici Interaction

Rac/Rop proteins play important roles in the regulation of cell growth and plant defense responses. However, the function of Rac/Rop proteins in wheat remains largely unknown. In this study, a small G protein gene, designated as TaRac6, was characterized from wheat (Triticum aestivum) in response to Puccinia striiformis f. sp. tritici (Pst) and was found to be highly homologous to the Rac proteins identified in other plant species. Transient expression analyses of the TaRac6-GFP fusion protein in Nicotiana benthamiana leaves showed that TaRac6 was localized in the whole cell. Furthermore, transient expression of TaRac6 inhibited Bax-triggered plant cell death (PCD) in N. benthamiana. Transcript accumulation of TaRac6 was increased at 24 h post-inoculation (hpi) in the compatible interaction between wheat and Pst, while it was not induced in an incompatible interaction. More importantly, silencing of TaRac6 by virus induced gene silencing (VIGS) enhanced the resistance of wheat (Suwon 11) to Pst (CYR31) by producing fewer uredinia. Histological observations revealed that the hypha growth of Pst was markedly inhibited along with more H₂O₂ generated in the TaRac6-silenced leaves in response to Pst. Moreover, transcript levels of TaCAT were significantly down-regulated, while those of TaSOD and TaNOX were significantly up-regulated. These results suggest that TaRac6 functions as a potential susceptibility factor, which negatively regulate the reactive oxygen species (ROS) burst in the wheat-Pst interaction.

The role of reactive oxygen species in the virulence of wheat leaf rust fungus Puccinia triticina

Reactive oxygen species (ROS) play an important role during host–pathogen interactions and are often an indication of induced host defence responses. In this study, we demonstrate for the first time that Puccinia triticina (Pt) generates ROS, including superoxide, H₂O₂ and hydroxyl radicals, during wheat infection. Through pharmacological inhibition, we found that ROS are critical for both Pt urediniospore germination and pathogenic development on wheat. A comparative RNA‐Seq analysis of different stages of Pt infection process revealed 291 putative Pt genes associated with the oxidation–reduction process. Thirty‐seven of these genes encode known proteins. The expressions of five Pt genes, including PtNoxA, PtNoxB, PtNoxR, PtCat and PtSod, were subsequently verified using RT‐qPCR analysis. The results show that the expressions of PtNoxA, PtNoxB, PtNoxR, PtCat and PtSod are up‐regulated during urediniospore germination. In comparison, the expressions of PtNoxA, PtNoxB, PtNoxR and PtCat are down‐regulated during wheat infection from 12 to 120 h after inoculation (HAI), whereas the expression of PtSod is up‐regulated with a peak of expression at 120 HAI. We conclude that ROS are critical for the full virulence of Pt and a coordinate down‐regulation of PtNox genes may be important for successful infection in wheat.

A stripe rust effector Pst18363 targets and stabilises TaNUDX23 that promotes stripe rust disease

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), poses a tremendous threat to the production of wheat worldwide. The molecular mechanisms of Pst effectors that regulate wheat immunity are poorly understood. In this study, we identified an effector Pst18363 from Pst that suppresses plant cell death in Nicotiana benthamiana and in wheat. Knocking down Pst18363 expression by virus-mediated host-induced gene silencing significantly decreased the number of rust pustules, indicating that Pst18363 functions as an important pathogenicity factor in Pst. Pst18363 was proven to interact with wheat Nudix hydrolase 23 TaNUDX23. In wheat, silencing of TaNUDX23 by virus-induced gene silencing increased reactive oxygen species (ROS) accumulation induced by the avirulent Pst race CYR23, whereas overexpression of TaNUDX23 suppressed ROS accumulation induced by flg22 in Arabidopsis. In addition, TaNUDX23 suppressed Pst candidate effector Pst322-trigged cell death by decreasing ROS accumulation in N. benthamiana. Knocking down of TaNUDX23 expression attenuated Pst infection, indicating that TaNUDX23 is a negative regulator of defence. In N. benthamiana, Pst18363 stabilises TaNUDX23. Overall, our data suggest that Pst18363 stabilises TaNUDX23, which suppresses ROS accumulation to facilitate Pst infection.

A Role for Zinc in Plant Defense Against Pathogens and Herbivores

Pests and diseases pose a threat to food security, which is nowadays aggravated by climate change and globalization. In this context, agricultural policies demand innovative approaches to more effectively manage resources and overcome the ecological issues raised by intensive farming. Optimization of plant mineral nutrition is a sustainable approach to ameliorate crop health and yield. Zinc is a micronutrient essential for all living organisms with a key role in growth, development, and defense. Competition for Zn affects the outcome of the host-attacker interaction in both plant and animal systems. In this review, we provide a clear framework of the different strategies involving low and high Zn concentrations launched by plants to fight their enemies. After briefly introducing the most relevant macro- and micronutrients for plant defense, the functions of Zn in plant protection are summarized with special emphasis on superoxide dismutases (SODs) and zinc finger proteins. Following, we cover recent meaningful studies identifying Zn-related passive and active mechanisms for plant protection. Finally, Zn-based strategies evolved by pathogens and pests to counteract plant defenses are discussed.

Advances in understanding obligate biotrophy in rust fungi

Contents Summary 1190 I. Introduction 1190 II. Rust fungi: a diverse and serious threat to agriculture 1191 III. The different facets of rust life cycles and unresolved questions about their evolution 1191 IV. The biology of rust infection 1192 V. Rusts in the genomics era: the ever-expanding list of candidate effector genes 1195 VI. Functional characterization of rust effectors 1197 VII. Putting rusts to sleep: Pucciniales research outlooks 1201 Acknowledgements 1202 References 1202 SUMMARY: Rust fungi (Pucciniales) are the largest group of plant pathogens and represent one of the most devastating threats to agricultural crops worldwide. Despite the economic importance of these highly specialized pathogens, many aspects of their biology remain obscure, largely because rust fungi are obligate biotrophs. The rise of genomics and advances in high-throughput sequencing technology have presented new options for identifying candidate effector genes involved in pathogenicity mechanisms of rust fungi. Transcriptome analysis and integrated bioinformatics tools have led to the identification of key genetic determinants of host susceptibility to infection by rusts. Thousands of genes encoding secreted proteins highly expressed during host infection have been reported for different rust species, which represents significant potential towards understanding rust effector function. Recent high-throughput in planta expression screen approaches (effectoromics) have pushed the field ahead even further towards predicting high-priority effectors and identifying avirulence genes. These new insights into rust effector biology promise to inform future research and spur the development of effective and sustainable strategies for managing rust diseases.

Host-Induced Gene Silencing: A Powerful Strategy to Control Diseases of Wheat and Barley

Wheat and barley are the most highly produced and consumed grains in the world. Various pathogens-viruses, bacteria, fungi, insect pests, and nematode parasites-are major threats to yield and economic losses. Strategies for the management of disease control mainly depend on resistance or tolerance breeding, chemical control, and biological control. The discoveries of RNA silencing mechanisms provide a transgenic approach for disease management. Host-induced gene silencing (HIGS) employing RNA silencing mechanisms and, specifically, silencing the targets of invading pathogens, has been successfully applied in crop disease prevention. Here, we cover recent studies that indicate that HIGS is a valuable tool to protect wheat and barley from diseases in an environmentally friendly way.

Silencing PsKPP4, a MAP kinase kinase kinase gene, reduces pathogenicity of the stripe rust fungus

Many obligately parasitic pathogens absorb nutrients from host plants via specialized infection structures, called haustoria and infection hyphae, to further colonization and growth in the host plant. In the wheat (Triticum aestivum) stripe rust fungus, Puccinia striiformis f. sp. tritici (Pst), the mitogen-activated protein kinase kinase (MAPKK) PsFUZ7 is involved in the regulation of haustorium formation and invasive growth. Here, we functionally characterized PsKPP4 of Pst, which is homologous to the yeast MAPKKK STE11. Similar to the silencing of PsFUZ7, the knockdown of PsKPP4 was detected in the vegetative hyphae and haustoria, resulting in the reduced pathogenicity of Pst. Pst urediniospores treated with the STE11 MAPKKK activation inhibitor produced deformed germ tubes. In addition, overexpression of PsKPP4 in fission yeast resulted in the production of fusiform cells and increased tolerance of yeast cells to oxidative stress. The transformation of PsKPP4 into the mst11 mutant of Magnaporthe oryzae partially restored mst11 function. The PsKPP4 protein contains a sterile alpha motif (SAM), Ras association (RA) and kinase domains, similar to its homologues in other fungi. Yeast two-hybrid assays revealed that the SAM domain is essential for the interaction between PsKPP4 and PsUBC2, a homologue of Ustilago maydis UBC2, known to interact with KPP4, which is associated with the regulation of the Fus3 cascade. Host-induced gene silencing of PsUBC2 reduced the pathogenicity of Pst slightly, indicating that PsUBC2 also plays a minor role in the regulation of the infection pathway of Pst. These observations indicate that PsKPP4, interacting with PsUBC2, may play an important role in the regulation of infection-related morphogenesis in Pst.

A novel citrate synthase isoform contributes infection and stress resistance of the stripe rust fungus

The early development of a rust fungus is dependent on the endogenous lipids stored in the urediniospores. After it establishes a parasitic relationship with the host, sugars absorbed from the host cells by haustoria become the primary nutrients. The tricarboxylic acid (TCA) cycle is essential to oxidize these nutrients. However, few studies have addressed the role of citrate synthase (CS), a rate-limiting enzyme of the TCA cycle, during the infection process of rust fungi. In this study, a CS gene from Puccinia striiformis f. sp. tritici (Pst), PsCS1, was cloned and characterized. Transcripts of PsCS1 and the enzyme activity of the CS were increased in the early Pst infection stage. Biochemical features and subcellular localization revealed that PsCS1 encoded a mitochondrial CS. Size exclusion chromatography, yeast two-hybrid and bimolecular fluorescence complementation experiments confirmed that PsCS1 could form a functional homo-octamer. The overexpression of PsCS1 enhanced the resistance of Escherichia coli to salt stress. The knockdown of PsCS1 using a host-induced gene silencing (HIGS) system blocked Pst growth in wheat. These results indicate that PsCS1 is required for nutrient metabolism in Pst and contributes to Pst infection by regulating ATP production and the supply of carbon sources.

The transcription factor PstSTE12 is required for virulence of Puccinia striiformis f. sp. tritici

Puccinia striiformis f. sp. tritici (Pst) is an obligate biotrophic fungus that causes extensive damage in wheat. The pathogen is now known to be a heteroecious fungus with an intricate life cycle containing sexual and asexual stages. Orthologues of the STE12 transcription factor that regulate mating and filamentation in Saccharomyces cerevisiae, as well as virulence in other fungi, have been extensively described. Because reliable transformation and gene disruption methods are lacking for Pst, knowledge about the function of its STE12 orthologue is limited. In this study, we identified a putative orthologue of STE12 from Pst in haustoria-enriched transcripts and designated it as PstSTE12. The gene encodes a protein of 879 amino acids containing three helices in the homeodomain, conserved phenylalanine and tryptophan sites, and two C₂ /H₂ -Zn²⁺ finger domains. Real-time reverse transcription-polymerase chain reaction (RT-PCR) analyses revealed that the expression of PstSTE12 was highly induced during the early infection stages and peaked during haustorium formation and the pycniospore stage in the aecial host barberry. Subcellular localization assays indicated that PstSTE12 is localized in the nucleus and functions as a transcriptional activator. Yeast one-hybrid assays revealed that PstSTE12 exhibits transcriptional activity, and that its C-terminus is necessary for the activation of transcription. PstSTE12 complemented the mating defect in an α ste12 mutant of S. cerevisiae. In addition, it partially complemented the defects of the Magnaporthe oryzae mst12 mutant in plant infection. Knocking down PstSTE12 via host-induced gene silencing (HIGS) mediated by Barley stripe mosaic virus (BSMV) resulted in a substantial reduction in the growth and spread of hyphae in Pst and weakened the virulence of Pst on wheat. Our results suggest that PstSTE12 probably acts at an intersection participating in the invasion and mating processes of Pst, and provide new insights into the comprehension of the variation of virulence in cereal rust fungi.

Candidate Effector Pst_8713 Impairs the Plant Immunity and Contributes to Virulence of Puccinia striiformis f. sp. tritici

Puccinia striiformis f. sp. tritici (Pst), the causal agent of stripe rust, is an obligate biotrophic pathogen responsible for severe wheat disease epidemics worldwide. Pst and other rust fungi are acknowledged to deliver many effector proteins to the host, but little is known about the effectors' functions. Here, we report a candidate effector Pst_8713 isolated based on the genome data of CY32 and the expression of Pst_8713 is highly induced during the early infection stage. The Pst_8713 gene shows a low level of intra-species polymorphism. It has a functional N-terminal signal peptide and its product was found in the host cytoplasm and nucleus. Co-infiltrations in Nicotiana benthamiana demonsrated that Pst_8713 was capable of suppressing cell death triggered by mouse pro-apoptotic protein-BAX or Phytophthora infestans PAMP-INF1. Overexpression of Pst_8713 in plants suppressed pattern-triggered immunity (PTI) -associated callose deposition and expression of PTI-associated marker genes and promoted bacterial growth in planta. Effector-triggered immunity (ETI) induced by an avirulent Pst isolate was weakened when we overexpressed Pst_8713 in wheat leaves which accompanied by reduction of reactive oxygen species (ROS) accumulation and hypersensitive response (HR). In addition, the host induced gene silencing (HIGS) experiment showed that knockdown of Pst_8713 weakened the virulence of Pst by producing fewer uredinia. These results indicated that candidate effector Pst_8713 is involved in plant defense suppression and contributes to enhancing the Pst virulence.

PSTha5a23, a candidate effector from the obligate biotrophic pathogen Puccinia striiformis f. sp. tritici, is involved in plant defense suppression and rust pathogenicity

During the infection of host plants, pathogens can deliver virulence-associated 'effector' proteins to promote plant susceptibility. However, little is known about effector function in the obligate biotrophic pathogen Puccinia striiformis f. sp. tritici (Pst) that is an important fungal pathogen in wheat production worldwide. Here, they report their findings on an in planta highly induced candidate effector from Pst, PSTha5a23. The PSTha5a23 gene is unique to Pst and shows a low level of intra-species polymorphism. It has a functional N-terminal signal peptide and is translocated to the host cytoplasm after infection. Overexpression of PSTha5a23 in Nicotiana benthamiana was found to suppress the programmed cell death triggered by BAX, PAMP-INF1 and two resistance-related mitogen-activated protein kinases (MKK1 and NPK1). Overexpression of PSTha5a23 in wheat also suppressed pattern-triggered immunity (PTI)-associated callose deposition. In addition, silencing of PSTha5a23 did not change Pst virulence phenotypes; however, overexpression of PSTha5a23 significantly enhanced Pst virulence in wheat. These results indicate that the Pst candidate effector PSTha5a23 plays an important role in plant defense suppression and rust pathogenicity, and also highlight the utility of gene overexpression in plants as a tool for studying effectors from obligate biotrophic pathogens.

TaARPC3, Contributes to Wheat Resistance against the Stripe Rust Fungus

The actin cytoskeleton participates in numerous cellular processes, including less-characterized processes, such as nuclear organization, chromatin remodeling, transcription, and signal transduction. As a key regulator of actin cytoskeletal dynamics, the actin related protein 2/3 complex (Arp2/3 complex) controls multiple developmental processes in a variety of tissues and cell types. To date, the role of the Arp2/3 complex in plant disease resistance signaling is largely unknown. Herein, we identified and characterized wheat ARPC3, TaARPC3, which encodes the C3 subunit of the Arp2/3 complex. Expression of TaARPC3 in the arc18 mutant of Saccharomyces cerevisiae Δarc18 resulted in complementation of stress-induced phenotypes in S. cerevisiae, as well as restore wild-type cell shape malformations. TaARPC3 was found predominantly to be localized in the nucleus and cytoplasm when expressed transiently in wheat protoplast. TaARPC3 was significantly induced in response to avirulent race of Puccinia striiformis f. sp. tritici (Pst). Knock-down of TaARPC3 by virus-induced gene silencing resulted in a reduction of resistance against Pst through a specific reduction in actin cytoskeletal organization. Interestingly, this reduction was found to coincide with a block in reactive oxygen species (ROS) accumulation, the hypersensitive response (HR), an increase in TaCAT1 mRNA accumulation, and the growth of Pst. Taken together, these findings suggest that TaARPC3 is a key subunit of the Arp2/3 complex which is required for wheat resistance against Pst, a process that is associated with the regulation of the actin cytoskeleton.

Characterization of a Ran gene from Puccinia striiformis f. sp. tritici involved in fungal growth and anti-cell death

Ran, an important family of small GTP-binding proteins, has been shown to regulate a variety of important cellular processes in many eukaryotes. However, little is known about Ran function in pathogenic fungi. In this study, we report the identification and functional analysis of a Ran gene (designated PsRan) from Puccinia striiformis f. sp. tritici (Pst), an important fungal pathogen affecting wheat production worldwide. The PsRan protein contains all conserved domains of Ran GTPases and shares more than 70% identity with Ran proteins from other organisms, indicating that Ran proteins are conserved in different organisms. PsRan shows a low level of intra-species polymorphism and is localized to the nucleus. qRT-PCR analysis showed that transcript level of PsRan was induced in planta during Pst infection. Silencing of PsRan did not alter Pst virulence phenotype but impeded fungal growth of Pst. In addition, heterologous overexpression of PsRan in plant failed to induce cell death but suppressed cell death triggered by a mouse BAX gene or a Pst Ras gene. Our results suggest that PsRan is involved in the regulation of fungal growth and anti-cell death, which provides significant insight into Ran function in pathogenic fungi.