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

Variability and functional characterization of the Phakopsora pachyrhizi Egh16-like effectors

Effector proteins in Phakopsora pachyrhizi (Pp), the causative agent of Asian Soybean rust, are involved in the infection process. A previous study identified a rust effector Egh16-like family based expression profile during the interaction with soybean. Herein, we scrutinized available the Pp genomes to validate the predicted Egh16-like family of Pp and identify new family members. We described 22 members of the Egh16-like gene family in the Pp MT2006 genome and 18 in the UFV02 and K8108 genomes, highlighting a family expansion. Family members have a small signal peptide, conserved cysteine-rich R/Y/FxC motifs in the C-terminal region, and a virulence-related Egh16-like domain and were able to suppress PTI related responses in Benthamiana. Phylogenetic analysis placed the family members into eight clusters, with members induced during the early stages of rust infection. Members of clusters VI and VII are present in different copy numbers in Pp genomes and suppressed PAMP-related responses.

Changes in planta K nutrient content altered the interaction pattern between Nicotiana benthamiana and Alternaria longipes

Potassium (K) fertilisation has frequently been shown to enhance plant resistance against pathogens, though the mechanisms remain elusive. This study investigates the interaction dynamics between Nicotiana benthamiana and the pathogen Alternaria longipes under different planta K levels. On the host side, adding K activated the expressions of three NLR (nucleotide-binding domain and leucine-rich repeat-containing proteins) resistance genes, including NbRPM1, NbR1B23 and NbNBS12. Silencing these NLRs attenuated resistance in high-K (HK, 40.8 g/kg) plant, whereas their overexpression strengthened resistance in low-K (LK, 23.9 g/kg) plant. Typically, these NLRs mainly strengthened plant resistance via promoting the expression of pathogenesis-related genes (PRs), ROS burst and synthesis of antifungal metabolites in HK plant. On the pathogen side, the expression of effectors HKCSP1, HKCSP2 and LKCSP were shown to be related to planta K content. A. longipes mainly expressed effectors HKCSP1 and HKCSP2 in HK plant to interfere host resistance. HKCSP1 physically interacted with NbRPM1 to promote the degradation of NbRPM1, then attenuated related resistance in HK N. benthamiana. Meanwhile, HKCSP2 directly interacted with NbPR5 to suppress resistance in HK plant. In LK plant, A. longipes mainly deployed LKCSP that interacted with NbR1B23 to interfere reduce resistance in N. benthamiana. Overall, our research insights that both pathogen and host mobilise distinct strategies to outcompete each other during interactions in different K nutrient environments.

Stripe rust effector Pst03724 modulates host immunity by inhibiting NAD kinase activation by a calmodulin

Puccinia striiformis f. sp. tritici (Pst) secretes effector proteins that enter plant cells to manipulate host immune processes. In this report, we present an important Pst effector, Pst03724, whose mRNA expression level increases during Pst infection of wheat (Triticum aestivum). Silencing of Pst03724 reduced the growth and development of Pst. Pst03724 targeted the wheat calmodulin TaCaM3-2B, a positive regulator of wheat immunity. Subsequent investigations revealed that Pst03724 interferes with the TaCaM3-2B-NAD kinase (NADK) TaNADK2 association and thus inhibits the enzyme activity of TaNADK2 activated by TaCaM3-2B. Knocking down TaNADK2 expression by virus-mediated gene silencing significantly increased fungal growth and development, suggesting a decrease in resistance against Pst infection. In conclusion, our findings indicate that Pst effector Pst03724 inhibits the activity of NADK by interfering with the TaCaM3-2B-TaNADK2 association, thereby facilitating Pst infection.

Salivary Protein Cyclin-Dependent Kinase-like from Grain Aphid Sitobion avenae Suppresses Wheat Defense Response and Enhances Aphid Adaptation

Aphids are insect pests that suck phloem sap and introduce salivary proteins into plant tissues through saliva secretion. The effector of salivary proteins plays a key role in the modulation of host plant defense responses and enhancing aphid host adaptation. Based on previous transcriptome sequencing results, a candidate effector cyclin-dependent kinase-like (CDK) was identified from the grain aphid Sitobion avenae. In this study, the function of SaCDK in wheat defense response and the adaptation of S. avenae was investigated. Our results showed that the transient overexpression of SaCDK in tobacco Nicotiana benthamiana suppressed cell death triggered by mouse pro-apoptotic protein-BAX or Phytophthora infestans PAMP-INF1. SaCDK, delivered into wheat cells through a Pseudomonas fluorescens-mediated bacterial type III secretion system, suppressed callose deposition in wheat seedlings, and the overexpression of SaCDK in wheat significantly decreased the expression levels of salicylic acid and jasmonic acid signaling pathway-related genes phenylalanine ammonia lyase (PAL), pathogenesis-related 1 protein (PR1), lipoxygenase (LOX) and Ω-3 fatty acid desaturase (FAD). In addition, aphid bioassay results showed that the survival and fecundity of S. avenae were significantly increased while feeding on the wheat plants carrying SaCDK. Taken together, our findings demonstrate that the salivary protein SaCDK is involved in inhibiting host defense response and improving its host adaptation, which lays the foundation to uncover the mechanism of the interaction of cereal aphids and host plants.

Glycine-serine-rich effector PstGSRE4 in Puccinia striiformis f. sp. tritici targets and stabilizes TaGAPDH2 that promotes stripe rust disease

Puccinia striiformis f. sp. tritici (Pst) secretes effector proteins that enter plant cells and manipulate host processes. In a previous study, we identified a glycine-serine-rich effector PstGSRE4, which was proven to regulate the reactive oxygen species (ROS) pathway by interacting with TaCZSOD2. In this study, we further demonstrated that PstGSRE4 interacts with wheat glyceraldehyde-3-phosphate dehydrogenase TaGAPDH2, which is related to ROS signalling. In wheat, silencing of TaGAPDH2 by virus-induced gene silencing increased the accumulation of ROS induced by the Pst virulent race CYR31. Overexpression of TaGAPDH2 decreased the accumulation of ROS induced by the avirulent Pst race CYR23. In addition, TaGAPDH2 suppressed Pst candidate elicitor Pst322-triggered cell death by decreasing ROS accumulation in Nicotiana benthamiana. Knocking down TaGAPDH2 expression attenuated Pst infection, whereas overexpression of TaGAPDH2 promoted Pst infection, indicating that TaGAPDH2 is a negative regulator of plant defence. In N. benthamiana, PstGSRE4 stabilized TaGAPDH2 through inhibition of the 26S proteasome-mediated destabilization. Overall, these results suggest that TaGAPDH2 is hijacked by the Pst effector as a negative regulator of plant immunity to promote Pst infection in wheat.

The interaction of two Puccinia striiformis f. sp. tritici effectors modulates high-temperature seedling-plant resistance in wheat

Wheat cultivar Xiaoyan 6 (XY6) has high-temperature seedling-plant (HTSP) resistance to Puccinia striiformis f. sp. tritici (Pst). However, the molecular mechanism of Pst effectors involved in HTSP resistance remains unclear. In this study, we determined the interaction between two Pst effectors, PstCEP1 and PSTG_11208, through yeast two-hybrid (Y2H), bimolecular fluorescence complementation (BiFC), and pull-down assays. Transient overexpression of PSTG_11208 enhanced HTSP resistance in different temperature treatments. The interaction between PstCEP1 and PSTG_11208 inhibited the resistance enhancement by PSTG_11208. Furthermore, the wheat apoplastic thaumatin-like protein 1 (TaTLP1) appeared to recognize Pst invasion by interacting with PSTG_11208 and initiate the downstream defence response by the pathogenesis-related protein TaPR1. Silencing of TaTLP1 and TaPR1 separately or simultaneously reduced HTSP resistance to Pst in XY6. Moreover, we found that PstCEP1 targeted wheat ferredoxin 1 (TaFd1), a homologous protein of rice OsFd1. Silencing of TaFd1 affected the stability of photosynthesis in wheat plants, resulting in chlorosis on the leaves and reducing HTSP resistance. Our findings revealed the synergistic mechanism of effector proteins in the process of pathogen infection.

Identification of gene modules and hub genes associated with Colletotrichum siamense infection in mango using weighted gene co-expression network analysis

Colletotrichum siamense is a hemibiotrophic ascomycetous fungus responsible for mango anthracnose. The key genes involved in C. siamense infection remained largely unknown. In this study, we conducted weighted gene co-expression network analysis (WGCNA) of RNA-seq data to mine key genes involved in Colletotrichum siamense-mango interactions. Gene modules of Turquoise and Salmon, containing 1039 and 139 respectively, were associated with C. siamense infection, which were conducted for further analysis. GO enrichment analysis revealed that protein synthesis, organonitrogen compound biosynthetic and metabolic process, and endoplasmic reticulum-related genes were associated with C. siamense infection. A total of 568 proteins had homologs in the PHI database, 370 of which were related to virulence. The hub genes in each module were identified, which were annotated as O-methyltransferase (Salmon) and Clock-controlled protein 6 (Turquoise). A total of 24 proteins exhibited characteristics of SCRPs. By using transient expression in Nicotiana benthamiana, the SCRPs of XM_036637681.1 could inhibit programmed cell death (PCD) that induced by BAX (BCL-2-associated X protein), suggesting that it may play important roles in C. siamense infection. A mango-C. siamense co-expression network was constructed, and the mango gene of XM_044632979.1 (auxin-induced protein 15A-like) was positively associated with 5 SCRPs. These findings help to deepen the current understanding of necrotrophic stage in C. siamense infection.

SmCSP4 from aphid saliva stimulates salicylic acid-mediated defence responses in wheat by interacting with transcription factor TaWKRY76

Aphid salivary proteins are critical in modulating plant defence responses. Grain aphid Sitobion miscanthi is an important wheat pest worldwide. However, the molecular basis for the regulation of the plant resistance to cereal aphids remains largely unknown. Here, we show that SmCSP4, a chemosensory protein from S. miscanthi saliva, is secreted into wheat plants during aphid feeding. Delivery of SmCSP4 into wheat leaves activates salicylic acid (SA)-mediated plant defence responses and subsequently reduces aphid performance by deterring aphid feeding behaviour. In contrast, silencing SmCSP4 gene via nanocarrier-mediated RNAi significantly decreases the ability of aphids to activate SA defence pathway. Protein-protein interaction assays showed that SmCSP4 directly interacts with wheat transcriptional factor TaWRKY76 in plant nucleus. Furthermore, TaWRKY76 directly binds to the promoter of SA degradation gene Downy Mildew Resistant 6 (DMR6) and regulates its gene expression as transcriptional activator. SmCSP4 secreted by aphids reduces the transcriptional activation activity of TaWRKY76 on DMR6 gene expression, which is proposed to result in increases of SA accumulation and enhanced plant immunity. This study demonstrated that SmCSP4 acts as salivary elicitor that is involved in activating SA signalling defence pathway of wheat by interacting with TaWRKY76, which provide novel insights into aphid-cereal crops interactions and the molecular mechanism on induced plant immunity.

Candidate effectors for leaf rust resistance gene Lr28 identified through transcriptome and in-silico analysis

Puccinia spp. causing rust diseases in wheat and other cereals secrete several specialized effector proteins into host cells. Characterization of these proteins and their interaction with host's R proteins could greatly help to limit crop losses due to diseases. Prediction of effector proteins by combining the transcriptome analysis and multiple in-silico approaches is gaining importance in revealing the pathogenic mechanism. The present study involved identification of 13 Puccinia triticina (Pt) coding sequences (CDSs), through transcriptome analysis, that were differentially expressed during wheat-leaf rust interaction; and prediction of their effector like features using different in-silico tools. NCBI-BLAST and pathogen-host interaction BLAST (PHI-BLAST) tools were used to annotate and classify these sequences based on their most closely matched counterpart in both the databases. Homology between CDSs and the annotated sequences in the NCBI database ranged from 79 to 94% and with putative effectors of other plant pathogens in PHI-BLAST from 24.46 to 54.35%. Nine of the 13 CDSs had effector-like features according to EffectorP 3.0 (≥0.546 probability of these sequences to be effector). The qRT-PCR expression analysis revealed that the relative expression of all CDSs in compatible interaction (HD2329) was maximum at 11 days post inoculation (dpi) and that in incompatible interactions (HD2329 + Lr28) was maximum at 3 dpi in seven and 9 dpi in five CDSs. These results suggest that six CDSs (>0.8 effector probability as per EffectorP 3.0) could be considered as putative Pt effectors. The molecular docking and MD simulation analysis of these six CDSs suggested that candidate Lr28 protein binds more strongly to candidate effector c14094_g1_i1 to form more stable complex than the remaining five. Further functional characterization of these six candidate effectors should prove useful for a better understanding of wheat-leaf rust interaction. In turn, this should facilitate effector-based leaf rust resistance breeding in wheat.

Tomato Aphid (Aphis gossypii) Secreted Saliva Can Enhance Aphid Resistance by Upregulating Signaling Molecules in Tomato (Solanum lycopersicum)

This study investigated the impact of Aphis gossypii watery saliva on the induction of tomato (Solanum lycopersicum) plant resistance. To examine the role of A. gossypii saliva, we collected watery saliva from A. gossypii after a 48 h feeding period on an artificial diet. SDS-PAGE resolving gel 12% was used to separate the salivary proteins. Relative expression of gene analysis revealed that the intrusion of A. gossypii saliva dripping onto S. lycopersicum leaves triggered robust defense responses mediated by a signaling molecule, i.e., salicylic acid, while the signaling molecule's jasmonic acid-dependent defense responses were moderately activated. Aphid saliva infiltrated S. lycopersicum leaves slowed the intrinsic rate of population growth of A. gossypii and significantly reduced the number of nymphs produced daily, compared to untreated leaves. During a choice test with untreated S. lycopersicum, aphids showed a repellent response towards saliva-infiltrated S. lycopersicum. Moreover, the (EPG) electrical penetration graph analysis demonstrated that the eating pattern of A. gossypii compared to untreated S. lycopersicum, that had been exposed to saliva was negatively impacted. These results provide compelling evidence for the involvement of salivary components of A. gossypii in inducing resistance against aphids in S. lycopersicum plants. Furthermore, the study underscores the crucial role of watery saliva in the intricate interactions between aphids and plants. The activation of pathways was also part of the defensive response (jasmonic acid (JA), salicylic acid (SA) signaling molecules). The findings of this research deliver valuable insights into the potential of watery aphid saliva as a natural defense mechanism against aphid infestations in S. lycopersicum crops.

Identification and Functional Analysis of CAP Genes from the Wheat Stripe Rust Fungus Puccinia striiformis f. sp. tritici

Cysteine-rich secretory proteins (C), antigen 5 (A), and pathogenesis-related 1 proteins (P) comprise widespread CAP superfamily proteins, which have been proven to be novel virulence factors of mammalian pathogenic fungi and some plant pathogens. Despite this, the identification and function of CAP proteins in more species of plant pathogens still need to be studied. This work presents the identification and functional analysis of CAP superfamily proteins from Puccinia striiformis f. sp. tritici (Pst), an important fungal pathogen that causes wheat stripe rust on wheat worldwide. A total of six CAP genes were identified in the Pst genome, designated as PsCAP1-PsCAP6. Five PsCAP proteins, including PsCAP1, PsCAP2, PsCAP3, PsCAP4, and PsCAP5, have N-terminal signal peptides secreted with the yeast signal sequence trap assay. Single-nucleotide polymorphism (SNP) analysis indicated that they showed a low level of intraspecies polymorphism. The expression abundance of PsCAP genes at different Pst infection stages was detected by RT-qPCR, and most of them were highly expressed during Pst infection on wheat and also Pst sexual reproduction on barberry (Berberis shensiana). Noticeably, the silencing of these six PsCAP genes by BSMV-mediated HIGS indicated that PsCAP1, PsCAP4, and PsCAP5 contribute significantly to Pst infection in wheat. These results indicate that PsCAP proteins may act as virulence factors during Pst infection, which also provides insights into Pst pathogenicity.

The Biological Roles of Puccinia striiformis f. sp. tritici Effectors during Infection of Wheat

Puccinia striiformis f. sp. tritici (Pst) is the causative agent of wheat stripe rust, which can lead to a significant loss in annual wheat yields. Therefore, there is an urgent need for a deeper comprehension of the basic mechanisms underlying Pst infection. Effectors are known as the agents that plant pathogens deliver into host tissues to promote infection, typically by interfering with plant physiology and biochemistry. Insights into effector activity can significantly aid the development of future strategies to generate disease-resistant crops. However, the functional analysis of Pst effectors is still in its infancy, which hinders our understanding of the molecular mechanisms of the interaction between Pst and wheat. In this review, we summarize the potential roles of validated and proposed Pst effectors during wheat infection, including proteinaceous effectors, non-coding RNAs (sRNA effectors), and secondary metabolites (SMs effectors). Further, we suggest specific countermeasures against Pst pathogenesis and future research directions, which may promote our understanding of Pst effector functions during wheat immunity attempts.

Molecular Mechanisms of the Co-Evolution of Wheat and Rust Pathogens

Wheat (Triticum spp.) is a cereal crop domesticated >8000 years ago and the second-most-consumed food crop nowadays. Ever since mankind has written records, cereal rust diseases have been a painful awareness in antiquity documented in the Old Testament (about 750 B.C.). The pathogen causing the wheat stem rust disease is among the first identified plant pathogens in the 1700s, suggesting that wheat and rust pathogens have co-existed for thousands of years. With advanced molecular technologies, wheat and rust genomes have been sequenced, and interactions between the host and the rust pathogens have been extensively studied at molecular levels. In this review, we summarized the research at the molecular level and organized the findings based on the pathogenesis steps of germination, penetration, haustorial formation, and colonization of the rusts to present the molecular mechanisms of the co-evolution of wheat and rust pathogens.

Two salivary proteins Sm10 and SmC002 from grain aphid Sitobion miscanthi modulate wheat defense and enhance aphid performance

The grain aphid Sitobion miscanthi is a serious pest of wheat that causes severe economic damage by sucking phloem sap and transmitting plant viruses. Here, two putative salivary effector homologs from S. miscanthi (Sm10 and SmC002) were selected based on sequence similarity to other characterized aphid candidate effectors. These effectors were then delivered into wheat cells separately via the type III secretion system of Pseudomonas fluorescens to elucidate their functions in the regulation of plant defenses and host fitness. The results showed that the delivery of either Sm10 or SmC002 into wheat plants significantly suppressed callose deposition and affected the transcript levels of callose synthase genes. The expression levels of salicylic acid (SA)-associated defense genes were upregulated significantly in wheat leaves carrying either Sm10 or SmC002. Moreover, LC-MS/MS analysis revealed that wheat SA levels significantly increased after the delivery of the two effectors. The results of aphid bioassays conducted on the wheat plants carrying Sm10 or SmC002 showed significant increases in the survival and fecundity of S. miscanthi. This study demonstrated that the Sm10 and SmC002 salivary effectors of S. miscanthi enhanced host plant susceptibility and benefited S. miscanthi performance by regulating wheat defense signaling pathways.

Wheat leaf rust fungus effector Pt13024 is avirulent to TcLr30

Wheat leaf rust, caused by Puccinia triticina Eriks. (Pt), is a global wheat disease threatening wheat production. Dissecting how Pt effector proteins interact with wheat has great significance in understanding the pathogenicity mechanisms of Pt. In the study, the cDNA of Pt 13-5-72 interacting with susceptible cultivar Thatcher was used as template to amplify Pt13024 gene. The expression pattern and structure of Pt13024 were analyzed by qRT-PCR and online softwares. The secretion function of Pt13024 signal peptide was verified by the yeast system. Subcellular localization of Pt13024 was analyzed using transient expression on Nicotiana benthamiana. The verification that Pt13024 inhibited programmed cell death (PCD) was conducted on N. benthamiana and wheat. The deletion mutation of Pt13024 was used to identify the virulence function motif. The transient transformation of wheat mediated by the type III secretion system (TTSS) was used to analyze the activity of regulating the host defense response of Pt13024. Pt13024 gene silencing was performed by host-induced gene silencing (HIGS). The results showed that Pt13024 was identified as an effector and localized in the cytoplasm and nucleus on the N. benthamiana. It can inhibit PCD induced by the Bcl-2-associated X protein (BAX) from mice and infestans 1 (INF1) from Phytophthora infestans on N. benthamiana, and it can also inhibit PCD induced by DC3000 on wheat. The amino acids 22 to 41 at N-terminal of the Pt13024 are essential for the inhibition of programmed cell death (PCD) induced by BAX. The accumulation of reactive oxygen species and deposition of callose in near-isogenic line TcLr30, which is in Thatcher background with Lr30, induced by Pt13024 was higher than that in 41 wheat leaf rust-resistant near-isogenic lines (monogenic lines) with different resistance genes and Thatcher. Silencing of Pt13024 reduced the leaf rust resistance of Lr30 during the interaction between Pt and TcLr30. We can conclude that Pt13024 is avirulent to TcLr30 when Pt interacts with TcLr30. These findings lay the foundation for further investigations into the role of Pt effector proteins in pathogenesis and their regulatory mechanisms.

The Puccinia striiformis effector Hasp98 facilitates pathogenicity by blocking the kinase activity of wheat TaMAPK4

The obligate biotrophic fungus Puccinia striiformis f. sp. tritici (Pst) employs virulence effectors to disturb host immunity and causes devastating stripe rust disease. However, our understanding of how Pst effectors regulate host defense responses remains limited. In this study, we determined that the Pst effector Hasp98, which is highly expressed in Pst haustoria, inhibits plant immune responses triggered by flg22 or nonpathogenic bacteria. Overexpression of Hasp98 in wheat (Triticum aestivum) suppressed avirulent Pst-triggered immunity, leading to decreased H₂ O₂ accumulation and promoting P. striiformis infection, whereas stable silencing of Hasp98 impaired P. striiformis pathogenicity. Hasp98 interacts with the wheat mitogen-activated protein kinase TaMAPK4, a positive regulator of plant resistance to stripe rust. The conserved TEY motif of TaMAPK4 is important for its kinase activity, which is required for the resistance function. We demonstrate that Hasp98 inhibits the kinase activity of TaMAPK4 and that the stable silencing of TaMAPK4 compromises wheat resistance against P. striiformis. These results suggest that Hasp98 acts as a virulence effector to interfere with the MAPK signaling pathway in wheat, thereby promoting P. striiformis infection.

Caveats of Using Bacterial Type Three Secretion Assays for Validating Fungal Avirulence Effectors in Wheat

Functional characterization of effector proteins of fungal obligate biotrophic pathogens, especially confirmation of avirulence (Avr) properties, has been notoriously difficult, due to the experimental intractability of many of these organisms. Previous studies in wheat have shown promising data suggesting the type III secretion system (T3SS) of bacteria may be a suitable surrogate for delivery and detection of Avr properties of fungal effectors. However, these delivery systems were tested in the absence of confirmed Avr effectors. Here, we tested two previously described T3SS-mediated delivery systems for their suitability when delivering two confirmed Avr effectors from two fungal pathogens of wheat, Puccinia graminis f. sp. tritici and Magnaporthe oryzae pathotype tritici. We showed that both effectors (AvrSr50 and AvrRmg8) were unable to elicit a hypersensitive response on wheat seedlings with the corresponding resistance gene when expressed by the Pseudomonas fluorescens "Effector to Host Analyser" (EtHAn) system. Furthermore, we found the utility of Burkholderia glumae for screening Avr phenotypes is severely limited, as the wild-type strain elicits nonhost cell death in multiple wheat accessions. These results provide valuable insight into the suitability of these systems for screening fungal effectors for Avr properties that may help guide further development of surrogate bacterial delivery systems in wheat. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Puccinia striiformis f. sp. tritici effectors in wheat immune responses

The obligate biotrophic fungus Puccinia striiformis f. sp. tritici, which causes yellow (stripe) rust disease, is among the leading biological agents resulting in tremendous yield losses on global wheat productions per annum. The combatting strategies include, but are not limited to, fungicide applications and the development of resistant cultivars. However, evolutionary pressure drives rapid changes, especially in its "effectorome" repertoire, thus allowing pathogens to evade and breach resistance. The extracellular and intracellular effectors, predominantly secreted proteins, are tactical arsenals aiming for many defense processes of plants. Hence, the identity of the effectors and the molecular mechanisms of the interactions between the effectors and the plant immune system have long been targeted in research. The obligate biotrophic nature of P. striiformis f. sp. tritici and the challenging nature of its host, the wheat, impede research on this topic. Next-generation sequencing and novel prediction algorithms in bioinformatics, which are accompanied by in vitro and in vivo validation approaches, offer a speedy pace for the discovery of new effectors and investigations of their biological functions. Here, we briefly review recent findings exploring the roles of P. striiformis f. sp. tritici effectors together with their cellular/subcellular localizations, host responses, and interactors. The current status and the challenges will be discussed. We hope that the overall work will provide a broader view of where we stand and a reference point to compare and evaluate new findings.

The salivary effector protein Sg2204 in the greenbug Schizaphis graminum suppresses wheat defence and is essential for enabling aphid feeding on host plants

Aphids secrete diverse repertoires of salivary effectors into host plant cells to promote infestation by modulating plant defence. The greenbug Schizaphis graminum is an important cereal aphid worldwide. However, the secreted effectors of S. graminum are still uncharacterized. Here, 76 salivary proteins were identified from the watery saliva of S. graminum using transcriptome and proteome analyses. Among them, a putative salivary effector Sg2204 was significantly up-regulated during aphid feeding stages, and transient overexpression of Sg2204 in Nicotiana benthamiana inhibited cell death induced by BAX or INF1. Delivering Sg2204 into wheat via the type III secretion system of Pseudomonas fluorescens EtAnH suppressed pattern-triggered immunity (PTI)-associated callose deposition. The transcript levels of jasmonic acid (JA)- and salicylic acid (SA)-associated defence genes of wheat were significantly down-regulated, and the contents of both JA and SA were also significantly decreased after delivery of Sg2204 into wheat leaves. Additionally, feeding on wheat expressing Sg2204 significantly increased the weight and fecundity of S. graminum and promoted aphid phloem feeding. Sg2204 was efficiently silenced via spray-based application of the nanocarrier-mediated transdermal dsRNA delivery system. Moreover, Sg2204-silenced aphids induced a stronger wheat defence response and resulted in negative impacts on aphid feeding behaviour, survival and fecundity. Silencing of Sg2204 homologues from four aphid species using nanocarrier-delivered dsRNA also significantly reduced aphid performance on host plants. Thus, our study characterized the salivary effector Sg2204 of S. graminum involved in promoting host susceptibility by suppressing wheat defence, which can also be regarded as a promising RNAi target for aphid control.

A Phakopsora pachyrhizi Effector Suppresses PAMP-Triggered Immunity and Interacts with a Soybean Glucan Endo-1,3-β-Glucosidase to Promote Virulence

Asian soybean rust, caused by the fungus Phakopsora pachyrhizi, is one of the most important diseases affecting soybean production in tropical areas. During infection, P. pachyrhizi secretes proteins from haustoria that are transferred into plant cells to promote virulence. To date, only one candidate P. pachyrhizi effector protein has been characterized in detail to understand the mechanism by which it suppresses plant defenses to enhance infection. Here, we aimed to extend understanding of the pathogenic mechanisms of P. pachyrhizi based on the discovery of host proteins that interact with the effector candidate Phapa-7431740. We demonstrated that Phapa-7431740 suppresses pathogen-associated molecular pattern-triggered immunity (PTI) and that it interacts with a soybean glucan endo-1,3-β-glucosidase (GmβGLU), a pathogenesis-related (PR) protein belonging to the PR-2 family. Structural and phylogenetic characterization of the PR-2 protein family predicted in the soybean genome and comparison to PR-2 family members in Arabidopsis thaliana and cotton, demonstrated that GmβGLU is a type IV β-1,3-glucanase. Transcriptional profiling during an infection time course showed that the GmβGLU mRNA is highly induced during the initial hours after infection, coinciding with peak of expression of Phapa-7431740. The effector was able to interfere with the activity of GmβGLU in vitro, with a dose-dependent inhibition. Our results suggest that Phapa-7431740 may suppress PTI by interfering with glucan endo-1,3-β-glucosidase activity. [Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 "No Rights Reserved" license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2022.

Evolution of pathogenicity in obligate fungal pathogens and allied genera

Obligate fungal pathogens (ascomycetes and basidiomycetes) and oomycetes are known to cause diseases in cereal crop plants. They feed on living cells and most of them have learned to bypass the host immune machinery. This paper discusses some of the factors that are associated with pathogenicity drawing examples from ascomycetes, basidiomycetes and oomycetes, with respect to their manifestation in crop plants. The comparisons have revealed a striking similarity in the three groups suggesting convergent pathways that have arisen from three lineages independently leading to an obligate lifestyle. This review has been written with the intent, that new information on adaptation strategies of biotrophs, modifications in pathogenicity strategies and population dynamics will improve current strategies for breeding with stable resistance.

Current Status and Future Perspectives of Genomics Research in the Rust Fungi

Rust fungi in Pucciniales have caused destructive plant epidemics, have become more aggressive with new virulence, rapidly adapt to new environments, and continually threaten global agriculture. With the rapid advancement of genome sequencing technologies and data analysis tools, genomics research on many of the devastating rust fungi has generated unprecedented insights into various aspects of rust biology. In this review, we first present a summary of the main findings in the genomics of rust fungi related to variations in genome size and gene composition between and within species. Then we show how the genomics of rust fungi has promoted our understanding of the pathogen virulence and population dynamics. Even with great progress, many questions still need to be answered. Therefore, we introduce important perspectives with emphasis on the genome evolution and host adaptation of rust fungi. We believe that the comparative genomics and population genomics of rust fungi will provide a further understanding of the rapid evolution of virulence and will contribute to monitoring the population dynamics for disease management.

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.

A Bacterial Type Three Secretion-Based Delivery System for Functional Characterization of Sporisorium scitamineum Plant Immune Suppressing Effector Proteins

The facultative biotrophic basidiomycete Sporisorium scitamineum causes smut disease in sugarcane. This study applied an assay to identify S. scitamineum candidate effectors (CEs) with plant immunity suppression activities by delivering them into Nicotiana benthamiana cells via the type-three secretion system of Pseudomonas fluorescens EtHAn. Six CEs were individually cloned into the pEDV6 vector and expressed by P. fluorescens EtHAn for translocation into the plant cells. Three CEs (g1052, g3890, and g5159) could suppress pattern-triggered immunity (PTI) responses with high reproducibility in different coinfiltration experiments with P. syringae pv. tomato DC3000. In addition, three CEs (g1052, g4549, and g5159) were also found to be AvrB-induced suppressors of effector-triggered immunity (ETI), demonstrating for the first time that S. scitamineum can defeat both PTI and ETI responses. A transcriptomic analysis at different stages of infection by the smut fungus of three sugarcane cultivars with contrasting responses to the pathogen revealed that suppressors g1052, g3890, g4549, and g5159 were induced at the early stage of infection. By contrast, the two CEs (g2666 and g6610) that did not exhibit suppression activities expressed only at the late stage of infection. Moreover, genomic structures of the CEs and searches for orthologs in other smut species suggested duplication events and further divergence in CEs evolution of S. scitamineum. Thus, the transient assay applied here demonstrated the potential of pEDV6 and P. fluorescens EtHAn as biological tools for identifying plant immune suppressors from S. scitamineum.

Transcriptome Analysis of Fusarium–Tomato Interaction Based on an Updated Genome Annotation of Fusarium oxysporum f. sp. lycopersici Identifies Novel Effector Candidates That Suppress or Induce Cell Death in Nicotiana benthamiana

Fusarium oxysporum f. sp. lycopersici (Fol) causes vascular wilt disease in tomato. Upon colonization of the host, Fol secretes many small effector proteins into the xylem sap to facilitate infection. Besides known SIX (secreted in xylem) proteins, the identity of additional effectors that contribute to Fol pathogenicity remains largely unexplored. We performed a deep RNA-sequencing analysis of Fol race 2-infected tomato, used the sequence data to annotate a published genome assembly generated via PacBio SMRT sequencing of the Fol race 2 reference strain Fol4287, and analysed the resulting transcriptome to identify Fol effector candidates among the newly annotated genes. We examined the Fol-infection expression profiles of all 13 SIX genes present in Fol race 2 and identified 27 new candidate effector genes that were likewise significantly upregulated upon Fol infection. Using Agrobacterium-mediated transformation, we tested the ability of 22 of the new candidate effector genes to suppress or induce cell death in leaves of Nicotiana benthamiana. One effector candidate designated Fol-EC19, encoding a secreted guanyl-specific ribonuclease, was found to trigger cell death and two effector candidates designated Fol-EC14 and Fol-EC20, encoding a glucanase and a secreted trypsin, respectively, were identified that can suppress Bax-mediated cell death. Remarkably, Fol-EC14 and Fol-EC20 were also found to suppress I-2/Avr2- and I/Avr1-mediated cell death. Using the yeast secretion trap screening system, we showed that these three biologically-active effector candidates each contain a functional signal peptide for protein secretion. Our findings provide a basis for further understanding the virulence functions of Fol effectors.

Infection Strategies and Pathogenicity of Biotrophic Plant Fungal Pathogens

Biotrophic plant pathogenic fungi are widely distributed and are among the most damaging pathogenic organisms of agriculturally important crops responsible for significant losses in quality and yield. However, the pathogenesis of obligate parasitic pathogenic microorganisms is still under investigation because they cannot reproduce and complete their life cycle on an artificial medium. The successful lifestyle of biotrophic fungal pathogens depends on their ability to secrete effector proteins to manipulate or evade plant defense response. By integrating genomics, transcriptomics, and effectoromics, insights into how the adaptation of biotrophic plant fungal pathogens adapt to their host populations can be gained. Efficient tools to decipher the precise molecular mechanisms of rust–plant interactions, and standardized routines in genomics and functional pipelines have been established and will pave the way for comparative studies. Deciphering fungal pathogenesis not only allows us to better understand how fungal pathogens infect host plants but also provides valuable information for plant diseases control, including new strategies to prevent, delay, or inhibit fungal development. Our review provides a comprehensive overview of the efforts that have been made to decipher the effector proteins of biotrophic fungal pathogens and demonstrates how rapidly research in the field of obligate biotrophy has progressed.

Pathogen effectors: What do they do at plasmodesmata?

Plants perceive an assortment of external cues during their life cycle, including abiotic and biotic stressors. Biotic stress from a variety of pathogens, including viruses, oomycetes, fungi, and bacteria, is considered to be a substantial factor hindering plant growth and development. To hijack the host cell's defence machinery, plant pathogens have evolved sophisticated attack strategies mediated by numerous effector proteins. Several studies have indicated that plasmodesmata (PD), symplasmic pores that facilitate cell-to-cell communication between a cell and neighbouring cells, are one of the targets of pathogen effectors. However, in contrast to plant-pathogenic viruses, reports of fungal- and bacterial-encoded effectors that localize to and exploit PD are limited. Surprisingly, a recent study of PD-associated bacterial effectors has shown that a number of bacterial effectors undergo cell-to-cell movement via PD. Here we summarize and highlight recent advances in the study of PD-associated fungal/oomycete/bacterial effectors. We also discuss how pathogen effectors interfere with host defence mechanisms in the context of PD regulation.

Prediction of effector proteins and their implications in pathogenicity of phytopathogenic filamentous fungi: A review

Plant pathogenic fungi encode and secrete effector proteins to promote pathogenesis. In recent years, the important role of effector proteins in fungi and plant host interactions has become increasingly prominent. In this review, the functional characterization and molecular mechanisms by which fungal effector proteins modulate biological processes and suppress the defense of plant hosts are discussed, with an emphasis on cell localization during fungal infection. This paper also provides a comprehensive review of bioinformatic and experimental methods that are currently available for the identification of fungal effector proteins. We additionally summarize the secretion pathways and the methods for verifying the presence effector proteins in plant host cells. For future research, comparative genomic studies of different pathogens with varying life cycles will allow comprehensive and systematic identification of effector proteins. Additionally, functional analysis of effector protein interactions with a wider range of hosts (especially non-model crops) will provide more detailed repertoires of fungal effectors. Identifying effector proteins and verifying their functions will improve our understanding of their role in causing disease and in turn guide future strategies for combatting fungal infections.

Verticillium dahliae CFEM proteins manipulate host immunity and differentially contribute to virulence

Background

Verticillium dahliae is a fungal pathogen that causes a vascular wilt on many economically important crops. Common fungal extracellular membrane (CFEM) domain proteins including secreted types have been implicated in virulence, but their roles in this pathogen are still unknown.

Results

Nine secreted small cysteine-rich proteins (VdSCPs) with CFEM domains were identified by bioinformatic analyses and their differential suppression of host immune responses were evaluated. Two of these proteins, VdSCP76 and VdSCP77, localized to the plant plasma membrane owing to their signal peptides and mediated broad-spectrum suppression of all immune responses induced by typical effectors. Deletion of either VdSCP76 or VdSCP77 significantly reduced the virulence of V. dahliae on cotton. Furthermore, VdSCP76 and VdSCP77 suppressed host immunity through the potential iron binding site conserved in CFEM family members, characterized by an aspartic acid residue in seven VdSCPs (Asp-type) in contrast with an asparagine residue (Asn-type) in VdSCP76 and VdSCP77. V. dahliae isolates carrying the Asn-type CFEM members were more virulent on cotton than those carrying the Asp-type.

Conclusions

In the iron-insufficient xylem, V. dahliae is likely to employ the Asp-type CFEM members to chelate iron, and Asn-type CFEM members to suppress immunity, for successful colonization and propagation in host plants.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01254-x.

Activation of Wheat Defense Response by Buchnera aphidicola-Derived Small Chaperone Protein GroES in Wheat Aphid Saliva

Salivary proteins secreted by aphids during feeding play an important role in regulating the plant defense response. We used mass spectrometry to identify 155 proteins from the wheat aphid, Sitobion miscanthi, among which 44 proteins were derived from the primary symbiont, Buchnera aphidicola. GroES, which is a highly abundant molecular chaperone that binds to GroEL, was detected in saliva. In vitro injection of purified GroES protein and overexpression of GroES in wheat leaves verified that GroES induced hydrogen peroxide accumulation and callose deposition in wheat and further activated the plant salic acid and jasmonic acid defense pathways. Our findings indicate that plants may have evolved new strategies to detect aphid attack and trigger defense responses by recognizing proteins derived from B. aphidicola, which is present in almost all aphid species.

Effectors of Puccinia striiformis f. sp. tritici Suppressing the Pathogenic-Associated Molecular Pattern-Triggered Immune Response Were Screened by Transient Expression of Wheat Protoplasts

Puccinia striiformis f. sp. tritici (Pst) is an important pathogen of wheat (Triticum aestivum L.) stripe rust, and the effector protein secreted by haustoria is a very important component involved in the pathogenic process. Although the candidate effector proteins secreted by Pst haustoria have been predicted to be abundant, few have been functionally validated. Our study confirmed that chitin and flg22 could be used as elicitors of the pathogenic-associated molecular pattern-triggered immune (PTI) reaction in wheat leaves and that TaPr-1-14 could be used as a marker gene to detect the PTI reaction. In addition, the experimental results were consistent in wheat protoplasts. A rapid and efficient method for screening and identifying the effector proteins of Pst was established by using the wheat protoplast transient expression system. Thirty-nine Pst haustorial effector genes were successfully cloned and screened for expression in the protoplast. We identified three haustorial effector proteins, PSEC2, PSEC17, and PSEC45, that may inhibit the response of wheat to PTI. These proteins are localized in the somatic cytoplasm and nucleus of wheat protoplasts and are highly expressed during the infection and parasitism of wheat.

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.

Candidate Effectors from Botryosphaeria dothidea Suppress Plant Immunity and Contribute to Virulence

Fungal effectors play important roles in host-pathogen interactions. Botryosphaeria dothidea is an ascomycetous fungus that is responsible for the diseases of hundreds of woody plant species, including apple ring rot, which seriously affects apples worldwide. However, little is known about the effectors of B. dothidea. In this study, we analyzed the B. dothidea genome and predicted 320 candidate effector genes, 124 of which were successfully amplified and cloned. We investigated the effects of these genes on plant cell death in Nicotiana benthamiana while using a transient expression system. Twenty-four hours after initial inoculation with Agrobacterium tumefaciens cells carrying candidate effectors, the infiltrated leaves were challenged with A. tumefaciens cells carrying the BAX gene. In total, 116 candidate effectors completely inhibited, while one partially inhibited, the programmed cell death (PCD) of N. benthamiana induced by BAX, whereas seven candidate effectors had no effect. We then further tested seven candidate effectors able to suppress BAX-triggered PCD (BT-PCD) and found that they all completely inhibited PCD triggered by the elicitors INF1, MKK1, and NPK1. This result suggests that these effectors were activated in order to suppress pathogen-associated molecular pattern-triggered immunity. The signal peptides of these candidate effectors exhibited secretory activity in yeast (pSUC2 vector). Moreover, the respective deletion of Bdo_11198 and Bdo_12090 significantly reduced the virulence of B. dothidea. These results suggest that these effectors play important roles in the interaction of B. dothidea with its hosts.

A Valsa mali Effector Protein 1 Targets Apple (Malus domestica) Pathogenesis-Related 10 Protein to Promote Virulence

To successfully colonize the plants, the pathogenic microbes secrete a mass of effector proteins which manipulate host immunity. Apple valsa canker is a destructive disease caused by the weakly parasitic fungus Valsa mali. A previous study indicated that the V. mali effector protein 1 (VmEP1) is an essential virulence factor. However, the pathogenic mechanism of VmEP1 in V. mali remains poorly understood. In this study, we found that the apple (Malus domestica) pathogenesis-related 10 proteins (MdPR10) are the virulence target of VmEP1 using a yeast two-hybrid screening. By bimolecular fluorescence (BiFC) and coimmunoprecipitation (Co-IP), we confirmed that the VmEP1 interacts with MdPR10 in vivo. Silencing of MdPR10 notably enhanced the V. mali infection, and overexpression of MdPR10 markedly reduced its infection, which corroborates its positive role in plant immunity against V. mali. Furthermore, we showed that the co-expression of VmEP1 with MdPR10 compromised the MdPR10-mediated resistance to V. mali. Taken together, our results revealed a mechanism by which a V. mali effector protein suppresses the host immune responses by interfering with the MdPR10-mediated resistance to V. mali during the infection.

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.

Proteinaceous effector discovery and characterization in filamentous plant pathogens

The complicated interplay of plant-pathogen interactions occurs on multiple levels as pathogens evolve to constantly evade the immune responses of their hosts. Many economically important crops fall victim to filamentous pathogens that produce small proteins called effectors to manipulate the host and aid infection/colonization. Understanding the effector repertoires of pathogens is facilitating an increased understanding of the molecular mechanisms underlying virulence as well as guiding the development of disease control strategies. The purpose of this review is to give a chronological perspective on the evolution of the methodologies used in effector discovery from physical isolation and in silico predictions, to functional characterization of the effectors of filamentous plant pathogens and identification of their host targets.

Smut fungi as a stratagem to characterize rust effectors: opportunities and challenges

The rust pathogens are one of the most complex fungi in the Basidiomycetes. The development of genomic resources for rust and other plant pathogens has opened the opportunities for functional genomics of fungal genes. Despite significant progress in the field of fungal genomics, functional characterization of the genome components has lacked, especially for the rust pathogens. Their obligate nature and lack of standard stable transformation protocol are the primary reasons for rusts to be one of the least explored genera despite its significance. In the recently sequenced rust genomes, a vast catalogue of predicted effectors and pathogenicity genes have been reported. However, most of these candidate genes remained unexplored due to the lack of suitable characterization methods. The heterologous expression of putative effectors in Nicotiana benthamiana and Arabidopsis thaliana has proved to be a rapid screening method for identifying the role of these effectors in virulence. However, no fungal system has been used for the functional validation of these candidate genes. The smuts, from the evolutionary point of view, are closely related to the rust pathogens. Moreover, they have been widely studied and hence could be a suitable model system for expressing rust fungal genes heterologously. The genetic manipulation methods for smuts are also well standardized. Complementation assays can be used for functional validation of the homologous genes present in rust and smut fungal pathogens, while the species-specific proteins can be expressed in the mutant strains of smut pathogens having reduced or no virulence for virulence analysis. We propose that smuts, especially Ustilago maydis, may prove to be a good model system to characterize rust effector proteins in the absence of methods to manipulate the rust genomes directly.

Effector Biology of Biotrophic Plant Fungal Pathogens: Current Advances and Future Prospects

The interaction of fungal pathogens with their host requires a novel invading mechanism and the presence of various virulence-associated components responsible for promoting the infection. The small secretory proteins, explicitly known as effector proteins, are one of the prime mechanisms of host manipulation utilized by the pathogen to disarm the host. Several effector proteins are known to translocate from fungus to the plant cell for host manipulation. Many fungal effectors have been identified using genomic, transcriptomic, and bioinformatics approaches. Most of the effector proteins are devoid of any conserved signatures, and their prediction based on sequence homology is very challenging, therefore by combining the sequence consensus based upon machine learning features, multiple tools have also been developed for predicting apoplastic and cytoplasmic effectors. Various post-genomics approaches like transcriptomics of virulent isolates have also been utilized for identifying active consortia of effectors. Significant progress has been made in understanding biotrophic effectors; however, most of it is underway due to their complex interaction with host and complicated recognition and signaling networks. This review discusses advances, and challenges in effector identification and highlighted various features of the potential effector proteins and approaches for understanding their genetics and strategies for regulation.

A novel effector CfEC92 of Colletotrichum fructicola contributes to glomerella leaf spot virulence by suppressing plant defences at the early infection phase

The ascomycete fungus Colletotrichum fructicola causes diseases on a broad range of plant species. On susceptible cultivars of apple, it induces severe early defoliation and fruit spots, named glomerella leaf spot (GLS), but the mechanisms of pathogenicity have remained elusive. Phytopathogens exhibit small secreted effectors to advance host infection by manipulating host immune reactions. We report the identification and characterization of CfEC92, an effector required for C. fructicola virulence. CfEC92 is a Colletotrichum-specific small secreted protein that suppresses BAX-triggered cell death in Nicotiana benthamiana. Accumulation of the gene transcript was barely detectable in conidia or vegetative hyphae, but was highly up-regulated in appressoria formed during early apple leaf infection. Gene deletion mutants were not affected in vegetative growth, appressorium formation, or appressorium-mediated cellophane penetration. However, the mutants were significantly reduced in virulence toward apple leaves and fruits. Microscopic examination indicated that infection by the deletion mutants elicited elevated deposition of papillae at the penetration sites, and formation of infection vesicles and primary hyphae was retarded. Signal peptide activity, subcellular localization, and cell death-suppressive activity (without signal peptide) assays suggest that CfEC92 could be secreted and perform virulence functions inside plant cells. RNA sequencing and quantitative reverse transcription PCR results confirmed that the deletion mutants triggered elevated host defence reactions. Our results strongly support the interpretation that CfEC92 contributes to C. fructicola virulence as a plant immunity suppressor at the early infection phase.

Revealing Differentially Expressed Genes and Identifying Effector Proteins of Puccinia striiformis f. sp. tritici in Response to High-Temperature Seedling Plant Resistance of Wheat Based on Transcriptome Sequencing

In the present study, we performed transcriptomic analysis to identify differentially expressed genes and effector proteins of Puccinia striiformis f. sp. tritici ( Pst ) in response to the high-temperature seedling-plant (HTSP) resistance in wheat. Experimental validation confirmed the function of the highest upregulated effector protein, PstCEP1. This study provides a key resource for understanding the biology and molecular basis of Pst responses to wheat HTSP resistance, and PstCEP1 may be used in future studies to understand pathogen-associated molecular pattern-triggered immunity and effector-triggered immunity processes in the Pst -wheat interaction system.

A putative effector UvHrip1 inhibits BAX-triggered cell death in Nicotiana benthamiana, and infection of Ustilaginoidea virens suppresses defense-related genes expression

Rice false smut (RFS), caused by Ustilaginoidea virens, is one of the most detrimental rice fungal diseases and pose a severe threat to rice production and quality. Effectors in U. virens often act as a set of essential virulence factors that play crucial roles in the interaction between host and the pathogen. Thus, the functions of each effector in U. virens need to be further explored. Here, we performed multiple alignment analysis and demonstrated a small secreted hypersensitive response-inducing protein (hrip), named UvHrip1, was highly conserved in fungi. The predicted SP of UvHrip1 was functional, which guided SUC secreted from yeast and was recognized by plant cells. The localization of UvHrip1 was mainly in the nucleus and cytoplasm monitored through the GFP fusion protein in Nicotiana benthamiana cells. uvhrip1 was drastically up-regulated in the susceptible cultivar LYP9 of rice during the pathogen infection, while did not in the resistant cultivar IR28. We also proved that UvHrip1 suppressed the mammalian BAX-induced necrosis-like defense symptoms in N. benthamiana. Furthermore, patterns of expression of defense-related genes, OsPR1#012 and OsPR10b, were regulated over U. virens infection in rice. Collectively, our data demonstrated that infection of U. virens suppresses defense-related genes expression and UvHrip1 was most likely a core effector in regulating plant immunity.

Alternate Hosts of Puccinia striiformis f. sp. tritici and Their Role

Understanding the interactions between the host and the pathogen is important in developing resistant cultivars and strategies for controlling the disease. Since the discovery of Berberis and Mahonia spp. as alternate hosts of the wheat stripe rust pathogen, Puccinia striiformis Westend. f. sp. tritici Erikss. (Pst), their possible role in generating new races of Pst through sexual reproduction has become a hot topic. To date, all the investigations about the role of alternate hosts in the occurrence of the wheat stripe rust epidemics revealed that it depends on alternate host species and environmental conditions. In this review, we summarized the current status of alternate hosts of Pst, their interactions with the pathogen, their importance in genetic diversity and disease epidemics. Most importantly, the recent research progress in understanding the role of alternate hosts of Pst is provided.

The Conserved Effector UvHrip1 interacts with OsHGW, and Infection of Ustilaginoidea virens Regulates Defense- and Heading Date-Related Signaling Pathway

Ustilaginoidea virens, which causes rice false smut (RFS), is one of the most detrimental rice fungal diseases and poses a severe threat to rice production and quality. Effectors in U. virens often act as a group of essential virulence factors that play crucial roles in the interaction between host and the pathogen. Thus, the functions of individual effectors in U. virens need to be further explored. Here, we demonstrated a small secreted hypersensitive response-inducing protein (hrip), named UvHrip1, which was highly conserved in U. virens isolates. UvHrip1 was also proven to suppress necrosis-like defense symptoms in N. benthamiana induced by the oomycete elicitor INF1. The localization of UvHrip1 was mainly in the nuclei and cytoplasm via monitoring the UvHrip1-GFP fusion protein in rice cells. Furthermore, Y2H and BiFC assay demonstrated that UvHrip1 interacted with OsHGW, which is a critical regulator in heading date and grain weight signaling pathways in rice. Expression patterns of defense- and heading date-related genes, OsPR1#051 and OsMYB21, were down-regulated over U. virens infection in rice. Collectively, our data provide a theory for gaining an insight into the molecular mechanisms underlying the UvHrip1 virulence function.

Functional analyses of small secreted cysteine-rich proteins identified candidate effectors in Verticillium dahliae

Secreted small cysteine-rich proteins (SCPs) play a critical role in modulating host immunity in plant-pathogen interactions. Bioinformatic analyses showed that the fungal pathogen Verticillium dahliae encodes more than 100 VdSCPs, but their roles in host-pathogen interactions have not been fully characterized. Transient expression of 123 VdSCP-encoding genes in Nicotiana benthamiana identified three candidate genes involved in host-pathogen interactions. The expression of these three proteins, VdSCP27, VdSCP113, and VdSCP126, in N. benthamiana resulted in cell death accompanied by a reactive oxygen species burst, callose deposition, and induction of defence genes. The three VdSCPs mainly localized to the periphery of the cell. BAK1 and SOBIR1 (associated with receptor-like protein) were required for the immunity triggered by these three VdSCPs in N. benthamiana. Site-directed mutagenesis showed that cysteine residues that form disulphide bonds are essential for the functioning of VdSCP126, but not VdSCP27 and VdSCP113. VdSCP27, VdSCP113, and VdSCP126 individually are not essential for V. dahliae infection of N. benthamiana and Gossypium hirsutum, although there was a significant reduction of virulence on N. benthamiana and G. hirsutum when inoculated with the VdSCP27/VdSCP126 double deletion strain. These results illustrate that the SCPs play a critical role in the V. dahliae-plant interaction via an intrinsic virulence function and suppress immunity following infection.

Citrus Postharvest Green Mold: Recent Advances in Fungal Pathogenicity and Fruit Resistance

As the major postharvest disease of citrus fruit, postharvest green mold is caused by the necrotrophic fungus Penicillium digitatum (Pd), which leads to huge economic losses worldwide. Fungicides are still the main method currently used to control postharvest green mold in citrus fruit storage. Investigating molecular mechanisms of plant-pathogen interactions, including pathogenicity and plant resistance, is crucial for developing novel and safer strategies for effectively controlling plant diseases. Despite fruit-pathogen interactions remaining relatively unexplored compared with well-studied leaf-pathogen interactions, progress has occurred in the citrus fruit-Pd interaction in recent years, mainly due to their genome sequencing and establishment or optimization of their genetic transformation systems. Recent advances in Pd pathogenicity on citrus fruit and fruit resistance against Pd infection are summarized in this review.

The progress of leaf rust research in wheat

Leaf rust (also called brown rust) in wheat, caused by fungal pathogen Puccinia triticina Erikss. (Pt) is one of the major constraints in wheat production worldwide. Pt is widespread with diverse population structure and undergoes rapid evolution to produce new virulent races against resistant cultivars that are regularly developed to provide resistance against the prevailing races of the pathogen. Occasionally, the disease may also take the shape of an epidemic in some wheat-growing areas causing major economic losses. In the recent past, substantial progress has been made in characterizing the sources of leaf rust resistance including non-host resistance (NHR). Progress has also been made in elucidating the population biology of Pt and the mechanisms of wheat-Pt interaction. So far, ∼80 leaf rust resistance genes (Lr genes) have been identified and characterized; some of them have also been used for the development of resistant wheat cultivars. It has also been shown that a gene-for-gene relationship exists between individual wheat Lr genes and the corresponding Pt Avr genes so that no Lr gene can provide resistance unless the prevailing race of the pathogen carries the corresponding Avr gene. Several Lr genes have also been cloned and their products characterized, although no Avr gene corresponding a specific Lr gene has so far been identified. However, several candidate effectors for Pt have been identified and functionally characterized using genome-wide analyses, transcriptomics, RNA sequencing, bimolecular fluorescence complementation (BiFC), virus-induced gene silencing (VIGS), transient expression and other approaches. This review summarizes available information on different aspects of the pathogen Pt, genetics/genomics of leaf rust resistance in wheat including cloning and characterization of Lr genes and epigenetic regulation of disease resistance.

An effector protein of the wheat stripe rust fungus targets chloroplasts and suppresses chloroplast function

Chloroplasts are important for photosynthesis and for plant immunity against microbial pathogens. Here we identify a haustorium-specific protein (Pst_12806) from the wheat stripe rust fungus, Puccinia striiformis f. sp. tritici (Pst), that is translocated into chloroplasts and affects chloroplast function. Transient expression of Pst_12806 inhibits BAX-induced cell death in tobacco plants and reduces Pseudomonas-induced hypersensitive response in wheat. It suppresses plant basal immunity by reducing callose deposition and the expression of defense-related genes. Pst_12806 is upregulated during infection, and its knockdown (by host-induced gene silencing) reduces Pst growth and development, likely due to increased ROS accumulation. Pst_12806 interacts with the C-terminal Rieske domain of the wheat TaISP protein (a putative component of the cytochrome b6-f complex). Expression of Pst_12806 in plants reduces electron transport rate, photosynthesis, and production of chloroplast-derived ROS. Silencing TaISP by virus-induced gene silencing in a susceptible wheat cultivar reduces fungal growth and uredinium development, suggesting an increase in resistance against Pst infection.

Stripe Rust Effector PstGSRE1 Disrupts Nuclear Localization of ROS-Promoting Transcription Factor TaLOL2 to Defeat ROS-Induced Defense in Wheat

Puccinia striiformis f. sp. tritici (Pst), a biotrophic plant pathogen, secretes numerous effectors to modulate host defense systems. Understanding the molecular mechanisms by which Pst effectors regulate wheat immunity is of great importance for the development of novel strategies for durable control of stripe rust. In this study, we identified a glycine-serine-rich effector gene, PstGSRE1, which is highly induced during early infection. Transgenic expression of PstGSRE1 RNAi constructs in wheat significantly reduced virulence of Pst and increased H₂O₂ accumulation in wheat. PstGSRE1 was shown to target the reactive oxygen species (ROS)-associated transcription factor TaLOL2, a positive regulator of wheat immunity. PstGSRE1 disrupted nuclear localization of TaLOL2 and suppressed ROS-mediated cell death induced by TaLOL2, thus compromising host immunity. This work reveals a previously unrecognized strategy whereby rust fungi exploit the PstGSRE1 effector to defeat ROS-associated plant defense by modulating the subcellular compartment of a host immune regulator and facilitate pathogen infection.

Rust pathogen effectors: perspectives in resistance breeding

Identification and functional characterization of plant pathogen effectors promise to ameliorate future research and develop effective and sustainable strategies for controlling or containing crop diseases. Wheat is the second most important food crop of the world after rice. Rust pathogens, one of the major biotic stresses in wheat production, are capable of threatening the world food security. Understanding the molecular basis of plant-pathogen interactions is essential for devising novel strategies for resistance breeding and disease management. Now, it has been established that effectors, the proteins secreted by pathogens, play a key role in plant-pathogen interactions. Therefore, effector biology has emerged as one of the most important research fields in plant biology. Recent advances in genomics and bioinformatics have allowed identification of a large repertoire of candidate effectors, while the evolving high-throughput tools have continued to assist in their functional characterization. The repertoires of effectors have become an important resource for better understanding of effector biology of pathosystems and resistance breeding of crop plants. In recent years, a significant progress has been made in the field of rust effector biology. This review describes the recent advances in effector biology of obligate fungal pathogens, identification and functional analysis of wheat rust pathogens effectors and the potential applications of effectors in molecular plant biology and rust resistance breeding in wheat.