Candidate effector proteins of the necrotrophic apple canker pathogen Valsa mali can suppress BAX-induced PCD

PcAvh87, a virulence essential RxLR effector of Phytophthora cinnamomi suppresses host defense and induces cell death in plant nucleus

Plants have developed intricate immune mechanisms to impede Phytophthora colonization. In response, Phytophthora secretes RxLR effector proteins that disrupt plant defense and promote infection. The specific molecular interactions through which Phytophthora RxLR effectors undermine plant immunity, however, remain inadequately defined. In this study, we delineate the role of the nuclear-localized RxLR effector PcAvh87, which is pivotal for the full virulence of Phytophthora cinnamomi. Gene expression analysis indicates that PcAvh87 expression is significantly upregulated during the initial infection stages, interacting with the immune responses triggered by the elicitin protein INF1 and pro-apoptotic protein BAX. Utilizing PEG/CaCl2-mediated protoplast transformation and CRISPR/Cas9-mediated gene editing, we generated PcAvh87 knockout mutants, which demonstrated compromised hyphal growth, sporangium development, and zoospore release, along with a marked reduction in pathogenicity. This underscores PcAvh87's crucial role as a virulence determinant. Notably, PcAvh87, conserved across the Phytophthora genus, was found to modulate the activity of plant immune protein 113, thereby attenuating plant immune responses. This implies that the PcAvh87-mediated regulatory mechanism could be a common strategy in Phytophthora species to manipulate plant immunity. Our findings highlight the multifaceted roles of PcAvh87 in promoting P. cinnamomi infection, including its involvement in sporangia production, mycelial growth, and the targeting of plant immune proteins to enhance pathogen virulence.

Valsa mali PR1-like protein modulates an apple valine-glutamine protein to suppress JA signaling-mediated immunity

Apple Valsa canker (AVC) is a devastating disease of apple (Malus × domestica), caused by Valsa mali (Vm). The Cysteine-rich secretory protein, Antigen 5, and Pathogenesis-related protein 1 (CAP) superfamily protein PATHOGENESIS-RELATED PROTEIN 1-LIKE PROTEIN c (VmPR1c) plays an important role in the pathogenicity of Vm. However, the mechanisms through which it exerts its virulence function in Vm-apple interactions remain unclear. In this study, we identified an apple valine-glutamine (VQ)-motif-containing protein, MdVQ29, as a VmPR1c target protein. MdVQ29-overexpressing transgenic apple plants showed substantially enhanced AVC resistance as compared with the wild type. MdVQ29 interacted with the transcription factor MdWRKY23, which was further shown to bind to the promoter of the jasmonic acid (JA) signaling-related gene CORONATINE INSENSITIVE 1 (MdCOI1) and activate its expression to activate the JA signaling pathway. Disease evaluation in lesion areas on infected leaves showed that MdVQ29 positively modulated apple resistance in a MdWRKY23-dependent manner. Furthermore, MdVQ29 promoted the transcriptional activity of MdWRKY23 toward MdCOI1. In addition, VmPR1c suppressed the MdVQ29-enhanced transcriptional activation activity of MdWRKY23 by promoting the degradation of MdVQ29 and inhibiting MdVQ29 expression and the MdVQ29-MdWRKY23 interaction, thereby interfering with the JA signaling pathway and facilitating Vm infection. Overall, our results demonstrate that VmPR1c targets MdVQ29 to manipulate the JA signaling pathway to regulate immunity. Thus, this study provides an important theoretical basis and guidance for mining and utilizing disease-resistance genetic resources for genetically improving apples.

Multi-Omics Approaches Provide New Insights into the Identification of Putative Fungal Effectors from Valsa mali

Pathogenic fungi secrete numerous effectors into host cells to manipulate plants' defense mechanisms. Valsa mali, a necrotrophic fungus, severely impacts apple production in China due to the occurrence of Valsa canker. Here, we predicted 210 candidate effector protein (CEP)-encoding genes from V. mali. The transcriptome analysis revealed that 146 CEP-encoding genes were differentially expressed during the infection of the host, Malus sieversii. Proteome analysis showed that 27 CEPs were differentially regulated during the infection stages. Overall, 25 of the 146 differentially expressed CEP-encoding genes were randomly selected to be transiently expressed in Nicotiana benthamiana. Pathogenicity analysis showed that the transient expression of VM1G-05058 suppressed BAX-triggered cell death while the expression of VM1G-10148 and VM1G-00140 caused cell death in N. benthamiana. In conclusion, by using multi-omics analysis, we identified potential effector candidates for further evaluation in vivo. Our results will provide new insights into the investigation of virulent mechanisms of V. mali.

Identification of Mycoviruses in the Pathogens of Fragrant Pear Valsa Canker from Xinjiang in China

As a common disease, canker seriously affects the yield and quality of fragrant pear due to the lack of effective control measures. Some fungi have been reported to harbor rich reservoirs of viral resources, and some mycoviruses can be used as biocontrol agents against plant diseases. In this study, 199 isolates were obtained from diseased branches of fragrant pear in the main production areas of Xinjiang. Among them, 134 belonged to Valsa spp., identified using morphological and molecular biological techniques, in which V. mali was the dominant species. The mycoviruses in Valsa spp. were further identified using metatranscriptomic sequencing and RT-PCR. The results revealed that a total of seven mycoviruses were identified, belonging to Botourmiaviridae, Endornaviridae, Fusariviridae, Hypoviridae, Mitoviridae, and Narnaviridae, among which Phomopsis longicolla hypovirus (PlHV) was dominant in all the sample collection regions. The Cryphonectria hypovirus 3-XJ1 (CHV3-XJ1), Botourmiaviridae sp.-XJ1 (BVsp-XJ1), and Fusariviridae sp.-XJ1 (Fvsp-XJ1) were new mycoviruses discovered within the Valsa spp. More importantly, compared with those in the virus-free Valsa spp. strain, the growth rate and virulence of the VN-5 strain co-infected with PlHV and CHV3-XJ1 were reduced by 59% and 75%, respectively, and the growth rate and virulence of the VN-34 strain infected with PlHV were reduced by 42% and 55%, respectively. On the other hand, the horizontal transmission efficiency of PlHV decreased when PlHV was co-infected with CHV3-XJ1, indicating that PlHV and CHV3-XJ1 were antagonistic. In summary, the mycoviruses in Valsa spp. were identified in Xinjiang for the first time, and three of them were newly discovered mycoviruses, with two strains yielding good results. These results will offer potential biocontrol resources for managing pear canker disease and provide a theoretical basis for the control of fruit tree Valsa canker disease.

Advances in understanding grapevine downy mildew: From pathogen infection to disease management

Plasmopara viticola is geographically widespread in grapevine-growing regions. Grapevine downy mildew disease, caused by this biotrophic pathogen, leads to considerable yield losses in viticulture annually. Because of the great significance of grapevine production and wine quality, research on this disease has been widely performed since its emergence in the 19th century. Here, we review and discuss recent understanding of this pathogen from multiple aspects, including its infection cycle, disease symptoms, genome decoding, effector biology, and management and control strategies. We highlight the identification and characterization of effector proteins with their biological roles in host-pathogen interaction, with a focus on sustainable control methods against P. viticola, especially the use of biocontrol agents and environmentally friendly compounds.

Antifungal Compound from the Predatory Bacterium Lysobacter enzymogenes Inhibits a Plant Pathogenic Fungus by Targeting the AAA ATPase VpVeb1

Heat-stable antifungal factor (HSAF) isolated from Lysobacter enzymogenes is considered a potential biocontrol agent. However, the target of HSAF in phytopathogenic fungi remains unclear. In this study, we investigated the target of HSAF in Valsa pyri that causes fatal pear Valsa canker. Thirty-one HSAF-binding proteins were captured and identified by surface plasmon resonance (SPR) and high-performance liquid chromatography-mass spectrometry (LC-MS/MS), and 11 deletion mutants were obtained. Among these mutants, only ΔVpVEB1 showed decreased sensitivity to HSAF. Additionally, ΔVpVEB1 exhibited significantly reduced virulence in V. pyri. Molecular docking and SPR results revealed that HSAF bound to threonine 569 and glycine 570 of VpVeb1, which are crucial for AAA ATPase activity. Another study showed that HSAF could decrease the ATPase activity of VpVeb1, leading to the reduced virulence of V. pyri. Taken together, this study first identified the potential target of HSAF in fungi. These findings will help us better understand the model of action of HSAF to fungi.

A prophage-encoded effector from "Candidatus Liberibacter asiaticus" targets ASCORBATE PEROXIDASE6 in citrus to facilitate bacterial infection

Citrus huanglongbing (HLB), associated with the unculturable phloem-limited bacterium "Candidatus Liberibacter asiaticus" (CLas), is the most devastating disease in the citrus industry worldwide. However, the pathogenicity of CLas remains poorly understood. In this study, we show that AGH17488, a secreted protein encoded by the prophage region of the CLas genome, suppresses plant immunity via targeting the host ASCORBATE PEROXIDASE6 (APX6) protein in Nicotiana benthamiana and Citrus sinensis. The transient expression of AGH17488 reduced the chloroplast localization of APX6 and its enzyme activity, inhibited the accumulation of reactive oxygen species (H₂ O₂ and O₂ ^- ) and the lipid oxidation endproduct malondialdehyde in plants, and promoted the proliferation of Pseudomonas syringae pv. tomato DC3000 and Xanthomonas citri subsp. citri. This study reveals a novel mechanism underlying how CLas uses a prophage-encoded effector, AGH17488, to target a reactive oxygen species accumulation-related gene, APX6, in the host to facilitate its infection.

VmPma1 contributes to virulence via regulation of the acidification process during host infection in Valsa mali

Valsa mali is a destructive phytopathogenic fungus that mainly infects apple and pear trees. Infection with V. mali results in host tissue acidification via the generation of citric acid, which promote invasion. Here, two plasma membrane H⁺-ATPases, VmPma1 and VmPma2, were identified in V. mali. The VmPma1 deletion mutant (∆VmPma1) displayed higher intracellular acid accumulation and a lower growth rate compared to the wild type. In contrast, the VmPma2 deletion mutant (∆VmPma2) showed no obvious phenotypic differences. Meanwhile, loss of VmPma1, but not VmPma2, in V. mali led to a significant decrease in growth under acidic or alkaline conditions compared with WT. More importantly, ∆VmPma1 showed a greater reduction in ATPase hydrolase activity and acidification of the external environment, more sensitivity to abiotic stress, and weaker pathogenicity than ∆VmPma2. This evidence indicates that VmPma1 is the main gene of the two plasma membrane H⁺-ATPases. Transcriptomic analysis indicated that many metabolic processes regulated by VmPma1 are strictly pH-regulated. Besides, we identified two genes (named VmAgn1p and Vmap1) that contribute to the pathogenicity of V. mali by differentially regulating external acidification capacity. Overall, our findings show that VmPma1 plays a pivotal role in pathogenicity by affecting the acidification of V. mali.

Comparative genomics of five Valsa species gives insights on their pathogenicity evolution

Valsa is a genus of ascomycetes within the Valsaceae family. This family includes many wood destructive pathogens such as the well known Valsa mali and Valsa pyri which cause canker diseases in fruit trees and threaten the global fruit production. Lack of genomic information of this family is impeding our understandings about their evolution and genetic basis of their pathogenicity divergence. Here, we report genome assemblies of Valsa malicola, Valsa persoonii, and Valsa sordida which represent close relatives of Valsa mali and Valsa pyri with different host preferences. Comparative genomics analysis revealed that segmental rearrangements, inversions, and translocations frequently occurred among Valsa spp. genomes. Gene families that exhibited gene copy expansions tended to be associated with secondary metabolism, transmembrane transport, and pyrophosphatase activities. Orthologous genes in regions lost synteny exhibited significantly higher rate of synonymous substitution (KS) than those in regions retained synteny. Moreover, among these genes, membrane transporter families associated with antidrug (MFS, DHA) activities and nutrient transportation (SP and APCs) activities were significantly over-represented. Lineage specific synonymous substitution (KS) and nonsynonymous substitution (KA) analysis based on the phylogeny constructed from 11 fungal species identified a set of genes with selection signatures in Valsa clade and these genes were significantly enriched in functions associated with fatty acid beta-oxidation, DNA helicase activity, and ATPase activity. Furthermore, unique genes that possessed or retained by each of the five Valsa species are more likely part of the secondary metabolic (SM) gene clusters. SM gene clusters conserved across five Valsa species showed various degrees of diversification in both identity and completeness. All 11 syntenically conserved SM clusters showed differential expression during the infection of apple branch with Valsa mali suggesting involvements of secondary metabolism in the pathogenicity of Valsa species.

Valsa mali secretes an effector protein VmEP1 to target a K homology domain-containing protein for virulence in apple

The K homology (KH) repeat is an RNA-binding motif that exists in various proteins, some of which participate in plant growth. However, the function of KH domain-containing proteins in plant defence is still unclear. In this study, we found that a KH domain-containing protein in apple (Malus domestica), HEN4-like (MdKRBP4), is involved in the plant immune response. Silencing of MdKRBP4 compromised reactive oxygen species (ROS) production and enhanced the susceptibility of apple to Valsa mali, whereas transient overexpression of MdKRBP4 stimulated ROS accumulation in apple leaves, indicating that MdKRBP4 is a positive immune regulator. Additionally, MdKRBP4 was proven to interact with the VmEP1 effector secreted by V. mali, which led to decreased accumulation of MdKRBP4. Coexpression of MdKRBP4 with VmEP1 inhibited cell death and ROS production induced by MdKRBP4 in Nicotiana benthamiana. These results indicate that MdKRBP4 functions as a novel positive regulatory factor in plant immunity in M. domestica and is a virulence target of the V. mali effector VmEP1.

VmMon1-Ccz1 Complex Is Required for Conidiation, Autophagy, and Virulence in Valsa mali

Apple Valsa canker caused by Valsa mali is a serious disease in eastern Asia, especially in China. In our previous proteomics study, monensin sensitivity 1 protein in Valsa mali (VmMon1) was identified to be significantly upregulated during V. mali infection. It was reported Mon1 protein formed a heterodimer called MC (Mon1-Ccz1) complex with caffeine, calcium, and zinc sensitivity 1 protein (Ccz1) in yeast. However, Ccz1 had not been identified in plant-pathogenic fungi such as Fusarium graminearum and Magnaporthe oryzae. Here, we identified a Ccz1 ortholog VmCcz1 in V. mali, by using DELTA-BLAST. The interaction of VmMon1 and VmCcz1 were verified using yeast two-hybrid assay, bimolecular fluorescence complementation, and co-immunoprecipitation assays. Further yeast three-hybrid screenings determined that VmRab7 (Ras-related protein in V. mali) interacted with the MC complex. Targeted gene deletion showed that the ∆VmMon1 and ∆VmCcz1 mutants were defective in vegetative growth, conidiation, and pathogenicity. In addition, both mutants were more sensitive to osmotic and oxidative stresses and intracellular protein transport inhibitors. Cytological examination revealed that the ∆VmMon1 and ∆VmCcz1 mutants were impaired in vacuole fusion and autophagy. More importantly, expression of pectinase genes decreased in both mutants compared with those of the wild type during infection. Overall, our study identified Mon1 and Ccz1 genes in V. mali and provided evidence that VmMon1 and VmCcz1 are critical components that modulate vacuole fusion and autophagy, thereby affecting the development, conidiation, and pathogenicity of V. mali. [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.

A Bursaphelenchus xylophilus Effector, BxSCD3, Suppresses Plant Defense and Contributes to Virulence

Bursaphelenchus xylophilus is the most economically important species of migratory plant-parasitic nematodes (PPNs) and causes severe damage to forestry in China. The successful infection of B. xylophilus relies on the secretion of a repertoire of effector proteins. The effectors, which suppress the host pine immune response, are key to the facilitation of B. xylophilus parasitism. An exhaustive list of candidate effectors of B. xylophilus was predicted, but not all have been identified and characterized. Here, an effector, named BxSCD3, has been implicated in the suppression of host immunity. BxSCD3 could suppress pathogen-associated molecular patterns (PAMPs) PsXEG1- and INF1-triggered cell death when it was secreted into the intracellular space in Nicotiana benthamiana. BxSCD3 was highly up-regulated in the early infection stages of B. xylophilus. BxSCD3 does not affect B. xylophilus reproduction, either at the mycophagous stage or the phytophagous stage, but it contributes to the virulence of B. xylophilus. Moreover, BxSCD3 significantly influenced the relative expression levels of defense-related (PR) genes PtPR-3 and PtPR-6 in Pinus thunbergii in the early infection stage. These results suggest that BxSCD3 is an important toxic factor and plays a key role in the interaction between B. xylophilus and host pine.

A Putative Effector LtCSEP1 from Lasiodiplodia theobromae Inhibits BAX-Triggered Cell Death and Suppresses Immunity Responses in Nicotiana benthamiana

Lasiodiplodia theobromae is a causal agent of grapevine trunk disease, and it poses a significant threat to the grape industry worldwide. Fungal effectors play an essential role in the interaction between plants and pathogens. However, few studies have been conducted to understand the functions of individual effectors in L. theobromae. In this study, we identified and characterized a candidate secreted effector protein, LtCSEP1, in L. theobromae. Gene expression analysis suggested that transcription of LtCSEP1 in L. theobromae was induced at the early infection stages in the grapevine. Yeast secretion assay revealed that LtCSEP1 contains a functional signal peptide. Transient expression of LtCSEP1 in Nicotiana benthamiana suppresses BAX-trigged cell death and significantly inhibits the flg22-induced PTI-associated gene expression. Furthermore, the ectopic expression of LtCSEP1 in N. benthamiana enhanced disease susceptibility to L. theobromae by downregulating the defense-related genes. These results demonstrated that LtCSEP1 is a potential effector of L. theobromae, which contributes to suppressing the plant’s defenses.

The Transcription Factor VpxlnR Is Required for the Growth, Development, and Virulence of the Fungal Pathogen Valsa pyri

Pears (Pyrus sp.) are widely cultivated in China, and their yield accounts for more than 60% of global pear production. The fungal pathogen Valsa pyri is a major causal agent of pear canker disease, which results in enormous losses of pear production in northern China. In this study, we characterized a Zn₂Cys₆ transcription factor that contains one GAL4 domain and a fungal-trans domain, which are present in VpxlnR. The vpxlnR gene expression was upregulated in the invasion stage of V. pyri. To investigate its functions, we constructed gene deletion mutants and complementary strains. We observed that the growth of the vpxlnR mutants was reduced on potato dextrose agar (PDA), Czapek plus glucose or sucrose compared with that of the wild-type strain. Additionally, vpxlnR mutants exhibited loss of function in fruiting body formation. Moreover, vpxlnR mutants were more susceptible to hydrogen peroxide (H₂O₂) and salicylic acid (SA) and were reduced in their virulence at the early infection stage. According to a previous study, VpxlnR-interacting motifs containing NRHKGNCCGM were searched in the V. pyri genome, and we obtained 354 target genes, of which 148 genes had Clusters of Orthologous Groups (COG) terms. PHI-BLAST was used to identify virulence-related genes, and we found 28 hits. Furthermore, eight genes from the 28 PHI-BLAST hits were further assessed by yeast one-hybrid (Y1H) assays, and five target genes, salicylate hydroxylase (VP1G_09520), serine/threonine-protein kinase (VP1G_03128), alpha-xylosidase (VP1G_06369), G-protein beta subunit (VP1G_02856), and acid phosphatase (VP1G_03782), could interact with VpxlnR in vivo. Their transcript levels were reduced in one or two vpxlnR mutants. Taken together, these findings imply that VpxlnR is a key regulator of growth, development, stress, and virulence through controlling genes involved in signaling pathways and extracellular enzyme activities in V. pyri. The motifs interacting with VpxlnR also provide new insights into the molecular mechanism of xlnR proteins.

A novel effector, CsSp1, from Bipolaris sorokiniana, is essential for colonization in wheat and is also involved in triggering host immunity

The hemibiotrophic pathogen Bipolaris sorokiniana causes root rot, leaf blotching, and black embryos in wheat and barley worldwide, resulting in significant yield and quality reductions. However, the mechanism underlying the host-pathogen interactions between B. sorokiniana and wheat or barley remains unknown. The B. sorokiniana genome encodes a large number of uncharacterized putative effector proteins. In this study, we identified a putative secreted protein, CsSp1, with a classic N-terminal signal peptide, that is induced during early infection. A split-marker approach was used to knock out CsSP1 in the Lankao 9-3 strain. Compared with the wild type, the deletion mutant ∆Cssp1 displayed less radial growth on potato dextrose agar plates and produced fewer spores, and complementary transformation completely restored the phenotype of the deletion mutant to that of the wild type. The pathogenicity of the deletion mutant in wheat was attenuated even though appressoria still penetrated the host. Additionally, the infectious hyphae in the deletion mutant became swollen and exhibited reduced growth in plant cells. The signal peptide of CsSp1 was functionally verified through a yeast YTK12 secretion system. Transient expression of CsSp1 in Nicotiana benthamiana inhibited lesion formation caused by Phytophthora capsici. Moreover, CsSp1 localized in the nucleus and cytoplasm of plant cells. In B. sorokiniana-infected wheat leaves, the salicylic acid-regulated genes TaPAL, TaPR1, and TaPR2 were down-regulated in the ∆Cssp1 strain compared with the wild-type strain under the same conditions. Therefore, CsSp1 is a virulence effector and is involved in triggering host immunity.

Mechanisms of Trichoderma longibrachiatum T6 Fermentation against Valsa mali through Inhibiting Its Growth and Reproduction, Pathogenicity and Gene Expression

Apple Valsa canker is one of the most serious diseases, having caused significant apple yield and economic loss in China. However, there is still no effective biological methods for controlling this disease. Our present study focused on the inhibitory activity and mechanisms of Trichoderma longibrachiatum (T6) fermentation on Valsa mali that causes apple Valsa canker (AVC). Our results showed that the T6 fermentation exhibited effective antifungal activity on the mycelial growth and conidia germination of V. mali, causing mycelium malformation and the hyphal disintegrating in comparison to the control. The activity of pathogenically related enzymes that are secreted from V. mali and the expression level of gene of V. mali were significantly inhibited and downregulated by treatment with T6 fermentation. In addition, the lesion area and number of pycnidia of V. mali formed on the branches were significantly reduced after treatment with the T6 fermentation through the pathogenicity test on the detached branches. Our results indicate that the possible mechanism of T6 fermentation against V. mali occurs through inhibiting its growth and reproduction, the pathogenic enzyme activity, and its related gene expression.

Two NIS1-like proteins from apple canker pathogen (Valsa mali) play distinct roles in plant recognition and pathogen virulence

Conserved effectors produced by phytopathogens play critical roles in plant-microbe interactions. NIS1-like proteins represent a newly identified family of effectors distributed in multiple fungal species. However, their biological functions in a majority of pathogenic fungi remain largely elusive and require further investigation. In this study, we characterized two NIS1-like proteins VmNIS1 and VmNIS2 from Valsa mali, the causal agent of apple Valsa canker. Both of these two proteins were predicted to be secreted. Using agroinfiltration, we found that VmNIS1 induced intense cell death, whereas VmNIS2 suppressed INF1 elicitin-triggered cell death in Nicotiana benthamiana. Treatment of N. benthamiana with VmNIS1 recombinant protein produced by Escherichia coli activated a series of immune responses and enhanced plant disease resistance against Phytophthora capsici. In contrast, VmNIS2 suppressed plant immune responses and promoted P. capsici infection when transiently expressed in N. benthamiana. Both VmNIS1 and VmNIS2 were shown to be highly induced at late stage of V. mali infection. By individually knocking out of these two genes in V. mali, however, only VmNIS2 was shown to be required for pathogen virulence as well as tolerance to oxidative stress. Notably, we further showed that C-terminal extension of VmNIS1 was essential for plant recognition and VmNIS2 may escape plant detection via sequence truncation. Our data collectively indicate that VmNIS1 and VmNIS2 play distinct roles in plant recognition and pathogen virulence, which provided new insights into the function of NIS1-like proteins in plant-microbe interactions.

Effectors of Plant Necrotrophic Fungi

Plant diseases caused by necrotrophic fungal pathogens result in large economic losses in field crop production worldwide. Effectors are important players of plant-pathogen interaction and deployed by pathogens to facilitate plant colonization and nutrient acquisition. Compared to biotrophic and hemibiotrophic fungal pathogens, effector biology is poorly understood for necrotrophic fungal pathogens. Recent bioinformatics advances have accelerated the prediction and discovery of effectors from necrotrophic fungi, and their functional context is currently being clarified. In this review we examine effectors utilized by necrotrophic fungi and hemibiotrophic fungi in the latter stages of disease development, including plant cell death manipulation. We define "effectors" as secreted proteins and other molecules that affect plant physiology in ways that contribute to disease establishment and progression. Studying and understanding the mechanisms of necrotrophic effectors is critical for identifying avenues of genetic intervention that could lead to improved resistance to these pathogens in plants.

Comparative transcriptomic analysis of candidate effectors to explore the infection and survival strategy of Bursaphelenchus xylophilus during different interaction stages with pine trees

BACKGROUND

The pine wood nematode (PWN), Bursaphelenchus xylophilus, is a devastating pathogen of many Pinus species in China. The aim of this study was to understand the interactive molecular mechanism of PWN and its host by comparing differentially expressed genes and candidate effectors from three transcriptomes of B. xylophilus at different infection stages.

RESULTS

In total, 62, 69 and 46 candidate effectors were identified in three transcriptomes (2.5 h postinfection, 6, 12 and 24 h postinoculation and 6 and 15 d postinfection, respectively). In addition to uncharacterized pioneers, other candidate effectors were involved in the degradation of host tissues, suppression of host defenses, targeting plant signaling pathways, feeding and detoxification, which helped B. xylophilus survive successfully in the host. Seven candidate effectors were identified in both our study and the B. xylophilus transcriptome at 2.5 h postinfection, and one candidate effector was identified in all three transcriptomes. These common candidate effectors were upregulated at infection stages, and one of them suppressed pathogen-associated molecular pattern (PAMP) PsXEG1-triggered cell death in Nicotiana benthamiana.

CONCLUSIONS

The results indicated that B. xylophilus secreted various candidate effectors, and some of them continued to function throughout all infection stages. These various candidate effectors were important to B. xylophilus infection and survival, and they functioned in different ways (such as breaking down host cell walls, suppressing host defenses, promoting feeding efficiency, promoting detoxification and playing virulence functions). The present results provide valuable resources for in-depth research on the pathogenesis of B. xylophilus from the perspective of effectors.

Unrelated Fungal Rust Candidate Effectors Act on Overlapping Plant Functions

Rust fungi cause epidemics that threaten the production of important plant species, such as wheat and soy. Melampsora larici-populina (Mlp) causes the poplar rust and encodes at least 1184 candidate effectors (CEs) whose functions are poorly known. In this study, we sequenced the transcriptome and used mass spectrometry to analyze the metabolome of Arabidopsis plants constitutively expressing 14 Mlp CEs and of a control line to discover alterations leading to plant susceptibility. We found 2299 deregulated genes across the experiment. Genes involved in pattern-triggered immunity, such as FRK1, PR1, RBOHD, and WRKY33, as well as AUX/IAA genes were down-regulated. We further observed that 680 metabolites were deregulated in at least one CE-expressing transgenic line, with “highly unsaturated and phenolic compounds” and “peptides” enriched among down- and up-regulated metabolites. Interestingly, transgenic lines expressing unrelated CEs had correlated patterns of gene and metabolite deregulation, while expression of CEs belonging to the same family deregulated different genes and metabolites. Thus, our results uncouple effector sequence similarity and function. This supports that effector functional investigation in the context of their virulence activity and effect on plant susceptibility requires the investigation of the individual effector and precludes generalization based on sequence similarity.

Genome Assembly and Transcriptome Analysis of the Fungus Coniella diplodiella During Infection on Grapevine (Vitis vinifera L.)

Grape white rot caused by Coniella diplodiella (Speg.) affects the production and quality of grapevine in China and other grapevine-growing countries. Despite the importance of C. diplodiella as a serious disease-causing agent in grape, the genome information and molecular mechanisms underlying its pathogenicity are poorly understood. To bridge this gap, 40.93 Mbp of C. diplodiella strain WR01 was de novo assembled. A total of 9,403 putative protein-coding genes were predicted. Among these, 608 and 248 genes are potentially secreted proteins and candidate effector proteins (CEPs), respectively. Additionally, the transcriptome of C. diplodiella was analyzed after feeding with crude grapevine leaf homogenates, which reveals the transcriptional expression of 9,115 genes. Gene ontology enrichment analysis indicated that the highly enriched genes are related with carbohydrate metabolism and secondary metabolite synthesis. Forty-three putative effectors were cloned from C. diplodiella, and applied for further functional analysis. Among them, one protein exhibited strong effect in the suppression of BCL2-associated X (BAX)-induced hypersensitive response after transiently expressed in Nicotiana benthamiana leaves. This work facilitates valuable genetic basis for understanding the molecular mechanism underlying C. diplodiella-grapevine interaction.

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.

A Sec-Dependent Secretory Protein of the Huanglongbing-Associated Pathogen Suppresses Hypersensitive Cell Death in Nicotiana benthamiana

"Candidatus Liberibacter asiaticus" (CLas) is a phloem-restricted Gram-negative bacterium that is the causal agent of citrus huanglongbing (HLB). In this study, we identified a CLas-encoded Sec-dependent secretory protein CLIBASIA_04405 that could contribute to the pathogenicity of this bacterium. The gene expression level of CLIBASIA_04405 was significantly higher in citrus than in psyllids. Transient overexpression of the mature CLIBASIA_04405 protein (m4405) in Nicotiana benthamiana leaves could suppress hypersensitive response (HR)-based cell death and H₂O₂ accumulation triggered by the mouse BAX and the Phytophthora infestans INF1. An alanine-substitution mutagenesis assay revealed the essential of amino acid clusters EKR^45-47 and DE^64-65 in cell death suppression. Challenge inoculation of the transgenic N. benthamiana-expressing m4405 with Pseudomonas syringae DC3000ΔhopQ1-1 demonstrated the greatly reduced bacterial proliferation. Remarkably, transcriptome profiling and RT-qPCR analysis disclosed that the gene expression of six small heat shock proteins (sHSPs), a set of plant defense regulators, were significantly elevated in the transgenic m4405 lines compared with those in wild-type N. benthamiana. In addition, the transgenic m4405 lines displayed phenotypes of dwarfism and leaf deformation. Altogether, these data indicated that m4405 was a virulence factor of CLas.

Adaptive regulation of virulence genes by microRNA-like RNAs in Valsa mali

MicroRNAs play important roles in the regulation of gene expression in plants and animals. However, little information is known about the action mechanism and function of fungal microRNA-like RNAs (milRNAs). In this study, combining deep sequencing, molecular and histological assays, milRNAs and their targets in the phytopathogenic fungus Valsa mali were isolated and identified. A critical milRNA, Vm-milR16, was identified to adaptively regulate the expression of virulence genes. Fourteen isolated milRNAs showed high expression abundance. Based on the assessment of a pathogenicity function of these milRNAs, Vm-milR16 was found to be a critical milRNA in V. mali by regulating sucrose non-fermenting 1 (VmSNF1), 4,5-DOPA dioxygenase extradiol (VmDODA), and a hypothetical protein (VmHy1). During V. mali infection, Vm-milR16 is downregulated, while its targets are upregulated. Overexpression of Vm-milR16, but not mutated Vm-milR16, significantly reduces the expression of targets and virulence of V. mali. Furthermore, deletion of VmSNF1, VmDODA and VmHy1 significantly reduce virulence of V. mali. All three targets seem to be essential for oxidative stress response and VmSNF1 is required for expression of pectinase genes during V. mali-host interaction. Our results demonstrate Vm-milRNAs contributing to the infection of V. mali on apple trees by adaptively regulating virulence genes.

Antifungal Mechanism of Dipicolinic Acid and Its Efficacy for the Biocontrol of Pear Valsa Canker

Valsa pyri is a fatal canker pathogen that causes significant reduction of crop yield in pear orchards. V. pyri invades the trunk phloem, and is difficult to control by chemical treatment. In this work, it was found for the first time that Bacillus subtilis-produced dipicolinic acid (DPA) exhibits antifungal activity against different canker pathogens, including Alteraria alternata, Botryosphaeria dothidea, Rhizoctonia solani, and V. pyri. Growth inhibition of V. pyri was observed at less than 5 mM concentration (pH = 5.6). DPA showed the highest antifungal activity at acidic pH values and in the presence of bivalent metals, such as zinc(II), cobalt(II), and copper(II). Measurement of mRNA expression levels and scanning electron microscope (SEM) observations revealed that DPA causes V. pyri apoptosis via inhibition of chitin biosynthesis and subsequent cell lysis. Interestingly, DPA showed high stability in the pear bark and was able to cross the pear tree bark into the phloem, protecting the internal phases of the pear trunk. In preventive applications, DPA reduced the canker symptoms of V. pyri on Cuigan pear trees by 90%. Taken together, an efficient strategy for the management of V. pyri-caused canker disease was developed using a novel antifungal agent, DPA, with strong antifungal activity and particular diffusion properties.

The Fungal-Specific Transcription Factor VpFSTF1 Is Required for Virulence in Valsa pyri

Valsa pyri is the causal agent of pear canker disease, which leads to enormous losses of pear production in eastern Asian, especially China. In this study, we identified a fungal-specific transcription factor 1 (termed as VpFSTF1) from V. pyri, which is highly conserved in fungi. To characterize its functions, we generated mutant and complementation strains in V. pyri and found that ΔVpFSTF1 mutants lost the ability to form fruiting bodies along with the reduced virulence. The radial growth of ΔVpFSTF1 mutant was sensitive to increasing concentrations of hydrogen peroxide (H₂O₂) and salicylic acid (SA). Moreover, RNA-sequencing (RNA-Seq) analysis of wild-type (WT) and ΔVpFSTF1 mutant strains was performed, and the results revealed 1,993 upregulated, and 2006 downregulated differentially expressed genes (DEGs) in the mutant. The DEGs were corresponding to the genes that are involved in amino acid metabolism, starch, and sucrose metabolism, gluconeogenesis, citrate cycle, and carbon metabolism. Interestingly, pathogen host interaction (PHI) analysis showed that 69 downregulated genes were related to virulence, suggesting that they might function downstream of VpFSTF1. Nine DEGs were further validated by quantitative reverse transcription-polymerase chain reaction (qRT-PCR), and the results were consistent with RNA-seq analysis. Furthermore, promoter regions were predicted, and VpFSTF1 binding activity was assessed. We demonstrated that five promoters are directly or indirectly targeted by VpFSTF1, including catalase-related peroxidase (VPIG_01209) and P450 family genes. Taken together, these findings indicate that VpFSTF1 is crucial for the virulence of V. pyri via direct or indirect regulation of downstream genes expression and lay an important foundation for understanding the molecular mechanism of V. pyri infection.

A Novel 'Candidatus Liberibacter asiaticus'-Encoded Sec-Dependent Secretory Protein Suppresses Programmed Cell Death in Nicotiana benthamiana

'Candidatus Liberibacter asiaticus' (CLas) is one of the causal agents of citrus Huanglongbing (HLB), a bacterial disease of citrus trees that greatly reduces fruit yield and quality. CLas strains produce an array of currently uncharacterized Sec-dependent secretory proteins. In this study, the conserved chromosomally encoded protein CLIBASIA_03875 was identified as a novel Sec-dependent secreted protein. We show that CLIBASIA_03875 contains a putative Sec- secretion signal peptide (SP), a 29 amino acid residue located at the N-terminus, with a mature protein (m3875) of 22 amino acids found to localize in multiple subcellular components of the leaf epidermal cells of Nicotiana benthamiana. When overexpressed via a Potato virus X (PVX)-based expression vector in N. benthamiana, m3875 suppressed programmed cell death (PCD) and the H2O2 accumulation triggered by the pro-apoptotic mouse protein BAX and the Phytophthora infestans elicitin INF1. Overexpression also resulted in a phenotype of dwarfing, leaf deformation and mosaics, suggesting that m3875 has roles in plant immune response, growth, and development. Substitution mutagenesis of the charged amino acid (D7, R9, R11, and K22) with alanine within m3875 did not recover the phenotypes for PCD and normal growth. In addition, the transiently overexpressed m3875 regulated the transcriptional levels of N. benthamiana orthologs of CNGCs (cyclic nucleotide-gated channels), BI-1 (Bax-inhibitor 1), and WRKY33 that are involved in plant defense mechanisms. To our knowledge, m3875 is the first PCD suppressor identified from CLas. Studying the function of this protein provides insight as to how CLas attenuates the host immune responses to proliferate and cause Huanglongbing disease in citrus plants.

Comparative genomics reveal pathogenicity-related loci in Pseudomonas syringae pv. actinidiae biovar 3

Bacterial canker of kiwifruit, is a severe global disease caused by Pseudomonas syringae pv. actinidiae (Psa). Here, we found that Psa biovar 3 (Psa3) was the only biovar consisting of three widely distributed clades in the largest Chinese kiwifruit cultivated area. Comparative genomics between the three clades revealed 13 polymorphic genes, each of which had multiple intra-clade variations. For instance, we confirmed that the polymorphic copA gene, which encodes a periplasmic protein CopA that is translocated by the Twin-arginine targeting (Tat) system, was involved in copper tolerance. We also found extensive variation in pathogenicity amongst strains within each genetically monomorphic clade. Accordingly, the pathogenic determinants of Psa3 were identified via a genomic comparison of phenotypically different strains within each clade. A case study of the high- and low-virulence strains in the clade 2 of Psa3 revealed that an hfq variant involved in in vitro growth and virulence, while a conserved locus 930 bp upstream of the hrpR gene in the Type III secretion system (T3SS) cluster was required for full pathogenicity on kiwifruit and elicitation of the hypersensitivity response on non-host Nicotiana benthamiana. The '-930' locus is involved in transcriptional regulation of hrpR/S and modulates T3SS function via the hierarchical 'HrpR/S-HrpL-T3SS/effector' regulatory cascade in Psa. Our results provide insights into the molecular basis underlying the genetic diversification and evolution of pathogenicity in Psa3 since kiwifruit canker emerged in China in the 1980s.

Hce2 domain-containing effectors contribute to the full virulence of Valsa mali in a redundant manner

Valsa mali is the causal agent of apple Valsa canker, a destructive disease in East Asia. Effector proteins play important roles in the virulence of phytopathogenic fungi, and we identified five Hce2 domain-containing effectors (VmHEP1, VmHEP2, VmHEP3, VmHEP4 and VmHEP5) from the V. mali genome. Amongst these, VmHEP1 and VmHEP2 were found to be up-regulated during the early infection stage and VmHEP1 was also identified as a cell death inducer through its transient expression in Nicotiana benthamiana. Although the deletion of each single VmHEP gene did not lead to a reduction in virulence, the double-deletion of VmHEP1 and VmHEP2 notably attenuated V. mali virulence in both apple twigs and leaves. An evolutionary analysis revealed that VmHEP1 and VmHEP2 are two paralogues, under purifying selection. VmHEP1 and VmHEP2 are located next to each other on chromosome 11 as tandem genes with only a 604 bp physical distance. Interestingly, the deletion of VmHEP1 promoted the expression of VmHEP2 and, vice versa, the deletion of VmHEP2 promoted the expression of VmHEP1. The present results provide insights into the functions of Hce2 domain-containing effectors acting as virulence factors of V. mali, and provide a new perspective regarding the contribution of tandem genes to the virulence of phytopathogenic fungi.

An Effector, BxSapB1, Induces Cell Death and Contributes to Virulence in the Pine Wood Nematode Bursaphelenchus xylophilus

The pine wood nematode (PWN) Bursaphelenchus xylophilus has caused serious damage to pine forests in China. Effectors secreted by phytonematodes play a role in host infection. We identified and characterized an effector, BxSapB1, based on the B. xylophilus transcriptome at the early stages of infection and the transient expression of proteins in Nicotiana benthamiana. BxSapB1 triggered cell death in N. benthamiana when secreted into the apoplast, and this effect was independent of N. benthamiana brassinosteroid-insensitive 1-associated kinase 1 (NbBAK1) and suppressor of BIR1-1 (NbSOBIR1). The signal peptide of BxSapB1 was proven to be functional in yeast using the yeast signal sequence trap system and BxSapB1 was strongly expressed in the subventral gland cells of B. xylophilus, as revealed by in-situ hybridization. In addition, based on local BLAST analysis, the BxSapB1 showed 100% identity to BUX.s00139.62, which was identified from the B. xylophilus secretome during Pinus thunbergii infection. BxSapB1 was upregulated in a highly virulent strain and downregulated in a weakly virulent strain of PWN at the early stages of infection. RNA interference assays showed that silencing BxSapB1 resulted in decreased expression of pathogenesis-related genes (PtPR-1b, PtPR-3, and PtPR-5) as well as delayed onset of symptoms in P. thunbergii infected by B. xylophilus. The combined data suggest that BxSapB1 can trigger cell death in N. benthamiana and that it contributes to the virulence in B. xylophilus during parasitic interaction.

Comparative transcriptomics of two Valsa pyri isolates uncover different strategies for virulence and growth

Valsa pyri, an ascomycete pathogen that is a member of the Valsaceae family (Sordariomycetes, Diaporthales), which causes pear or apple canker and leads to tree death and massive yield losses. Here, we selected two V. pyri isolates (Vp14 and Vp297) that exhibited different invasion abilities for transcriptomics analyses. Compared toVp297, Vp14 had stronger virulence and spread faster on host-like nutrients. Four samples, including mycelium or infectious mycelium, of the two isolates were sequenced. Clean reads were mapped to the V. pyri genome, and 12490 transcripts and 178 new genes were identified. There were dramatically fewer differentially expressed genes (DEGs) in Vp14 than in Vp297. According to GO and COG annotations, there were both more up- and down-regulated genes in Vp297 than in Vp14 except for genes involved in amino acid transport and metabolism, carbohydrate transport and metabolism, peroxidases and so on. Specific up-regulated DEGs, including genes encoding cell wall degrading enzymes and genes involved in nitrogen metabolism and peroxidases which play crucial roles in virulence and infectious growth, were especially enriched inVp14. These results indicate that the Vp14 isolate may infect its host and take up nutrition more efficiently, reflecting a stronger ability for invasion or infectious growth. Our analysesindicate that a successful V. pyri infection involves multiple instances of transcriptome remodeling to regulate gene functions. Comparative transcriptomics between isolates of V. pyri may aid in our understanding of the virulence mechanism of this pathogen.

Two members of the velvet family, VmVeA and VmVelB, affect conidiation, virulence and pectinase expression in Valsa mali

Velvet protein family members are important fungal-specific regulators which are involved in conidial development, secondary metabolism and virulence. To gain a broader insight into the physiological functions of the velvet protein family of Valsa mali, which causes a highly destructive canker disease on apple, we conducted a functional analysis of two velvet protein family members (VmVeA and VmVelB) via a gene replacement strategy. Deletion mutants of VmVeA and VmVelB showed increased melanin production, conidiation and sensitivity to abiotic stresses, but exhibited reduced virulence on detached apple leaves and twigs. Further studies demonstrated that the regulation of conidiation by VmVeA and VmVelB was positively correlated with the melanin synthesis transcription factor VmCmr1. More importantly, transcript levels of pectinase genes were shown to be decreased in deletion mutants compared with those of the wild-type during infection. However, the expression of other cell wall-degrading enzyme genes, including cellulase, hemi-cellulase and ligninase genes, was not affected in the deletion mutants. Furthermore, the determination of pectinase activity and immunogold labelling of pectin demonstrated that the capacity for pectin degradation was attenuated as a result of deletions of VmVeA and VmVelB. Finally, the interaction of VmVeA with VmVelB was identified through co-immunoprecipitation assays. VmVeA and VmVelB play critical roles in conidiation and virulence, probably via the regulation of the melanin synthesis transcription factor VmCmr1 and their effect on pectinase gene expression in V. mali, respectively.

Transcription factor VmSeb1 is required for the growth, development, and virulence in Valsa mali

Transcription factor Seb1 contains two C2H2 zinc finger motifs which are similar to the Msn2/4 of Saccharomyces cerevisiae. The homologous proteins of Seb1 function to regulate the response to various stresses or decomposing and utilizing pectin in some fungi. In this study, we characterized a homologue of Seb1 gene, VmSeb1, in Valsa mali, which causes a highly destructive bark disease on apple. VmSeb1 deletion mutant showed a drastic reduction in growth rate in vitro. It is also important for conidiation because VmSeb1 deletion mutant formed more pycnidia on PDA medium. Deletion mutant of VmSeb1 increased melanin genes expression. In addition, the sensitivity to oxidative stress increased and cell wall inhibitor in VmSeb1 deletion mutant, as its growth was more severely inhibited by H₂O₂ and Congo red than that in the wild-type. The virulence assay showed that the lesion length caused by the VmSeb1 deletion mutant was smaller compared to wild-type on detached apple twigs. However, expression of pectinase genes and pectinase activity in deletion mutant were the same as those of the wild-type during infection. These results indicate that VmSeb1 plays important roles in growth, asexual development, response to oxidative stress, maintenance of cell wall integrity, and virulence. However, VmSeb1 is not involved in the regulation of pectinase genes expression in V. mali.

Deletion of Endo-β-1,4-Xylanase VmXyl1 Impacts the Virulence of Valsa mali in Apple Tree

Valsa mali, a parasitic fungus, is a destructive pathogen of apple tree that causes heavy economic losses in China. The pathogen secretes various cell wall-degrading enzymes (CWDEs) that degrade plant cell-wall components, and thus facilitate its entry into host cells. Therefore, functional analysis of the genes encoding CWDEs is necessary to understand virulence of V. mali toward apple tree. Here, we identified and cloned an endo-β-1,4-xylanase gene, VmXyl1 in V. mali. The full-length cDNA of VmXyl1 is 1626 bp containing 5'- and 3'-non-coding regions, as well an open reading frame of 1320 bp that encodes a protein with a calculated molecular mass and an isoelectric point of 43.8 kDa and 4.4, respectively. The predicted amino acid sequences showed significant homology to a family GH10 of glycosyl hydrolases. The apple branch extract and beechwood xylan, but not glucose, induced the expression of VmXyl1. Furthermore, VmXyl1 had high expression levels in the apple tree bark during the pathogen infection. The deletion of VmXyl1 did not affect mycelia growth; however, it significantly reduced pycnidia formation in V. mali. The deletion strains showed a reduced virulence toward apple leaves and twigs. Moreover, the mutant strains had reduced endo-β-1,4-xylanase activity and growth when cultured using beechwood xylan as the only carbon source. Reintroducing wild-type VmXyl1 into the mutant strains rescued the defect phenotype. We conclude that VmXyl1 determines the virulence of V. mali toward apple tree. These results provide valuable insight into the plant-pathogen molecular interactions.

Valsa mali Pathogenic Effector VmPxE1 Contributes to Full Virulence and Interacts With the Host Peroxidase MdAPX1 as a Potential Target

The Valsa canker, caused by Valsa mali (V. mali), is a destructive disease of apple in Eastern Asia. Effector proteins are important for fungal pathogenicity. We studied a candidate effector VmPxE1 isolated based on the genome information of V. mali. By using the yeast invertase secretion assay system, VmPxE1 was shown to contain a signal peptide with secretory functions. VmPxE1 can suppress BCL-2-associated X protein (BAX)-induced cell death with a high efficacy of 92% in Nicotiana benthamiana. The expression of VmPxE1 was upregulated during the early infection stage and deletion of VmPxE1 led to significant reductions in virulence on both apple twigs and leaves. VmPxE1 was also shown to target an apple ascorbate peroxidase (MdAPX1) by the yeast two-hybrid screening, bimolecular fluorescence complementation and in vivo co-immunoprecipitation. Sequence phylogenetic analysis suggested that MdAPX1 was an ascorbate peroxidase belonging to a subgroup of heme-dependent peroxidases of the plant superfamily. The ectopic expression of MdAPX1 in the mutant of VmPxE1 significantly enhanced resistance to H₂O₂, while the presence of VmPxE1 seems to disturb MdAPX1 function. The present results provide insights into the functions of VmPxE1 as a candidate effector of V. mali in causing apple canker.

Comparative genome and transcriptome analyses reveal adaptations to opportunistic infections in woody plant degrading pathogens of Botryosphaeriaceae

Botryosphaeriaceae are an important fungal family that cause woody plant diseases worldwide. Recent studies have established a correlation between environmental factors and disease expression; however, less is known about factors that trigger these diseases. The current study reports on the 43.3 Mb de novo genome of Lasiodiplodia theobromae and five other genomes of Botryosphaeriaceae pathogens. Botryosphaeriaceous genomes showed an expansion of gene families associated with cell wall degradation, nutrient uptake, secondary metabolism and membrane transport, which contribute to adaptations for wood degradation. Transcriptome analysis revealed that genes involved in carbohydrate catabolism, pectin, starch and sucrose metabolism, and pentose and glucuronate interconversion pathways were induced during infection. Furthermore, genes in carbohydrate-binding modules, lysine motif domain and the glycosyl hydrolase gene families were induced by high temperature. Among these genes, overexpression of two selected putative lignocellulase genes led to increased virulence in the transformants. These results demonstrate the importance of high temperatures in opportunistic infections. This study also presents a set of Botryosphaeriaceae-specific effectors responsible for the identification of virulence-related pathogen-associated molecular patterns and demonstrates their active participation in suppressing hypersensitive responses. Together, these findings significantly expand our understanding of the determinants of pathogenicity or virulence in Botryosphaeriaceae and provide new insights for developing management strategies against them.

A mitogen-activated protein kinase gene (VmPmk1) regulates virulence and cell wall degrading enzyme expression in Valsa mali

Mitogen-activated protein kinases (MAPKs) play critical roles in the regulation of different developmental processes and hydrolytic enzyme production in many fungal plant pathogens. In this study, an FUS3/KSS1-related MAPK gene, VmPmk1, was identified and characterized in Valsa mali, which causes a highly destructive canker disease on apple. VmPmk1 deletion mutant showed a significant reduction in growth rate in vitro, and could not produce pycnidium, indicating that the MAPK gene is important for growth and asexual development. Also, VmPmk1 played a significant role in response to oxidative stress and in the maintenance of cell wall integrity. More importantly, when deletion mutant was inoculated onto detached apple leaves and twigs, an obvious decrease in lesion size was observed. Furthermore, expression of many cell wall degrading enzyme (CWDE) genes declined in the VmPmk1 deletion mutant during infection. VmPmk1 deletion mutant also showed a significant reduction in activities of CWDEs in both induced media and infection process. Finally, the determination of immunogold labeling of pectin demonstrated that the capacity of degradation pectin was attenuated due to the deletion of VmPmk1. These results indicated that VmPmk1 plays important roles in growth, asexual development, response to oxidative stress, and maintenance of cell wall integrity. More importantly, VmPmk1 is involved in pathogenicity of V. mali mainly by regulating CWDE genes expression.

Effects of Temperature, Humidity, and Wound Age on Valsa mali Infection of Apple Shoot Pruning Wounds

Valsa canker, caused by Valsa mali, is a destructive disease of apple in China. The pathogen infects apple branches, mainly through pruning wounds, and causes branch and tree death. To determine the conditions required for V. mali infection through pruning wounds and growth within the xylem, pruning wounds on 1- to 4-year-old apple branches were inoculated with conidia in vitro under artificially controlled conditions and in vivo in the orchard. The effects of temperature, wetness duration, and wound age on conidial infection through pruning wounds as well as hyphal growth in the xylem were examined. The results showed that, after invading through pruning wounds, V. mali hyphae grew along xylem vessels, tracheids, and rays, expanding longitudinally and laterally. The hyphae could enter adjacent xylem vessels and tracheids through micropores to form a dense hyphal network. Wetness duration did not exhibit an essential effect on conidial infection from pruning wounds. Conidia spread to pruning wounds with rainwater could infect the xylem without any other extra moisture. Temperature for V. mali conidia infection through pruning wounds and hyphal extension in the xylem ranged from 5 to 35°C, with the optimum at 20°C. Pruning wounds made in late March were susceptible to V. mali infection in March, April, and May; the susceptibility was markedly deceased by June, and the pathogen could barely infect through the pruning wounds in November. The infected pruning wounds began to show symptoms from the spring of the following year. More than half of the observed Valsa canker lesions emerged in the spring of the second year, and new canker twigs were also developed from the inoculations in the spring of the third year. March, April, and May are the critical periods for protecting pruning wounds against infection by V. mali in China, and coating pruning wounds with protective film immediately after pruning is an easy and effective measure to protect the pruning wounds.

Horizontal gene transfer drives adaptive colonization of apple trees by the fungal pathogen Valsa mali

Horizontal gene transfer (HGT) often has strong benefits for fungi. In a study of samples from apple canker in Shaanxi Province, China, diverse microbes, along with the necrotrophic pathogen Valsa mali, were found to colonize the apple bark, thus providing ample opportunity for HGT to occur. In the present study, we identified 32 HGT events in V. mali by combining phyletic distribution-based methods with phylogenetic analyses. Most of these HGTs were from bacteria, whereas several others were from eukaryotes. Three HGTs putatively functioned in competition with actinomycetes, some of which showed a significant inhibitory effect on V. mali. Three HGTs that were probably involved in nitrogen uptake were also identified. Ten HGTs were thought to be involved in pathogenicity because they were related to known virulence factors, including cell wall-degrading enzymes and candidate effector proteins. HGT14, together with HGT32, was shown to contribute to bleomycin resistance of V. mali.These results suggest that HGT drives the adaptive evolution of V. mali. The HGTs identified here provide new clues for unveiling the adaptation mechanisms and virulence determinants of V. mali.

Studying the Mechanism of Plasmopara viticola RxLR Effectors on Suppressing Plant Immunity

The RxLR effector family, produced by oomycete pathogens, may manipulate host physiological and biochemical events inside host cells. A group of putative RxLR effectors from Plasmopara viticola have been recently identified by RNA-Seq analysis in our lab. However, their roles in pathogenesis are poorly understood. In this study, we attempted to characterize 23 PvRxLR effector candidates identified from a P. viticola isolate “ZJ-1-1.” During host infection stages, expression patterns of the effector genes were varied and could be categorized into four different groups. By using transient expression assays in Nicotiana benthamiana, we found that 17 of these effector candidates fully suppressed programmed cell death elicited by a range of cell death-inducing proteins, including BAX, INF1, PsCRN63, PsojNIP, PvRxLR16 and R3a/Avr3a. We also discovered that all these PvRxLRs could target the plant cell nucleus, except for PvRxLR55 that localized to the membrane. Furthermore, we identified a single effector, PvRxLR28, that showed the highest expression level at 6 hpi. Functional analysis revealed that PvRxLR28 could significantly enhance susceptibilities of grapevine and tobacco to pathogens. These results suggest that most P. viticola effectors tested in this study may act as broad suppressors of cell death to manipulate immunity in plant.