A Phytophthora sojae cytoplasmic effector mediates disease resistance and abiotic stress tolerance in Nicotiana benthamiana

Effector MoSDT1 enhances Magnaporthe oryzae virulence and plays a dual role in regulating rice defense

C2H2 zinc effectors are a class of pathogen proteins that play a dual role in plant-pathogen interactions, promoting pathogenicity and enhancing plant defense. In our previous research, we identified Magnaporthe oryzae Systemic Defense Trigger 1 (MoSDT1) as a C2H2 zinc effector that activates rice (Oryza sativa) defense when overexpressed in rice. However, its regulatory roles in pathogenicity and defense require further investigation. In this study, we generated an MoSDT1 overexpressing strain and 2 knockout strains of M. oryzae to assess the impact of MoSDT1 on pathogenicity, rice defense, and phenotypic characteristics. Our analyses revealed that MoSDT1 substantially influenced vegetative growth, conidia size, and conidiation, and was crucial for the virulence of M. oryzae while suppressing rice defense. MoSDT1 localized to the nucleus and cytoplasm of rice, either dependent or independent of M. oryzae delivery. Through RNA-seq, scRNA-seq, and ChIP-seq, we identified that MoSDT1 modulates rice defense by regulating the phosphorylation and ubiquitination of various rice signaling proteins, including transcription factors, transcription repressors, kinases, phosphatases, and the ubiquitin system. These findings provide valuable insights into the regulatory mechanisms of C2H2 zinc finger effector proteins and offer important foundational information for utilizing their target genes in disease resistance breeding and the design of targets for disease management.

Complexity of responses to ionizing radiation in plants, and the impact on interacting biotic factors

In nature, plants are simultaneously exposed to different abiotic (e.g., heat, drought, and salinity) and biotic (e.g., bacteria, fungi, and insects) stresses. Climate change and anthropogenic pressure are expected to intensify the frequency of stress factors. Although plants are well equipped with unique and common defense systems protecting against stressors, they may compromise their growth and development for survival in such challenging environments. Ionizing radiation is a peculiar stress factor capable of causing clustered damage. Radionuclides are both naturally present on the planet and produced by human activities. Natural and artificial radioactivity affects plants on molecular, biochemical, cellular, physiological, populational, and transgenerational levels. Moreover, the fitness of pests, pathogens, and symbionts is concomitantly challenged in radiologically contaminated areas. Plant responses to artificial acute ionizing radiation exposure and laboratory-simulated or field chronic exposure are often discordant. Acute or chronic ionizing radiation exposure may occasionally prime the defense system of plants to better tolerate the biotic stress or could often exhaust their metabolic reserves, making plants more susceptible to pests and pathogens. Currently, these alternatives are only marginally explored. Our review summarizes the available literature on the responses of host plants, biotic factors, and their interaction to ionizing radiation exposure. Such systematic analysis contributes to improved risk assessment in radiologically contaminated areas.

PcLRR-RK3, an LRR receptor kinase is required for growth and in-planta infection processes in Phytophthora capsici

Receptor protein kinases (RPKs) critically provide the basic infrastructure to sense, perceive, and conduct the signalling events at the cell surface of organisms. The importance of LRR-RLKs has been well studied in plants, but much less information has been reported in oomycetes. In this work, we have silenced the PcLRR-RK3 and characterised its functional importance in Phytophthora capsici. PcLRR-RK3 was predicted to encode signal peptides, leucine-rich repeats, transmembrane, and kinase domains. PcLRR-RK3-silenced transformants showed impaired colony growth, decreased deformed sporangia, and reduced zoospores count. The mycelium of silenced transformants did not penetrate within the host tissues and showed defects in the pathogenicity of P. capsici. Interestingly, gene silencing also weakens the ability of zoospores germination and penetration into host tissues and fails to produce necrotic lesions. Furthermore, PcLRR-RK3 localisation was found to be the plasma membrane of the cell. Altogether, our results revealed that PcLRR-RK3 was required for the regulation of vegetative growth, zoospores penetration, and establishment into host leaf tissues.

The nuclear effector MoHTR3 of Magnaporthe oryzae modulates host defence signalling in the biotrophic stage of rice infection

Fungal effectors play a pivotal role in suppressing the host defence system, and their evolution is highly dynamic. By comparative sequence analysis of plant-pathogenic fungi and Magnaporthe oryzae, we identified the small secreted C₂ H₂ zinc finger protein MoHTR3. MoHTR3 exhibited high conservation in M. oryzae strains but low conservation among other plant-pathogenic fungi, suggesting an emerging evolutionary selection process. MoHTR3 is exclusively expressed in the biotrophic stage of fungal invasion, and the encoded protein localizes to the biotrophic interfacial complex (BIC) and the host cell nucleus. The signal peptide crucial for MoHTR3' secretion to the BIC and the protein section required for its translocation to the nucleus were both identified by a functional protein domain study. The host-nuclear localization of MoHTR3 suggests a function as a transcriptional modulator of host defence gene induction. After ΔMohtr3 infection, the expression of jasmonic acid- and ethylene-associated genes was diminished in rice, in contrast to when the MoHTR3-overexpressing strain (MoHTR3ox) was applied. The transcript levels of salicylic acid- and defence-related genes were also affected after ΔMohtr3 and MoHTR3ox application. In pathogenicity assays, ΔMohtr3 was indistinguishable from the wild type. However, MoHTR3ox-infected plants showed diminished lesion formation and hydrogen peroxide accumulation, accompanied by a decrease in susceptibility, suggesting that the MoHTR3-induced manipulation of host cells affects host-pathogen interaction. MoHTR3 emphasizes the role of the host nucleus as a critical target for the pathogen-driven manipulation of host defence mechanisms and underscores the ongoing evolution of rice blast's arms race.

Lysobacter enzymogenes prevents Phytophthora infection by inhibiting pathogen growth and eliciting plant immune responses

The Phytophthora pathogen causes enormous damage to important agricultural plants. This group of filamentous pathogens is phylogenetically distant from fungi, making them difficult to control by most chemical fungicides. Lysobacter enzymogenes OH11 (OH11) is a biocontrol bacterium that secretes HSAF (Heat-Stable Antifungal Factor) as a broad-spectrum antifungal weapon. Here, we showed that OH11 could also control a variety of plant Phytophthora diseases caused by three major oomycetes (P. sojae, P. capsici and P. infestans). We provided abundant evidence to prove that OH11 protected host plants from Phytophthora pathogen infection by inhibiting mycelial growth, digesting cysts, suppressing cyst germination, and eliciting plant immune responses. Interestingly, the former two processes required the presence of HSAF, while the latter two did not. This suggested that L. enzymogenes could prevent Phytophthora infection via multiple previously unknown mechanisms. Therefore, this study showed that L. enzymogenes could serve as a promising alternative resource for promoting plant resistance to multiple Phytophthora pathogens.

Differential Colonization of the Plant Vasculature Between Endophytic Versus Pathogenic Fusarium oxysporum Strains

Plant xylem colonization is the hallmark of vascular wilt diseases caused by phytopathogens within the Fusarium oxysporum species complex. Recently, xylem colonization has also been reported among endophytic F. oxysporum strains, resulting in some uncertainty. This study compares xylem colonization processes by pathogenic versus endophytic strains in Arabidopsis thaliana and Solanum lycopersicum, using Arabidopsis pathogen Fo5176, tomato pathogen Fol4287, and the endophyte Fo47, which can colonize both plant hosts. We observed that all strains were able to advance from epidermis to endodermis within 3 days postinoculation (dpi) and reached the root xylem at 4 dpi. However, this shared progression was restricted to lateral roots and the elongation zone of the primary root. Only pathogens reached the xylem above the primary-root maturation zone (PMZ). Related to the distinct colonization patterns, we also observed stronger induction of callose at the PMZ and lignin deposition at primary-lateral root junctions by the endophyte in both plants. This observation was further supported by stronger induction of Arabidopsis genes involved in callose and lignin biosynthesis during the endophytic colonization (Fo47) compared with the pathogenic interaction (Fo5176). Moreover, both pathogens encode more plant cell wall-degrading enzymes than the endophyte Fo47. Therefore, observed differences in callose and lignin deposition could be the combination of host production and the subsequent fungal degradation. In summary, this study demonstrates spatial differences between endophytic and pathogenic colonization, strongly suggesting that further investigations of molecular arm-races are needed to understand how plants differentiate friend from foe. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Engineering plant immune circuit: walking to the bright future with a novel toolbox

Plant pathogens destroy crops and cause severe yield losses, leading to an insufficient food supply to sustain the human population. Apart from relying on natural plant immune systems to combat biological agents or waiting for the appropriate evolutionary steps to occur over time, researchers are currently seeking new breakthrough methods to boost disease resistance in plants through genetic engineering. Here, we summarize the past two decades of research in disease resistance engineering against an assortment of pathogens through modifying the plant immune components (internal and external) with several biotechnological techniques. We also discuss potential strategies and provide perspectives on engineering plant immune systems for enhanced pathogen resistance and plant fitness.

Integrative transcriptome analysis revealed the pathogenic molecular basis of Rhizoctonia solani AG-3 TB at three progressive stages of infection

Rhizoctonia solani has a broad host range and results in significant losses in agricultural production. Here, an integrated transcriptomic analysis was performed to reveal the critical genes responsible for the pathogenesis of R. solani AG-3 TB on Nicotiana tabacum at different infection stages. The results showed that various differential expressed genes (DEGs) were enriched in fatty acid metabolism, amino sugar, carbon metabolism, and cellular carbohydrate biosynthetic process at the early (6–12 hpi), middle (24–36 hpi), and late stage (48–72 hpi) of infection. Specifically, several critical genes such as shikimate kinase that were involved in the biosynthesis of an important fungal toxin, phenylacetic acid (PAA) showed markedly increase at 24 hpi. Additionally, the genes expression levels of carbohydrate-active enzymes (CAZymes) and cell wall degrading enzymes (CWDEs) were significantly increased at the late infection stage. Furthermore, we identified 807 potential secreted proteins and 78 small cysteine-rich proteins, which may function as fungal effectors and involved in the pathogenicity. These results provide valuable insights into critical and potential genes as well as the pathways involved in the pathogenesis of R. solani AG-3 TB.

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.

A putative effector of the rubber-tree powdery mildew fungus has elicitor activity that can trigger plant immunity

A putative powdery mildew effector can elicit defense responses including reactive oxygen species and callose accumulations in model plants Nicotiana benthamiana and Arabidopsis thaliana and host plant Hevea brasiliensis. Powdery mildew fungi cause severe diseases in many agricultural plants, such as the mildew fungus Erysiphe quercicola infecting the rubber tree (Hevea brasiliensis), causing latex yield losses. However, effectors of E. quercicola were rarely functionally characterized. In this study, we identified a highly specific candidate-secreted effector protein, EqCSEP04187, from E. quercicola. This putative effector is expressed at the late stage but not the early stage during infection. The constitutive expression of EqCSEP04187 in model plants Nicotiana benthamiana and Arabidopsis thaliana elicited defense responses, as did transient expression of EqCSEP04187 in protoplasts of H. brasiliensis. Introducing EqCSEP04187 into another H. brasiliensis-associated fungal pathogen, Colletotrichum gloeosporioides, inhibited H. brasiliensis infection, and infection by E. quercicola was decreased in the A. thaliana eds1 mutant expressing EqCSEP04187. Further analysis suggests that these reductions in infection were the consequences of EqCSEP04187 eliciting defense responses. Our study suggests that this putative effector has elicitor activity that can improve plant resistance.

Reactive oxygen species and nitric oxide as mediators in plant hypersensitive response and stomatal closure

Nitric oxide (NO) and reactive oxygen species (ROS) have attracted considerable interest from plant pathologists since they regulate plant defenses via the hypersensitive response (HR) and stomatal closure. Here, we introduce the regulatory mechanisms of NO and ROS bursts and discuss the role of such bursts in HR and stomatal closure. It showed that epidermal sections of leaves respond to pathogens by the rapid and intense production of intracellular ROS and NO. Oxidative stress and H₂O₂ induce stomatal closure. Catalase and peroxidase-deficient plants are also hyperresponsive to pathogen invasion, suggesting a role for H₂O₂ in HR-mediated cell death. The analysis reveals that ROS and NO play important roles in stomatal closure and HR that involves multiple pathways. Therefore, multi-disciplinary and multi-omics combined analysis is crucial to the advancement of ROS and NO research and their role in plant defense mechanism.

Two polygalacturonase-inhibiting proteins (VrPGIP) of Vigna radiata confer resistance to bruchids (Callosobruchus spp.)

Bruchids (Callosobruchus spp.) are destructive storage pests of mung beans (Vigna radiata). Bruchids infest mature seeds during storage and in the field causing heavy losses. Bruchid resistance in mung bean has been characterized as a dominant trait controlled by a single gene. Several independent mapping studies showed that the Br locus on chromosome 5 was a key quantitative trait loci (QTL) involved in bruchid resistance. Two polygalacturonase-inhibitor protein (PGIP) family genes, VrPGIP1 and VrPGIP2, located in the Br locus may be the primary genes responsible for bruchid resistance in mung bean but no experimental proof is available. We isolated the VrPGIP1 and VrPGIP2 genes from bruchid resistant mung bean cultivar V2802 and purified the proteins by prokaryotic expression. Both VrPGIP1 and VrPGIP2 had polygalacturonase inhibitor activity and both of the PGIP proteins conferred resistance to bruchids in an artificial seed test system. VrPGIPs can inhibit the enzyme activity of polygalacturonase present in males, females and fourth instar larvae of C. maculatus. These results demonstrated that VrPGIP1 and VrPGIP2 play a critical role in bruchid resistance probably through inhibiting polygalacturonase activity.

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.

Editing of an effector gene promoter sequence impacts plant-Phytophthora interaction

Pathogen avirulence (Avr) effectors interplay with corresponding plant resistance (R) proteins and activate robust plant immune responses. Although the expression pattern of Avr genes has been tied to their functions for a long time, it is still not clear how Avr gene expression patterns impact plant-microbe interactions. Here, we selected PsAvr3b, which shows a typical effector gene expression pattern from a soybean root pathogen Phytophthora sojae. To modulate gene expression, we engineered PsAvr3b promoter sequences by in situ substitution with promoter sequences from Actin (constitutive expression), PsXEG1 (early expression), and PsNLP1 (later expression) using the CRISPR/Cas9. PsAvr3b driven by different promoters resulted in distinct expression levels across all the tested infection time points. Importantly, those mutants with low PsAvr3b expression successfully colonized soybean plants carrying the cognate R gene Rps3b. To dissect the difference in plant responses to the PsAvr3b expression level, we conducted RNA-sequencing of different infection samples at 24 h postinfection and found soybean immune genes, including a few previously unknown genes that are associated with resistance. Our study highlights that fine-tuning in Avr gene expression impacts the compatibility of plant disease and provides clues to improve crop resistance in disease control management.

Plant immunity in signal integration between biotic and abiotic stress responses

Plants constantly monitor and cope with the fluctuating environment while hosting a diversity of plant-inhabiting microbes. The mode and outcome of plant-microbe interactions, including plant disease epidemics, are dynamically and profoundly influenced by abiotic factors, such as light, temperature, water and nutrients. Plants also utilize associations with beneficial microbes during adaptation to adverse conditions. Elucidation of the molecular bases for the plant-microbe-environment interactions is therefore of fundamental importance in the plant sciences. Following advances into individual stress signaling pathways, recent studies are beginning to reveal molecular intersections between biotic and abiotic stress responses and regulatory principles in combined stress responses. We outline mechanisms underlying environmental modulation of plant immunity and emerging roles for immune regulators in abiotic stress tolerance. Furthermore, we discuss how plants coordinate conflicting demands when exposed to combinations of different stresses, with attention to a possible determinant that links initial stress response to broad-spectrum stress tolerance or prioritization of specific stress tolerance.

Integrative transcriptome analysis discloses the molecular basis of a heterogeneous fungal phytopathogen complex, Rhizoctonia solani AG-1 subgroups

Rhizoctonia solani is a fungal species complex that causes necrotrophic crop diseases. It comprises several anastomosis groups, some of which include intra-subgroups, such as AG-1 IA and AG-1 IB, exhibiting varying pathogenicity. Owing to its heterozygous and multinucleate features, genomic analyses of R. solani are still challenging, and understanding of its genetic diversity and genic components is limited. In this study, in order to elucidate the molecular basis of this phytopathogen complex, an integrated transcriptome analysis was undertaken for three subgroups of AG-1, i.e. AG-1 IA, AG-1 IB, and AG-1 IC. Sequence variations suggested substantial evolutionary distances within AG-1. Transcript simple sequence repeats showed comparable characteristics among AG-1, but contained polymorphic sites. Intra-subgroup polymorphisms suggested varying genic heterozygosity within AG-1, suggesting their independent evolutionary trajectory. Sequences of pathogenic factors, phytotoxin biosynthesis pathway enzymes, secreted lignocellulosic enzymes, secreted reactive oxygen species detoxification enzymes, apoplastic/cytoplasmic effector candidates, were conserved among those subgroups. d_N/d_S ratios of a secretome subset suggested core secreted proteins in AG-1 and distinct evolution of Cys-rich small secreted proteins after differentiation of AG-1 subgroups. Identification of likely pathogenic factors including allergen protein homologues, oxidative phosphorylation and ethylene biosynthesis pathways, and diversification of polysaccharide monooxygenases provides molecular insight into key genomic components that play a role in R. solani pathogenesis.

PpCRN7 and PpCRN20 of Phythophthora parasitica regulate plant cell death leading to enhancement of host susceptibility

BACKGROUND

Phytophthora species secrete cytoplasmic effectors from a family named Crinkler (CRN), which are characterised by the presence of conserved specific domains in the N- and C-terminal regions. P. parasitica causes disease in a wide range of host plants, however the role of CRN effectors in these interactions remains unclear. Here, we aimed to: (i) identify candidate CRN encoding genes in P. parasitica genomes; (ii) evaluate the transcriptional expression of PpCRN (Phytophthora parasitica Crinkler candidate) during the P. parasitica interaction with Citrus sunki (high susceptible) and Poncirus trifoliata (resistant); and (iii) functionally characterize two PpCRNs in the model plant Nicotiana benthamiana.

RESULTS

Our in silico analyses identified 80 putative PpCRN effectors in the genome of P. parasitica isolate 'IAC 01/95.1'. Transcriptional analysis revealed differential gene expression of 20 PpCRN candidates during the interaction with the susceptible Citrus sunki and the resistant Poncirus trifoliata. We have also found that P. parasitica is able to recognize different citrus hosts and accordingly modulates PpCRNs expression. Additionally, two PpCRN effectors, namely PpCRN7 and PpCRN20, were further characterized via transient gene expression in N. benthamiana leaves. The elicitin INF-1-induced Hypersensitivity Response (HR) was increased by an additive effect driven by PpCRN7 expression, whereas PpCRN20 expression suppressed HR response in N. benthamiana leaves. Despite contrasting functions related to HR, both effectors increased the susceptibility of plants to P. parasitica.

CONCLUSIONS

PpCRN7 and PpCRN20 have the ability to increase P. parasitica pathogenicity and may play important roles at different stages of infection. These PpCRN-associated mechanisms are now targets of biotechnological studies aiming to break pathogen's virulence and to promote plant resistance.

Overexpression of Magnaporthe Oryzae Systemic Defense Trigger 1 (MoSDT1) Confers Improved Rice Blast Resistance in Rice

The effector proteins secreted by a pathogen not only promote virulence and infection of the pathogen, but also trigger plant defense response. Therefore, these proteins could be used as important genetic resources for transgenic improvement of plant disease resistance. Magnaporthe oryzae systemic defense trigger 1 (MoSDT1) is an effector protein. In this study, we compared the agronomic traits and blast disease resistance between wild type (WT) and MoSDT1 overexpressing lines in rice. Under control conditions, MoSDT1 transgenic lines increased the number of tillers without affecting kernel morphology. In addition, MoSDT1 transgenic lines conferred improved blast resistance, with significant effects on the activation of callose deposition, reactive oxygen species (ROS) accumulation and cell death. On the one hand, overexpression of MoSDT1 could delay biotrophy-necrotrophy switch through regulating the expression of biotrophy-associated secreted protein 4 (BAS4) and Magnaporthe oryzaecell death inducing protein 1 (MoCDIP1), and activate plant defense response by regulating the expression of Bsr-d1, MYBS1, WRKY45, peroxidase (POD), heat shock protein 90 (HSP90), allenoxide synthase 2 (AOS2), phenylalanine ammonia lyase (PAL), pathogenesis-related protein 1a (PR1a) in rice. On the other hand, overexpression of MoSDT1 could increase the accumulation of some defense-related primary metabolites such as two aromatic amino acids (L-tyrosine and L-tryptohan), 1-aminocyclopropane carboxylic acid, which could be converted to ethylene, vanillic acid and L-saccharopine. Taken together, overexpression of MoSDT1 confers improved rice blast resistance in rice, through modulation of callose deposition, ROS accumulation, the expression of defense-related genes, and the accumulation of some primary metabolites.

Identification and Functional Characterization of an Effector Secreted by Cronartium ribicola

Cri-9402 was identified as a protein effector from Cronartium ribicola, based on the effect of its expression on growth of Pseudomonas syringae Psm ES4326 introduced into transiently transformed tobacco leaves and stably transformed Arabidopsis seedlings. In tobacco leaves transiently expressing its coding sequence, growth of P. syringae Psm ES4326 was inhibited. Expression of pathogenesis-related (PR) protein 2 (PR2), PR4a, endochitinase B, hypersensitive-related 201 (HSR201), HSR203J, and proteinase inhibitor 1 was upregulated but expression of PR1, coronatine insensitive 1, and abscisic acid 1 was significantly suppressed. In transformed Arabidopsis seedlings, the effector stimulated growth of P. syringae Psm ES4326; significantly suppressed expression of PR1, PR2, nonexpresser of pathogenesis-related genes 1 (NPR1), NPR3, NPR4, phytoalexin deficient 4, and salicylic acid induction deficient 2; and enhanced expression of plant defensin 1.2 (PDF1.2). The above results showed that the majority of responses to this effector in tobacco leaves were converse to those in transformed Arabidopsis. We could conclude that Cri-9402 promoted disease resistance in tobacco leaves and disease susceptibility in Arabidopsis seedlings. Its transcript was mainly expressed in aeciospores of C. ribicola and was probably involved in production or germination of aeciospores, and it was an effector potentially functioning in white pine-blister rust interactions.

Effects of exogenous salicylic acid and pH on pathogenicity of biotrophy-associated secreted protein 1 (BAS1)-overexpressing strain, Magnaporthe oryzae

Abiotic stress can influence the interactions between a pathogen and its host. In this paper, we analyzed the effects of salicylic acid (SA) and pH on the morphological development and pathogenicity of Magnaporthe oryzae, the pathogen that causes rice (Oryza sativa) blast. A strain of rice blast that overexpresses biotrophy-associated secreted protein 1 (BAS1) and a wild-type (WT) strain were pretreated with different levels of pH and different concentrations of SA to analyze M. oryzae colony growth, sporulation, spore germination, dry weight of hypha, and appressorium formation. Disease incidence and the expression of defense-related genes in infected rice were analyzed after pretreatment with pH 5.00 or pH 8.00 and 200 μM SA. The results showed that both SA and pH had some influence on morphological development, including sporulation and appressorium formation of the BAS1-overexpression strain. In the 200 μM SA pretreatment, there was a lower incidence of disease and higher expression levels of the rice defense-related genes PR1a, PAL, HSP90, and PR5 on leaves inoculated with the BAS1-overexpession strain compared with the WT strain, whereas, LOX2 appeared to be downregulated in the BAS1-overexpession strain compared with the WT. In both pH treatments, disease incidence and expression of HSP90 were higher and the expression of PR1a and PR10a and LOX2 and PAL was lower in leaves inoculated with the BAS1-overexpression strain compared with leaves inoculated with the WT strain. We conclude that SA and pH affect morphological development of the BAS1-overexpression blast strain, but that these factors have little influence on the pathogenicity of the strain, indicating that BAS1-overexpression may have enhanced the tolerance of this rice blast strain to abiotic stressors. This work suggests new molecular mechanisms that exogenous SA and pH affect the interactions between M. oryzae and rice.

Effects of exogenous salicylic acid and pH on pathogenicity of biotrophy-associated secreted protein 1 (BAS1)-overexpressing strain, Magnaporthe oryzae

Abiotic stress can influence the interactions between a pathogen and its host. In this paper, we analyzed the effects of salicylic acid (SA) and pH on the morphological development and pathogenicity of Magnaporthe oryzae, the pathogen that causes rice (Oryza sativa) blast. A strain of rice blast that overexpresses biotrophy-associated secreted protein 1 (BAS1) and a wild-type (WT) strain were pretreated with different levels of pH and different concentrations of SA to analyze M. oryzae colony growth, sporulation, spore germination, dry weight of hypha, and appressorium formation. Disease incidence and the expression of defense-related genes in infected rice were analyzed after pretreatment with pH 5.00 or pH 8.00 and 200 μM SA. The results showed that both SA and pH had some influence on morphological development, including sporulation and appressorium formation of the BAS1-overexpression strain. In the 200 μM SA pretreatment, there was a lower incidence of disease and higher expression levels of the rice defense-related genes PR1a, PAL, HSP90, and PR5 on leaves inoculated with the BAS1-overexpession strain compared with the WT strain, whereas, LOX2 appeared to be downregulated in the BAS1-overexpession strain compared with the WT. In both pH treatments, disease incidence and expression of HSP90 were higher and the expression of PR1a and PR10a and LOX2 and PAL was lower in leaves inoculated with the BAS1-overexpression strain compared with leaves inoculated with the WT strain. We conclude that SA and pH affect morphological development of the BAS1-overexpression blast strain, but that these factors have little influence on the pathogenicity of the strain, indicating that BAS1-overexpression may have enhanced the tolerance of this rice blast strain to abiotic stressors. This work suggests new molecular mechanisms that exogenous SA and pH affect the interactions between M. oryzae and rice.

GmDAD1, a Conserved Defender Against Cell Death 1 (DAD1) From Soybean, Positively Regulates Plant Resistance Against Phytophthora Pathogens

Initially identified as a mammalian apoptosis suppressor, defender against apoptotic death 1 (DAD1) protein has conserved plant orthologs acting as negative regulators of cell death. The potential roles and action mechanisms of plant DADs in resistance against Phytophthora pathogens are still unknown. Here, we cloned GmDAD1 from soybean and performed functional dissection. GmDAD1 expression can be induced by Phytophthora sojae infection in both compatible and incompatible soybean varieties. By manipulating GmDAD1 expression in soybean hairy roots, we showed that GmDAD1 transcript accumulations are positively correlated with plant resistance levels against P. sojae. Heterologous expression of GmDAD1 in Nicotiana benthamiana enhanced its resistance to Phytophthora parasitica. NbDAD1 from N. benthamiana was shown to have similar role in conferring Phytophthora resistance. As an endoplasmic reticulum (ER)-localized protein, GmDAD1 was demonstrated to be involved in ER stress signaling and to affect the expression of multiple defense-related genes. Taken together, our findings reveal that GmDAD1 plays a critical role in defense against Phytophthora pathogens and might participate in the ER stress signaling pathway. The defense-associated characteristic of GmDAD1 makes it a valuable working target for breeding Phytophthora resistant soybean varieties.

Random mutagenesis screen shows that Phytophthora capsici CRN83_152-mediated cell death is not required for its virulence function(s)

With the increasing availability of plant pathogen genomes, secreted proteins that aid infection (effectors) have emerged as key factors that help to govern plant-microbe interactions. The conserved CRN (CRinkling and Necrosis) effector family was first described in oomycetes by their capacity to induce host cell death. Despite recent advances towards the elucidation of CRN virulence functions, the relevance of CRN-induced cell death remains unclear. In planta over-expression of PcCRN83_152, a CRN effector from Phytophthora capsici, causes host cell death and boosts P. capsici virulence. We used these features to ask whether PcCRN83_152-induced cell death is linked to its virulence function. By randomly mutating this effector, we generated PcCRN83_152 variants with no cell death (NCD) phenotypes, which were subsequently tested for activity towards enhanced virulence. We showed that a subset of PcCRN83_152 NCD variants retained their ability to boost P. capsici virulence. Moreover, NCD variants were shown to have a suppressive effect on PcCRN83_152-mediated cell death. Our work shows that PcCRN83_152-induced cell death and virulence function can be separated. Moreover, if these findings hold true for other cell death-inducing CRN effectors, this work, in turn, will provide a framework for studies aimed at unveiling the virulence functions of these effectors.

A Perspective on CRN Proteins in the Genomics Age: Evolution, Classification, Delivery and Function Revisited

Plant associated microbes rely on secreted virulence factors (effectors) to modulate host immunity and ensure progressive infection. Amongst the secreted protein repertoires defined and studied in pathogens to date, the CRNs (for CRinkling and Necrosis) have emerged as one of only a few highly conserved protein families, spread across several kingdoms. CRN proteins were first identified in plant pathogenic oomycetes where they were found to be modular factors that are secreted and translocated inside host cells by means of a conserved N-terminal domain. Subsequent localization and functional studies have led to the view that CRN C-termini execute their presumed effector function in the host nucleus, targeting processes required for immunity. These findings have led to great interest in this large protein family and driven the identification of additional CRN-like proteins in other organisms. The identification of CRN proteins and subsequent functional studies have markedly increased the number of candidate CRN protein sequences, expanded the range of phenotypes tentatively associated with function and revealed some of their molecular functions toward virulence. The increased number of characterized CRNs also has presented a set of challenges that may impede significant progress in the future. Here, we summarize our current understanding of the CRNs and re-assess some basic assumptions regarding this protein family. We will discuss the latest findings on CRN biology and highlight exciting new hypotheses that have emanated from the field. Finally, we will discuss new approaches to study CRN functions that would lead to a better understanding of CRN effector biology as well as the processes that lead to host susceptibility and immunity.

A Phytophthora sojae effector PsCRN63 forms homo-/hetero-dimers to suppress plant immunity via an inverted association manner

Oomycete pathogens produce a large number of effectors to promote infection. Their mode of action are largely unknown. Here we show that a Phytophthora sojae effector, PsCRN63, suppresses flg22-induced expression of FRK1 gene, a molecular marker in pathogen-associated molecular patterns (PAMP)-triggered immunity (PTI). However, PsCRN63 does not suppress upstream signaling events including flg22-induced MAPK activation and BIK1 phosphorylation, indicating that it acts downstream of MAPK cascades. The PsCRN63-transgenic Arabidopsis plants showed increased susceptibility to bacterial pathogen Pseudomonas syringae pathovar tomato (Pst) DC3000 and oomycete pathogen Phytophthora capsici. The callose deposition were suppressed in PsCRN63-transgenic plants compared with the wild-type control plants. Genes involved in PTI were also down-regulated in PsCRN63-transgenic plants. Interestingly, we found that PsCRN63 forms an dimer that is mediated by inter-molecular interactions between N-terminal and C-terminal domains in an inverted association manner. Furthermore, the N-terminal and C-terminal domains required for the dimerization are widely conserved among CRN effectors, suggesting that homo-/hetero-dimerization of Phytophthora CRN effectors is required to exert biological functions. Indeed, the dimerization was required for PTI suppression and cell death-induction activities of PsCRN63.

Overexpression of a Phytophthora Cytoplasmic CRN Effector Confers Resistance to Disease, Salinity and Drought in Nicotiana benthamiana

The Crinkler (CRN) effector family is produced by oomycete pathogens and may manipulate host physiological and biochemical events inside host cells. Here, PsCRN161 was identified from Phytophthora sojae based on its broad and strong cell death suppression activities. The effector protein contains two predicted nuclear localization signals and localized to nuclei of plant cells, indicating that it may target plant nuclei to modify host cell physiology and function. The chimeric gene GFP:PsCRN161 driven by the Cauliflower mosaic virus (CaMV) 35S promoter was introduced into Nicotiana benthamiana. The four independent PsCRN161-transgenic lines exhibited increased resistance to two oomycete pathogens (P. parasitica and P. capsici) and showed enhanced tolerance to salinity and drought stresses. Digital gene expression profiling analysis showed that defense-related genes, including ABC transporters, Cyt P450 and receptor-like kinases (RLKs), were significantly up-regulated in PsCRN161-transgenic plants compared with GFP (green fluorescent protein) lines, implying that PsCRN161 expression may protect plants from biotic and abiotic stresses by up-regulation of many defense-related genes. The results reveal previously unknown functions of the oomycete effectors, suggesting that the pathogen effectors could be directly used as functional genes for plant molecular breeding for enhancement of tolerance to biotic and abiotic stresses.