The Magnaporthe oryzae effector Avr-PikD suppresses rice immunity by inhibiting an LSD1-like transcriptional activator

Avirulence effectors(Avrs),encoded by plant pathogens,can be recognized by plants harboring the corresponding resistance proteins,thereby initiating effector-triggered immunity(ETI).In susceptible plants,however,Avrs can function as effectors,facilitating infection via effector-triggered susceptibility(ETS).Mechanisms of Avr-mediated ETS remain largely unexplored.Here we report that the Magnaporthe oryzae effector Avr-PikD enters rice cells via the canonical cytoplasmic secretion pathway and suppresses rice basal defense.Avr-PikD interacts with an LSD1-like transcriptional activator AKIP30 of rice,and AKIP30 is also a positive regulator of rice immunity,whereas Avr-PikD impedes its nuclear localization and suppresses its transcriptional activity.In summary,M.oryzae delivers Avr-PikD into rice cells to facilitate ETS by inhibiting AKIP30-mediated transcriptional regulation of immune response against M.oryzae.

The processed C-terminus of AvrRps4 effector suppresses plant immunity via targeting multiple WRKYs

Pathogens generate and secrete effector proteins to the host plant cells during pathogenesis to promote virulence and colonization.If the plant carries resistance(R)proteins that recognize pathogen effectors,effector-triggered immunity(ETI)is activated,resulting in a robust immune response and hypersensitive response(HR).The bipartite effector AvrRps4 from Pseudomonas syringae pv.pisi has been well studied in terms of avirulence function.In planta,AvrRps4 is processed into two parts.The Cterminal fragment of AvrRps4(AvrRps4^(C))induces HR in turnip and is recognized by the paired resistance proteins AtRRS1/AtRPS4 in Arabidopsis.Here,we show that AvrRps4^(C)targets a group of Arabidopsis WRKY,including WRKY46,WRKY53,WRKY54,and WRKY70,to induce its virulence function.Indeed,AvrRps4^(C)suppresses the general binding and transcriptional activities of immune-positive regulator WRKY54 and WRKY54-mediated resistance.AvrRps4^(C)interferes with WRKY54's binding activity to target gene SARD1 in vitro,suggesting WRKY54 is sequestered from the SARD1 promoter by AvrRps4^(C).Through the interaction of Avr Rps4^(C)with four WRKYs,AvrRps4 enhances the formation of homo-/heterotypic complexes of four WRKYs and sequesters them in the cytoplasm,thus inhibiting their function in plant immunity.Together,our results provide a detailed virulence mechanism of AvrRps4 through its C-terminus.

Erratum to “Plant Defense against Necrotrophic Pathogens” [American Journal of Plant Sciences 11(12) (2020) 2122-2138]

The original online version of this article (Ghozlan, M.H., EL-Argawy, E., Tokgöz, S., Lakshman, D.K. and Mitra, A. (2020) Plant Defense against Necrotrophic Pathogens. American Journal of Plant Sciences, 11, 2122-2138. https://doi.org/10.4236/ajps.2020.1112149) was published mistakenly without another co-author, Nikita Gambhir. In this regard, we revise authors and “how to cite” sections by adding her name.

Recent Progress in Understanding PAMP- and Effector-Triggered Immunity against the Rice Blast Fungus Magnaporthe oryzae

Rice blast, caused by the fungal pathogen Magnaporthe oryzae, is one of the most destructive diseases of rice worldwide. The rice-M, oryzae pathosystem has become a model in the study of plant-fungal interactions because of its scientific advancement and economic importance. Recent studies have identified a number of new pathogen- associated molecular patterns （PAMPs） and effectors from the blast fungus that trigger rice immune responses upon perception. Interaction analyses between avirulence effectors and their cognate resistance proteins have provided new insights into the molecular basis of plant-fungal interactions. In this review, we summarize the recent research on the characterization of those genes in both M. oryzae and rice that are important for the PAMP- and effector-triggered immunity recognition and signaling processes. We also discuss future directions for research that will further our understanding of this pathosystem.

Translational Regulation of Metabolic Dynamics during Effector-Triggered Immunity

Recent studies have shown that global translational reprogramming is an early activation event in pattern-triggered immunity,when plants recognize microbe-associated molecular patterns.However,it is not fully known whether translational regulation also occurs in subsequent immune responses,such as effector-triggered immunity(ETI).In this study,we performed genome-wide ribosome profiling in Arabidopsis upon RPS2-mediated ETI activation and discovered that specific groups of genes were translationally regulated,mostly in coordination with transcription.These genes encode enzymes involved in aromatic amino acid,phenylpropanoid,camalexin,and sphingolipid metabolism.The functional significance of these components in ETI was confirmed by genetic and biochemical analyses.Our findings provide new insights into diverse translational regulation of plant immune responses and demonstrate that translational coordination of metabolic gene expression is an important strategy for ETI.

水稻抗稻瘟病天然免疫机制及抗病育种新策略

稻瘟病是水稻最严重的病害之一,由子囊菌(Magnaporthe oryzae)引起。利用抗病品种是防治稻瘟病最经济、最有效的措施。近年来,稻瘟病已发展为研究植物与病原真菌分子互作机制的模式系统,在水稻与稻瘟菌互作和寄主抗性分子生物学、基因组学和蛋白组学等领域取得了一系列重要的研究成果。文章综述了近年来水稻抗稻瘟病两种天然免疫机制,即病原菌相关分子模式诱导和效应蛋白诱导的抗病机制研究的最新进展,讨论了GWAS、TALLEN、CRISPR和HIGS等基因组研究新方法和新技术在水稻抗病育种中的应用,并对目前稻瘟病抗性机制研究和抗病育种中的问题和挑战进行了探讨和展望。

植物天然免疫系统研究进展

很多植物病原菌严重地损害植物的生长和繁殖。植物与病原体协同进化过程中,也逐渐形成了一系列复杂高效的保护机制来抵御病原物的侵染。植物中抵抗外界微生物刺激所形成的系统被称为植物天然免疫系统,可分为两个层次。第1个层次是植物模式识别受体(PRRs)识别病原相关分子模式(PAMPs),触发病原相关分子模式触发的免疫反应(PTI),激活植物体中促丝裂原活化蛋白激酶(MAPK)信号通路使植物产生早期应答反应。PTI适应性较广,可识别和响应包括非致病菌的许多类微生物。第2个层次是病原菌产生效应因子抑制基础免疫响应PTI,而植物产生针对性更强的抗性蛋白(R蛋白)识别效应因子,并通过效应因子触发型免疫(ETI)来重建植物的抗性。笔者综述了近年来植物天然免疫系统的研究进展,认为随着对植物天然免疫系统研究的深入,应重视PTI和ETI的结合利用,有效扩大植物抗菌谱,改良植物ETI抗性。

大豆胞囊线虫效应蛋白Hg16B09对植物免疫反应的抑制作用

为明确大豆胞囊线虫效应蛋白Hg16B09是否对植物免疫反应具有调控作用,本研究借助本氏烟草叶片瞬时表达Hg16B09,利用免疫激发子触发PTI和ETI反应,考察激发子诱导烟草叶片细胞过敏性坏死反应发生、活性氧爆发以及防御相关基因的表达变化情况。结果表明:Hg16B09在烟草细胞中瞬时表达能够显著抑制鞭毛蛋白flg22诱导H;O;的产生及防御基因PTI5、WRKY22-A、WRKY22-B和ACRE31的表达,其表达量相比对照分别降低了45%、61%、67%和61%。Hg16B09也能抑制由丁香假单胞杆菌Pst DC3000和线虫无毒效应子Rbp-1触发的HR发生,并减少了细胞坏死面积;qRT-PCR分析显示,Hg16B09对Pst DC3000激活的SA途径防御基因PR1a、PR2、WRKY51及PI1上调表达具有显著抑制作用,与对照相比表达量分别降低了67%、71%、48%和81%。以上结果表明Hg16B09对植物的PTI和ETI免疫反应具有双重抑制作用,说明调控寄主免疫防御是Hg16B09促进SCN寄生的一个关键毒性功能。

小麦条锈菌效应蛋白Hasp58抑制植物免疫的功能分析

为明确小麦条锈菌吸器特异表达效应蛋白Hasp58抑制植物免疫的功能,利用PCR获得Hasp58的全长cDNA序列;借助生物信息学软件预测Hasp58的信号肽;将Hasp58构建到pGR106载体用于农杆菌(Agrobactrium tumefacien)介导的烟草瞬时表达系统,明确Hasp58抑制细胞坏死的毒性功能;将Hasp58构建到pCambia1302载体用于烟草细胞定位,明确Hasp58定位情况;将Hasp58构建到pEDV6载体用于荧光假单胞杆菌(Pseudomonas fluorescens)介导的小麦瞬时表达系统,对小麦胼胝质、过敏性坏死面积、活性氧、菌丝面积、菌丝长度进行统计分析,明确Hasp58抑制寄主植物PAMPs引发的免疫反应(PTI)和效应蛋白诱导的免疫反应(ETI)功能。结果表明,条锈菌Hasp58的cDNA全长为1 026 bp,编码342个氨基酸,N端含26_aa的信号肽;通过农杆菌侵染,在烟草中瞬时表达Hasp58,发现其能够抑制由BAX诱导的细胞坏死,为条锈菌的候选效应蛋白;烟草细胞定位发现,含有信号肽和缺失信号肽的Hasp58与GFP融合表达均定位在细胞质,推测该效应蛋白在胞质作用;利用细菌的三型分泌系统在小麦中瞬时表达Hasp58,发现其能够抑制荧光假单胞杆菌引起的胼胝质积累,导致由无毒性条锈菌小种CYR23引起的小麦过敏性坏死面积和活性氧减少,使菌丝面积和长度增加。推测小麦条锈菌效应蛋白Hasp58可抑制寄主植物的PAMPs引发的免疫反应(PTI)和效应蛋白诱导的免疫反应(ETI),并促进自身侵染。

Direct acetylation of a conserved threonine of RIN4 by the bacterial effector HopZ5 or AvrBsT activates RPM1-dependent immunity in Arabidopsis

Plant pathogenic bacteria deliver effectors into plant cells to suppress immunity and promote pathogen survival;however, these effectors can be recognised by plant disease resistance (R) proteins to activate innate immunity. The bacterial acetyltransferase effectors HopZ5 and AvrBsT trigger immunity in Arabidopsis thaliana genotypes lacking SUPPRESSOR OF AVRBST-ELICITED RESISTANCE 1 (SOBER1). Using an Arabidopsis accession, Tscha-1, that naturally lacks functional SOBER1 but is unable to recognise HopZ5, we demonstrate that RESISTANCE TO P. SYRINGAE PV MACULICOLA 1 (RPM1) and RPM1-INTERACTING PROTEIN 4 (RIN4) are indispensable for HopZ5- or AvrBsT-triggered immunity. Remarkably, T166 of RIN4, the phosphorylation of which is induced by AvrB and AvrRpm1, was directly acetylated by HopZ5 and AvrBsT. Furthermore, we demonstrate that the acetylation of RIN4 T166 is required and sufficient for HopZ5- or AvrBsT-triggered RPM1-dependent defence activation. Finally, we show that SOBER1 interferes with HopZ5- or AvrBsT-triggered immunity by deacetylating RIN4 T166. We have thus elucidated detailed molecular mechanisms underlying the activation and suppression of plant innate immunity triggered by two bacterial acetyltransferases, HopZ5 and AvrBsT from different bacterial pathogens.

Intra-strain Elicitation and Suppression of Plant Immunity by Ralstonia solanacearum Type-III Effectors in Nicotiana benthamiana

Effector proteins delivered inside plant cells are powerful weapons for bacterial pathogens,but this exposes the pathogen to potential recognition by the plant immune system.Therefore,the effector repertoire of a given pathogen must be balanced for a successful infection.Ralstonia solanacearum is an aggressive pathogen with a large repertoire of secreted effectors.One of these effectors,RipE1,is conserved in most R.solanacearum strains sequenced to date.In this work,we found that RipE1 triggers immunity in N.benthamiana,which requires the immune regulator SGT1,but not EDS1 or NRCs.Interestingly,RipE1-triggered immunity induces the accumulation of salicylic acid(SA)and the overexpression of several genes encoding phenylalanine-ammonia lyases(PALs),suggesting that the unconventional PALmediated pathway is responsible for the observed SA biosynthesis.Surprisingly,RipE1 recognition also induces the expression of jasmonic acid(JA)-responsive genes and JA biosynthesis,suggesting that both SA and JA may act cooperatively in response to RipE1.We further found that RipE1 expression leads to the accumulation of glutathione in plant cells,which precedes the activation of immune responses.R.solanacearum secretes another effector,RipAY,which is known to inhibit immune responses by degrading cellular glutathione.Accordingly,RipAY inhibits RipE1-triggered immune responses.This work shows a strategy employed by R.solanacearum to counteract the perception of its effector proteins by plant immune system.

水稻-病原菌互作途径研究进展

水稻稻瘟病和白叶枯病分别由真菌病原菌Magnaporthe oryzae(M.oryzae)和细菌病原菌Xanthomonas oryzae pv.oryzae(Xoo)引起,是造成世界范围内水稻减产的主要病因,水稻-稻瘟病菌及水稻-白叶枯病原菌互作已成为研究植物-病原菌互作的模式系统。本研究归纳了目前已克隆的抗稻瘟病及白叶枯病基因与其分子结构和功能,概括了近年来鉴定的一些病原菌相关分子(Pathogen-associated molecular patterns,PAMPs)及稻瘟病菌和白叶枯菌分泌的效应蛋白,并总结了针对稻瘟病菌和白叶枯菌介导的病原物分子诱导的抗病反应(PAMP-triggered immunity,PTI)和效应蛋白诱导的抗病性(Effectortriggered immunity,ETI)及其信号传导途径的研究成果,指出效应蛋白-抗病蛋白间互作将为探索植物-病原菌间互作提供新的分子基础,并为水稻抗病育种实践提供借鉴与指导。

Suppression of ETI by PTI priming to balance plant growth and defense through an MPK3/MPK6-WRKYs-PP2Cs module

Pattern-triggered immunity(PTI)and effector-triggered immunity(ETI)are required for host defense against pathogens.Although PTI and ETI are intimately connected,the underlying molecular mechanisms remain elusive.In this study,we demonstrate that flg22 priming attenuates Pseudomonas syringae pv.tomato DC3000(Pst)AvrRpt2-induced hypersensitive cell death,resistance,and biomass reduction in Arabidopsis.Mitogen-activated protein kinases(MAPKs)are key signaling regulators of PTI and ETI.The absence of MPK3 and MPK6 significantly reduces pre-PTI-mediated ETI suppression(PES).We found that MPK3/MPK6 interact with and phosphorylate the downstream transcription factor WRKY18,which regulates the expression of AP2C1 and PP2C5,two genes encoding protein phosphatases.Furthermore,we observed that the PTI-suppressed ETI-triggered cell death,MAPK activation,and growth retardation are significantly attenuated in wrky18/40/60 and ap2c1 pp2c5 mutants.Taken together,our results suggest that the MPK3/MPK6-WRKYs-PP2Cs module underlies PES and is essential for the maintenance of plant fitness during ETI.

Plant Defense against Necrotrophic Pathogens

Necrotrophic pathogenic bacteria, fungi and oomycetes are widely distributed and are responsible for significant crop losses. Host plants deploy different defense mechanisms and appropriate immune responses to defend them against these pathogens. Regardless of the pathogen’s lifestyle, infection activates plant immune responses either through Pathogen/Microbe Associated Molecular Pattern (P/MAMP) or through Effector Triggered Immunity (ETI). However, as R-genes are not usually associated with resistance to necrotrophs, resistance is largely dependent on the balanced interplay between crucial phytohormones in complex signaling pathways involving jasmonic acid (JA), ethylene, salicylic acid (SA) and abscisic acid (ABA). An increase in salicylic acid levels enhances susceptibility to necrotrophic pathogens but promotes resistance to hemibiotrophs, whereas a deficiency in SA or SA signaling has either no significant impact or affects resistance only at the primary infection site. The same fashion is observed for JA signaling system that appears to elicit resistance against diseases caused by necrotrophic pathogens and can trigger systemic immunity conferring resistance against them. On the other hand, ABA can play a positive or negative role in plant defense responses to necrotrophs as ABA-mediated defense responses are dependent on specific plant-pathogen interactions. Understanding plant immune response against necrotrophic pathogens may lead to the development of resistant or tolerant crop cultivars.

泛素化修饰在水稻与稻瘟病菌互作中的研究进展

稻瘟病菌Magnaporthe oryzae引起的稻瘟病是水稻Oryza sativa生产中的重大真菌病害,对水稻与稻瘟病菌互作机制的解析有助于挖掘优异抗性基因,为水稻育种提供抗稻瘟病基因资源。泛素化是真核生物中保守存在的重要蛋白质翻译后修饰(post-translational modification,PTM),参与植物生长发育和胁迫响应等过程。随着生物技术的不断发展,水稻与稻瘟病菌互作研究不断深入,在泛素化修饰组学、泛素化互作组以及蛋白功能等相关方面的研究取得了长足进展,近年来已鉴定到的多种水稻泛素修饰相关蛋白在水稻免疫过程中发挥着重要作用。该文在简要介绍泛素化途径的基础上,对E2泛素结合酶、E3泛素连接酶和去泛素化酶等参与水稻病原相关分子模式触发的免疫(pathogen-associated molecular pattern-triggered immunity,PTI)或效应蛋白触发的免疫(effector-triggered immunity,ETI)的研究进展进行综述,并对该领域亟待解决的科学问题以及未来的发展方向进行探讨,以期为泛素化修饰在植物免疫过程中的深入研究提供参考与借鉴。

Plant Immune Mechanisms:From Reductionistic to Holistic Points of View

After three decades of the amazing progress made on molecular studies of plant-microbe interactions(MPMI),we have begun to ask ourselves"what are the major questions still remaining?"as if the puzzle has only a few pieces missing.Such an exercise has ultimately led to the realization that we still have many more questions than answers.Therefore,it would be an impossible task for us to project a coherent"big picture"of the MPMI field in a single review.Instead,we provide our opinions on where we would like to go in our research as an invitation to the community to join us in this exploration of new MPMI frontiers.

Robust transcriptional indicators of immune cell death revealed by spatiotemporal transcriptome analyses

Recognition of a pathogen by the plant immune system often triggers a form of regulated cell death traditionally known as the hypersensitive response(HR).This type of cell death occurs precisely at the site of pathogen recognition,and it is restricted to a few cells.Extensive research has shed light on how plant immune receptors are mechanistically activated.However,two central key questions remain largely unresolved:how does cell death zonation take place,and what are the mechanisms that underpin this phenomenon?Consequently,bona fide transcriptional indicators of HR are lacking,which prevents deeper insight into its mechanisms before cell death becomes macroscopic and precludes early or live observation.In this study,to identify the transcriptional indicators of HR we used the paradigmatic Arabidopsis thaliana–Pseudomonas syringae pathosystem and performed a spatiotemporally resolved gene expression analysis that compared infected cells that will undergo HR upon pathogen recognition with bystander cells that will stay alive and activate immunity.Our data revealed unique and time-dependent differences in the repertoire of differentially expressed genes,expression profiles,and biological processes derived from tissue undergoing HR and that of its surroundings.Furthermore,we generated a pipeline based on concatenated pairwise comparisons between time,zone,and treatment that enabled us to define 13 robust transcriptional HR markers.Among these genes,the promoter of an uncharacterized AAA-ATPase was used to obtain a fluorescent reporter transgenic line that displays a strong spatiotemporally resolved signal specifically in cells that will later undergo pathogen-triggered cell death.This valuable set of genes can be used to define cells that are destined to die upon infection with HR-triggering bacteria,opening new avenues for specific and/or high-throughput techniques to study HR processes at a single-cell level.

Research on ADR1s helps understanding the plant immune network

Plant innate immunity begins with the recognition of pathogens by plasma membrane localized pattern-recognition receptors(PRRs)and intracellular nucleotide-binding domain leucine-rich repeat containing receptors(NLRs),which lead to pattern-triggered immunity(PTI)and effector-triggered immunity(ETI),respectively.For a long time,PTI and ETI have been regarded as two independent processes although they share multiple components and signal outputs.Increasing evidence shows an intimate link between PTI and ETI.PTI and ETI mutually potentiate each other,and this is essential for robust disease resistance during pathogen infection.An ancient class of NLRs called RNLs,so named because they carry a Resistance to Powdery Mildew 8(RPW8)-like coiled-coil(CC)domain in the N terminus,has emerged as a key node connecting PTI and ETI.RNLs not only act as helper NLRs that signal downstream of sensor NLRs,they also directly mediate PTI signaling by associating with PRR complexes.Here,we focus on Activated Disease Resistance 1(ADR1),a subclass of RNLs,and discuss its role and mechanism in plant immunity.

两类免疫受体强强联手筑牢植物免疫防线

植物先天免疫系统在抵御病原菌入侵过程中发挥至关重要的作用,主要包括两个层次,即病原菌相关分子模式和效应因子分别触发的PTI和ETI免疫反应。PTI和ETI分别由植物细胞膜表面模式识别受体(PRRs)和胞内免疫受体(NLRs)激活,具有特异的激活机制,但是两者激活的下游免疫事件相互重叠。PTI和ETI是否为泾渭分明的两道防线,以及ETI与PTI下游事件为何如此相似,一直是植物免疫领域最受关注的问题之一。最近,中国科学院分子植物科学卓越创新中心辛秀芳团队与合作者利用拟南芥(Arabidopsis thaliana)与丁香假单胞杆菌(Pseudomonas syringae)互作系统对PTI和ETI在机制上的联系进行了研究。他们发现PRRs和共受体参与ETI,而活性氧的产生是联系PRRs和NLRs所介导的免疫早期信号事件。他们还发现NLRs信号能够迅速增强PTI关键因子的转录和蛋白水平,PTI的增强在ETI免疫反应中不可或缺。该研究从机制上解析了植物免疫领域中长期悬而未决的PTI与ETI相似性之谜,是该领域的一项突破性进展,为未来作物分子设计育种提供了新的启示。

小麦条锈菌细胞质效应子Hasp8抑制植物基础免疫

为明确小麦条锈菌Puccinia striiformis f. sp. tritici效应子Hasp8的功能,以小麦条锈菌CYR31侵染水源11小麦叶片的cDNA为模板,通过PCR技术扩增获得Hasp8基因的完整编码区序列,采用qRTPCR技术测定Hasp8基因的表达情况,利用农杆菌介导马铃薯X病毒(potato virus X,PVX)瞬时表达体系在烟草上验证Hasp8基因是否能够抑制由Bax蛋白诱导的细胞坏死,将Hasp8基因构建到pCAMBIA1302载体用于在烟草上瞬时表达Hasp8进行亚细胞定位以明确该效应蛋白的作用空间;并将Hasp8基因构建到p EDV6载体上用于荧光假单胞菌Pseudomonas fluorescens介导的瞬时表达系统以明确该效应蛋白是否能够抑制病原相关分子模式引发的免疫反应和效应蛋白诱导的免疫反应。结果表明,Hasp8基因编码117个氨基酸,N端含22 aa的信号肽;qRT-PCR结果显示,Hasp8基因在小麦条锈菌CYR31侵染小麦水源11的早期上调表达。烟草瞬时表达Hasp8基因能够抑制由Bax诱导的细胞坏死;烟草上亚细胞定位发现效应子Hasp8定位于细胞质和细胞核;荧光假单胞菌介导的抑制植物免疫试验结果显示效应子Hasp8能够抑制由荧光假单胞菌诱导的胼胝质积累,并且抑制由小麦条锈菌无毒性生理小种CYR23引起的过敏性坏死和活性氧积累,促进病菌的侵染,菌丝面积和菌丝长度增加。