The Role of TIR-NBS and TIR-X Proteins in Plant Basal Defense Responses

Toll/interleukin receptor (TIR) domain-containing proteins encoded in the Arabidopsis (Arabidopsis thaliana) genome include the TIR-nucleotide binding site (TN) and TIR-unknown site/domain (TX) families. We investigated the function of these proteins. Transient overexpression of five TX and TN genes in tobacco (Nicotiana benthamiana) induced chlorosis. This induced chlorosis was dependent on ENHANCED DISEASE RESISTANCE1, a dependency conserved in both tobacco and Arabidopsis. Stable overexpression transgenic lines of TX and TN genes in Arabidopsis produced a variety of phenotypes associated with basal innate immune responses; these were correlated with elevated levels of salicylic acid. The TN protein AtTN10 interacted with the chloroplastic protein phosphoglycerate dehydrogenase in a yeast (Saccharomyces cerevisiae) two-hybrid screen; other TX and TN proteins interacted with nucleotide binding-leucine-rich repeat proteins and effector proteins, suggesting that TN proteins might act in guard complexes monitoring pathogen effectors.Innate immunity is a primary defense mechanism in plants that functions to protect against a variety of biotic stresses (Eitas and Dangl, 2010). The plant basal immune system comprises pattern or pathogen recognition receptors that can recognize a variety of plant pathogens by identifying specific pathogen-associated molecular patterns (PAMPs; Tsuda and Katagiri, 2010). This recognition of PAMPs by plant pattern recognition receptors triggers PAMP-triggered immunity or plant basal immunity (Jones and Dangl, 2006; Zipfel, 2008). Well-known PAMPs or microbe-associated molecular patterns recognized by plants include bacterial flagellin, cold shock proteins, and elongation factor Tu. To suppress PAMP-triggered immunity, plant pathogens secrete an array of virulence factors such as type III effector proteins, while plant resistance (R) proteins function to recognize the effector molecules (Römer et al., 2009; Lewis et al., 2010; Tsuda and Katagiri, 2010; Zhang et al., 2012). Specific recognition of a pathogen effector by a plant R protein triggers a second type of immune response called effector-triggered immunity, resulting in an incompatible reaction (Qi et al., 2011; Sohn et al., 2012; Tahir et al., 2012).The most commonly known plant R proteins are the nucleotide-binding (NB) site Leucine-rich repeat (LRR) proteins that plants use to detect effector proteins. The NB is often called NB-ARC because of sequence similarities to the human apoptotic protease-activating factor APAF1 and Caenorhabditis elegans homolog CELL DEATH PROTEIN4 (Lukasik and Takken, 2009). Plant NB-LRR proteins often also have, at the N terminus, a Toll/Interleukin-1 receptor (TIR) or coiled coil (CC) domain (Meyers et al., 2003). In animal TIR proteins, this domain is more commonly located at the C terminus and is linked by a transmembrane domain to an N-terminal LRR domain (Torto et al., 2002). In Drosophila spp. and other microbes, a TIR domain has been shown to play an important role in the activation of antifungal immune responses (Jenkins and Mansell, 2010). Toll-like receptors (TLRs) perform an integral role in the activation of antimicrobial responses in many animals (Radhakrishnan and Splitter, 2010).In plants, two additional TIR-containing protein families, TIR-NB site (TN) and TIR-unknown/random (TX), were identified, which are distinct from the longer TIR-NB-LRR (TNL) R protein homologs (Meyers et al., 2002). TN proteins contain TIR and NBS domains but lack LRRs, while TX proteins lack both NBS and LRR domains, yet often have a small and variable C-terminal domain (Meyers et al., 2002). In the Arabidopsis (Arabidopsis thaliana) ecotype Columbia (Col-0) genome, there are 30 TX genes and 21 TN genes (Meyers et al., 2003). The crystal structure of a TIR domain from an Arabidopsis TN protein (At1g72930/{"type":"entrez-protein","attrs":{"text":"NP_177436","term_id":"15218629","term_text":"NP_177436"}}NP_177436) contains a compact globular fold resembling the mammalian (TLR1 and MYELOID DIFFERENTIATION PRIMARY RESPONSE GENE88 [MYD88]) and bacterial TIR domain proteins (Chan et al., 2010). Although plant TIR domains share less than 20% sequence identity with the human TLR domains, the structures of the TIR domain in plants, mammalian TLRs, and bacterial TIR domain proteins have strong similarity (Chan et al., 2010).A high proportion of TX and TN genes were previously reported to be in complex clusters with TNL genes; these clusters were found to be duplicated to multiple locations in the genome (Meyers et al., 2002). The existence of genetically linked pairs or sets of genes such as RESISTANCE TO PERONOSPORA PARASITICA2A (RPP2A)-RPP2B, RESISTANCE TO PSEUDOMONAS SYRINGAE1 (RPS1)-RPS4, LEAF RUST RESISTANCE GENE10 (LR10)-RESISTANCE GENE ANALOGUE2 (RGA2), RICE BLAST RESISTANCE GENE AT PIK LOCUS1 (PIKM-1)-TS-PIKM2-TS, and RICE BLAST RESISTANCE GENE AT PI LOCUS1 (PI5-1)-PI5-2 in the genomes and their role in disease resistance suggests that these linked genes are required to effect a defense response in plants (Eitas and Dangl, 2010). The genomic pairing of the TNL genes with TX or TN genes suggests a role of the tightly linked TN protein in the function of its cognate TNL protein or proteins (and vice versa).The specific direct or indirect interaction between an R gene and a corresponding avirulence (Avr) gene in the characterized pairs of interaction resulted in an immune response in the form of localized programmed cell death, called the hypersensitive response (HR; Burch-Smith et al., 2007; Caplan et al., 2008). The recognition of avirulence proteins from pathogens by the cognate R proteins induces a cascade of changes that increases the levels of salicylic acid (SA), jasmonic acid (JA), phenyl ammonium lyase, and systemin (Liu et al., 2010). The production of several of these biochemical signals is known to trigger multiple convergent ‘R’-gene signaling pathways, leading to programmed cell death and further changes in gene expression patterns (Vlot et al., 2008a, 2008b). Structure function analysis of Arabidopsis R proteins RPS4 (Zhang et al., 2004) and RPP1A (Michael Weaver et al., 2006) have shown that TIR and NBS domains of the proteins without the LRR domain (TNL truncations) could be sufficient to induce HR. Studies using overexpression of plant R genes (particularly the truncated TNL genes) suggest that the TIR and NBS domains by themselves might be sufficient to induce HR and to initiate plant defense responses (Michael Weaver et al., 2006; Swiderski et al., 2009).In this study, we present experimental and computational data that are collectively consistent with a role for Arabidopsis TX and TN proteins in plant defenses. For example, the ability of the TX and TN genes to induce HR responses upon transient expression is dependent on ENHANCED DISEASE RESISTANCE1 (EDS1). This EDS1 dependency in induced HR was demonstrated in both tobacco (Nicotiana benthamiana) and in Arabidopsis. Stable transgenic overexpression in Arabidopsis of TX and TN genes resulted in a variety of phenotypes involved with basal innate immune responses that are dependent on SA. We also demonstrated the interaction of TX and TN proteins with plant pathogenic elicitor proteins and other plant signal transduction proteins.     

Molecular Analysis of Plant Defense Responses to Plant Pathogens

A number of inducible plant responses are believed to contribute to disease resistance. These responses include the hypersensitive reaction, phytoalexin synthesis, and the production of chitinase, glucanase, and hydroxyproline-rich glycoproteins. Because of the coordinate induction of these responses, it has been difficult to determine whether they are functional defense responses, and if they are, how they specifically contribute to disease resistance. Recent developments in molecular biology have provided experimental techniques that will reveal the specific contribution of each response to disease resistance. In this paper, we describe a strategy to determine if the hypersensitive reaction is a functional plant defense mechanism.     

茉莉素的信号转导与植物的防御反应

文章从茉莉素信号途径及其与其他信号途径交叉作用的角度 ,概括了茉莉素在植物防御反应中的作用     

高等植物防卫反应的信号传导

高等植物对病原微生物的防卫反应包括植物细胞对病原菌的识别,胞内信号的转换与传导,防卫反应的开启与SAR抗性的形成等。本文对高等植物防卫反应信号传导的研究进展进行了综述。     

拟南芥转录因子bHLH17负调控茉莉素介导的抗性反应

植物激素茉莉素作为抗性信号调控植物对腐生性病原菌和昆虫的抗性, 作为发育信号调控植物根的生长、雄蕊发育、表皮毛形成和叶片衰老。茉莉素受体COI1识别茉莉素分子, 进而与JAZ蛋白互作并诱导其降解, 继而调控多种茉莉素反应。拟南芥(Arabidopsis thaliana) IIId亚组bHLH转录因子(bHLH3、bHLH13、bHLH14和bHLH17)是JAZ的一类靶蛋白。与野生型相比, IIId亚组bHLH转录因子的单突变体对灰霉菌和甜菜夜蛾的抗性无明显差异, 而四突变体对灰霉菌和甜菜夜蛾的抗性增强。该文通过高表达bHLH17并研究其对灰霉菌和甜菜夜蛾的抗性反应, 结果显示, 被灰霉菌侵染的bHLH17高表达植株较野生型表现出更严重的病症。取食bHLH17高表达植株叶片的甜菜夜蛾幼虫体重大于取食野生型叶片的幼虫体重。bHLH17高表达抑制了茉莉素诱导的抗性相关基因(Thi2.1)和伤害响应基因(VSP2、AOS、JAZ1、JAZ9和JAZ10)的表达。原生质体转化实验显示bHLH17通过其N端行使转录抑制功能。研究结果表明, IIId亚组bHLH转录抑制因子bHLH17高表达会负调控茉莉素介导的对灰霉菌和甜菜夜蛾的抗性。     

拟南芥AMP1负调控植物对高盐胁迫的反应

高盐胁迫严重影响植物的生长发育及农作物产量,因此鉴定盐胁迫响应相关基因至关重要。拟南芥的AMP1编码一个推测的谷氨酸羧肽酶,参与植物的生长发育、光形态建成与种子休眠。研究证明了AMP1的一个新功能,它的缺失提高了缺失突变体amp1的抗高盐胁迫的能力,研究证明amp1突变体的强抗高盐胁迫表型一方面是由于在高盐胁迫下amp1突变体比野生型中积累了更多的甜菜碱和脯氨酸降低了突变体细胞的水势,另一方面高盐胁迫条件下amp1突变体中高盐胁迫响应的下游基因RD29A,以及AHA3的表达量也高于野生型,后者可促进Na+的外排;高盐条件能够对植物造成氧化胁迫,研究发现AMP1的缺失还上调了抗氧化相关基因ZAT10/12的表达量,进而降低了在高盐胁迫条件下amp1突变体内过氧化物的积累水平,减轻对细胞的损伤和生长的抑制,这些都提高了amp1突变体的抗高盐胁迫的能力。以上结果证明在拟南芥中AMP1负调控植物对高盐胁迫的反应过程。     

Plant NB-LRR signaling: upstreams and downstreams

Plant disease resistance proteins commonly belong to the nucleotide binding-leucine rich repeat (NB-LRR) protein family. These specialized immune proteins mediate recognition of diverse pathogen-derived effector proteins and initiate potent defense responses. NB-LRRs exhibit a multidomain architecture and each domain appears to have discrete functions depending on the stage of NB-LRR signaling. Novel proteins that were found to interact with the core HSP90 chaperone complex regulate accumulation and activation of NB-LRR immune receptors. Recent studies have also advanced our understanding of how accessory proteins contribute to NB-LRR activation. The dynamic nature of NB-LRR localization to different subcellular compartments before and after activation suggests that NB-LRRs may activate immune responses in multiple parts of the cell. In this review we highlight recent advances in understanding NB-LRR function.     

Signal Molecules for Plant Defense Responses to Biotic Stress

Plants are capable of recognizing the penetrating pathogens and of responding to their attack by the activation of the defense systems. Signal transduction from the receptor to the cell genome is required for this activation. Recently, signal molecules have been found, which are involved in the signal transduction triggered in response to biotic stress. The data accumulated imply the presence of a complex and well-coordinated signal network in plant cells. This net controls plant defense responses to pathogen attacks.     

The Hypervariable Amino-Terminus of P1 Protease Modulates Potyviral Replication and Host Defense Responses

The replication of many RNA viruses involves the translation of polyproteins, whose processing by endopeptidases is a critical step for the release of functional subunits. P1 is the first protease encoded in plant potyvirus genomes; once activated by an as-yet-unknown host factor, it acts in cis on its own C-terminal end, hydrolyzing the P1-HCPro junction. Earlier research suggests that P1 cooperates with HCPro to inhibit host RNA silencing defenses. Using Plum pox virus as a model, we show that although P1 does not have a major direct role in RNA silencing suppression, it can indeed modulate HCPro function by its self-cleavage activity. To study P1 protease regulation, we used bioinformatic analysis and in vitro activity experiments to map the core C-terminal catalytic domain. We present evidence that the hypervariable region that precedes the protease domain is predicted as intrinsically disordered, and that it behaves as a negative regulator of P1 proteolytic activity in in vitro cleavage assays. In viral infections, removal of the P1 protease antagonistic regulator is associated with greater symptom severity, induction of salicylate-dependent pathogenesis-related proteins, and reduced viral loads. We suggest that fine modulation of a viral protease activity has evolved to keep viral amplification below host-detrimental levels, and thus to maintain higher long-term replicative capacity.     

反式-2-己烯醛在植物防御反应中的作用

反式-2-己烯醛是绿色植物释放的一种小分子挥发性物质,在调节植物生长发育和抵抗各种环境胁迫中发挥重要作用。已有研究表明,反式-2-己烯醛可抑制植物根系生长,具有较高的抑菌和抗虫活性,也可以作为植物间的“信使”来传递防御信号。该文系统综述了反式-2-己烯醛的生物合成、代谢途径及其在生物胁迫防御反应中的重要作用,提出了研究...     

植物防御反应中活性氧的产生和作用

活性氧在植物抗病性中起着重要的作用。本文将对其在防御反应中的产生和作用进行简要的论述。     

IBR5 Modulates Temperature-Dependent,R Protein CHS3-Mediated Defense Responses in Arabidopsis

Plant responses to low temperature are tightly associated with defense responses. We previously characterized the chilling-sensitive mutant chs3-1 resulting from the activation of the Toll and interleukin 1 receptor-nucleotide binding-leucine-rich repeat (TIR-NB-LRR)-type resistance (R) protein harboring a C-terminal LIM (Lin-11, Isl-1 and Mec-3 domains) domain. Here we report the identification of a suppressor of chs3, ibr5-7 (indole-3-butyric acid response 5), which largely suppresses chilling-activated defense responses. IBR5 encodes a putative dual-specificity protein phosphatase. The accumulation of CHS3 protein at chilling temperatures is inhibited by the IBR5 mutation. Moreover, chs3-conferred defense phenotypes were synergistically suppressed by mutations in HSP90 and IBR5. Further analysis showed that IBR5, with holdase activity, physically associates with CHS3, HSP90 and SGT1b (Suppressor of the G2 allele of skp1) to form a complex that protects CHS3. In addition to the positive role of IBR5 in regulating CHS3, IBR5 is also involved in defense responses mediated by R genes, including SNC1 (Suppressor of npr1-1, Constitutive 1), RPS4 (Resistance to P. syringae 4) and RPM1 (Resistance to Pseudomonas syringae pv. maculicola 1). Thus, the results of the present study reveal a role for IBR5 in the regulation of multiple R protein-mediated defense responses.     

Characterization of Peanut Germin-Like Proteins,AhGLPs in Plant Development and Defense

Background

Germin-like superfamily members are ubiquitously expressed in various plant species and play important roles in plant development and defense. Although several GLPs have been identified in peanut (Arachis hypogaea L.), their roles in development and defense remain unknown. In this research, we study the spatiotemporal expression of AhGLPs in peanut and their functions in plant defense.

Results

We have identified three new AhGLP members (AhGLP3b, AhGLP5b and AhGLP7b) that have distinct but very closely related DNA sequences. The spatial and temporal expression profiles revealed that each peanut GLP gene has its distinct expression pattern in various tissues and developmental stages. This suggests that these genes all have their distinct roles in peanut development. Subcellular location analysis demonstrated that AhGLP2 and 5 undergo a protein transport process after synthesis. The expression of all AhGLPs increased in responding to Aspergillus flavus infection, suggesting AhGLPs'' ubiquitous roles in defense to A. flavus. Each AhGLP gene had its unique response to various abiotic stresses (including salt, H₂O₂ stress and wound), biotic stresses (including leaf spot, mosaic and rust) and plant hormone stimulations (including SA and ABA treatments). These results indicate that AhGLPs have their distinct roles in plant defense. Moreover, in vivo study of AhGLP transgenic Arabidopsis showed that both AhGLP2 and 3 had salt tolerance, which made transgenic Arabidopsis grow well under 100 mM NaCl stress.

Conclusions

For the first time, our study analyzes the AhGLP gene expression profiles in peanut and reveals their roles under various stresses. These results provide an insight into the developmental and defensive roles of GLP gene family in peanut.     

Plant NB-LRR immune receptors: from recognition to transcriptional reprogramming

Both plants and animals contain nucleotide-binding domain and leucine-rich repeat (NB-LRR)-type immune receptors that function during defense against pathogens. Unlike animal NB-LRRs that recognize general pathogen or microbe-associated molecular patterns (PAMPs or MAMPs), plant NB-LRR immune receptors have evolved the ability to specifically recognize a wide range of effector proteins from different pathogens. Recent research has revealed that plant NB-LRRs are incredibly adaptive in their ways of pathogen recognition and defense initiation. This review focuses on the remarkable variety of functions, recognition mechanisms, subcellular localizations, and host factors associated with plant NB-LRR immune receptors.     

MAPK和活性氧参与植物抗病防卫反应的信号转导

促分裂原活化蛋白激酶（MAPK）级联途径和活性氧参与调控植物过敏性细胞死亡。本文介绍促分裂原活化蛋白激酶级联途径在植物抗病防卫反应信号转导中的作用研究进展,并对活性氧积累与MAPK之间的关系作了分析。     

Understanding Plant Development and Stress Responses through Integrative Approaches

As the name reflects, integrative plant biology is the core topic of JIPB. In the past few years JIPB has been pursuing the development of this area, to assist the scientific community to bring together all possible research tools to understand plant growth, development and stress responses in micro- and macro-scales. As part of these efforts, JIPB and Yantai University organized the 1st International Symposium on Integrative Plant Biology in the seaside town of Yantai during August 10-12,2009 (Figure 1). The symposium was co-sponsored by Botanical Society of China, Chinese Society for Cell Biology, Genetics Society of China, and Chinese Society for Plant Physiology.