Cell polarization,a crucial process in fungal defence

Plant cells responding to fungal attack undergo large morphological alterations, along with rapid and extensive metabolic reprogramming. Cytological analysis of single infected plant cells revealed a large complexity of interdependent, rapid and dynamic changes of a multitude of cellular components. Among these changes are major rearrangements of the cytoskeleton, translocation of cytoplasm and of the cell nucleus to the fungal penetration site, and local apposition of barrier material around this site, which results in massive cell-wall reinforcement. If this first line of defence is overcome by the pathogen, in many cases, it is followed by hypersensitive plant cell death, which stops growth of the penetrating fungus and finally leads to its death. The speed and magnitude of the initial defence response appear to be crucial to plant disease resistance.     

The reactive oxygen species network pathways: an essential prerequisite for perception of pathogen attack and the acquired disease resistance in plants

Availability of complete Arabidopsis(Arabidopsis thaliana) and rice(Oryza sativa) genome sequences, together with molecular recourses of functional genomics and proteomics have revolutionized our understanding of reactive oxygen species (ROS) signalling network mediating disease resistance in plants. So far, ROS have been associated with aging, cellular and molecular alteration in animal and plant cells. Recently, concluding evidences suggest that ROS network is essential to induce disease resistance and even to mediate resistance to multiple stresses in plants. ROS are obligatory by-products emerging as a result of normal metabolic reactions. They have the potential to be both beneficial and harmful to cellular metabolism. Their dual effects on metabolic reactions are dosage specific. In this review we focus our attention on cellular ROS level to trigger beneficial effects on plant cells responding to pathogen attack. By exploring the research related contributions coupled with data of targeted gene disruption, and RNA interference approaches, we show here that ROS are ubiquitous molecules of redox-pathways that play a crucial role in plant defence mechanism. The molecular prerequisites of ROS network to activate plant defence system in response to pathogen infections are here underlined. Bioinformatic tools are now available to scientists for high throughput analysis of cellular metabolisms. These tools are used to illustrate crucial ROS-related genes that are involved in the defence mechanism of plants. The review describes also the emerging findings of ROS network pathways to modulate multiple stress resistance in plants.     

Interactions between tobacco mosaic virus and the tobacco N gene

The interaction between tobacco mosaic virus (TMV) and tobacco harbouring the N gene is a classical system for studying gene-for-gene interactions in disease resistance. The N gene confers resistance to TMV by mediating defence responses that function to limit viral replication and movement. We isolated the N gene and determined that N belongs to the nucleotide-binding-site-leucine-rich-repeat (NBS-LRR) class of plant disease resistance genes, and encodes both full-length and truncated proteins. Sequence homologies and mutagenesis studies indicated a signalling role for the N protein similar to that seen for proteins involved in defence responses in insects and mammals. The N gene confers resistance to TMV in transgenic tomato, demonstrating the use of the NBS-LRR class of disease resistance genes in engineering crop resistance. From the pathogen side of this interaction, the TMV 126 kDa replicase protein has been implicated as the avirulence factor that triggers N-mediated defence responses. We employed Agrobacterium-mediated expression strategies to demonstrate that expression of the putative helicase region of the replicase protein is sufficient to elicit N-mediated defences. The thermosensitivity of the N-mediated response to TMV is retained when induced by expression of this replicase fragment. Thus, both components of this gene-for-gene interaction are now available for studies that address the molecular mechanisms involved in N-mediated TMV resistance.     

AtCPK1 calcium‐dependent protein kinase mediates pathogen resistance in Arabidopsis

In mammals, lipid bodies play a key role during pathological and infectious diseases. However, our knowledge on the function of plant lipid bodies, apart from their role as the major site of lipid storage in seed tissues, remains limited. Here, we provide evidence that a calcium‐dependent protein kinase (CPK) mediates pathogen resistance in Arabidopsis. AtCPK1 expression is rapidly induced by fungal elicitors. Loss‐of‐function mutants of AtCPK1 exhibit higher susceptibility to pathogen infection compared to wild‐type plants. Conversely, over‐expression of AtCPK1 leads to accumulation of salicylic acid (SA) and constitutive expression of SA‐regulated defence and disease resistance genes, which, in turn, results in broad‐spectrum protection against pathogen infection. Expression studies in mutants affected in SA‐mediated defence responses revealed an interlocked feedback loop governing AtCPK1 expression and components of the SA‐dependent signalling pathway. Moreover, we demonstrate the dual localization of AtCPK1 in lipid bodies and peroxisomes. Overall, our findings identify AtCPK1 as a component of the innate immune system of Arabidopsis plants.     

Role of extracytoplasmic leucine rich repeat proteins in plant defence mechanisms

Plant-pathogen interactions involve highly complex series of reactions in disease development. Plants are endowed with both, resistance and defence genes. The activation of defence genes after contact with avirulence gene products of pathogens depends on signals transduced by leucine-rich repeats (LRRs) contained in resistance genes. Additionally, LRRs play roles for various actions following ligand recognition. Polygalacturonase inhibiting proteins (PGIPs), the only plant LRR protein with known ligands, are pectinase inhibitors, bound by ionic interactions to the extracellular matrix (ECM) of plant cells. They have a high affinity for fungal endopolygalacturonases (endoPGs). PGIP genes are organised in families encoding proteins with similar physical characteristics but different specificities. They are induced by infection and stress related signals. The molecular basis of PG-PGIP interaction serves as a model to understand the evolution of plant LRR proteins in recognising non-self-molecules. Extensins form a different class of structural proteins with repetitive sequences. They are also regulated by wounding and pathogen infection. Linkage of extensins with LRR motifs is highly significant in defending host tissues against pathogen invasion. Overexpression of PGIPs or expression of several PGIPs in a plant tissue, and perhaps manipulation of extensin expression could be possible strategies for disease management.     

植物病毒载体在植病互作研究中的应用

在植物与病原菌互作的研究中,植物抗性基因和病原菌无毒基因的研究是两个重要的热点。利用植物病毒沉默载体构建的VIGS(Virus Induced Gene Silencing)体系研究植物的防御机制;利用植物病毒表达载体克隆和研究病原菌的无毒基因,将使我们更深刻地理解植物和病原菌互作的分子机理,最终为培育番茄白粉病持久抗性品种打下理论基础。对植物病毒载体的研究进行了综述并就我们承担的课题进行了讨论。     

Molecular mechanisms involved in bacterial speck disease resistance of tomato

An important recent advance in the field of plant-microbe interactions has been the cloning of genes that confer resistance to specific viruses, bacteria, fungi or nematodes. Disease resistance (R) genes encode proteins with predicted structural motifs consistent with them having roles in signal recognition and transduction. The future challenge is to understand how R gene products specifically perceive defence-eliciting signals from the pathogen and transduce those signals to pathways that lead to the activation of plant defence responses. In tomatoes, the Pto kinase (product of the Pto R gene) confers resistance to strains of the bacterial speck pathogen, Pseudomonas syringae pv. tomato, that carry the corresponding avirulence gene avrPto. Resistance to bacterial speck disease is initiated by a mechanism involving the physical interaction of the Pto kinase and the AvrPto protein. This recognition event initiates signalling events that lead to defence responses including an oxidative burst, the hypersensitive response and expression of pathogenesis-related genes. Pto-interacting (Pti) proteins have been identified that appear to act downstream of the Pto kinase and our current studies are directed at elucidating the roles of these components.     

Screening for resistance against Pseudomonas syringae in rice-FOX Arabidopsis lines identified a putative receptor-like cytoplasmic kinase gene that confers resistance to major bacterial and fungal pathogens in Arabidopsis and rice

Approximately 20,000 of the rice-FOX Arabidopsis transgenic lines, which overexpress 13,000 rice full-length cDNAs at random in Arabidopsis, were screened for bacterial disease resistance by dip inoculation with Pseudomonas syringae pv. tomato DC3000 (Pst DC3000). The identities of the overexpressed genes were determined in 72 lines that showed consistent resistance after three independent screens. Pst DC3000 resistance was verified for 19 genes by characterizing other independent Arabidopsis lines for the same genes in the original rice-FOX hunting population or obtained by reintroducing the genes into ecotype Columbia by floral dip transformation. Thirteen lines of these 72 selections were also resistant to the fungal pathogen Colletotrichum higginsianum. Eight genes that conferred resistance to Pst DC3000 in Arabidopsis have been introduced into rice for overexpression, and transformants were evaluated for resistance to the rice bacterial pathogen, Xanthomonas oryzae pv. oryzae. One of the transgenic rice lines was highly resistant to Xanthomonas oryzae pv. oryzae. Interestingly, this line also showed remarkably high resistance to Magnaporthe grisea, the fungal pathogen causing rice blast, which is the most devastating rice disease in many countries. The causal rice gene, encoding a putative receptor-like cytoplasmic kinase, was therefore designated as BROAD-SPECTRUM RESISTANCE 1. Our results demonstrate the utility of the rice-FOX Arabidopsis lines as a tool for the identification of genes involved in plant defence and suggest the presence of a defence mechanism common between monocots and dicots.     

Putting knowledge of plant disease resistance genes to work.

Plant disease resistance genes trigger defence mechanisms upon recognition of pathogen compatibility factors, which are encoded by avirulence genes. Isolation of the barley powdery mildew resistance gene Mla opens the door to understanding the extensive allelic diversity of this locus. Completion of the Arabidopsis genome sequence enables the analysis of the complete set of R-gene homologues in a flowering plant. A new R gene, RPW8, conferring resistance in Arabidopsis to powdery mildew, reveals a new class of protein associated with pathogen recognition. New prospects for using R-gene polymorphism in agriculture are becoming apparent.     

Advances in understanding the mechanism of resistance to anthracnose and induced defence response in tea plants

The tea plant (Camellia sinensis) is susceptible to anthracnose disease that causes considerable crop loss and affects the yield and quality of tea. Multiple Colletotrichum spp. are the causative agents of this disease, which spreads quickly in warm and humid climates. During plant–pathogen interactions, resistant cultivars defend themselves against the hemibiotrophic pathogen by activating defence signalling pathways, whereas the pathogen suppresses plant defences in susceptible varieties. Various fungicides have been used to control this disease on susceptible plants, but these fungicide residues are dangerous to human health and cause fungicide resistance in pathogens. The problem-solving approaches to date are the development of resistant cultivars and ecofriendly biocontrol strategies to achieve sustainable tea cultivation and production. Understanding the infection stages of Colletotrichum, tea plant resistance mechanisms, and induced plant defence against Colletotrichum is essential to support sustainable disease management practices in the field. This review therefore summarizes the current knowledge of the identified causative agent of tea plant anthracnose, the infection strategies and pathogenicity of C. gloeosporioides, anthracnose disease resistance mechanisms, and the caffeine-induced defence response against Colletotrichum infection. The information reported in this review will advance our understanding of host–pathogen interactions and eventually help us to develop new disease control strategies.     

A novel simple extracellular leucine-rich repeat (eLRR) domain protein from rice (OsLRR1) enters the endosomal pathway and interacts with the hypersensitive-induced reaction protein 1 (OsHIR1)

Receptor-like protein kinases (RLKs) containing an extracellular leucine-rich repeat (eLRR) domain, a transmembrane domain and a cytoplasmic kinase domain play important roles in plant disease resistance. Simple eLRR domain proteins structurally resembling the extracellular portion of the RLKs may also participate in signalling transduction and plant defence response. Yet the molecular mechanisms and subcellular localization in regulating plant disease resistance of these simple eLRR domain proteins are still largely unclear. We provided the first experimental evidence to demonstrate the subcellular localization and trafficking of a novel simple eLRR domain protein (OsLRR1) in the endosomal pathway, using both confocal and electron microscopy. Yeast two-hybrid and in vitro pull-down assays show that OsLRR1 interacts with the rice hypersensitive-induced response protein 1 (OsHIR1) which is localized on plasma membrane. The interaction between LRR1 and HIR1 homologs was shown to be highly conserved among different plant species, suggesting a close functional relationship between the two proteins. The function of OsLRR1 in plant defence response was examined by gain-of-function tests using transgenic Arabidopsis thaliana . The protective effects of OsLRR1 against bacterial pathogen infection were shown by the alleviating of disease symptoms, lowering of pathogen titres and higher expression of defence marker genes.     

Transcriptomic and Proteomic Analyses of Resistant Host Responses in Arachis diogoi Challenged with Late Leaf Spot Pathogen,Phaeoisariopsis personata

Late leaf spot is a serious disease of peanut caused by the imperfect fungus, Phaeoisariopsis personata. Wild diploid species, Arachis diogoi. is reported to be highly resistant to this disease and asymptomatic. The objective of this study is to investigate the molecular responses of the wild peanut challenged with the late leaf spot pathogen using cDNA-AFLP and 2D proteomic study. A total of 233 reliable, differentially expressed genes were identified in Arachis diogoi. About one third of the TDFs exhibit no significant similarity with the known sequences in the data bases. Expressed sequence tag data showed that the characterized genes are involved in conferring resistance in the wild peanut to the pathogen challenge. Several genes for proteins involved in cell wall strengthening, hypersensitive cell death and resistance related proteins have been identified. Genes identified for other proteins appear to function in metabolism, signal transduction and defence. Nineteen TDFs based on the homology analysis of genes associated with defence, signal transduction and metabolism were further validated by quantitative real time PCR (qRT-PCR) analyses in resistant wild species in comparison with a susceptible peanut genotype in time course experiments. The proteins corresponding to six TDFs were differentially expressed at protein level also. Differentially expressed TDFs and proteins in wild peanut indicate its defence mechanism upon pathogen challenge and provide initial breakthrough of genes possibly involved in recognition events and early signalling responses to combat the pathogen through subsequent development of resistivity. This is the first attempt to elucidate the molecular basis of the response of the resistant genotype to the late leaf spot pathogen, and its defence mechanism.     

New insights into the regulation of plant immunity by amino acid metabolic pathways

Besides defence pathways regulated by classical stress hormones, distinct amino acid metabolic pathways constitute integral parts of the plant immune system. Mutations in several genes involved in Asp‐derived amino acid biosynthetic pathways can have profound impact on plant resistance to specific pathogen types. For instance, amino acid imbalances associated with homoserine or threonine accumulation elevate plant immunity to oomycete pathogens but not to pathogenic fungi or bacteria. The catabolism of Lys produces the immune signal pipecolic acid (Pip), a cyclic, non‐protein amino acid. Pip amplifies plant defence responses and acts as a critical regulator of plant systemic acquired resistance, defence priming and local resistance to bacterial pathogens. Asp‐derived pyridine nucleotides influence both pre‐ and post‐invasion immunity, and the catabolism of branched chain amino acids appears to affect plant resistance to distinct pathogen classes by modulating crosstalk of salicylic acid‐ and jasmonic acid‐regulated defence pathways. It also emerges that, besides polyamine oxidation and NADPH oxidase, Pro metabolism is involved in the oxidative burst and the hypersensitive response associated with avirulent pathogen recognition. Moreover, the acylation of amino acids can control plant resistance to pathogens and pests by the formation of protective plant metabolites or by the modulation of plant hormone activity.     

Light acclimation,retrograde signalling,cell death and immune defences in plants

This review confronts the classical view of plant immune defence and light acclimation with recently published data. Earlier findings have linked plant immune defences to nucleotide‐binding site leucine‐rich repeat (NBS‐LRR)‐dependent recognition of pathogen effectors and to the role of plasma membrane‐localized NADPH‐dependent oxidoreductase (AtRbohD), reactive oxygen species (ROS) and salicylic acid (SA). However, recent results suggest that plant immune defence also depends on the absorption of excessive light energy and photorespiration. Rapid changes in light intensity and quality often cause the absorption of energy, which is in excess of that required for photosynthesis. Such excessive light energy is considered to be a factor triggering photoinhibition and disturbance in ROS/hormonal homeostasis, which leads to cell death in foliar tissues. We highlight here the tight crosstalk between ROS‐ and SA‐dependent pathways leading to light acclimation, and defence responses leading to pathogen resistance. We also show that LESION SIMULATING DISEASE 1 (LSD1) regulates and integrates these processes. Moreover, we discuss the role of plastid–nucleus signal transduction, photorespiration, photoelectrochemical signalling and ‘light memory’ in the regulation of acclimation and immune defence responses. All of these results suggest that plants have evolved a genetic system that simultaneously regulates systemic acquired resistance (SAR), cell death and systemic acquired acclimation (SAA).     

Molecular genetics of disease resistance in cereals

AIMS: This Botanical Briefing attempts to summarize what is currently known about the molecular bases of disease resistance in cereal species and suggests future research directions. SCOPE: An increasing number of resistance (R) genes have been isolated from rice, maize, wheat and barley that encode both structurally related and unique proteins. This R protein diversity may be attributable to the different modus operandi employed by pathogen species in some cases, but it is also a consequence of multiple defence strategies being employed against phytopathogens. Mutational analysis of barley has identified additional genes required for activation of an R gene-mediated defence response upon pathogen infection. In some instances very closely related barley R proteins require different proteins for defence activation, demonstrating that, within a single plant species, multiple resistance signalling pathways and different resistance strategies have evolved to confer protection against a single pathogen species. Despite the apparent diversity of cereal resistance mechanisms, some of the additional molecules required for R protein function are conserved amongst cereal and dicotyledonous species and even other eukaryotic species. Thus the derivation of functional homologues and interacting partner proteins from other species is contributing to the understanding of resistance signalling in cereals. The potential and limit of utilizing the rice genome sequence for further R gene isolation from cereal species is also considered, as are the new biotechnological possibilities for disease control arising from R gene isolation. CONCLUSIONS: Molecular analyses in cereals have further highlighted the complexity of plant-pathogen co-evolution and have shown that numerous active and passive defence strategies are employed by plants against phytopathogens. Many advances in understanding the molecular basis of disease resistance in cereals have focused on monogenic resistance traits. Future research targets are likely to include less experimentally tractable, durable polygenic resistances and nonhost resistance mechanisms.     

植物抗病的分子生物学基础

随着分子生物学的不断发展,人们已逐步了解植物寄主与病原之间的相互作用及植物抗病的分子机理。植物受病原侵染后出现两种类型的卫反应：局部防卫反应（过敏反应）和系统获得性防卫反应。本质素、植保素、活性氧、水杨酸等物质已被证明了在植物抗病中起了重要作用。抗病基因和防卫基因的诱导表达构成了防卫反应的遗传基础。本文综述了近年来抗病的分子生物学研究进展,并对其发展和应用前景作了展望。     

Ethylene perception via ETR1 is required in Arabidopsis infection by Verticillium dahliae

Vascular wilts caused by Verticillium spp. are very difficult to control and, as a result, are the cause of severe yield losses in a wide range of economically important crops. The responses of Arabidopsis thaliana mutant plants impaired in known pathogen response pathways were used to explore the components in defence against Verticillium dahliae . Analysis of the mutant responses revealed enhanced resistance in etr1-1 [ethylene (ET) receptor mutant] plants, but not in salicylic acid-, jasmonic acid- or other ET-deficient mutants, indicating a crucial role of ETR1 in defence against this pathogen. Quantitative polymerase chain reaction analysis revealed that the decrease in symptom severity shown in etr1-1 plants was associated with significant reductions in the growth of the pathogen in the vascular tissues of the plants, suggesting that impaired perception of ET via ETR1 results in increased disease resistance. Furthermore, the activation and increased accumulation of the PR-1 , PR-2 , PR-5 , GSTF12 , GSTU16 , CHI-1 , CHI-2 and Myb75 genes, observed in etr1-1 plants after V. dahliae inoculation, indicate that the outcome of the induced defence response of etr1-1 plants seems to be dependent on a set of defence genes activated on pathogen attack.