Possible role of phytocassane,rice phytoalexin,in disease resistance of rice against the blast fungus Magnaporthe grisea

In addition to momilactone, phytocassanes A through E (diterpene phytoalexins) were detected in rice leaves in fields suffering from rice blast. Furthermore, phytocassane accumulation was most abundant at the edges of necrotic lesions, indicating that the phytoalexins prevent subsequent spread of the fungus from the infected site. In pot experiments the pattern of phytocassane accumulation in rice leaves in an incompatible interaction (infection with an avirulent race of Magnaporthe grisea) was more rapidly induced than in a compatible interaction (infection with a virulent race of M. grisea).     

Inheritance of dsRNAs in the rice blast fungus, Magnaporthe grisea

The 1.6 and 1.8 kbp dsRNAs have been found in the rice blast fungus, Magnaporthe grisea strain MG01. These dsRNA molecules are located in cytoplasm of the fungal cells and maintained stably during vegetative growth. Three crosses between dsRNA free and dsRNA containing strains including a parental cross, sib-mating and back cross were made to follow the inheritance of dsRNAs during sexual reproduction. Approximately 10% of ascospore progenies (11 out of 105) contained dsRNAs from all three crosses. These data indicate that dsRNAs of M. grisea are inherited at a low frequency and not in a Mendelian fashion.     

Characterization of infected process and primary mechanism in rice Acuce defense against rice blast fungus,Magnaporthe oryzae

Plant Molecular Biology - Identification of infection process and defense response during M. oryzae infecting Acuce. Magnaporthe oryzae is a destructive rice pathogen. Recent studies have focused...     

A hybrid machine model of rice blast fungus,Magnaporthe grisea

The fungus, Magnaporthe grisea (Rice blast fungus) is a major agricultural problem affecting rice and related food crops. The way that the fungus invades the host plant and propagates itself is a very important scientific problem and recent advances in research into the genetic basis of these processes can be used to build a simple partial model using hybrid computational modelling techniques. The possible potential benefits of doing this include the use of computer simulation and automated analysis through techniques such as model checking to understand the complex behaviour of such systems. The example is a metaphor for the process of trying to integrate and understand much of the vast amounts of genomic and other data that is being produced in current molecular biology research.     

水稻受稻瘟菌侵染后发病初期的细胞学反应

采用致病性不同的3个稻瘟菌株接种水稻IR64品种。结果显示：在抗性反应中,病菌接种后水稻细胞中形成原生质颗粒,逐渐向病菌侵入部位聚集;原生质浓缩及沉积的形成、细胞坏死与病菌侵染菌丝的受抑制在时间和空间上是一致的。在中度抗性反应中,原生质颗粒化的时间延迟。在感病反应中,尚未观察到水稻细胞质浓缩现象。病菌侵染后寄主细胞在蓝光激发下的自发荧光表明,在寄主细胞中有多酚类物质的产生和积累。抗性反应中稻瘟菌接种后多酚类物质产生早,荧光范围大而强;中度抗性反应中,多酚类物质产生迟,荧光范围小而弱;而在感病反应中,难以观察到寄主细胞的自发荧光。胼胝质、磷脂酶Dγ(phospholipase Dγ,PLDγ)产生和积累的趋势与多酚类积累的情况大致是相似的。不同互作类型寄主细胞中多酚类物质、胼胝质和PLDγ产生和积累的差异表明,这些物质在水稻的抗病性中起了重要作用。     

A novel elicitor identified from Magnaporthe oryzae triggers defense responses in tobacco and rice

Key message

Our studies indicate a potential important elicitor candidate which can aid in the fight against a worldwide disease, rice blast.

Abstract

In this study, we report the purification, identification, characterization, and gene cloning of a novel hypersensitive response-inducing protein elicitor (MoHrip2) secreted from an important pathogenic fungus, Magnaporthe oryzae. The protein fraction was isolated from the culture filtrate of M. oryzae and identified by de novo sequencing. The elicitor-encoding gene mohrip2 was cloned following sequence comparison and PCR amplification. This 459-bp gene encodes a 152-residue polypeptide that contains an 18-residue signal peptide and exhibits a pI of 4.72 and an apparent molecular mass of 16 kDa. The hypothetical protein, MoHrip2, was expressed in Escherichia coli, and both the recombinant and the endogenous protein caused necrotic lesions in tobacco leaves. In addition to phenolic compound deposition and alkalization of the extracellular medium, MoHrip2 also induced hydrogen peroxide production and nitric oxide accumulation in tobacco cells. Moreover, rice seedlings treated with MoHrip2 exhibited pronounced resistance to M. oryzae compared with control seedlings.     

NAC转录因子在植物抗病和抗非生物胁迫反应中的作用

NAC转录因子是植物特有的一类转录因子,其共同特点是在N端含有一段高度保守、由约150个氨基酸组成的NAC结构域,而C端为高度变异的转录调控区。研究表明,NAC转录因子不仅参与植物生长发育的调控,而且在植物抗逆反应中具有重要的调控作用。文章着重介绍NAC转录因子在植物抗逆反应中的作用及其调控机制,并简要讨论NAC转录因子生物学功能的研究方向。     

Ascherxanthone B from Aschersonia luteola, a new antifungal compound active against rice blast pathogen Magnaporthe grisea

Aims:  Fungicide resistance now exists in the rice blast fungus, Magnaporthe grisea , necessitating the need for new active agents. Fungi isolated from habitats in Thailand were screened with reference to this problem.
Methods and Results:  A new, reliable in vitro screening system based on a microdilution plate format was set up using a virulent strain of M. grisea THL 16. Culture broth extracts from approximately 800 fungal strains were investigated, one of these, Aschersonia luteola BCC 8774, was found to produce an active fungicidal compound, ascherxanthone B, with an IC₉₀ value of 0·58 μg ml⁻¹ (0·95 μmol l⁻¹). An in vivo study of anti-blast efficacy of ascherxanthone B showed a positive effect in disease reduction.
Conclusions:  Previous report has shown that a species of Aschersonia produces ascherxanthone A. Research on the species, A. luteola BCC 8774, led to the discovery of related novel metabolite, ascherxanthone B with fungicidal properties.
Significance and Impact of the Study:  Current methods of rice blast control seem to fail leading to increase in crop losses. Our discovery of the anti-blast activity shown by ascherxanthone B is the first step in the development of a potentially novel fungicide.     

OsCPK20 positively regulates Arabidopsis resistance against Pseudomonas syringae pv. tomato and rice resistance against Magnaporthe grisea

Calcium-dependent protein kinases are important decoders of calcium signals in plants, which are involved in plant immunity. We report isolation and functional characterization of a pathogen-responsive OsCPK20 gene in rice. The expression of OsCPK20 in rice was significantly induced following treatment with a Magnaporthe grisea elicitor. Overexpression of constitutively active OsCPK20 in Arabidopsis enhanced the resistance to infection with Pseudomonas syringae pv. tomato, associated with elevated expression of both SA- and JA-related defense genes. Similarly, transgenic rice plants containing constitutively active OsCPK20 exhibited enhanced resistance to blast fungus M. grisea. The enhanced resistance in the transgenic Arabidopsis and rice was associated with activated expression of both SA- and JA-related defense genes. We also found that OsCPK20 was significantly induced by drought stress, indicating that OsCPK20 might be involved in plant response to drought stress. Taken together, our results indicate that rice OsCPK20 positively regulates Arabidopsis resistance against Pseudomonas syringae pv. tomato and rice resistance against M. grisea, and that it may enhance disease resistance by activating both SA- and JA-dependent defense responses.     

Actin in penetration pegs of the fungal rice blast pathogen,Magnaporthe grisea

Summary The penetration peg is the structure used byMagnaporthe grisea to pierce the surface of rice leaves or very hard nonbiodegradable substrates. Penetration pegs produced by appressoria in vitro were examined by electron microscopy and immunofluorescence microscopy using various fluorophore labeled anti-actins. Freeze-substitution preparation of appressoria at early stages of substrate penetration showed that peg cytoplasm consisted primarily of a zone of exclusion, excluding even ribosomes, and was continuous with a similar region in the appressorium. Apical vesicles were, however, observed in short, presumably elongating pegs. Immunofluorescence microscopy was used to demonstrate binding of a monoclonal anti-actin to penetration peg cytoplasm, following permeabilization of appressoria by means of a brief sonication. Occasional filaments and ca. 300 nm diameter plaques were labeled in appressorial cytoplasm. Western blot analysis of germ tube extracts showed that the monoclonal probe bound predominantly to a single band with a molecular weight similar to that of rabbit muscle actin. Preincubation of the antibody with actin virtually eliminated peg labeling. We conclude that the penetration peg contains actin which may play a role in the formation of the zone of exclusion.Abbreviations PE polyethylene - Tris tris(hydroxymethyl)-amino-methane - TBS Tris-buffered saline - TBS-B Tris buffered saline plus 3% bovine serum albumin     

Ferricrocin synthesis in Magnaporthe grisea and its role in pathogenicity in rice

Iron is an essential element for the growth of nearly all organisms. In order to overcome the problem of its low bioavailability, microorganisms (including fungi) secrete siderophores, high-affinity iron chelators. As the acquisition of iron is also a key step in infection processes, siderophores have been considered as potential virulence factors in several host–pathogen interactions. Most fungi produce siderophores of the hydroxamate-type, which are synthesized by non-ribosomal peptide synthetases (NRPSs). Magnaporthe grisea , the causal agent of rice blast disease, produces ferricrocin as intracellular storage siderophore and excretes coprogens. In the M. grisea genome we identified SSM1 , an NRPS gene, and a gene encoding an l -ornithine N5-monooxygenase ( OMO1 ) that is clustered with SSM1 and responsible for catalysing the first step in siderophore biosynthesis, the N⁵ hydroxylation of ornithine. Disruption of SSM1 confirmed that the gene encodes ferricrocin synthetase. Pathogenicity of these mutants towards rice was reduced, suggesting a role of this siderophore in pathogenicity of M. grisea .     

Microsatellite markers for population studies of the rice blast fungus,Magnaporthe grisea

We developed nine new microsatellite markers for rice blast (Magnaporthe grisea) population studies. These markers were used in addition to nine microsatellite markers previously developed by our group for mapping purpose. Altogether, the 18 markers were used in multiplex PCR (polymerase chain reaction) to characterize six populations from different geographical origins. The average number of alleles per locus across populations ranged from 1.2 to 7 and the total number of alleles detected from 2 to 19. Based on this large range of polymorphism, this set of markers is expected to be useful for different kind of population studies at different geographical scales.     

Informativeness of dinucleotide repeat-based primers in fungal pathogen of rice Magnaporthe grisea

Microsatellites or simple sequence repeats (SSRs) occur ubiquitously and show complex patterns in length, motif size and sequence. Among SSRs, dinucleotide repeats occur in high abundance in fungi with shorter length as compared to other organisms. In this study, multilocus profiles obtained in Magnaporthe grisea, a model plant pathogen were evaluated. The results showed lower rate of polymorphism by (GT)(n)/(TG)(n) repeat-based primers and suggested occurrence of (GA)(n)/(AG)(n) repeats as integral repeats and (TC)(n)/(CT)(n) and (AC)(n)/(CA)(n) as non-integral repeats. Low repeat length variation was found to be correlated with less number of repeat motifs. The study provides an insight into the possibility of molecular coevolution of mobile elements and dinucleotide repeats in fungi. The study could be applied to other species for wider applications including evolutionary and population genetics.     

Proteomic analysis of pathogen-responsive proteins from rice leaves induced by rice blast fungus, Magnaporthe grisea

Proteomic approaches using two-dimensional gel electrophoresis (2-DE) were adopted to identify proteins from rice leaf that are differentially expressed in response to the rice blast fungus, Magnaporthe grisea. Microscopic observation of inoculated leaf with M. grisea revealed that callose deposition and hypersensitive response was clearly visible in incompatible interactions but excessive invading hypha with branches were evident in compatible interactions. Proteins were extracted from leaves 24, 48, and 72 hours after rice blast fungus inoculation. Eight proteins resolved on the 2-DE gels were induced or increased in the inoculated leaf. Matrix-assisted laser desorption/ionization-time of flight analysis of these differentially displayed proteins showed them to be two receptor-like protein kinases (RLK), two beta-1.3-glucanases (Glu1, Glu2), thaumatin-like protein (TLP), peroxidase (POX 22.3), probenazole-inducible protein (PBZ1), and rice pathogenesis-related 10 (OsPR-10). Of these proteins, RLK, TLP, PBZ, and OsPR-10 proteins were induced more in the incompatible interactions than in compatible ones. A phytohormone, jasmonic acid also induced all eight proteins in leaves. To confirm whether the expression profile is equal to the 2-DE data, seven cDNA clones were used as probes in Northern hybridization experiments using total RNA from leaf tissues inoculated with incompatible and compatible rice blast fungal races. The genes encoding POX22.3, Glu1, Glu2, TLP, OsRLK, PBZ1, and OsPR-10 were activated in inoculated leaves, with TLP, OsRLK, PBZ1, and OsPR-10 being expressed earlier and more in incompatible than in compatible interactions. These results suggest that early and high induction of these genes may provide host plants with leading edges to defend themselves. The localization of two rice PR-10 proteins, PBZ1 and OsPR-10, was further examined by immunohistochemical analysis. PBZ1 accumulated highly in mesophyll cells under the attachment site of the appressorium. In contrast, OsPR-10 expression was mainly localized to vascular tissue.     

Detection of Magnaporthe grisea in infested rice seeds using polymerase chain reaction

AIM: To develop a diagnostic assay based on polymerase chain reaction for the detection of Magnaporthe grisea from infested rice seeds. METHODS AND RESULTS: Primers were designed based on the nucleotide sequence of the mif 23, an infection-specific gene of M. grisea. The primers amplified target DNA from genetically and geographically diverse isolates of the pathogen. The lowest concentration of template DNA that led to amplification was 20 rhog. No PCR product was detected when DNA from other fungi was used, indicating the specificity of the primers. With this PCR based seed assay, M. grisea was detected in rice seedlots with infestation rates as low as 0.2%. CONCLUSION: The PCR detection of M. grisea is simple, rapid, specific, sensitive and suitable for the routine detection of the pathogen in infested seeds. SIGNIFICANCE AND IMPACT OF THE STUDY: Introduction of the blast fungus into new areas where it has not been previously recorded could be avoided by the detection of infested seedlots. A PCR-based seed assay could facilitate risk assessment of naturally infested rice seeds; help design management programs and optimize fungicide use.     

In vitro antifungal activity of antifungalmycin 702, a new polyene macrolide antibiotic, against the rice blast fungus Magnaporthe grisea

Antifungalmycin 702, a novel polyene macrolide antibiotic produced by Streptomyces padanus JAU4234, strongly inhibited mycelial growth of the rice blast fungus, Magnaporthe grisea, with EC₅₀ of 37 μg/ml and EC₉₀ of 136 μg/ml. Significant reduction in the number of conidia was observed at above 20 μg/ml. Conidia germination and appressorium formation were also suppressed and were not viable with >40 μg/ml. When treated with antifungalmycin 702, hyphae morphology became irregular. Based on microscopic examination, antifungalmycin 702 may exert its antifungal activity by changing the structure of cell membranes and the cytoskeleton and interacting with the organelles. Antifungalmycin 702 thus has potential as a new fungicide in the treatment of rice blast disease.