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The hypersensitive response (HR) of plants is one of the earliest responses to prevent pathogen invasion. A brown dot lesion on a leaf is visual evidence of the HR against the blast fungus Magnaporthe oryzae in rice, but tracking the browning process has been difficult. In this study, we induced the HR in rice cultivars harboring the blast resistance gene Pit by inoculation of an incompatible M. oryzae strain, which generated a unique resistance lesion with a brown ring (halo) around the brown fungal penetration site. Inoculation analysis using a plant harboring Pit but lacking an enzyme that catalyzes tryptamine to serotonin showed that high accumulation of the oxidized form of serotonin was the cause of the browning at the halo and penetration site. Our analysis of the halo browning process in the rice leaf revealed that abscisic acid enhanced biosynthesis of serotonin under light conditions, and serotonin changed to the oxidized form via hydrogen peroxide produced by light. The dramatic increase in serotonin, which has a high antioxidant activity, suppressed leaf damage outside the halo, blocked expansion of the browning area and attenuated inhibition of plant growth. These results suggest that serotonin helps to reduce biotic stress in the plant by acting as a scavenger of oxygen radicals to protect uninfected tissues from oxidative damage caused by the HR. The deposition of its oxide at the HR lesion is observed as lesion browning. [ABSTRACT FROM AUTHOR]

The pathogenicity of progeny from crosses among three Chinese isolates of Magnaporthe grisea collected from rice was tested on three Japanese differentials (Ishikarishiroke, Aichiasahi, K 59) having the blast resistance genes Pii, Pia, and Pit, respectively. Monogenic control was demonstrated for avirulence to the differentials. To identify resistance genes corresponding to the avirulence genes, the resistance and susceptibility in F3 lines of the cultivars in response to the parents of the crosses were analyzed genetically. The three avirulence genes identified, designated Avr-Pii, Avr-Pia, and Avr-Pit, appear to correspond to resistance genes Pii, Pia, and Pit, respectively. The monogenic control of avirulence in the fungus and monogenic dominant resistance in rice cultivars supports a gene-for-gene relation in the Pii-, Pia-, or Pit-dependent resistance to the rice blast fungus in rice cultivars. [ABSTRACT FROM AUTHOR]

Discusses the interspecific relationships between the Sogatella furcifera and the Pyricularia frisea via the host plants. Likelihood that the Sogatella furcifera will develop blast lesion caused by the Pyricularia frisea; Incidence of the rice blast disease caused by the Pyricuralia grisea on rice infested previously by male adults or female adults of the white-backed planthopper.

The article discusses the avirulence genes AVR-Pia and AVR-Pii partial mapping in the rice blast fungus Magnaporthe oryzae. The study utilized 31 restriction fragment length polymorphism (RFLP) markers from existing M. oryzae genetic maps in constructing genetic map based on a cross between fertile isolates. The genetic linkage between AVR-Pii or AVR-Pia and RFLP markers is determined, in which gene AVR-Pii was found linked to two markers and AVR-Pia was linked to three markers.

A small and extra chromosome of 1.6 Mb was previously identified in a Pyricularia oryzae strain, 84R-62B. To understand a role of the 1.6 Mb chromosome in the pathogenic changeability of P. oryzae, we performed experiments designed to characterize the 1.6 Mb chromosome in the present study. A gene family encoding secreted protein Pex31s in P. oryzae consists of five homologs, Pex31- A to - E. Among them, Pex31- A and - D are known to be recognized by Pik- m and Pik/Pik- m/ Pik- p, respectively. In the present study, we identified Pex31- A and - D in the genome of 84R-62B. Segregation analyses using an F population between 84R-62B and another rice blast strain, Y93-245c-2, revealed a strong linkage between the two homologs and the 1.6 Mb chromosome of 84R-62B. A CHEF-Southern analysis revealed an association between the 1.6 Mb chromosome and the homologs, indicating that both homologs are located on the 1.6 Mb chromosome of 84R-62B. The loss of the 1.6 Mb chromosome was observed in subcultures of a F progeny, F1-327. These subcultures concomitantly acquired virulence on Pik, Pik- m, and Pik- p. The present study is the first report showing that loss of a small and extra chromosome leads to pathogenic mutation of P. oryzae and may provide a new insight into the mechanisms generating pathogenic variation of this fungus. [ABSTRACT FROM AUTHOR]

The rice cultivar 'Chumroo' is commonly cultivated in the mid- and high-altitude areas of Bhutan. This cultivar has shown durable blast resistance in that area, without evidence of breakdown, for over 20 years. Chumroo was inoculated with 22 blast isolates selected from the race differential standard set of Japan. The cultivar showed resistance to all the isolates. To identify the resistance gene(s), Chumroo was crossed with a susceptible rice cultivar, Koshihikari. The F plants of the cross showed resistance. Segregation analyses of 300 F family lines fitted the segregation ratio of 1:2:1 and indicated that a single dominant gene controls the resistance to a blast isolate Ao 92-06-2 (race 337.1). The Chumroo resistance locus (termed Pi46(t)) was mapped between two SSR markers, RM6748 and RM5473, on the terminal region of the long arm of chromosome 4, using linkage analysis with SSR markers. The nearest marker, RM5473, was linked to the putative resistance locus at a map distance of 3.2 cM. At the chromosomal region, no true resistance genes were identified, whereas two field resistance genes were present. Therefore, we designated Pi46(t) as a novel blast resistance locus. [ABSTRACT FROM AUTHOR]

Magnaporthe oryzae is the causal agent of rice blast disease, a devastating problem worldwide. This fungus has caused breakdown of resistance conferred by newly developed commercial cultivars. To address how the rice blast fungus adapts itself to new resistance genes so quickly, we examined chromosomal locations of AVR-Pita, a subtelomeric gene family corresponding to the Pita resistance gene, in various isolates of M. oryzae (including wheat and millet pathogens) and its related species. We found that AVR-Pita (AVR-Pita1 and AVR-Pita2) is highly variable in its genome location, occurring in chromosomes 1, 3, 4, 5, 6, 7, and supernumerary chromosomes, particularly in rice-infecting isolates. When expressed in M. oryzae, most of the AVR-Pita homologs could elicit Pita-mediated resistance, even those from non-rice isolates. AVR-Pita was flanked by a retrotransposon, which presumably contributed to its multiple translocation across the genome. On the other hand, family member AVR-Pita3, which lacks avirulence activity, was stably located on chromosome 7 in a vast majority of isolates. These results suggest that the diversification in genome location of AVR-Pita in the rice isolates is a consequence of recognition by Pita in rice. We propose a model that the multiple translocation of AVR-Pita may be associated with its frequent loss and recovery mediated by its transfer among individuals in asexual populations. This model implies that the high mobility of AVR-Pita is a key mechanism accounting for the rapid adaptation toward Pita. Dynamic adaptation of some fungal plant pathogens may be achieved by deletion and recovery of avirulence genes using a population as a unit of adaptation. [ABSTRACT FROM AUTHOR]

In order to clone and analyse the avirulence gene AVR-Pia from Japanese field isolates of Magnaporthe oryzae, a mutant of the M. oryzae strain Ina168 was isolated. This mutant, which was named Ina168m95-1, gained virulence towards the rice cultivar Aichi-asahi, which contains the resistance gene Pia. A DNA fragment (named PM01) that was deleted in the mutant and that co-segregated with avirulence towards Aichi-asahi was isolated. Three cosmid clones that included the regions that flanked PM01 were isolated from a genomic DNA library. One of these clones (46F3) complemented the mutant phenotype, which indicated clearly that this clone contained the avirulence gene AVR-Pia. Clone 46F3 contained insertions of transposable elements. The 46F3 insert was divided into fragments I–VI, and these were cloned individually into a hygromycin-resistant vector for the transformation of the mutant Ina168m95-1. An inoculation assay of the transformants revealed that fragment V (3.5 kb) contained AVR-Pia. By deletion analysis of fragment V, AVR-Pia was localized to an 1199-bp DNA fragment, which included a 255-bp open reading frame with weak homology to a bacterial cytochrome- c-like protein. Restriction fragment length polymorphism analysis of this region revealed that this DNA sequence co-segregated with the AVR-Pia locus in a genetic map that was constructed using Chinese isolates. [ABSTRACT FROM AUTHOR]

The population structure of Magnaporthe oryzae from green foxtail ( Setaria viridis) in Japan was examined by DNA fingerprint analyses using the transposable elements MGR586 and MAGGY as probes. Fifteen M. oryzae isolates from green foxtail were collected from 11 Japanese prefectures so that a macrogeographic population of this pathogen is represented. All the 15 isolates were sorted into distinct haplotypes by DNA fingerprint analyses with both probes. Furthermore, similarities between the DNA fingerprint profiles of the 15 isolates were exclusively low; i.e., if lineages are arbitrarily established based on greater than 70% similarities in isolates, the 15 isolates could be categorized into 13 distinct lineages by DNA fingerprinting with both probes. We also examined the MGR586 DNA fingerprint variations of this pathogen in 9 microgeographic populations each of which contained 20 to 24 isolates collected from a 1 m2 or 50 m2 area. In all the 9 populations, more than 2 haplotypes, which shared less than 70% similarities, were identified in the DNA fingerprint profiles. These results suggested that M. oryzae isolates from the green foxtail in Japan possessed a complex lineage structure, even at the microgeographic scale. [ABSTRACT FROM AUTHOR]

Twenty-eight Pyricularia isolates from two wild foxtails—green foxtail ( Setaria viridis) and giant foxtail ( S. faberii)—in Japan were taxonomically characterized by DNA analyses, mating tests, and pathogenicity assays. Although most of the isolates failed to produce perithecia in mating tests with Magnaporthe oryzae, a diagnostic polymerase chain reaction-restriction fragment length polymorphism phenotype of M. oryzae was detected in the beta-tubulin genomic region in all isolates. The pathogenicity assays revealed that host ranges of the isolates were similar to those of isolates from foxtail millet ( S. italica), which were exclusively pathogenic on foxtail millet. In addition to the 28 isolates from wild foxtails, 22 Pyricularia isolates from 11 other grasses were analyzed by RFLP using single-copy sequences as probes. In a dendrogram constructed from the RFLP data, isolates that were previously identified as M. oryzae formed a single cluster. All the wild foxtail isolates formed a subcluster with foxtail millet isolates within the M. oryzae cluster. From these results, we conclude that Pyricularia isolates from the wild foxtails are closely related to isolates from foxtail millet and should be classified into the Setaria pathotype of M. oryzae. [ABSTRACT FROM AUTHOR]

A Japanese differential rice cultivar K60 was tested with 114 F1 cultures of Magnaporthe oryzae from a cross between isolates 84R-62B and Y93-245c-2. Segregation patterns of avirulence and virulence in the progeny suggested that avirulence on cv. K60 was controlled by a single gene derived from 84R-62B and tentatively named AvrK60. In the F1 population, AvrK60 cosegregated with avirulence gene AvrPik on a small 1.6-Mb chromosome of 84R-62B and with the 1.6-Mb chromosome itself. Therefore, we suggest that, along with AvrPik, AvrK60 is also located on the 1.6-Mb chromosome of 84R-62B. [ABSTRACT FROM AUTHOR]