The role of ethylene (ET) in resistance to infection with blast fungus (Magnaporthe grisea) in rice (Oryza sativa) is poorly understood. To study it, we quantified ET levels after inoculation, using young rice plants at the four-leaf stage of rice cv Nipponbare (wild type) and its isogenic plant (IL7), which contains the Pi-i resistance gene to blast fungus race 003. Small necrotic lesions by hypersensitive reaction (HR) were formed at 42 to 72 h postinoculation (hpi) in resistant IL7 leaves, and whitish expanding lesions at 96 hpi in susceptible wild-type leaves. Notable was the enhanced ET emission at 48 hpi accompanied by increased 1-aminocyclopropane-1-carboxylic acid (ACC) levels and highly elevated ACC oxidase (ACO) activity in IL7 leaves, whereas only an enhanced ACC increase at 96 hpi in wild-type leaves. Among six ACC synthase (ACS) and seven ACO genes found in the rice genome, OsACS2 was transiently expressed at 48 hpi in IL7 and at 96 hpi in wild type, and OsACO7 was expressed at 48 hpi in IL7. Treatment with an inhibitor for ACS, aminooxyacetic acid, suppressed enhanced ET emission at 48 hpi in IL7, resulting in expanding lesions instead of HR lesions. Exogenously supplied ACC compromised the aminooxyacetic acid-induced breakdown of resistance in IL7, and treatment with 1-methylcyclopropene and silver thiosulfate, inhibitors of ET action, did not suppress resistance. These findings suggest the importance of ET biosynthesis and, consequently, the coproduct, cyanide, for HR-accompanied resistance to blast fungus in young rice plants and the contribution of induced OsACS2 and OsACO7 gene expression to it.In monocot plants, the mechanism of disease resistance, including the roles of defense signal compounds for resistance (R)-gene-mediated resistance, such as salicylic acid (SA), ethylene (ET), and jasmonic acid (JA), has not been well elucidated. To study the mechanism, the Japonica rice (Oryza sativa) cv Nipponbare is an attractive model because of recent developments in genomic and molecular information, such as the Rice Genome Research Program (http//rgp.dna.affrc.go.jp; International Rice Genome Sequencing Project, 2005). Rice blast fungus (Magnaporthe grisea) is an extensively studied pathogen whose infection seriously affects rice yields worldwide. Genetic studies have identified 13 major R genes to blast fungi in rice plants, and standard rice cultivars with individual R genes and corresponding standard blast fungal races have been prepared to identify the race of blast fungus and R genes in a rice cultivar (Yamada et al., 1976;Kiyosawa, 1984). In Japan, attempts have been made to generate individual isogenic lines that contain a specific R gene with the same genetic background as practical rice cultivars, such as cv Nipponbare (Ise and Horisue, 1988). Actually, multilines containing compatible and incompatible lines were reported to be effective for disease control in the field (Browning and Frey, 1969) and, to our knowledge, there is no information to date on the breakdown of disease resistanc...

Members of the Fusarium graminearum species complex are important cereal pathogens worldwide and belong to one of at least nine phylogenetically distinct species. We examined 298 strains of the F. graminearum species complex collected from wheat or barley in Japan to determine the species and trichothecene chemotype. Phylogenetic analyses and species-diagnostic polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLPs) revealed the presence and differential distribution of F. graminearum sensu stricto (s. str.) and F. asiaticum in Japan. F. graminearum s. str. is predominant in the north, especially in the Hokkaido area, while F. asiaticum is predominant in southern regions. In the Tohoku area, these species co-occurred. Trichothecene chemotyping of all strains by multiplex PCR revealed significantly different chemotype compositions of these species. All 50 strains of F. graminearum s. str. were of a 15- or 3-acetyl deoxynivalenol type, while 173 (70%) out of 246 strains of F. asiaticum were of a nivalenol type. The possibility of gene flow between the two species was investigated by use of 15 PCR-RFLP markers developed in this study. However, no obvious hybrids were detected from 98 strains examined, including strains collected from regions where both species co-occur.

Class III plant peroxidases are believed to function in diverse physiological processes including disease resistance and wound response, but predicted low substrate specificities and the presence of 70 or more isoforms have made it difficult to define a specific physiological function(s) for each gene. To select pathogen-responsive POX genes, we analyzed the expression profiles of 22 rice POX genes after infection with rice blast fungus. The expression of 10 POX genes among the 22 genes was induced after fungal inoculation in both compatible and incompatible hosts. Seven of the 10 POX genes were expressed at higher levels in the incompatible host than in the compatible host 6-24 h after inoculation by which time no fungus-induced lesions have appeared. Organ-specific expression and stress-induced expression by wounding and treatment with probenazole, an agrichemical against blast fungus, jasmonic acid, salicylic acid and 1-aminocyclopropane-1-carboxylate, a precursor of ethylene, indicated that rice POXs have individual characteristics and can be classified into several types. A comparison of the amino acid sequences of POXs showed that multiple isoforms with a high sequence similarity respond to stress in different or similar ways. Such redundant responses of POX genes may guarantee POX activities that are necessary for self-defense in plant tissues against environmental stresses including pathogen infection.

The rice line WSS2 which was derived from the Vietnamese indica variety Tetep, displays a high partial resistance to sheath blight. Quantitative trait locus (QTL) analysis of the resistance using simple sequence repeat (SSR) and sequence-tagged site (STS) markers was conducted in a BC 1 F 1 population derived from the cross Hinohikari/WSS2//Hinohikari. Sheath blight resistance in this population and its cross-parents was studied using syringe inoculation. Two QTLs for sheath blight resistance (qSB-3 and qSB-12) were identified on chromosomes 3 and 12. Their resistance alleles were derived from the resistant parent WSS2. These QTLs totally explained 29.6 % of the phenotypic variation. Sheath blight resistance was significantly correlated with culm length and heading date. Among the QTLs for culm length and heading date, qCL-3 for culm length was located in the same region as qSB-3, and the remaining QTLs were not linked to qSB-12. Thus, it was reasonable to assume that qSB-12 would enable to breed a rice variety resistant to sheath blight.