| Rice blast disease, caused by the filamentous fungus Magnaporthe grisea (Hebert) Barr. (anamorph, Pyricularia grisea [Cooke] Sacc), is one of the most devastating diseases of rice, Oryza sativa L. For most rice-growing areas, rice blast disease management relies on the frequent application of new resistant rice cultivars. However, the management of rice blast through the use of disease-resistant cultivars has become difficult (formidable) because of the highly variable nature of the pathogen as well as favorable environmental conditions for infection during the crop season. Therefore, genetic diversity in the M. grisea population was studied to facilitate the rice breeding for durable resistance and the disease integrated management. And genetic analysis and mapping of avirulence genes from the cross between M. grisea isolates were also studied for aiding to the positional cloning and further functional studies of avirulence genes.A total of 482 isolates were collected from 13 areas in China. Each isolate was subjected to DNA fingerprint analysis using pot2-rep-PCR. At a 55% genetic similarity leve, 482 isolates (381 haplotypes) were grouped into 11 lineages. This DNA fingerprinting analysis indicated that the M. grisea populations cannot be delineated into region-specific groups. Analyses of the number of alleles (na), Nei's gene diversity, unbiased genetic distance, and Shannon's Information index among 13 natural populations showed that clusters were not related to the geographic distance between populations with the exception of the Ningxi (NX) and Jilin (JL) cluster. Among northern populations, NX and JL were more similar to one another than to other populations. Pathogen populations consisting of 121 isolates from China were grouped into 53 pathotypes on the basis of disease reaction in differential rice lines. Isolates assayed for pathotypes were detected based on disease reactions; a relationship between pathotypes and fingerprint groups was not obvious. High frequency of virulence was found on the rice line Shin2 (Pi-k~s, Pi-sh) followed by PiNo.4 (Pi-ta~2, Pi-sh) and Kl (Pi-ta), while it was low on Kanto 51 (Pi-k +?), K3 (Pz-k~h), and Fujisaka (Pi-i, Pi-sh). Virulence was rare on Toridl (Pi-z', Pi-sh). Tetep (Pi-k~h+?) waspredicted to be a highly effective, as none of the isolates infected this line. These blast-resistant rice lines can be used in resistance breeding for the effective management of rice blast in the respective locations of China.Six hundred and seventy isolates of Magnaporthe grisea collected during 1998-2003 from 13 provinces in China were tested for their mating type and fertility with four standard tested isolates. Results showed that two mating types, MAT1-1 and MAT 1-2, exist in China. The percentage of fertile isolates was 40.3% on average, amongst the fertile isolates, 21.9% for MAT1-1, and 18.4% for MAT 1-2. The rate of perfect stage formation was different among the tested isolates. Since both mating type genes had already been cloned, a PCR assay was carried out to determine the mating type of all tested M. grisea isolates, it was found that MAT1-1 was 62.5% and MAT1-2 was 37.5% among the 670 isolates. In most of the tested locations, two mating types existed together. It was suggested that though the sexual stage rarely occurred in natural condition, the sexual recombination might still be a potential mechanism for the genetic diversity of Magnaporthe grisea.The segregation of avirulence/virulence was studied in 87 F1 progeny isolates of Magnaporthe grisea from a cross of two isolates, Guy 11 and JS153, using 25 known resistance gene differential rice cultivars. The segregation ratio of avirulence to virulence was 1:1, 3:1, 1:3 and 7:1 on differential rice varieties, respectively. The segregation ratio indicated that the avirulence and virulence to different varieties were controlled by different avirulence genes. As F1 hybrid progeny were further backcrossed with their parents and sib-crossed between progeny, we haved developed 10 populations. The x ~2 test of goodness-of-fit for... |
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