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The avirulence gene AVR-Pita in Magnaporthe grisea prevents the fungus from infecting rice cultivars carrying the disease resistance gene Pi-ta. Insertion of Pot3 transposon into the promoter of AVR-Pita caused the gain of virulence toward Yashiro-mochi, a rice cultivar containing Pi-ta, which demonstrated the ability of Pot3 to move within the M. grisea genome. The appearance of Pot3 in M. grisea seems to predate the diversification of various host-specific forms of the fungus.

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Colletotrichum lagenarium and Magnaporthe grisea are plant pathogenic fungi that produce melanin during the appressorial differentiation stage of conidial germination and during the late stationary phase of mycelial growth. Here, we report the identification of genes for two unique transcription factors, CMR1 (Colletotrichum melanin regulation) and PIG1 (pigment of Magnaporthe), that are involved in melanin biosynthesis. Both Cmr1p and Pig1p contain two distinct DNA-binding motifs, a Cys2His2 zinc finger motif and a Zn(II)2Cys6 binuclear cluster motif. The presence of both these motifs in a single transcriptional regulatory protein is unique among known eukaryotic transcription factors. Deletion of CMR1 in C. lagenarium caused a defect in mycelial melanization, but not in appressorial melanization. Also, cmr1Delta mutants do not express the melanin biosynthetic structural genes SCD1 and THR1 during mycelial melanization, although the expression of these two genes was not affected during appressorial melanization.

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Genetic mapping showed that the rice blast avirulence gene AVR-Pita is tightly linked to a telomere on chromosome 3 in the plant pathogenic fungus Magnaporthe grisea. AVR-Pita corresponds in gene-for-gene fashion to the disease resistance (R) gene Pi-ta. Analysis of spontaneous avr-pita(-) mutants indicated that the gene is located in a telomeric 6.5-kb BglII restriction fragment. Cloning and DNA sequencing led to the identification of a candidate gene with features typical of metalloproteases. This gene is located entirely within the most distal 1.5 kb of the chromosome. When introduced into virulent rice pathogens, the cloned gene specifically confers avirulence toward rice cultivars that contain Pi-ta. Frequent spontaneous loss of AVR-Pita appears to be the result of its telomeric location. Diverse mutations in AVR-Pita, including point mutations, insertions, and deletions, permit the fungus to avoid triggering resistance responses mediated by Pi-ta. A point mutation in the protease consensus sequence abolishes the AVR-Pita avirulence function.

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The rice blast resistance (R) gene Pi-ta mediates gene-for-gene resistance against strains of the fungus Magnaporthe grisea that express avirulent alleles of AVR-Pita. Using a map-based cloning strategy, we cloned Pi-ta, which is linked to the centromere of chromosome 12. Pi-ta encodes a predicted 928-amino acid cytoplasmic receptor with a centrally localized nucleotide binding site. A single-copy gene, Pi-ta shows low constitutive expression in both resistant and susceptible rice. Susceptible rice varieties contain pi-ta(-) alleles encoding predicted proteins that share a single amino acid difference relative to the Pi-ta resistance protein: serine instead of alanine at position 918. Transient expression in rice cells of a Pi-ta(+) R gene together with AVR-Pita(+) induces a resistance response. No resistance response is induced in transient assays that use a naturally occurring pi-ta(-) allele differing only by the serine at position 918. Rice varieties reported to have the linked Pi-ta(2) gene contain Pi-ta plus at least one other R gene, potentially explaining the broadened resistance spectrum of Pi-ta(2) relative to Pi-ta. Molecular cloning of the AVR-Pita and Pi-ta genes will aid in deployment of R genes for effective genetic control of rice blast disease.

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Mutants of Magnaporthe grisea harboring a defective gene for 1,3, 8-trihydroxynaphthalene reductase retain the capability to produce scytalone, thus suggesting the existence of a second naphthol reductase that can catalyze the reduction of 1,3,6, 8-tetrahydroxynaphthalene to scytalone within the fungal melanin biosynthetic pathway. The second naphthol reductase gene was cloned from M. grisea by identification of cDNA fragments with weak homology to the cDNA of trihydroxynaphthalene reductase. The amino acid sequence for the second naphthol reductase is 46% identical to that of trihydroxynaphthalene reductase. The second naphthol reductase was produced in Esherichia coli and purified to homogeneity. Substrate competition experiments indicate that the second reductase prefers tetrahydroxynaphthalene over trihydroxynaphthalene by a factor of 310; trihydroxynaphthalene reductase prefers trihydroxynaphthalene over tetrahydroxynaphthalene by a factor of 4.2. On the basis of the 1300-fold difference in substrate specificities between the two reductases, the second reductase is designated tetrahydroxynaphthalene reductase. Tetrahydroxynaphthalene reductase has a 200-fold larger K(i) for the fungicide tricyclazole than that of trihydroxynaphthalene reductase, and this accounts for the latter enzyme being the primary physiological target of the fungicide. M. grisea mutants lacking activities for both trihydroxynaphthalene and tetrahydroxynaphthalene reductases do not produce scytalone, indicating that there are no other metabolic routes to scytalone.

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We have initiated a mutational analysis of pathogenicity in the rice blast fungus, Magnaporthe grisea, in which hygromycin-resistant transformants, most generated by restriction enzyme-mediated integration (REMI), were screened for the ability to infect plants. A rapid primary infection assay facilitated screening of 5,538 transformants. Twenty-seven mutants were obtained that showed a reproducible pathogenicity defect, and 18 of these contained mutations that cosegregated with the hygromycin resistance marker. Analysis of eight mutants has resulted in the cloning of seven PTH genes that play a role in pathogenicity on barley, weeping lovegrass, and rice. Two independent mutants identified the same gene, PTH2, suggesting nonrandom insertion of the transforming DNA. These first 7 cloned PTH genes are described.

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Arthroscopic rotator cuff repairs using titanium-alloy suture anchors are a new treatment option for active patients with shoulder injuries. Shoulder arthroscopy and arthroscopic repair procedures are alternative treatments to traditional open surgical procedures for Bankart lesions and rotator cuff tears. Distinct advantages of arthroscopic repair techniques include decreased patient tissue trauma and morbidity rates and shortened recovery and rehabilitation periods.

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Genetic analysis of host specificity in the rice blast fungus (Magnaporthe grisea) identified a single gene, PWL2 (for Pathogenicity toward Weeping Lovegrass), that exerts a major effect on the ability of this fungus to infect weeping lovegrass (Eragrostis curvula). The allele of the PWL2 gene conferring nonpathogenicity was genetically unstable, with the frequent appearance of spontaneous pathogenic mutants. PWL2 was cloned based on its map position. Large deletions detected in pathogenic mutants guided the gene cloning efforts. Transformants harboring the cloned PWL2 gene lost pathogenicity toward weeping lovegrass but remained fully pathogenic toward other host plants. Thus, the PWL2 host species specificity gene has properties analogous to classical avirulence genes, which function to prevent infection of certain cultivars of a particular host species. The PWL2 gene encodes a glycine-rich, hydrophilic protein (16 kD) with a putative secretion signal sequence. The pathogenic allele segregating in the mapping population, pwl2-2, differed from PWL2 by a single base pair substitution that resulted in a loss of function. The PWL2 locus is highly polymorphic among rice pathogens from diverse geographic locations.

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We have identified genes for pathogenicity toward rice (Oryza sativa) and genes for virulence toward specific rice cultivars in the plant pathogenic fungus Magnaporthe grisea. A genetic cross was conducted between the weeping lovegrass (Eragrostis curvula) pathogen 4091-5-8, a highly fertile, hermaphroditic laboratory strain, and the rice pathogen O-135, a poorly fertile, female-sterile field isolate that infects weeping lovegrass as well as rice. A six-generation backcrossing scheme was then undertaken with the rice pathogen as the recurrent parent. One goal of these crosses was to generate rice pathogenic progeny with the high fertility characteristic of strain 4091-5-8, which would permit rigorous genetic analysis of rice pathogens. Therefore, progeny strains to be used as parents for backcross generations were chosen only on the basis of fertility. The ratios of pathogenic to nonpathogenic (and virulent to avirulent) progeny through the backcross generations suggested that the starting parent strains differ in two types of genes that control the ability to infect rice. First, they differ by polygenic factors that determine the extent of lesion development achieved by those progeny that infect rice. These genes do not appear to play a role in infection of weeping lovegrass because both parents and all progeny infect weeping lovegrass. Second, the parents differ by simple Mendelian determinants, "avirulence genes," that govern virulence toward specific rice cultivars in all-or-none fashion. Several crosses confirm the segregation of three unlinked avirulence genes, Avr 1-CO39, Avr 1-M201 and Avr1-YAMO, alleles of which determine avirulence on rice cultivars CO39, M201, and Yashiro-mochi, respectively. Interestingly, avirulence alleles of Avr1-CO39, Avr1-M201 and Avr1-YAMO were inherited from the parent strain 4091-5-8, which is a nonpathogen of rice. Middle repetitive DNA sequences ("MGR sequences"), present in approximately 40-50 copies in the genome of the rice pathogen parent, and in very low copy number in the genome of the nonpathogen of rice, were used as physical markers to monitor restoration of the rice pathogen genetic background during introgression of fertility. The introgression of highest levels of fertility into the most successful rice pathogen progeny was incomplete by the sixth generation, perhaps a consequence of genetic linkage between genes for fertility and genes for rice pathogenicity. One chromosomal DNA segment with MGR sequence homology appeared to be linked to the gene Avr1-CO39. Finally, many of the crosses described in this paper exhibited a characteristic common to many crosses involving M. grisea rice pathogen field isolates.(ABSTRACT TRUNCATED AT 400 WORDS)

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We have identified a family of dispersed repetitive DNA sequences in the genome of Magnaporthe grisea, the fungus that causes rice blast disease. We have named this family of DNA sequences "MGR" for M. grisea repeat. Analysis of five MGR clones demonstrates that MGR sequences are highly polymorphic. The segregation of MGR sequences in genetic crosses and hybridization of MGR probes to separated, chromosome-size DNA molecules of M. grisea shows that this family of sequences is distributed among the M. grisea chromosomes. MGR sequences also hybridize to discrete poly(A)+ RNAs. Southern blot analysis using a MGR probe can distinguish rice pathogens from various sources. However, MGR sequences are not highly conserved in the genomes of M. grisea field isolates that do not infect rice. These results suggest that host selection for a specific pathogen genotype has occurred during the breeding and cultivation of rice.

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