RESUMO
Resistance of barley (Hordeum vulgare) to the powdery mildew fungus Erysiphe graminis f.sp. hordei is conferred by several dominant genes, but also by recessive alleles of the Mlo locus mapping on the long arm of chromosome 4. In addition, this single-factor-mediated resistance is active against all known physiological races of the parasite. Thus the mechanism underlying mlo-mediated resistance should differ substantially from that mediated by the dominant genes. A positional cloning strategy to isolate the Mlo gene from the barley genome, the size of which is almost double the size of the human genome, has been designed. The AFLP technique was employed to identify markers tightly linked to the Mlo locus and to produce a local high-resolution genetic map. The use of this high-volume marker technology allowed the rapid screening of approximately 250,000 loci for linkage to Mlo. A large number of Mlo-linked AFLP markers were identified, one of which cosegregated with Mlo on the basis of more than 4000 meiotic events. A four-genome-equivalent barley YAC library (average insert size 480 kb) was constructed and screened with this cosegregating marker. Four YACs containing this marker were isolated and subsequent characterization by AFLP-based physical mapping allowed the physical delimitation of the Mlo locus to a DNA segment of 30 kb.
Assuntos
Mapeamento Cromossômico , Genes de Plantas/genética , Hordeum/genética , Proteínas de Plantas/genética , Cromossomos/genética , Cromossomos Artificiais de Levedura/genética , Clonagem Molecular , Impressões Digitais de DNA , Enzimas de Restrição do DNA/metabolismo , Fungos/patogenicidade , Biblioteca Gênica , Ligação Genética , Marcadores Genéticos/genética , Doenças das PlantasRESUMO
Mutation-induced recessive alleles (mlo) of the barley Mlo locus confer a leaf lesion phenotype and broad spectrum resistance to the fungal pathogen, Erysiphe graminis f. sp. hordei. The gene has been isolated using a positional cloning approach. Analysis of 11 mutagen-induced mlo alleles revealed mutations leading in each case to alterations of the deduced Mlo wild-type amino acid sequence. Susceptible intragenic recombinants, isolated from mlo heteroallelic crosses, show restored Mlo wild-type sequences. The deduced 60 kDa protein is predicted to be membrane-anchored by at least six membrane-spanning helices. The findings are compatible with a dual negative control function of the Mlo protein in leaf cell death and in the onset of pathogen defense; absence of Mlo primes the responsiveness for the onset of multiple defense functions.
