Plant & Animal Genome IX Conference

Workshop: Barley
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THE GENE-RICH Mla-REGION CONTAINS DIFFERENTIALLY EXPRESSED, DEFENSE GENE FAMILIES INTERSPERSED WITH NESTED RETROTRANSPOSONS

¹ USDA-ARS, Department of Plant Pathology, Iowa State University, Ames, IA 50011-1020

² Interdepartmental Genetics Program, Iowa State University, Ames, IA 50011-1020

³ Clemson University Genomics Institute, Clemson, SC 29634

⁴ Sainsbury Laboratory, John Innes Centre, Norwich NR4 7UH, U.K.

⁵ MPI für Züchtungsforschung, D-50829 Köln, Germany

The barley Mla locus on chromosome 5 (1H) confers multiple resistance specificities to the powdery mildew fungus, Blumeria (=Erysiphe) graminis f. sp. hordei. In order to clone Mla alleles by map-based methods, we constructed a BAC contig from the cultivar Morex [Wei et al. (1999) Genetics 153:1929-1948] and determined the complete DNA sequence of two overlapping BACs that encompassed the Mla locus. Computational analysis of the 261-kb, Mla-spanning sequence has revealed approximately 30 putative genes. Among these 30 candidate ORFs are three families of NBS-LRR resistance gene homologs (RGHs) and six chymotrypsin inhibitor genes. Coding regions are positioned in two gene islands bridged by nested retrotransposons. Probes from the Mla-RGH1 family were used to identify three classes of cDNAs from C.I. 16151, a cultivar containing the Mla6 allele. The first of these cDNAs is predicted to encode a full-length CC-NBS-LRR protein and the other two are truncated variants. A cosmid containing a gene corresponding to the full-length candidate cDNA was used in a single-cell expression assay to complement AvrMla6-dependent, resistance specificity to B. graminis in barley and wheat. This assay was also used to substantiate previous genetic data that the Mla6 allele requires the signaling pathway component, Rar1, for function. Comparison of MLA6 and the Rar1-independent MLA1 protein reveals 91.2% sequence identity, demonstrating that highly related CC-NBS-LRR proteins encoded by alleles at the Mla locus can dictate similar powdery mildew resistance phenotypes, yet require distinct signaling components. Research supported by USDA-NRI/CGP grant 98-35300-6169.