Submitted to: Plant Genome Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 1/16/2002
Publication Date: N/A
Technical Abstract: Proteins that initiate defense against disease often share conserved signaling motifs in plants and animals. In addition to conservation of protein domains, genes with related functions are frequently clustered in the genome. To determine the organization and content of a major defense gene complex in plants, we determined the complete sequence of a 261-kb BAC Ccontig from barley cultivar (cv.) Morex that spans the Mla (powdery mildew resistance complex. The Mla region is comprised of thirty-two protein-encoding genes and two tRNAser genes. Sixteen of the protein-coding sequences are plant-defense related; 12 of these are associated with defense against powdery mildew disease, but function in different signaling pathways. These defense-related and other protein-encoding sequences are organized as three gene-rich islands separated by two nested complexes of transposable elements and a gene-poor region. A heterochromatic knob-like sequence, designated KL1HS1, is positioned proximal to Mla and is comprised of a gene-poor core with seventeen families of diverse tandem repeats that overlap a hypermethylated, but transcriptionally active, gene-dense island. Sequence similarity analysis of these repeats indicates that bidirectional expansion has resulted in a positional shift of KL1HS1. Our results indicate that the present Mla region evolved over a period greater than 7 million years through a series of duplication and inversion events in addition to random and nested transposon insertion. We present a model for the evolution of the Mla plant-defense complex, which profiles several emerging features of large cereal genomes.