Submitted to: The Plant Cell
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/2002
Publication Date: N/A
Interpretive Summary: Limited contiguous DNA sequence from grain cereal plants, especially those with genomes larger than that of humans, has prevented a full understanding of how their genomes are organized. This lack of knowledge negatively impacts crop-breeding programs. To evaluate the organization of a major disease defense complex in the 5000-Megabase barley genome, we determined the complete 261-kilobase sequence that spans the Mla (powdery mildew) resistance complex. This investigation revealed that half of the 32 genes are involved in plant defense responses. The defense genes are grouped together in three interwoven resistance complexes interspersed with two complexes of transposable elements or "jumping DNA." It has long been known that many disease resistance genes in barley, wheat, and rye are clustered together on chromosomes. These data provide for a general mechanism of genome expansion in plants and how physically dissimilar sequences have been grouped together in functional groups. This is the longest contiguous DNA sequence described in small grain cereal crops. The results described in this manuscript will impact scientists who work on genome organization, defense genes and their role, and transposable elements.
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 evaluate 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 locus. Among the 32 protein-encoding genes, 16 are plant-defense related; 12 of these are associated with defense against powdery mildew disease, but function in different signaling pathways. The Mla region is organized as three gene-rich islands separated by two nested complexes of transposable elements and a 45-kb gene-poor region. A heterochromatic knob-like sequence, designated KL1HS1, is positioned directly 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 bi-directional expansion has resulted in a positional shift of KL1HS1. Our results indicate that the present Mla region evolved over 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 region, which profiles several emerging features of large cereal genomes.