Genome Dynamics and Evolution of theMla(Powdery Mildew) Resistance Locus in Barley[W]

Abstract

Genes that confer defense against pathogens often are clustered in the genome and evolve via diverse mechanisms. To evaluate the organization and content of a major defense gene complex in cereals, we determined the complete sequence of a 261-kb BAC contig from barley cv Morex that spans the Mla (powdery mildew) resistance locus. Among the 32 predicted genes on this contig, 15 are associated with plant defense responses; 6 of these are associated with defense responses to 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-like region is positioned directly proximal to Mla and is composed of a gene-poor core with 17 families of diverse tandem repeats that overlap a hypermethylated, but transcriptionally active, gene-dense island. Paleontology analysis of long terminal repeat retrotransposons indicates that the present Mla region evolved over a period of Ͼ 7 million years through a variety of duplication, inversion, and transposon-insertion events. Sequence-based recombination estimates indicate that R genes positioned adjacent to nested long terminal repeat retrotransposons, such as Mla , do not favor recombination as a means of diversification. We present a model for the evolution of the Mla region that encompasses several emerging features of large cereal genomes.

A physical, genetic and functional sequence assembly of the barley genome

“…These tended to cluster towards the distal regions of barley chromosomes ( Supplementary Fig. 17), including a major group on barley chromosome 1HS, colocalizing with the MLA powdery mildew resistance gene cluster 28 . Biased allocation to recombination-rich regions provides the genomic environment for generating sequence diversity required to cope with dynamic pathogen populations 29,30 .…”

Recognition and signal transduction in the expression of Phytophthora resistance in soybean [Glycine max (L) Merr]

“…Plant R genes often occur in clusters, and genes within one cluster are mostly derived from a common ancestor (Richly et al 2002). Three distinct CC-NBS-LRR gene families were identified in the Mla locus within a 240-kb region (Wei et al 1999;Wei et al 2002). Plants have to generate novel resistance specificities to combat the quickly evolved pathogens.…”

“…It has been hypothesized that transposable elements play a major role in the expansion and diversification of disease resistance gene family ). The abundance of retroelements has been observed on several R genes or RGA loci, such as barley powdery mildew resistance gene, Mla, and Citrus virus resistance gene, Ctv (Wei et al 2002;Yang et al 2003). The variability among 14 rice Xa21 gene members has been considered to be evolved mainly from the rearrangements mediated by transposon-like elements .…”

“…Some complex resistance gene loci carry related but highly polymorphic sequences. Three distinct CC-NBS-LRR gene families were identified in the Mla locus within a 240-kb region (Wei et al 1999;Wei et al 2002). In addition to these studies on cloned functional R genes, resistance gene analogues (RGAs) identified from different plant species also showed the clustering feature (Aarts et al 1998a;Graham et al 2002a;Irigoyen et al 2004;Kanazin et al 1996;Leister et al 1996;Monosi et al 2004;Shen et al 1998).…”

“…Long contiguous sequences containing several R genes or resistance gene analogues (RGA) have been determined (Brunner et al 2003;Graham et al 2002;Wei et al 2002;Yang et al 2003). These sequences provided insights into the mechanisms of R gene evolution and generation of novel recognition specificity.…”