Plant Cell Advance Online Publication
Published on July 25, 2002; 10.1105/tpc.002238
Received February 9, 2002
Accepted April 30, 2002
Genome Dynamics and Evolution of the Mla (Powdery Mildew) Resistance Locus
in Barley
Fusheng Wei 1, Rod A. Wing 2, and Roger P. Wise 3*
1
Interdepartmental Genetics Program and Department of Plant Pathology, Iowa State
University, Ames, Iowa 50011-1020
2
Clemson University Genomics Institute, Clemson, South Carolina 29634
3
Interdepartmental Genetics Program and Department of Plant Pathology, Iowa State
University, Ames, Iowa 50011-1020; Corn Insects and Crop Genetics Research,
United States Department of Agriculture, Agricultural Research Service, Iowa State
University, Ames, Iowa 50011-1020
* To whom correspondence should be addressed. E-mail: rpwise{at}iastate.edu.
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 >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.
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