ARS | Publication request: Evolution of New Disease Specificity at a Single Resistance Locus in a Crop-Weed Complex: Reconstitution of the Lr21 Gene in Wheat.

Submitted to: Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 3, 2009
Publication Date: June 1, 2009
Citation: Huang, L., Brooks, S.A., Li, W., Fellers, J.P., Nelson, J., Gill, B.S. 2009. Evolution of New Disease Specificity at a Single Resistance Locus in a Crop-Weed Complex: Reconstitution of the Lr21 Gene in Wheat. Genetics 182:595-602.

Interpretive Summary: Little is known about the evolution of new specificities in plant disease resistance genes. In this study, we reconstructed a possible evolutionary history of the wheat leaf rust resistance gene, Lr21. In wild populations of a wheat ancestor, Aegilops tauschii, both functional and nonfunctional versions of Lr21 were found. The functional version of the gene appeared to be a chimera resulting from recombination between the two nonfunctional versions. The two nonfunctional versions also occur in bread wheat varieties Wichita and Fielder, respectively. These two susceptible varieties were crossed and one out of 5876 progeny was resistant to leaf rust. When the resistant plant was tested, it had a chimeric recombination of the versions from the parents. This was the first demonstration of the evolution of a new resistance gene specificity by rearrangement of two nonfunctional resistance genes. This might be exploited in the design of new resistance genes by recombining existing genes.

Technical Abstract: Leaf-rust resistance gene Lr21, present in modern varieties of hexaploid wheat, originated in goatgrass Aegilops tauschii Coss., the D genome donor of wheat. The goatgrass donor was collected in Iran where it grows as a weed in wheat fields as part of the native agricultural ecosystem. In order to elucidate the origin of the gene, we analyzed sequences of Lr21 and lr21 alleles from hexaploid wheat and Ae. tauschii collected along the Caspian Sea of Iran and Azerbaijan. Three founding nonfunctional haplotypes, H1, H2, and H3 were identified, of which Lr21 is a chimera of H1 and H2. Two hexaploid wheat cultivars, Fielder and Wichita, carry respectively the H1 and H2 haplotypes. We attempted to reconstitute a functional Lr21 allele by crossing these two cultivars. Rust inoculation of 5876 F2 progeny revealed a single resistant plant that proved to carry the H1H2 haplotype as a result of intragenic recombination. These findings suggest that plants can reuse “dead” alleles to generate new disease resistance specificity and lead to a “death–recycle” model of plant resistance-gene evolution at simple loci. They support a role for crop–weed complexes in native agricultural ecosystems in the evolution of resistance.