Leaf rust is the most destructive disease of wheat worldwide. Genetic resistance is the preferred strategy for disease control. However, genetic resistance often breaks down and there is a need to understand the molecular basis of resistance expression. The objective of this study was molecular cloning of leaf rust-resistance gene Lr21. Lr21 was originally transferred from Aegilops tauschii (accession RL5289), the D-genome donor of common wheat (Triticum aestivum L.). An allele of Lr21, previously designated as Lr40, was also transferred from Ae. tauschii (TA1649) into the wheat variety Wichita (WI) and released as wheat germplasms WGRC2 and WGRC7. High-resolution mapping of Lr21 from the crosses WGRC2/WI and WGRC7/WI showed that the clone KSUD14, and the 3′ and 5′ portions of cDNA isolated by gene-specific primers designed from KSUD14 cosegregated with Lr21 except in one plant (97-87-43). In 97-87-43, two recombination events within a 1,416-bp interval were detected in the KSUD14 Lr21 candidate-gene region. The recombined gene has a two-nucleotide deletion in the coding region resulting in a frameshift and encoding a truncated protein. The cosmid clone 69-7-1, which was isolated from the TA1649-derived cosmid library and contained the Lr21 candidate gene, conferred resistance to rust culture PRTUS6 in the transgenic plant 1410 upon transformation of the susceptible cv. Fielder. This provided conclusive evidence for the cloning of Lr21. The gene encodes 1,080 amino acids and contains a central nucleotide-binding site and 13 imperfect leucine-rich repeats. Lr21 belongs to the NBS-LRR class of resistance genes. The map-based molecular cloning of Lr21 is a breakthrough in gene cloning from a huge and complex genome of wheat and offers an opportunity for novel manipulation of Lr21 for more durable leaf rust resistance.
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