Microsatellite mapping of adult-plant leaf rust resistance gene <Emphasis Type="Italic">Lr22a</Emphasis> in wheat

This study was conducted to identify microsatellite markers (SSR) linked to the adult-plant leaf rust resistance gene Lr22a and examine their cross-applicability for marker-assisted selection in different genetic backgrounds. Lr22a was previously introgressed from Aegilops tauschii Coss. to wheat (Triticum aestivum L.) and located to chromosome 2DS. Comparing SSR alleles from the donor of Lr22a to two backcross lines and their recurrent parents showed that between two and five SSR markers were co-introgressed with Lr22a and the size range of the Ae. tauschii introgression was 9-20 cM. An F(2) population from the cross of 98B34-T4B x 98B26-N1C01 confirmed linkage between the introgressed markers and Lr22a on chromosome 2DS. The closest marker, GWM296, was 2.9 cM from Lr22a. One hundred and eighteen cultivars and breeding lines of different geographical origins were tested with GWM296. In total 14 alleles were amplified, however, only those lines predicted or known to carry Lr22a had the unique Ae. tauschii allele at GWM296 with fragments of 121 and 131 bp. Thus, GWM296 is useful for selecting Lr22a in diverse genetic backgrounds. Genotypes carrying Lr22a showed strong resistance to leaf rust in the field from 2002 to 2006. Lr22a is an ideal candidate to be included in a stack of leaf rust resistance genes because of its strong adult-plant resistance, low frequency of commercial deployment, and the availability of a unique marker.