Contributed by Colin W. Hiebert
Leaf rust resistance gene Lr22a (1) was introgressed from Aegilops tauschii to chromosome arm 2DS of common wheat. Lr22a is an adult plant resistance (APR) gene that shows a high level of resistance, comparable to seedling resistance gene, which contrasts with the slow-rusting phenotypes of others APR genes, like Lr34. All three Canadian cultivars that carry the Lr22a allele showed leaf rust resistance that is better than the individual genetic components of their resistance. In no case Lr22a was deployed as the only source of leaf rust resistance.
The diploid Aegilops tauschii var. strangulata RL5271 (2n = 14,DD), carrying Lr22a, was previously crossed to the tetraploid Canthatch (2n = 2x = 28, AABB) to produce the synthetic hexaploid RL5404. Later this line was the donor line of Lr22a in backcrossing programs aimed to fine map and find suitable markers for breeding.
Hiebert et al. (2) determined that Xgwm296-2D and Xgwm455 alleles from A. tauschii RL5271 remained associated with Lr22a through at least 13 cycles of recombination in common wheat. The closest locus is Xgwm296-2D, 2.9 cM distal to Lr22a, which yields two amplification products (131 and 121 bp) in RL5271. These alleles were only found in lines carrying Lr22a, while lines lacking this gene amplified a variety of products of different sizes (see below, expected products).
In a collection of Canadian hard-red spring wheat cultivars, those expected to carry Lr22a, including AC Minto, 5500HR, and 5600HR, showed strong resistance to P. triticina in the field from 2002 to 2006. Furthermore, no isolates of P. triticina that are virulent to Lr22a have been found in Canadian virulence surveys between 2002 and 2005 (3).
Lr22a is deployed in the Canadian cultivar AC Minto and putatively in 5500HR and 5600HR. These cultivars have only occupied small percentages (0.13–0.95%) of the wheat production area in Canada from 1998 to 2006. In the US there are no reports of cultivars carrying this gene (JA Kolmer, personal communication). The absence of virulence on Lr22a could be explained in part by its lack of exposure to P. triticina, making it an interesting alternative for gene pyramiding.
Methods for Xgwm296
WMS296-F 5'- AAT TCA ACC TAC CAA TCT CTG -3'
WMS296-R 5'- GCC TAA TAA ACT GAA AAC GAG -3'
- Denaturing step: 94°C, 2 min
- Amplification step (30 cycles):
- 94°C, 1 min
- 55°C, 1 min
- 73°C, 50 sec
- Extension step: 73°C, 5 min
PCR reaction mix:
- PCR buffer 1X
- 0.2 mM each dNTP
- 1.5 mM MgCl2
- 10 pmol each primer
- 1 U Taq DNA polymerase
- 50 ng genomic DNA
PCR products were separated on 5% denaturing polyacrylamide gels in TBE buffer (89 mM tris, 89 mM boric acid, 20 mM EDTA) at 85 W for 2 h, and silver stained.
PCR amplification of Xgwm296-2D in RL5271, the donor of Lr22a, yields two products of 131 and 121 bp. In a screening of more than a hundred lines from different geographical origins a total of 14 alleles were found, the sizes of the 2DS alleles not linked to Lr22a ranged were between 167 and 135 bp. Some of these lines amplify a single fragment while have two 2DS alleles. Therefore, it is important to know the genotypes of the parental lines when using Xgwm296-2D for molecular breeding.
There is another Xgwm296 locus on chromosome 2A; however, it amplifies a larger fragment of 176 +/- 4 bp that does not overlap with any of the alleles of the 2DS locus.
The gel picture on the right shows the Xgwm296 alleles observed in RL5271 (Lr22a donor), RL6044 (Thatcher*7//Tetra-Canthatch/RL5271) which also carries Lr22a, and the recurrent parent Thatcher,
1. Telocentric mapping in hexaploid wheat of genes for leaf rust resistance and other characters derived from Aegilops squarrosa. Rowland GG, Kerber ER. In: Canadian Journal of Genetics and Cytology, 1974, 16:137–144
2. Microsatellite mapping of adult-plant leaf rust resistance gene Lr22a in wheat. Hiebert CW, Thomas JB, Somers DJ, McCallum BD, Fox SL. In: Theoretical and Applied Genetics, 2007, 115:877–884. DOI:10.1007/s00122-007-0604-3
3. Physiologic specialization of wheat leaf rust (Puccinia triticina) in Canada in 2002. McCallum BD, Seto-Goh P. In: Canadian Journal of Plant Pathology, 2005, 27:90–95. DOI:10.1080/07060660509507199