Contributed by Gina Brown-Guedira and Suhkwinder Singh
Host plant resistance is an economical and environmentally sound method of control of leaf rust, caused by the fungus Puccinia triticina Eriks, which is one of the most serious diseases of wheat (Triticum aestivum L.) worldwide. Wild relatives of wheat, including the tetraploid Triticum timopheevii ssp. armeniacum (hereafter referred to as T. armeniacum), represent an important source of genes for resistance to leaf rust (3).
Mapping: A leaf rust resistance gene transferred to germplasm line KS96WGRC36 (TAM 107*4/ TA 870) from TA 870 of T. armeniacum was linked to microsatellite markers Xgwm382 (6.7 cM) and Xgdm87 (9.4 cM) on wheat chromosome arm 2BL. This new leaf rust resistance gene is designated Lr50.
Markers for Lr50
Lr50 is flanked by to microsatellite markers Xgwm382 (6.7 cM) and Xgdm87 (9.4 cM) on wheat chromosome arm 2B long.
Primers for GWM382 (2)
WMS382-F 5'- GTC AGA TAA CGC CGT CCA AT -3' WMS382-R 5'- CTA CGT GCA CCA CCA TTT TG -3'
Primers for GDM87 (1)
GDM87-left 5'- AAT AAT GTG GCA GAC AGT CTT GG -3'
GDM87-right 5'- CCA AGC CCC AAT CTC TCT CT -3'
Components of PCR reaction
Final volume: 25µl reactions containing,
- 50 ng genomic DNA
- 12.5 pmol each primer
- 0.2 mM each dNTP
- 0.625 U Taq
- 1x PCR buffer
- 2 mM MgCl2
PCR conditions (for both sets of primers):
- Denaturing step: 4 min at 94°C
- Amplification step: (30 cycles)
- 30 s at 94°C
- 30 s at 60°C
- 30 s at 72°C
- Extension step: 5 min at 72°C.
PCR products are resolved on 2.3% agarose gels (GeneMate GenePure HiRes Agarose - ISC BioExpress Cat# E-3115-500). Some susceptible cultivars have microsatellite bands of similar mobility as the resistant germplasm. Therefore, a preliminary screening is necessary to test the existence of polymorphism between the donor and the selected recurrent parents. For example, see the bands produced by line TAM 107 for Xgdm87 in panel B.
The Lr50 gene has been transferred from four accessions of T. armeniacum, TA870, TA874, TA895 and TA145 to three hard winter wheat backgrounds, Karl 92 (red), TAM 107 (red) and Arlin (white). The Iine U2657 (Karl92*3/TA874) is being used as the Lr50germplasm for backcrossing. This line has not been formally released but is available upon request from Gina Brown-Guedira
Additional information. Field testing
Virulence to Lr50 exists in races of P. triticina in North America. The races PNMQ and MBRL are virulent on seedlings of lines having Lr50 . However, low to intermediate infections types have been observed on adult plants of wheat having this gene when evaluated for multiple years in the field, indicating that virulence is not common in the southern U.S. Great Plains. Selection pressure on the pathogen imposed by deployment of genes such as Lr9, Lr24, and Lr41 that are also ineffective against the PNMQ race of P. triticina may result in an increase in virulence to Lr50 in the region. This will limit the usefulness of this gene unless it is deployed in combination with other effective genes. Identification of microsatellite markers linked to Lr50 provides a tool to incorporate this gene into pyramids that include other effective resistance genes.
1. Isolation and mapping of microsatellite markers specific for the D genome of bread wheat. Pestsova E, Ganal MW, Röder M.S. In: Genome, 2000, 43(4)689-697. DOI: 10.1139/g00-042
2. A microsatellite map of wheat . Röder MS, Korzun V, Wendehake K, Plashke J, Tixier MH, Leroy P, Ganal MW. In: Genetics, 1998, 149:2007-2023. [Journal link]
3. Performance and mapping of leaf rust resistance transferred to wheat from Triticum timopheevii ssp. armeniacum. Brown-Guedira GL, Singh S, Fritz AK . In: Phytopathology, 2003, 93-784-789. DOI: 10.1094/PHYTO.2003.93.7.784