The Swiss winter bread wheat cultivar "Forno" has been showing near immune levels of leaf rust resistance since its release in 1986. At least six different loci contribute to this phenotype (1). The largest fraction of the phenotipic variation (33-43%) is explained by Lr34. Regretfully, this gene is associated with leaf tip necrosis, an undesirable trait in Western Europe breeding programs. The same authors determined that the second largest source of variation (28–32%) was due to a loci on chromosome arm 1BS, named QLr.sfr-1BS (1). This QTL is not linked to leaf tip necrosis and provides partial adult plant resistance (APR).
Later, Singla et al. (2) found that QLr.sfr-1BS is a new leaf rust resistance gene and designated it Lr75. Additionally, they mapped the gene with greater resolution in a 4.3 cM interval flanked by the SSR loci gwm604 and swm271. Historically, several APR genes have been more durable than all-stage resistance genes. But since Lr75 provides a partial resistance, it is an interesting target for stacking with other APR genes.
Lr75 is located on chromosome arm 1BS, between the distal marker gwm604 (1.6 cm) and proximal marker swm271 (2.7 cM).Primers sequences:
swm271-F1 5'- GTC CAT TCG GCG CTA GAT CG -3' swm271-R1 5'- CTG GCT CCG GCA CCT TAT CA -3' gwm604-F 5'- TAT ATA GTT CAA TAT GAC CCG -3' gwm604-R 5'- ATC TTT TGA ACC AAA TGT G -3'
Marker swm271 was first reported by Singla et al. (2), while gwm604 has been in use for a longer time, and its alleles are known to vary across germplasm (see for example here). In consequence, potential users are strongly advised to check these markers in their germplasm of interest before using them in their breeding programs. In case of lack of polymorphism, there are other markers mapping further away in both directions from Lr75, but their mapping distances make them less attractive for breeding (see fig 4. of Ref. 2).
Conditions presented here should be considered only as a starting point of the PCR optimization for individual laboratories.
1. Dissection of quantitative and durable leaf rust resistance in Swiss winter wheat reveals a major resistance QTL in the Lr34 chromosomal region. Schnurbusch T, Paillard S, Schori A, Messmer M, Schachermayr G, Winzeler M, Keller B. In: Theoretical and Applied Genetics, 2004, 108:477–484. DOI: 10.1007/s00122-003-1444-4.
2. Characterization of Lr75: a partial, broad-spectrum leaf rust resistance gene in wheat. Singla J, Lüthi l, Wicker T, Bansal U, Krattinger SG, Keller B. In: Theoretical and Applied Genetics, 2017, 130:1-12.