Mapping Quantitative Trait Loci for Slow-rusting Traits in Wheat

Leaf rust of wheat is caused by Puccinia triticina. Slow rusting is an important component of durable resistance to leaf rust and often expressed in the form of long latent period, and low infection type and severity. Selecting for slow rusting traits is an effective approach to improve durability of rust resistance in wheat cultivars. The objective of this study was to identify quantitative trait loci (QTLs) associated with the slow rusting traits in U.S. wheat CI13227 using DNA markers. A population of recombinant inbred lines (RILs) was developed from the cross of CI 13227/Suwon and evaluated for final rust severity (FRS), area under disease progress curve (AUDPC), infection type (IT), and latent period (LP) in a field (Purdue University IN) and a greenhouse (Kansas State University, KS) experiment.  The population was analyzed with single-nucleotide polymorphism (SNP) markers derived from genotyping-by-sequence. A total of 1677 SNP and 90 simple sequence repeat (SSR) markers were analyzed in the population. Among the RILs, the frequency distributions for the slow rust traits were continuous, and significant correlations were observed between the greenhouse and field experiments for all the traits (r = 0.44 to 0.66, p<0.001). A common QTL with the largest effect for all slow-rusting traits measured was detected on chromosome 3B in both the field and greenhouse experiments, and this QTL explained 11 to 37% of phenotypic variations for different traits. In the field experiment, two QTLs with a minor effect on 2B and 7B were significant for all traits and one minor QTL on chromosome 2A was significant for only FRS. In the greenhouse experiment, QTL on chromosome 4A was significant for all traits except infection type, which explained 11 to 16 % of phenotypic variations for different traits. The QTL on 3B showed a consistent major effect in both field and greenhouse experiments, thus it is the most important QTL for slow rusting in CI13227. DNA markers closely associated with the QTL have potential to be used in marker-assisted selection (MAS) for slow rusting traits in breeding programs.