Locating QTL conferring resistance against net blotch, leaf rust, and stripe rust in the wild barley nested association mapping (NAM) population HEB-25

Abstract

Net blotch, caused by the fungus Pyrenophora teres f. teres, Puccinia hordei, causing leaf rust, and Puccinia striiformis f. sp. hordei, the causal agent of stripe rust, are important fungal diseases of barley with the potential to cause severe yield losses and a reduction in feed and malting quality. The identification of QTL conferring resistance is the basis for targeted breeding approaches aiming to improve resistance of modern barley cultivars against these fungi. Initial domestication from its wild progenitor Hordeum vulgare subsp. spontaneum and extensive breeding resulted in a loss of genetic diversity in modern elite barley varieties. Many modern barley varieties are similar in their genotype resulting in an increased risk of occurrence of severe epidemics. There is an urgent need to broaden the genetic basis of resistance of modern barley cultivars to ensure a stable production. Wild barley accessions possessing high allelic richness have shown to be a valuable source of resistance alleles no longer present in the current breeding pool. In the framework of this thesis, the NAM population HEB-25 was utilized to detect QTL conferring resistance against net blotch, leaf rust, and stripe rust. Screening of HEB-25 in two-year field trials revealed the presence of a high genetic diversity within HEB-25 and allowed for the identification of HEB-lines with a high degree of resistance to one or more of the evaluated fungi. NAM applying the 9k iSelect barley chip performed independently for all three fungi resulted in the identification of a high number of QTL conferring resistance on all chromosomes with predominantly small effect. In case of net blotch six QTL, eight QTL for stripe rust and two QTL for leaf rust, are considered novel showing no overlap with previously reported resistance QTL. Estimation of parent-specific QTL effects indicates the presence of alleles with increasing or decreasing effect on genotype resistance, respectively. Leucine-rich repeat, NB-ARC, and serine/threonine-protein kinase-like genes were found at high frequency in the QTL intervals and due to their important role in plant defense response represent putative candidate genes causing the QTL effect. HEB-25 showed to be well suited for the detection of QTL conferring resistance to net blotch, leaf rust, or stripe rust and represents a valuable source for improving genetic diversity and resistance of modern barley cultivars against these fungi.