Dissection of the Barley 2L1.0 Region Carrying the ‘Laevigatum’ Quantitative Resistance Gene to Leaf Rust Using Near-Isogenic Lines (NIL) and subNIL

Abstract

Partial resistance to leaf rust (Puccinia hordei G. H. Otth) in barley is a quantitative resistance that is not based on hypersensitivity. This resistance hampers haustorium formation, resulting in a long latency period in greenhouse tests. The three most consistent quantitative trait loci (QTL) uncovered in the L94 x 'Vada' mapping population were introgressed by marker-assisted backcrossing into the susceptible L94 background to obtain near-isogenic lines (NIL). We also developed the reciprocal Vada-NIL for the susceptibility alleles of those QTL. The QTL Rphq2 affected latency period of P. hordei more than the QTL Rphq3 and Rphq4. The NIL confirmed the contribution of Rphq2 to partial resistance by prolonging the latency period by 28 h on L94-Rphq2 and shortening the latency period by 23 h on Vada-rphq2. On the basis of flanking restriction fragment length polymorphism-based markers, Rphq2 appeared to be located near the telomeric end of the long arm of chromosome 2H, in a physical region of high recombination, making it the target QTL for map-based cloning. Microscopic observations on the NIL confirmed the nonhypersensitive nature of the resistance conferred by Rphq2. A high-resolution genetic map of the Rphq2 region was constructed using a population of 38 subNIL with overlapping L94 introgressions in Vada background across the region. Rphq2 mapped approximately 2 centimorgans (cM) proximal from the MlLa locus. By bulked segregant analysis and use of synteny with rice, we developed additional markers and fine-mapped Rphq2 to a genetic interval of 0.11 cM that corresponds to a stretch of sequence of, at most, 70 kb in rice. Analysis of this rice sequence revealed predicted genes encoding two proteins with unknown function, retrotransposon proteins, peroxidase proteins, and a protein similar to a mitogen-activated protein kinase kinase kinase (MAP3K). Possible homologs of those peroxidases and MAP3K in barley are candidates for the gene that contributes to partial resistance to P. hordei.

The phenotypic expression of QTLs for partial resistance to barley leaf rust during plant development

“…L94-Rphq2, L94-Rphq3 and L94-Rphq4 contained Vada introgressed fragments of 4.6, 22.6 and 10.8 centiMorgans (cM), respectively. Vada-rphq2, Vadarphq3 and Vada-rphq4 contained L94 introgressed fragments of 5.2, 45.8 and 12.7 cM, respectively (Van Berloo et al 2001;Marcel et al 2007). In each NIL, the target QTL was the major source of genetic variation for partial resistance to leaf rust because of the absence of other segregating QTLs.…”

“…This has a distinct advantage in, for example, Weld evaluations (Marcel et al 2008). QTL-NILs are further instrumental in Wne mapping of the QTL responsible for the phenotypic eVect (Marcel et al 2007). We set out to use the QTL-NILs to characterize the dependence of each QTL on the host plant growth stage.…”

“…Recently, the Vada alleles of the three largest-eVect QTLs from the above-described study were introgressed into the susceptible L94 line, and the L94 alleles were introgressed into the resistant cultivar Vada by marker-assisted backcrossing to obtain NILs (Van Berloo et al 2001;Marcel et al 2007). The QTL eVects found in the L94 £ Vada mapping population were consistently conWrmed in the two complementary sets of NILs, including their dependence on plant developmental stage (Marcel et al 2007(Marcel et al , 2008.…”

Selecting a set of wild barley introgression lines and verification of QTL effects for resistance to powdery mildew and leaf rust

“…So far, no report is available on the verification of QTL data by a complete or nearly complete barley IL set. However, there are several studies that confirmed QTL effects for powdery mildew resistance genes (Czembor and Czembor 2004) and leaf rust resistance QTLs (van Berloo et al 2001;Marcel et al 2007Marcel et al , 2008 by NILs. Bernacchi et al (1998b) pointed out that the verification of QTLs with NILs can be characterized with regard to the stability of the QTL effect across environments and the magnitude of the effect.…”

Marker assisted separation of resistance genes Rph22 and Rym16 Hb from an associated yield penalty in a barley: Hordeum bulbosum introgression line

“…For barley leaf rust (Puccinia hordei Otth. ), there have been twenty quantitative trait loci (QTL) uncovered which contribute to this 'slow rusting' type resistance (Marcel et al 2007(Marcel et al , 2008Qi et al 1998Qi et al , 2000. The best understood 'slow rusting' system is from the cultivar 'Vada', which when crossed with the susceptible line 'L94' resulted in the identification and genetic mapping of six QTL conditioning this response (Qi et al 1998).…”