Isolate specificity of quantitative trait loci for partial resistance of barley to Puccinia hordei confirmed in mapping populations and near‐isogenic lines

Abstract

Summary• Partial resistance is considered race-nonspecific and durable, consistent with the concept of 'horizontal' resistance. However, detailed observations of partial resistance to leaf rust (Puccinia hordei) in barley (Hordeum vulgare) revealed small cultivar × isolate interactions, suggesting a minor-gene-for-minor-gene interaction model, similar to so-called 'vertical' resistance.• Three consistent quantitative trait loci (QTLs), labelled Rphq2, Rphq3 and Rphq4, that were detected in the cross susceptible L94 × partially resistant Vada have been incorporated into the L94 background to obtain near-isogenic lines (NILs). Three isolates were used to map QTLs on seedlings of the L94 × Vada population and to evaluate the effect of each QTL on adult plants of the respective NILs under field conditions.• Rphq2 had a strong effect in seedlings but almost no effect in adult plants, while Rphq3 was effective in seedlings and in adult plants against all three isolates. However, Rphq4 was effective in seedlings and in adult plants against two isolates but ineffective in both development stages against the third, demonstrating a clear and reproducible isolate-specific effect. The resistance governed by the three QTLs was not associated with a hypersensitive reaction.• Those results confirm the minor-gene-for-minor-gene model suggesting specific interactions between QTLs for partial resistance and P. hordei isolates.

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

“…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.…”

“…The susceptible line L94, the Rphq3 is consistently eVective in both seedlings and adult plants. In the L94 £ Vada mapping population, also Qi et al (1998) and Marcel et al (2008) found a strong eVect of Rphq2, a medium eVect of Rphq3 and no or hardly signiWcant eVect of Rphq4 in the seedling stage. In adult plants, Rphq2 was reported to give a very low and just signiWcant eVect in both greenhouse and Weld test (Qi et al 1998) if measured in the mapping population.…”

“…In adult plants, Rphq2 was reported to give a very low and just signiWcant eVect in both greenhouse and Weld test (Qi et al 1998) if measured in the mapping population. In a Weld test on the QTL-NILs, Rphq2 also gave a small eVect on infection severity (Marcel et al 2008), which varied according to the isolate tested. In the QTL-NILs test by Marcel et al (2008), the eVect of Rphq4 tended to be smaller than that of Rphq3, in contrast with results on the mapping population reported by Qi et al (1998).…”

“…In a Weld test on the QTL-NILs, Rphq2 also gave a small eVect on infection severity (Marcel et al 2008), which varied according to the isolate tested. In the QTL-NILs test by Marcel et al (2008), the eVect of Rphq4 tended to be smaller than that of Rphq3, in contrast with results on the mapping population reported by Qi et al (1998). This indicates that phenotypic expression of Rphq4 may not be consistently expressed.…”

“…As a consequence, pairs of NILs are more eYcient, and allow one to use more experimental replications compared with mapping populations, as fewer plants are required per experiment. 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).…”