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

Abstract Partial resistance is generally considered to be a durable form of resistance. In barley, Rphq2, Rphq3 and Rphq4 have been identiWed as consistent quantitative trait loci (QTLs) for partial resistance to the barley leaf rust pathogen Puccinia hordei. These QTLs have been incorporated separately into the susceptible L94 and the partially resistant Vada barley genetic backgrounds to obtain two sets of near isogenic lines (NILs). Previous studies have shown that these QTLs are not eVective at conferring disease resistance in all stages of plant development. In the present study, the two sets of QTL–NILs and the two recurrent parents, L94 and Vada, were evaluated for resistance to P. hordei isolate 1.2.1 simultaneously under greenhouse conditions from the Wrst leaf to the Xag leaf stage. EVect of the QTLs on resistance was measured by development rate of the pathogen, expressed as latency period (LP). The data show that Rphq2 prolongs LP at the seedling stage (the Wrst and second leaf stages) but has almost no eVect on disease resistance in adult plants. Rphq4 showed no eVect on LP until the third leaf stage, whereas Rphq3 is consistently eVective at prolonging LP from the Wrst leaf to the Xag leaf. The changes in the eVectiveness of Rphq2 and Rphq4 happen at the barley tillering stage (the third to fourth leaf stages). These results indicate that multiple disease evaluations of a single plant by repeated inoculations of the fourth leaf to the Xag leaf should be conducted to precisely estimate the eVect of Rphq4. The present study conWrms and describes in detail the plant development-dependent eVectiveness of partial resistance genes and, consequently, will enable a more precise evaluation of partial resistance regulation during barley development