Resistance loci affecting distinct stages of fungal pathogenesis: use of introgression lines for QTL mapping and characterization in the maize - Setosphaeria turcicapathosystem

Abstract

BackgroundStudies on host-pathogen interactions in a range of pathosystems have revealed an array of mechanisms by which plants reduce the efficiency of pathogenesis. While R-gene mediated resistance confers highly effective defense responses against pathogen invasion, quantitative resistance is associated with intermediate levels of resistance that reduces disease progress. To test the hypothesis that specific loci affect distinct stages of fungal pathogenesis, a set of maize introgression lines was used for mapping and characterization of quantitative trait loci (QTL) conditioning resistance to Setosphaeria turcica, the causal agent of northern leaf blight (NLB). To better understand the nature of quantitative resistance, the identified QTL were further tested for three secondary hypotheses: (1) that disease QTL differ by host developmental stage; (2) that their performance changes across environments; and (3) that they condition broad-spectrum resistance.ResultsAmong a set of 82 introgression lines, seven lines were confirmed as more resistant or susceptible than B73. Two NLB QTL were validated in BC4F2 segregating populations and advanced introgression lines. These loci, designated qNLB1.02 and qNLB1.06, were investigated in detail by comparing the introgression lines with B73 for a series of macroscopic and microscopic disease components targeting different stages of NLB development. Repeated greenhouse and field trials revealed that qNLB1.06Tx303 (the Tx303 allele at bin 1.06) reduces the efficiency of fungal penetration, while qNLB1.02B73 (the B73 allele at bin 1.02) enhances the accumulation of callose and phenolics surrounding infection sites, reduces hyphal growth into the vascular bundle and impairs the subsequent necrotrophic colonization in the leaves. The QTL were equally effective in both juvenile and adult plants; qNLB1.06Tx303 showed greater effectiveness in the field than in the greenhouse. In addition to NLB resistance, qNLB1.02B73 was associated with resistance to Stewart's wilt and common rust, while qNLB1.06Tx303 conferred resistance to Stewart's wilt. The non-specific resistance may be attributed to pleiotropy or linkage.ConclusionsOur research has led to successful identification of two reliably-expressed QTL that can potentially be utilized to protect maize from S. turcica in different environments. This approach to identifying and dissecting quantitative resistance in plants will facilitate the application of quantitative resistance in crop protection.

Unraveling Genomic Complexity at a Quantitative Disease Resistance Locus in Maize

“…In a QTL study of the recombinant inbred line (RIL) population Ki14 3 B73 evaluated for three foliar fungal diseases, NLB, GLS, and SLB, a 33-Mb region spanning bins 1.05 and 1.06 was the only locus identified that conferred resistance to all three diseases (Zwonitzer et al 2010). A number of QTL studies for NLB resistance in maize have identified QTL at bin 1.06, ranging in physical size from 3 to 30 Mb (Freymark et al 1993;Welz et al 1999;Wisser et al 2006;Chung et al 2010bChung et al , 2011Van Esbroeck et al 2010;Poland et al 2011). Additionally, bin 1.06 harbors the dominant Stewart's wilt resistance gene Sw1 (Ming et al 1999).…”

“…The importance of genes localized to maize bin 1.06 in resistance to both NLB and Stewart's wilt has been described in multiple mapping populations. Using a population of Tx303 3 B73 introgression lines (Szalma et al 2007), Chung et al (2010b showed that the NLB resistance QTL at 1.06 protects against fungal penetration.To explore the genomic complexity of this important region, we constructed high-resolution mapping populations at this locus and evaluated NLB and Stewart's wilt resistance, using a set of Tx303 3 B73 near-isogenic lines (NILs) (Szalma et al 2007;Chung et al 2010b). Fine mapping allowed us to dissect the linkage relationship between the major-effect Stewart's wilt QTL and the minor-effect NLB QTL and to identify candidate genes.…”

“…The importance of genes localized to maize bin 1.06 in resistance to both NLB and Stewart's wilt has been described in multiple mapping populations. Using a population of Tx303 3 B73 introgression lines (Szalma et al 2007), Chung et al (2010b showed that the NLB resistance QTL at 1.06 protects against fungal penetration.…”

“…These studies have consistently implicated the region spanning from 180 to 205 Mb. Lines carrying a Tx303 introgression at this interval were found to be associated with resistance to NLB and Stewart's wilt (families TBBC3-38 and TBBC-39 of the TBBC3 population) (Chung et al 2010b). Based on genetic background and seed availability, TBBC3-38_19E, TBBC3-38_15A, and TBBC3-38_17G were selected for fine mapping.…”

Association mapping and genetic control for northern leaf blight (Exserohilum turcicum) resistance in maize lines

“…In the present study, we used QTLs on these chromosomes, as well as a QTL on chromosome 3 (Ogliari et al, 2007) and a QTL on chromosome 8 (Chung et al, 2010a;2010b;Wang et al, 2012).…”