Abstract
The fungus Parastagonospora nodorum is the causal agent of septoria nodorum leaf blotch (SNB) and glume blotch which are common in many wheat growing regions in the world. The disease is complex and could be explained by multiple interactions between necrotrophic effectors secreted by the pathogen and matching susceptibility genes in wheat. An Australian P. nodorum population was clustered into five groups with contrasting properties. This study was set to identify their pathogenicity profiles using a diverse wheat panel of 134 accessions which are insensitive to SnToxA and SnTox1 in both in vitro and in vivo conditions. SNB seedling resistance/susceptibility to five representative isolates from the five clusters, responses to crude culture-filtrates (CFs) of three isolates and sensitivity to SnTox3 semi-purified effector together with 11,455 SNP markers have been used for linkage disequilibrium (LD) and association analyses. While quantitative trait loci (QTL) on 1D, 2A, 2B, 4B, 5B, 6A, 6B, 7A, 7D chromosomes were consistently detected across isolates and conditions, distinct patterns and isolate specific QTL were also observed among these isolates. In this study, SnTox3–Snn3-B1 interaction for the first time in Australia and SnTox3–Snn3-D1 interaction for the first time in bread wheat were found active using wild-type isolates. These findings could be due to new SnTox3 haplotype/isoform and exotic CIMMYT/ICARDA and Vavilov germplasm used, respectively. This study could provide useful information for dissecting novel and different SNB disease components, helping to prioritise research targets and contributing valuable information on genetic loci/markers for marker-assisted selection in SNB resistance wheat breeding programme.
Highlights
This Snn3-D1 receptor has never been identified on the D sub-genome of hexaploid wheat with the exception of some synthetic hexaploid wheats and SnTox3–Snn3-B1 interaction has never been detected in Australian conditions
quantitative trait locus (QTL) mapping suffers from limited power in detecting small QTL effects[38], limited variation which exists between the two parental lines, as well as large QTL intervals which depend on the population sizes and recombination frequencies[39]
To facilitate the septoria nodorum leaf blotch (SNB) resistance breeding processes and avoid laborious and time-consuming phenotyping, molecular markers linked to resistance or susceptibility genes/QTL or strong marker-trait-associations (MTAs) detected from Genome-wide association studies (GWAS) analysis should be used for marker-assisted selection and gene pyramiding
Summary
The disease occurs due to multiple interactions between proteinaceous necrotrophic effectors (NEs) secreted by P. nodorum and corresponding dominant susceptibility genes in the host (Snn)[6,7] Effects from these interactions are normally additive[8,9,10], epistasis between interactions has been frequently o bserved[10,11,12,13,14]. For SNB seedling resistance/susceptibility, numerous QTL identified on 19 of the 21 wheat chromosomes by genetic mapping have been r eported[33]. The objectives of the current study are to evaluate the pathogenicity profiles of recent Australian P. nodorum representatives from the five identified groups and to identify resistance/susceptibility sources from a wheat panel selected from CIMMYT/ICARDA and Vavilov collections[42] which are insensitive to ToxA and SnTox[1]. Information on markers tightly linked to the identified resistance will be obtained for further evaluation of their uses in marker-assisted selection programs
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