Abstract
BackgroundSclerotinia sclerotiorum is a necrotrophic fungus that causes Sclerotinia head rot (SHR) in sunflower, with epidemics leading to severe yield losses. In this work, we present an association mapping (AM) approach to investigate the genetic basis of natural resistance to SHR in cultivated sunflower, the fourth most widely grown oilseed crop in the world.ResultsOur association mapping population (AMP), which comprises 135 inbred breeding lines (ILs), was genotyped using 27 candidate genes, a panel of 9 Simple Sequence Repeat (SSR) markers previously associated with SHR resistance via bi-parental mapping, and a set of 384 SNPs located in genes with molecular functions related to stress responses. Moreover, given the complexity of the trait, we evaluated four disease descriptors (i.e, disease incidence, disease severity, area under the disease progress curve for disease incidence, and incubation period). As a result, this work constitutes the most exhaustive AM study of disease resistance in sunflower performed to date.Mixed linear models accounting for population structure and kinship relatedness were used for the statistical analysis of phenotype-genotype associations, allowing the identification of 13 markers associated with disease reduction. The number of favourable alleles was negatively correlated to disease incidence, disease severity and area under the disease progress curve for disease incidence, whereas it was positevily correlated to the incubation period.ConclusionsFour of the markers identified here as associated with SHR resistance (HA1848, HaCOI_1, G33 and G34) validate previous research, while other four novel markers (SNP117, SNP136, SNP44, SNP128) were consistently associated with SHR resistance, emerging as promising candidates for marker-assisted breeding. From the germplasm point of view, the five ILs carrying the largest combination of resistance alleles provide a valuable resource for sunflower breeding programs worldwide.
Highlights
Sclerotinia sclerotiorum is a necrotrophic fungus that causes Sclerotinia head rot (SHR) in sunflower, with epidemics leading to severe yield losses
A total of 27 Candidate genes (CGs) were suitable for amplification and direct sequencing in the eight sunflower inbred breeding lines (ILs) used as a core set (CS) for initial polymorphism discovery
Unlike previous association mapping (AM) approaches to SHR, that investigated from eight to sixteen CGs [17, 18], here we evaluated 27 CGs, a panel of nine Simple Sequence Repeat (SSR) previously associated with SHR resistance via bi-parental mapping and a set of 384 SNPs located in genes related to biotic and abiotic stress responses, such as hypersensitive response, jasmonic-acid and auxin mediated signaling pathways, response to oxidative stress, calcium associated genes, germin-like proteins, among others [9]
Summary
Sclerotinia sclerotiorum is a necrotrophic fungus that causes Sclerotinia head rot (SHR) in sunflower, with epidemics leading to severe yield losses. A number of genome-wide association studies resulted in the identification of loci related to flowering time and plant architecture in sunflower [19,20,21], their power to detect associations for more complex traits, such as quantitative disease resistance, has not been exploited yet. In this context, the use of a larger number of candidate genes by means of medium to high-throughput technologies may serve as a suitable stepping-stone for future large-scale exploration of defense responses to fungal pathogens. According to Neupane et al [23], sunflower chromosome 13, followed by chromosomes 9, 4 and 2, contains the highest number of nucleotide Binding Site—Leucine-Rich Repeat (NBS-LRR) genes, which encode disease resistance proteins involved in plant defense
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