Genome-wide association and targeted transcriptomic analyses reveal loci and candidate genes regulating preharvest sprouting in barley

KEY MESSAGE

Genome-wide association study of diverse barley genotypes identified loci, single nucleotide polymorphisms and candidate genes that control seed dormancy and therefore enhance resistance to preharvest sprouting. Preharvest sprouting (PHS) causes significant yield and quality loss in barley and it is strongly associated with the level of seed dormancy. This study performed genome-wide association study using a collection of 255 diverse barley genotypes grown over four environments to identify loci controlling dormancy/PHS. Our phenotypic analysis revealed substantial variation in germination index/dormancy levels among the barley genotypes. Marker-trait association and linkage disequilibrium (LD) decay analyses identified 16 single nucleotide polymorphisms (SNPs) and two QTLs associated with dormancy/PHS, respectively, on chromosome 3H and 5H explaining 6.9% to 11.1% of the phenotypic variation. QTL.5H consist of 14 SNPs of which 12 SNPs satisfy the FDR threshold of α = 0.05, and it may represent the SD2 locus. The QTL on 3H consists of one SNP that doesn't satisfy FDR (α = 0.05). Genes harbouring the significant SNPs were analyzed for their expression pattern in the seeds of selected dormant and non-dormant genotypes. Of these genes, HvRCD1, HvPSRP1 and HvF3H exhibited differential expression between the dormant and non-dormant seed samples, suggesting their role in controlling seed dormancy/PHS. Three SNPs located within the differentially expressed genes residing in QTL.5H explained considerable phenotypic variation (≥ 8.6%), suggesting their importance in regulating PHS resistance. Analysis of the SNP marker data in QTL.5H identified a haplotype for PHS resistance. Overall, the study identified loci, SNPs and candidate genes that control dormancy and therefore play important roles in enhancing PHS resistance in barley through marker-assisted breeding.

QTL x environment modeling of malting barley preharvest sprouting

HvMKK3 alleles are temperature sensitive and are major contributors to environmental stability of preharvest sprouting in barley. Preharvest sprouting (PHS) can severely damage barley (Hordeum vulgare L.) malting quality, but PHS resistance is often negatively correlated with malting quality. Seed dormancy is closely related to PHS. Increased temperature during grain fill can decrease seed dormancy in barley, but genetic components of seed dormancy temperature sensitivity are poorly understood. Six years of PHS data were used to fit quantitative trait locus (QTL) x environment mixed models incorporating marker data from seed dormancy genes HvAlaAT1, HvGA20ox1, and HvMKK3 and weather covariates in spring and winter two-row malting barley. Variation in winter barley PHS was best modeled by average temperature range during grain fill and spring barley PHS by total precipitation during grain fill. Average high temperature during grain fill also accurately modeled PHS for both datasets. A highly non-dormant HvMKK3 allele determined baseline PHS susceptibility and HvAlaAT1 interactions with multiple HvMKK3 alleles conferred environmental sensitivity. Polygenic variation for PHS within haplotype was detected. Residual genotype and QTL by environment interaction variance indicated additional environmental and genetic factors involved in PHS. These models provide insight into genotype and environmental regulation of barley seed dormancy, a method for PHS forecasting, and a tool for breeders to improve PHS resistance.

Quantitative Trait Loci and Maternal Effects Affecting the Strong Grain Dormancy of Wild Barley (Hordeum vulgare ssp. spontaneum)

Wild barley (Hordeum vulgare ssp. spontaneum) has strong grain dormancy, a trait that may enhance its survival in non-cultivated environments; by contrast, cultivated barley (Hordeum vulgare ssp. vulgare) has weaker dormancy, allowing uniform germination in cultivation. Malting barley cultivars have been bred for especially weak dormancy to optimize their use in malt production. Here, we analyzed the genetic mechanism of this difference in seed dormancy, using recombinant inbred lines (RILs) derived from a cross between the wild barley accession 'H602' and the malting barley cultivar 'Kanto Nakate Gold (KNG)'. Grains of H602 and KNG harvested at physiological maturity and dried at 30°C for 7 days had germination of approximately 0 and 100%, respectively. Analysis of quantitative trait loci (QTL) affecting grain dormancy identified the well-known major dormancy QTL SD1 and SD2 (located near the centromeric region and at the distal end of the long arm of chromosome 5H, respectively), and QTL at the end of the long arm of chromosome 4H and in the middle of the long arm of chromosome 5H. We designated these four QTL Qsd1-OK, Qsd2-OK, Qsdw-4H, and Qsdw-5H, and they explained approximately 6, 38, 3, and 13% of the total phenotypic variation, respectively. RILs carrying H602 alleles showed increased dormancy levels for all QTL. The QTL acted additively and did not show epistasis or QTL-environment interactions. Comparison of QTL locations indicated that all QTL except Qsdw-5H are likely the same as the QTL previously detected in the doubled haploid population from a cross between the malting cultivar 'Haruna Nijo' and 'H602.' We further examined Qsd2-OK and Qsdw-5H by analyzing the segregation of phenotypes and genotypes of F₂ progenies derived from crosses between RILs carrying specific segments of chromosome 5H from H602 in the KNG background. This analysis confirmed that the two genomic regions corresponding to these QTL are involved in the regulation of grain dormancy. Germination tests of F₁ grains derived from reciprocal crosses between H602 and KNG revealed that the H602 strong dormancy phenotype shows maternal inheritance with incomplete dominance. These results provide new insight into the mechanisms regulating grain dormancy in barley.