Genomics of drought tolerance in flax (Linum usitatissimum L.)

Flax (Linum usitatissimum L.), one of the eight founder crops of agriculture, is a versatile plant that has been a source of fiber and oil for millennia. Flax production, however, is constrained by various factors, including moisture stress. Yet, variations in the level of tolerance to different moisture regimes exists among flax germlines, thereby providing opportunities for breeders.
To understand the global scale eco-geographic adaptation and to further gain insight into the genetic architecture of drought-related traits leading towards marker-assisted breeding and/or genomic selection in flax, a genome-wide investigation using high density single nucleotide polymorphism (SNP) data was performed in two sets of flax genotypes: a core (n=407) and a mini-core (n=115) collection, both representing the major world flax growing regions. The core collection was used to demonstrate haplotype distribution and possible driving forces that have shaped the population structure of the crop. The mini-core was used to perform genotype-phenotype association for drought-related traits through genome-wide association studies (GWAS) based on phenotypic values generated in two experiments. In the first experiment, 16 early root and shoot traits were evaluated using a semi-hydroponic pouch system in a controlled environment, while in the second experiment 11 drought-related traits were evaluated under irrigated and non-irrigated conditions. For the latter, GWAS was performed using the adjusted values of six of the measured traits that significantly responded to the watering regime, as well as six calculated stress indices for each trait.
The genetic structure analysis clustered the core collection into four major groups: Temperate, South Asian, Mediterranean and Abyssinian. The Temperate group comprised the majority of the core collection and was dominated by accessions from Eurasia, the Americas and Oceania. Despite their limited representation, the other three groups harbored a high concentration of endemic haplotypes that can be attributed to the long history of flax cultivation
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in these regions hypothesized to be the centers of diversification the crop. More in-depth samplings from these regions is thus expected to broaden the genetic diversity of the flax core collection substantially. Genetic variation within morphotypes is more pronounced than between the fiber and oil types, reflecting selection of genotypes for dually elite traits i.e. both for seed and fiber. The overall result suggests the contribution of use-directed human selection and eco-geographic variations along the latitudinal gradient as major forces in shaping the genetic structure of the crop.
The two GWAS experiments, i.e., early root-shoot growth and field-drought-tolerance-related traits, yielded 228 and 144 quantitative trait nucleotides (QTNs), respectively, that were associated with at least one trait. Most loci, 100 kb up- and downstream of the associated QTN, harbored genes predicted to play roles in modulating the associated trait or traits. In the root-shoot GWAS, genes predicted to encode GRAS transcription factors, mitogen-activated protein kinases (MAPKs), and auxin related lateral organ boundary proteins were present at root trait QTN loci, while QTN loci associated with shoot traits mainly harbored genes involved in photo-morphogenesis and plant immunity. QTN loci of drought tolerant traits consistently harbored genes known to mediate responses to abiotic stress, including drought, heat and salt, suggesting the importance of these loci for stress tolerance to be considered as candidate QTL of the traits associated with.