Genome-Wide Association Study of Salt Tolerance at the Seed Germination Stage in Flax (Linum usitatissimum L.).

Soil salinization seriously affects the growth and distribution of flax. However, there is little information about the salt tolerance of flax. In this study, the salt tolerance of 200 diverse flax accessions during the germination stage was evaluated, and then the Genome-wide Association Study (GWAS) was carried out based on the relative germination rate (RGR), relative shoot length (RSL) and relative root length (RRL), whereby quantitative trait loci (QTLs) related to salt tolerance were identified. The results showed that oil flax had a better salt tolerance than fiber flax. A total of 902 single nucleotide polymorphisms (SNPs) were identified on 15 chromosomes. These SNPs were integrated into 64 QTLs, explaining 14.48 to 29.38% (R2) of the phenotypic variation. In addition, 268 candidate genes were screened by combining previous transcriptome data and homologous gene annotation. Among them, Lus10033213 is a single-point SNP repeat mapping gene, which encodes a Glutathione S-transferase (GST). This study is the first to use GWAS to excavate genes related to salt tolerance during the germination stage of flax. The results of this study provide important information for studying the genetic mechanism of salt tolerance of flax, and also provide the possibility to improve the salt tolerance of flax.

Genome-wide analysis of genomic imprinting in the endosperm and allelic variation in flax.

Genomic imprinting is an epigenetic phenomenon that causes biased expression of maternally and paternally inherited alleles. In flowering plants, genomic imprinting predominantly occurs in the triploid endosperm and plays a vital role in seed development. In this study, we identified 248 candidate imprinted genes including 114 maternally expressed imprinted genes (MEGs) and 134 paternally expressed imprinted genes (PEGs) in flax (Linum usitatissimum L.) endosperm using deep RNA sequencing. These imprinted genes were neither clustered in specific chromosomal regions nor well conserved among flax and other plant species. MEGs tended to be expressed specifically in the endosperm, whereas the expression of PEGs was not tissue-specific. Imprinted single nucleotide polymorphisms differentiated 200 flax cultivars into the oil flax, oil-fiber dual purpose flax and fiber flax subgroups, suggesting that genomic imprinting contributed to intraspecific variation in flax. The nucleotide diversity of imprinted genes in the oil flax subgroup was significantly higher than that in the fiber flax subgroup, indicating that some imprinted genes underwent positive selection during flax domestication from oil flax to fiber flax. Moreover, imprinted genes that underwent positive selection were related to flax functions. Thirteen imprinted genes related to flax seed size and weight were identified using a candidate gene-based association study. Therefore, our study provides information for further exploration of the function and genomic variation of imprinted genes in the flax population.

Resequencing 200 Flax Cultivated Accessions Identifies Candidate Genes Related to Seed Size and Weight and Reveals Signatures of Artificial Selection.

Seed size and weight are key traits determining crop yield, which often undergo strongly artificial selection during crop domestication. Although seed sizes differ significantly between oil flax and fiber flax, the genetic basis of morphological differences and artificial selection characteristics in seed size remains largely unclear. Here we re-sequenced 200 flax cultivated accessions to generate a genome variation map based on chromosome assembly reference genomes. We provide evidence that oil flax group is the ancestor of cultivated flax, and the oil-fiber dual purpose group (OF) is the evolutionary intermediate transition state between oil and fiber flax. Genome-wide association studies (GWAS) were combined with LD Heatmap to identify candidate regions related to seed size and weight, then candidate genes were screened based on detailed functional annotations and estimation of nucleotide polymorphism effects. Using this strategy, we obtained 13 candidate genes related to seed size and weight. Selective sweeps analysis indicates human-involved selection of small seeds during the oil to fiber flax transition. Our study shows the existence of elite alleles for seed size and weight in flax germplasm and provides molecular insights into approaches for further improvement.