Flax has been cultivated for its oil and fiber for thousands of years. However, it remains unclear how the modifications of agronomic traits occurred on the genetic level during flax cultivation. In this study, we conducted genome-wide variation analyses on multiple accessions of oil-use, fiber-use, landraces, and pale flax to identify the genomic variations during flax cultivation. Our findings indicate that, during flax domestication, genes relevant to flowering, dehiscence, oil production, and plant architecture were preferentially selected. Furthermore, regardless of origins, the improvement of the modern oil-use flax preceded that of the fiber-use flax, although the dual selection on oil-use and fiber-use characteristics might have occurred in the early flax domestication. We also found that the expansion of MYB46/MYB83 genes may have contributed to the unique secondary cell wall biosynthesis in flax and the directional selections on MYB46/ MYB83 may have shaped the morphological profile of the current oil-use and fiber-use flax.

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Auxin response factors (ARFs) are critical components of the auxin signaling pathway, and are involved in diverse plant biological processes. However, ARF genes have not been investigated in flax (Linum usitatissimum L.), an important oilseed and fiber crop. In this study, we comprehensively analyzed the ARF gene family and identified 33 LuARF genes unevenly distributed on the 13 chromosomes of Longya-10, an oil-use flax variety. Detailed analysis revealed wide variation among the ARF family members and predicted nuclear localization for all proteins. Nineteen LuARFs contained a complete ARF structure, including DBD, MR, and CTD, whereas the other fourteen lacked the CTD. Phylogenetic analysis grouped the LuARFs into four (I–V) clades. Combined with sequence analysis, the LuARFs from the same clade showed structural conservation, implying functional redundancy. Duplication analysis identified twenty-seven whole-genome-duplicated LuARF genes and four tandem-duplicated LuARF genes. These duplicated gene pairs’ Ka/Ks ratios suggested a strong purifying selection pressure on the LuARF genes. Collinearity analysis revealed that about half of the LuARF genes had homologs in other species, indicating a relatively conserved nature of the ARFs. The promoter analysis identified numerous hormone- and stress-related elements, and the qRT-PCR experiment revealed that all LuARF genes were responsive to phytohormone (IAA, GA3, and NAA) and stress (PEG, NaCl, cold, and heat) treatments. Finally, expression profiling of LuARF genes in different tissues by qRT-PCR indicated their specific functions in stem or capsule growth. Thus, our findings suggest the potential functions of LuARFs in flax growth and response to an exogenous stimulus, providing a basis for further functional studies on these genes.

Flaxseed, which is rich in lignan, α-linolenic acid, dietary fiber, and several minerals, is an important food and nutrition source. In this study, trials were conducted at Yongdeng over two consecutive years (2018 and 2019), with three cultivars (Longyaza 1, Longya 14, and Zhangya 2) and five application rates of phosphorus (P) (0, 40, 80, 120, and 160 kg P2O5 ha−1). We examined the effects of P on the seed yield, and the yields and contents of dietary fiber, lignan, iron (Fe), zinc (Zn), manganese (Mn), and copper (Cu). We found that P fertilization positively influenced yields of seed and levels of lignan, Fe, and Cu, showing average increases of 15, 20, 24, and 28%, respectively, compared with plants not given P over the 2-year study. Additionally, P fertilization resulted in increased concentrations of Fe and Cu in flaxseed of 8 and 2%, respectively. P fertilization negatively affected the levels of dietary fiber, Zn, and Mn, which were reduced by 7, 11, and 7%, respectively, in comparison with the control. In conclusion, the results demonstrated that appropriate P application is an effective strategy for improving yields of seed, lignan, Fe, and Cu in flax production and for enhancing concentrations of Fe and Cu in flax.

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