Genome-Wide Identification and Expression Analysis of Auxin Response Factor Gene Family in Linum usitatissimum

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’ K(a)/K(s) 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.