Genome-Wide Identification and Characterization of Auxin Response Factor (ARF) Gene Family Involved in Wood Formation and Response to Exogenous Hormone Treatment in Populus trichocarpa

Auxin is a key regulator that virtually controls almost every aspect of plant growth and development throughout its life cycle. As the major components of auxin signaling, auxin response factors (ARFs) play crucial roles in various processes of plant growth and development. In this study, a total of 35 genes were identified, and their phylogenetic relationships, chromosomal locations, synteny relationships, exon/intron structures, -elements, conserved motifs, and protein characteristics were systemically investigated. We also analyzed the expression patterns of these genes and revealed that 16 of them, including , , , , - , , , , , , , and , were preferentially expressed in primary stems, while 15 of them, including , , , , , , - , , , , , , and , participated in different phases of wood formation. In addition, some genes, with at least one -element related to indole-3-acetic acid (IAA) or abscisic acid (ABA) response, responded differently to exogenous IAA and ABA treatment, respectively. Three PtrARF proteins, namely PtrARF18, PtrARF23, and PtrARF29, selected from three classes, were characterized, and only PtrARF18 was a transcriptional self-activator localized in the nucleus. Moreover, Y2H and bimolecular fluorescence complementation (BiFC) assay demonstrated that PtrARF23 interacted with PtrIAA10 and PtrIAA28 in the nucleus, while PtrARF29 interacted with PtrIAA28 in the nucleus. Our results provided comprehensive information regarding the gene family, which will lay some foundation for future research about genes in tree development and growth, especially the wood formation, in response to cellular signaling and environmental cues.