Genome-Wide Identification and Expression Analysis of Auxin Response Factor (ARF) Gene Family in Betula pendula

As the key transcription factors regulating auxin responsive genes expression, auxin response factors (ARFs) play critical roles in diverse aspects of plant growth and development. Betula pendula is a valuable ornamental tree, and the information on ARF gene family of B. pendula is needed for better understanding. The publication of the genome sequence of B. pendula enable to analyze the bioinformatics information and expression pattern of BpeARF gene family on the genome-wide basis. In this study, physical and chemical properties, chromosome location, phylogenetic relationship, gene structure, conserved domain, motif composition, and cis-acting element of BpeARF gene family were analyzed, and expression patterns of BpeARF genes were investigated using completely random design in different tissues and under exogenous NAA and drought treatments. A total of 17 BpeARF genes was identified from B. pendula genome, which were unevenly distributed on 13 chromosomes and encoded adequate proteins ranging from 613 to 1135 amino acids in length. Three BpeARF gene pairs were formed by segmental duplication, and the Ka/Ks values of these BpeARF gene pairs were less than 1. According to the phylogenetic relationship among B. pendula, Betula platyphylla, Populus trichocarpa, and Arabidopsis thaliana, the BpeARF genes were divided into four classes, and the intron/exon structure, conserved domain, and motif composition showed high similarity among the BpeARF genes within the same class. The cis-acting elements in the promoter regions of BpeARF genes were related to tissue development, hormone response, and stress resistance. Quantitative real-time PCR exhibited diverse expression patterns of BpeARF genes in different tissues and in response to exogenous auxin treatment and drought stress. The expressions of one, ten, seven, and three BpeARF genes were the high levels in buds, young leaves, stems, and roots, respectively. Under exogenous NAA treatment, six BpeARF genes in stems and roots were upregulated expression at all timepoints. Under drought stress, BpeARF7 and BpeARF15 were upregulated in stems and roots, and BpeARF5 and BpeARF6 were downregulated in leaves, stems, and roots. Our results provided valuable information for the classification and putative functions of BpeARF gene family, which may be helpful for selecting candidate genes and verifying gene function in the genetic engineering of birch trees in further research.