Complete Genome Expression Analysis of the Auxin Response Factor Gene Family in Sandalwood and Their Potential Roles in Drought Stress

Auxin response factors (ARFs) are essential transcription factors in plants that play an irreplaceable role in controlling the expression of auxin response genes and participating in plant growth and stress. The ARF gene family has been found in Arabidopsis thaliana, apple (Malus domestica), poplar (Populus trichocarpa) and other plants with known whole genomes. However, S. album (Santalum album L.), has not been studied. In this study, we analyzed and screened the whole genome of S. album and obtained 18 S. album ARFs (SaARFs), which were distributed on eight chromosomes. Through the prediction of conserved domains, we found that 13 of the 18 SaARFs had three intact conserved domains, named DBD, MR, Phox and Bem1 (PB1), while the extra five SaARFs (SaARF3, SaARF10, SaARF12, SaARF15, SaARF17) had only two conserved domains, and the C-terminal PB1 domain was missing. By establishing a phylogenetic tree, 62 ARF genes in S. album, poplar and Arabidopsis were divided into four subgroups, named I, II, III and IV. According to the results of collinearity analysis, we found that ten of the eighteen ARF genes were involved in five segmental duplication events and these genes had short distance intervals and high homology in the SaARF gene family. Finally, tissue-specific and drought-treatment expression of SaARF genes was observed by quantitative real-time polymerase chain reaction (qRT-PCR), and six genes were significantly overexpressed in haustorium. Meanwhile we found SaARF5, SaARF10, and SaARF16 were significantly overexpressed under drought stress. These results provide a basis for further analysis of the related functions of the S. album ARF gene and its relationship with haustorium formation.