## 1 Introduction  
Auxin is the first discovered plant hormone, one that regulates plant growth and development extensively. The function of auxin depends on a signal transduction pathway involving a variety of transcription factors, one of the most important of which is auxin response factor (ARF). Since the ARF was discovered in Arabidopsis in 1997, ARF has been found in Arabidopsis (_Arabidopsis thaliana_), rice (_Oryza sativa_), maize (_Zea mays_ L.), tomato (_Solanum lycopersicum_), grapes (_Vitis vinifera_), and other plants. The numbers of ARF genes that have been reported in various species are summarized in Table 1. ARFs were identified as large gene families and showed high homology in the same species. As a key factor in plant auxin signaling, ARFs can specifically bind to the auxin response element (AuxRE) in the promoter region of the auxin response genes and activate or repress the expression of auxin response gene, which in turn triggers many auxin-mediated physiological effects. \begin{table}
\begin{tabular}{c c} \hline \hline
**Species** & **Gene No.** \\ \hline _Agare americana_ & 32 \\ _Ananus comosus_ & 20 \\ _Arabidopsis thaliana_ & 23 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the number of ARF genes in species that have been reported. ARFs are structurally similar, with most members containing three regions: DBD (DNA-binding domain), MR (middle region), and PB1 (Phox and Bern 1). As shown in Figure 1, DBD is a DNA-binding domain directly involved in binding to AuxRE elements, while the MR region is responsible for activation or repression of target genes. It is believed that the amino acid composition and sequence length of MR region determine whether activation or inhibition. If this region is rich in proline, serine, and threonine it is able to \begin{table}
\begin{tabular}{c c} \hline \hline
**Species** & **Gene No.** \\ \hline _Aquilegia aerulaa_ & 12 \\ _Bamana_ & 47 \\ _Beta vulgaris_ & 16 \\ _Boehmeria nivea_ & 14 \\ _Brachypodium distachyon_ & 24 \\ _Brassica rapa_ & 31 \\ _Capsicum annuum_ & 22 \\ _Carica papaya_ & 15 \\ _Cicer arietinu_ & 28 \\ _Citrus clententina_ & 17 \\ _Citrus sinensis_ & 22 \\ _Cucumis melo_ & 17 \\ _Cucumis sativus_ & 15 \\ _Dendrobitn officinal_ & 14 \\ _Dimoarpus longau_ & 17 \\ _Eucalyptus granddis_ & 17 \\ _Fagopyrun tataricum_ & 20 \\ _Fragaria vesca_ & 17 \\ _Glycine max_ & 55 \\ _Gossypium raimondii_ & 35 \\ _Hordeum vulgare_ & 20 \\ _Jatrupula curcas_ & 17 \\ _Litchi chimensis_ & 39 \\ _Malus domestica_ & 33 \\ _Manihot esculenta_ & 18 \\ _Marchantia polymorpha_ & 3 \\ _Medicago truncatula_ & 24 \\ _Mimulus guttatus_ & 19 \\ _Mulberry_ & 17 \\ _Oryza sativa_ & 25 \\ _Osmanthus fragrans_ & 50 \\ _Phaseolus vulgaris_ & 25 \\ _Phyllostachys edulis_ & 24 \\ _Physcomittella patens_ & 12 \\ _Pourits trichoronapu_ & 39 \\ _Prunus mutne_ & 17 \\ _Prunus persica_ & 17 \\ _Prunus sibrica_ & 14 \\ _Rafflesia cantlevi_ & 9 \\ _Ricinus communuis_ & 17 \\ _Selagilella mocelladorffii_ & 7 \\ _Selaria italica_ & 23 \\ _Solanum lycopenesum_ & 22 \\ _Solanum tuberosum_ & 20 \\ _Sorghum bicolor_ & 25 \\ _Tamarix chitensis_ & 12 \\ _Thobroma cacao_ & 19 \\ _Tritticum aestivum_ & 27 \\ _Vitis vinifera_ & 19 \\ _Zea mays_ & 36 \\ _Ziziphus jujuba_ & 16 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Cont. produce inhibition, while activator-class ARFs often have glutamine-rich and leucine-rich MR regions. The PB1 domain can bind to Aux/IAA proteins to form heterodimers [1; 2]. Some reports show that Aux/IAA and ARFs can form not only dimers but also larger complexes (oligomers), noting that oligomerization of Aux/IAA proteins may be essential for the inhibition of ARF proteins and only sufficient amounts of Aux/IAA proteins can exert the inhibitory effect of ARF proteins. The PB1 domain contributes to the oligomerization of ARFs and is important for Aux/IAA-ARF-mediated gene regulation [3; 4].