## 2 ARF Is Involved in Regulating Leaf Development  
Most plants can perform photosynthesis by themselves, absorb carbon dioxide and release oxygen at the same time, and synthesize organic substances. This process is the prerequisite for all organisms on the earth to obtain energy. Leaves are the most important place for photosynthesis of plants. Important indicators such as the shape, size, and chlorophyll content of plant leaves usually determine whether photosynthesis can proceed normally and play a key role in the efficiency of photosynthesis [6]. In addition, in the leaves of plants, there are usually many other physiological activities, such as respiration. Generally speaking, there are stomata on the back of the leaves to support high-intensity respiration. Plants mainly maintain their own metabolic balance through these two activities on the leaves to maintain vitality. In addition, the transpiration also requires the participation of leaves.  
At present, there have been a large number of reports showing the plant auxin plays a critical role in the life activity of leaves. A moderate increase in the concentration of auxin will stimulate the growth of leaves, while exceeding the optimal concentration will have an inhibitory effect. Auxin response factor (ARF), as a key regulator in the process of auxin signal transduction, directly participates in the regulation process of auxin on plant leaves in many links [7; 8]. _Arabidopsis thaliana_ is one of the first species to be used to study the regulation of ARF. In Arabidopsis, AtARF is already a well-studied member of the ARF family, and its regulatory role in leaves has been widely reported. AtARF2, AtARF3, AtARF4, and MONOPTEROS (MP)/ARF5 are involved in the regulation of auxin-responsive gene expression and leaf development. MP can promote the expression of _WUSCHEL-RELATED HOMEOBOX1 (WOX1)_ and _PRESSED FLOWER (PRS)_ directly, promoting leaf flattening. However, the expression of _WOX1_ and _PRS_ can be repressed by several other members of the ARF family, AtARF2/3/4, as shown in Figure 2. These ARFs are all able to bind directly to the promoters of _WOX1_ and _PRS_[6]. [PERSON] et al. [7] reported that ARF7 and ARF3 act synergistically with ARF5 in leaf formation, indicating a synergistic interaction, and these gene combinations work in parallel to maintain meristem organization and induce leaf formation. In addition, some studies have indicated that auxin induces leaf expansion by activating NPH4/ARF7 and ARF19 [8] (Figure 2). In tomato, [PERSON] et al. [9] found that tomato leaf development was co-regulated by several members of the ARF family, including SIARF6A/8A/8B/24. In addition, SIARF6A/24 can influence the process of leaflet initiation by binding to the promoter of _PIN-FORMED1 (SIPIN1)_, which affects the local accumulation of auxin. There are also reports that the chlorophyll content in leaves and fruit is significantly increased in tomato mutant lines of overexpressing SIARF6A. While downregulation of SIARF6A reduces the chlorophyll content [10]. ARF also plays a significant role in the leaves of some other species. In rice, OsARF19 controls rice leaf angles by binding to the promoters of the auxin early response gene _OsGH3_-5 and brassinosteroid (BR) response gene _OsBRI1_[11].  
Leaf senescence is a necessary process of plant metabolism, accompanied by physiological changes such as the decrease of ribonucleucleic acid content, the destruction of chloroplasts, and the decrease of photosynthesis capacity. ARF also plays a regulatory role in the process. The _AtARF1_ and _AtARF2_ genes in Arabidopsis are involved in regulating leaf senescence, which is independent of the ethylene and cytokinin pathways [12; 13]. In tomato, overexpression of _SIARF2_ accelerates leaf senescence [14]. In Iitchi, [PERSON] et al. [15] concluded that LcARF2 may be related to the senescence of litchi leaves.