## 5 ARF Is Involved in Regulating Fruit Development and Ripening  
Research on fruit development and ripening has always been a hot topic. Human demand for high-quality fruits drives researchers to conduct in-depth research on the formation, development, and quality improvement of fruits, and its economic value is also huge. Plant fruits generally include two parts: pericarp and seed. Pericarp can be divided into exocarp, mesocarp, and endocarp, and seeds play a key role in the process of propagation and reproduction. The process of fruit development depends on cell division and cell expansion. After the completion of the pollination and ovary fertilization process, the carpel tissue around the ovary begins to expand rapidly [35]. The research on fruit includes the entire process from the occurrence of flowers to the later fruit preservation. For a long time, it has been believed that plant hormones play an important regulatory role in fruit growth and development, especially auxin, ethylene, and abscisic acid. Some progress has been made in recent years regarding the role of ARF in the regulation of tomato fruit development and ripening. Figure 3 shows a model of ARFs regulating tomato fruit development and ripening. SIARF2 is an important part of tomato maturation regulatory network. Overexpression of _SIARF2_ enhances the expression of ethylene biosynthesis genes, increasing ethylene production and accelerating fruit ripening. However, downregulation or silencing of SIARF2 will lead to maturation defects and even inhibit maturity [36; 37]. SIARF4 affects many physiological indicators of tomato fruits. For example, GOLDEN2-LIKE (GLK) transcription factors are required for the regulation of chloroplast and chlorophyll levels [38], and SIARF4 may control the accumulation of chlorophyll and the structure of the fruit cell wall through transcriptional inhibition of GLK1 expression in tomato fruits, as shown in Figure 3. Chlorophyll content is a key feature of immature fruit that affects the nutrient composition and flavor of mature fruit. Inhibition of SIARF4 expression can make tomato fruits show a dark green immature phenotype, and the resulting fruit is harder than the wild type. In addition, SIARF4 is also a major participant in mediating the control of auxin on sugar metabolism [39]. SIARF10 plays a similar role to SIARF4 and regulates chlorophyll and sugar accumulation during tomato fruit development through transcriptional activation of SIGLK1 [40]. SIARF10 is also involved in seed development and the regulation of fruit size and shape [41]. Another auxin response factor that has an effect on the chlorophyll content of tomato fruits is SIARF6A. The chlorophyll content in fruit was significantly increased in tomato mutant with overexpression of SIARF6A. In addition, ethylene production was inhibited, and fruit ripening rate decreased [10]. When pollination occurs, SIARF5 plays a role in fruit germination [42]. SIARF7 negatively regulates both fruit set and fruit development, slowing the plant auxin response during fruit growth [43; 44]. In addition, SIARF8 and SIARF9 in tomato play a role in regulating fruit development and growth, and in negatively regulating cell division during early fruit development, respectively [40; 45]. A number of auxin response factors have also been reported in other species, and they also impact fruit development and ripening. In Arabidopsis, AtARF2 regulates silique ripening, whereas AtARF8 is a negative regulator of fruit development [12,46]. In strawberries, FaARF6a is considered the most likely candidate among ARFs involved in the regulation of auxin activity in mature flower receptacles, interacting with FaAux/IAA11 [5]. Similar to SIARF4 in tomato, PpARF4 in peaches have an effect on sugar metabolism and cell wall degradation, and thus may be involved in regulating fruit ripening. According to [PERSON], CiARF1 and CiARF18 in citrus sinensis are present in the peel and pulp, respectively, and have constitutive expression patterns during fruit development and ripening [47].