## 4 Hormonal Regulation of Fruit Shapes  
Hormones can respond to environmental and endogenous signals throughout almost the entire life of a plant. Plant hormones directly or indirectly affect the division and expansion of fruit cells in the early stages of fruit development, to regulate fruit growth, and ultimately determine the size and shape of fruits [103; 104; 105]. There are many plant hormones in nature, including auxin, cytokin (CK), GA, ethylene, and The abscisic acid (ABA). Several of them have been reported to affect fruit shape. Auxin plays an important role in the development of fleshy fruits [106; 107; 108]. [PERSON] et al. [107] measured the endogenous hormone content of fruits in different cucumber inbred lines. They found that in various developmental stages, the IAA content of the fruit is positively correlated with the fruit size and cell growth [109]. In _Solanum_, the LAI589 near-isogenic lines, which have elongated pear-shaped ovaries and fruits, were treated with auxin (2,4-D) before anthesis, resulting in an increased cell number and size of the ovary/fruit proximal end [110]. The auxin synthesis gene _CsYUC10b_ is associated with fruit curvature, and its overexpression can induce the formation of straight fruits [109]. In addition, _SIIAA17_, an auxin transcriptional repressor of Auxin/indoleacetic acid (Aux/IAA), can lead to large fruits in tomatoes [103]. _SIARF9_ and _BnaA9.ARF18_ belongs to the Auxin response factors (ARFs) family, and they can regulate fruit size in tomatoes and _Brassica napus_, respectively [103; 110]. In addition to auxin, other hormones can work with certain regulatory factors to regulate fruit shape [111]. CKs mainly regulate cell division in plants and there is a positive correlation between them, which is consistent with the fruit cell division activity [112; 113; 114]. The CK biosynthetic gene _CYP735A_ can change the cell volume and biomass accumulation to affect the fruit size [115]. In cucumbers, trans-Zeatin riboside (tZR) decreases in the initial stage of fruit development and is mainly involved in the cell division process at the early stage of ovary development. The content of zeatin (ZT) increases in the early stage of fruit development after flowering, which can promote the horizontal expansion of cells in the early stage [107]. GA may regulate flowering and stimulate fruit and seed development [114]. Exogenous application of GA can induce cell enlargement and lead to parthenocarpy [116; 117]. In cucumbers, GA can promote cell expansion during fruit development [118] and antagonize IAA in the mid-to-early stage (9-12 days after anthesis); it may inhibit fruit elongation by hindering cell division [105]. During the development of the tomato fruit, the GA accumulation is consistent with the direction of cell division and expansion in the early stage [119]. Ethylene is mostly related to fruit ripening in tomatoes [120], but recent reports have shown that it can regulate the length of the cucumber fruit. Ethylene content also affects plant development [119]. 1-aminocyclopropane-1 carboxylate synthase 2 (ACS2) can catalyze the biosynthesis of ethylene [119]; the ethylene-decreased cucumber mutant _acs2_ shortens the FL. However, in _sf1_ mutants, excessive ethylene content can also lead to the same phenotype [60]. Therefore, an excessive or inadequate amount of ethylene can affect the FL. The _ACS_ homologous gene _CmAC57_ is related to the round fruits of melons [5; 121]. These results showed that the dose-dependent function of ethylene in Cucurbitaceae is relatively conserved. In a separate study on tomatoes, several Aux/IAA-like genes (_DR1_, _DR3_, _DR4_, and _DR8_) related to IAA transcript accumulation were differentially regulated by ethylene [122]. ABA is a plant growth inhibitory hormone [123], [PERSON] et al. [124] pointed out that in tomatoes, the ABA-deficient mutant _not/flc_ has a small fruit and cell shape [123; 125].