## 2 Factors Influencing the Regeneration of Cucumber Plants
### Exogenous Hormone  
During plant regeneration, the content of endogenous hormones is in a state of dynamic change. Therefore, callus differentiation and bud regeneration are greatly affected by the content and proportion of key hormones such as auxin and cytokinit (CTK) [34; 35; 36]. In bitter melon, relatively high zeatin (ZT) concentrations as well as low indole-3-acetic acid (IAA)/ZT appeared to be associated with bud formation [34]. A similar phenomenon was observed in barley, with the highest levels of IAA, trans-ZT, and cis-ZT among the varieties with the highest rates of regeneration [36]. The content of endogenous hormones in cucumber plants cultured in vitro was measured by [PERSON] [31], who observed that 0-6 d and 15-18 d were the periods when calluses appeared in large numbers. The level of endogenous IAA increased significantly during this period, indicating that this auxin has a promoting effect on cucumber callus induction. Interestingly, the endogenous abscisic acid (ABA) content changed dramatically during this process, suggesting that ABA is also involved in the process of cucumber regeneration [27]. [PERSON] et al. [16] found that changes of endogenous hormones were different in different genotypes of cucumber, and only the CTK content was significantly positively correlated with the differentiation rate. The qPCR results showed that expression of _CYP35A_ (involved in the regulation of cytokinin synthesis) is 18.08 times higher in highly differentiated than in poorly differentiated material. These results show that CTK is the critical factor affecting the regeneration activity of cucumber tissues [16]. Because different hormones have different effects, and different genotypes have different responses to exogenous hormones [16], researchers usually add various concentrations of different hormone combinations at each stage during the cucumber regeneration process to screen for optimal effects. In related reports, IAA, naphthalene acetic acid (NAA), gibberellic acid (GA), indole-3-butyric acid, 6-benzylamino purine (6-BA), and ZT have been used for the in vitro culture of cucumber tissues [11; 37; 38]. In the bud induction stage, the addition of exogenous auxin may promote callus formation rather than adventitious bud differentiation due to the high auxin content of cucumber itself, so auxin and its analogs are rarely used. Currently, the combination of 6-BA and ABA is frequently used to induce shoot regeneration in the process of cucumber transformation (Table 1). The combination of two hormones has been shown to effectively inhibit the formation of callus and directly produce bud clumps [39]. However, the molecular mechanism of ABA-stimulated shoot induction and the interaction between ABA and 6-BA in the cucumber remain unknown. Some studies have speculated that ABA might be involved in regulating the water content of explants that do not become vitrified and/or activating stress tolerance genes, resulting in an increase in regeneration efficiency [18; 40]. Interestingly, this effect appears to be seen only in cucumbers, whereas the addition of ABA to other cucurbit crops has been found to inhibit explant growth [40]. After the induction of adventitious buds is complete, they enter the rooting stage via elongation culture. Because such adventitious shoots have strong rooting ability, it is generally believed that only solid MS medium can give optimal rooting effects [23; 24]. However, it was also observed that some materials did not take root well with MS medium alone [29]; therefore, auxin and its analogs were often added to the medium in some studies. These could promote the rooting of shoots and accelerate the rooting process of adventitious shoots [27; 41]. In addition, thidiazuron is an exogenous hormone commonly used in the culture of unfertil cucumber cells and ovules and plays a key role in inducing the stage of explant callus formation to somatic embryogenesis [42].