## 5 Conclusions and Future Perspectives  
Since the advent of the cucumber genetic transformation, numerous researchers have been attracted to this topic, and therefore the transgenic technology has been improved and developed continuously. At present, transgenic systems of some cucumber lines have been established, but the efficiency of genetic transformation is universally low. Thus, the lowest reported transformation efficiency was 0.1% [55], while the highest was 26% [21]. This finding has many reasons. First, the few cucumber lines that have been used to obtain transgenic plants were selected easily, which has limited the use of some new varieties. Second, the concentration of antibiotics added to the screening medium has been too high, so some transformed cells have been killed at an early stage of differentiation. Third, the position of regeneration buds differs from that of _Agrobacterium_ infection. Fourth, there are few studies on environmental factors such as culture temperature, pH of the medium, light conditions, air conditions, and their interactions. Last, there has been little research on some details, such as types of basal medium, curing agents, organic additives, antioxidants, and surfactants. In most of the existing reports, a genotype with a higher regeneration rate was selected first, and many parameters affecting genetic transformation efficiency, such as co-cultivation conditions, _Agrobacterium_ strains, and bacterial cell density, were optimized gradually. Finally, excellent transformation systems suitable for certain genotypes have been established. In summary, most studies have focused on using cotyledons and cotyledon nodes as explants, adding different concentrations of 6-BA, ABA, and AgNO\({}_{3}\); causing _Agrobacterium_ infection using soaking or vacuum infiltration; and obtaining transgenic plants by direct bud regeneration. In particular, although the use of cotyledon nodes has become more frequent in recent years, cotyledons remain the most efficient explants to obtain transgenic cucumber lines, and the transformation efficiency has been up to 21%. In addition, vacuum infiltration is gradually replacing traditional immersion infection. This is a trend that will improve transformation conditions to achieve high frequency for cotyledons mediated by _Agrobacterium_ strain EHA105 under vacuum. In recent years, the method of using nanomaterials to deliver biomolecules into plants has gradually become well known. DNA has been delivered successfully to spinach, tobacco, watercress, and _Arabidopsis_ using chitosan-wrapped single-walled carbon nanotubes, suggesting that this could be a general mechanism applicable to different plants [79]. In addition, it was demonstrated that nanoparticles can also carry short interfering (si)RNA into plants as a carrier to exert the gene silencing effect, and delivery of siRNA does not seem to require the nanocarriers to enter plant cells, but only needs them to be released in the apoplast to function [80]. It is foreseeable that with the application of nanotechnology in cucumber in the future, the in-depth study of safety screening markers, and the continuous optimization of CRISPR/Cas9 technology, the problems currently plaguing researchers will be solved gradually. Genomic transformation is an important tool for exploring the relationship between gene function and important agronomic traits of cucumber. Using overexpression and RNAi and gene editing technology, the molecular mechanisms of some important agronomic traits have been unraveled. To meet people's growing demand for high-quality, nutritious, and safe cucumber products, research on gene mining and regulatory network analysis with important breeding value should be increased in the future. Conceptualization, L.M., H.W. and P.Z. ; methodology, L.L. ; software, X.W. ; validation, Y.Z., X.W. and S.Z. ; formal analysis, J.T. ; investigation, H.L. ; resources, H.L. ; data curation, L.L. ; writing--original draft preparation, J.T. and L.L. ; writing, review and editing, H.L., L.M., H.W. and P.Z.