Background: Cucumber (Cucumis sativus) is one of the most important vegetable crops in the world. As conventional breeding of cucumber is very challenging, genetic engineering is an alternative option to introduce important traits such as enhanced stress resistance and nutritional value. However, the efficiency of the transformation system depends on genotypes, transformation conditions, selection agents, etc. This study aims to speed up the process of Agrobacterium-mediated transformation of

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... cumber. ' Xintai mici ', a very popular and typical north China-type cucumber variety, was transformed with Agrobacterium GV3101. The strain carried pCAMBIA2300s plasmid, a double vector with the marker gene of neomycin phosphotransferase II ( npt II). Results: The research results indicated that cefotaxime sodium was suitable for inhibiting Agrobacterium in the stage of screening and bud elongation. Timentin was best used during rooting stage. Furthermore, 25 mg/L kanamycin was used in the early stage of screening and increased to 50 mg/L for further screening. At the bud elongation and rooting stage, 75 and 100 mg/L kanamycin was used respectively to improve the screening efficiency. In order to obtain the highest regeneration frequency of resistant buds, 50, 150, and 100 μM acetosyringone were added in the pre-culture medium, infection solution, and co-culture medium respectively. To confirm the presence of the transgenes, DNA from npt II transgenic cucumber plants was analyzed by polymerase chain reaction after transplanting resistant regenerated plants. Conclusions: We finally achieved an 8.1% conversion, which was among the highest values reported until date using cucumber ' Xintai mici '. Thus an effective protocol for Agrobacterium tumefaciens -mediated genetic transformation of cucumber was optimized.