Identification of l-ascorbic acid metabolic genes in wild Cucumis species and the key genes involved in high content of AsA in C. hystrix fruits

• Compared the genes in AsA metabolic pathway between wild Cucumis relatives and cultivated species (cucumber and melon) for the first time, and found the wild relatives averagely have more biosynthesis genes than cultivated species. • Report a wild Cucumis species C. hystrix with ascorbic acid content three times higher than that in cucumber fruit, which could be a valuable source for breeding high AsA varity. • Four candidate genes were identified for the high AsA content in C. hystrix fruit. Cucumber ( Cucumis sativus L.) is a globally important vegetable crop, and the ascorbic acid (AsA) content in fruit serves one of the key quality indexes for fresh consumed fruits. Nevertheless, the AsA content in the present cultivars is very low, and the genetic variation is narrow as well, posing challenges for breeding high AsA content in cucumber. C. hystrix Chakr. is a unique wild relative having cross compatibility with cucumber. We have found that the AsA content in C. hystrix is three times higher than in cucumber, which give us opportunity to identify the AsA metabolic genes for the genetic improvement of AsA content. In this study, three wild Cucumis relatives, including C. sativus var. hardwickii. (the wild form of cultivated cucumber), C. hystrix and C. metuliferus , along with cultivated cucumber and melon were used for identification of AsA metabolic genes by whole genome identification and transcriptome comparative analysis. The results indicated that the average number of AsA biosynthesis genes in wild relatives is 52, more than that of 47 for cultivated species. With tissue-specific expression analysis, eight highest expressed genes found in C. hystrix fruit while only two genes in cucumber fruit. The comparative transcriptome analysis of AsA metabolism pathway in C. hystrix and cucumber indicated that the high AsA content in C. hystrix is regulated by the higher gene expression in l -galactose biosynthesis pathway genes ( GME2.2 and GMP1 ) and lower gene expression of recycling and oxidation genes ( AO1.2 and AO1.3 ). These results provide a genetic basis for genetic improvement of AsA in Cucumis crops. Furthermore, identification of the genes involved in high AsA content metabolism pathway in C. hystrix provides effective candidate genes for genetic improvement of cucumber with high AsA content.