High frequency somatic embryogenesis and plant regeneration from hypocotyl and leaf explants of gherkin (Cucumis anguria L.)

Gherkin (Cucumis anguria L.) is an important commercial vegetable crop. An efficient protocol for plantlet regeneration from hypocotyl and leaf explants through direct somatic embryogenesis was developed. High frequency of somatic embryos (21.6 and 34.0) were obtained from hypocotyl and leaf explants when cultured on Murashige and Skoog (MS) salts plus B5 vitamins (MSB₅) supplemented with 5% sucrose, 1.5mgL^-1 2,4-D, 0.5mgL^-1 BAP and 150μM l-glutamine. Initial culturing of embryos in the dark conditions for two weeks, followed by four weeks under light resulted in a higher frequency of embryo formation when compared to continuous light conditions. Histological observation showed that the somatic embryos originated from single cells of the epidermal layer. Histological evidence on formation of shoot and root poles during conversion of the embryos confirmed that these structures were true somatic embryos. Maturation of somatic embryos occurred on plant growth regulator-free MSB₅ semi-solid medium containing 3% sucrose and 0.5% (w/v) activated charcoal (AC). Conversion of embryos into plantlets was achieved on MSB₅ medium supplemented with 3% sucrose, 0.3% AC and 0.5mgL^-1 gibberellic acid (GA₃). Sucrose was found to be the best carbon source for SE induction, maturation and germination. Ninety percent of embryos were converted into normal plantlets. The plantlets were successfully acclimatized in the greenhouse with 95% survival rate and transferred to ex vitro conditions which developed with normal phenotypes. This regeneration protocol assured successful embryo induction and plantlet conversion. This is the first report for the induction of direct somatic embryogenesis from hypocotyl and leaf explants of C. anguria. The result of this study is beneficial for genetic transformation and mass clonal propagation.