DEVELOPMENTAL BIOLOGY/MORPHOGENESIS Efficacious somatic embryogenesis and fertile plant recovery from shoot apex explants of onion (Allium cepa. L.) M. Ramakrishnan & S. Antony Ceasar & V. Duraipandiyan & Melvin A. Daniel & S. Ignacimuthu Received: 1 August 2012 / Accepted: 7 March 2013 / Editor: Jerry Ranch # The Society for In Vitro Biology 2013 Abstract An efficient somatic embryogenesis and regener- ation system was developed for the first time in onion using shoot apex explants. These explants were used to initiate callus in Murashige and Skoog (MS) medium supplemented with 4.0 mg l -1 2,4-dichlorophenoxyacetic acid. The induc- tion frequency of primary callus in this medium was 85.3%. The primary calli were then transferred onto medium supplemented with 2.0 mg l -1 2,4-dichlorophenoxyacetic acid. Following two biweekly subcultures, embryogenic callus formed. Inclusion of a low concentration of 6- benzylaminopurine in the subculture medium promoted the formation of embryogenic callus. The addition of 2.0 mg l -1 glycine, 690 mg l -1 proline, and 1.0 g l -1 casein hydrolysate also increased the frequency of callus induction and em- bryogenic callus formation. The highest frequency of em- bryogenic callus (86.9%) and greatest number of somatic embryos (26.3 per callus) were obtained by the further addition of 8.0 mg l -1 silver nitrate. Somatic embryos formed plantlets on regeneration medium supplemented with 1.5 mg l - 1 6-benzylaminopurine; addition of 2.0 mg l -1 glycine to the regeneration medium promoted a high frequency of regeneration (78.1%) and plantlet forma- tion (28.7 plants per callus). The regenerated plantlets were transferred to half-strength MS medium supplemented with 1.5 mg l -1 indole-3-butyric acid for root development; the maximum frequency of root formation was 87.7% and the average number of roots was 7.6 per shoot. The regenerated plantlets were successfully grown to maturity after harden- ing in the soil. This is the first report of somatic embryo- genesis and regeneration from shoot apex explants of onion. Keywords Onion . Shoot apex . Embryogenic callus . Somatic embryo . Plant regeneration Introduction Allium cepa L. is the most economically important member of the genus Allium. It is cultivated worldwide as a condi- ment, vegetable, and medicinal plant with many different uses. Taxonomically, it belongs to Allium section cepa (Mill.) Prokh. This section comprises nine wild species besides the cultivated A. cepa (common or bulb onion) and Allium fistulosum (bunching onion). A. cepa and related vegetables such as garlic (Allium sativum) and leek (Allium ampeloprasum) have an annual global trade in bulbs worth US$400 million (Brewster 1994). Genetic engineering to introduce new genes into plants has been achieved in many economically important crops, but few reports exist for onion (A. cepa var. cepa) (Aswath et al. 2006). Successful plant transformation requires a plant regeneration system, a DNA delivery system, and a selection system to select transgenic cells. For onion, the characterization of these three aspects of transformation is still incomplete. In vitro culture and plant regeneration from Allium imma- ture embryo and mature zygotic embryo explants have been widely reported (Dunstan and Short 1978; Hussey and Falavigna 1980; Phillips and Luteyn 1983; van der Valk et al. 1992; Eady et al. 1998; Eady and Lister 1998; Zheng et al. 1998, 1999). For the micropropagation of onion, suspension cultures are generally preferable to culture on solid medium because callus growth on solid medium is very slow. Regenerable suspension cells have been reported in A. cepa M. Ramakrishnan : S. A. Ceasar : V. Duraipandiyan : M. A. Daniel : S. Ignacimuthu (*) Division of Plant Biotechnology, Entomology Research Institute, Loyola College, Chennai, India 600 034 e-mail: entolc@hotmail.com S. A. Ceasar Institute of Integrative and Comparative Biology, Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT, UK V. Duraipandiyan Department of Botany and Microbiology, College of Science, King Saud University, Riyadh, Kingdom of Saudi Arabia 11451 In Vitro Cell.Dev.Biol.—Plant DOI 10.1007/s11627-013-9510-3