Efficient plant regeneration of watermelon (Citrullus lanatus Thunb.) via somatic embryogenesis and assessment of genetic fidelity using ISSR markers

A simple and effective somatic embryogenic system was established for watermelon (Citrullus lanatus) cv. 'Arka Manik'. Embryogenic callus was obtained from leaf explants of 20-d-old in vitro-grown seedlings cultured on embryogenic callus induction medium. The highest frequency of embryogenic callus induction (96.8%) occurred on Murashige and Skoog (MS) medium supplemented with 2.44 mu M 2,4-dichlorophenoxyacetic acid (2,4-D) and 2.27 mu M thidiazuron (TDZ). Transfer of embryogenic calluses with proembryogenic masses to embryo maturation medium led to the asynchronous development of somatic embryos (SEs), which progressed from the globular stage to the cotyledonary stage. The maximum number of SEs/explant (16.1 +/- 0.24) was obtained on MS medium supplemented with 2.44 mu M 2,4-D, 2.27 mu M TDZ, and 30 g L-1 sucrose. Plantlet conversion from cotyledonary-stage SEs was tested on different strengths of MS medium (quarter-, half-, and full-strength) lacking plant growth regulators. The highest frequencies of germination (91.5%) and survivability (82.1%) of plantlets were achieved on full-strength MS medium. Transverse sections of embryogenic callus revealed SE development from callus cells near the epidermis. Secondary SEs occasionally formed from globular-shaped primary embryos. Genetic fidelity of mother plants and ex vitro plants was confirmed by inter-simple sequence repeat (ISSR) markers. The present study is the first report on the use of molecular markers in in vitro culture of watermelon. The developed protocol facilitates rapid production of true-to-type watermelon plants by somatic embryogenesis and thus could serve to generate effective target material for genetic transformation protocols.