PLANT-REGENERATION IN SWEET-POTATO (IPOMOEA-BATATAS L, CONVOLVULACEAE)

The application of new techniques for improvement of sweet potato crop s, particularly including the exploitation of somaclonal variation, ge ne transfer by genetic transformation and somatic hybridization, requi res the control of plant regeneration from tissue cultures. Shoots can easily be regenerated from explants of stems, petioles, leaves and ro ots, while callus cultures do not produce any shoots. The potential of somatic embryogenesis and plant regeneration via embryogenesis was ev aluated for 10 cultivars of sweet potato. Protocols for plant regenera tion from cultured protoplasts have also been developed. Since mesophy ll was resistant to enzyme digestion, fragments of stems and petioles, callus and cell suspensions were used as source of protoplasts of swe et potato. Series of transfers of protoplast-derived calluses, particu larly those which had been obtained from in vitro plants, to media con taining a high level of zeatin resulted in successful formation of sho ots in only two sweet potato cultivars. In addition, the embryogenic p otential was irreversibly lost through protoplast culture, since proto plasts isolated from embryogenic cell suspensions developed into non-e mbryogenic callus. Consequently, an alternative protocol is being succ essfully developed to improve plant regeneration from cultured protopl asts of sweet potato, involving first root formation from which shoots can then be regenerated. Preliminary evaluation in field conditions i n Gabon revealed that plants regenerated from cultured protoplasts exh ibited a great genetic variability in their growth and tuber formation in particular.