Improved Production Procedures for Somatic Embryos of Sweetpotato for a Synthetic Seed System

Embryogenic callus of sweetpotato ( Ipomoea batatas (L.) Lam.) disassociates in liquid medium to form a heterogeneous population of embryogenic and nonembry- ogenic cell aggregates of varying sizes. To improve embryo production, such cell ag- gregate populations were obtained by manually fragmenting calli 5 to 10 mm in size into liquid medium. The resulting suspensions were analyzed and the embryogenic fraction identified. The percentage of embryogenic aggregates and the percentage of aggregates forming embryos decreased with decreasing aggregate size. Thus, 76% of the 710- to 1000-µm-diame ter aggregates but only 14% of the 180- to 250-µm aggre- gates had embryogenic potential. However, only 20% of the 710- to 1000-µm aggregates and only 2% of the 180- to 250-µm aggregates actually formed embryos. Conversely, embryogenic callus and embryo production per milligram of cultured embryogenic callus increased quadratically with decreasing aggregate size. Individual torpedo-stage embryos were produced from cell aggregates 180 to 250 µm in size. Sweetpotato, a major food crop and po- tential biomass crop, is conventionall y prop- agated using stem cuttings. This propagation method is costly since intensive labor and large nurseries and storage facilities are re- quired. The lack of resistance to virus dis- eases further complicates the maintenance and multiplication of virus-free stocks. Mass production of disease-free plant material can be expected using tissue culture. The pro- duction of synthetic seed, through somatic embryogenesis, and direct seeding would re- duce the production cost of this vegetatively propagated crop. The application of somatic embryogenesis to synthetic seed production will require many