Somatic embryogenesis and genetic uniformity of regenerated cassava plants from low-temperature preserved secondary somatic cotyledons

Low plant regeneration and transformation frequencies in cassava have been attributed to recycling of cassava secondary somatic embryos for long periods. This study examined the suitability of low-temperature storage of dehydrated cassava secondary somatic embryos as an alternative short-term conservation approach to recycling. The study included two experiments: in the first one, somatic embryogenesis from cotyledons of dehydrated secondary somatic embryos stored at six temperatures for 4, 8 and 12 months was established; and in the second, plant regeneration via shoot organogenesis using somatic cotyledon explants obtained in the first experiment was examined. The growth, photosynthetic pigment contents and genetic stability of the regenerated plants were assessed. At each storage temperature, the survival of explants, somatic embryogenesis, shoot induction and elongation decreased with increases in storage period. At 4 months after storage (MAS), explant survival, frequency of somatic embryogenesis, shoot induction, elongation and rooting had the best responses at 16°C and 20°C. Similarly, the growth and photosymthetic pigment contents of plants regenerated from somatic embryos preserved at 16 °C and Z0 °C recorded the best results at 4 MAS. However, at 8 and 12 MAS, the best explant survival, soma- tic embryogenesis, shoot induction and elongation were recorded at 16°C. Eight RAPD primers produced a total of 56 distinct and scorable bands, ranging from 250 hp to 3000 bp in size. An random amplified DNA (RAPD) analysis showed a uniform banding profile among regenerants and to that of the mother plant, indicating no genetic variation among regenerants and between the regenerants and the mother plant. The data presented here suggest that cassava secondary somatic embryos dehydrated to 50% moisture content should be preserved at l€r20°C for 4 months; for 8 and 12 months storage, the embryos should be stored at 16°C. No genetic alteration was detected among the regenerants. Therefore, the method is a suitable means for short-term conservation of cassava secondary somatic embryos.