SOMATIC EMBRYO CYCLING - EVALUATION OF A NOVEL TRANSFORMATION AND ASSAY SYSTEM FOR SEED-SPECIFIC GENE-EXPRESSION IN SOYBEAN

Somatic embryo cycling, a modification of soybean somatic embryogenic suspension culture, was developed as an efficient and rapid method of producing tissue suitable for stable transformation of soybean germpla sm by biolistic particle bombardment. Instead of using immature seed e xplants, cotyledon-staged somatic embryo hypocotyls were placed on aux in-containing medium, where they initiated new somatic embryos primari ly from single epidermal cells. By bombarding hypocotyls prior to init iation of subsequent embryo formation, we have effectively transformed soybean somatic embryos with the reporter genes neomycin phosphotrans ferase, P-glucuronidase, and a mammalian stearyl CoA delta-9 desaturas e, controlled by a seed-specific promoter. These embryos contain signi ficantly reduced levels of saturated palmitic and stearic fatty acids, and significant amounts of monounsaturated palmitoleic acid, which is not normally abundant in soybean seeds. This study demonstrates the e ffectiveness of somatic embryo cycling for soybean transformation, and for testing expression of genes for seed-specific proteins. Abnormal flower development in recovered plants is a limitation for application of the technique to produce transgenic seed at present.