Agrobacterium tumefaciens - mediated transformation of soybean Glycine max (L.) Merrill. using immature zygotic cotyledon explants

Agrobacterium tumefaciens-mediated transformation of soybean Glycine max (L.) Merrill. cv. Jack using immature zygotic cotyledons was investigated to identify important factors that affected transformation efficiency and resulted in the production of transgenic soybean somatic embryos. The factors evaluated were initial immature zygotic cotyledon size,Agrobacterium concentration during inoculation and co-culture and the selection regime. Our results showed that 8- to 10-mm zygotic cotyledons exhibited a higher transformation rate, as indicated by transient GUS gene expression, whereas the smaller zygotic cotyledons, at less than 5 mm, died shortly after co-cultivation. However, the smaller zygotic cotyledon explants were found to have a higher embryogenic potential. Analysis ofAgrobacterium and immature cotyledon explant interactions involved twoAgrobacterium concentrations for the inoculation phase and three co-culture regimes. No differences in explant survival or somatic embyogenic potential were observed between the twoAgrobacterium concentrations tested. Analysis of co-culture regimes revealed that the shorter co-culture times resulted in higher explant survival and higher somatic embryo production on the explants, whereas the co-culture time of 4 days severely reduced survival of the cotyledon explants and lowered their embryogenic potential. Analysis of selection regimes revealed that direct placement of cotyledon explants on hygromycin 25 mg/l was detrimental to explant survival, whereas 10 mg/l gave continued growth and subsequent somatic embryo development and plant regeneration. The overall transformation frequency in these experiments, from initial explant to whole plant, was 0.03 %. Three fertile soybean plants were obtained during the course of these experiments. Enzymatic GUS assays and Southern blot hybridizations confirmed the integration of T-DNA and expression of the GUS-intron gene in the three primary transformants. Analysis of 48 progeny revealed that three copies of the transgene were inherited as a single Mendelian locus.