Comparison of direct and indirect embryogenesis protocols, biolistic gene transfer and selection parameters for efficient genetic transformation of sugarcane

The comparison of direct and indirect somatic embryogenesis (DSE and ISE) for biolistic transformation of sugarcane by minimal expression cassettes indicated the highest transformation efficiency with ISE (2.2 independent transgenic plants per shot) and the most rapid production of transgenic plants with DSE (12 weeks from explant to plants in soil). Microprojectiles of 0.3 mu m diameter produced 5 times more transgenic lines than 1 mu m microprojectiles when used at the same weight basis per shot. A significant reduction of the number of hybridization signals in the Southern blot was also observed with 0.3 mu m microprojectiles when compared to 1.0 mu m microprojectiles. This suggests that the lower DNA carrying capacity and greater number of the smaller microprojectiles contributes to more transgenic lines with less complex transgene integration. When geneticin sulfate, was used for selection following DSE, significantly more (4.8 times) transgenic plants were produced than with paromomycin sulfate and an equal number of non-expressing plants were produced with both selection agents. In conclusion, optimization of two alternative morphogenic routes for regeneration (DSE and ISE), biolistic and selection parameters generated rooted, transgenic plants of a commercially important sugarcane cultivar with simple transgene integration and within 12 or 19 weeks of culture initiation, respectively. Reducing the complexity of the transgene integration and minimizing the time in tissue culture will likely enhance the performance of the transgenic events.