Transposition of the maize Ds element from a viral vector to the rice genome

The geminivirus miscanthus streak virus (MiSV) was used as a gene vector to study the transposition of the maize Ds element in rice protoplasts. Efficient excision of the Ds from the MiSV vector was observed only when the MiSV vector was allowed to replicate and the plasmid expressing the transposase gene encoded by Ac was co-transfected. Under the same condition, the Ds carrying a hygromycin phosphotransferase gene (Ds::HPT) was also efficiently excised. Hygromycin-resistant calli were obtained by culturing these transfected protoplasts in order to examine the transposition of the excised Ds::HPT into the rice genome. In five out of 16 calli examined, the Ds::HPT, but not the vector sequence, was integrated into the rice genome and 8 bp target site duplication typical of Ac/Ds transposition was generated. These results show that the Ds::HPT inserted in the MiSV vector transposed directly into the rice genome. This demonstrates the direct transposition of a cloned plant transposable element into the plant genome. Implications of these finding are discussed.