Transferring foreign DNA into plant cells by biolistic and Agrobacterium -mediated methods may result in random integration of different copy numbers of the transgene, and different proportions of intact vs. rearranged copies of the transgene. This may, in turn, affect transgene expression levels. To test the above hypothesis, we first introduced the same plasmid, pAc1PG-CAM, into rice (BX)Oryza sativa L.) calli separately by the biolistic method and by the Agrobacterium -mediated method. To show whether different plasmids may affect the results, we also introduced pTOK233 by the Agrobacterium -mediated method and pJPM44 by the biolistic method. Transgene expression of R0 plants was monitored by histochemical analysis of GUS activity. Transgene copy number was determined by Southern blot analysis after digesting genomic DNA with an enzyme that has a unique cutting site within the input plasmid. The total genomic DNA was also digested by a two-cut enzyme (the cuts are located at two sides of a given transgene expression cassette), followed by Southern blotting analysis, for determining the number of intact transgene expression cassettes. Our data showed that Agrobacterium -mediated transformation resulted in lower transgene copy number (average between 2.1 and 2.3) in transgenic rice plants, compared with those plants obtained by the biolistic method (average between 4.2 and 5.6). The frequency of DNA rearrangement in expression cassettes is lower in transgenic rice plants obtained by the Agrobacterium -mediated method than those obtained by the biolistic method. The average rearrangement frequency is 0.07 to 0.106 for the Agrobacterium -mediated method, and 0.57 to 0.66 for the biolistic method. Our results suggest that it is better to compare the number of intact expression cassettes instead of the total copy number of the transgene in demonstrating their influence on the level of transgene expression. This is the first report on the frequency of expression cassette rearrangement in transgenic plants transformed with the same plasmid by two different transformation methods.
基因枪法和农杆菌介导法对水稻转基因拷贝数和基因重排概率的影响
程在全* 黄兴奇* Ray WU**
(Department of Molecular Bioligy and Genetics,Cornell Unxersity,Ithaca,NY 14853,USA)
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