Hereditary Behavior of bar Gene Cassette is Complex in Rice Mediated by Particle Bombardment

Particle bombardment transformation using minimal gene cassette (containing the promoter, open reading frame and terminator) is the novel trend in plant genetic transformation, and its use helps to alleviate the undesirable effects of plasmid vector backbone sequences on transgenic plants. In the present article, studies related to the hereditary behavior of bar gene cassette in T₁ to T₃ generations of the transgenic rice (Oryza sativa L.) lines transformed by particle bombardment have been discussed. The selectable marker bar gene cassette that integrated with the rice genome had multiple copies and showed complex segregation behaviors including the presence of ‘false homozygotes’, with abnormal segregation ratios ranging from 35:1 to 144:1 (Basta-resistant: sensitive plants) in their progenies. In five out of ten original transgenic lines, bar gene can be stably transmitted as a dominant gene to self-pollinated T₂ progeny. The homozygotes were obtained in three transgenic lines in T₁ generation regardless of the multiple-copy integration patterns ofbar gene. Southern blotting analysis showed that multiple copies of bar gene cassette were linked, which formed transgene arrays in the host rice genome. The authors also observed stable transmission of integration patterns of bar gene cassette, as obtained from Southern blotting analysis, in the regularly segregated transgenic rice lines and loss of gene in an irregularly segregated transgenic line. The segregation behavior varied among the transgenic progenies that exhibited similar Southern hybridization patterns of bar gene. On the basis of these results, the multiple-copy integration, gene lost, and gene expression interaction were the major reasons for the complex segregation behaviors of bar gene cassette in transgenic rice plants.