Evaluation of codA, tms2, and ABRIN-A as negative selectable markers in transgenic tobacco and rice

The codA and tms2 genes are used as efficient conditional negative selectable markers (NSMs) in several dicotyledonous plants. We evaluated both genes under control of the CaMV 35S promoter for their effectiveness as conditional NSMs. The ABRIN-A chain gene from Abrus precatorius was evaluated as a nonconditional NSM. The efficacies of codA, tms2, and ABRIN-A as NSMs were compared in transgenic rice and tobacco. Tobacco leaf discs and scutellum-derived callus of rice were transformed with the three genes. Leaf discs of T₀ transgenic tobacco plants and the T₁ seedlings of transgenic rice plants, both transformed with codA, showed a pronounced reduction in growth in the presence of the substrate 5-fluorocytosine. The tms2 gene was inferred to act as a nonconditional NSM in tobacco since all the recovered hygromycin-resistant transgenic tobacco plants harbored only truncated transferred DNAs (T-DNAs) with deletions of the tms2 gene. The T₁ transgenic rice seedlings transformed with tms2 showed a drastic reduction in shoot and root growth in the presence of the substrate naphthaleneacetamide. Both codA and tms2 genes served as good conditional NSMs in rice. The ABRIN-A gene proved to be a good nonconditional NSM in tobacco since all recovered hygromycin-resistant plants harbored only truncated T-DNAs with deletions of the ABRIN-A gene. Twelve transgenic rice plants, which harbored the complete ABRIN-A gene, displayed normal growth suggesting that ABRIN-A is not toxic to rice.