Generation of selectable marker-free transgenic rice resistant to chewing insects using two co-transformation systems

To produce selectable marker-free (SMF) transgenic rice resistant to chewing insects, the Bacillus thuringiensis cryIA(c) gene (Bt) was introduced into two elite japonica rice varieties by using two Agrobacterium-mediated co-transformation systems. One system is with a single mini-twin T-DNA binary vector in one Agrobacterium strain, which consists of two separate T-DNA regions, one carrying the Bt while the other contains the selectable marker gene, hygromycin resistant gene (HPT). The other system uses two separate binary vectors in two separate Agrobacterium cultures, containing the Bt or HPT gene on individual plasmids. A lot of independent transgenic rice lines harboring both Bt and selectable marker genes were obtained. The results showed that the co-transformation frequency of the Bt gene and HPT gene was much higher by using the mini-twin T-DNA vector system (29.87%) than that by the two separate binary vector systems (4.52%). However, the frequency of the SMF transgenic rice plants obtained from the offspring of co-transgenic plants (21.74%) was lower for the mini-twin T-DNA vector system than that for the latter (50-60%). The data of ELISA implied that the expressed Bt proteins were quantitated as 0.025-0.103% of total leaf soluble proteins in the transgenic plant. Therefore, several elite transgenic rice lines, free of the selectable marker gene, were chosen. The results from both in vitro and in vivo insect bioassays indicated that the SMF transgenic rice was shown to be highly resistant to the striped stem borer and rice leaf folder. Moreover, in a natural field condition without any insecticide applied, all the transgenic rice plants were found to be not injured by the rice leaf folder, whereas the wild types were impaired seriously.