Development of insect-resistant transgenic indica rice with a synthetic cry1C* gene

Stemborers and leaffolders are two groups of lepidopteran pests that cause severe damage to rice in many areas of the world. In this study, a cry1C* gene encoding Bacillus thuringiensis (Bt) δ-endotoxin was synthesized by codon optimization as the first step towards gene stacking in our resistance management strategy of transgenic rice. Agrobacterium-mediated transformation of this gene into Minghui 63 (Oryza sativa L.), an elite indica CMS restorer line, produced 120 independently transformed plants, 19 of which had a single-copy transgene. Preliminary screening of T₁ families of these 19 transformants in the field identified five lines showing a high level of resistance to leaffolders (Cnaphalocrocis medinalis) and stemborers. Hybrids were produced by crossing these five lines with Zhenshan 97A, the male-sterile line for Shanyou 63, the most widely cultivated hybrid in China. These five lines and their hybrids were highly resistant to yellow stemborer (Tryporyza incertulas) as revealed by an insect bioassay. The content of Cry1C* protein varied considerably among the five lines as well as among the corresponding hybrids. T1c-19, a line showing the highest content of Cry1C* protein, and its hybrid were tested in the field for insect resistance and agronomic performance and found to be highly resistant to stemborers and leaffolders throughout the growth period, resulting in a significantly increased grain yield compared with the respective controls. These results indicate that T1c-19 can be used for production of insect-resistant hybrid rice and as a germplasm for gene stacking to produce rice plants with two toxins.