Generation of selectable marker-free soft transgenic rice with transparent kernels by downregulation of SSSII-2

Abstract The amylose content (AC) of rice endosperm starch varies from 0 to 35%, and is associated with rice cooking and eating quality. Soft rice has low AC, generally between 6% and 15%, and its eating quality is high whether it is consumed hot or cold. However, the appearance quality of current soft rice cultivars needs to be improved, especially opaque endosperm. Conventional genetic engineering has improved some agronomic traits of soft rice varieties, but not endosperm appearance. In the present study, a RNAi construct of the soluble starch synthase II-2 ( SSSII-2 ) and the hygromycin phosphotransferase ( HPT ) gene were introduced into an elite japonica rice variety, Kangtiaowuyunjing (KWY8) by co-transformation. Several selectable marker-free (SMF) transgenic lines were obtained, and SSSII-2 expression was significantly downregulated in selected transgenic lines, resulting in lower AC of the endosperm. The physicochemical properties of the transgenic rice kernels, including gel consistency (GC) and rapid visco analyzer (RVA) profile, differed significantly from those of wild-type rice and were similar to those of a soft rice variety, Nanjing 46 (NJ46). These findings indicate that the cooking, eating, and processing qualities of transgenic rice are comparable to those of NJ46. However, the transgenic rice endosperm retained a transparent appearance under low-moisture conditions. Thus, SMF SSSII-2 RNAi rice provides a resource for breeding soft rice with transparent endosperm.