Combined Hybridization and Evaluation of High-Lysine Rice: Nutritional and Physicochemical Qualities and Field Performance

Rice, as a major food crop, provides necessary energy and nutrition for humans and livestock. However, its nutritional value is affected by lysine. Using point mutation, we previously obtained AK2 (aspartokinase) and DHDPS1 (dihydrodipicolinate synthase) genes insensitive to lysine feedback inhibition and constructed transgenic lines AK2-52 and DHDPS1-22, which show increased lysine synthesis, as well as Ri-12, which shows decreased lysine degradation by inhibiting rice lysine ketoglutarate reductase/saccharopine dehydrogenase (LKR/SDH) activity. In this study, further transgenic lines were hybridized and evaluated. The lysine content of mature seeds from pyramid lines PRD and PRA increased 32.5- and 29.8-fold, respectively, compared with the wild-type, while the three-gene pyramiding line PRDA had a moderate lysine content. The total lysine, total free lysine, and total protein contents of PRD and PRA also increased and had no obvious impact on the physical and chemical quality, seed appearance, and main agronomic traits. Meanwhile, comparative analysis with polygenic polymeric lines GR containing bacterial AK (lysC) and DHDPS (dapA) genes revealed differences in the way bacterial and endogenous rice AK and DHDPS regulate lysine biosynthesis. These results provide a reference for further evaluation and commercialization of high-lysine transgenic rice.