Fixation of hybrid vigor in rice: opportunities and challenges

Rice, one of most important crops, is emerging not only as a model plant for molecular biological studies in cereal crops, but also as a model system for hybrid breeding methodologies, which include three-line system, two-line system and one-line system, corresponding to inter-varietal heterosis, inter-subspecific heterosis and heterosis of distant hybridization based on enhancing heterosis level, respectively. The developmental goal and direction of rice breeding are to simplify procedure of hybrid breeding and to enhance heterosis level. One-line breeding is not only to use distance cross for enhancing heterosis but also to lock hybrid vigor and to produce permanent hybrid. Rice of early generation stability, sporophytic apomictic rice or gametophytic apomictic rice might be used as a genetic tool for one-line breeding, and also represent three main methods and three development stages in one-line breeding.