Development and prospect of hybrid japonica rice in China

The extension of hybrid japonica rice is of great significance to increase rice production and ensure grain security in China. The low productivity of parental materials and the narrow genetic distance between parents resulted in the insufficient superiority of hybrid japonica rice in China, and the yield of japonica rice cannot be improved significantly compared with that of conventional japonica rice. The low yield of hybrid seed production and few super japonica hybrid combinations directly limited the widely extension of hybrid japonica rice. In this paper, the history of development of japonica hybrid rice breeding in China was reviewed, including the discovery and utilization of sterile lines such as Dian type and BT type, as well as the breeding of hybrid japonica restorer lines (C57 and C418) with high combining ability via “indica-japonica bridge” method. In addition, the key technology advances of increasing hybrid seed production of japonica rice and utilization of heterosis were summarized as following: (1) The increase of stigma exsertion rate and enhancement of stigma viability lead to high probability of successful fertilization, thus resulting in the increase of hybrid rice seed production. Significant correlations were showed between stigma exsertion rate and the ratio of floret length and width, stigma exsertion rate and column length, stigma exsertion rate and angle. Great progresses on stigma exsertion have been achieved from the studies on the genetic and molecular level (i.e marker-assisted selection). (2) To further increase the utilization of heterosis in hybrid japonica rice, the studies on heterotic groups and patterns for making hybrid combinations from the selection of superior parents in japonica rice were conducted. Based on the different heterotic groups of parental lines of japonica rice, several models for making hybrid combinations were proposed, including the model of heterosis complementation of indica and japonica rice, the model of the interactions of thermo-sensitive and photo-period-sensitive genes, and the model of grain structure complementation. These models emphasize comprehensive characteristics in rice breeding, including the utilization of indica-japonica intersubspecific heterosis, the applications of wide compatibility genes and restorer genes in rice, the anti-senility of leaves and roots, the applications of plant type such as large panicles and good grain-filling characteristics, and resistance for abiotic stress such as lodging resistance. Therefore, this study provides technical suggestions and strategies for further breeding and application of japonica hybrid rice.