Rice Genetics IV - IRRI books - International Rice Research Institute

Molecular tools for achievingsynthetic apomixis in hybrid riceJ. Bennett, X.-Z. Bi, A. Kathiresan, and G.S. KhushApomixis is asexual reproduction through seed. It occurs in more than 300plant species but appears to be absent from the genus Oryza. One of themost important applications of apomixis in rice would be to develop truebreedinghybrid genotypes that would allow poor farmers to benefit from hybridyield advantage. Although considerable progress has been made in mappingloci that control apomixis in other members of the family Poaceae, nogene capable of inducing apomixis has yet been isolated for introduction intorice. This chapter reports on the development of molecular tools to achieve asynthetic apomixis in rice, in which the sexual embryo would be replaced byan asexual embryo induced in the nucellus. We cover two objectives of theresearch: induction of the nucellar embryo and ablation of the zygotic embryo.We emphasize the tasks of identifying embryo-inducing genes, placingthem under the control of nucellus-specific promoters, and assaying theirefficacy in rice.Hybrid rice accounts for about 65% of rice production in China and is increasinglycommon in Korea, India, the United States, and several other countries (Li and Yuan1999). The main incentive for planting hybrid rice is the yield advantage over the bestinbred lines (Virmani 1994). This yield advantage is now 15–20% for indica-indicaand japonica-japonica hybrids, but the recent development of indica-tropical japonicahybrids could further increase yield heterosis (Khush et al 1998).The main disincentive to planting hybrid rice is the cost of buying expensivehybrid seed each season. Although the cost of hybrid seed is declining in China (Liand Yuan 1999), it remains high in tropical countries. Many rice farmers in developingcountries are accustomed to reducing the cost of inbred seed by setting aside aportion of their production each year for sowing in the following season. If this practicewere adopted for hybrid rice, farmers would notice a decline in yield advantageand plant uniformity, as the heterozygosity of the F 1 generation declined in the F 2 andlater generations. However, if the heterozygosity of hybrid rice could be fixed byMolecular tools for achieving synthetic apomixis in hybrid rice 377