Apomixis in rice and prospects for its use in heterosis breeding

摘要

Apomixis is being explored as a new frontier to exploit hybrid vigor and to develop true-breeding hybrid rice varieties. Apomixis is common in grasses and in several polyploid plant species and is controlled by one or a few genes. Among the major cereals, maize, wheat,and pearl millet have apomictic relatives. But there is no clear evidence of apomixis in rice. A-genome diploid wild relatives have been examined based on studies of crosses with dominant marker stocks.Reports on cyto-embryological studies also lack genetic evidence for the occurrence of apomixis in rice. We proposed three strategies to develop apomictic rice: (1) screening germplasm of tetraploid wild species as a source of apomixis and transferring the apomictic trait to rice cultivars, (2) inducing apomictic mutants in rice through mutagenesis,and (3) developing apomictic rice using molecular approaches. We have screened 108 accessions of tetraploid Oryza species for apospory (multiple embryo sac development) and 86 accessions for diplospory (based on callose detection), including five related genera. But none of the accessions showed any evidence of apomixis. We have also undertaken the second approach to induce apomictic mutants. Mutagenized populations derived from treating seeds and fertilized egg cells with gamma rays, ethyl methane sulphonate, and N-methyl-N-nitrosourea are being screened. We have selected a dominant purple leaf mutant of rice for identification of the apomictic mutants following mutagenesis. IRRI is collaborating with advanced laboratories to develop apomictic rice through molecular approaches. Molecular markers linked to the apomictic mode of reproduction have been identified in progenies of maize (Tripsacum) and in crosses of sexual and apomictic wild species of Pennisetum. Cloning of the gene(s) for apomixis is under way from apomictic plant species such as Tripsacum, Pennisetum, Brachiaria,and Cenchrus. Once such genes become available, they will be introduced into elite breeding lines of rice using transformation technology.We are also exploring the possibility to identify dividing nucellar cells capable of forming adventitious embryos in a transgenic rice line (HSK-1). Apomixis will increase the efficiency of heterosis breeding in producing many true-breeding hybrids compared with those produced by using three-line or two-line hybrid breeding systems.The availability of a large number of hybrids will increase genetic diversity and reduce genetic vulnerability. Moreover, the possible vulnerability to pests and diseases because of narrow cytoplasmic male sterility sources will also be eliminated. The development of apomictic rice would enable resource-poor farmers in developing countries to adopt high-yielding hybrid rice technology. This would lead to an increase in area planted to hybrid rice, resulting in higher productivity and production.