Abstract:Spontaneous genic male sterility will be a best genetic tool for hybrid seed production if uniform male sterile (ms, hereafter) lines are available. Some strategies were suggested to obtain such lines by means of cloning and transformation of male fertility (MF, hereafter) genes of spontaneous genic male sterility: (1) transferring MF gene into plastid genome of ms plants to suppress ms gene. The male fertile transgenic plants can be used as the maintainers to reproduce the genic male sterile lines when crossed to ms plants. (2) To comprise a lethal gene such as ribosomal inhibitor protein gene to transiently activate promoters that are active only in late embryogensis, then splice it with the MF gene, and transfer them into ms plants. The spikelets of transgenic plants may be fertile at heading stage and killed by the lethal gene when matured. When the transgenic plants are crossed with ms plants, their hybrid seeds with MF gene will be killed. The others with same nuclear as ms plants will survive. Immature seeds with the lethal gene can be harvested before maturing as the maintainer. (3) Gene switch system such as Cre/lox can snip out a stretch of DNA in plant genome. Its repressor protein prevents Cre recombinase gene from being switched on. Such stimulant as the antibiotic tetracycline can block the repressor protein. We can splice a DNA sequence that can be recognized by Cre recombinase to each end of a MF gene. By transferring this stretch of DNA and the gene switch system into ms plants, we can obtain a transgenic plant having homozygous MF gene, ms gene and the gene switch system. The transgenic plant may reproduce seeds like a conventional variety.
When it is treated with the stimulant, the repressor protein will be blocked. The recombinase gene can work and snip out the MF gene. The fertile transgenic plants become male sterile then and can be used as the female parent for hybrid seeds production. (4)Take the native promoter from MF gene and replace it with a promoter that only functions in response to a chemical (such as antibiotic tetracycline), transfer the recombinant into ms line. The transgenic line would be fertile when the inducing chemical is present and used for the multiplication of ms plants. For hybrid seeds production the chemical will not be applied. Artificial male sterile genes and their restoration genes are also suggested to be used in the strategies.