Maiko Akasaka

Development and characterization of transgenic dominant male sterile rice toward an outcross-based breeding system

Genomic selection is attracting attention in the field of crop breeding. To apply genomic selection effectively for autogamous (self-pollinating) crops, an efficient outcross system is desired. Since dominant male sterility is a powerful tool for easy and successive outcross of autogamous crops, we developed transgenic dominant male sterile rice (Oryza sativa L.) using the barnase gene that is expressed by the tapetum-specific promoter BoA9. Barnase-induced male sterile rice No. 10 (BMS10) was selected for its stable male sterility and normal growth characteristics. The BMS10 flowering habits, including heading date, flowering date, and daily flowering time of BMS10 tended to be delayed compared to wild type. When BMS10 and wild type were placed side-by-side and crossed under an open-pollinating condition, the seed-setting rate was <1.5%. When the clipping method was used to avoid the influence of late flowering habits, the seed-setting rate of BMS10 increased to a maximum of 86.4%. Although flowering synchronicity should be improved to increase the seed-setting rate, our results showed that this system can produce stable transgenic male sterility with normal female fertility in rice. The transgenic male sterile rice would promote a genomic selection-based breeding system in rice.

Development of transgenic male-sterile rice by using anther-specific promoters identified by comprehensive screening of the gene expression profile database ‘RiceXPro’

Because genomic selection is designed for the population breeding of allogamous species, a successive outcrossing system is required for efficient use of genomic selection in autogamous crops, such as Oryza sativa L. (rice). Transgenic and dominant male-sterility is a suitable tool for efficient outcrossing of autogamous crops. Though there have been some reports of dominant male-sterile rice developed using transgenic technology, the flowering habit was substandard. Here, to isolate promoters that, when linked to a lethal gene, induce dominant male-sterility while retaining a good flowering habit, we identified 38 candidate genes with anther-specific expression by using the ‘RiceXPro’ database. We then evaluated the abilities of the near-upstream regions of these genes to induce male-sterility when linked to the lethal gene barnase and introduced into the rice cultivar ‘Nipponbare’. Seven of the 38 promoters induced clear dominant male-sterility; promoters expressed in the later stage of anther development induced male-sterility while retaining better flowering habits when compared to ones expressed in the early stage. These seven promoters could potentially be used to facilitate development of an efficient outcross-based breeding system in rice.