Molecular marker-assisted selection of the Pi9 gene for improving blast resistance of rice sterile line Rongfeng A

Objective This study aimed to use the molecular marker-assisted selection technology to improve the resistance to rice blast for the three-line cytoplasmic male sterile （CMS） line Rongfeng A and its maintainer line Rongfeng B，so as to provide new resistant parents for the heterosis utilization in the three-line system.Method Using indica rice line 75-1-127 as the donor parent of the broad-spectrum and durable blast-resistant gene Pi9， and Rongfeng B and Rongfeng A as the recipient parent， the functional codominant marker SPL-1 of gene Pi9 was used to conduct molecular-assisted selection and breeding for improving the blast resistance of Rongfeng A and its maintainer line Rongfeng B. The blast resistance， sterility characteristics， agronomic traits， and yield traits of the improved lines were determined and analyzed.Result The amplification product of SPL-1 marker from Rongfeng B or Rongfeng A genomes was about 1300 bp in length，while the product from 75-1-127 genome was around 740 bp in length. This marker exhibited obvious polymorphism and clear and stable bands between the donor and recipient parents. Greenhouse inoculation assay was performed using fourteen Magnaporthe oryzae strains and the results showed that the resistance frequency of Rongfeng B or Rongfeng A was only 28.57 %，comparing to 85.71 % of 75-1-127. The field investigation in infected nursery showed that Rongfeng B and Rongfeng A were highly susceptible to seedling blast （grade 4），while the three lines of Rongfeng B-Pi9， the three lines of Rongfeng B-Pi9 ， and 75-1-127 were highly resistant to seedling blast （grade 0）. Field investigations of sterility showed that Rongfeng A and its three A-Pi9 improved sterile lines exhibited both sterility rate and degree at 100%， with typical sterility as the main type of pollen sterility， but Rongfeng B and its three improved maintainer lines Rongfeng B-Pi9 displayed normal pollen fertility. The stigma exsertion rate， bilateral stigma exsertion rate， and total stigma exsertion rate in the three Rongfeng A-Pi9 lines were higher than those in the recipient parent Rongfeng A. Among the three pairs of sterile lines/maintainer lines， Rongfeng A-Pi9-1 and its maintainer line Rongfeng B-Pi9-1 demonstrated the optimal comprehensive agronomic yield traits.Conclusion A new rice sterile line Rongfeng A-Pi9-1 based on three-line system and its maintainer line Rongfeng B-Pi9-1 were bred through backcross breeding assisted by the functional marker SPL-1. As they have high blast resistance， excellent sterility characteristics， and good agronomic and yield traits， they could be used as blast-resistant parental lines in hybrid breeding.