Generation and characterization of bentazon susceptible mutants of commercial male sterile lines and evaluation of their utility in hybrid rice production

Abstract

Male sterile lines are required to have complete male sterility during hybrid seed production to ensure high seed purity in rice. However, unexpected temperature fluctuations during the reproductive stage could cause partial fertility reversion of certain types of male sterile rice, resulting in decreased hybrid seed purity. Herbicide susceptible male sterile lines have been sought to solve the problem through the use of herbicide spray at seedling stage of hybrid to eliminate seedlings of selfed sterile line. Two such male sterile lines, one CMS line and one P/TGMS line, which are otherwise excellent, were subjected to gamma-ray mutagenesis for bentazon susceptibility. A cost-effective system with a ∼20-fold workload reduction was first developed for bentazon susceptible mutant selection; three bentazon susceptible mutant lines were subsequently selected and identified to carry independent mutations of the CYP81A6 gene: one 1-bp deletion, one 2-bp deletion and one 11-bp deletion, which all would result in truncated and dysfunctional CYP81A6 enzymes. Two molecular markers were also developed for the latter two mutations, which are expected to facilitate the efficient use of the mutations in future breeding programs. No significant negative effects on other agronomic traits of both the mutants and their F1 hybrids with other varieties were observed, which indicated that the mutant lines could directly replace their parental lines in commercial hybrid rice production. Herbicide test also proved that all contaminated bentazon susceptible seedlings could be completely killed while hybrid seedlings were not affected. Taking together, our study demonstrated that bentazon susceptible mutants of practical use for maintaining high hybrid rice purity in paddy fields could be developed by gamma-ray mutagenesis of commercial male sterile lines aided with an efficient selection protocol.