Use of N-methyl-N-nitrosourea treatment of fertilized egg cells for saturation mutagenesis of rice

The efficiency of N-methyl-N-nitrosourea (MNU) treatment of fertilized egg cells at the single-cell stage in rice (Oryza sativa L.) was about twice that of dry seeds. The proportion of recessive iso-chlorophyll mutants in the segregating progeny of M₁ plants decreased remarkably with treatment at later stages of embryonic development, perhaps because of the presence of diplontic selection and chimera formation when older, multi-cellular embryos were mutagenized. Panicle sterility of the M₁ plants showed a linear relationship with the frequency of chlorophyll mutations in the progeny. The mutagenicity of N-ethyl-N-nitrosourea was similar to that of MNU, but N-methyl-N′-nitro-N-nitrosoguanidine and N-ethyl-N′-nitro-N-nitrosoguanidine were less effective. Several thousand rice mutants affecting endosperm, culm length, heading date, and chlorophyll were obtained by MNU mutagenesis. The frequency of certain types of endosperm mutants seemed to differ according to the precise timing of treatment. The mutation rate was calculated as 7.4 × 10^-6 per nucleotide, and the mutations were evenly distributed over the gene regions examined. These results indicate that our rice mutant library, generated by MNU mutagenesis, is a promising resource for identifying mutations in any rice gene.