Whole-Genome Sequencing of Rice Mutant Library Members Induced by N-Methyl-N-Nitrosourea Mutagenesis of Fertilized Egg Cells

Abstract

Although targeted genome editing technology has become a powerful reverse genetic approach for accelerating functional genomics, conventional mutant libraries induced by chemical mutagens remain valuable for plant studies. Plants containing chemically induced mutations are simple yet effective genetic tools that can be grown without regard for biosafety issues. Whole-genome sequencing of mutant individuals reduces the effort required for mutant screening, thereby increasing their utility. In this study, we sequenced members of a mutant library of Oryza sativa cv. Nipponbare derived from treating single fertilized egg cells with N-methyl-N-nitrosourea (MNU). By whole-genome sequencing 266 M1 plants in this mutant library, we identified a total of 0.66 million induced point mutations. This result represented one mutation in every 146-kb of genome sequence in the 373 Mb assembled rice genome. These point mutations were uniformly distributed throughout the rice genome, and over 70,000 point mutations were located within coding sequences. Although this mutant library was a small population, nonsynonymous mutations were found in nearly 61% of all annotated rice genes, and 8.6% (3248 genes) had point mutations with large effects on gene function, such as gaining a stop codon or losing a start codon. WGS showed MNU-mutagenesis using rice fertilized egg cells induces mutations efficiently and is suitable for constructing mutant libraries for an in silico mutant screening system. Expanding this mutant library and its database will provide a useful in silico screening tool that facilitates functional genomics studies with a special emphasis on rice.