TKC-MC: An Effective Strategy for Generating Heritable Heterozygous Mutations in Essential Genes in Rice

The CRISPR/Cas9 gene-editing technology has been widely used in defining gene functions and crop improvement. However, some genes are essential for plant growth and development. Loss-of-function homozygous mutations in essential genes lead to plant death or sterility. Mutations in essential genes need to be maintained and propagated in heterozygous plants. CRISPR/Cas9 technology is highly efficient in generating homozygous or bi-allelic mutations at T0 generation in rice, making it difficult to generate useful genetic materials for essential genes using traditional gene editing technology. In this study, we designed Transgene-Killer CRISPR (TKC)-mediated mismatch-spacer targeting (TKC-M) to efficiently generate heritable heterozygous mutations in essential genes in rice. Leveraging our earlier transgenic offspring self-elimination TKC platform, TKC-M relied on timely self-elimination of Cas9 and engineered gRNA-target mismatches to enrich heritable heterozygous or mosaic incomplete-edited T0 mutants and heterozygous progeny. We found that the sensitivity of targets to spacer mismatch(es) varies. A single-base mismatch at gRNA positions 11 or 17 yielded abundant heritable heterozygotes in sensitive targets. For insensitive targets, dual mismatches at positions 8 and 15 maximised heritable heterozygotes. Co-transformation of rice with TKC vectors carrying gRNA without mismatches (G1), gRNA with a mismatch at position 11 (M11) and M8 + M15 spacers, termed TKC-M Cocktail (TKC-MC) significantly increased the incomplete-edited mutant ratio compared with using G1 alone. This work establishes a technical foundation for generating mutant libraries that cover every single gene in a plant genome and for in-depth research on essential genes.