Efficiency of potato genome editing: Targeted mutation on the genes involved in starch biosynthesis using the CRISPR/dMac3-Cas9 system

Potato (Solanum tuberosum L.) has a tetraploid genome. To make a mutant lacking a specific gene function, it is necessary to introduce mutations into all four gene alleles. To achieve this goal, we developed a powerful genome editing tool, CRISPR/dMac3-Cas9, which installed the translation enhancer dMac3 that greatly increased the translation of the downstream open reading frame. The CRISPR/dMac3-Cas9 system employing three guide RNAs (gRNAs) greatly elevated the frequency of the generation rate of mutation. This system enabled to create the 4-allele mutants of granule-bound starch synthase (GBSS) and starch branching enzyme (SBE). These mutants indicated functionally defective features, suggesting that we succeeded in efficient genome editing of the potato tetraploid genome. Here, we show the effect of the number of gRNAs for efficient mutagenesis of the target gene using the mutants of the GBSS1 gene. CRISPR/dMac3-Cas9 employing three gRNA genes achieved a higher mutation efficiency than the CRISPR/dMac3-Cas9 with two gRNAs, suggesting being influenced by the dose effect of the number of gRNAs at the target region. The alleles of the SBE3 gene contained SNPs that caused sequence differences in the gRNAs but these gRNAs functioned efficiently. However, many rearrangement events and large deletions were induced. These results support the importance of accurate binding of gRNA to the target sequence, which may lead to a hint to avoid the unexpected mutation on the off-target sites.