Optimization of CRISPR/Cas9 ribonucleoprotein delivery into cabbage protoplasts for efficient DNA-free gene editing

The CRISPR/Cas9-based gene editing system for the direct delivery of pre-assembled Cas9 ribonucleoproteins (RNPs), consisting of a Cas9 nuclease and a single guide RNA (sgRNA), into plant protoplasts enables DNA-free gene editing without introducing foreign gene into plants. Here, we described the optimization of CRISPR/Cas9 RNPs delivery into cabbage protoplasts for efficient DNA-free gene editing. We determined the insertion and deletion (indel) frequency of BoMYBL2-1, a negative regulatory gene for anthocyanin biosynthesis in cabbage (Brassica oleracea var. capitata). We optimized the molar ratio of Cas9 to sgRNA and the incubation time of RNP–protoplast transfection to enhance the indel frequency under various conditions. Based on the BoMYBL2-1 nucleotide sequences, we designed nine sgRNAs to target BoMYBL2-1. Our in vitro digestion assay showed that all sgRNAs were able to cleave the targeted fragment. When the sgRNA and Cas9 proteins were subsequently transfected into protoplasts isolated from cabbage cotyledons, the deep sequencing results showed that the indel frequency of sgRNAs in BoMYBL2-1 was the highest (7.4%) with sgRNA3. We compared various molar ratios of Cas9 and sgRNA and incubation times of RNP–protoplast transfection to optimize transfection and ensure high indel frequency. The highest frequency was observed when the Cas9:sgRNA ratio was 1:10. Furthermore, when the incubation time for RNP–protoplast transfection was 1 min and 3 min, the indel frequency was higher than 25%. Altogether, these results provide valuable information on the optimized conditions for high-efficiency gene editing using CRISPR/Cas9 RNP delivery into cabbage protoplasts.