Enhancement of the CRISPR/Cas9-Based Genome Editing System in Lettuce (Lactuca sativa L.) Using the Endogenous U6 Promoter

The CRISPR/Cas9 system has been widely applied as a precise gene-editing tool for studying gene functions as well as improving agricultural traits in various crop plants. Here, we optimized a gene-editing system in lettuce (Lactuca sativa L.) using the endogenous U6 promoter and proved that the PHOT2 gene is a versatile target gene. We isolated the LsU6-10 promoter from 10 U6 snRNA genes identified from the lettuce genome database for comparison with the AtU6-26 promoter that has been used to drive sgRNAs in lettuce. Two CRISPR/Cas9 vectors were constructed using the LsU6-10 and AtU6-26 promoters to drive sgRNA361 to target the PHOT2 gene. The chloroplast avoidance response was defective in lettuces with biallelic mutations in the targeted PHOT2 gene, as in the Arabidopsis phot2 mutant. The PHOT2 gene mutations were stably heritable from the R0 to R2 generations, and the high gene-editing efficiency enabled the selection of transgene-free lines in the R1 generation and the establishment of independent phot2 mutants in the R2 generation. Our results suggest that the LsU6-10 promoter is more effective than the AtU6-26 promoter in driving sgRNA for the CRISPR/Cas9 system in lettuce and that PHOT2 is a useful target gene to verify gene editing efficiency without any detrimental effects on plant growth, which is often a consideration in conventional target genes.