Heat-shock inducible CRISPR/Cas9 system generates heritable mutations in rice

Summary Transient expression of CRISPR/Cas9 is an effective approach for limiting its activities and improving its precision in genome editing. Here, we describe the heat-shock inducible CRISPR/Cas9 system for controlled genome editing, and demonstrate its efficiency in the model crop, rice. Using a soybean heat-shock protein gene promoter and the rice U3 promoter to express Cas9 and sgRNA, respectively, we developed the heat-shock (HS) inducible CRISPR/Cas9 system, and tested its efficacy in targeted mutagenesis. Two loci were targeted by transforming rice with HS-CRISPR/Cas9 vectors, and the presence of targeted mutations was determined before and after the HS treatment. We found only a low rate of targeted mutagenesis before HS, but an increased rate of mutagenesis after HS treatment among the transgenic lines. Specifically, only ∼11% of transformants showed characteristic insertions-deletions at the ambient room temperature, but a higher percentage (∼45%) of callus lines developed mutations after a few days of HS treatment. Analysis of regenerated plants harboring HS-CRISPR/Cas9 revealed that targeted mutagenesis was suppressed in the plants but induced by HS, which was detectable by Sanger sequencing after several weeks of HS treatments. Most importantly, the HS-induced mutations were transmitted to the progeny at a high rate, generating monoallelic and biallelic mutant lines that independently segregated from Cas9. Taken together, this work shows that HS-CRISPR/Cas9 is a controlled and reasonably efficient platform for genome editing, and therefore, a promising tool for limiting genome-wide off-target effects and improving the precision of genome editing. Significance Statement A method for the temporal control on gene editing based on the use of heat-shock induced expression of CRISPR/Cas9 is described, which was efficient in producing heritable mutations in the rice genome. We assume this method will be useful for targeting essential genes and improving the precision of CRISPR/Cas9.