Precision genome editing in plants via gene targeting and subsequent break-induced single-strand annealing

Abstract

Genome editing via artificial nucleases such as CRISPR/Cas9 has become popular in plants now. However, small insertions or deletions are major mutations and nucleotide substitutions rarely occur when DNA cleavage is induced. To induce nucleotide substitutions, a base editor utilizing dead or nickase-type Cas9 fused with deaminase have been developed. However, the direction and position of practical substitution are still limited. In this context, homologous recombination (HR)-mediated gene targeting (GT) has advantages because any mutations existing on the donor DNA are copied and passed onto the endogenous DNA. As HR-mediated GT is extremely rare in higher plants, positive-negative selection has been used to isolate cells in which GT has occurred. After successful selection, positive selection marker is no longer needed and should ideally be eliminated. In a previous study, we reported a seamless piggyBac-transposon-mediated marker elimination system. Precision marker elimination efficiency in this system is very high. The piggyBac transposon integrates into the host genome at TTAA elements and excises without leaving a footprint at the excised site, so a TTAA sequence is necessary at the location of a positive selection marker. To compensate for this limitation, we have developed a novel marker elimination system using an I-SceI break and subsequent single-strand annealing (SSA)-mediated DNA repair system.

Keywords: gene targeting; genome editing; herbicide resistance; homologous recombination; phytoene desaturase; positive-negative selection; single-strand annealing.