Precise Editing of a Target Base in the Rice Genome Using a Modified CRISPR/Cas9 System.

CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/CRISPR-associated Cas9) has been widely used in genome editing in a variety of organisms, including rice (Cong et al., 2013; Feng et al., 2013). The majority of the editing events reported in plants typically result in an abundance of random insertions and deletions (Indels) at the target locus through the error-prone non-homologous end joining pathway (NHEJ) (Feng et al., 2013; Mao et al., 2013; Shan et al., 2013), making genome editing an extremely valuable tool for plant functional genomics research. Most known agriculturally important traits, however, are conferred by point mutations (Huang et al., 2010; Hu et al., 2015). Development of a technique that enables precise and efficient base replacement in the target locus, rather than stochastic disruption of the gene, will greatly facilitate precision plant molecular breeding. Homology-directed repair (HDR) is a potential approach to achieve base replacement by providing a homologous DNA template during genome editing. However, HDR is extremely inefficient due to competition by NHEJ, the dominant pathway to repair double-strand breaks (DSBs) in plants, and Indels are generally much more abundant outcomes, making HDR a rather ineffective method to achieve base replacement in plants (Li et al., 2013; Mao et al., 2013).