Single-nucleotide polymorphism (SNP) is an abundant form of genetic variation among individuals in plant species, which has been believed to have effects on the gene function responsible for phenotypic diversity. Utilization of functional SNPs associated with diverse important agronomic traits is of great value in crop improvements. Recently, a simple base editing system, which introduces a single-nucleotide change in genes of interest for generating gain-of-function mutants, has been developed in plants (Li et al., 2017, Lu and Zhu, 2017, Ren et al., 2017), in which Cas9(D10A) nickase is engineered with the rat cytosine deaminase (APOBEC1) and the uracil glycosylase inhibitor (UGI). Under the guidance of sgRNA, the Cas9n fusion protein is directed to the target locus in the genome, deaminating cytidines (C) in the editing window (13–19 bp before the PAM sequence) into uridines (U) and enabling the introduction of thymines (T), adenines (A) or guanines (G) during DNA replication (Li et al., 2017, Lu and Zhu, 2017, Ren et al., 2017). However, consistent with the observation that the AID/APOBEC family members have differential sensitivities to the 5′ nucleotide preceding the target C (Beale et al., 2004, Komor et al., 2016, Komor et al., 2017), APOBEC1 shows a striking preference for TC, and the editing efficiency of GC, in general, is quite low. To broaden the application of base editing technology in rice, which is characterized by high GC content, here we further optimize our rBE system with human AID (hAID), another type of cytosine deaminase with a priority on GC and AC (Beale et al., 2004), to introduce desired point mutations in the target region for gene correction or modification as a proof of concept.

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