An update on precision genome editing by homology-directed repair in plants

Beneficial alleles derived from local landraces or related species, or even orthologs from other plant species, are often caused by differences of one or several single-nucleotide polymorphisms or indels in either the promoter region or the encoding region of a gene and often account for major differences in agriculturally important traits. Clustered regularly interspaced short palindromic repeats-associated endonuclease Cas9 system (CRISPR/Cas9)-mediated precision genome editing enables targeted allele replacement or insertion of flag or foreign genes at specific loci via homology-directed repair (HDR); however, HDR efficiency is low due to the intrinsic rare occurrence of HDR and insufficient DNA repair template in the proximity of a double-stranded break (DSB). Precise replacement of the targeted gene with elite alleles from landraces or relatives into a commercial variety through genome editing has been a holy grail in the crop genome editing field. In this update, we briefly summarize CRISPR/Cas-mediated HDR in plants. We describe diverse strategies to improve HDR efficiency by manipulating the DNA repair pathway, timing DSB induction, and donor delivery, and so on. Lastly, we outline open questions and challenges in HDR-mediated precision genome editing in both plant biological research and crop improvement.