The Problem of the Low Rates of CRISPR/Cas9-Mediated Knock-ins in Plants: Approaches and Solutions

The main number of genome editing events in plant objects obtained during the last decade with the help of specific nucleases zinc finger (ZFN), transcription activator-like effector nucleases (TALEN), and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas are the microindels causing frameshift and subsequent gene knock-out. The knock-ins of genes or their parts, i.e., the insertion of them into a target genome region, are between one and two orders of magnitude less frequent. First and foremost, this is associated with the specific features of the repair systems of higher eukaryotes and the availability of the donor template in accessible proximity during double-strand break (DSB) repair. This review briefs the main repair pathways in plants according to the aspect of their involvement in genome editing. The main methods for increasing the frequency of knock-ins are summarized both along the homologous recombination pathway and non-homologous end joining, which can be used for plant objects.