Concerns regarding 'off-target' activity of genome editing endonucleases

Genome editing (GE) tools ensure targeted mutagenesis and sequence-specific modification in plants using a wide resource of customized endonucleases; namely, zinc-finger nucleases (ZFNs), and transcription activator-like effector nucleases (TALENs), and the CRISPR (clustered regularly interspaced short palindromic repeats)/Cas (CRISPR-associated protein) system. Among these, in recent times CRISPR/Cas9 has been widely used in functional genomics and plant genetic modification. A significant concern in the application of GE tools is the occurrence of 'off-target' activity and induced mutations, which may impede functional analysis and gene activity studies. Moreover, the 'off-target' activity results in either not reported or unknown, difficult to detect, produce non-quantifiable cellular signaling and physiological effects. In the past few years, several experimental methods have been developed to identify undesired mutations and to curtail 'off-target' cleavage. Improvement in target specificity and minimizing 'off-target' activity will offer better applications of GE technology in plant biology and crop improvement.