Genome editing in crop improvement: Present scenario and future prospects

摘要

Genome editing refers to a process by which a specific chromosomal sequence is changed. The edited chromosomal sequence may comprise an insertion of at least one nucleotide, a deletion of at least one nucleotide, and/or a substitution of at least onenucleotide. Genome editing is a relatively new technology that is gaining importance as a tool for crop improvement because of its advantages over routinely used methods of genetic engineering. Genome-editing technology is precise and efficient. Genome editing is now considered a safe technique because no foreign sequences are left behind in the final genome-edited organism (GEO). Genome editing involves the induction of double-stranded breaks (DSBs) at specific sites of DNA, which turns on endogenous repair mechanisms—homol-ogy-dependent repair (HDR)—when homologous sequences are present, and nonhomologous end-joining (NHEJ) in the absence of homologous sequences. During repair, site-specific mutations are produced. A range of molecular tools for inducing DSBs at specific sites of a genome is available with genome editors. One category of such molecular scissors include engineered and programmable site-specific nucleases (SSNs), such as meganucleases (MNs), also known as homing nucleases (HNs), zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and RNA-guided nuclease (RGN) systems, the most widely used RGN being the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated system 9 (CRISPR/Cas9), and DNA-guided nuclease (DGN) system, i.e., A/gAgo (an acronym for Natronobacterium gregoryi Argonaute). Transposons and Group II intron retro-transposition have also been employed in genome editing. Some new genome-editing approaches have also emerged under the umbrella of triplex technology, which are based on antisense technology and make use of diverse types of oligonucleotide-linked nucleases, triplex-forming oligonucleotides, nucleic acid analogs, peptide nucleic acids, and aptamers for providing homologous sequences for HDR. Some engineered animal viruses such as lentiviruses, adeno-associated viruses, recombinant adeno-associated viruses, and adenoviruses (AdVs) and plant viruses such as RNA viruses, tobacco rattle virus (TRV), andsingle-stranded DNA (ssDNA) viruses called geminiviruses have been used as genome-editing devices that act as delivery vehicles of SSNs.