Origin of the genome editing systems: application for crop improvement

In the last years, genome editing technologies led to a new era of genome engineering, enabling an effective, precise, and rapid engineering of the plant genomes. Four types of engineered nucleases are used for genome editing: engineered homing endonucleases/meganucleases (EMNs), zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and CRISPR (clustered regularly interspaced short palindromic repeats)/Cas (CRISPR-associated). All these nucleases have one thing in common: to induce double-strand breaks (DSBs) at definite genomic loci. DSB activates the DNA repair mechanisms through the recruitment of DNA repair factors by the cell, that can act in one of two repair pathways: homologous-direct repair (HDR) or non-homologous end-joining (NHEJ). The most common pathway used to repair DSBs in plants (NHEJ) is more mutagenic than the other alternative pathway (HDR). The development of CRISPR/Cas systems has generated a genome editing revolution in plant genetics and breeding. In the present review, the history of CRISPR/Cas methods is reported. Thereafter, the advances in genome editing technologies using CRISPR/Cas9 developed rapidly, with base editing systems for transition substitution using a combination of Cas9 nickase and either cytosine or adenosine deaminase. Moreover, technologies for gene targeting and prime editing systems using DNA or RNA as donors have also been developed. Successful examples where CRISPR/Cas approaches have been used to improve the biotic and abiotic stress tolerance, and traits related to yield and plant architecture have been discussed.