**4. CRISPR**/**Cas9-Mediated GE in Plants**

To date, numerous efforts have been successfully employed for targeted gene editing in model as well as in major crop plants via the CRISPR/Cas9-based GE toolbox. Many factors have been reported that affect the editing ability of the CRISPR/Cas9 system including targeted DNA, GC contents, Cas9 codons, sgRNA structure, and expression of cas9 and sgRNA. All these factors must be highly optimized to achieve greater efficiency of CRISPR/Cas9 system [24].

## *4.1. Designing the CRISPR*/*Cas9 Delivery System*

In the past, numerous attempts have been made at gene editing in plants, but the efficiency of CRIPSR/Cas9 was low [25,26]. With the improvements in technology, many highly efficient vector delivery systems for CRISPR/Cas9 have been designed for plant GE, such as supersession of viral infection, gene disruption of cis-elements, genomic deletion, gene knockout, and multiplex genome editing. Continuous progress in this editing toolkit has allowed more precise, accurate, and targeted delivery of the Cas9 system into plant cells, which include discovery of new Cas9 variants, efficient screening methods for knockout mutants, vector selection, and construction and employment of the most appropriate delivery system for the Cas9 expression cassette. In this section, we describe the construction, screening, and delivery of the CRISPR/Cas9 system into plant cells.
