**2. CRISPR/Cas9 gene-editing system in plants**

CRISPR/Cas systems are split into two classes and five kinds based on the Cas protein classification. The *Streptococcus pyogenes* type II CRISPR/SpCas9 system has

**Figure 1.**

*The workflow of CRISPR/Cas9-based gene editing in plants.*


### **Table 1.**

*List of research on crop quality improvement by using CRISPR/Cas gene-editing technology.*

### **Figure 2.** *Mechanism of CRISPR/Cas9 system.*

been adapted and evolved as a powerful GE tool for several purposes [2]. The Cas9 protein and the guide RNA (gRNA or sgRNA) are the two main components. CRISPR RNA (crRNA) and trans-activating crRNA are both made up of gRNA (tracrRNA). A 20-nt fragment (also known as spacer, complementary to a certain region of target genes) is followed by a protospacer adjacent motif (PAM) in the target genes of interest in the gRNA. Cas9 nuclease generates DSBs to three base pairs upstream of the PAM motif under the supervision of gRNA [23]. Gene deletion or loss of protein function is common outcome of NHEJ cleavage repair [24]. **Figure 1** indicates the general steps of CRISPR/Cas9 gene editing technique.

Using CRISPR/Cas9, several scientists have been working to improve crop yield, quality, and stress resistance. CRISPR/Cas9-mediated genome editing has been reported in 41 food crop species, 15 industrial crops, 6 oil crops, 8 ornamental crops, 1 fiber crop, and 1 feed crop (**Table 1**) [25]. Mechanism of CRISPR/Cas9 system breakage is presenting in **Figure 2**.
