Delivery of Marker‐Free DNA to Plant Genome by the Transgenic Selection‐Associated Fragment Elimination (T‐SAFE) System

The presence of a selection marker in transgenic plants has raised public concerns regarding health safety. We have developed a CRISPR/Cas9-based DNA delivery system termed transgenic selection-associated fragment elimination (T-SAFE). The T-SAFE system comprises four cassettes: the selection marker, CRISPR/Cas9, spacer-plus-protospacer adjacent motif (SP), and the cargo. The first two cassettes, the selection marker and CRISPR/Cas9, are collectively referred to as SCC. The SCC is flanked by two identical SPs derived from the fruit fly <i>Ebony</i> gene, which efficiently facilitate the SCC cleavage and subsequently lead to self-elimination of the selection marker upon integration of exogenous DNA into the plant genome. To inhibit the production of a functional Cas9 protein in bacteria, the IV2 intron of the potato ST-LS1 gene has been incorporated into the Cas9 gene. Additionally, the Cas9 gene is driven by a reproductive cell-specific or inducible promoter to avoid SCC cleavage in nonreproductive plant cells. These innovative features allow the T-SAFE system to achieve an elimination efficiency of the selection marker ranging from 10%-30% in <i>Arabidopsis</i> and 5%-8% in rice, with a DNA delivery capacity of approximately 10 kb. This approach offers a safe means for genetically modifying plants.