Technological breakthroughs in generating transgene-free and genetically stable CRISPR-edited plants

CRISPR/Cas9 gene-editing technologies have been very effective in editing target genes in all major crop plants and offer unprecedented potentials in crop improvement.A major challenge in using CRISPR gene-editing technology for agricultural applications is that the target gene-edited crop plants need to be transgene free to maintain trait stability and to gain regulatory approval for commercial production.In this article,we present various strategies for generating transgene-free and target geneedited crop plants.The CRISPR transgenes can be removed by genetic segregation if the crop plants are reproduced sexually.Marker-assisted tracking and eliminating transgenes greatly decrease the time and labor needed for identifying the ideal transgene-free plants.Transgenes can be programed to undergo self-elimination when CRISPR genes and suicide genes are sequentially activated,greatly accelerating the isolation of transgene-free and target gene-edited plants.Transgene-free plants can also be generated using approaches that are considered non-transgenic such as ribonucleoprotein transfection,transient expression of transgenes without DNA integration,and nano-biotechnology.Here,we discuss the advantages and disadvantages of the various strategies in generating transgene-free plants and provide guidance for adopting the best strategies in editing a crop plant.