Gene-editing leaving no traces of transgenic DNA is widely possible by using clever plant breeding tricks with pollen to eliminate the editing machinery.

Key finding:

In conclusion, we report that HI-Edit enables widespread application of genome-editing technologies for crop improvement. Compared with existing methods, HI-Edit is faster and more effective at delivering edits to advanced breeding materials, thereby avoiding the delay and high cost of introgression. Importantly, HI-Edit offers a path to transgene-free edited inbred lines, a major consideration for regulatory approval of genome-edited crops. We used HI-Edit to enable genome editing of monocots (maize and wheat) and dicots (Arabidopsis) by both maternal and paternal HI mechanisms. The MATL knockout system could be extended to rice and other mono-cots, and CENH3 and wide-crossing HI-Edit systems may be used in both monocots and dicots. The rate of haploid editing was high for CENH3 (17% of haploids edited) and for five out of six maize lines (more than 3% of haploids were edited).

Overall summary:

Genome editing using CRISPR–Cas9 works efficiently in plant cells, but delivery of genome-editing machinery into the vast majority of crop varieties is not possible using established methods. We co-opted the aberrant reproductive process of haploid induction (HI) to induce edits in nascent seeds of diverse monocot and dicot species. Our method, named HI-Edit, enables direct genomic modification of commercial crop varieties. HI-Edit was tested in field and sweet corn using a native haploid-inducer lineand extended to dicots using an engineered CENH3 HI system. We also recovered edited wheat embryos using Cas9 delivered by maize pollen. Our data indicate that a transient hybrid state precedes uniparental chromosome elimination in maize HI. Edited haploid plants lack both the haploid-inducer parental DNA and the editing machinery. Therefore, edited plants could be used in trait testing and directly integrated into commercial variety development.

Letter
One-step genome editing of elite crop germplasm during haploid induction
Tim Kelliher and others Nature Biotechnology volume 37, pages287–292 (2019)