Efficient production of gene-edited onion (Allium cepa) plants using biolistic delivery of cas9 RNPs and transient expression constructs

Delivery of Cas9/sgRNA RNPs and DNA vectors designed to transiently express morphogenic regulatory genes or an antibiotic resistance gene enabled efficient recovery of gene edited onion lines. We developed a protocol for efficiently producing gene edited onion plants that does not depend on stable transformation. The process makes use of transient gene expression to enrich for gene editing among plants regenerated from immature embryos bombarded with ribonucleoprotein (RNP) complexes composed of CRISPR-associated protein 9 (Cas9) and single guide RNAs (sgRNAs). We used the Allium cepa Downy Mildew Resistant 6 (AcDMR6) gene as the target gene for our studies and produced a total of 47 onion plants with edited AcDMR6 alleles, including 13 homozygous plants, 12 biallelic plants, 7 heterozygous plants, and 15 chimeric plants. The most effective strategy for producing gene edited onion plants involved co-delivering plasmids encoding a hygromycin-resistance protein and plant developmental regulator genes along with the Cas9/sgRNA RNPs followed by transient Hygromycin selection for 48 h. Using this approach, up to 12% of the regenerated onion plants carried edited alleles of AcDMR6. By comparison, no editing was observed among the 146 plants regenerated from explants transfected with the Cas9/sgRNA RNPs alone. The strategy we describe here for using transient gene expression to enrich for gene editing in onion could potentially be extended to other crop species as well.