Targeted Gene Editing in Wheat During Haploid Production via Wide Hybridization with Transgenic Maize Expressing Cas9 and Guide RNA.

The clustered regularly interspersed short palindromic repeats (CRISPR)/Cas9 system is an efficient and versatile genome engineering tool, which has been widely used for targeted mutagenesis and gene functional characterization in various organisms. This system is simple because it only requires a Cas9 enzyme serving as a nuclease and guide RNA (gRNA) containing a 20-nt sequence matching the target gene. Delivery of a vector expressing Cas9 and gRNA or the preassembled Cas9/gRNA complex as a ribonucleoprotein (RNP) into plant cells for gene targeting are usually via the biolistic- or Agrobacterium-mediated approach. However, most wheat genotypes suffer from low efficiency of callus induction and plant regeneration from explants receiving the vector or RNP delivered by the biolistic- or Agrobacterium-mediated transformation method, limiting the application of genome editing systems in many commercially grown wheat varieties. Here, we describe a stepwise protocol for targeted gene editing in wheat via wide hybridization with transgenic maize expressing Cas9 and gRNA. A binary vector expressing Cas9 and gRNA is constructed and used for Agrobacterium-mediated transformation to generate transgenic maize plants, which are used to pollinate emasculated spikes of wheat varieties. After fertilization, the maize chromosomes enter the transient hybrid zygote and the transgene (T-DNA) on a maize chromosome expresses the Cas9 enzyme and gRNA, which forms an RNP complex to edit the target gene in wheat genome. After several cell divisions, maize chromosomes in the hybrid zygote are eliminated, resulting in formation of haploid wheat embryos with the target gene edited, which can be rescued by embryo culture technique to produce haploid plants. Doubled haploid (DH) wheat plants with homozygous gene mutations are developed by chromosome doubling through colchicine treatment of the haploid plants. The wheat × maize hybridization combined with the CRISPR/Cas9 system provides a one-step approach for generating DH lines with the target gene edited from any wheat genotypes of interest.