Genome editing in maize: Toward improving complex traits in a global crop

Abstract

Recent advances in genome editing have enormously enhanced the effort to develop biotechnology crops for more sustainable food production. CRISPR/Cas, the most versatile genome-editing tool, has shown the potential to create genome modifications that range from gene knockout and gene expression pattern modulations to allele-specific changes in order to design superior genotypes harboring multiple improved agronomic traits. However, a frequent bottleneck is the delivery of CRISPR/Cas to crops that are less amenable to transformation and regeneration. Several technologies have recently been proposed to overcome transformation recalcitrance, including HI-Edit/IMGE and ectopic/transient expression of genes encoding morphogenic regulators. These technologies allow the eroding of the barriers that make crops inaccessible for genome editing. In this review, we discuss the advances in genome editing in crops with a particular focus on the use of technologies to improve complex traits such as water use efficiency, drought stress, and yield in maize.

Figure 1 -. Multiplex approaches for genome editing of multiple genes or multiple sites of a single gene. A. A single sgRNA is designed to target multiple genes with conserved domains/sequences (green boxes). B. Multiple sgRNAs are designed and simultaneously delivered, targeting sequences in different genes (colored boxes). C. Multiple sgRNAs may target different sections of a single gene, resulting in the deletion of specific regions between the target sites.

Figure 2 -. Using CRISPR/Cas to edit the promoter region of target genes. A. Cis -regulatory elements (CREs) present upstream of a given gene may act as enhancers (purple, red, and orange boxes) or repressors (green and blue boxes), modulating gene expression. B. Multiplex genome editing approach: multiple sgRNAs targeting different CREs may result in stochastic mutations in the promoter region, resulting in alleles with different expression patterns/levels. This method may ultimately lead to lines with a phenotypic gradient.

Figure 3 -. In trans genome editing in maize. First, a haploid-inducer (HI) line (amenable to transformation) is equipped with the CRISPR/Cas machinery targeting a specific locus (A). Next, HI pollen is used to pollinate plants from a non-transformable genotype (B). After fertilization, the CRISPR/Cas machinery encoded by the male parental genome edits the female genome (C). The male genome is degraded, resulting in a haploid embryo containing only the female genome (D). Chromosome doubling is achieved by applying chemical agents, resulting in a non-transgenic double haploid plant harboring the edited female genome (E).