A multiplex guide RNA expression system and its efficacy for plant genome engineering.

BACKGROUND

The CRISPR system is composed of a Cas9 endonuclease (Cas9) and a single-stranded guide RNA (gRNA) harboring a target-specific sequence. Theoretically, Cas9 proteins could cleave as many targeted loci as gRNAs bind in a genome.

RESULTS

We introduce a PCR-free multiple gRNA cloning system for editing plant genomes. This method consists of two steps: (1) cloning the annealed products of two single-stranded oligonucleotide fragments harboring a complimentary target-binding sequence on each strand between tRNA and gRNA scaffold sequences in a pGRNA vector; and (2) assembling tRNA-gRNA units from several pGRNA vectors with a plant binary vector containing a Cas9 expression cassette using the Golden Gate assembly method. We validated the editing efficiency and patterns of the multiplex gRNA expression system in wild tobacco () protoplasts and in transformed plants by performing targeted deep sequencing. Two proximal cleavages by Cas9-gRNA largely increased the editing efficiency and induced large deletions between two cleavage sites.

CONCLUSIONS

This multiplex gRNA expression system enables high-throughput production of a single binary vector and increases the efficiency of plant genome editing.