High-efficiency gene targeting in hexaploid wheat using DNA replicons and CRISPR/Cas9.

The ability to edit plant genomes through gene targeting (GT) requires efficient methods to deliver both sequence-specific nucleases (SSNs) and repair templates to plant cells. This is typically achieved using Agrobacterium T-DNA, biolistics or by stably integrating nuclease-encoding cassettes and repair templates into the plant genome. In dicotyledonous plants, such as Nicotinana tabacum (tobacco) and Solanum lycopersicum (tomato), greater than 10-fold enhancements in GT frequencies have been achieved using DNA virus-based replicons. These replicons transiently amplify to high copy numbers in plant cells to deliver abundant SSNs and repair templates to achieve targeted gene modification. In the present work, we developed a replicon-based system for genome engineering of cereal crops using a deconstructed version of the wheat dwarf virus (WDV). In wheat cells, the replicons achieve a 110-fold increase in expression of a reporter gene relative to non-replicating controls. Furthermore, replicons carrying CRISPR/Cas9 nucleases and repair templates achieved GT at an endogenous ubiquitin locus at frequencies 12-fold greater than non-viral delivery methods. The use of a strong promoter to express Cas9 was critical to attain these high GT frequencies. We also demonstrate gene-targeted integration by homologous recombination (HR) in all three of the homoeoalleles (A, B and D) of the hexaploid wheat genome, and we show that with the WDV replicons, multiplexed GT within the same wheat cell can be achieved at frequencies of ~1%. In conclusion, high frequencies of GT using WDV-based DNA replicons will make it possible to edit complex cereal genomes without the need to integrate GT reagents into the genome.

Research progress of genome editing and derivative technologies in plants.

Genome editing technologies using engineered nucleases have been widely used in many model organisms. Genome editing with sequence-specific nuclease (SSN) creates DNA double-strand breaks (DSBs) in the genomic target sites that are primarily repaired by the non-homologous end joining (NHEJ) or homologous recombination (HR) pathways, which can be employed to achieve targeted genome modifications such as gene mutations, insertions, replacements or chromosome rearrangements. There are three major SSNs─zinc finger nuclease (ZFN), transcription activator-like effector nuclease (TALEN) and clustered regularly interspaced short palindromic repeats/CRISPR-associated 9 (CRISPR/Cas9) system. In contrast to ZFN and TALEN, which require substantial protein engineering to each DNA target, the CRISPR/Cas9 system requires only a change in the guide RNA. For this reason, the CRISPR/Cas9 system is a simple, inexpensive and versatile tool for genome engineering. Furthermore, a modified version of the CRISPR/Cas9 system has been developed to recruit heterologous domains that can regulate endogenous gene expression, such as activation, depression and epigenetic regulation. In this review, we summarize the development and applications of genome editing technologies for basic research and biotechnology, as well as highlight challenges and future directions, with particular emphasis on plants.

Creation of fragrant rice by targeted knockout of the OsBADH2 gene using TALEN technology.

Fragrant rice is favoured worldwide because of its agreeable scent. The presence of a defective badh2 allele encoding betaine aldehyde dehydrogenase (BADH2) results in the synthesis of 2-acetyl-1-pyrroline (2AP), which is a major fragrance compound. Here, transcription activator-like effector nucleases (TALENs) were engineered to target and disrupt the OsBADH2 gene. Six heterozygous mutants (30%) were recovered from 20 transgenic hygromycin-resistant lines. Sanger sequencing confirmed that these lines had various indel mutations at the TALEN target site. All six transmitted the BADH2 mutations to the T1 generation; and four T1 mutant lines tested also efficiently transmitted the mutations to the T2 generation. Mutant plants carrying only the desired DNA sequence change but not the TALEN transgene were obtained by segregation in the T1 and T2 generations. The 2AP content of rice grains of the T1 lines with homozygous mutations increased from 0 to 0.35-0.75 mg/kg, which was similar to the content of a positive control variety harbouring the badh2-E7 mutation. We also simultaneously introduced three different pairs of TALENs targeting three separate rice genes into rice cells by bombardment and obtained lines with mutations in one, two and all three genes. These results indicate that targeted mutagenesis using TALENs is a useful approach to creating important agronomic traits.

Genome editing in rice and wheat using the CRISPR/Cas system.

Targeted genome editing nucleases, such as zinc-finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs), are powerful tools for understanding gene function and for developing valuable new traits in plants. The clustered regularly interspersed short palindromic repeats (CRISPR)/Cas system has recently emerged as an alternative nuclease-based method for efficient and versatile genome engineering. In this system, only the 20-nt targeting sequence within the single-guide RNA (sgRNA) needs to be changed to target different genes. The simplicity of the cloning strategy and the few limitations on potential target sites make the CRISPR/Cas system very appealing. Here we describe a stepwise protocol for the selection of target sites, as well as the design, construction, verification and use of sgRNAs for sequence-specific CRISPR/Cas-mediated mutagenesis and gene targeting in rice and wheat. The CRISPR/Cas system provides a straightforward method for rapid gene targeting within 1-2 weeks in protoplasts, and mutated rice plants can be generated within 13-17 weeks.

Simultaneous editing of three homoeoalleles in hexaploid bread wheat confers heritable resistance to powdery mildew.

Sequence-specific nucleases have been applied to engineer targeted modifications in polyploid genomes, but simultaneous modification of multiple homoeoalleles has not been reported. Here we use transcription activator-like effector nuclease (TALEN) and clustered, regularly interspaced, short palindromic repeats (CRISPR)-Cas9 (refs. 4,5) technologies in hexaploid bread wheat to introduce targeted mutations in the three homoeoalleles that encode MILDEW-RESISTANCE LOCUS (MLO) proteins. Genetic redundancy has prevented evaluation of whether mutation of all three MLO alleles in bread wheat might confer resistance to powdery mildew, a trait not found in natural populations. We show that TALEN-induced mutation of all three TaMLO homoeologs in the same plant confers heritable broad-spectrum resistance to powdery mildew. We further use CRISPR-Cas9 technology to generate transgenic wheat plants that carry mutations in the TaMLO-A1 allele. We also demonstrate the feasibility of engineering targeted DNA insertion in bread wheat through nonhomologous end joining of the double-strand breaks caused by TALENs. Our findings provide a methodological framework to improve polyploid crops.

An efficient TALEN mutagenesis system in rice.

Targeted gene mutagenesis is a powerful tool for elucidating gene function and facilitating genetic improvement in rice. TALENs (transcription activator-like effector nucleases), consisting of a custom TALE DNA binding domain fused to a nonspecific FokI cleavage domain, are one of the most efficient genome engineering methods developed to date. The technology of TALENs allows DNA double-strand breaks (DSBs) to be introduced into predetermined chromosomal loci. DSBs trigger DNA repair mechanisms and can result in loss of gene function by error-prone non-homologous end joining (NHEJ), or they can be exploited to modify gene function or activity by precise homologous recombination (HR). In this paper, we describe a detailed protocol for constructing TALEN expression vectors, assessing nuclease activities in vivo using rice protoplast-based assays, generating and introducing TALEN DNAs into embryogenic calluses of rice and identifying TALEN-generated mutations at targeted genomic sites. Using these methods, T0 rice plants resulting from TALEN mutagenesis can be produced within 4-5 months.

[CRISPR/Cas: a novel way of RNA-guided genome editing].

Bacteria and archaea have evolved an adaptive immune system, known as type II prokaryotic clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) system, which uses short RNA to direct the degradation of target sequences present in invading viral and plasmid DNAs. Recent advances in CRISPR/Cas system provide an improved method for genome editing, showing robust and specific RNA-guided endonuclease activity at targeted endogenous genomic loci. It is the latest technology to modify genome DNA specifically and effectively following zinc finger nucleases (ZFNs) and TALE nucleases (TALENs). Compared with ZFNs and TALENs, CRISPR/Cas is much simpler and easier to engineer. This review summarizes recent progress, and discusses the prospects of CRISPR/Cas system, with an emphasis on its structure, principle, applications and potential challenges.