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Plant Genome Editing with TALEN and CRISPR Aimee Malzahn1, Levi Lowder2 and Yiping Qi1,3*

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Plant Genome Editing with TALEN and CRISPR Aimee Malzahn1, Levi Lowder2 and Yiping Qi1,3* Malzahn et al. Cell Biosci (2017) 7:21 DOI 10.1186/s13578-017-0148-4 Cell & Bioscience REVIEW Open Access Plant genome editing with TALEN and CRISPR Aimee Malzahn1, Levi Lowder2 and Yiping Qi1,3* Abstract Genome editing promises giant leaps forward in advancing biotechnology, agriculture, and basic research. The process relies on the use of sequence specifc nucleases (SSNs) to make DNA double stranded breaks at user defned genomic loci, which are subsequently repaired by two main DNA repair pathways: non-homologous end join- ing (NHEJ) and homology directed repair (HDR). NHEJ can result in frameshift mutations that often create genetic knockouts. These knockout lines are useful for functional and reverse genetic studies but also have applications in agriculture. HDR has a variety of applications as it can be used for gene replacement, gene stacking, and for creating various fusion proteins. In recent years, transcription activator-like efector nucleases and clustered regularly inter- spaced palindromic repeats (CRISPR) and CRISPR associated protein 9 or CRISPR from Prevotella and Francisella 1 have emerged as the preferred SSNs for research purposes. Here, we review their applications in plant research, discuss current limitations, and predict future research directions in plant genome editing. Keywords: Plant genome editing, TALEN, CRISPR, Cas9, Cpf1, NHEJ, HDR Background edited crops are not regulated as genetically modifed Te feld of genome editing is experiencing rapid growth (GM) [2]. as new methods and technologies continue to emerge. Using genome editing to boost agriculture productivity is Genome editing relies on DNA repair needed as the world population is expected to grow to 9.6 DNA damage occurs naturally in all cells either due to billion by 2050 while the amount of arable land decreases exogenous factors, such as UV radiation, or endogenous [1]. Besides potential for boosting crop yields, genome agents such as metabolic by-products and free radicals. editing is now one of the best tools for carrying out A double-strand break (DSB) is the most lethal type of reverse genetics and is emerging as an especially versatile DNA damage and must be repaired before DNA replica- tool for studying basic biology. tion, which has led to the evolution of two major DNA Genome edited plants are diferentiated from repair pathways in eukaryotes: non-homologous end- conventional transgenic plants as they may not joining and homology-directed repair [3–6] (Fig. 1). incorporate foreign DNA. Although genome editing Non-homologous end-joining (NHEJ) is an error-prone can be used to introduce foreign DNA into the genome, repair pathway. When a DSB occurs, NHEJ can quickly, it may simply involve changes of a few base pairs in the although often imprecisely, be used in two ways to repair plant’s own DNA. Tis distinction makes genome editing the break. In classical NHEJ (Fig. 1a), several diferent a novel and powerful breeding tool that has promising proteins (e.g. Ku70 and Ku80) bind to broken DNA ends applications in agriculture, especially when genome and are joined together by a ligase that can result in the insertion or deletion (indel) of nucleotides. In micro- homology-based alternative NHEJ (Fig. 1b), 5′ ends are cut until 3′ overhangs with homology are created. DNA *Correspondence: [email protected] strands then bind at their complementary sequence, and 1 Department of Plant Science and Landscape Architecture, University faps of non-homologous DNA are excised. Tis typically of Maryland, College Park, MD 20742, USA Full list of author information is available at the end of the article results in deletions as DNA between homologous sections © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Malzahn et al. Cell Biosci (2017) 7:21 Page 2 of 18 Fig. 1 Major DNA repair pathways in plants. Non-homologous end joining (NHEJ) and homology directed repair (HDR) are two main repair pathways. Classical NHEJ may lead to insertions or deletions, while microhomology based alternative NHEJ always results in deletions. Homology directed repair is less efcient, but can result in precise integration of a donor DNA template into the genome are removed. NHEJ often leads to frameshift mutations forescent tagging. Tere are many exciting applications which can result in premature stop codons, rendering in basic and applied science using HDR. For example, genes non-functional (Fig. 1a, b). Tis is helpful for creat- HDR was used to engineer an herbicide resistant trait in ing knockout plants useful for reverse genetic studies, but tobacco plants [9]. can also create desirable agricultural traits. For example, a powdery mildew resistant wheat line was created by Rapid evolution of sequence specifc nucleases knocking out three redundant MLO genes [7]. (SSNs) for plant genome editing Te second DNA repair pathway is homology directed Meganucleases, or homing endonucleases, are site repair (HDR) which relies on template DNA. Homolo- specifc endonucleases found in eukaryotes, archaea, gous recombination is an important process that occurs and bacteria which recognize DNA sequences over in somatic cells to repair DSBs and in meiotically dividing 12 bp long [10]. Several hundred meganucleases have cells to exchange genetic material between parental chro- been discovered and they can be divided into four mosomes. Te most common conservative HDR mecha- families: LAGLIDADG, His-Cys box, GIY-YIG, and the nism in plants, which repairs almost all DSBs in somatic HNH family [10]. Te LAGLIDADG family consists of cells, is the synthesis-dependent strand annealing (SDSA) popular meganucleases I-CreI and I-SceI. Originally, pathway [4, 8] (Fig. 1c). As a DSB occurs, 3′ overhangs meganucleases were only able to target a single sequence are extended from the break site. A 5′ end invades the and thus were not capable of targeting endogenous homologous strand forming a D-loop. Synthesis flls in genes. After it was discovered that only a few amino the gaps using homologous DNA as a template, and the acid residues make direct contact with nucleotides, the 3′ end reanneals with the second 3′ end without cross- binding specifcity was successfully altered for targeting over. Te result is a precisely integrated template or endogenous genes. For example, targeted mutagenesis “donor” DNA strand. In nature, template DNA in the was successfully achieved in maize with de novo- form of a sister chromatid or homologous chromosome engineered meganucleases [11]. However, DNA binding is not always available, which may hinder HDR. However, properties of meganucleases cannot be completely synthetic template DNA can be provided exogenously separated from their nuclease activity, making them and used for gene insertion, replacement, or epitope/ difcult to engineer and use in research. Malzahn et al. Cell Biosci (2017) 7:21 Page 3 of 18 Zinc fnger nucleases (ZFNs) function as dimers and each monomer is a fusion protein of a zinc fnger DNA binding domain and a non-specifc FokI nuclease domain [12, 13]. A zinc fnger is formed by repeated groupings of cysteine and histidine residues and recognize 3 nucleotides (nt). Each ZFN monomer is typically composed of 3 or 4 zinc fngers, recognizing 9 or 12 nt DNA. Te zinc fngers are thought to be modular, making it possible to recognize a long stretch of DNA by putting multiple zinc fngers together [14, 15]. However, ZFNs based on modular assembly typically have poor activity and high toxicity [16, 17], suggesting there is context dependency among neighboring fngers. Tis context dependency in ZFN engineering has been largely addressed by a proprietary platform developed by Sangamo Bioscience [18] and by academically developed platforms such as “OPEN” [19] and “CoDA” [20]. “OPEN” or “CoDA” generated ZFNs were later used for generating mutants and studying DNA repair mechanisms in the model plant Arabidopsis [21–23]. Te possibility of engineering transcription activator- Fig. 2 TALEN and CRISPR-Cas9. a A TALEN is composed of two mono- like (TAL) efectors for DNA targeting was realized in mers with each containing a TALE DNA binding domain and a FokI 2009 when their DNA binding mechanism was discov- nuclease domain. Fok1 dimerizes to create a double-strand break. b ered [24, 25]. TAL efectors in nature are introduced CRISPR-Cas9 is a two-component system composed of Cas9 and a into plant host cells by the bacterium Xanthomonas via gRNA. Once Cas9 fnds a PAM site, if the gRNA binds to the DNA, a the type III secretion system, where they alter host gene double break occurs three base pairs upstream the PAM expression to meet the bacteria’s needs. In the nucleus, TAL efectors bind target genes’ promoters within 60 base pairs of start codons and activate transcrip- found to function as an adaptive immune system in tion [24]. Te DNA binding central repeat domain of bacteria and archaea against invading viruses, phages and each TAL efector is composed of a few to 33.5 repeats plasmids [34–36]. Te bacteria can protect themselves which are typically made of 34 amino acids [26]. Using using a series of CRISPR associated (Cas) proteins that a β-glucuronidase (GUS) reporter in tobacco, Boch et al. cleave viral DNA, insert pieces of viral DNA into their discovered repeat variable diresidue (RVD) at positions own genomes, and then use certain Cas9 protein(s) 12 and 13 of each repeat determines nucleotide binding paired with RNA transcribed from the viral DNA library specifcity [25].
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