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Article summary
Site-directed genome engineering in
higher plants has great potential for basic research and molecular breeding. Here, we describe a method for site-directed mutagenesis of the Arabidopsis nuclear genome that efficiently generates heritable mutations using the
RNA-guided endonuclease (RGEN) derived from bacterial clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 (CRISPR associated) protein system. To induce mutagenesis in proliferating tissues during embryogenesis and throughout the plant life cycle, the single guide RNA (sgRNA) and Cas9 DNA endonuclease were expressed from the U6 snRNA and INCURVATA2 promoters, respectively. After Agrobacterium-mediated introduction of T-DNAs encoding RGENs that targets FLOWERING LOCUS T (FT) and SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 4 genes, somatic mutagenesis at the targeted loci was observed in T1 transformants. In the results of FT-RGEN, T1 plants often showed late flowering indicative of the presence of large somatic sectors in which the FT gene is mutated on both chromosomes. DNA sequencing analysis estimated that about 90 % of independent chromosomal DNA fragments carried mutations in the analyzed tissue of a T1 plant showing late flowering. The most frequently detected somatic polymorphism showed a high rate of inheritance in T2 plants, and inheritance of less frequent polymorphisms was also observed. As a result, late-flowering plants homozygous for novel, heritable null alleles of FT including a 1 bp insertion or short deletions were recovered in the following T2 and T3 generations. Our results demonstrate that dividing tissue-targeted mutagenesis using RGEN provides an efficient heritable genome engineering method in A. thaliana.
Reference:Hyun, Youbong, et al. "Site-directed mutagenesis in Arabidopsis thaliana using dividing tissue-targeted RGEN of the CRISPR/Cas system to generate heritable null alleles." Planta (2014): 1-14.
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Engineered nucleases can be used to induce site-specific double-strand breaks (DSBs) in plant genomes. Thus, homologous recombination (HR) can be enhanced and targeted mutagenesis can be achieved by error-prone non-homologous end-joining (NHEJ). Recently, the bacterial CRISPR/Cas9 system was used for DSB induction in plants to promote HR and NHEJ. Cas9 can also be engineered to work as a nickase inducing single-strand breaks (SSBs). Here authors show that only the nuclease but not the nickase is an efficient tool for NHEJ-mediated mutagenesis in plants. This group demonstrate the stable inheritance of nuclease-induced targeted mutagenesis events in the ADH1 and TT4 genes of Arabidopsis thaliana at frequencies from 2.5 up to 70.0%. Deep sequencing analysis revealed NHEJ-mediated DSB repair in about a third of all reads in T1 plants. In contrast, applying the nickase resulted in the reduction of mutation frequency by at least 740-fold. Nevertheless, the nickase is able to induce HR at similar efficiencies as the nuclease or the homing endonuclease I–SceI. Two different types of somatic HR mechanisms, recombination between tandemly arranged direct repeats as well as gene conversion using the information on an inverted repeat could be enhanced by the nickase to a similar extent as by DSB-inducing enzymes. Thus, the Cas9 nickase has the potential to become an important tool for genome engineering in plants. It should not only be applicable for HR-mediated gene targeting systems but also by the combined action of two nickases as DSB-inducing agents excluding off-target effects in homologous genomic regions.
Reference:Fauser, Friedrich, Simon Schiml, and Holger Puchta. "Both CRISPR/Cas‐based nucleases and nickases can be used efficiently for genome engineering in Arabidopsis thaliana." The Plant Journal (2014).
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Targeted genome engineering (also known as genome editing) has emerged as an alternative to classical plant breeding and transgenic (GMO) methods to improve crop plants. Until recently, available tools for introducing site-specific double strand DNA breaks were restricted to zinc finger nucleases (ZFNs) and TAL effector nucleases (TALENs). However, these technologies have not been widely adopted by the plant research community due to complicated design and laborious assembly of specific DNA binding proteins for each target gene. Recently, an easier method has emerged based on the bacterial type II CRISPR (clustered regularly interspaced short palindromic repeats)/Cas (CRISPR-associated) immune system.
The CRISPR/Cas system allows targeted cleavage of genomic DNA guided by a customizable small noncoding RNA, resulting in gene modifications by both non-homologous end joining (NHEJ) and homology-directed repair (HDR) mechanisms. In this review, researchers from The Sainsbury Laboratory summarize and discuss recent applications of the CRISPR/Cas technology in plants.
Belhaj, Khaoula, et al. "Plant genome editing made easy: targeted mutagenesis in model and crop plants using the CRISPR/Cas system." Plant methods 9.1 (2013): 39.
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Genetic modification, including plant breeding, has been widely used to improve crop yield and quality, as well as to increase disease resistance. Targeted genome engineering is expected to contribute significantly to future varietal improvement, and genome editing technologies using zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9/single guide RNA (sgRNA) have already been successfully used to genetically modify plants. However, to date, there has been no reported use of any of the current genome editing approaches in sweet orange, an important fruit crop.
In this study, we first developed a novel tool, Xcc-facilitated agroinfiltration, for enhancing transient protein expression in sweet orange leaves. We then successfully employed Xcc-facilitated agroinfiltration to deliver Cas9, along with a synthetic sgRNA targeting the CsPDS gene, into sweet orange. DNA sequencing confirmed that the CsPDS gene was mutated at the target site in treated sweet orange leaves. The mutation rate using the Cas9/sgRNA system was approximately 3.2 to 3.9%. Off-target mutagenesis was not detected for CsPDS-related DNA sequences in our study. This is the first report of targeted genome modification in citrus using the Cas9/sgRNA system—a system that holds significant promise for the study of citrus gene function and for targeted genetic modification.
Jia, Hongge, and Nian Wang. "Targeted Genome Editing of Sweet Orange Using Cas9/sgRNA." PloS one 9.4 (2014): e93806.
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