Abstract

Development of CRISPR/Cas9 transient gene editing screening tools in plant biology has been hindered by difficulty of delivering high quantities of biologically active single guide RNAs (sgRNAs). Furthermore, it has been largely accepted that in vivo generated sgRNAs need to be devoid of extraneous nucleotides, which has limited sgRNA expression by delivery vectors. Here, we increased cellular concentrations of sgRNA by transiently delivering sgRNAs using a Tobacco mosaic virus-derived vector (TRBO) designed with 5′ and 3′ sgRNA proximal nucleotide-processing capabilities. To demonstrate proof-of-principle, we used the TRBO-sgRNA delivery platform to target GFP in Nicotiana benthamiana (16c) plants, and gene editing was accompanied by loss of GFP expression. Surprisingly, indel (insertions and deletions) percentages averaged nearly 70% within 7 d postinoculation using the TRBO-sgRNA constructs, which retained 5′ nucleotide overhangs. In contrast, and in accordance with current models, in vitro Cas9 cleavage assays only edited DNA when 5′ sgRNA nucleotide overhangs were removed, suggesting a novel processing mechanism is occurring in planta. Since the Cas9/TRBO-sgRNA platform demonstrated sgRNA flexibility, we targeted the N. benthamiana NbAGO1 paralogs with one sgRNA and also multiplexed two sgRNAs using a single TRBO construct, resulting in indels in three genes. TRBO-mediated expression of an RNA transcript consisting of an sgRNA adjoining a GFP protein coding region produced indels and viral-based GFP overexpression. In conclusion, multiplexed delivery of sgRNAs using the TRBO system offers flexibility for gene expression and editing and uncovered novel aspects of CRISPR/Cas9 biology.

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