What is the available evidence for the range of applications of genome-editing as a new tool for plant trait modification and the potential occurrence of associated off-target effects: a systematic map
Background: Within the last decades, genome-editing techniques such as CRISPR/Cas, TALENs, Zinc-Finger Nucleases, Meganucleases, Oligonucleotide-Directed Mutagenesis and base editing have been developed enabling a precise modifcation of DNA sequences. Such techniques provide options for simple, time-saving and cost-efective applications compared to other breeding techniques and hence genome editing has already been promoted for a wide range of plant species. Although the application of genome-editing induces less unintended modifcations (of-targets) in the genome compared to classical mutagenesis techniques, of-target efects are a prominent point of criticism as they are supposed to cause unintended efects, e.g. genomic instability or cell death. To address these aspects, this map aims to answer the following question: What is the available evidence for the range of applications of genome-editing as a new tool for plant trait modifcation and the potential occurrence of associated of-target efects? This primary question will be considered by two secondary questions: One aims to overview the market-oriented traits being modifed by genome-editing in plants and the other explores the occurrence of of-target efects. Methods: A literature search in nine bibliographic databases, Google Scholar, and 47 web pages of companies and governmental agencies was conducted using predefned and tested search strings in English language. Articles were screened on title/abstract and full text level for relevance based on pre-defned inclusion criteria. The relevant information of included studies were mapped using a pre-defned data extraction strategy. Besides a descriptive summary of the relevant literature, a spreadsheet containing all extracted data is provided. Results: Altogether, 555 relevant articles from journals, company web pages and web pages of governmental agencies were identifed containing 1328 studies/applications of genome-editing in model plants and agricultural crops in the period January 1996 to May 2018. Most of the studies were conducted in China followed by the USA. Genomeediting was already applied in 68 diferent plants. Although most of the studies were basic research, 99 diferent market-oriented applications were identifed in 28 diferent crops leading to plants with improved food and feed quality, agronomic value like growth characteristics or increased yield, tolerance to biotic and abiotic stress, herbicide tolerance or industrial benefts. 252 studies explored of-target efects. Most of the studies were conducted using CRISPR/ Cas. Several studies frstly investigated whether sites in the genome show similarity to the target sequence and secondly analyzed these potential of-target sites by sequencing. In around 3% of the analyzed potential of-target sites, unintended mutations were detected. Only a few studies conducted of-target analyses using unbiased detection methods (e.g. whole genome sequencing). No of-target efects that could be correlated to the genome-editing process were identifed in these studies. Conclusions: The rapid adoption in plant breeding was demonstrated by a considerable number of market oriented applications (crops and traits) described in publications worldwide. Studies investigating of-target efects are very heterogeneous in their structure and design. Therefore, an in-depth assessment regarding their weight of evidence is mandatory.