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Short title: Relevance of off-target changes in genome editing 1
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Co-Corresponding Authors: Peter L. Morrell ([email protected]); Robert M. Stupar 3
([email protected]), Department of Agronomy and Plant Genetics, 1991 Upper Buford Circle, 4
411 Borlaug Hall, St. Paul, MN 55108-6026 USA 5
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Full title: Plant genome editing and the relevance of off-target changes 7
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Author Names and Affiliations: Nathaniel Grahama,b, Gunvant B. Patilc,1, David M. Bubeckd, 9
Raymond C. Doberte, Kevin C. Glenne, Annie T. Gutsched, Sandeep Kumard, John A. Lindbof, 10
Luis Maasg, Gregory D. Mayd, Miguel E. Vega-Sancheze, Robert M. Stuparc, Peter L. Morrellc 11
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a Department of Genetics, Cell Biology and Development, University of Minnesota, St. Paul, MN 13
55108 USA 14
b Pairwise, Durham, NC 27709 USA 15
c Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, MN 55108 16
USA 17
d Corteva Agriscience™, Johnston, IA 50131, USA 18
e Bayer Crop Science, Chesterfield, MO 63017 USA 19
f HM Clause, Davis, CA 95618 USA 20
g Enza Zaden Research USA, San Juan Bautista, CA 95045 USA 21
1 Present address: Department of Plant and Soil Science, Texas Tech University, Lubbock, TX 22
79409. 23
Plant Physiology Preview. Published on May 26, 2020, as DOI:10.1104/pp.19.01194
Copyright 2020 by the American Society of Plant Biologists
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One Sentence Summary: With well-designed protocols and guide RNAs, off-target changes 24
induced by site-directed nucleases are negligible with fewer genetic differences than from 25
standing variation or induced mutagenesis. 26
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Conflict of interest statement: Some of the authors are employees of Corteva Agriscience, 28
Bayer Crop Science, HM Clause, Enza Zaden, or Pairwise, which are companies involved in 29
targeted genetic modification of plants. R.M.S. is a co-inventor of a patent in the plant gene-30
editing space. All other authors report no conflicts of interest relevant to this article. 31
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List of author contributions: The authors contributed equally in conceiving, drafting, editing, 33
and critical review of the text, tables, and figures of this review. 34
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Abstract 36
Site-directed nucleases (SDNs) used for targeted genome editing are powerful new tools to 37
introduce precise genetic changes into plants. Like traditional approaches, such as conventional 38
crossing and induced mutagenesis, genome editing aims to improve crop yield and nutrition. 39
Next-generation sequencing studies demonstrate that across their genomes, populations of 40
crop species typically carry millions of single nucleotide polymorphisms and many copy number 41
and structural variants. Spontaneous mutations occur at rates of approximately 10-8 to 10-9 per 42
site per generation, while variation induced by chemical treatment or ionizing radiation results in 43
higher mutation rates. In the context of SDNs, an off-target change or edit is an unintended, 44
non-specific mutation occurring at a site with sequence similarity to the targeted edit region. 45
SDN-mediated off-target changes can contribute to a small number of additional genetic 46
variations compared to those that occur naturally in breeding populations or are introduced by 47
induced-mutagenesis methods. Recent studies show that using computational algorithms to 48
design genome editing reagents can mitigate off-target edits in plants. Finally, crops are subject 49
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to strong selection to eliminate off-type plants through well-established multigenerational 50
breeding, selection, and commercial variety development practices. Within this context, off-51
target edits in crops present no new safety concerns compared to other breeding practices. The 52
current generation of genome editing technologies is already proving useful to develop new 53
plant varieties with consumer and farmer benefits. Genome editing will likely undergo improved 54
editing specificity along with new developments in SDN delivery and increasing genomic 55
characterization, further improving reagent design and application. 56
Introduction: Plant Genetic Variability 57
Genetic differences between individuals are the basis of adaptation and evolution. Plant 58
breeding, as a form of directed evolution, has a long history of using genetic diversity for crop 59
improvement. During the process of crop domestication, humans selected individual plants with 60
favorable traits that resulted from novel mutations or standing variation in the ancestral species. 61
The process of selecting plant varieties with favorable characteristics for cultivation and 62
consumption continues to the present day. Modern plant breeding is a more directed process 63
than the crop improvement that occurred through the history and pre-history of most cultivated 64
species, but it continues to involve lengthy development cycles. Progress in our understanding 65
of the genetic basis of traits and refined approaches to select superior progeny have contributed 66
to increased yield for most major crops. Globally, crop yields have increased by greater than 67
one percent each year for much of the past century (Xu et al., 2017). Technological innovations, 68
including those that increased our understanding of nucleic acid sequence and function, have 69
contributed to this progress. Advances have primarily been incremental, with few step changes 70
(Bernardo, 2016). The relatively recent introduction of genome editing, which permits targeted 71
genetic changes, is part of a continuum of development of plant breeding techniques (Lusser et 72
al., 2012). Its primary appeal is that, in contrast to established mutagenesis techniques, 73
changes can be targeted to desired genes. 74
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Over the past several decades, there has been rapid development of genome editing 75
techniques (also referred to as gene editing or genome engineering) (Gaj et al., 2013; 76
Jaganathan et al., 2018; Zhang et al., 2018). These technologies make use of site-directed 77
nucleases (SDNs) and have broad applicability, including applications in many crop plants 78
(Podevin et al., 2013; Voytas, 2013; Weeks et al., 2016). As described in greater detail below, 79
SDNs are enzymes that create DNA double-strand breaks (DSB) at specific, pre-determined 80
nucleotide sequences sites, permitting targeted genetic modifications (Voytas, 2013; Weeks et 81
al., 2016). The most robust and best-characterized SDNs are the Zinc-Finger Nucleases (ZFN), 82
Transcription Activator-Like Effector Nucleases (TALEN), and the type II Clustered Regularly-83
Interspaced Short Palindromic Repeats-CRISPR-associated proteins (e.g., Cas9 or Cas12a) 84
systems, hereafter referred to as “CRISPR” (Gaj et al., 2013; Jaganathan et al., 2018; Zhang et 85
al., 2018). In all cases, the targeted DSBs generated by these precision nucleases are repaired 86
by the cell’s native DNA break repair mechanisms: the non-homologous end-joining (NHEJ) or 87
the homologous recombination (HR) pathways (Danner et al., 2017). 88
One area of consideration for genome editing of plants, animals, and in human therapeutic 89
applications is the potential for “off-target” edits that could lead to unintended mutations at 90
untargeted loci in the genome. An off-target edit is an unintended, non-specific mutation that 91
occurs at a site with sequence similarity to the targeted edit region. Off-target edits are distinct 92
from untargeted mutations that occur at sites without similarity to the target region and that are 93
usually associated with spontaneous or other type of background variation. The fidelity of SDNs 94
to edit only the targeted sequence has been questioned, particularly in mammalian genome 95
editing applications (Carroll, 2011; Pattanayak et al., 2011; Carroll, 2013; Wu et al., 2014; 96
Zhang et al., 2015; Tsai and Joung, 2016; Kosicki et al., 2018). The potential to introduce DNA 97
sequence changes at non-targeted loci in patients undergoing genome editing procedures has 98
become an essential consideration for clinical and disease treatment applications (Araki and 99
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Ishii, 2016). Furthermore, unintended mutations in treated human somatic cells could pose 100
health risks such as unregulated cell proliferation (cancer) (Cox et al., 2015). Off-target edits 101
were detected in the early stages of CRISPR-Cas9 research involving human cancer cell lines 102
(Fu et al., 2013). The off-target edit frequency in human cells ranged from 0 to 150 mutations 103
per genome, and this frequency was closely linked to the single-guide RNA (sgRNA) specificity 104
(Frock et al., 2015). The potential for off-target edits and rate at which they could occur in plants 105
are discussed below. 106
Plants differ from animals in substantive ways that alter the impact of induced changes. First, 107
somatic cell changes in plants differ in consequences from those in mammals. Somatic changes 108
in a portion of the plant are less likely to affect critical (irreplaceable) tissues. Unlike many 109
animals, genetic changes in juvenile plants can be transmitted to reproductive tissues (Schmid-110
Siegert et al., 2017). However, plants frequently develop multiple independent reproductive 111
structures, only a portion of which may be affected by any new mutations. Also, breeding to 112
develop new lines for commercial release involves an intensive process of selection of individual 113
plants with useful phenotypes while eliminating individuals with undesirable mutations or 114
phenotypes (commonly known as “off-types”) (ASTA, 2016; Glenn et al., 2017; Kaiser et al., 115
2020). For these reasons, off-target edits in crops present fewer safety concerns than those that 116
could arise with therapeutic applications of genome editing. 117
The goal of this review is a quantitative comparison of genetic variation that could arise through 118
genome editing with the variation that exists in crop species or is generated by current breeding 119
practices. The review examines our current understanding of the nature and abundance of 120
standing and induced genetic variation in crops and how they compare to the potential for off-121
target changes during genome editing. 122
Sources and Types of Inherent (Standing) Genomic Variation in Plants 123
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Before the identification of DNA as the molecular basis for the transmission of heritable genetic 124
variation, crop domestication and plant breeding involved visible inspection and selection of 125
desired phenotypes. There was limited knowledge of the types and extent of underlying 126
genomic variation in crop plants and their wild relatives (Zohary, 1999). The development of 127
genomic sequencing methodologies has provided an unprecedented understanding of the 128
molecular basis of diversity in plant species, ranging from single nucleotide variants to macro-129
scale events such as genomic rearrangements or whole genome duplication leading to 130
polyploidy and transposon-directed DNA transposition (Wendel et al., 2016; Gabur et al., 2019). 131
Our understanding of the primary sources of genetic diversity, namely mutations and 132
recombination, and the cellular processes involved in generating this variation, have increased 133
as both DNA sequencing technologies and genome assemblies have improved (Bevan et al., 134
2017). Mutation creates new variants while recombination arranges variants into new 135
combinations (Morrell et al., 2006); both processes rely upon endogenous DNA repair 136
mechanisms (see Box 1 on double-strand DNA break repair). Genetic drift and both positive and 137
purifying selection remove variants from populations, maintaining an equilibrium that permits 138
variation to persist at relatively constant levels in stable populations. Much of the process of 139
plant breeding involves the creation of varieties that combine favorable variants from their 140
parental lines. 141
[INSERT TEXT BOX - The Importance of Double-Strand Break Repairs] 142
Reference genomes and assessment of genomic variation 143
Standing variation in a species can be estimated at the whole genome level by comparing a 144
sample of individuals using Next Generation Sequencing (NGS). High-throughput DNA 145
sequencing, whole-genome reference sequences and assembly, and improved bioinformatic 146
tools have provided an unprecedented understanding of the variation in populations of the 147
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world’s major field and vegetable crop species (Morrell et al., 2011; Huang and Han, 2014; 148
Bevan et al., 2017). NGS and reference-based read mapping have also been used to identify de 149
novo sequence variation in lines accumulating induced and spontaneous mutations over several 150
generations (Ossowski et al., 2010; Belfield et al., 2012; Henry et al., 2014). Many of the field 151
crop and vegetable species that constitute a major part of the global diet now have a high-152
quality reference genome sequence (Michael and VanBuren, 2015). Reference genomes have 153
several limitations, the most apparent being that all genes or gene variants are not present in 154
any single accession (Hirsch et al., 2016; Jiao et al., 2017). Means of accounting for structural 155
variation and gene content are rapidly evolving, but generally involve representing multiple 156
genomes of a single species in a graph-like structure (Church et al., 2015; Paten et al., 2017). 157
A list of the classes of variation detected in cultivated plants is presented in Table 1. Single 158
nucleotide variations (SNVs) (referred to as single nucleotide polymorphisms (SNPs) when 159
segregating in populations), are the most abundant class (Innan et al., 1996). Because of their 160
abundance, SNPs are the most commonly genotyped polymorphism for breeding applications 161
and comparative studies in crops (Morrell et al., 2011). Insertion and deletion (indels) changes 162
are also common forms of variation, but because indel events are superimposed over one 163
another, over evolutionary time it becomes increasingly difficult to distinguish the boundaries of 164
individual insertion or deletion events (Clegg and Morrell, 2004). These naturally occurring 165
changes are similar to those induced by SDNs, with SNVs being equivalent to base edits (see 166
below) and indels resembling the gene knockouts induces by CRISPR edits. 167
[INSERT TABLE 1 HERE] 168
It is important to note that most whole genome sequencing studies to date have used short-read 169
sequencing technologies (Gilad et al., 2009). As a result, the diversity in breeding populations 170
due to structural variation such as variation in transposable element location and abundance, 171
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presence-absence variation (PAV) and gene copy number variants (CNV) has been difficult to 172
measure (Muñoz-Diez et al., 2012). PAV and CNV typically refer to changes that include genes 173
and thus are functionally equivalent to SDN-mediated gene insertion or deletion. Although 174
structural variants are less common than SNPs, they are an important source of variation 175
(Gabur et al., 2019; Schiessl et al., 2019). There are many examples of gene presence/absence 176
or copy number differences impacting agronomic traits including submergence tolerance in rice 177
(Oryza sativa) (Xu et al., 2006), high oleic acid content in sunflower oil (Schuppert et al., 2006), 178
soybean cyst nematode resistance (Cook et al., 2012), and seed coat pigmentation in soybean 179
(Glycine max) (Todd and Vodkin, 1996). Because of the density of naturally occurring variation, 180
intra- and inter-specific crosses of plants that occur during plant breeding can introduce millions 181
of SNPs (see Figure 1), in addition to thousands of PAV or CNV sequence variations into 182
resulting progeny. As more long-read sequencing technologies are deployed, more accurate 183
measurements of the CNV and PAV in breeding populations will be possible. 184
Sources of spontaneous de novo genome variation 185
Table 2 reports current estimates of standing genomic variation in select crop species. 186
Environmental conditions and breeding and genetic practices such as tissue culture propagation 187
(described below) can further impact the rate at which mutations occur. The sources, types, and 188
rates of de novo genomic variation are reviewed below. 189
[INSERT TABLE 2 HERE] 190
Spontaneous Nucleotide Variation: 191
De novo mutations arise primarily from errors in the DNA replication process. DNA repair 192
processes (NHEJ and HR) can also result in new mutations, including variants that arise due to 193
environmentally-triggered DNA damage (Jiang et al., 2014; Nisa et al., 2019). Whole-genome 194
sequencing of mutation accumulation lines of Arabidopsis (Arabidopsis thaliana) estimated a 195
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mutation rate of 7 x 10-9 base substitutions per site per sexual generation (Ossowski et al., 196
2010), a rate confirmed in a more recent report (Weng et al., 2019). This rate estimate in 197
Arabidopsis is consistent with earlier estimates of a mutation rate of 5.9 x 10-9 substitutions per 198
site per year based on phylogenetic divergence in monocots (Gaut et al., 1996). Most of the 199
mutations reported by Ossowski et al. (2010) were SNPs, though 1-3 bp indel mutations were 200
also detected at a rate of 4.0 +/-1.6 x 10-10 indels per site per generation. Deletions larger than 3 201
bp occurred at a rate of 0.5 x 10-9 per site per generation, and the average larger deletion size 202
was hundreds of bp per event (Ossowski et al., 2010). In a similar study in rice, the mutation 203
rate was estimated at 5.4 x 10-8 per site per diploid genome (Tang et al., 2018). In maize (Zea 204
mays), the spontaneous mutation rate was reported to be 2.2-3.9 x 10-8 per site per generation 205
(Yang et al., 2017). The general trend for single nucleotide mutation rates in plants is on the 206
order of 10-8 to 10-9 per site per generation, similar to estimates for humans (Nachman and 207
Crowell, 2000; Roach et al., 2010). 208
Transposons as a source of variation: 209
In many plant species, transposable elements (TE) and the remnants of past TE insertion 210
events make up the majority of the genome (Sahebi et al., 2018). For example, TEs make up 211
approximately 85% of the maize genome, 76% of the pepper genome, and between 20-40% of 212
the rice genome, depending upon the rice cultivar (Dubin et al., 2018; Anderson et al., 2019). 213
Transposons are DNA elements that can auto-excise and re-insert in the genome. They have 214
the potential to disrupt genes or create rearrangements. They can also result in complete copies 215
of genetic elements being placed in other areas of the genome. Transposons can have a 216
profound impact on genome structure and gene function (Lisch, 2012). Some types of 217
transposons are known to be activated by environmental conditions (e.g., temperature) and by 218
breeding practices, (e.g., tissue culture, described below) (Vitte et al., 2014; Rey et al., 2016). 219
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Accurate measurements of rates of heritable transposition in a single generation in plants have 220
not been determined. 221
Summary of inherent genomic variation 222
NGS has dramatically improved the estimation of genome-wide standing variation and made it 223
possible to estimate rates of de novo mutations, especially SNVs. Numerous studies have 224
examined the genomic variation between individuals in breeding populations for well-studied 225
crops like maize (Hufford et al., 2012; Lu et al., 2015; Bukowski et al., 2017), soybean (Li et al., 226
2014; Maldonado dos Santos et al., 2016; Valliyodan et al., 2016), and rice (Zhao et al., 2018). 227
To date, short-read sequencing technology has been the principal tool used for resequencing 228
plant genomes. However, short-read sequence technologies are of more limited utility for 229
detection of large structural variants (such as PAV or CNV) (Gilad et al., 2009; Morrell et al., 230
2011; Fuentes et al., 2019). As long-read sequencing becomes a more standard component of 231
comparative studies and additional reference-quality genomes from individual species are 232
reported, diversity due to large structural changes will become more readily accessible (Gabur 233
et al., 2019; Schiessl et al., 2019). 234
Sources of Induced Genomic Variation in Plants 235
Although naturally-occurring genetic variants have been the primary source of plant genetic 236
diversity used for crop domestication and breeding, geneticists have developed approaches to 237
augment natural plant genomic diversity and accelerate the development of improved cultivars. 238
Genetic variation induced by chemical mutagens and ionizing radiation has been used in plant 239
breeding since early in the 20th century (Friedberg et al., 2006). Somaclonal variation induced 240
by tissue culture has also been a source of variation used in plant breeding. Mutation breeding 241
is considered cost-effective, ubiquitously applicable, non-hazardous, environmentally friendly, 242
and continues to benefit plant breeders and consumers globally (Ahloowalia et al., 2004). Many 243
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types of mutagenic and ionizing agents have been evaluated to maximize genomic variation 244
while minimizing detrimental or lethal effects in plants (Sikora et al., 2011). As described below, 245
each type of mutagenic agent tends to have a predominant type of mutation induced across the 246
genome. 247
The mutations induced by chemical and ionizing radiation occur throughout the genome, but at 248
an increased rate compared to spontaneous mutations (Belfield et al., 2012; Henry et al., 2014). 249
It can be difficult to select individuals with mutations for the desired phenotype, yet lacking 250
mutations that produce undesirable phenotypes (Bolon et al., 2011; Saika et al., 2011). Mutant 251
lines that carry numerous new mutations have been developed for genetic studies and breeding 252
improved varieties in many crops (Tables 3 and 4). Over 3,200 mutant varieties, including 253
cereals (1,584; 49.5%), flowers/ornamentals (700; 21.9%), legumes (480; 15%) and others 254
(435; 13.6%), have been released for commercial use in more than 210 plant species in over 70 255
countries (Figure 2). 256
[INSERT TABLES 3 and 4 HERE] 257
Mutant population screening is commonly used in rice breeding, resulting in 828 registered lines 258
in the Mutant Variety Database (Kharkwal et al., 2004; Shu et al., 2012; Wang and Jia, 2014). 259
Most of the wheat (Triticum aestivum) varieties used for making Italian pasta were derived in 260
part through mutation breeding (Micke et al., 1990; Mba, 2013). For some crops, mutation 261
breeding is more widely utilized for crop improvement within specific geographic regions. For 262
example, nearly 50% of the improved soybean cultivars grown in Vietnam are derived from 263
induced mutations (Vinh et al., 2009). Mutagenesis has also been used to introduce desirable 264
traits in horticultural crops. For example, researchers have used induced mutations to develop 265
variation for tomato (Solanum lycopersicum) fruit size (Just et al., 2013; Park et al., 2014), 266
starch content in potato (Solanum tuberosum) (Muth et al., 2008), and virus resistance in 267
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peppers (Capsicum annuum) (Ibiza et al., 2010). Several grapefruit (Citrus x paradisi Macfad.) 268
varieties, including popular red flesh varieties, were developed using induced mutation (Mba, 269
2013). Although induced mutant plant varieties have enjoyed considerable global commercial 270
success and public acceptance, the molecular and physiological basis for the improved 271
characteristics in these varieties is generally unknown (Shu et al., 2012). 272
The genomic changes induced by mutagenesis or tissue culture derived plants have been 273
evaluated in numerous studies using NGS techniques (Tables 3 and 4). In addition to WGRS 274
methods (Table 3), TILLING studies have been used to estimate induced mutation density by 275
collecting sequencing data on a limited number of genes from a large population of individual 276
mutagenized plants (Table 4). Exome capture and whole exome resequencing studies also rely 277
on targeted sequencing of a portion of the genome, namely the protein coding regions (exons), 278
of mutagenized plant populations. 279
For example, mutations induced via fast neutron irradiation generate a wide variety of mutations 280
that differ in size and copy number, including single nucleotide variants, deletions, insertions, 281
inversions, translocations, and duplications (Bolon et al., 2014; Li et al., 2017; Jo and Kim, 282
2019). Li et al. (2017) generated and deep-sequenced a fast neutron mutant population (1,504 283
lines) in the model rice cultivar, Kitaake. The estimated mutation frequency analyzed was 61 284
mutations/plant (1.6 x 10-7 per bp, as shown in Table 3) with varied types of mutations observed 285
in 58% of all known rice genes. Induced mutations can differ from spontaneous mutations in 286
subtle ways. Transversions (a purine base substitutes for a pyrimidine base, or vice versa) were 287
found to be more frequent in fast neutron populations (Belfield et al., 2012). Transitions (purine 288
to purine or pyrimidine to pyrimidine substitutions) are concentrated at pyrimidine dinucleotide 289
sites, suggesting covalent linkages as a molecular mechanism for these mutations (Belfield et 290
al., 2012). Other mutagens may generate a narrower range of mutation types (Tables 3 and 4; 291
Figure 2). For example, EMS primarily creates transitions at guanine sites (Henry et al., 2014). 292
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These changes have a slightly elevated probability of being flanked at the 5' neighboring 293
nucleotide by adenine or guanine and the 3' side by cytosine (Henry et al., 2014). There is also 294
evidence that some plant genomes can tolerate elevated mutation rates better than others. The 295
hexaploid wheat and oat polyploid genomes, possessing multiple redundant alleles, can tolerate 296
a larger number of accumulated mutations without any notable impact on survival or agronomic 297
performance (Stadler, 1929; Krasileva et al., 2017; Hussain et al., 2018). 298
Genome variation induced by conventional tissue culture practices 299
Many modern breeding programs use tissue culture or in vitro culture systems to propagate 300
plants, preserve elite genotypes, or for production of pure lines using double haploidy. Similar to 301
chemical and irradiation-induced mutations, in vitro culture methods can also generate de novo 302
genetic variability (referred to as somaclonal variation) (Neelakandan and Wang, 2012). Several 303
studies have examined the rate of de novo sequence variation of endogenous genes in plants 304
regenerated through tissue culture. Jiang et al. (2011) regenerated 28 Arabidopsis plants from 305
root explants (living cells transferred to culture medium) from a single Columbia (Col-0) 306
laboratory strain (Jiang et al., 2011). Five regenerated plants were sequenced and analyzed, 307
and on average, each plant had ~30 de novo sequence changes (totaling 21 indels of 2 bp or 308
fewer, and 131 single nucleotide variants substitutions across the five plants) (Jiang et al., 309
2011). The calculated somaclonal mutation frequencies of spontaneous mutations in this 310
experiment was between 4 x 10-7 and 24 x 10-7 mutations per nucleotide. Tang et al. (2018) 311
reported a mutation rate of 1.86 x 10-7 in rice plants regenerated through tissue culture. These 312
values are nearly two orders of magnitude higher than the rate Ossowski et al. (2010) estimated 313
from greenhouse-grown plants derived from single seed descent (Shaw et al., 2000). 314
Somaclonal variation provides another source of genomic variability in plants (Krishna et al., 315
2016). Nearly two hundred commercial varieties from approximately 55 plant species (e.g., 316
horticultural crops, cereals, legumes, medicinal, and aromatic plants) were derived through 317
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somaclonal variation (reviewed by (Neelakandan and Wang, 2012; Krishna et al., 2016)). During 318
the in vitro culture process, many regenerated plants develop differences in appearance relative 319
to the parental genotype. Changes induced can include heritable genetic and epigenetic 320
alterations (Bednarek et al., 2007; Machczyńska et al., 2015; Krishna et al., 2016). As an 321
example, in the popular cultivar of banana (Musa acuminata) ‘Cavendish,’ the occurrence of off-322
types (different from the parental clone) from in vitro cultured plantlets ranges from 6 to 38% 323
(Sahijram et al., 2003). The genomic variation induced during in vitro regeneration is influenced 324
by media composition, length of incubation and subculture, explant origin, and genotype. 325
Genome-wide DNA polymorphisms or structural variations resulting from tissue culture have 326
been observed in soybean (Anderson et al., 2016), barley (Hordeum vulgare) (Bednarek et al., 327
2007), rice (Miyao et al., 2012; Zhang et al., 2014), potato (Fossi et al., 2019), papaya (Carica 328
papaya) (Kaity et al., 2009), banana (Ray et al., 2006), and several other plant species. 329
Bednarek et al. (2007) assessed somaclonal variation in barley and identified 6% gene 330
expression variation (1.7% of this variation was due to nucleotide changes and the balance was 331
due to altered methylation). Recently, Li et al. (2019) performed whole-genome sequencing of 332
cotton plants derived from in vitro culture and CRISPR-Cas9 edited plants. They determined 333
that most of the mutations were induced either by somaclonal variation or were attributable to 334
heterogeneity present in parental plants (Li et al., 2019). Tissue culture and somaclonal 335
variation have been successfully used to generate valuable heritable genomic changes in crops, 336
and have been widely accepted by breeders for germplasm development and release of new 337
varieties. 338
In mutation breeding (i.e., irradiation, chemical, or somaclonal), efforts are focused on 339
identifying beneficial and novel phenotypes. However, in addition to the beneficial mutation(s), 340
other “coincident” mutations or rearrangements can be generated elsewhere in the genome 341
(Figure 3). While most of these mutations will have negligible or no phenotypic effect, it can be 342
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necessary to remove undesirable mutations with observable deleterious effects (off-types) 343
through backcrossing and selection. Each generation of backcrossing is expected to reduce the 344
presence of coincident mutations by 50%. However, given their number and genomic 345
distribution, some mutations may persist through this process. This process of backcrossing and 346
selection is one reason why mutation breeding has been successfully used for decades with a 347
recognized history of safe use (Food and Drug Administration, 1992). 348
Genome Editing as a Source of Variation 349
The advantage of classical mutation breeding is that it allows for the rapid generation of 350
genomic variation that can be used as a source of traits not currently identified in a crop’s 351
germplasm. The drawback of these methods is that variation is induced randomly both at 352
desired locations and across the genome. Because of these factors, screening, isolating, and 353
introgressing desired mutations into elite germplasm requires large populations and lengthy 354
breeding development timelines. Thus, classical mutation breeding as a tool to generate 355
variation is of great value but is relatively inefficient. 356
Genome editing is a targeted and more exact form of mutation breeding. These tools, based on 357
the use of SDNs, provide researchers the ability to modify sequences at specific locations within 358
the genome. SDNs generate targeted DSBs that, regardless of the SDN used, are repaired by 359
the same native DNA break repair mechanisms, the NHEJ or HR pathways described 360
previously. As described below and depicted in Figure 1, these methods generally result in 361
many fewer mutations elsewhere in the genome relative to earlier mutagenic techniques. SDNs 362
continue to be improved to further reduce the likelihood of non-targeted mutations (Zhao and 363
Wolt, 2017; Hahn and Nekrasov, 2018). 364
The genome editing toolbox and its utility 365
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Though still a relatively new technology, genome editing using SDNs has demonstrated great 366
promise as a tool for crop improvement. Recent reviews catalog the increasing number of 367
phenotypes and characteristics in crop plants that have been generated using SDNs 368
(Jaganathan et al., 2018; Zhang et al., 2018; Chen et al., 2019). Some of these traits were 369
derived from loss-of-function mutations within coding regions or gene regulatory regions that 370
resulted from NHEJ-mediated repair leading to mutations (insertions/deletions) at the break site. 371
Other traits were derived from specific nucleotide insertion or substitution, and gene/allele 372
replacement edits, which can be generated by repairing SDN-generated DSBs with a DNA 373
donor template (Townsend et al., 2009; Li et al., 2015; Petolino et al., 2016; Filler Hayut et al., 374
2017; Shi et al., 2017; Yu et al., 2017). Several types of SDNs that have been used in plants 375
rely on either protein/DNA or RNA/DNA binding to provide editing specificity. As shown in Figure 376
4 (Panel A-B), ZFN and TALEN are chimeric proteins consisting of DNA recognition/binding 377
domains fused to the catalytic domain of the FokI nuclease (Christian et al., 2010; Carroll, 2011; 378
Weeks et al., 2016). TALENs are derived from transcription activator-like (TAL) effectors found 379
in the plant pathogen Xanthomonas. TAL effectors bind to specific DNA sequences by using 380
tandem amino acid repeats. Variations in these repeats, knowns as the repeat variable di-381
residues (RVDs), allow for specificity to nucleotide sequences. For both ZFNs and TALENs, 382
specificity for a chosen target sequence is improved by expanding the length of DNA targets 383
(e.g., 18 to 32 bp for the paired nuclease) and the spacer (i.e., the distance between paired 384
nucleases). The designed proteins interact as heteromeric pairs, with DSBs occurring only when 385
both protein halves are bound at the same genomic location, further increasing the specificity of 386
these reagents for creating DSBs only at the desired site. 387
RNA-Guided Endonucleases (RGENs) function as a protein complex that is directed to a 388
genomic target site through a short non-coding guide RNA (gRNA). The nuclease protein and 389
the gRNA are transcribed separately and then form a complex in the cell nucleus. A portion of 390
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17
the gRNA contains a sequence that is directly complementary to a genomic target site that 391
guides the induction of DSB formation at that location. In the case of the CRISPR system, Cas9 392
or another Cas or Cpf nuclease recognizes the gRNA-DNA pair, thus targeting this site for a 393
DSB (Figure 4, Panel C) (Jinek et al., 2012; Cong et al., 2013; Weeks et al., 2016). The gRNA 394
of the most widely used Cas9, Streptococcus pyogenes Cas9 (SpCas9), binds to a 20 bp 395
genomic site, termed the protospacer, that is adjacent to a triplet DNA sequence known as the 396
protospacer adjacent motif (PAM). Unlike complex protein engineering needed for DNA binding 397
using ZFNs and TALENs, Cas9 can be programmed to new target sites by changing the spacer 398
sequence of the guide RNA, which simplifies vector design and construction while substantially 399
reducing the cost. Due to this ease of design and greater efficiency, CRISPR has become the 400
SDN of choice for genome editing (Globus and Qimron, 2018). 401
Recently developed CRISPR editing systems do not rely on DSB formation to induce targeted 402
changes, but instead involve chimeric protein units consisting of a disabled nuclease and an 403
additional protein domain. In these systems, the disabled nuclease is used to target the 404
additional protein units to specific genomic locations for different applications. Applications 405
include targeted base editing with deaminase domains (Komor et al., 2016; Gaudelli et al., 406
2017; Adli, 2018), transcriptional knock down using repressors (Qi et al., 2013; Thakore et al., 407
2015), targeted DNA methylation and histone modification (Kearns et al., 2015; Stepper et al., 408
2016), gene activation using strong transcriptional activators (Cheng et al., 2013), activation of 409
developmentally silent endogenous loci through chromatin looping (Deng et al., 2014; Morgan et 410
al., 2017), DNA transposition (Strecker et al., 2019), site-specific recombination (Standage-411
Beier et al., 2019), and prime editing (Anzalone et al., 2019). The targeting components of the 412
nucleases are still intact, allowing for nucleotide changes in a site-directed manner. For 413
example, cytosine and adenine base editors (converts C to T and A to G, respectively) fuse a 414
nickase-type Cas9 with a deaminase domain and thus do not induce DSBs. Both are useful 415
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18
genome editing tools, however the cytosine base editor has been reported to induce off-target 416
editing both in animal cells (Zuo et al., 2019) and in plants ((Jin et al., 2019); discussed in more 417
detail below). 418
Designing genome editing protocols and methods to optimize editing specificity and 419
reduce the potential for off-target edits 420
Optimizing the specificity of genome editing methods can be achieved in multiple ways, often 421
through altering SDNs delivery into plants. At present, the most common practice involves 422
stably integrating a transgene into the plant genome that expresses the SDN “reagents” (protein 423
or protein/guide RNA complex that is encoded by the transgene) and template DNA molecule 424
for HR edits. The transgene(s) encoding the SDN components are typically integrated at a locus 425
that is unlinked to the gene(s) being edited. A subsequent outcross or selfing generation results 426
in the independent assortment of unlinked genes to segregate the SDN-encoding transgene 427
away from the edited target sequence. Resulting progeny plants can be screened to select non-428
transgenic plants carrying the targeted edit and lacking the SDN-encoding insert (Gao et al., 429
2016; Char et al., 2017). The resulting plant variety is often molecularly indistinguishable from 430
varieties with mutations that either occur naturally or are produced by induced mutagenic 431
techniques. It should be noted that chromosomal re-arrangements due to T-DNA insertions 432
have been observed (Clark and Krysan, 2010; Hu et al., 2017), but these can generally be 433
detected by next-generation sequencing-based methods and removed by segregation and 434
selection. Transient delivery methods, which seek to limit the overall exposure of the genome to 435
the SDN and avoid the stable integration of a transgene, have also been developed in plants 436
(Woo et al., 2015; Kelliher et al., 2019). The most common transient delivery method uses a 437
ribonucleoprotein (RNP) complex that consists of a purified RGEN protein bound to a guide 438
RNA (Kim et al., 2017; Andersson et al., 2018). The complex is delivered directly to plant cells, 439
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19
through bombardment or other means (Ran et al., 2017), so that the active protein can cause a 440
DSB, but the SDN-encoding DNA is not integrated into the genome (Chen et al., 2018). 441
Off-target edits, and the means to reduce such edits, have received a great deal of attention in 442
the human therapeutic and medical fields (Cho et al., 2014; Shen et al., 2014; Lee et al., 2016; 443
Akcakaya et al., 2018; Lee et al., 2018; Moon et al., 2019) and are also of interest in the plant 444
science community (Feng et al., 2014; Wolt et al., 2016; Tang et al., 2018; Jin et al., 2019; Li et 445
al., 2019; Young et al., 2019). As described in the Introduction, changes observed following 446
genome editing protocols can be split into those that occur at sites without similarity to the 447
targeted region, referred to as untargeted mutations, and “off-target edits” that occur at sites 448
with sequences similar to the targeted region. Numerous studies discussed below have found 449
that the potential for and level of off-target activity in plants can be predicted by comparing the 450
sequence of the target site to the rest of the genome. SDNs bind to specific nucleotide 451
sequences, with the most likely location of potential off-target editing to occur at sites with 452
nucleotide-sequence level similarity to the intended target (Jiang and Doudna, 2017). 453
Bioinformatics prediction tools have been designed to predict sites that may result in an off-454
target edit. Programs have been developed for all of the major SDNs [PROGNOS (Fine et al., 455
2014); TALE-NT (Doyle et al., 2012); CAS-OFF Finder (Bae et al., 2014)], and are routinely 456
included in the design of SDN-based editing protocols (Liu et al., 2017; Listgarten et al., 2018; 457
Minkenberg et al., 2019). Prediction algorithms are useful for identifying potential off-target sites 458
in well-sequenced genomes. Plant species with limited reference genome sequence data may 459
be poorly assessed by these tools. For plant species without a comprehensive genome 460
sequence, it may be difficult to predict potential off-target SDN activity. However, the amount of 461
variation introduced would likely be less than traditional mutagenesis techniques classically 462
utilized in plant breeding strategies (Figure 1). As described in more detail below, it can also be 463
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20
difficult to assess and discriminate between bona fide off-target SDN activity versus variation 464
attributable to transformation or standing variation. 465
Off-target Edit Evaluation and Frequency 466
The approaches reported for the identification of off-target edits vary widely and can impact the 467
level of off-target activity observed. As noted above, many studies rely on algorithms to predict 468
the optimal on-target activity and potential off-target edit sites based on similarity with the gRNA 469
sequence. The most common method used to assess whether the off-target activity has 470
occurred at these site(s) is PCR amplification, followed by amplicon sequencing. PCR 471
approaches provide a rapid method for determining if SDN-mediated off-target edits have 472
occurred at a predicted or suspected site. However, it is reliant on accurate predictions of 473
potential off-target sites, which in turn relies on a well-sequenced source or reference genome 474
for thorough analysis. Several alternative, “non-biased” methods to assess and detect DSB and 475
edits have been developed for off-target edit analysis (reviewed in (Zischewski et al., 2017)), 476
although few have been evaluated in plants. 477
Recently, high-throughput whole genome resequencing (WGRS) has been used to evaluate off-478
target edits and untargeted mutations in plants. While WGRS is the most comprehensive 479
strategy currently available, it requires that researchers carefully control for differences between 480
the reference genome and the experimental line, heterogeneity in experimental lines, and 481
sequencing errors (Michno and Stupar, 2018). WGRS cannot always distinguish between 482
standing variants and de novo untargeted mutations generated by tissue culture or SDN 483
treatments or attributable to sequencing miscalls. Several recent studies have used WGRS to 484
examine genome-wide mutational profiles in genome-edited rice (Tang et al., 2018), maize (Lee 485
et al., 2019), and cotton (Li et al., 2019). A large-scale study examined the amount of variation 486
that occurs due to various genome editing techniques (i.e., RGENs, CRISPR-Cas9 and Cpf1) 487
(Tang et al., 2018). The study reported WGRS of 69 individual plants that had been regenerated 488
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21
via tissue culture (with and without Agrobacterium treatment), edited with various SDNs 489
reagents, or received no treatment (field grown). WGRS analysis revealed the presence of 490
variation (e.g., SNPs, indels) throughout the genome in all plants, though more so in plants that 491
had gone through the tissue culture process. Relative to plants that went through tissue culture, 492
there was no increase in the variation observed from the plants that had been exposed to SDN 493
treatment. The exception was in plants edited with a gRNA purposefully designed to be more 494
likely to elicit off-target edits (Tang et al., 2018). The detected off-target edits in these plants 495
were predicted by the program Cas-OFFinder (Bae et al., 2014), confirming that intentionally 496
inadequate guide design led to the observed off-target activity. A recent study in maize 497
determined that regardless of the SDN delivery method (Agrobacterium, particle bombardment, 498
or RNP), proper design of gRNAs leads to undetectable levels of off-target edits (Young et al., 499
2019). Studies examining rice (Tang et al., 2018), cotton (Li et al., 2019), and maize (Lee et al., 500
2019; Young et al., 2019) report that nearly all of the observed variation from RGEN 501
experiments is attributable to tissue culture or natural variation, and not the delivered nucleases. 502
Tang et al. (2018) found that plants regenerated via tissue culture contained ~100-150 SNVs 503
and ~40-80 indels. The number of de novo variants attributable to tissue-culture and SDN 504
delivery is small relative to standing variation that differentiates individual lines (Table 2). 505
A recent study has applied the WGRS approach to evaluate the off-target edit frequency of base 506
editors in rice (Jin et al., 2019). For potential off-target edits, the authors found that SNVs were 507
not significantly different between control plants (exposed to tissue culture with no editing 508
machinery) and adenine base edited plants. The rate of off-target edits was significantly higher 509
only in plants generated with cytosine (C to T) base editors, irrespective of whether a gRNA was 510
supplied. The percentage of changes that were C to T transitions was higher in plants 511
containing cytosine base editing constructs, suggesting these reagents were inducing 512
background variation. A larger number of off-target mutations suggests that the source of off-513
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22
target variation was not the SDN, but the enzymatic domain of the cytosine base editing 514
complex. Previous studies had found that the cytosine deaminase domain, and the additional 515
uracil glycosylase inhibitor domain used in cytosine base editors, generate higher background 516
levels of C to T changes (Radany et al., 2000; Harris et al., 2002). The studies suggest that 517
increased optimization of the cytosine base editor would be necessary to minimize off-target 518
activity. However, it is worth noting that when compared to the frequencies of mutagenesis 519
reported in rice for somaclonal variation (Miyao et al., 2011, Tang et al, 2018) or other forms of 520
induced mutagenesis (see Tables 3 and 4), mutation rates with the cytosine base editor are 521
comparable to methods used in conventional and mutation breeding. This study suggests that, 522
as new protein motifs and editing strategies are developed, the intended activity and potential 523
for off-target activity should be evaluated. 524
WGRS studies (Tang et al., 2018; Jin et al., 2019; Li et al., 2019) show that the amount of 525
induced background variation as a result of introducing editing enzymes into plants is generally 526
low, especially for editing applications with SDNs. The identification of high background 527
mutation rates for C-to-T base editor enzymes (Jin et al., 2019) suggests the need for careful 528
screening of the effects of new approaches. Taken together, these results suggest that WGRS 529
is best applied to new approaches and may not be necessary for routine application of SDNs, 530
as unintended activity will generally be predictable by computational algorithms that allow for the 531
optimal design of gRNAs (Tang et al., 2018; Lee et al., 2019; Young et al., 2019). Indeed, 532
conventional mutagenesis methods have been found to be safe using existing agronomic 533
evaluations, generally in the absence of WGRS evaluation. 534
Conclusions 535
Whole-genome examinations of variability in plants and improvements in understanding of gene 536
function are occurring concurrently with advances in genome editing technology. Targeted 537
genome editing utilizing SDN techniques are the most recent option for the introduction of 538
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23
precise genetic variation for crop improvement (Figure 5). SDNs are designed to introduce 539
mutations with a high degree of sequence specificity, resulting in fewer unintended genomic 540
changes than earlier mutagenesis techniques. As the understanding of nuclease activity has 541
increased, SDN protocols and associated prediction algorithms to more specifically target 542
intended sequences have improved and been aided by the increasing number of well-annotated 543
and sequenced genomes. NGS has facilitated a more thorough understanding of plant genomes 544
and the genetic changes that accompany the development of new plant varieties. These 545
methods allow more accurate estimations of de novo genomic variation. Each plant generation 546
has new genetic variants that are the result of a range of naturally-occurring processes. The 547
potential for, and possible effects of, SDN-mediated off-target changes must be put in the 548
context of naturally occurring standing variation in crops and mutations induced or introduced 549
during plant breeding practices (Table 5). 550
[INSERT TABLE 5 HERE] 551
The history of safe consumption of foods from plants has been built on the fact that many 552
mutations, regardless of origin, have no phenotypic effect such that breeders cannot remove 553
these “neutral” genetic changes from plant populations. By comparison, plants displaying off-554
type phenotypes due to unintended mutations are eliminated or ameliorated through well-555
established, multigenerational breeding, selection, and commercial variety development 556
practices (Figure 5). Varieties developed through genome editing will be subjected to the same 557
screening and selection practices as is used for improved varieties developed using other 558
sources of genetic variation (Figure 5). While off-target edits are deemed to be unacceptable for 559
therapeutic applications of genome editing, off-target edits in the context of crops are smaller in 560
magnitude than that of current crop improvement methods, such as conventional or mutation 561
breeding (see Outstanding Questions). The current generation of genome editing technologies 562
has facilitated the efficient generation of desirable genomic variation and new plant varieties that 563
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24
would have been more challenging to achieve through other breeding or molecular approaches. 564
Genome editing technology continues to be refined, with improved editing specificity, 565
developments in the delivery of editing enzymes, and ever-increasing genomic characterization. 566
Acknowledgments 567
The authors wish to acknowledge Wayne A. Parrott for comments on an earlier version of the 568
manuscript and Emily E. Vonderharr for copy editing of the text. We also thank Allison Devitre 569
and Emily Turnbough for their help with figure art. Funding provided by USDA / National 570
Institute of Food and Agriculture to Robert M. Stupar and Peter L. Morrell (2015-33522-24096 571
and 2019-33522-30200). 572
573
Box 1: The importance of double-strand break repairs 574
Box 1 cited articles: Pacher and Puchta, 2017; Puchta, 2005; Schuermann et al., 2005; 575
Waterworth et al., 2010; Waterworth et al., 2011; Waterworth et al., 2016; Friedberg et al., 2006; 576
Voytas, 2013; Pacher et al., 2017; Brunet and Jasin, 2018; Čermák et al., 2017; Knoll et al., 577
2014; Gaut et al., 2007; Hajjar and Hodgkin, 2007 578
579
580
581
582
583
584
585
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25
586
TABLES 587
Table 1. Common types of genomic variation in the breeding populations of field crops 588
and vegetables. 589
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26
Variant type Variation Description/comment
Nucleotide
polymorphisms
SNP
Single Nucleotide
Polymorphism
A variant at a single position where one base
replaces another base. SNPs arise from errors
during DNA replication or repair. Also referred to
as single nucleotide variant (SNV) or single base
substitution (SBS).
MNP
Multi-nucleotide
polymorphisms
Changes at 2 or more adjacent nucleotides MNPs
are relatively rare.
Structural
variants
Indel
Insertion/deletions
Sites where DNA is lost (deletion) or gained
(insertion) typically describing relatively small (1-
20 bp but up to 1000 bp) changes. Usually
classified as sequence differences smaller than
PAV.
SSR
Simple sequence repeats
SSRs are repeats of 2 to 6 nucleotides that vary
in the number of repeat motifs present. Also
referred to as short tandem repeats (STRs) or
microsatellites.
Can be considered a type of CNV
CNV
Copy number variation
Sequences with variable copy number between
individuals. The term CNV generally applies genic
regions. Can be due to duplications, deletions or
insertions. Simple sequence repeats (SSR) can
be considered as very small CNVs. Typically,
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27
CNVs refer to larger sequence repeats (>1kb for
example) and may encompass one or more
genes or parts of genes.
PAV
Presence/Absence
Variation
Large sequence block (>1 kb) present in one
line/individual but completely missing in another
line. Like CNV, PAV typically refers to a region
containing genes and can be thought of as the
extreme example of CNV.
TE
Transposable elements
Mobile genetic elements. The number and
location of each element frequently differ from the
reference genome.
Large Structural Variation
(SV) and ploidy (whole
and partial genome
duplication)
Includes DNA rearrangements such as
chromosomal inversions, translocations, partial
genome duplications, duplications, etc.
590
591
592
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28
Table 2: Estimated genomic diversity in the breeding populations of select field and 593
vegetable crops. Not all forms of genomic variation were assessed for all species listed. This is 594
denoted by ND (Not Determined). In addition, because of the short-read sequencing technologies used, 595
the number of PAV/CNV structural variants, representing large deletions, inversions and duplications, is 596
likely underestimated. 597
598
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29
Plant
Species
Evaluated
sample size
and description
Estimate
d
Genome
Sizea
(Mbp)
Estimate
d SNPs
present
(million)
Average
Pairwise
Diversityb
Estimate
d Indels
(million)
Structur
al
Variants
(CNV &
PAV)
References
Bean
(Phaseolus
vulgaris)
37 521 45.0 ~5.0 x 10-
3
Mesoame
rican
~1.7 x 10-
3 Andean
2.1 18.5 k (Schmutz et
al., 2014;
Lobaton et al.,
2018)
Wild
Cabbage
(Brassica
oleracea)
10 (1 wild
species)
489 4.8 ND ND 11.5 k
(Golicz et al.,
2016)
Chickpea
(Cicer
arietinum)
35 738 2.0 ND 0.3
9.9 k (Thudi et al.,
2016)
Cucumber
(Cucumis
sativus)
115 194 3.3 ND 0.03 0.8 k (Zhang et al.,
2015)
(Qi et al.,
2013)
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30
Maize
(Zea mays)
2815 accessions
13 lines
2,400 0.7
2.91
~6.0 x 10-
3
ND
0.4
1.1 M (Gore et al.,
2009; Romay
et al., 2013;
Wang et al.,
2017)
Rice (Oryza
sativa ssp.
Japonica)
3010 375 29.0 ~1.0 x 10-
3
2.4 94 k (Liu et al.,
2017; Wang et
al., 2018)
Soybean
(Glycine
max)
106 (Landraces,
US cultivars and
wild)
978 10.4 1.8 x 10-3
0.7 7.9 k (Valliyodan et
al., 2016)
Tomato
(Solanum
lycopersicu
m)
69 (incl 12 wild
species)
950 17.0
~2 x 10-4
3.6 1.7 k (Causse et al.,
2013; Sauvage
et al., 2017)
Pepper
(Capsicum
annuum)
20 (3 wild
species)
3,500 9.8 ~2.6 x 10-
3
0.24 ND (Aguilar-
Meléndez et
al., 2009; Qin
et al., 2014)
599
a Estimated genome sizes from references or https://plants.ensemble.org April 2019. 600
b Pairwise diversity (also referred to as nucleotide diversity) is the average number of nucleotide 601
differences per site between any two DNA sequences in all possible pairs in the sample population 602
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31
(Tajima, 1983). It is a measure that allows for diversity/variation to be compared across organisms that 603
vary widely in genome size. 604
605
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32
Table 3: Examples of induced mutation types and frequency in food crops and plant model species evaluated via whole 606
genome resequencing (WGRS) analyses 607
Plant
[Assembled
Genome Size] a
Mutagen
No. Plants
analyzed
Mean mutations
per plant b (per
diploid genome)
Total mutation
frequency c
(mutations per
nucleotide site per
experiment)
Reference
Rice
(Oryza sativa)
[375 Mb]
FN
1,504
29 SNPs
26 Indels
3 translocations
3 inversion
/duplications
61 total
[1.6 x 10-7
]
(Li et al., 2017)
GR 6
2,419 SNPs
452 Indels
69 SV
383 CNV
3,323 total
[8.86 x 10-6
] (Li et al., 2016)
Tissue culture 2
114 SNPs
36 Indels
150 total
1.86 x 10-7
(Tang et al., 2018)
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33
Sorghum
(Sorghum
bicolor)
[708 Mb]
EMS 256 7,660 SNPs [1.1 x 10-5
] (Jiao et al., 2016)
Soybean
(Glycine max)
[978 Mb]
EMS 12 12,796 SNPs
12,854 SNPs (Md) [1.3 x 10
-5] (Tsuda et al., 2015)
Tomato
(Solanum
lycopersicum)
[950 Mb]d
EMS 4
1348 SNPs
17 Indels
1365 total
[1.4 x 10-6
]
(Shirasawa et al., 2016)
GR 3 137 SNPs
40 Indels
177 total
[1.9 x 10-7
]
Lotus japonica
[472 Mb] d
EMS 2 2,201 SNPs [4.7 x 10-6
] (Mohd-Yusoff et al.,
2015)
Arabidopsis
thaliana
[135 Mb]
GR 6
11 SNPs (8.5 Md)
7 Indels (6.5 Md)
18 total
3.6×10 -7
(Belfield et al., 2012)
Tissue culture 5
26 SNP (18 Md)
4 Indels (5 Md)
30 total
4.2×10−7
-24.2×10−7
(Jiang et al., 2011)
608
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34
Abbreviations: EMS: Ethyl Methanesulphonate; FN: Fast Neutron; NaN3-MNU: sodium azide-methyl-nitro urea; GR: Gamma Ray; Indel: 609
Insertion/deletion; SNP: single nucleotide polymorphism; SV: structural variant; CNV: copy number variation; WGRS: whole genome 610
resequencing; kb: kilobase; Mb: Megabase. 611
a Assembled genome size based on EnsemblPlants database (http://plants.ensembl.org) unless otherwise noted. 612
b Mean number of mutations per analyzed plant are either values as published or calculated based on total mutations observed across all 613
plants divided by the number of plants analyzed. All plants listed are diploids and studies confirmed full (>90%) genomic coverage unless 614
otherwise noted in footnote c. For some studies, median values (Md) for number of mutations per plant could also be calculated. 615
c Total mutation frequency represents total mutations per nucleotide site per experiment as published or numbers in brackets were calculated by 616
dividing total mutations per plant by the assembled genome size. As noted, total mutation frequency was not limited to a single type of mutation. 617
d Average genome sequence coverage was <90% for tomato (86% coverage; 815 Mb) and Lotus japonica (67% coverage; 315 Mb); partial 618
genomic coverage values were used in normalizing number of mutations per diploid genome and in calculating total mutation frequency. 619
620
621
622
623
624
625
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Table 4: Examples of induced mutation types and frequency in food crops using TILLING or exome resequencing genomic 626
analyses 627
Plant
[Assembled
Genome Size] a
Mutagen
No. Plants
analyzed
Reported mutation
density
[calculated mutation
frequency] b
Number of
genes/genomic
region analyzed c
[total Mb
analyzed]
Reference
Rice
(Oryza sativa)
[375 Mb]
NaN3-MNU 768 1 SNP/265 kb
[3.8×10-6
]
10 genes
[7.9 Mb total] (Till et al., 2007)
Maize
(Zea mays)
[2,400 Mb]
EMS 1,086 1 SNP/774 kb
[1.3 x 10-6
]
Whole exome
resequence
[139 Mb/plant]
(Lu et al., 2018)
Wheat
(Triticum
aestivum)
[14,540 Mb]
EMS 1,535 tetraploid
1,200 hexaploid
23 SNPs/Mb (4n)
[2.3 x 10-5
]
33 SNPs/Mb (6n)
[3.3 x 10-5
]
Whole exome
resequence
[119 Mb/plant (4n)
162 Mb/plant (6n)]
(Krasileva et al.,
2017)
Barley
(Hordeum NaN3 3,148
1 SNP/374 kb
[2.7 x 10-6
]
4 genes
[8.2 Mb total]
(Talamè et al.,
2008)
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36
vulgare)
[8,050 Mb]
Cotton
(Gossypium
hirsutum)
[2,100 Mb]
EMS 8,000 M2 lines
(2 varieties)
1 SNP/153 kb
[6.5 x 10-6
]
1 SNP/326 kb
[3.1 x 10-6
]
8 genes
[58.1 Mb/variety]
(Aslam et al.,
2016)
Tomato
(Solanum
lycopersicum)
[950 Mb]
EMS 1,352 M3 lines 1 SNP/322 kb
[3.1 x 10-6
]
7 genes
[10.9 Mb total]
(Minoia et al.,
2010)
628
629
Abbreviations: EMS: Ethyl Methanesulphonate; NaN3-MNU: sodium azide-methyl-nitro urea; TILLING: Targeting Induced Local Lesions In 630
Genomes; SNP: single nucleotide polymorphism; kb: kilobase; Mb: Megabase. 631
a Assembled genome size based on EnsemblPlants database (http://plants.ensembl.org). 632
b Mutation density (mutations/kb or Mb) is as reported and was determined by dividing the total genomic region evaluated (kb) by the number of 633
nucleotide changes observed. Calculated mutation frequency is the arithmetic calculation of the mutations per nucleotide site per experiment. 634
c Number of gene or genomic regions analyzed in mutagenized plants are as reported. The total DNA length analyzed across all plants and 635
genes (for TILLING) or per plant (for exome sequencing) is also included. 636
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37
637
Table 5. Examples of genetic variation introduced by different breeding methods, as 638
reported in recent studies in rice. 639
a Wang et al., 2018 640
b Tang et al., 2018 641
c Li et al., 2017 642
643
Variation Type Breeding Method
Re-mixing standing
variants
de novo variants
Cross pollinationa Self pollinationb
Classical
mutagenesis
(Fast
neutron)c
Tissue
cultureb
Genome
editingb
Number of single
nucleotide variants
introduced
2 × 106 23 43 114 0
Number of
insertions/deletions
introduced
> 5 × 103 18 48 36 1
Total number of
mutations
introduced
> 2 × 106 41 91 150 1
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38
Figure Legends 644
Figure 1: Comparison of the average number of SNPs and indels per genome (individual) 645
introduced into tomato by different breeding strategies. The data represents approximate 646
number of SNPs between the genome sequence of S. lycopersicum (Hz 1706 reference 647
genome) and other cultivars or wild relatives which have been used in breeding of modern 648
tomato cultivars. After selfing (X Self), each individual plant in the next generation will have (on 649
average) approximately 6 random single nucleotide polymorphisms (SNPs) (assuming the de 650
novo rate of spontaneous heritable SNP formation in tomato is similar to that of lab-grown 651
Arabidopsis (Ossowski et al., 2010). Most modern elite tomato lines commercially grown 652
typically have four or more disease resistance genes that have been introgressed by crossing 653
with wild tomato species such as (Solanum pennellii or S. pimpinellifolium) (Foolad, 654
2007). These initial elite x wild species hybridization events introduced millions to tens of 655
millions of SNPs in addition to indels, copy number (CNV) and presence/absence variation 656
(PAV) (Aflitos et al., 2014). Crosses with more closely related domestic tomato lines or 657
landraces (i.e., cultivar [cv.] San Marzano) will introduce (on average) hundreds of thousands of 658
SNPs/indels (Ercolano et al., 2014). Creating random variation by treating seeds with the 659
chemical mutagen ethyl methanesulphonate (EMS) typically introduces thousands of SNPs per 660
individual (Minoia et al., 2010). Treating cells with a well-designed gene editing reagent 661
(including a guide RNA homologous to target sequence and adjacent protospacer adjacent 662
motif [PAM]) can create a single SNP or indel at a precise, pre-determined location. Crossing 663
with wild or closely related species can also introduce additional indels, CNV and PAV into the 664
genome, which is not considered in this figure. 665
Figure 2. Number of officially released mutant varieties (top 20 countries) showing direct 666
release of improved varieties (orange bars) and mutants used as breeding material (blue 667
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39
bars). Note: European Union countries denoted by asterisks. Data Source: Mutant Variety 668
Database, https://mvd.iaea.org. 669
Figure 3: Overview of sources genomic variation in crop plants. The simplified gene 670
models depict observed variation among individual crop species. The comparison of 671
inherent/standing (genotype X and Y) and induced variation (chemical and irradiation) with the 672
reference genome shows a range of intended and unintended mutations, including single 673
nucleotide polymorphisms (SNPs), small indels, transposon insertions/movement, or a large 674
segmental deletion. However, the intended edit induced via a site-directed nuclease (SDN) 675
occurs at the target site. Note: not drawn to the scale; intergenic variation not shown. 676
Figure 4. Most widely used SDNs. A. TALENs are composed of a DNA binding domain and 677
the nuclease Fok1; the DNA binding domain contains an array of nearly identical protein 678
subdomains each varying at two specific amino acids, known as the repeat variable di-residues 679
(RVDs); specific RVDs recognize unique bases on the target DNA molecule. B. ZFNs are also 680
composed of DNA binding domains and the Fok1 nuclease; each ZFN is composed of three 681
zinc-finger domains that are custom designed to recognize a triplet of DNA bases on the target 682
sequence. For TALENs and ZFNs, two subunits are needed per target region, each binding 683
closely spaced DNA sequences. C. The Cas9 nuclease binds to the target sequence via an 684
RNA molecule, known as the sgRNA; the 5’ region of the sgRNA, the proto-spacer, is typically 685
20 nucleotides long and is complementary to the target DNA; a PAM sequence is also required 686
for recognition (shown here in bold and underlined). 687
Figure 5: An overview of conventional and modern mutation breeding approaches for 688
crop improvement. The figure illustrates the systematic approach for introducing intended and 689
unintended genomic variation in crops for trait discovery, germplasm development and 690
commercial release of new crop varieties. After the introduction of genomic variation, the 691
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40
mutant populations go through a series of selection and testing to remove undesirable 692
mutations and phenotypic variations. The best performing line is selected for subsequent 693
commercial release. This breeding process is common to all methods used for crop product 694
development (Kaiser et al., 2020). 695
696
697
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1248
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ADVANCES
• The importance and potential impact of “off-target” genetic changes during genome editing are subjects of ongoing debate.
• Recent studies have demonstrated that with appropriate design of genome editing reagents, off-target edits in plants are nearly undetectable.
• Off-target edits with base editing proteins can be minimized through optimization of the base-editing enzyme domain.
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OUTSTANDING QUESTIONS
• What additional technical and non-technical roadblocks will need to be overcome to facilitate further research and commercial use of plant genome editing tools?
• As new genome editing tools and more facile reagent delivery methods are developed, what is an appropriate level of evaluation for potential off-target activity in plants?
• How can the plant science community effectively convey to society at large the relevance of potential off-target genomic changes when placed in the context of highly variable and dynamic plant genomes?
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BOX 1. The Importance of Double-Strand Break Repairs Double-strand breaks (DSB) in DNA occur as a part of routine biological functions in all organisms (Pacher and Puchta, 2017). DNA DSB repair is crucial to many ordinary cellular functions such as DNA replication, homologous recombination, and in response to transposon replication and movement (Puchta, 2005; Schuermann et al., 2005; Waterworth et al., 2011). DSBs also occur during the seed dormancy process and subsequent seed germination is dependent on the effective repair of DSBs (Waterworth et al., 2010; Waterworth et al., 2011; Waterworth et al., 2016). Mutagenesis (including radiation-based techniques such as fast-neutron mutagenesis) and genome editing with SDNs generate DSBs (Friedberg et al., 2006; Voytas, 2013) which are repaired by endogenous repair mechanisms. Regardless of their origin, DSB in a somatic plant cell typically initiate break repair by one of two primary mechanisms; NHEJ or homology-dependent repair (HDR) (Pacher and Puchta, 2017). NHEJ is the predominant repair mechanism in plant somatic cells, accounting
for nearly all repair events (Puchta, 2005). Repair typically results in restoration of the original sequence, but small (typically 1-20 base pair (bp)) insertions or deletions (indels) also occur (Waterworth et al., 2011; Brunet and Jasin, 2018). It has been recently shown in plants that only 15% of DSBs generated by CRISPR-Cas9 are repaired imperfectly, leading to mutations (Čermák et al., 2017). HDR in plant somatic cells is rare (Knoll et al., 2014) but is of critical importance during meiosis leading to recombination between intact homologous regions. Sequences used as templates in HDR can be allelic (from homologous chromosomes), ectopic (from non-homologous chromosomes), or intrachromosomal (from the same chromosome) (Puchta, 2005). Homologous and non-homologous recombination following DSB repair contributes to genomic rearrangements and has allowed the introgression of many beneficial traits from crop wild relatives into modern cultivars by conventional cross-breeding (Gaut et al., 2007; Hajjar and Hodgkin, 2007).
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7,000,000
4,000,000
170,000 ~3,000~6
X Self X S. pennelli X S. pimpin-ellifolium
X cv. SanMarzano
EMS Genome Editing
1
Target Sequence
Genomic DNA Cas9
Guide RNA
PAM
H3C
O
S OO
CH3
I I
I I
Appr
oxim
ate N
umbe
r of
SNPs
Crossing Plants Treat in Lab
Random, Unknown and Uncharacterized Genetic Changes Precise Change www.plantphysiol.orgon August 23, 2020 - Published by Downloaded from
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0
100
200
300
400
500
600
700
800No
. of m
utan
t var
ieties
Direct use of an induced mutant
Breeding material
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Reference genome
Inherent Variation (genotype X)
Inherent Variation (genotype Y)
Mutation breeding (chemical)
Mutation breeding (irradiation)
Genome Editing (SDN)
GOI
Gene SNP Small InDel Space between genes Large deletion Transposons GOI: Gene of Interest www.plantphysiol.orgon August 23, 2020 - Published by Downloaded from Copyright © 2020 American Society of Plant Biologists. All rights reserved.
Fokl
Fokl
5 ` -ACGTAGCGTCATCGCCACTAGCGTATGATCGTACTGTGCAGTTGTGGTTTGTCTACCGTA3 ` -TGCATCGCAGTAGCGGTGATCGCATACTAGCATCAGACGTCAACACCAAACAGATGGCAT
Fokl
HD NI NG HD NN HD HD NI HD NG NI NN HD NN NGRepeat Variable Di-residues
Target DNA
DNA-binding domain nuclease
C A T C G C C A C T A G C G T
A. TAL effector nucleases (TALENs)
Fokl
5 ` -CGCCACTAGCGTATGATCGTACTGTGCAGTTGTGGTTTG3 ` -CGCGTGATCGCATACTAGCATCAGACGTCAACACCAAAC
DNA-binding domainnuclease
B. Zinc-finger nucleases (ZFN)Fokl
C. Clustered Regularly-Interspaced Short Palindromic Repeats (CRISPR) Associated Protein 9 (Cas9)
5 ` -ACGTAGCGT
5 `-CATCGCCACTAGCGTATGAT
CATCGCCACTAGCGTATGAT
GTAGCGGTGATCGCATACTA3 ` -TGCATCGCA
CGGACTG- 3´GCCTCAG- 5´
-3´
Cas9 nuclease
Proto-spacer Adjacent Motif (PAM)
sgRNA proto-spacersequence
sgRNA
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Introduction of genetic variation for crop improvement
Conventional breeding Genome editingChemical/radiation mutagenesis
Nucleotidepolymorphisms
Gene content polymorphisms
Chromosomalchanges
SNPs Indels CNV/PAV Deletions Duplications InversionsTranslocations
Selection & testing• Removal of off-types• Phenotypic characteristics• Performance attributes• Processing features• End-user attributes
Genotyping & Inbreeding Small-scale field trials
Large-scale field trials
Crosses & multi-year, multi-location field trials
Commercial release
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