17 MARCH 2017 • VOL 355 ISSUE 6330 1123 SCIENCE sciencemag.org
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Rapid increases in food production can be
achieved by enhancing crop pest resistance,
particularly for East and West Africa, where
pesticide use is low and pests can cause yield
losses averaging more than 50% (8). CRISPR/
Cas has been used to enhance resistance to
bacterial blight in rice in a laboratory setting
by disrupting the promoter of two SWEET
genes (9). A later, laboratory-based study
used a similar approach to promote powdery
mildew resistance in wheat by disrupting the
susceptibility gene MLO-A1 (10). Disrupting
all three MLO genes conferred broad
spectrum resistance to powdery mildew,
highlighting the power of multiplexed
genome editing in polyploid crops.
Altered rainfall patterns caused by climate
change may also lead to substantial yield losses
because more than 70% of global agriculture
relies on rain (2). Researchers addressing
this challenge recently used genome editing
to enhance drought tolerance in maize by
editing a previously unidentified promoter
to increase expression of the ARGOS8 gene
(11). ARGOS8 down-regulates the growth-inhibiting hormone ethylene, enhancing
plant growth and yield under drought stress.
In tomato, flowering time can be manipulated
by using CRISPR/Cas to generate early-yielding varieties by disrupting the flower-repressing SP5G gene (12). Field trials showed
that genome-edited plants could be harvested
2 weeks earlier than control plants. Introducing early flowering into crops in this way may
facilitate their adaption to changing climate
regimes by shifting crucial windows in plant
development to match climate patterns.
Although these genome-edited plants have
undergone field trials, they have not yet been
commercialized. Genome editing may also
enable the precise engineering of complex
traits such as photosynthesis efficiency and
yield component traits such as seed weight
and number.
However, progress in crop improvement
through genome editing is limited by
technical and sociopolitical constraints.
Despite the wealth of genomic data available
for major crops, researchers have yet to
broadly connect genotype with phenotype
information, model the behavior of gene
networks, characterize regulatory elements,
and develop databases to integrate and
analyze this information. Knowing what to
edit to improve a crop therefore presents
an ongoing limitation and challenge. When
gene targets are kno wn, genes can be knocked
out by cleaving double-stranded DNA and
relying on error-prone DNA repair pathways
to introduce mutations. Although disrupting
genes in this way is straightforward, making
specific DNA sequence changes is more
challenging because a DNA template must
be delivered into the cell and integrated
by using the less efficient error-free repair
pathways. Increasing the amount of DNA
template or the concentrations of error-
free repair pathway machinery may help
overcome this limitation.
Furthermore, the DNA target site specificity
of genome editing remains controversial, with
evaluation of the frequency of unintended off-target editing in crops showing inconclusive
results (13). Because many crops are polyploid
and contain high levels of repetitive DNA,
the number of potential off-target edits is
higher than in animals. Off-target editing
can be reduced by using variants of the Cas
enzyme with increased target site specificity
(14) or by delivering Cas and guide RNA in
the form of ribonucleoprotein complexes
that are not integrated into the genome and
degrade rapidly after editing (15). In sexually
reproducing crops, off-target edits can also be
bred out by crossing edited individuals with
parental lines.
The growth of genome-edited crops also
faces sociopolitical challenges, including
government regulation, public acceptance,
and adoption by producers such as
smallholder farmers. Although genetically
modified organisms (GMOs) are extensively
regulated in many countries, organisms
edited by using CRISPR/Cas without
permanent introduction of transgenes are
not currently regulated by the United States.
The legal status of genome edited crops
remains contentious in the European Union,
and a decision on their regulation is unlikely
before 2018 (16). If regulatory authorities
evaluate genome-edited crops that do not
contain transgenes as non-GMOs, they will
be more cost effective for seed companies to
commercialize, and a greater variety of traits
and species may be targeted.
However, public acceptance of genome-
edited crops is also required for their
development to avoid the backlash that
occurred previously with GMO crops.
Smallholder farmers may also be unable
to adopt new crop varieties because of lack
of access to credit and market for seeds.
Clarification on the regulation of genome-
edited crops is urgently needed to support
their development, and open public debate
is required to give the public confidence
in the safety and benefits of these crops.
Coordinated efforts to help provide improved
varieties to smallholder farmers and
accelerate their adoption are also crucial
to increase food security, particularly in
developing countries. j
REFERENCES AND NOTES
1. United Nations, World Population Prospects: The 2015
Revision (Population Division of the Department of
Economic and Social Affairs, 2015).
2. J. R. Porter et al. , in Climate Change 2014: Impacts,
Adaptation, and Vulnerability. Part A: Global and Sectoral
Aspects. Contribution of Working Group II to the Fifth
Assessment Report of the Intergovernmental Panel on
Climate Change, C. B. Field et al. , Eds. (Cambridge Univ.
Press, 2014), pp. 485–533.
3. M.Jinek et al., Science 337,816(2012).
4. R. E. Evenson, D. Gollin,Science 300, 758 (2003).
5. D. Tilman, C.Balzer,J.Hill, B.L.Befort, Proc.Natl.Acad.Sci.
U. S. A.108, 20260 (2011).
6. IPCC, Climate Change 2014: Impacts, Adaptation, and
Vulnerability. Part B: Regional Aspects. Contribution of
Working Group II to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change.
V. R. Barros et al., Eds. (Cambridge Univ. Press, 2014).
7. L. Cong et al. , Science339, 819 (2013).
8. E. C. Oerke, J. Agric. Sci. 144, 31 (2006).
9. W.Z.Jiang et al., Nucleic Acids Res.41,e188(2013).
10. Y. P. Wang et al. , Nat. Biotechnol. 32, 947 (2014).
11. J. Shi et al., Plant Biotechnol. J.15, 207 (2016).
12. S. Soyk et al. , Nat. Genet. 49, 162 (2017).
13. Q. W.Shan etal., Nat.Biotechnol.31,686(2013).
14. B. P. Kleinstiver et al. , Nature 529, 490 (2016).
15. Z.Liangetal., Nat.Commun.8,14261(2017).
16. See www.nature.com/news/gene-editing-in-legal-
limbo-in-europe-1.21515
ACKNOWLEDGMENTS
A.S. was supported by an International Postgraduate Research
Scholarship awarded by the Australian government and
Australian Research Council project LP130100925.
10.1126/science.aal4680
Conventional crop
breeding cycle
Crop traits are combined via
recombination over multiple
generations to produce
improved varieties.
CRISPR/Cas-assisted
crop breeding cycle
Crops with diferent edits of
known targets are produced
in a single step, and selected
for advanced trials based on
phenotypic traits.
Advanced
feld trials
Improved
variety
Parental
cross of elite
varieties
Phenotypic and
genotypic selection
of candidate lines
Preliminary
feld trials
Intercrossing and/or
backcrossing of
breeding population
Genome
editing of
elite variety
Functional
genomics
provides new
editing targets
12
34
Crop improvement schemes
Gene editing tools such as CRISPR/Cas can
improve crops more quickly than traditional
approaches if the nucleotide sequence and
function of the target site are known.
