MINI REVIEW
published: 26 May 2022
doi: 10.3389/fpls.2022.904829
Clustered Regularly Interspaced Short
Palindromic Repeats-Associated
Protein System for Resistance Against
Plant Viruses: Applications and
Perspectives
Fredy D. A. Silva * and Elizabeth P. B. Fontes *
Department of Biochemistry and Molecular Biology/Bioagro, National Institute of Science and Technology in Plant-Pest
Interactions, Universidade Federal de Viçosa, Viçosa, Brazil
Edited by:
Peng Zhang,
Center for Excellence in Molecular
Plant Sciences (CAS), China
Reviewed by:
Rahul Mahadev Shelake,
Gyeongsang National University,
South Korea
*Correspondence:
Fredy D. A. Silva
fredy.silva@ufv.br
Elizabeth P. B. Fontes
bbfontes@ufv.br
Specialty section:
This article was submitted to
Plant Biotechnology,
a section of the journal
Frontiers in Plant Science
Received: 25 March 2022
Accepted: 03 May 2022
Published: 26 May 2022
Citation:
Silva FDA and Fontes EPB (2022)
Clustered Regularly Interspaced
Short Palindromic RepeatsAssociated Protein System for
Resistance Against Plant Viruses:
Applications and Perspectives.
Front. Plant Sci. 13:904829.
doi: 10.3389/fpls.2022.904829
Different genome editing approaches have been used to engineer resistance against plant
viruses. The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPRassociated protein (Cas; CRISPR/Cas) systems to create pinpoint genetic mutations have
emerged as a powerful tool for molecular engineering of plant immunity and increasing
resistance against plant viruses. This review presents (i) recent advances in engineering
resistance against plant viruses by CRISPR/Cas and (ii) an overview of the potential host
factors as targets for the CRISPR/Cas system-mediated broad-range resistance and
immunity. Applications, challenges, and perspectives in enabling the CRISPR/Cas system
for crop protection are also outlined.
Keywords: CRISPR/Cas, genome editing, resistance to viruses, susceptibility genes, virus-host interactions,
plant antiviral immunity
INTRODUCTION
The agronomic impact caused by phytopathogens imposes severe yield losses on many important
crops worldwide. Agricultural productivity reduction is a recurrent problem due to diseases
caused by phytopathogens; viruses are among the principal constraints to crop productivity
in a world impacted by accelerated climate change (Savary et al., 2019; Amari et al., 2021).
Advances in plant genome editing technology have achieved remarkable breakthroughs in
many fields and have been used in plant biotechnology as a tool to improve several traits in
an unprecedented way. Part of this progress results from the use of clustered regularly interspaced
short palindromic repeats (CRISPR) and CRISPR-associated genes (Cas), CRISPR/Cas system
as a tool for genome editing, modulating gene regulation, epigenetic editing, and chromatin
engineering (Cong et al., 2013; Doudna and Charpentier, 2014; Adli, 2018; Melo et al., 2021).
CRISPR/Cas systems have provided the means to engineer different aspects of the molecular
biology’s central dogma (CD) involved in gene regulation, which will undoubtedly accelerate
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crop improvement (Pramanik et al., 2021a). CRISPR/Cas current
applications include gene discovery, introgression, generation
of biotic/abiotic stress-resistant crops, plant cell factories, and
delayed senescence (Pramanik et al., 2021a). CRISPR/Cas systems
have also enhanced the plant immunity and resistance against
phytopathogenic viruses by targeting viral genome sequences
or host recessive genes in the plant genome (Borrelli et al.,
2018; Zahir and Mahfouz, 2021). The first strategy relies on
the CRISPR/Cas system harboring sequences that target specific
regions of viral genomes. The genome of most plant virus
families is composed of RNA; however, some families comprise
DNA virus species (Sastry et al., 2019). Based on the genome
nature, the plant viruses are classified into 26 families
encompassing six different groups: (+) sense ssRNA viruses,
(−) sense ssRNA viruses, (+/−) sense ssRNA viruses, dsRNA
viruses, (+) sense ssDNA viruses, and (+/−) sense ssDNA
viruses. Some economically relevant plant viruses include species
from the Virgaviridae, Tospoviridae, Geminiviridae, Bromoviridae,
and Potyviridae families, such as Tobacco mosaic virus (TMV),
Tomato spotted wilt virus (TSWV), Tomato yellow leaf curl
virus (TYLCV), Cucumber mosaic virus (CMV), Potato virus
Y (PVY), African cassava mosaic virus (ACMV), Plum pox
virus (PPV), Brome mosaic virus (BMV), and Bean golden
mosaic virus (BGMV; Rybicki, 2015; Silva et al., 2017).
Geminiviridae is one large family of plant viruses divided into
nine genera bearing agronomic interest because geminivirus
species can infect several mono- and dicotyledonous plants,
including maize, tomato, potato, cucumber, cassava, pepper,
bean, and cotton (Mansoor et al., 2006; Sastry et al., 2019).
Several strategies have been used to control plant viruses.
Approaches may be based on traditional techniques, including
prophylaxis to prevent virus spread. Other strategies use chemicals
to control virus dispersion by natural insect vectors or the
removal of infected plants. Additionally, the genomic-assisted
selection of resistant cultivars obtained by plant breeding has
been used with success (Pérez-de-Castro et al., 2012). More
recently, crop transgenic lines expressing small interference
RNAs (siRNAs) and RNA interference (RNAi) targeted to viral
sequences have been extensively used to obtain resistance
(Loriato et al., 2020; Rubio et al., 2020). Successful RNAi-based
transgenic plant immunity strategies include the engineered
resistance against TYLCV and BGMV (Aragão and Faria, 2009;
Leibman et al., 2015). Nevertheless, the emergence of the
CRISPR technology with customizable specificities of the
RNA-guided nucleases (RGNs), like Cas9, has made targeted
genome editing the mainstream method employed by plant
virologists to obtain resistance to viruses in several crops (Ali
et al., 2015). In addition to simplicity, versatility, and rapid
nature, the CRISPR/Cas technology has efficiently modified
several viral genomes and endogenous genes in a large variety
of crop hosts. CRISPR/Cas-meditated genome interference
systems have generated resistance in plants against several
viruses, including bean yellow dwarf virus (BeYDV; Baltes
et al., 2015), beet severe curly top virus (BSCTV; Ji et al.,
2015), tomato yellow leaf curl virus (TYLCV; Tashkandi et al.,
2018), African cassava mosaic virus (ACMV; Mehta et al.,
2019), cotton leaf curl Multan virus (CLCuMuV; Yin et al., 2019),
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chili leaf curl virus (ChiLCV; Roy et al., 2019), cauliflower
mosaic virus (CaMV; Liu et al., 2018), and cucumber mosaic
virus (CMV; Zhang et al., 2018). Although the potential of
this strategy is unquestionable, limitations due to off-target
editing effects, the rapid evolution of mutants resulting from
the mutagenic nature of the CRISPR/Cas system, and the
possibility of generating viral escapes in a short time are under
constant debate (Mehta et al., 2019). To overcome these issues,
new recently discovered CRISPR/Cas systems and multiple
gRNAs targeted to different sites have been employed (Shafiq
et al., 2021; Zahir and Mahfouz, 2021). In addition, the recessive
resistance mediated by potential host susceptibility factors has
been considered a promising alternative for applying the CRISPR/
Cas system toward broad-range resistance and immunity. This
review describes briefly some genome editing tools employed
as antiviral strategies and primarily advances in CRISPR/
Cas-mediated resistance against plant viruses by targeting viral
genomes and/or host susceptibility/recessive resistant genes.
MOLECULAR EDITING TOOLS TO
IMPROVE PLANT IMMUNITY AGAINST
VIRUSES
Different molecular approaches have been employed to improve
plant immunity against viruses. Among those, the nucleases
zinc-finger nucleases (ZFNs) and transcription activator-like
effector nucleases (TALENs) have taken a prominent place as
genome editing techniques (Bibikova et al., 2003; Cheng et al.,
2015). ZFNs are fusion proteins of zinc-finger transcriptional
activators and Fok1 endonuclease, whereas, in TALENs, the
Fok1 endonuclease is linked to a bacterial TALE protein. Both
designed endonucleases are well-characterized tools for targeting
effectors to a specific genome region; they require a specific
amino acid sequence (a zinc-finger or TALE motif) that
recognizes a DNA sequence of the genome. However, some
drawbacks make the use of ZFNs and TALENs limited. Because
ZFNs require an amino acid sequence to specify the target
site, there is a need for multiple designs to recognize different
regions in the genome. Likewise, a single TALE motif recognizes
one nucleotide, and hence an array of TALENs is required to
associate with longer DNA sequences (Becker and Boch, 2021).
Furthermore, in both approaches, the target specificity derives
from protein-DNA association; thereby, they can only edit
targeted DNA viruses, not being used to edit several plant
RNA virus genomes. In addition, replication is partially inhibited
for some plant viruses because they can only target a single
site (Zahir and Mahfouz, 2021).
The CRISPR/Cas system is not limited as ZFNs and TALENs.
Due to the feasibility of its mechanism, CRISPR/Cas has become
an alternative tool for controlling viral infections by directly
editing viral genomes or host factors. CRISPR/Cas was first
described as an immune system of archaea and bacteria for
defense against viruses by specific interaction of short-viral
sequences based on complementarity (Labrie et al., 2010). The
system consists of an RNA sequence complementary to the
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CRISPR/Cas for Resistance to Virus
target sequence known as the spacer or CRISPR RNA (crRNA),
and a scaffold sequence followed by the CAS protein known
as the trans-activating crRNA (tracrRNA; Deltcheva et al., 2011;
Gardiner et al., 2022). The most used system is the CRISPR/
Cas9 (Figure 1). The mechanism requires a single guide (sg)
RNA containing a fusion of 20 nucleotide spacer and scaffold
sequence that directs the Cas9 endonuclease to a specific region
of the genomic DNA. Additionally, a short NGG sequence
and a protospacer adjacent motif (PAM) are required. Cas9
promotes a double-strand break that will be repaired by the
host cell resulting in an insertion or deletion that can potentially
disrupt the open reading frame of the targeted gene (Doudna
and Charpentier, 2014; Jiang and Doudna, 2017).
A recently developed system, the PAMless SpCas9 variant
with relaxed nucleotide preference, has overcome these sequence
limitations by increasing the number of possible CRISPR/Cas
targets (Walton et al., 2020; Ren et al., 2021). Advantages and
applications of the CRISPR/Cas systems include (i) improvement
of plant immunity by targeting RNA or DNA viral genome
with single or multiplex targets (Hussain et al., 2018), (ii)
engineering recessive resistance by editing CRISPR/Cas-targeted
host-factors required for viral replication or movement
FIGURE 1 | A schematic model for engineering resistance to plant viruses provided by the clustered regularly interspaced short palindromic repeats (CRISPR)/
CRISPR-associated protein (CRISPR/Cas) systems. Plant genome transformed with CRISPR/Cas9 system expresses a functional Cas9 protein complex Cas9/
gRNA. After Geminivirus infection (1), the viral single-stranded DNA (ssDNA) is delivered into the cytoplasm and translocated to the nucleus. The host nuclear
machinery assists the complementary strand synthesis resulting in the viral replication to double-stranded DNA (dsDNA) (2), producing multiple viral copies. The
Cas9 protein complex Cas9/gRNA binds to the viral genome (3), which is assisted by a short sequence of 20 nucleotides that directs the Cas9 endonuclease to a
specific region of the genomic DNA where it acts as a molecular scissor. A protospacer adjacent motif (PAM) is required. The action of Cas9 results in a doublestrand break, and virus replication is disrupted by preventing access to replication proteins (4; I), introducing pinpoint mutations in the viral genome (II), or disrupting
the genome by cleavage of dsDNA (III). The CRISPR/Cas system mutagenic property may generate some viral variants. Alternatively, the Cas9 protein complex with
multiple gRNAs can target plant host factors to disrupt genes important for viral replication or movement (5). Combining multiplex CRISPR/Cas systems, such as
Cas13 and Cas9, is a possible alternative to avoid viral escapes and targeting RNA viruses. After RNA virus infection (6), the viral mRNA interacts with CRISPR/
Cas13 system through a short CRISPR RNA (crRNA; 7). The Cas13-crRNA complex is RNA-guided RNA-targeted, and the cleavage of the vRNA induces vRNA
degradation (8) and disrupts viral infection. The figure was created with BioRender.com. Cas, CRISPR-associated; CRISPR, clustered regulatory interspaced short
palindromic repeats; dsDNA, double-stranded DNA; ssDNA, single-stranded DNA; sgRNA, single guide RNA; vDNA, viral DNA; CRNA, CRISPR RNA; ssRNA,
single-stranded RNA; tracrRNA, trans-activating crRNA; and PAM, protospacer adjacent motif.
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(Cao et al., 2020), (iii) recovery of plants with viral symptoms
after infection, as a diagnostic system in plants, and (iv) also
the generation of edited non-transgenic crops (Aman et al., 2020).
V-A CRISPR-Cas (Cas12a/Cpf1) with low homology to canonical
Cas12a nucleases, is a typical example of these new nucleases
(Liu et al., 2020). CRISPR/Cas14a is a compact nuclease isolated
from archaea, which can be targeted to a single-stranded DNA
(ssDNA) genome. The sequence-independence and unrestricted
cleavage mechanism make CRISPR/Cas14a a potential tool for
engineering resistance against plant ssDNA viruses (Khan et al.,
2019). Finally, an alternative strategy employs a combination
of CRISPR/Cas systems (Cas3, Cas9, Cas12, Cas13, and Cas14)
as a multiplex to enhance plant immunity. These advances in
molecular technologies make CRISPR/Cas a powerful tool for
improving plant immunity against viruses.
CRISPR/Cas-MEDIATED VIRUS GENOME
EDITING TO CONTROL INFECTION IN
PLANTS
Plant viruses are the most diverse phytopathogens globally,
impacting cultivated crops. Due to the simplicity of their
genome, composed mainly of RNA, plant viruses evolve rapidly.
The CRISPR/Cas system can mediate genome interference in
DNA or RNA genomes, providing an efficient strategy to control
plants viruses (Aman et al., 2020). Accordingly, the CRISPR/
Cas9 system has generated plant immunity against viruses in
several crops, including beans, tomato, cassava, cotton, chili,
wheat, cucumber, and soybean. Nicotiana benthamiana plants
expressing Cas9-sgRNA to the targeted bean yellow dwarf virus
(BeYDV) genome displayed reduced virus load and symptoms
(Baltes et al., 2015). Likewise, tomato plants expressing Cas9sgRNA targeting TYLCV coat protein (CP) or replicase (Rep)
sequences were resistant to TYLCV (Tashkandi et al., 2018).
Recently, the use of the CRISPR/Cas9 system with multiple
sgRNAs, which target essential conserved regions for replication
of viral genomes, has improved the resistance of plants against
several viruses, including cotton leaf curl Multan virus
(CLCuMuV; Yin et al., 2019), wheat dwarf virus (WDV; Kis
et al., 2019), and soybean mosaic virus (SMV; Zhang et al.,
2020). Likewise, the expression of Cas9-sgRNA targeting ACMV
transcription activator (AC2) and replication enhancer (AC3)
sequences generated moderate resistance to the begomovirus
in cassava (Mehta et al., 2019). Limitations of the CRISPR/
Cas9 systems targeting viral genomes include the possibility
of generating viral escapes and variants capable of replication
(Mehta et al., 2019; Aman et al., 2020). In addition to CRISPR/
Cas9, other CRISPR/Cas variants, including Cas3, Cas9, Cas12,
Cas13, and Cas14, have potentially been deployed to improve
plant immunity (Figure 1). For example, CRISPR/Cas3 system
can be used as multiplex targets for double-stranded DNA
viruses (dsDNA) or RNA viruses using multiplex sites (Aman
et al., 2020). In another study, plants have been engineered
using the CRISPR/Cas13 system that targeted TMV and turnip
mosaic virus (TuMV) genomes, enhancing plant immunity
against these plant RNA viruses (Aman et al., 2018). Furthermore,
CRISPR/Cas13 was able to protect potato plants from potato
virus Y (PVY; Zhan et al., 2019).
The CRISPR/Cas12 is another system used for resistance
to viruses. CRISPR/Cas12 can target both dsDNA and ssDNA
viruses (Aman et al., 2020). Applications of CRISPR/Cas12a
system include the detection of plant viruses in different crops
such as apples and tomato (Cho et al., 2016; Aman et al.,
2020; Mahas et al., 2021). The technology has advanced with
engineered Cas nucleases to improve their efficiency and precision
for the next generation CRISPR editing technologies. The
engineered nuclease CRISPR-MAD7 system, a Class 2 type
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CRISPR/Cas SYSTEMS-MEDIATED
HOST GENOME EDITING TO IMPROVE
PLANT IMMUNITY AGAINST PLANT
VIRUSES
Plant viruses are obligate intracellular parasites that require
the host cellular machinery to translate their viral genome,
replicate, and spread to neighbor cells (Kumar, 2019). Many
plant host factors are crucial for viral infections and have
been extensively studied as potential targets for controlling
plant diseases. Indeed, recessive resistance can be achieved
either by silencing a negative regulator of plant defense or a
host gene essential for infection. For resistance to viruses, the
latter has predominated and been identified as loss-ofsusceptibility mutants. The first identified natural recessive
resistant genes against RNA viruses mapped to mutations in
eukaryotic translation initiation factors eIF4E and eIF4G genes
(Calil and Fontes, 2017). Due to its simplicity and accuracy,
the CRISPR/Cas systems have been used as a powerful tool
to mediate host genome editing and improve plant immunity
against plant viruses in several crops (Table 1). CRISPR/Cas9
sgRNA targeting N′ and C′ termini of eukaryotic translation
initiation factor eIF4E gene has induced broad-spectrum
resistance against the potyviruses zucchini yellow mosaic virus
(ZYMV) in cucumber, papaya ringspot mosaic virus-W
(PRSV-W) in papaya, and immunity to ipomovirus cucumber
vein yellowing virus (CVYV; Chandrasekaran et al., 2016).
Due to the physiological importance of translation, the induction
of specific pinpoint mutations using CRISPR/Cas is a strategy
to avoid deleterious effects by mutating translation initiation
genes. Sequence-specific mutations of eIF(iso)4E from Arabidopsis
thaliana by CRISPR/Cas9 provided resistance to TuMV (Pyott
et al., 2016). CRISPR/Cas9 editing eIF4G in rice has induced
resistance to rice tungro spherical virus (RTSV; Macovei et al.,
2018). The CRISPR/nCas9 cytidine deaminase system introduced
a single mutation in the eIF4E1 generating resistant plants to
clover yellow vein virus (ClYVV; Bastet et al., 2019). Simultaneous
CRISPR/Cas9-mediated editions of eIF4E isoforms nCBP-1 and
nCBP-2 reduced cassava brown streak virus (CBSV) symptoms
and severity (Gomez et al., 2019). In addition to translation
initiation factors as targets for CRISPR/Cas-mediated
resistance to RNA viruses, the nuclear protein coilin, and
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TABLE 1 | Summary of CRISPR/Cas system mediating resistance to plant virus by targeting host factors.
CRISPR/Cas system
Plant species
Target host factor
Genus/Plant virus
References
CRISPR/Cas9
Cucumber (Cucumis sativus L.)
Cucumber (Cucumis sativus L.)
CRISPR/Cas9
Cucumber (Cucumis sativus L.)
CRISPR/Cas9
Arabidopsis thalaiana
CRISPR/Cas9
Rice (Oriza sativa)
CRISPR/nCas9 cytidine
deaminase
Arabidopsis thaliana
Potyvirus/Cucumber vein
yellow virus (CuVYV)
Potyvirus/Zucchini yellow
mosaic virus (ZYMV)
Potyvirus/Papaya ring spot
virus-W (PRSV-W)
Potyvirus/Turnip mosaic virus
(TuMV)
Tungrovirus/Rice tungo
spherical virus (RTSV)
Potyvirus/Clover yellow vein
virus (RTSV)
Chandrasekaran et al., 2016
CRISPR/Cas9
CRISPR/Cas9
Cassava (Manihot esculenta
Crantz)
Potato (Solanum tuberosum)
Soya bean [Glycine max (L.)
Merr.]
Host factor eukaryotic translation initiation
factor 4E (eIF4E)
Host factor eukaryotic translation initiation
factor 4E (eIF4E)
Host factor eukaryotic translation initiation
factor 4E (eIF4E)
Host factor eukaryotic translation initiation
factor eIF(iso)4E
Host factor eukaryotic translation initiation
factor eIF4G
Substitutions encoded by a Pisum
sativum eIF4E virus-resistance allele into
the Arabidopsis thaliana eIF4E1
Simultaneous editions of IF4E isoforms
nCBP-1 and nCBP-2
Nuclear Coilin
Multiple targets of isoflavanoids pathway
Ipomovirus/Cassava brown
streak virus (CBSV)
Potyvirus/Potato virus Y (PVY)
Potyvirus/Soya bean mosaic
virus (SMV)
Gomez et al., 2019
Begomovirus/Tomato yellow
leaf curl virus (TYLCV)
Pramanik et al., 2021b
CRISPR/Cas9
CRISPR/Cas9
CRISPR/Cas9
Tomato (Solanum lycopersum)
flavone-3-hydrolases (GmF3H1,
GmF3H2)
flavone synthase II (GmFNSII-1)
Susceptibility (S-gene) factor
flavanone-3-hydroxylase (F3H)/flavone synthase II (FNSII) genes
have also been used as targets for resistance to RNA virus of
the Potyviridae family. Editing coilin by the CRISPR/Cas9
system increased the resistance of edited potato lines to PVY
(Makhotenko et al., 2019). Also, the CRISPR/Cas9 mediated
multiplex gene-editing technology has been employed to target
flavone-3-hydroxylases [Glycine max (Gm)F3H1 and GmF3H2]
and flavone synthase II (GmFNSII-1) genes as a metabolic
engineering approach that resulted in increased isoflavone
content and enhanced resistance of edited soybean plants to
soybean mosaic virus (SMV; Zhang et al., 2020).
Despite these reports, the application of CRISPR/Cas for
host genome editing in plant immunity has been limited because
of the restricted repertoire of characterized naturally loss-ofsusceptibility mutants or recessive-resistant genes. In the lack
of known recessive resistant genes, a loss-of-function mutation
in susceptibility genes, which will not cause deleterious effects
on plant growth and productivity, can be an alternative target
for CRISPR/Cas-mediated host immunity. In fact, the inactivation
of a necessary host factor for infection is supposed to account
for recessively inherited disease resistance to plant viruses. For
the ssDNA bipartite begomoviruses, two susceptibility genes,
the endosomal NSP-interacting syntaxin-6 domain-containing
protein (NISP), and NSP-interacting GTPase (NIG), which are
involved in the intracellular traffic of viral DNA, may be targets
for enhancing resistance in crops (Carvalho et al., 2008; GouveiaMageste et al., 2021). Silencing of NISP enhanced resistance
to cabbage leaf curl virus (CabLCV) in Arabidopsis without
yield penalty, an essential property for considering susceptibility
genes as targets for engineering recessive resistance. Accordingly,
the silenced lines display lower DNA viral load and attenuated
symptoms and are phenotypically indistinguishable from the
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Chandrasekaran et al., 2016
Chandrasekaran et al., 2016
Pyott et al., 2016
Macovei et al., 2018
Bastet et al., 2019
Makhotenko et al., 2019
Zhang et al., 2020
control lines under normal conditions (Gouveia-Mageste et al.,
2021). Likewise, CRISPR/Cas9 sgRNA has been employed to
target the susceptibility gene (S-gene) SIPelo for the monopartite
begomovirus TYLCV inducing resistance in edited tomato
plants against the virus (Pramanik et al., 2021b). Silencing the
S-gene suppressed viral DNA accumulation and restricted the
systemic spread of TYLCV to non-inoculated leaves (Pramanik
et al., 2021b). Collectively, these results demonstrate the potential
of CRISPR/Cas systems to generate host-mediated immunity
to DNA and RNA viruses by targeting susceptibility genes or
resistant recessive genes. The efficiency of the CRISPR/Cas
systems in introducing mutagenesis in multiple target sites
offers a precise genome editing technology for engineering a
variety of transgene-free resistant crops.
CONCLUSION AND FUTURE
PERSPECTIVES
Clustered regularly interspaced short palindromic repeats/Cas
systems have a central role in plant biotechnology as an accurate
molecular tool for editing genomes, rapidly improving desired
traits, creating new plant varieties, and enhancing plant immunity
against phytopathogens. The use of CRISPR/Cas systems is
suitable for mediating viral genome editing while maintaining
the biological functions of cells. Additionally, CRISPR/Cas
systems have the potential to edit host factors, improving plant
immunity against plant viruses. Nevertheless, a drawback in
CRISPR/Cas-mediated host genome editing to enhance plant
immunity is the limited repertoire of well-characterized recessive
resistant genes or host susceptibility genes in which mutations
are not likely to cause host growth defects. A better understanding
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CRISPR/Cas for Resistance to Virus
of the host-virus interactome will expand the use of CRISPR/
Cas for editing host susceptibility genes, which may be more
efficient targets for durable resistance against viruses.
Meanwhile, advances in the next generation CRISPR editing
technology variants, such as the CRISPR-MAD7 system and
engineered nucleases Cas12a, increase the accuracy, range of
possibilities, and applications. Engineered Cas MAD7-RR,
MAD7-RVR, and M-AFID (MAD7-APOBEC fusion-induced
deletion) increase the targeting range of MAD7 by creating
predictable deletions from 5′-deaminated Cs to the MAD7cleavage site. This new CRISPR-MAD7 system has an efficiency
of up to 65.6%, as demonstrated in mutant rice and wheat
plants (Lin et al., 2021). MAD7 can expand the CRISPR toolbox
for genome engineering due to its highly efficient target to
gene disruption and insertions, different protospacer adjacent
motifs, and small-guide RNA requirements (Liu et al., 2020).
Other advances in CRISPR/Cas systems have improved precision
and provided multiple edited sites in viral genomes toward
reaching a lower risk of generating viral escapes or new variants.
Using new CRISPR/Cas systems (Cas3, Cas12, Cas13, and
Cas14) as multiplex sgRNAs targeting different sites is a new
and more efficient strategy to improve broad-spectrum resistance,
prevent viral infections, and control disease in the field.
AUTHOR CONTRIBUTIONS
FDAS and EPBF wrote the drafts with input from both authors.
All authors contributed to the article and approved the submitted
version.
FUNDING
This work was partially supported by the Council for Advanced
Professional Training (CAPES)—88887.511855/2020-00, National
Council for Scientific and Technological Development (CNPq)—
441955/2019-3, and Fapemig.
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