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Biology and Biotechnology of Environmental Stress Tolerance in Plants, Volume 3

specific Zinc Finger (ZF) domains (Beerli & Barbas, 2002; Lindhout et

al., 2006). Researchers have extensively used this approach to fine-tune

gene expression in different systems including plants (Maeder et al., 2008;

Foley et al., 2009; Townsend et al., 2009; Mitsuda et al., 2011; vanTol et

al., 2017). However, several major problems have been identified in the

ZF domains functions. For example, designing and construction of ZF

domains is labor intensive and extremely time-consuming and lack the

desired target specificity. Machens et al. (2017) pointed out these inherent

problems with ZF domains and further questioned the precise functioning

of these domains for any endogenous promoter. Thus, desirable efficiency

for regulation of any specific transcription module is not fully achieved

(Machens et al., 2017).

On the other hand, Transcription activator-like effectors (TALEs)

is another promising alternative approach to ZF domains. Studies have

demonstrated this approach comparative target-oriented and more precise

in TF engineering (Boch, 2011; Bogdanove & Voytas, 2011). Bagdanove

& Voytas (2011) showed that TALEs have a relatively higher degree of

specificity compared to ZF domains. The reason for high specificity is that

TALEs are specifically designed using 18 out of 34 amino acids repeats that

specify contiguous DNA nucleotides (Bogdanove & Voytas, 2011). TALEs

have demonstrated advantages over ZF domains. However, constructing a

new target specific protein for designing specific DBDs in TALEs is cumber­

some. This limits the engineering of efficient ATFs for generating quick

knockouts/mutations. However, many attempts have introduced several

new characters in TALE design to increase its efficiency (Zhang et al., 2014;

Lowder et al., 2017; Schwartz et al., 2017). Another problem is that TALEs

have been shown to be sensitive to cytosine methylation and unsuitable

for targets with CpG sites (Bogdanove & Voytas, 2011). CRISPR-Cas9

(clustered regularly interspaced short palindromic repeat-associated protein

9) technology represents another landmark discovery in tailoring the

precise regulation of gene expression. This technology has circumvented

ZF and TALEs related problems using a catalytically inactive/dead CRISPR

associated protein 9 (dCas9) domain to engineer ATFs (Cheng et al., 2013;

Lowder et al., 2015; Wyvekens et al., 2015). It is experimentally proved

that dCas9 protein provide effective control of gene expression in a variety

of prokaryotes and eukaryotes (Jinek et al., 2012; Wiedenheft et al., 2012;

Cho et al., 2013; Cong et al., 2013; DiCarlo et al., 2013; Friedland et al.,

2013; Hwang et al., 2013; Jiang et al., 2013a, b; Mali et al., 2013) including

plant systems (Feng et al., 2013, 2014; Li et al., 2013a; Miao et al., 2013;