NPR1 gene that leads to the induction of various defense genes such as PR proteins,
phytoalexins, reactive oxygen species, peroxidase, and other related enzymes
(Zhang et al. 2019). As a result, in this chapter, the structure and classification of
WRKY as well as downstream regulation/interaction with the same or other TFs,
proteins, and small RNAs to regulate the defense gene for various plant pathogens
such as fungi, bacteria, and viruses have been summarized. We have also provided
important highlights of understanding the regulation and advancement in crop
implement strategies by using WRKY TFs for the management of phytopathogens.
11.2 WRKY TFs: Structure
A specific DNA-binding domain is the distinguishing character of WRKY TFs
which consists of a greatly conserved partly protruding region of almost 60 amino
acids in plants (Duan et al. 2015). In this region, there is an almost invariable
sequence at N terminal of protein, WRKYGQK (W; Tryptophan, R; Arginine, K;
Lysine, Y; Tyrosine, G; Glycine Q; Glutamine, K; Lysine).WRKY TFs generally
bind to a considerably conserved region of DNA identified as the W-box elements
having the conserved motif TGACC/Tsingaly or in tandem repeats of the promoter
region of defense genes (Chen et al. 2019). Recently, the WRKY domain structure
with W-box as a binding site was identified and showed four-stranded β-sheet makes
grove of DNA in an unusual manner where the plane of β-sheet is almost at right
angles to the helical axis of the DNA (Yamasaki et al. 2012). Whereas C-terminal of
protein consists of 4–5 stranded anti-parallel β-sheets of zinc-finger-like motif
(Cx4-HxC or 5Cx22-23HxH or Cx7Cx23) (Alves et al. 2014). A tryptophan
amino acid makes the core structure of the conserved WRKYGQK sequence,
whereas the rest of amino acids were bind to DNA. The glycine amino acid helps
in creating protrudes that make grove for binding to W-box by hydrophobic interaction with the methyl groups of thymine nitrogenous bases of the DNA. Mutation in
the thymine base or Zn
2+ -binding site drastically reduced the DNA-binding activity
by a disorder of the active structure of DNA-binding domain protein (Yamasaki et al.
2013). Only in very few crops like Arabidopsis (WRIY), soybean (WHQY), potato
(WHKC and WRKC), black cottonwood (FRKY), tomato (WRKR, WIKY, WSKY,
and WQKY), and French bean (ARKM, WWKN, and WRMY), WRKY proteins
have been reported to have changes in the conserved sequence of WRKY (Mohanta
et al. 2016).
11.3 Classification of WRKY TFs
Based on the total number of WRKY domains and presence/absence of zinc-fingerlike motif, WRKY protein is classified into three groups, i.e., group I, group II, and
group III (Chen et al. 2019). Group I members consist of two WRKY domains,
whereas group II members have one WRKY domain along with Cys2-His2-type of
zinc-finger motif. Group II members are further divided into five subgroups IIa to IIe
11 Role of WRKY Transcription Factor Superfamily in Plant Disease Management
337
phytoalexins, reactive oxygen species, peroxidase, and other related enzymes
(Zhang et al. 2019). As a result, in this chapter, the structure and classification of
WRKY as well as downstream regulation/interaction with the same or other TFs,
proteins, and small RNAs to regulate the defense gene for various plant pathogens
such as fungi, bacteria, and viruses have been summarized. We have also provided
important highlights of understanding the regulation and advancement in crop
implement strategies by using WRKY TFs for the management of phytopathogens.
11.2 WRKY TFs: Structure
A specific DNA-binding domain is the distinguishing character of WRKY TFs
which consists of a greatly conserved partly protruding region of almost 60 amino
acids in plants (Duan et al. 2015). In this region, there is an almost invariable
sequence at N terminal of protein, WRKYGQK (W; Tryptophan, R; Arginine, K;
Lysine, Y; Tyrosine, G; Glycine Q; Glutamine, K; Lysine).WRKY TFs generally
bind to a considerably conserved region of DNA identified as the W-box elements
having the conserved motif TGACC/Tsingaly or in tandem repeats of the promoter
region of defense genes (Chen et al. 2019). Recently, the WRKY domain structure
with W-box as a binding site was identified and showed four-stranded β-sheet makes
grove of DNA in an unusual manner where the plane of β-sheet is almost at right
angles to the helical axis of the DNA (Yamasaki et al. 2012). Whereas C-terminal of
protein consists of 4–5 stranded anti-parallel β-sheets of zinc-finger-like motif
(Cx4-HxC or 5Cx22-23HxH or Cx7Cx23) (Alves et al. 2014). A tryptophan
amino acid makes the core structure of the conserved WRKYGQK sequence,
whereas the rest of amino acids were bind to DNA. The glycine amino acid helps
in creating protrudes that make grove for binding to W-box by hydrophobic interaction with the methyl groups of thymine nitrogenous bases of the DNA. Mutation in
the thymine base or Zn
2+ -binding site drastically reduced the DNA-binding activity
by a disorder of the active structure of DNA-binding domain protein (Yamasaki et al.
2013). Only in very few crops like Arabidopsis (WRIY), soybean (WHQY), potato
(WHKC and WRKC), black cottonwood (FRKY), tomato (WRKR, WIKY, WSKY,
and WQKY), and French bean (ARKM, WWKN, and WRMY), WRKY proteins
have been reported to have changes in the conserved sequence of WRKY (Mohanta
et al. 2016).
11.3 Classification of WRKY TFs
Based on the total number of WRKY domains and presence/absence of zinc-fingerlike motif, WRKY protein is classified into three groups, i.e., group I, group II, and
group III (Chen et al. 2019). Group I members consist of two WRKY domains,
whereas group II members have one WRKY domain along with Cys2-His2-type of
zinc-finger motif. Group II members are further divided into five subgroups IIa to IIe
11 Role of WRKY Transcription Factor Superfamily in Plant Disease Management
337