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which contains the amino acid sequence WRKY (tryptophan-arginine-lysine-tyrosine)
at its amino-terminal end and a putative zinc fi nger motif at its carboxy-terminal
end. Most of the WRKY proteins contain one WRKY domain, while some of the
WRKY proteins have two WRKY domains (Eulgem et al.
2000
; Maeo et al.
2001
;
Zheng et al.
2006
). WRKYs are subdivided into three subgroups (Eulgem et al.
2000
). Members of group I have two WRKY domains, whereas members of groups
II and III have one WRKY domain. Group III domains contain a Cx
7
CX
23
HXC pat-
tern of zinc ligands which is distinct from the Cx
4-5
CX
22-23
HXH zinc fi nger pattern
of group I and group II WRKY domains (Maeo et al.
2001
; Knoth et al.
2007
).
Members of the
Arabidopsis
WRKY group III transcription factors are part of
different plant defense signaling pathways (Kalde et al.
2003
)
WRKY is localized to the nucleus of plant cells and recognizes DNA molecules
containing the
TTGACC
W-box sequence (Zheng et al.
2006
). The WRKY domain
binds specifi cally to various W box elements containing a (C/T)TGAC(C/T) core
sequence. The promoters of a large number of defense-related genes contain W-box
sequences that are recognized by WRKY proteins and the WRKY transcription factors
have been shown to be necessary for the inducible expression of these defense genes
(Eulgem et al.
1999
,
2000
; Yu et al.
2001
; Shimono et al.
2007
; van Verk et al.
2008
).
Several WRKY transcription factors have been shown to be involved in activation
of SA biosynthesis genes in plant immune responses (van Verk et al.
2011
).
CaM binds to the Ca
2+
-dependent CaM-binding domain of several WRKY tran-
scription factors. CaM binds specifi cally to the Ca
2+
-dependent CaM-binding
domain of AtWRKY7 transcription factor (Park et al.
2005
). WRKY7 is a member
of the WRKYIId subfamily, and all members of this subfamily including WRKY11,
WRKY15, WRKY17, WRKY21, WRKY39, and WRKY74 were found to interact
with Ca
2+
/CaM (Park et al.
2005
). Other WRKYs, WRKY43, WRKY45, WRKY50,
and WRKY 53, have been shown to interact with different isoforms of CaM in a
Ca
2+
-dependent manner (Popescu et al.
2007
).
The defense signal salicylic acid induces expression of
WRKY7
gene, suggesting
that the
WRKY
gene is involved in plant's defense response. The transcription factor
WRKY7 negatively regulates the expression of defense-related genes in
Arabidopsis
(Kim et al.
2006
). The CaM binding transcription factors WRKY11 and WRKY17
act as negative regulators of basal resistance in
Arabidopsis thaliana
(Journot-
Catalino et al.
2006
). Another transcription factor interacting with CAM is
WRKY45, which is involved in triggering plant defense responses. Transgenic rice
plants overexpressing the transcription factor
WRKY45
showed enhanced resistance
to the blast pathogen
Magnaporthe oryzae
(Shimono et al.
2007
). All these WRKYs
are CaM binding proteins.
4.14.13
Calmodulin-Binding MYB Transcription Factors
Several
MYB
transcription factor genes are found in plants. They are characterized by
the presence of a highly conserved MYB domain at their N-termini (Du et al.
2009
).
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