based on the presence of amino acid motifs additional in the WRKY domain. Group
III members also contain one WRKY domain with Cys2-His/Cysor Cys2-His2 type
of zinc-finger motif (Eulgem et al. 2000). The criteria of classification method are
exclusively based on protein structure; however, this classification does not concern
about evolution, origin, and duplications of the gene for WRKY TFs. So, in the year
2005, Zhang and Wang (2005) again reclassified WRKY TFs into five groups, i.e.,
group 1, group 2_a + 2_b, groups 2_c, group 2_d + 2_e, and group 3, based on
phylogenetic analysis, domains conservation, and intron position in WRKY domain.
However, based on the insertion position in an intron, WRKY TFs were again
classified into two groups where group 1 includes R-type of the intron in the
WRKY domain, whereas group 2 members include V-type of an intron (Zhang
and Wang 2005).
Initially, WRKY TF was identified for the regulation of sporamin and β-amylase
production from sweet potato as SPF1 (Ishiguro and Nakamura 1994). However,
later, Rushton and group identified three different WRKY TFs (WRKY1, WRKY2,
and WRKY3) from parsley in a stress response against elicitor Pep25 of
Phytophthora parasitica and given the name of “worky” in 1996 (Rushton et al.
1996). This opened a way for identification of stress-responsive WRKY TFs, which
has resulted now in a superfamily. They formerly considered to be reported from
plants only, but they have also found in protists (Giardia lamblia) and Metazoa
(Dictyostelium discoideum) (Finatto et al. 2018).
11.4 Regulation of WRKY TFs
11.4.1 Kinases
MAP kinases can regulate most of WRKY TF by upregulation of various defense
genes against several pathogens (Aamir et al. 2018). After recognition of PAMP or
MAMP molecules, plants trigger a series of events in MAPKKK signaling which
leads to activation of AtWRKY33 (Qiu et al. 2008). Then AtWRKY33 activates
phytoalexin producing defense gene naming PAD3 (phytoalexin deficient 3) that
produces camalexin and provides defense against Pseudomonas syringae by disruption of bacterial membranes (Rogers et al. 1996). AtWRKY33 could be activated by
two other MAP kinases, i.e., MPK3 and MPK6, which provide defense against
Botrytis cinerea by producing camalexin (Mao et al. 2011). Other kinases like
calcium-dependent protein kinases like CPK11 and CPK4 modulate AtWRKY28
against P. syringae pv. tomato. It induces PKS2 (SOS2-like protein kinase 5) gene
which phosphorylates NPR1 (non-expressor of pathogenesis-related gene 1) gene
that in turn induces systemic resistance (Gao and He 2013). This interaction again
induces a couple of WRKY genes like AtWRKY38 and AtWRKY62, which ultimately
induces plant defense genes (Xie et al. 2010). Similarly, in rice, OsWRKY45 WRKY
TF regulated by MAPK provides resistance against Magnaporthe oryzae and
Xanthomonas oryzae pv. oryzae by induction of SAR pathway (Nakayama et al.
2013). Likewise, SlWRKY33 interacts with MAPK5 and provides resistance against
338
L. S. Rajput et al.
III members also contain one WRKY domain with Cys2-His/Cysor Cys2-His2 type
of zinc-finger motif (Eulgem et al. 2000). The criteria of classification method are
exclusively based on protein structure; however, this classification does not concern
about evolution, origin, and duplications of the gene for WRKY TFs. So, in the year
2005, Zhang and Wang (2005) again reclassified WRKY TFs into five groups, i.e.,
group 1, group 2_a + 2_b, groups 2_c, group 2_d + 2_e, and group 3, based on
phylogenetic analysis, domains conservation, and intron position in WRKY domain.
However, based on the insertion position in an intron, WRKY TFs were again
classified into two groups where group 1 includes R-type of the intron in the
WRKY domain, whereas group 2 members include V-type of an intron (Zhang
and Wang 2005).
Initially, WRKY TF was identified for the regulation of sporamin and β-amylase
production from sweet potato as SPF1 (Ishiguro and Nakamura 1994). However,
later, Rushton and group identified three different WRKY TFs (WRKY1, WRKY2,
and WRKY3) from parsley in a stress response against elicitor Pep25 of
Phytophthora parasitica and given the name of “worky” in 1996 (Rushton et al.
1996). This opened a way for identification of stress-responsive WRKY TFs, which
has resulted now in a superfamily. They formerly considered to be reported from
plants only, but they have also found in protists (Giardia lamblia) and Metazoa
(Dictyostelium discoideum) (Finatto et al. 2018).
11.4 Regulation of WRKY TFs
11.4.1 Kinases
MAP kinases can regulate most of WRKY TF by upregulation of various defense
genes against several pathogens (Aamir et al. 2018). After recognition of PAMP or
MAMP molecules, plants trigger a series of events in MAPKKK signaling which
leads to activation of AtWRKY33 (Qiu et al. 2008). Then AtWRKY33 activates
phytoalexin producing defense gene naming PAD3 (phytoalexin deficient 3) that
produces camalexin and provides defense against Pseudomonas syringae by disruption of bacterial membranes (Rogers et al. 1996). AtWRKY33 could be activated by
two other MAP kinases, i.e., MPK3 and MPK6, which provide defense against
Botrytis cinerea by producing camalexin (Mao et al. 2011). Other kinases like
calcium-dependent protein kinases like CPK11 and CPK4 modulate AtWRKY28
against P. syringae pv. tomato. It induces PKS2 (SOS2-like protein kinase 5) gene
which phosphorylates NPR1 (non-expressor of pathogenesis-related gene 1) gene
that in turn induces systemic resistance (Gao and He 2013). This interaction again
induces a couple of WRKY genes like AtWRKY38 and AtWRKY62, which ultimately
induces plant defense genes (Xie et al. 2010). Similarly, in rice, OsWRKY45 WRKY
TF regulated by MAPK provides resistance against Magnaporthe oryzae and
Xanthomonas oryzae pv. oryzae by induction of SAR pathway (Nakayama et al.
2013). Likewise, SlWRKY33 interacts with MAPK5 and provides resistance against
338
L. S. Rajput et al.