Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 358

proteins are released from MPK4. The unbound WRKY33 targets the promoter of a
defense-related gene PAD3 ( PHYTOALEXIN DEFICIENT3 ) for transcriptional
activation (Qiu et al. 2008a , b ).
Bethke et al. ( 2009b ) identifi ed a transcription factor of the Ethylene Response
Factor (ERF) family, ERF104, which interacted with the Arabidopsis MAPK
MPK6. The ERF104 was found to be a nuclear substrate involved in plant defense
and MPK6 binds with ERF104. The continued binding of MPK6 to ERF104 might
constrain physical interactions with subsequent ERF104 targets and impringe on its
role in transcription activation (Bethke et al. 2009b ). The release of ERF104 from
MPK6 in the nucleus required rapid ET signaling (Bethke et al. 2009b ). Bethke
et al. ( 2009a ) suggested that the PAMP fl g22 signal network includes one pathway
for MPK6 to target ERF104 directly through phosphorylation and on a separate
branch, to stimulate ET production, which triggers a yet unknown mechanism (that
is dependent on EIN2 and the EIN3/EIL members) for the release of ERF104 from
MPK6 in the nucleus. The released transcription factor ERF104 may activate tran-
scription of defense genes. These results suggest that transcription factor release
after MAPK activation in the nucleus controls the downstream gene expression.

7.21 Role of MAPK Signaling Cascade in Triggering

Phytoalexin Biosynthesis

Phytoalexins are key components in plant defense responses and several elicitors
are known to trigger production of phytoalexins (Vidhyasekaran 2007 ). MAPK sig-
naling cascades have been shown to activate the phytoalexin camalexin biosynthesis
(Ren et al. 2008 ). The Arabidopsis MAPKKK /MEKK1-MKK4/MKK5-MPK3/

MPK6 cascade has been shown to trigger the camalexin biosynthesis (Fig. 7.7 ).
Both MPK3 and MPK6 play important role in triggering biosynthesis of camalexin.
MPK3/MPK6 cascade coordinates the induction of multiple genes in the camalexin
biosynthetic pathway. The camalexin biosynthetic genes include the genes encoding
anthranilate synthase

α

subunits (ASA and ASB), phosphoribosylanthranilate
transferase (PAT), indole-3-glycerolphosphate synthase (IGPS), tryptophan synthase
α

α and β

subunits (TSA) and TSB), and the P450 enzymes CYP79B2, CYP79B3, and
CYP71B15 (PAD3). The induction of all of these genes was partially compromised
in mpk3 / mpk6 mutant plants (Ren et al. 2008 ).

and β

7.22 Role of MAPK Signaling Cascade in Stomatal Immune

Response

Stomata serve as passive ports of bacterial entry during infection. They constitute
one entry point for bacteria, which need to reach apoplastic spaces to multiply and
cause disease (Nicaise et al. 2009 ). The stomata in the Arabidopsis leaf epidermis