Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 341

7.5 MAPK Pathways Involved in Defense Signal

Transduction May Be Interconnected

Rather than linear pathways, multiple interconnected MAPK pathways may be
required to transmit PAMP elicitor signals and integrate defense responses (Pedley
and Martin 2005 ; Mészáros et al. 2006 ). Two separate MAPK pathways including
MAPKKK - NtMEK2 - WIPK and MAPKKK - NtMEK2 - SIPK have been
shown to be involved in defense response signaling in tobacco (Liu et al. 2003 ).
Both these pathways are interconnected in induction of defense signals. SIPK,
NtMEK2 and the upstream MAPKKK pre-exist in cells. Upon recognition of
elicitor signals, NtMEK2 is activated by its upstream MAPKKK, which in turn
activates the pre-existing SIPK. Activation of SIPK turns on the transcription of
WIPK gene, which leads to the accumulation of WIPK protein. The newly synthe-
sized WIPK protein is then activated by the NtMEK2 which is activated by a puta-
tive MAPKKK (Fig. 7.3 ; Liu et al. 2003 ). WIPK triggers HR-related cell death
probably by the production of H 2 O 2 . SIPK besides activating synthesis of WIPK,
activates transcription of various defense genes, particularly 3-hydroxy-3-methyl-
glutaryl CoA reductase ( HMGR ) and phenylalanine ammonia lyase ( PA L ) genes
encoding key enzymes in the phytoalexin and salicylic acid biosynthesis path-
ways (Yang et al. 2001 ; Liu et al. 2003 ).
Two independent MAPK pathways, MEKK1 - MKK1 - MPK4 and MEKK1 -
MKK4/5 - MPK3/6, have been shown to be interconnected in defense signaling
system in Arabidopsis (Mészáros et al. 2006 ). Both these pathways have com-
mon MAPKK, MEKK1, but their MAPKKs differ. The fi rst pathway has MKK1
as MAPKK, while the second one has the MAPKK pair, MKK4 and MKK5.
MKK1 specifi cally interacts with, and phosphorylates MPK4 (Teige et al. 2004 ),
while it cannot phosphorylate the MAPK pair MPK3 and MPK4 (Mészáros
et al. 2006 ). MKK4/5 can phosphorylate MPK3/4. It has been suggested that
MEKK1 tethers MPK4 and MKK1 through its N- and C-terminal domains,
respectively. This would prevent the association of MKK4/MKK5 until MEKK1
is activated by elicitor signals and dissociates the MKK1-MPK4 complex
(Mészáros et al. 2006 ). The activated MEKK1 in the fi rst pathway after sequen-
tial phosphorylation of MKK1 and MPK4 activates the second pathway. The
MPK4 pathway positively regulates ethylene/JA signaling system and negatively
regulates SA-mediated signaling system (Petersen et al. 2000 ; Liu and Zhang
2004 ). The MPK3/MPK6 pathway is involved in JA/ET signaling system and
also in activation of WRKY22 and WRKY29 transcription factors (Asai et al.
2002 ; Takahashi et al. 2007a ).
Another MAP kinase cascade in Arabidopsis thaliana involves MEKK1 - MKK2 -
MPK4/MPK6 (Teige et al. 2004 ). Transgenic Arabidopsis plants overexpressing a
MAPKK, MKK2, were developed (Teige et al. 2004 ). The MAPKK MKK5 also
showed enhanced expression levels in MKK2 overexpressing plants. MKK5 is an
activator of MPK3 and MPK6. Upregulation of MKK5 in the MKK2 overexpressor
lines indicates some level of crosstalk between these two pathways (Teige et al. 2004 ).