Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 346

7.10 MAPKs May Function Downstream of G-Proteins,

Ca 2+ , ROS, SA, ABA, and NO Signaling Pathways

Silencing of a small GTPase, OsRac1 , by RNA interference or loss of function
mutation of the heterotrimeric G-protein -subunit gene resulted in strong reduction

of the OsMAPK6 protein levels and of kinase activation by the elicitor in rice
(Lieberherr et al. 2005 ). These results suggest that both small G protein and hetero-
trimeric G protein act upstream of MAPK in induction of defense genes such as PA L
in rice (Lieberherr et al. 2005 ). It has been reported that G

α

functions upstream of
OsRac1 (Suharsono et al. 2002 ) and the MAPK may act downstream of OsRac1 in
rice (Lieberherr et al. 2005 ).
The signaling cascade initiated by the DAMP/HAMP elicitor AtPep1 leads to
expression of defense genes in a Ca 2+ -dependent manner in Arabidopsis (Qi et al.
2010 ). The endogenous elicitor AtPep1 after binding with its PRR AtPepR1 acti-
vates plant membrane inwardly conducting Ca 2+ permeable channels in mesophyll
cells, resulting in cytosolic Ca 2+ elevation (Qi et al. 2010 ). The resulting Ca 2+ signature
triggers the expression of the MAPK MPK3. The results suggest that AtPep-
dependent expression of MPK3 is mediated by the Ca 2+ signaling pathway.
H 2 O 2 activates a MAPKK kinase from alfalfa, OMTK1 (oxidative stress-activated
MAP triple-kinase 1) (Nakagami et al. 2004 ). JA activates MKK3 - MPK6 cascade in
Arabidopsis (Takahashi et al. 2007a ) and OsBIMK1, a rice MAP kinase (Song and
Goodman 2002 ). ABA activates p48 MAPK in pea (Uppalapati et al. 2004 ) and rice
MAPK gene OsMAPK5 (Xiong and Yang 2003 ). Salicylic acid activates a 48-kD MAP
kinase and SIPK in tobacco (Zhang and Klessig 1997 ; Zhang and Liu 2001 ) and p48 and
p44 MAPKs in pea (Uppalapati et al. 2004 ). Both nitric oxide (NO) and SA activated
SIPK in tobacco (Kumar and Klessig 2000 ). Studies with transgenic NahG tobacco
revealed that SA is required in the NO-mediated induction of SIPK. These observations
suggest that SIPK may function downstream of SA in the NO signaling pathway (Kumar
and Klessig 2000 ). Collectively these studies suggest that MAPKs may function down-
stream of G-proteins, Ca 2+ infl ux, ROS, NO, SA, JA, ET, and ABA signaling.

α

7.11 Some MAPKs May Act Upstream of SA, JA,

and ET Signaling Pathways

There are also reports that JA or SA or ethylene may not be able to induce expres-
sion of MAPKs and they may act only at downstream of MAPK. JA or SA could not
activate transcription of TIPK ( Trichoderma -induced protein kinase) in cucumber,
even at high concentrations (Shoresh et al. 2005 ). JA was unable to induce expres-
sion of LeMPK3 in tomato (Mayrose et al. 2004 ) or activate WIPK or its alfalfa
homolog, SAMK (Bögre et al. 1997 ; Kumar and Klessig 2000 ). WIPK in tobacco is
not activated by ethylene (Kumar and Klessig 2000 ). Ethylene did not affect expres-
sion of the tomato LeMPK3 (Mayrose et al. 2004 ). The tomato MAPKs LeMPK1