Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 347

and LeMPK2 function upstream of JA biosynthesis (Kandoth et al. 2007 ). These
studies suggest that MAPKs may act at upstream of various signaling systems in
different plants.

7.12 Some MAP Kinases Act Downstream

of Phosphoinositide (PI) Signal Transduction Pathway

Some MAP kinases have been shown to act downstream of phosphoinositide (PI)
signal transduction pathway. In this pathway, PI is phosphorylated by phosphati-
dylinositol kinase to form phosphatidylinositol-4-phosphate (PIP), which is
phosph orylated by phosphatidylinositol-4-monophosphate kinase to form phospha-
tidylinositol-4,5-bisphosphate (PIP2). The PIP2 is hydrolysed by phospholipase C
into inositol trisphosphate (IP3) and diacylglycerol (DAG). It has been reported that
a fungal elicitor induced a rapid and biphasic increase in levels of PIP2 and IP3 in
pea that was apparent within 15 min (Toyoda et al. 1993 ). Neomycin, a known inhib-
itor of phospholipase C blocked the elicitor-induced accumulation both of IP3 and
the phytoalexin pisatin. These results suggest that rapid changes in PI metabolism are
indispensable in the defense signaling system in pea plants (Toyoda et al. 1993 ). In
another related study, increased activation of two MAP kinases, p44 and p48 kinases,
by a fungal pathogen elicitor within 15-30 min was observed (Uppalapati et al.
2004 ). The elicitor-induced p44 kinase activation was inhibited by a MAPKK inhibi-
tor, PD098059 and this inhibition was correlated with the suppression of elicitor-
induced expression of the defense gene PA L encoding phenylalanine ammonia-lyase.
These observations suggest that the MAPK is involved in defense signaling system
in pea (Uppalapati et al. 2004 ). Pre-treatment of epicotyls of pea plants with the
phospholipase C inhibitor neomycin completely suppressed elicitor-induced p44
kinase activation and subsequent phytoalexin accumulation (Uppalapati et al. 2004 ).
A DAG kinase which degrades the DAG was shown to activate the p44 kinase. This
was demonstrated using a DAG kinase inhibitor, R59022, and this inhibitor potenti-
ated the elicitor-induced MAP kinase activities (Uppalapati et al. 2004 ). Toyoda
et al. ( 2000 ) reported that inhibition of DAG kinase potentiated the PA L gene activa-
tion and accumulation of phytoalexin in pea. These results suggest that DAG may act
as a second messenger and inhibition of breakdown of DAG by DAG kinase may
increase induction of the defense gene and defense compound. It demonstrates that
the MAP kinase acts downstream of DAG in the PI metabolism (Fig. 7.4 ).

7.13 MAP Kinase Cascades May Act Either Upstream

or Downstream of ROS Signaling System

ROS may act either upstream or downstream of MAPK pathways The overexpression
of a constitutively active MAPK kinase (MAPKK), which activates endogenous
SIPK and WIPK, induced the NbrbohB ( Nicotiana benthamiana respiratory burst