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proteins (Jones et al. 1995 ). It has been shown that 14-3-3 proteins bind to
phosphorylated Ser residues present within one of a small number of consensus
sequences found in many of the proteins with which they interact (Yaffe et al. 1997 ;
Yaffe 2002 ).
MAPKKK
is a positive regulator of immunity-associated programmed cell death
(PCD) in tomato and Nicotiana benthamiana . A 14-3-3 protein, TFI7, has been identi-
fi ed as a MAPKKK
α
- interacting protein in tomato (Oh et al. 2010 ). TFI7 protein
contains a phosphopeptide binding motif, which was found to be essential for the
interaction with MAPKKK
α
in vivo and also the PCD-enhancing activity of TFI7.
A 14-3-3 binding motif, including a putative phosphorylated Ser-535, is present in the
C-terminal region of MAPKKK
α
reduced
interaction with TFI7 and both PCD-eliciting ability and stability of MAPKKK
α
. An S535A substitution in MAPKKK
α
α
.
Coexpression of the 14-3-3 protein with tomato MAPKKK
α
enhanced MAPKKK
α
-mediated PCD. Coexpression TF17 with MAPKKK
in vivo caused increased
accumulation of the kinase and enhanced phosphorylation of two MAP kinases
(Oh et al. 2010 ). Collectively, these results suggest that the 14-3-3 protein enhances
signaling ability of MAPKKK
α
α
in activating plant innate immune responses.
7.7
Role of MAPKs in Priming Plants for Augmented
Defense Gene Activation
Some dormant MAPKs have been suggested to be important components required
for priming in Arabidopsis and the prestress deposition of these inactive kinases
may be a possible mechanism of priming during development of systemic acquired
resistance (Beckers et al. 2009 ). MPK3, and functionally redundant MPK6,
have been found to be important components for full priming in Arabidopsis .
The resistance-inducing avirulent strains of P . syringae pv. tomato DC3000 and
P . syringae pv. phaseolicola induced SA accumulation and MPK3 expression. Both
SA and the SA-related compounds benzothiadiazole (BTH) and 4-chloro-SA acti-
vate MPK3 gene expression and induce priming and SAR. In contrast, another
SA-related compound 3-hydroxybenzoic acid did not induce MPK3 gene expres-
sion, priming and disease resistance. This strong correlation between the ability of
avirulent bacteria and various SA-related compounds to activate MPK3 gene expres-
sion and their capacity to prime plants for augmented defense gene activation and
induced resistance suggests that MPK3 plays a role in priming (Beckers et al. 2009 ).
Similarly another MAPK gene, MPK6 was also found to be involved in priming
process. However, the BTH-induced accumulation of MPK6 transcript and protein
was less pronounced. Both MPK3 and MPK6 accumulate in an inactive form during
priming of Arabidopsis with BTH (Beckers et al. 2009 ). Both MPK3 and MPK6
displayed greater activity in Arabidopsis plants which are primed and subsequently
challenged with the virulent P . syringae pv. maculicola . These two enzymes were
more strongly activated in primed plants than in nonprimed plants. Priming of
defense gene expression and induced resistance were lost or reduced in mpk3 or
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