Calcium Ion Signaling System: Calcium Signatures and Sensors - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 252

4.21.4.2 Arabidopsis Calmodulin Binding Protein CBP60g

Is Involved in SA Biosynthesis

A calmodulin binding protein, CBP60g, has been shown to be involved in activating SA
biosynthesis (Wang et al. 2009 ). Overexpression of CBP60g in Arabidopsis caused
elevated SA accumulation, increased expression of the defense genes, and enhanced
defense responses, and enhanced resistance to Pseudomonas syringae (Wan et al. 2012 ).
CBP60g has been shown to participate in SA signaling biosynthesis and accumulation
(Wang et al. 2009 ). It has been suggested that the signal coming from CBP60g may act
upstream from SA synthesis, as SA levels are reduced in cbp60g mutants (Wang et al.
2009 ). The effect of cbp60g mutant in SA biosynthesis was most similar to that of pad4
mutant, suggesting that CBP60 may act upstream of PAD4 (Wang et al. 2009 ). PAD4, a
key regulator of SA signaling system, contributes to SA levels. The pad4 mutant plants
showed reduced accumulation of SA after PAMP treatment (Tsuda et al. 2008 ). PAD4
is a key regulator acting at upstream of SA (Lippok et al. 2007 ). Arabidopsis plants car-
rying pad4 mutations have a defect in accumulation of SA upon pathogen infection
(Zhou et al. 1998 ). PAD4 is required for amplifi cation of weak signals to a level suffi -
cient for activation of SA signaling (Jirage et al. 1999 ). The PAD4 protein sequence
displays similarity to triacyl glycerol lipases and other esterases (Jirage et al. 1999 ). It
was also observed that the effect of cbp60g mutant in SA biosynthesis was almost simi-
lar to that of sid2 mutant (Wang et al. 2009 ). It suggests that CBP60g may also act
upstream of SID2, an isochorismate synthase that is involved in biosynthesis of SA
(Wang et al. 2009 , 2011 ). Isochorismate synthase encoded by SID2 is essential for the
biosynthesis of salicylic acid in response to pathogen challenge (Garcion et al. 2008 ;
Truman and Glazebrook 2012 ). Both the calmodulin binding protein CBP60g and its
closest homolog, the non-calmodulin binding SARD1 (for SYSTEMIC ACQUIRED
RESISTANCE DEFICIENT1), have been shown to bind to the promoter region of SID2
(Zhang et al. 2010 ). CBP60g is strongly induced in response to PAMPs treatment (Wang
et al. 2009 ). Plants carrying cbp60g null mutations were compromised in the induction
of SID2 and accumulation of SA (Wang et al. 2009 ). A central domain of CBP60g was
found to bind to an oligomer with the sequence GAAATTTTGG selected from the SID2
promoter (Zhang et al. 2010 ). PAMPs triggered signaling is greatly affected by the loss
of CBP60g (Wang et al. 2011 ). Loss of CBP60g severely impacts the plants ability to
produce SA in response to bacterial inoculation and renders the plant susceptible to
infection. CBP60 was shown to bind specifi cally to a 10mer oligonucleotide with the
sequence GAAATTTTGG (Truman and Glazebrook 2012 ). These results suggest that
the calmodulin binding protein CBP60g binds with SID2 gene and promotes SA biosyn-
thesis through activation of SID2 (Fig. 4.11 ).

4.21.5 Jasmonate Signaling System

Calcium signaling has been shown to act upstream of jasmonate (JA) biosynthesis
pathway. PAMP elicitor signals activate a receptor-coupled G-protein and the acti-
vated G-proteins further switch on calcium ion channels (Zhao and Sakai 2003 ).