Calcium Ion Signaling System: Calcium Signatures and Sensors - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 243

CaM-like
domain with 4
EF hands

Amino-terminal
variable domain

Catalytic domain

Short
domain
of
variable
length

Auto-inhibitory domain
with binding sites for
CaM-like domain

Fig. 4.7 Structure of calcium-dependent protein kinase

results in kinase activation (Harmon et al. 2000 ). These results suggest Ca 2+ binding
to CDPK is essential for the activation of the enzyme.
PAMP/elicitor may be involved in enhanced transcription of CDPK genes, prob-
ably through the action of calcium signature. The tobacco CDPK gene NtCDPK1
was found to be induced by fungal elicitors (Yoon et al. 1999 ). Two other CDPK
genes from tobacco, NtCDPK2 and NtCDPK3 , showed mRNA up-regulation after
the application of the Avr9 race-specifi c elicitor (Romeis et al. 2001 ). Fungal elici-
tors triggered the transcription of the tomato LeCDPK1 gene (Chico et al. 2002 ). A
CDPK gene in maize, ZmCPK10 , was induced both during a fungal infection and
after treatment with fungal elicitors (Murillo et al. 2001 ). Activation of the ZmCPK10
gene was very rapid. ZmCPK10 transcripts could be detected 5 min after elicitation
and reached maximum levels at 30 min after treatment (Murillo et al. 2001 ).
Several CDPKs have been reported in plants. Arabidopsis thaliana has 34 genes
that encode CDPKs (Hrabak et al. 2003 ). Several CDPK genes have been detected
in soybean, rice, tomato, maize, and Arabidopsis (Harmon et al. 2001 ). Individual
isoforms of CDPKs may have different functions and participate in multiple distinct
signaling pathways (Harmon et al. 2001 ). Depending on the calcium signatures
(variations in [Ca 2+ ] cyt concentration, oscillations, and waves), specifi c CDPK isoforms
are activated (Ludwig et al. 2004 ).
The Nicotiana benthamiana CDPK, NtCDPK2, induced enhanced ROS produc-
tion and activation of defense-related genes in N. benthamiana . By contrast, a
homologous isoform, NtCDPK3, did not induce the HR-associated cell death. The
results suggest that NtCDPK2 kinase, but not the closely related NtCDPK3 protein,
is specifi cally involved in the defense response (Ludwig et al. 2004 ). Dependent on
the incoming signals, NtCDPK2 enzyme activation varied in strength and duration
(Romeis et al. 2001 ). A short and weak NtCDPK2 activation results in the induction
of the wound signaling pathway, whereas a much stronger and sustained elicitation
leads to defense response signaling system (Romeis et al. 2001 ). Several CDPK
genes have been detected in maize. However, only one specifi c CDPK gene,
ZmCPK10 gene is transcriptionally activated in response to both fungal infection
and treatment with fungal elicitors/PAMPs. This gene was found to be involved in
activation of defense signaling pathways, leading to the induction of PR genes
(Murillo et al. 2001 ).