Biology Reference
In-Depth Information
Chapter 4
Calcium Ion Signaling System:
Calcium Signatures and Sensors
Abstract
Plant innate immune system is a potential surveillance system against
possible attack by pathogens. The invading pathogen's signature (pathogen associ-
ated molecular pattern [PAMP]) is perceived by pattern recognition receptors
(PRRs) of host plants. The informations generated by the PAMP alarm signals
activate the plant innate immune system. The plant immune system uses several
second messengers to encode information generated by the PAMPs and deliver the
information downstream of PRRs to proteins which decode and interpret the signals
and initiate defense gene expression. Calcium ion is an important intracellular
second messenger and carries the PAMP signal downstream to initiate immune
responses. Transient changes in permeability of the plasma membrane to Ca
2+
and
infl ux of extracellular Ca
2+
through various plasma membrane-resident ion channels
are the earliest events in defense signaling system. Different PAMPs trigger Ca
2+
infl ux through activation/'opening' of specifi cally different ion channels. Massive
infl ux of Ca
2+
occurs within a few minutes after PAMP treatment. The PAMP-
activated calcium signaling is modulated by calcium signatures. Ca
2+
signatures
(single calcium transients, oscillations, or waves) are generated in the cytosol, and
in noncytosolic locations including the nucleus and chloroplast through the coordi-
nated action of Ca
2+
infl ux and effl ux pathways. PAMP signals induce an oscillation
in the cytosolic free Ca
2+
concentration and the information encoded in the induced
transient Ca
2+
changes is decoded by an array of Ca
2+
-binding proteins that serve as
Ca
2+
sensors. The Ca
2+
sensors identifi ed in plants include calmodulins (CaMs),
CaM-like and CaM-related proteins, calcineurin B-like (CBL) proteins, Ca
2+
-
dependent protein kinases (CDPKs), and Ca
2+
-binding proteins without EF hands.
Specifi c calcium signatures are recognized by different calcium sensors to trans-
duce calcium-mediated signals into downstream events. The information encoded
by specifi c calcium signatures is decoded by an array of sensors and the extracel-
lular signals are transmitted to cellular calcium-dependent effectors. Thus the extra-
cellular PAMP signals are transmitted to the calcium-dependent effectors through
calcium sensors resulting in activation of the complex immune signaling network.
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