Calcium Signalling in Plants

Peter D Bickerton, University of Manchester, Manchester, UK
Jon K Pittman, University of Manchester, Manchester, UK

Published online: May 2012

DOI: 10.1002/9780470015902.a0023722

Full Article on Wiley Online Library

Ca²⁺ is an important signal transduction molecule that has been shown to regulate responses to a large number of environmental stimuli in plants and control many developmental processes. These stimuli induce the formation of Ca²⁺ signals within a cell, which are generated through the action of Ca²⁺ release and uptake from and into internal cellular stores or the apoplast by the activity of Ca²⁺ channels, pumps and exchangers. These signals take the form of elevations of Ca²⁺ with specific spatio‐temporal characteristics which are thought to denote the initial stimulus and mediate an appropriate cellular response. Information is therefore hypothesised to be encoded in these signals, which are decoded and relayed to downstream gene expression regulators and protein kinases via an array of Ca²⁺‐binding sensor proteins.

Key Concepts:

Calcium signals allow plants to specifically sense and respond to environmental stimuli.
Calcium signals are generated through the coordinated action of calcium influx channels and calcium efflux transporters.
A large network of calcium‐binding proteins act as calcium sensors and relay calcium signals to downstream effector proteins.

Keywords: Arabidopsis; calcium; calcium transport; calmodulin; protein kinases; signal transduction; transcription factors

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Article Title:	Calcium Signalling in Plants
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	Figure 1. Schematic highlighting the key steps in Ca²⁺ signalling pathways. Stimuli such as pathogen elicitors, ABA, H₂O₂ and cold, show specific cytosolic Ca²⁺ signatures after influx of Ca²⁺. Ca²⁺ binds to sensors such as calmodulin (CaM), CaM‐like proteins (CML), calcium‐dependent protein kinases (CDPK), and calcineurin B‐like proteins (CBL) (which forms a complex with Ca²⁺‐independent protein kinase (CIPK)). Ca²⁺ sensors activate transcription factors (TF) and other proteins either by direct binding or through phosphorylation (P). TFs bind DNA and cause up‐ or down‐regulation of gene expression. CAMTA, CaM‐binding transcription activator; CBP60, CaM‐binding protein; TGA, basic leucine zipper (bZIP) transcription factor; MYB, plant homologue of animal myeloblastosis TFs; WRKY, TF with conserved domain containing these amino acids.
	Figure 2. The distribution of Ca²⁺ transporters within a typical plant cell. Plants possess Ca²⁺‐permeable influx channels (blue rectangles), as well as vacuolar Ca²⁺/H⁺ exchangers (green cylinder) and Ca²⁺ pumps (red ovals) for extruding Ca²⁺. All named proteins are from Arabidopsis thaliana. ACA, autoinhibited Ca²⁺‐ATPase; CAX, Ca²⁺/H⁺ exchanger; CNGC, cyclic nucleotide gated channel; DACC, depolarisation‐activated Ca²⁺ channel; ECA, ER‐type Ca²⁺‐ATPase; GLR, glutamate receptor‐like channel; HACC, hyperpolarisation‐activated Ca²⁺ channel; IP₃R‐like, inositol‐1,4,5‐trisphosphate receptor‐like channel; MCC, mechanosensitive Ca²⁺ channel; NAADPR, nicotinic acid adenine dinucleotide phosphate receptor channel; RyR‐like, cyclic ADP‐ribose‐activated ryanodine receptor‐like channel; TPC, two‐pore channel, a slow‐activating vacuolar (SV) Ca²⁺ channel; VVCC, vacuolar voltage‐gated Ca²⁺ channel. Courtesy of Dr. James Connorton.
	Figure 3. Putative pathways for pathogen‐mediated, Ca²⁺‐dependent activation of plant defence responses. Binding of an elicitor (AtPep) to a receptor (AtPepR1) activates adenylyl cyclase (AC) and/or guanylyl cyclase (GC), which generate cAMP or cGMP, respectively. Increase in cAMP and cGMP concentration activates cyclic nucleotide gated channels (CNGC), which causes influx of Ca²⁺ into the cytosol. Ca²⁺ binding to calmodulin (CaM) and calcium‐dependent protein kinases (CDPK) indirectly activates the hypersensitive response (HR), defence‐related gene expression and increased salicylic acid (SA) production. CaM activates NO synthase, producing NO which acts in conjunction with H₂O₂ generated from CDPK‐activated NADPH oxidase in order to stimulate HR and gene expression. CaM binding to transcription factors (TF) (e.g. CAMTA, WRKY and CBP60) activates gene expression, while CDPK may phosphorylate TFs which can then bind to DNA (dashed arrows). Red line depicts inhibition of CNGC activity by Ca²⁺/CaM.

Calcium Signalling in Plants

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