Chapter 3 : System of Subsystems
3.6 Summary: A System with Subsystems
Guard cell signalling is a complex system that is coherently organized to control the stomatal aperture. These are signal perception, sphingolipid signalling, inositol signalling, ion
channels, ion channel regulatory proteins, Ca2+ signalling and NO signalling. This system
comprises an interconnected set of subsystems made of proteins, lipids, small molecules and various other conditions of the guard cells. The interconnections among the subsystems facilitate the communication flow of the whole system to respond to the signals by means of a collection of feedback processes and their relative dominance in time and space.
There are seven self-organized functional subsystems linked together to carry the ABA signal to close stomata. In systems thinking, this tight binding by feedback loops, which function differently in time and space with a defined hierarchy, provides the ABA network with better resilience to function in a variable environment with minimum time delay.
Ion channel signalling and Ca2+ signalling, being hubs in the system, are tightly connected
with many of the other subsystems. Of these two, the largest number of positive and/or negative
feedback loops connect Ca2+ signalling to the majority of subsystems, placing it as the core
reinforcement in the system.
Table 3.1: Regulatory mechanisms of ABA signalling network considered in the wiring diagram in Figure 3.5 (coloured according to modules in Figure 3.5)
Node
Regulation Target Source
Sphingolipid Signalling
SK ABA Not clear
SK PA Increase the specificity constant to promote substrate binding S1P SK Phosphorylation of Sphingosine to produce S1P
GPA1 S1P Addition of GTP permitting PLD release
PLD GPA1 Binding of GTP to GPA1 (Gα-GTP)dissociates GPA1 from PLD PLD NO May be S-nitrosylation of Cys residues or nitration of Tyr residues PLD CA Strengthen enzyme-substrate binding/targeting membrane localization of proteins
PA PLD Facilitates the hydrolysis of P-choline to produce PA
RCN1 PA Not clear
Signal Perception via ABA
PYR ABA Physical binding (conformational changes)
CuAO ABA Induce biosynthesis
PI4P ABA Phosphoinositide turnover PI4P PA Phosphoinositide turnover PI3P ABA Phosphoinositide turnover
PIP2 PI4P Phosphorylation
Inositol Signalling PLC ABA Possibly via phosphorylation
PLC CA Ca2+ driven activity and membrane targeting
InSP3 PLC
PLC hydrolyse PIP2 to produce InSP3
InSP3 PIP2
InSP6 InSP3 Phosphorylation by respective kinases InSP6 MRP5 Transport InSP6 to the vacuole
Osmoregulatory Enzymes/molecules
pH SnRK2 Not clear
PP2C PYR Physical binding (conformational changes)
PP2C PA Decreases the phosphatase activity and/or membrane-tethering role
PP2C ROS Oxidization
PP2C GPX3 Possibly by oxidation
PP2C pH Regulating phosphatase activity
PP2C ROP11 Physically binds to protect from Receptor inhibition
PP2C ABH1 Not clear
ROP11 ABA Not clear
ROP11 ERA1 Changing localization
SnRK2 PP2C Dephosphorylation
MAPK ROS May be through post-translational modification of redox sensitive proteins (cysteine oxidation)
MAPK CDPK Not Known
MAPK CA Not Known
MAPK PP2C Dephosphorylation
CDPK CA Ca
2+ binding to the N-terminal lobe
separates auto inhibitory domain from the active site
CDPK PP2C Dephosphorylation
RbOH SnRK2 Phosphorylation
RbOH CDPK Phosphorylation
RbOH CA Cabinding to ROS production is not clear 2+ binds to EF-hand motif of RbOH but the significance of this
RbOH RCN1 Not clear
RbOH PA Physically binds and stimulates
RbOH PI4P Not clear
RbOH PI3P Not clear
RbOH pH Not clear
ROS CuAO Catabolism of CuAO (oxidation) ROS RBOH O2 RBOH �⎯⎯⎯⎯⎯⎯⎯�O2-(ROS)
ROS GPX3 H2 O2 GPX3 �⎯⎯⎯⎯⎯⎯⎯⎯� H2O
GPX3 ROS High levels of system ROS lead the activation of the ROS scavenging NO Signalling
NO ROS Not clear
NO CaM Ca2+- bound CaM interact NOS like Enzyme to produce NO
CADPR CGMP
β-NAD+ ADP-ribosyl cyclase�⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯�cADPR
Activate ADP-ribosyl cyclase via G-kinase to stimulate cADPR synthesis
CGMP NO Activates soluble guanylyl cyclase resulting cGMP by nitrosylation
Ion Channels SLAC1 SnRK2 Phosphorylation
SLAC1 CDPK Phosphorylation
SLAC1 PP2C Dephosphorylation
SLAC1 MAPK Not clear
SLAC1 ERA1 Protein farnesylation
SLAC1 MRP5 Not clear
SLAC1 Malate Increasing current noise
TPC1 CA Ca2+ induced Ca2+ release by binding to EF-hands TPC1 DEPOLAR Voltage gated regulation
GORK NO Nytrosylation
GORK pH Membrane delimited pathway
GORK ROS Post-translational modification increasing current intensity GORK DEPOLAR Setting active potential for channel opening
QUAC SnRK2 Phosphorylation
QUAC DEPOLAR Voltage gated regulation
ATALMT6 CA Ca2+-dependent current activation
KAT1 CDPK Phosphorylation
KAT1 SnRK2 Phosphorylation
QUAC Malate Malate is a substrate for R-type ion channels
TPK1 CA Conformational changes induced by binding into EF-hands
TPK1 pH Not clear
Malate PEPC
Catalyses the b-carboxylation of PEP to yield oxaloacetate (OAA) and inorganic phosphate (the branch-point step in the malate- accumulation pathway)
PEP(C3) → OAA(C4) → Malate
Malate QUAC Transport malate Malate ATALMT6 Transport malate
AHA1 PP1 Through phosphorylation, displace C-terminal auto inhibitory domain AHA1 CA Post-translational modulation
AHA1 pH Not clear
Calcium Signalling CA Icas Influx of Ca2+ to cytosol CA CADPR Influx of Ca2+ to cytosol CA Ica Influx of Ca2+ to cytosol CA InSP6 Influx of Ca2+ to cytosol
CA Ca2+-ATPase Efflux of Ca2+ from cytosol
CA CAX1 Efflux of Ca2+ from cytosol
CaATPase CaM Suppress auto-inhibitory action by binding into N-terminus CaM CA Ca2+ binding enhances catalytic activity
Ica ABH1 Not known
Ica ERA1 May be through farnesylation
Ica MRP5 Not Known
Ica ROS ROS modify Ica channel proteins through directly and/or through additional intermediate proteins Ica DEPOLAR Changes in electrical potential
CBL CA Ca2+ binding for activation CAX1 CIPK May mask auto-inhibitory domain
SCAB1 ABA Not clear
SCAB1 InSP6 Not clear
Icas ACTIN Stretch activation
CIPK CBL Mask auto-inhibitory domain and facilitate localization CIPK PP2C Replace CBL protein to enhance auto inhibition PP1 PA Inhibits the phosphatase activity
ABPS CA Capping and depolymerization
ABPS PI3P Inactivate actin stabilization and facilitate depolymerization
ABPS PI4P
ACTIN ARP23 Actin nucleation
ACTIN SCAB1 Stabilizes actin filaments
ACTIN ATRAC1 Not clear
ACTIN ROS Depolymerize actin filaments via bonds weakening of inter-monomer ACTIN CA Facilitate depolymerization
ACTIN ABPS Stabilize actin filaments
PEPC Malate Binding to inhibitory site (feedback inhibitor-binding site)
ATRAC1 PP2C Not clear
ARP23 PIP2 PIP2 concentration may induce a ‘switch’ for N-WASP mediated Arp2/3 actin polymerization
ARP23 ABA Not clear
DEPOLAR CA Accumulation of positive ions makes membrane more positive DEPOLAR TPK1 Addition of K+ to cytosol from the vacuole
DEPOLAR AHA1 Facilitate membrane hyperpolarization
DEPOLAR SLAC1 Removal of negative charges makes membrane more positive
DEPOLAR QUAC Removal of negative charges makes membrane more positive CLOSURE GORK Removal of K+ (osmotic regulation)
CLOSURE ACTIN Rearrangement of cytoskeleton
CLOSURE Malate Removal of malate (osmotic regulation) CLOSURE SLAC1 Removal of Cl- (osmotic regulation)