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Progressive stimulus-response mode
Progressive stimulus-response mode
Ethylene
Ethylene
C
Ca
ETR 2
PLD
P
PI kinase
PL
L
D
C
Ca
PA
P
L
D
MT
CTR1
FFOX
DG
H
X
MAPK
cascade
MAPK
FFA
ascade
P
L
D
C
Ca
X
Gene
expression
Nucleus
Metabolon
FFA
D
DG
C
Ca
PI kinase
P
PL
P
PA
PLD
C
Ca
ETR 1
Transient stimulus-
Transient stimulus-
response mode
Ethylene
response mode
Fig. 9.21
A model describing potential events involved in the ethylene signal transduction pathway indicating
the role of PLD. Ethylene binding to the receptor is proposed to activate a PI kinase that in turn results in the
generation of a voltage difference across the membrane. Voltage-sensitive calcium channels may open in response to
the voltage difference, increasing the cytosolic calcium level. PLD can bind to the membrane at the anionic domains
created by PI-kinase activity or in response to the increased cytosolic calcium levels. In the progressive stimulus-
response model, the hormonal stimulation continues ultimately inactivating calcium and proton pumps (H-X,
Ca-X). Excessive cytosolic calcium would ultimately cause senescence. In the transient stimulus-response model,
cytosolic calcium level is maintained at homeostatic levels. Lipid catabolites are recycled back to phospholipids.
ETR1 and ETR2 are designation of receptors with potentially distinct function. CTR-1 is a protein kinase that acts
downstream from the receptor. PLD could exist as a metabolon (left bottom) in combination with phosphatidate
phosphatase, calmodulin, and lipolytic acyl hydrolase, or even bound to microtubules (MT) (DG, diacylglycerol;
FFA, free fatty acid; FFOX, fatty acid oxidation products; PA, phosphatidic acid; PL, phospholipid).
is promoted by calcium. Under normal conditions of growth and development, this switch
would be tightly regulated (transient stimulus-response model) due to cross talk with other
signals (Lohrmann and Harter, 2002). After the initiation of a programmed senescence event
(progressive stimulus-response model), such tight regulation could be lost and would even-
tually lead to high cytosolic calcium and lowered cytosolic pH, as the calcium ATPase and
proton ATPase are inhibited, respectively (Paliyath and Thompson, 1988; Paliyath et al.,
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