Chemistry Reference
In-Depth Information
3.1 Phospholipid Signal
Phospholipids provide the structural basis of the cell membrane, but they can also
be cell regulators (Xue and Chen
2009
). The hydrolysis of phospholipids by
phospholipases is often the first step in the generation of messengers. Additionally,
the phospholipid transduction pathway involves the generation of secondary
messengers from the hydrolysis or changes in the phosphorylation state of phos-
pholipids present in the plasma membrane (Xue and Chen
2009
). The hydrolysis of
phosphatidylinositol 4,5-bisphosphate (PIP
2
) in the cell membrane by the action of
phospholipase C (PLC) results in the production of two important secondary
messengers: the cytosolic inositol 1,4,5 trisphosphate (IP
3
) and diacylglycerol
(DAG), which remains associated with the plasma membrane (Fig.
2
) (Xue and
Chen
2009
). The compound IP
3
is soluble in water therefore diffuses quickly
throughout the cytosol, where it can be phosphorylated by inositide kinases to
form inositol hexaphosphate (IP
6
). The main function of IP
3
/IP
6
is to mobilize
Ca
2+
from the intracellular stores. As a result of this mobilization, a transient
increase in cytosolic Ca
2+
occurs that can mediate a cellular response indirectly
through binding to calmodulin as well as other calcium-binding proteins, and
activation or directly through binding to specific enzymes, such as calcium-
dependent kinases (CDPK) or other proteins that bind Ca
2+
. DAG, in turn, is an
insoluble lipid and remains in the membrane (Gross et al.
1992
) and can activate a
class of proteins called protein kinase C (PKC) (Legendre et al.
1993
). However,
in plants, PKC has not been identified with any member of this family of kinases.
There is evidence that DAG quickly becomes phosphorylated by DAG kinase
(DGK) to produce phosphatidic acid (PA) (Munnik and Testerink
2008
). Thus, two
signaling pathways are initiated by PLC: the elevation of cytosolic Ca
2+
, which
results in the modulation of response elements sensitive to Ca
2+
, and the effect of
PA and other lipids on the activity, relocation or binding of different proteins (Xue
and Chen
2009
; Munnik and Testerink
2008
). It should be noted that PA could also
be formed by the hydrolysis of membrane lipids, mainly phosphatidylcholine (PC),
by the action of phospholipase D (PLD) (Fig.
2
) (Munnik and Testerink
2008
).
Studies of physiological function involving the enzyme PLD have revealed that
it can be rapidly activated. The stimuli that regulate PLD include hormones, growth
factors, cytokinins, extracellular matrix components, antigens and stimuli, such as
freezing, wounds, plant-pathogen interactions, dehydration and salt stress (Wang
2002
; Bargmann and Munnik
2006
). The secondary messengers generated from
these stimuli are important in the plant cell responses to environmental changes or
extreme conditions (Xue and Chen
2009
). The activation of PLC and PLD phos-
pholipases has been linked to several signaling processes in plants, such as hor-
monal and stress responses (Chapman
1998
; Munnik et al.
1998
; Wang
2001
).
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