Biology Reference
In-Depth Information
Although membrane depolarization-activated Ca
2+
-permeable channels are
common in plants, the presence of hyperpolarization-activated Ca
2+
-permeable
channels has also been reported in plasma membrane of certain plant cells including
tomato (Gelli and Blumwald
1997
) and
Arabidopsis
(Pei et al.
2000
) cells.
Hyperpolarization-activated cation channels are activated at voltages more negative
than about −100 to −150 mV (White and Broadley
2003
). Reactive oxygen species
increase the activity of these channels through the activity of NADPH oxidase
(Pei et al.
2000
; Murata et al.
2001
; Foreman et al.
2003
). Abscisic acid (ABA)
activates a hyperpolarization-dependent Ca
2+
-permeable channel in the plasma
membrane of
Arabidopsis
guard cells, leading to Ca
2+
infl ux and to an increase in
[Ca
2+
]
cyt
(Hamilton et al.
2000
; Pei et al.
2000
). ABA has been shown to increase the
level of ROS in an NADPH-dependent manner and ROS is known to stimulate
hyperpolarization-activated Ca
2+
infl ux current in the plasma membrane termed I
Ca
(Pei et al.
2000
). Reactive oxygen species (ROS) has been shown to stimulate
hyperpolarization-activated Ca
2+
-permeable channels and cause a transient increase
in cytoplasmic calcium ([Ca
2+
]
cyt
) content in
Arabidopsis
(Pei et al.
2000
).
4.3.2
Cyclic Nucleotide-Gated Ion Channels (CNGCs)
Cyclic nucleotide-gated ion channels (CNGCs) have been found in plant cell plasma
membrane (Leng et al.
1999
; Maathuis and Sanders
2001
; Sanders et al.
2002
;
Kaplan et al.
2007
; Baxter et al.
2008
; Ma and Berkowitz
2011
; Qi et al.
2010
;
Abdel-Hamid et al.
2011
; Moeder et al.
2011
). Twenty genes encoding putative
CNGCs have been detected in
Arabidopsis
(Talke et al.
2003
). The
HLM1
gene
encodes a cyclic nucleotide-gated channel, CNGC4 in
Arabidopsis
(Balagué et al.
2003
).
AtCNGC11
and
AtCNGC12
genes encode two CNG ion channels involved
in host defense response (Yoshioka et al.
2006
).
DND1
gene encodes AtCNGC2
that allows passage of Ca
2+
, K
+
, and other cations (Clough et al.
2000
). A CNG ion
channel has been identifi ed in barley and it was homologous to the
Arabidopsis
HLM1. Ten other members of the barley CNG channel gene family have been
identifi ed (Rostoks et al.
2006
).
CNG ion channels are heterotetrameric in structure. Six membrane-spanning
domains fl anked by hydrophilic amino and carboxy terminus have been detected
in these proteins. A putative cyclic nucleotide binding domain was located in the
carboxy terminus (Zhong et al.
2003
). AtCNGC subunits share many similarities
with voltage-gated outward rectifying K
+
-selective ion channel proteins, includ-
ing a cytoplasmic N terminus, six membrane-spanning regions, a pore domain,
and a cytoplasmic C terminus. AtCNGCs are gated primarily by binding of
cAMP (cyclic adenosine monophosphate) or cGMP (cyclic guanosine-3
-
cyclic monophosphate) rather than by voltage (Yoshioka et al.
2006
). AtCNGC4
is activated by both cGMP and cAMP (Balagué et al.
2003
). Cyclic nucleotides
cAMP and cGMP have been shown to be linked with Ca
2+
signaling (Moutinho
et al.
2001
).
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