Agriculture Reference
In-Depth Information
CNGC18 (Ali et al.
2007
; Wang et al.
2013
; Gao et al.
2014
). CNGC2 is mainly
permeable to monovalent cation except Na
+
, and functions mainly in innate
immunity responses (Ali et al.
2007
; Leng et al.
2002
). CNGC5 and CNGC6 are
mainly permeable to divalent cation, including Ca
2
+
(Wang et al.
2013
). However,
the disruption of
CNGC5
and
CNGC6
, two genes highly expressed in
Arabidopsis
guard cells, does not obviously inhibit the activity of ABA-activated Ca
2
+
chan-
nels (Wang et al.
2013
). For GLRs, it has been reported that the overexpression
of
Arabidopsis
GLR3.1
led to the impairment of external Ca
2
+
-induced stomatal
closure and imposed-Ca
2
+
oscillation-programmed long-term stomatal closure
without affecting cytosolic Ca
2
+
kinetics (Cho et al.
2009
). However, external
Ca
2
+
-induced stomatal closure differs from ABA-induced stomatal closure,
because the disruption of Ca
2
+
sensor (CAS) blocks external Ca
2
+
-induced sto-
matal closure without affecting ABA responses in
Arabidopsis
guard cells (Han
et al.
2003
). Therefore, GLR3.1 and the three CNGCs described above seem not to
be essential for ABA signaling and may not be ABA-activated hyperpolarization-
activated inward Ca
2
+
channels. Annexins are ubiquitous soluble proteins, some of
them can behave as Ca
2
+
channels, and potentially involve in cytosolic Ca
2
+
and
ROS signaling in plant kingdom (Mortimer et al.
2008
). But annexins are less ana-
lyzed compared to CNGC and GLR in plants. Annexin 1 involves in drought-stress
responses in
Arabidopsis
(Konopka-Postupolska et al.
2009
), and the expression of
annexins in
Mimosa
pudica
is sensitive to ABA (Hoshino et al.
2004
). The iden-
tity of annexins as plasma membrane Ca
2
+
channels in guard cells has not been
reported yet, and more work is needed to characterize the functions of annexins.
Together, the journey for seeking ABA-activated Ca
2
+
channels located in the
plasma membrane of guard cells as well as the associated encoding genes just
began, and we may have a long way to go.
There are two sources for cytosolic Ca
2
+
increases and oscillation: the influx
of external Ca
2
+
through plasma membrane Ca
2
+
channels and Ca
2
+
release from
intracelullar Ca
2
+
stores, such as endocytoplasmic reticulum and vacuole in plant
cells. ABA can trigger cytosolic Ca
2
+
increase in an InsP6-dependent manner in
guard cells (Lemtiri-Chlieh et al.
2003
; Lemtiri-Chlieh and Berkowitz
2004
), indi-
cating the involvement of intracellular Ca
2
+
release in ABA-induced stomata clo-
sure. Several types of ion channels, including TPC1 (two pore channel 1) and
chloride channel (CLC) family, were identified as vacuole membrane channels, and
some of them involve in intracellular Ca
2
+
release in
Arabidopsis
guard cells (De
Angeli et al.
2006
; Peiter et al.
2005
). The vacuole membrane-located ion channels
will be discussed in “Channels in the intracellular membrane” section below.
+
15.6.2 Plasma Membrane K
Channels
Several channel families being permeable to K
+
were found in plants, especially
in
Arabidopsis
, including HKT (high-affinity K
+
transporter), KUP/HAK/KTs,
CNGCs (cyclic nucleotide-gated channels), and Kir1- and shaker-type K
+
channel
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