Agriculture Reference
In-Depth Information
cGMP and cAMP are well known as important second messengers in mamma-
lian cells for several decades. Cyclic nucleotide-gated channels (CNGCs) were pro-
posed as the downstream targets of cGMP and cAMP because CNGCs contain a
nucleotide-binding domain and a calmodulin-binding domain, and can be activated
by cyclic nucleotides by direct binding in mammalian cells. cGMP was reported
functioning in guard cells downstream of ROS for the activation of Ca
2
+
influx in
Arabidopsis
(Dubovskaya et al.
2011
). But whether cyclic nucleotides involve in
ABA signaling transduction in guard cells is still under debate because guanylate
cyclase and adenylate cyclase could not be identified in plants so far. Recently,
8-nitro-cGMP was reported as a signaling molecule and involves in ABA signaling
in
Arabidopsis
guard cells (Joudoi et al.
2013
). The ABA- and NO-induced produc-
tion of 8-nitro-cGMP was detected in guard cells in the presence of ROS (Joudoi
et al.
2013
). 8-nitro-cGMP can induce stomatal closure alone, but 3′,5′-cyclic
monophosphate (8-bromo-cGMP) did not (Joudoi et al.
2013
). CNGCs are the most
possible downstream targets of cyclic nucleotides in plants. CNGC homologs in
plants were identified many years ago, and the model plant
Arabidopsis
has a large
CNGC family which is composed of 20 members. The functions of cyclic nucleo-
tides and CNGCs will be reviewed below in Ca
2
+
channel section because CNGCs
are proposed as Ca
2
+
channel candidates in plant cells.
15.6 Channels in the Membrane Systems of Guard
Cells Function as a Motor as Well as Regulators
for ABA-induced Stomatal Closure
Stomatal movement is driven by the changes of osmotic turgor pressure in guard
cells. The changes of turgor pressure result from ion flux through plasma mem-
brane and vacuole membrane of guard cells and consequent water flow. Different
types of ion channels are the main functional proteins mediating ion flux across
the biological membranes in guard cells. ABA is a main stimulating factor induc-
ing stomatal closure and preventing stomatal opening in response to drought stress
through a signaling network, in which ion channels are the key components. ABA
first induces cytosolic free Ca
2
+
increases and oscillation mainly by activating
hyperpolarization-activated Ca
2
+
channels in the plasma membrane of guard cells
(Pei et al.
2000
; Allen et al.
2000
), and partially by triggering Ca
2
+
release from
intracellular Ca
2
+
stores (Tang et al.
2007
). Cytosolic Ca
2
+
functions as an essen-
tial second messenger to mediate ABA signaling to further stimulate downstream
components, including outward anion channels and potassium channels. Anion
channels and outward-rectifying K
+
channels mediate osmotic ion efflux which
consequently leads to the water loss and stomatal closure. Therefore, ion channels
play essential roles for ABA-induced stomatal closure and ABA-related inhibition
of stomatal opening. In the past decades, different channels and associated genes
were identified, and the channel properties were thoroughly analyzed using elec-
trophysiological and other techniques.
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