ABA Regulation of Stomatal Movement - Abscisic Acid: Metabolism, Transport and Signaling - page 290

15.6.1 Plasma Membrane Ca 2 + Channels
Ca 2 + imaging and electrophysiological techniques provide powerful tools for the
analysis of Ca 2 + ion channels in guard cells. The application of external ABA
and Ca 2 + can trigger cytosolic Ca 2 + increases and oscillation, which can be
monitored using calcium sensor yellow cameleon expressing in vivo (Allen et al.
2000 , 2001 ). Hyperpolarization voltage-dependent inward Ca 2 + currents have
been recorded in guard cell protoplasts, and ABA can activate the Ca 2 + channel
currents by shifting activating voltage to more positive potential (Hamilton et al.
2000 ; Pei et al. 2000 ), suggesting the presence of ABA-activated inward Ca 2 +
channels in the plasma membrane of Arabidopsis guard cells. Further research
found that ABA can induce the production of ROS, which consequently activate
the hyperpolarization-activated Ca 2 + channels in Arabdiopsis guard cells (Pei
et al. 2000 ). Two NADPH oxidases AtrBOHD and AtrBOHF were identified as
the essential players for ROS production in Arabidopsis guard cells by transferring
electrons from NADPH to electron receptors (Kwak et al. 2003 ). The disruption of
the two protein encoding genes impaired ABA-induced ROS production, cytosolic
Ca 2 + increases and hyperpolarization-activated inward Ca 2 + currents (Kwak et al.
2003 ), suggesting that ROS are important mediators for ABA signaling as Ca 2 +
channel activators in guard cells. In addition, glutathione peroxidase 3 can regulate
ROS homeostasis in guard cells and consequently affect the activity of Ca 2 + chan-
nels, and the mutation of this gene in Arabidopsis impaired the ABA activation of
hyperpolarization-activated Ca 2 + channel currents (Miao et al. 2006 ). Some cal-
cium/calmodulin-dependent protein kinases (CDPKs) (Mori et al. 2006 ; Zhu et al.
2007 ), and phosphatases (Miao et al. 2006 ; Kohler and Blatt 2002 ) are important
ABA signaling regulators in guard cells as described above. The mutations of
CPK3 and CPK6 impaired the ABA- and hyperpolarization-activated inward Ca 2 +
channel currents in Arabidopsis guard cells (Mori et al. 2006 ), and the application
of phosphatase antagonist okadaic acid (OA) and calyculin A (CA) increased the
open probability of the Ca 2 + channels in the excised plasma membrane patches
of Vicia faba guard cell protoplast (Kohler and Blatt 2002 ). Despite the numer-
ous analysis on the Ca 2 + channel activity and the importance of cytosolic Ca 2 + as
a second messenger for ABA signaling, the identities of plasma membrane Ca 2 +
channels activated by ABA in guard cells are still unknown. How the regulating
factors involve in the activation of ABA/ROS-activated inward Ca 2 + channels in
plant guard cells remains to be addressed.
The most interesting candidates for the plasma membrane Ca 2 + channels in
plant guard cells are mainly from three channel families, including CNGCs, glu-
tamate receptors (GLR), and annexins (Ma 2011 ; Ward et al. 2009 ). It has been
reported that cGMP involves in ABA-induced stomatal closure and functions
downstream of ROS to activate cytosolic Ca 2 + increases in Arabidopsis guard
cells (Dubovskaya et al. 2011 ), suggesting a role of CNGCs as Ca 2 + chan-
nels. Few CNGCs were identified as cyclic nucleotide-activated cation channels
in Arabidopsis in the past few years, including CNGC2, CNGC5, CNGC6, and