ABA Regulation of Stomatal Movement - Abscisic Acid: Metabolism, Transport and Signaling - page 296

proposed that FV channels function as K + channel mediating K + release from
vacuole to cytoplasm, and the luminal K + in vacuole regulates the activity of
FV channels in a feedback manner. The more K + is released from vacuole, the
lower the FV channel activity is (Pottosin and Martínez-Est￩vez 2003 ). So far, the
genetic identities of FV channels are still unknown, and further study is needed to
find out the genetic identity and whether FV involves in ABA signaling in guard
cells.
Malate and nitrate are the most abundant anions mainly stored in vacuole in
plant cells, and both anions involve in stomatal movement as osmotic solutes.
Therefore, anion channels or transporters must exist in the vacuole membrane of
guard cells. In tonoplast, anion channels are mainly from two channel families:
CLC and ALMT. For ALMT family, ALMT9 was identified as a malate-activated
chloride-permeable channel located in the tonoplast of guard cells in Arabidopsis
(Kovermann et al. 2007 ; De Angeli et al. 2013 ), and the disruption of ALMT9
impairs stomata opening without affecting ABA- and dark-induced stomatal clo-
sure (De Angeli et al. 2013 ). CLC family has seven members in Arabidopsis . Four
CLC members localize in vacuole, two CLC members localize in Golgi, and one
localizes in chloroplast. The Arabidopsis vacuole CLC channels include CLCa,
CLCb, CLCc, and CLCg. AtCLCc is a real channel being permeable to nitrate,
chloride, and citrate and involves in stomatal movement (Harada et al. 2004 ). The
disruption of AtCLCc gene inhibits vacuole chloride currents and impairs light-
induced stomatal opening and ABA-induced stomatal closure (Jossier et al. 2010 ).
AtCLCa is the most well-analyzed CLC member. The disruption of AtCLCa
leads to 50 % reduction of nitrate in root and shoot (Geelen et al. 2000 ). Further
research confirmed that AtCLCa functions as a tonoplast NO 3 /H + exchanger, not

a channel (De Angeli et al. 2006 ). Whether CLCa, CLCb, and CLCg involve in
stomatal movement needs further investigation.
Vacuole is important for ABA-induced stomatal closure, but not all vacuole
channels are related to ABA signaling cascade because the disruption of some
vacuole channel/transporter genes does not impair ABA-induced stomatal closure.
Nevertheless, it is still reasonable to conclude that vacuole channels play critical
roles in guard cells.

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15.7 Conclusion

In the past few decades, fantastic research identified many important components
for ABA signaling in guard cells and finally revealed the simple but relatively
complete ABA signaling chain in guard cells. Briefly, the ABA signaling chain for
ABA-induced stomatal closure can be summarized as the following: First, ABA
binds to ABA receptors to trigger the protein interaction between ABA recep-
tors and PP2Cs to inhibit the phosphatase activity of PP2Cs; second, the inhibi-
tion of PP2Cs by ABA receptors releases SnRKs and CPKs from the inhibition
by PP2Cs, and the kinases are then autophosphorylated and auto-activated; third,