ABA Regulation of Stomatal Movement - Abscisic Acid: Metabolism, Transport and Signaling - page 294

signaling, phosphatases ABI1 or ABI2 , inhibits OST1-activated SLAC1 currents
in Xenopus oocytes (Geiger et al. 2009 ; Lee et al. 2009 ). Ion selectivity analysis
showed that SLAC1 has a large permeability to NO 3 and Cl − , but its permeability

to malate is quite weak (Geiger et al. 2009 ). These results finally ensured the iden-
tity of SLAC1 as a plasma membrane S-type anion channel in guard cells, and also
uncovered a mechanism of how SLAC1 is regulated in ABA-induced stomatal clo-
sure. Further research found that Arabidopsis ost1 mutant guard cells showed par-
tially reduced S-type anion channel currents (Geiger et al. 2009 ), supporting that the
regulating mechanisms revealed in Xenopus oocytes function in guard cells. slac1
mutants are insensitive to ABA strongly, but ABA still induces a slight stomatal clo-
sure slowly (Negi et al. 2008 ; Vahisalu et al. 2008 ), suggesting that other plasma
membrane anion channels are functioning in guard cells. Further study found that a
homolog of SLAC1, SLAH3, involves in stomatal closure by mediating NO 3

−

−

efflux

(Geiger et al. 2011 ).
As described above, R-type anion channels are another type of anion channels
mediating anion efflux in guard cells, but the genetic identities are completely
unknown for decades. Recently, ALMT12, a member of aluminum-activated
malate transporter (ALMT) family, was found to be required for stomatal clo-
sure (Sasaki et al. 2010 ), and further analysis revealed that ALMT12 represents
a R-type anion channel in Arabidopsis guard cells (Meyer et al. 2010 ). ALMT12
locates in the plasma membrane of guard cells, and the stomatal closure of
atalmt12 mutant is partially insensitive to darkness, CO 2 , and ABA (Meyer et al.
2010 ). Patch clamping analysis showed a reduced R-type anion channel currents
in atalmt12 guard cell protoplast in the presence of external malate, and R-type
anion currents were also recorded in Xenopus oocytes expressing ALMT12
gene (Meyer et al. 2010 ). These studies ensured the identity of ALMT12 as a
R-type anion channel in Arabidopsis guard cells. ALMT9, another member of
ALMT family, involves in stomatal closure and was identified as a malate chan-
nel (Kovermann et al. 2007 ). But ALMT9 locates in the vacuole membrane, not
plasma membrane of guard cells and other types of plant cells (Kovermann et al.
2007 ). Whether more members of ALMT family function in guard cells as plasma
membrane anion channels needs further analysis.
SLAC1 and SLAH3 are mainly permeable to Cl − and nitrate, not malate;
ALMT9 is mainly permeable to malate, not Cl − ; and ALMT12 is permeable to
Cl − and can be activated by external malate. These data suggest that differ-
ent anion channels, including SLAC1, SLAH3, ALMT12, and ALMT9, function
cooperatively to drive anion efflux and stomatal closure in response to ABA.

15.6.4 Channels in the Intracellular Membranes

Ion channels in the plasma membrane of guard cells have been extensively ana-
lyzed. However, ion channels are also present in intracellular biological mem-
brane, especially in vacuole membrane. Vacuole is the main intracellular organ