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anion channel in stomatal signaling (see earlier description in CDPK and SnRK
sections). This study shows that GHR1 is an important player in ABA and H
2
O
2
signaling for stomatal movement.
Tanaka et al. (
2012
) identified an
Arabidopsis
receptor-like kinase, ARCK1,
as a negative ABA signaling regulator during the postgermination growth.
ARCK1 is a receptor-like cytosolic protein kinase, which belongs to the
cysteine-rich repeat RLK (CRK) subfamily. The extracellular region of the cyto-
solic CRK protein contains two copies of the DUF26 domain containing four
conserved cysteines, three of which form the motif C-8X-C-2X-C, which is pre-
sumed to be involved in formation of the 3D structure of the protein through
disulfide bonds and plays roles in protein-protein interactions. Another member
of the CRK subfamily, CRK36, interacts with ARCK1 in the plasma membrane,
phosphorylates ARCK1 in vitro, and functions as a negative ABA signaling
regulator during the postgermination growth (Tanaka et al.
2012
). These results
suggest that CRK36 and ARCK1 form a functional complex in the plasma mem-
brane to regulate ABA signaling during the postgermination growth (Tanaka
et al.
2012
).
Sheng Luan's group (Yu et al.
2012
) identified a receptor-like kinase,
FERONIA (FER), a positive regulator of auxin-promoted growth (Duan et al.
2010
), as a negative regulator of ABA signaling. They described a cross talk
mechanism between ABA- and auxin-signaling pathways where FER suppress
ABA response through activation of ABI2, a negative regulator of ABA sign-
aling. The plasma membrane-localized FER kinase interacts with guanine
exchange factors GEFs including GEF1, GEF4, and GEF10, which, in turn, acti-
vate GTPase ROP11. The ROP11 protein interacts with the A-type PP2C ABI2
phosphatase and enhances its activity, thereby linking the FER pathway with the
inhibition of ABA signaling (for the cytosolic ABA receptors PYR/PYL/RCAR
and A-type PP2C-mediated ABA signaling cascades, see Cutler et al.
2010
, and
Chap.
6
of this topic). Interestingly, at the same time, an independent group
reports that ROP11 GTPase negatively regulates ABA signaling by protecting
ABI1 phosphatase activity from inhibition by the ABA receptor RCAR1/PYL9
in
Arabidopsis
(Li et al.
2012a
,
b
). Thus, this FER-GEFs-ROP11-PP2C linked
pathway may function in parallel with the PYR/PYL/RCAR-PP2C-coupled
pathway (Fig.
8.2
); however, it remains unknown whether FER may function as
a cell surface receptor for ABA.
Although candidate plasma membrane receptors for ABA, such as GPCR-
type G proteins GTG1/GTG2 (Pandey et al.
2009
), and cytosolic ABA receptors
PYR/PYL/RCAR-mediated signaling pathway, have been described, an ABA-
signaling pathway from cell surface signal perception to downstream signaling
processes remains to be elucidated. Can RLKs serve as a plasma membrane ABA
receptor? Genetic and cell biological studies are required in the future to further
identify members of the RLK family and their targets involved in ABA signal-
ing in order to uncover primary signaling events that take place in the cell sur-
face for ABA perception and thus to understand the complicated ABA-signaling
pathways.
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