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the different members (Antoni et al. 2012 ; Gonzalez-Guzman et al. 2012 ). The
expression level of PYL3 and PYL10 to PYL13 is extremely low in the whole
plant level, while the expression of PYR1 and the rest of PYL1 to PYL9 could
be detected in both vegetative and reproductive tissues at different levels (Antoni
et al. 2012 ; Gonzalez-Guzman et al. 2012 ).
This functional diversity of different members of PYR/PYL/RCAR receptors
has been supported by several recent reports. A study revealed that PYL8 inter-
acts with at least five PP2Cs, namely HAB1, HAB2, ABI1, ABI2, and PP2CA,
and plays a non-redundant role for the regulation of root ABA sensitivity; the
single pyl8 mutant shows ABA insensitivity in root growth (Antoni et al. 2013 ).
Further, Zhao et al. ( 2014 ) found that PYL8 promotes lateral root growth inde-
pendently of the core ABA-SnRK2 signaling pathway by enhancing the activities
of MYB77 and its paralogs, MYB44 and MYB73, to augment auxin signaling.
Yin et al. ( 2013 ) provided genetic evidence that four ABA receptors PYR1, PYL1,
PYL2, and PYL4 are not sufficient for ABA-induced stomatal opening inhibi-
tion in Arabidopsis : ABA-induced stomatal closure is impaired in the pyr1 pyl1
pyl2 pyl4 quadruple ABA receptor mutant, whereas ABA inhibition of the open-
ing of the mutant's stomata remains intact; ABA substantially inhibits blue light-
induced phosphorylation of H + -ATPase in guard cells in both the mutant and
the wild type (Yin et al. 2013 ). This suggests that stomatal opening and closure
in response to ABA are differentially regulated, and other more members of the
PYR/PYL/RCAR receptors, or other different classes of ABA receptors, such as
ABAR/CHLH or GTG1/GTG2, are required for ABA-induced inhibition of stoma-
tal opening.
6.6 Summary
Recent identification of the receptors or candidate receptors for ABA, local-
ized to plasma membrane, cytosol, and chloroplast, confirms previous observa-
tions that ABA signal is perceived both inter- and intracellularly. ABAR/CHLH
is a chloroplast-membrane protein involved in multiple functions including chlo-
rophyll biosynthesis, plastid-to-nucleus retrograde signaling and ABA signal-
ing. Multiple lines of evidence demonstrate ABA-binding ability of ABAR,
though structural study will be needed to resolve the interaction of ABAR with
ABA at the subcellular level. Increasing evidence well supports that ABAR is a
crucial player in ABA signaling, which antagonizes a group of WRKY transcrip-
tion repressors (WRKY18/40/60) to relieve ABA-responsive genes of inhibition.
The ABAR-WRKY40 coupled mechanism may be inhibited by CPN20, a chloro-
plast interaction partner of ABAR. High level of ABA inhibits the ABAR-CPN20
interaction, which in turn promotes the ABAR-WRKY40 interaction to trigger
the downstream signaling to repress WRKY40 expression and finally to relieve
ABA-responsive genes of inhibition (Fig. 6.1 ). Recent discovery of a PPR protein
SOAR1 that functions as a key regulator of ABA signaling downstream of ABAR
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