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Fig. 6.3 A PYR/PYL/RCAR-mediated linear ABA signaling pathway. Under basal ABA levels,
clade A PP2Cs function as negative regulators to repress ABA signaling, through either dephos-
phorylation of SnRK2s or interaction with other targets. ABA binding to the PYR/PYL/RCAR
receptors inactivates PP2Cs, leading to the activation of SnRK2s through phosphorylation by
BIN2 kinases, which subsequently phosphorylate downstream targets, such as members of the
ABF transcription factors or regulatory components of the stomatal aperture, such as the anion
channels SLAC1, which finally induces a diversity of ABA responses. Arrows denote positive
regulation or activation, and bars denote negative regulation or repression. The solid lines and
dotted lines indicate direct effects and indirect effects, respectively. Question mark indicates
unconfirmed link
downstream of SnRK2s (Furihata et al. 2006 ; Fujii et al. 2007 , 2009 ; Fujii and Zhu
2009 ; Fujita et al. 2009 , 2013 ; Sirichandra et al. 2010 ; see also Chaps. 8 and 11 ) . The
SnRK2s directly activate, through phosphorylation, the transcription factors that bind
to ABA-responsive promoter elements and regulate gene expression (see Chap. 11 ) .
In the guard cells, SnRK2s regulate stomatal aperture by modulating the slow anion
channel SLAC1 and SLAH3, K + inward channel KAT1 (Lee et al. 2009 , 2013 ; Geiger
et al. 2009 , 2010 , 2011 ; Brandt et al. 2012 ; Acharya et al. 2013 ; see also Chap. 8 ) .
Together, these results establish a core signaling pathway, in which
PYR/PYL/RCAR-PP2C-SnRK2 is the core ABA signaling module, which func-
tions to regulate downstream ABFs or ion channels of guard cells to induce a
diversity of ABA responses (Cutler et al. 2010 ; Umezawa et al. 2010 ; Fig. 6.3 ).
6.5.3 Functional Diversity of the Members of
PYR/PYL/RCAR Family
Among the fourteen members of the PYR/PYL/RCAR family, the receptor nature
of PYL13/RCAR7 is questioned. A study showed that PYL13/RCAR7 is lack of
the crucial lysine residue for ABA binding and has no physical interaction with
PP2Cs (Joshi-Saha et al. 2011 a), and another two studies reported that PYL13
did not bind ABA because of changes in conserved amino acid residues involved
in ABA binding and selectively inhibits PP2CA independent of ABA (Li et al.
2013 ; Zhao et al. 2013 ). However, a recent report showed that PYL13/RCAR7
inhibits the activities of a number of PP2Cs such as PP2CA and ABI2 in an
 
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