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approaches showed that the loss-of-function mutants of
FyPP
display ABA-
hypersensitive phenotypes in seed germination and postgermination growth (Dai
et al.
2013
). The FyPP proteins directly interact with and dephosphorylate ABI5;
and further genetic evidence demonstrated an antagonistic interaction between
FyPP1/3 with SnRK2 in regulating ABA responses (Dai et al.
2013
). These find-
ings describe a model that FyPP1 and FyPP3 function as two negative regulators
of ABA signaling through dephosphorylating ABI5 to counteract SnRK2s that
phosphorylate ABI5 to promote ABA signaling (Fig.
8.2
).
The high degree of genetic redundancy caused by manifold possible combina-
tions of functional complexes may have limited the experimental evidence concern-
ing processes regulated by PP2As. Further identification of more members of PP2As
or PP2A heterotrimer combinations involved in ABA signaling will be necessary.
Additionally, it merits to be answered whether PP2As may function as the PP2A-
like protein PP6 to cooperate with SnRK2s to catalyze reversible ABI5 phosphoryla-
tion in ABA signaling, or whether they may function, in a manner similar to PP2Cs,
as negative regulators directly downstream of the PYR/PYL/RCAR receptors for
ABA and upstream of SnRK2s in ABA signaling. The elucidation of functional
mechanisms of PP2As will deepen our understanding of ABA-signaling pathway.
8.7 Concluding Remarks and Perspectives
Extensive research efforts with integrative approaches have led to consider-
able progresses to understand highly complex ABA-signaling network involving
reversible protein phosphorylation catalyzed by protein kinases and phosphatases.
A wide array of protein kinases and phosphatases have been identified as crucial
players in ABA signaling, among which CDPKs, SnRKs, MAPKs, RLKs, PP2Cs,
and PP2As are relatively best characterized and their functional mechanisms in
ABA signaling begin to be understood (Fig.
8.2
). Particularly, a PYR/PYL/RCAR-
PP2C-SnRK2s-ABI5/ABFs-linked protein dephosphorylation-phosphoryla-
tion signaling cascade has been described, and most recent studies supplement
this pathway with both a reversible phosphorylation of SnRK2s catalyzed by
BIN2/BILs and PP2Cs and a reversible ABI5 phosphorylation catalyzed by
SnRK2s and FyPP1/3 (Fig.
8.2
). Additionally, some intriguing signaling cascades,
such as CDPK-SLAC1/SLAH3-coupled signaling and ABA-ROS-MPK9/12 sign-
aling cascade in guard cells, and a plasma membrane RLK-mediated FER-GEFs-
ROP11-PP2C linked pathway, have been described (Fig.
8.2
). However, many
more regulatory interactions of protein kinases and phosphatases in ABA signal-
ing are probably still hidden. Further studies will be needed to provide compelling
evidence to link these signaling processes to the bigger picture of ABA-signaling
pathway/network from the primary perception events of ABA signal to the most
downstream ABA-specific physiological response. Special attention should be
attracted to elucidating the possible cooperation of the identified CDPKs, SnRKs,
MAPKs, RLKs, PP2Cs, and PP2As, and establishing molecular, cellular, and
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