Protein Kinases and Phosphatases Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 147

Several efforts have been made to explore functional mechanism of the
Arabidopsis CIPK15-mediated ABA signaling. It was reported that CIPK15 interacts
with two type-A phosphatases 2C ABI1 and ABI2 (Guo et al. 2002 ), both of which
are the most characterized negative regulators of ABA signaling (Leung et al. 1994 ,
1997 ; Meyer et al. 1994 ; Gosti et al. 1999 ; Merlot et al. 2001 ; and also see the sec-
tions as follows), suggesting that they may possibly act as Ca 2 + -sensors downstream
or upstream of the ABI1 and ABI2 phosphatases when forming a protein CBL-CIPK
complex for perceiving calcium signal. Additionally, an AP2 transcription factor ERF7
that negatively regulates ABA response was shown to be a kinase substrate of CIPK15
(Song et al. 2005 ), suggesting that CIPK15 may regulate ABA signaling more directly
by phosphorylating transcription factor and modulating gene expression.
Additionally, SnRK3.11/CIPK24/SOS2, a key regulator of plant response to salt
stress (Liu et al. 2000 ), was shown to interact with the type-A protein phosphatase
2C ABI2, suggesting that the CIPK kinase and ABI2 phosphatase may possibly con-
trol phosphorylation status of each other to relay salt signaling (Ohta et al. 2003 ).
The SOS pathway is one of the CBL-CIPK signaling pathways conferring plant salt
tolerance. The CBL4/SOS3 calcium sensor (Liu and Zhu 1998 ) activates the kinase
SnRK3.11/SOS2 in response to salt stress, and the SOS2 kinase phosphorylates and
activates SOS1 (Shi et al. 2000 ), a plasma membrane sodium/proton antiporter (Qiu
et al. 2002 ), which keeps potassium homeostasis in plant cells under high salt stress.
The interaction between ABI2 and SnRK3.11/SOS2 may constitute a point though
which ABA exerts its effect on plant salt tolerance.
In contrast with the CIPK1, CIPK3, and CIPK15 that function as negative ABA-
signaling regulators, CIPK6/SnRK3.14 and CIPK20/PKS8/SnRK3.6 were shown
to be positive regulators of ABA signaling in Arabidopsis , which may antagonizes
CIPK1/3/15 in ABA-signaling pathway. CIPK20/SnRK3.6/PKS18 (T169D, constitu-
tively active form) overexpression lines show ABA-hypersensitive phenotypes, whereas
RNAi plants show ABA-insensitive phenotypes (Gong et al. 2002 ). Overexpression of
an active form of Brassica napus CIPK member BnCIPK6M in Arabidopsis confers
ABA-hypersensitive phenotypes, while downexpression of its homologous gene the
Arabidopsis CIPK6 results in ABA-insensitive phenotypes (Chen et al. 2012 ).
As described earlier, Arabidopsis genome encodes at least 10 members in CBL
and 25 members in CIPK gene family, which provides a high level of diversity,
complexity, and flexibility in the function of the CBL-CIPK network. Further
researches need to identify new members of CIPK-mediating ABA signaling and
particularly upstream regulators/downstream targets to elucidate their functional
mechanisms in ABA-signaling pathways.
8.3.5 SnRKs and ABA Signaling: A Summary
SnRKs are a class of Ser/Thr kinases and are recognized as a family member
of the CDPK-SnRK superfamily, which can be subdivided into three subfami-
lies: SnRK1, SnRK2, and SnRK3. The Arabidopsis genome contains 38 SnRKs,