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water transport, transcription, and cell signaling (Harper and Harmon 2005 ; Cheng
et al. 2002b ; Curran et al. 2011 ; Boudsocq and Sheen 2013 ; Schulz et al. 2013 ),
which helps to unravel mechanisms of CDPK functions.
In summary, the multigene family-encoded CDPK members and diversity in a
broad range of their characteristics, such as expression pattern, subcellular locali-
zation, and substrate specificities, are likely to provide functional specificity and
redundancy in mediating Ca 2 + signaling in developmental and environmental
responses of plant cells.
8.2.2 CDPKs: Hubs in ABA Signaling
In recent years, major progress has been made to uncover the central roles of
CDPKs in triggering downstream cellular events in response to ABA. An ear-
lier report showed that the constitutively ectopic expression of two Arabidopsis
CDPKs AtCPK10/CDPK1 ( Arabidopsis gene identifier number At1g18890) and
AtCPK30/CDPK1a (At1g74740) from subgroup III (Hrabak et al. 2003 ; Boudsocq
and Sheen 2013 ) in maize leaf protoplasts activated a stress- and ABA-inducible
HVA1 promoter, showing the connection of CDPKs to ABA-signaling pathway
(Sheen 1996 ). Later, the function of the AtCPK10 in ABA signaling was confirmed
genetically: AtCPK10 regulates ABA-mediated stomatal movement in response to
drought stress possibly by interacting with a heat-shock protein HSP1 (Zou et al.
2010 ). Constitutively, active AtCPK10 also induces endogenous stress- and ABA-
inducible genes in Arabidopsis leaf cells (Boudsocq and Sheen 2013 ), suggesting
a conservation of specific CDPK functions in dicots and monocots. This transcrip-
tional induction is likely to be mediated by the ABA-responsive transcription fac-
tors, ABFs, which were identified as in vitro substrates for several CDPKs from
subgroups I and III (Zhu et al. 2007 ; Choi et al. 2005 ). An Arabidopsis CDPK,
AtCPK32, was shown to interact with and activate an ABA-induced transcription
factor ABF4, resulting in the induction of ABF4 target genes, and constitutive over-
expression of AtCPK32 resulted in ABA-hypersensitive phenotypes in ABA-induced
inhibition of seed germination (Choi et al. 2005 ). The Arabidopsis CPK3 and CPK6
were identified as key, positive regulators in ABA-regulated stomatal signaling (Mori
et al. 2006 ), but it has been unknown whether these two CDPK members regulate
the ABFs or other ABA-responsive transcription factors through which they modu-
late expression of ABA-responsive genes. Yu et al. ( 2006 ) isolated an ABA-induced
CDPK from grape berry, ACPK1, which phosphorylates a P-type ATPase in vitro,
and was further shown to confer ABA-hypersensitive phenotypes and drought tol-
erance when ectopically expressed in Arabidopsis (Yu et al. 2007 ). The two closet
homologs of ACPK1 in Arabidopsis , AtCPK4 and AtCPK11 from subgroup I
(Hrabak et al. 2003 ; Boudsocq and Sheen 2013 ), were shown to function as posi-
tive regulators in all the major ABA responses including seed germination, seedling
growth, guard cell regulation, and plant tolerance to drought and salt stresses (Zhu
et al. 2007 ). Plants overexpressing AtCPK4 or AtCPK11 showed hypersensitivity to
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