Protein Kinases and Phosphatases Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 137

ABA and increased tolerance to drought, whereas the cpk4 or cpk11 mutant exhibits
the opposite phenotypes and the cpk4 cpk11 double mutant shows stronger ABA-
related phenotypes than the single mutants. AtCPK4 and AtCPK11 phosphorylate
ABF1 and ABF4 (Zhu et al. 2007 ; Lynch et al. 2012 ), and AtCPK4 also phospho-
rylates ABF2 (Lu et al. 2013 ). The two CDPKs regulate expression of a subset of
ABA-responsive genes probably through these ABA-responsive transcription fac-
tors, which correlates with ABA-related phenotypes and altered drought and salt tol-
erance in the overexpressing lines and loss-of-function mutants (Zhu et al. 2007 ). By
contrast, the Arabidopsis CPK12, the closest homolog of AtCPK4/AtCPK11 in sub-
group I (Hrabak et al. 2003 ; Boudsocq and Sheen 2013 ), was reported to function
as a negative regulator of ABA signaling in seed germination and postgermination
growth, and to phosphorylate ABF1, ABF4 as well as a key, negative ABA-signaling
regulator ABA insensitive 2 (ABI2), suggesting that different members of the CDPK
family may constitute a regulation loop by functioning positively and negatively in
ABA signal transduction (Zhao et al. 2011a , b ). Together, all these findings suggest
that ABA-induced transcriptional reprogramming via ABFs is likely to be a key fea-
ture of the CDPK-mediated ABA signaling (Boudsocq and Sheen 2013 ).
In crop plants, a homolog of the Arabidopsis CPK4/11/12 in maize, ZmCPK11,
was reported to regulate ABA-induced antioxidant defense (Ding et al. 2013 ),
suggesting that these CDPK members may regulates multiple ABA responses in
both abiotic and biotic stresses. In rice, two members of CDPK, OsCPK12 and
OsCPK21, were reported to be positive regulators of ABA signaling in response to
salt stresses (Asano et al. 2011 , 2012 ). The rice plants overexpressing OsCPK12
or OsCPK21 showed increased salt tolerance and ABA sensitivity in seedling
growth, and the expression levels of several ABA- and high salinity-inducible
genes were enhanced in these transgenic plants. However, the loss-of-function
oscpk12 mutant and OsCPK12 RNAi lines showed wild-type ABA-related pheno-
types, suggesting that functional redundancy among OsCPK12 and other CDPKs
exists in the ABA-signaling pathway in rice (Asano et al. 2011 , 2012 ). It remains
unclear, however, whether these rice CDPKs regulates gene expression through
ABA-responsive transcription factors like the Arabidopsis ABFs to mediate ABA
signaling, which needs studies in the future.
Although the role of CDPK phosphorylation has not been thoroughly inves-
tigated, other transcription factors than ABFs and proteins of diverse iden-
tity involved in ABA signaling have also been identified as CDPK substrates in
vitro. The Arabidopsis CPK11 was shown to interact physically with a nuclear
zinc finger transcription factor AtDi19-1 (drought-induced 19-1, Rodriguez
et al. 2006 ; Curran et al. 2011 ; Liu et al. 2013 ) and a heat-shock protein HSP1
(Uno et al. 2009 ). The Di19-1 and Di19-2 of cotton ( Gossypium hirsutum ) were
reported to regulate plant response to ABA and salt stress (Li et al. 2010 ). A more
recent report showed that AtDi19 functions as a transcriptional activator and
was involved in Arabidopsis responses to drought stress through upregulation of
pathogenesis-related PR1 , PR2 , and PR5 gene expressions, and this transactiva-
tion activity of AtDi19 is enhanced by AtCPK11 (Liu et al. 2013 ). This suggests
that AtCPK11 may interact with both ABFs (ABF1/4) and AtDi19 transcription