In that CBL1 affects CBF gene expression

In order to quickly
respond to changing environmental stimuli and counteract the negative
regulation via MPK3/6, the CALCIUM/CALMODULIN-REGULATED RECEPTOR-LIKE KINASE 1
(CRLK1) and CRLK2 are activated, promoting freezing tolerance in plants (Yang et al., 2010;
Zhao et al., 2017). CRLKs
interact with MITOGEN-ACTIVATED PROTEIN KINASE KINASE KINASE1 (MEKK1) which
activates the MITOGEN-ACTIVATED PROTEIN KINASE KINASE2 (MKK2) – MPK4 cascade (Yang et al., 2010;
Furuya et al., 2013; Zhao et al., 2017). Cold-responsive
MKK2 was already described as a positive regulator of cold responses (Teige et al., 2004). The
subsequent phosphorylation and activation of MPK4 inhibits the MPK3/6-mediated
phosphorylation and degradation of ICE1 (Zhao et al., 2017). Ca2+-signals are also perceived via other
transduction mechanisms, such as a CALCINEURIN B-LIKE PROTEIN (CBL) / CBL-INTERACTING
PROTEIN KINASE (CIPK) -module. In the context of freezing tolerance, Ca2+ is
sensed by CBL1, which was described to have a negative effect on freezing
tolerance (Cheong et al., 2003). It was further speculated that CBL1 affects CBF gene
expression in a dose-responsive manner (Albrecht et al., 2003).
Recently, Huang et al., 2011 identified the respective CBL1 interaction
partner, CIPK7, both of which are induced by cold and affect cold responses (Huang et al., 2011). Potentially CBL1 interferes with another CBL/CIPK
module, comprising CBL9 and CIPK3, involved in cold signaling as a response to
ABA -signaling (Cheong et al., 2003;
Kim et al., 2003; Pandey et al., 2008). Further
the involvement of CALCIUM-DEPENDENT PROTEIN KINASES (CDPK) was shown (Bohmer and Romeis,
2007; Komatsu et al., 2007).

ICE1 stability is additionally
regulated in a light-dependent manner. Darkness facilitates constitutively photomorphogenic1
(COP1)-mediated ubiquitination and proteasomal degradation of ICE1, which also
leads to reduced production of stomata (Lee et al., 2017;
Salomé, 2017).
Light-signaling and COP1-involvement indicates that the freezing tolerance
module is an integration point for different environmental stimuli, linking
adaptation to abiotic stresses and stomatal development.

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This further becomes
evident by the interaction between ICE1 and SPEECHLESS (SPCH) (Kanaoka et al., 2008), a group Ia bHLH transcription factor responsible for
the asymmetric cell divisions and production of paired guard cells in
Arabidopsis (Heim et al., 2003;
MacAlister et al., 2007). SPCH
itself undergoes MPK3/ MPK6-mediated phosphorylation, which mostly has a
repressive effect on SPCH activity. The failure of the non-phosphorylatable
mutant S193A to rescue spch-3 knock-out
phenotype demonstrated, that phosphorylation of one Ser193, however, is
required for full activity (Lampard et al., 2008). MPK-regulated phosphorylation of SPCH is controlled
by different stimuli, such as brassinosteroids (BRs). The presence of BRs lead
to degradation of the BRASSINOSTEROID-INSENSITIVE 2 (BIN2) (Jonak and Hirt, 2002;
Peng et al., 2008; Kim et al., 2009),
which can no longer phosphorylate and hence inactivate MAPKKK YODA (YDA) (Bergmann et al., 2004;
Lukowitz et al., 2004; Kim et al., 2012). YDA
is upstream of MKK4/MKK5 – MPK3/MPK6 (Wang et al., 2007), initiating MPK signaling and hence inhibiting SPCH
activation by phosphorylation (Lampard et al., 2008). The degradation of BIN2 also interferes with the
BIN2 mediated phosphorylation, and hence inactivation, of MKK4 and MKK5,
consequently leading to reduced stomatal development (Khan et al., 2013). Therefore,
BRs impede stomatal development.

An opposing effect mediated
by the same stimuli can be demonstrated by the finding that BIN2 phosphorylates
SPCH directly (Gudesblat et al.,
2012). BR-mediated
degradation of BIN2 hence leads to an impaired phosphorylation of SPCH and stomatal
development can be initiated (Gudesblat et al.,
2012). Besides
the mostly negative regulation of SPCH activity by the BRs- or MPK signaling
pathways, it was recently shown that CYCLIN-DEPENDENT KINASE A;1 (CDKA;1)
phosphorylation of SPCH Ser186 is needed for mutant complementation (Yang et al., 2015). This
ambivalent regulation of SPCH is a well-studied example of how complex the
activity of a bHLH transcription factor can be regulated and fine-tuned.

Hence, protein
phosphorylation by one kinase at different residues can have opposing effects
on the activity, similarly to the effects that different stimuli, such as
hormones, have. The complexity can further get elevated by the integration of
different kinases, modifying the same protein in parallel. Subsequently, the need
arises to understand the molecular mechanism that alters protein activity in
response to a phosphorylation event.

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