The its proteasomal degradation (Duek et al.,

The activity of bHLH
transcription factor ABA-RESPONSIVE KINASE SUBSTRATE1 (AKS1), connected to
guard cell regulation under drought stress, was shown to be diminished upon
ABA-mediated phosphorylation, leading to an interrupted binding to the promoter
of the target gene POTASSIUM CHANNEL IN
stomatal opening (Leonhardt et al.,
2004; Takahashi et al., 2013). KAT1 promoter binding is only possible
when AKS1 forms homo-multimers. Hence, phosphorylation of AKS1 interrupts the multimerization
and renders the monomer inactive (Takahashi et al.,


In addition to an interrupted
DNA binding capacity, phosphorylation can also lead to the degradation or
stabilization of a protein, which is another well-studied mechanism to
fine-tuned protein activity. The light-inducible, atypical bHLH protein Long Hypocotyl in Far-Red1 (HFR1) is
involved in photomorphogenesis, combining phytochrome A (phyA)-mediated far-red
light- and CRYPTOCHROME1
(cry-1) mediated blue light signaling (Fairchild et al.,
2000; Fankhauser and Chory, 2000; Soh et al., 2000; Duek and Fankhauser, 2003). HFR1
interacts with COP1 during darkness, which leads to its proteasomal degradation
(Duek et al., 2004;
Jang et al., 2005; Yang et al., 2005). Reciprocal
results propose that phosphorylation of HFR1 promotes on one hand its
degradation in a COP1 dependent manner (Duek et al., 2004), while on the other hand, light-dependent phosphorylation
of preferentially Ser122 by casein
kinase II (CKII) renders the protein more stable (Park et al., 2008).

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HFR1 in addition interacts
with the negative regulator of light signaling Phytochrome-interacting factor 3 (PIF3), whose protein
stability is also regulated via proteasomal degradation, similar to other PIF
proteins (Fairchild et al.,
2000; Monte et al., 2004; Park et al., 2004; Shen et al., 2005; Al-Sady et al.,
2006; Nozue et al., 2007; Shen et al., 2007; Lorrain et al., 2008). Active
forms of phytochromes (Pfr), which are localized in the nucleus after
light-induced translocation from the cytosol, interact with PIFs. Pfr-mediated
PIF phosphorylation leads to subsequent proteasomal degradation and initiation
of photomorphogenesis (Bhattacharya et al.,
2017). It
was reported that BRs have an impact on the PIF regulation by BIN2-mediated
phosphorylation of PIF4 (Bernardo-García et
al., 2014). In
addition to CK2 modifying PIF1 (Bu et al., 2011), also Photoregulatory
Protein Kinase 1 (PPK1) was shown to phosphorylate PIF3 (Ni et al., 2017). Interestingly, phytochromes can also act as kinases,
as shown for Avena sativa phyA,
phosphorylating PIF3 (Shin et al., 2016).


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