As and allow a fine-tuned signal transduction to


As sessile organisms, it is of importance for plants
to sense nutrient availability in the soil. PTMs enable a quick response to
changing nutrient occurrences, such as Fe deficiency, and allow a fine-tuned
signal transduction to adjust respective gene expression accordingly. Exemplary,
histone PTMs are described to be involved in Fe homeostasis. Under sufficient
Fe conditions, the gene expression of BHLH038/039/100/101 is reduced due to
SHK1-BINDING PROTEIN1 (SKB1)-mediated dimethylation of histone 4 arginine 3 (H4R3),
suggesting a negative effect on Fe uptake (Fan et al., 2014). Further it was reported, that GENERAL
CONTROL NONREPRESSED PROTEIN5 (GCN5)-mediated acetylation and HISTONE
DEACETYLASE7 (HDA7)-mediated deacetylation of FRD3 has a direct impact on Fe root-to-shoot transportation (Xing et al., 2015).

Besides, ubiquitination-dependent regulation of several
proteins involved in Fe-uptake or homeostasis was shown. As described before,
PYELs are targeted for ubiquitination and subsequent proteasomal degradation by
BTS (Selote et al., 2015). Also, one PYEL homologue in Malus domestica, MdbHLH104, was shown to
be ubiquitinated and subsequently degraded via the proteasome to avoid Fe
over-accumulation under Fe sufficient conditions (Zhao et al., 2016).

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A constant ubiquitin-mediated turnover was also
reported for FIT, securing the accessibility of a pool of active FIT within the
cell. Hence, the cell can remain responsive to incoming signals (Lingam et al., 2011; Meiser et al., 2011; Sivitz et
al., 2011).
Ubiquitination also affects protein localization and stability of IRON
REGULATED TRANSPORTER (IRT1). IRT1 DEGRADATION FACTOR1 (IDF1)-facilitated mono-ubiquitination
of lysines K154 and K179 is needed for clathrin mediated endocytosis of IRT1
from the plasma membrane to the trans-Golgi
network. There, IRT1 both undergoes exocytosis and is either re-located to the
PM or is marked for vacuolar degradation (Kerkeb et al., 2008; Barberon et al., 2011; Shin et
al., 2016).
However it is speculated, that two more lysine residues are targets for
ubiquitination (Shin et al., 2013) and that ubiquitination is
dependent on prior phosphorylation events (Ivanov and Bauer, 2017).

The MAP kinase cascade is involved in the regulation
of the Fe deficiency response via ethylene signaling. Under Fe limited
conditions, MPK3 and MPK6 gene expression and their
corresponding enzymatic activity was increased. mpk3 and mpk6 knock-out mutant
plants were chlorotic and displayed a decreased Fe content. Molecular analysis
revealed a decreased expression of ACC SYNTHASE (ACS) genes ACS2 and ACS6, responsible for the production of the ethylene precursor ACC,
and reduced Fe deficiency responses (Ye et al., 2015). Further
it was shown, that MPK3 and MPK6-mediated phosphorylation of ACS2 and ACS6
prevents their proteasomal degradation (Liu and Zhang, 2004; Joo et al., 2008; Han et al.,
2010). Another
MPK3 and MPK6 substrate is the transcription factor WRKY33 (Mao et al., 2011). Upon
phosphorylation, WRKY33 can bind to ACS2
and ACS6 promotors and induce gene
expression (Li et al., 2012). Also
other kinases promote ACS2 and ACS6 gene expression such as CALCIUM-DEPENDENT PROTEIN KINASE
(CPK)  CPK5 and CPK6 (Li et al., 2017). Transcription of ACS genes were shown to be upregulated
under Fe -limited conditions (Chae and Kieber, 2005; Garcia et al., 2010; Ye et
al., 2015). This
supports the observed increase of ethylene biosynthesis in plant roots upon Fe
deficiency (Romera et al., 1999; Romera and Alcantara, 2004). Further
it is known, that FIT gene expression
is reduced in ethylene omitted conditions (Lucena et al., 2006; Garcia et al., 2010). Hence,
ethylene is not only a positive regulator of FIT gene expression but also FIT protein stability, mediated by EIN3/EIL1
protein-protein interaction (Lucena et al., 2006; Garcia et al., 2010; Lingam et
al., 2011).

Recently it was shown that Fe deficiency causes the
accumulation of cytosolic Ca2+ within cells of the elongation and
root -hair zone in primary Arabidopsis roots. In addition, lsk knock-out mutant plants, which contain lesion in CIPK23 gene, displayed a stronger
chlorosis, reduced chlorophyll- and Fe content, altered root structure as well
as reduced FRO2 activity than WT under Fe deficient conditions. This led to the
conclusion, that kinases from the CIPK -family are involved in Fe accumulation
and uptake, by positively mediating FRO2 activity (Tian et al., 2016). CBL1
and CBL9 were identified to act as respective Ca2+ sensors upstream
of CIPK23 in this process (Xu et al., 2006; Tian et al., 2016).

Under low Fe, the plasma membrane ATPase AHA2 reduces
the soil pH and facilitates the solubilization of Fe3+ from soil
particles (Santi and Schmidt, 2009). Hence, differential regulation of
AHA2 via protein phosphorylation indirectly affects Fe accumulation and uptake.
AHA2 interaction with 14-3-3 proteins has an activating effect on the proton
pump (Jahn et al., 1997; Oecking et al., 1997; Baunsgaard
et al., 1998; Fullone et al., 1998).
Phosphorylation of C-terminal threonine, Thr947, located within a Tyr-Thr-Val
recognition site for 14-3-3 proteins, facilitates the interaction of both
proteins and activates AHA2 (Fuglsang et al., 1999; Kinoshita and Shimazaki, 1999;
Svennelid et al., 1999; Maudoux et al., 2000). In
addition, the binding of the peptide hormone plant
peptide containing sulfated tyrosine 1 (PSY1) to the respective LEUCINE-RICH
REPEAT RECEPTOR KINASE (LRR-RK) PSY1R, has a positive effect on AHA2 activity.
The activated PSY1R interacts with and phosphorylates AHA2 at Thr881, assisting
protein extrusion (Fuglsang et al., 2014). Pump activation by Thr881
phosphorylation appears to be independent of subsequent 14-3-3 binding (Niittyla et al., 2007). The
phosphorylation of C-terminal Ser931 by the CBL2 – CIPK11 module however causes
an inhibition of AHA2 activity by prohibiting the interaction with 14-3-3
proteins. cikp11 loss-of-function
mutants display an increase in tolerance towards high external pH (Fuglsang et al., 2007). Flagellin-mediated
phosphorylation of Ser899 also inhibits AHA2 action (Nuhse et al., 2007). In addition, peptide RAPID
ALKALINIZATION FACTOR (RALF)-triggered activation of the cell surface receptor
FERONIA also prompts Ser899 phosphorylation (Haruta et al., 2014).

The described phosphorylation of AHA2 is conveyed by
different overlapping environmental and biochemical stimuli, which mediate the
signal integration into downstream actions, either activating or inactivating
the enzyme (Haruta et al., 2015).

 

These examples show, which potential
post-translational modifications have on the regulation of signaling pathways. Hence,
it is of interest to expand this knowledge, putting a focus on
phosphorylation-based protein regulation, since it is probable that more
proteins involved in Fe deficiency signaling might undergo post-translational
control.

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