As sessile organisms, it is of importance for plantsto sense nutrient availability in the soil.
PTMs enable a quick response tochanging nutrient occurrences, such as Fe deficiency, and allow a fine-tunedsignal transduction to adjust respective gene expression accordingly. Exemplary,histone PTMs are described to be involved in Fe homeostasis. Under sufficientFe conditions, the gene expression of BHLH038/039/100/101 is reduced due toSHK1-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 GENERALCONTROL NONREPRESSED PROTEIN5 (GCN5)-mediated acetylation and HISTONEDEACETYLASE7 (HDA7)-mediated deacetylation of FRD3 has a direct impact on Fe root-to-shoot transportation (Xing et al., 2015).
Besides, ubiquitination-dependent regulation of severalproteins involved in Fe-uptake or homeostasis was shown. As described before,PYELs are targeted for ubiquitination and subsequent proteasomal degradation byBTS (Selote et al., 2015). Also, one PYEL homologue in Malus domestica, MdbHLH104, was shown tobe ubiquitinated and subsequently degraded via the proteasome to avoid Feover-accumulation under Fe sufficient conditions (Zhao et al., 2016).A constant ubiquitin-mediated turnover was alsoreported for FIT, securing the accessibility of a pool of active FIT within thecell.
Hence, the cell can remain responsive to incoming signals (Lingam et al., 2011; Meiser et al., 2011; Sivitz etal., 2011).
Ubiquitination also affects protein localization and stability of IRONREGULATED TRANSPORTER (IRT1). IRT1 DEGRADATION FACTOR1 (IDF1)-facilitated mono-ubiquitinationof lysines K154 and K179 is needed for clathrin mediated endocytosis of IRT1from the plasma membrane to the trans-Golginetwork. There, IRT1 both undergoes exocytosis and is either re-located to thePM or is marked for vacuolar degradation (Kerkeb et al., 2008; Barberon et al., 2011; Shin etal.
, 2016).However it is speculated, that two more lysine residues are targets forubiquitination (Shin et al., 2013) and that ubiquitination isdependent on prior phosphorylation events (Ivanov and Bauer, 2017). The MAP kinase cascade is involved in the regulationof the Fe deficiency response via ethylene signaling. Under Fe limitedconditions, MPK3 and MPK6 gene expression and theircorresponding enzymatic activity was increased.
mpk3 and mpk6 knock-out mutantplants were chlorotic and displayed a decreased Fe content. Molecular analysisrevealed 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). Furtherit was shown, that MPK3 and MPK6-mediated phosphorylation of ACS2 and ACS6prevents their proteasomal degradation (Liu and Zhang, 2004; Joo et al.
, 2008; Han et al.,2010). AnotherMPK3 and MPK6 substrate is the transcription factor WRKY33 (Mao et al., 2011). Uponphosphorylation, WRKY33 can bind to ACS2and ACS6 promotors and induce geneexpression (Li et al., 2012). Alsoother 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 upregulatedunder Fe -limited conditions (Chae and Kieber, 2005; Garcia et al., 2010; Ye etal., 2015). Thissupports the observed increase of ethylene biosynthesis in plant roots upon Fedeficiency (Romera et al., 1999; Romera and Alcantara, 2004). Furtherit is known, that FIT gene expressionis 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/EIL1protein-protein interaction (Lucena et al.
, 2006; Garcia et al., 2010; Lingam etal., 2011). Recently it was shown that Fe deficiency causes theaccumulation of cytosolic Ca2+ within cells of the elongation androot -hair zone in primary Arabidopsis roots. In addition, lsk knock-out mutant plants, which contain lesion in CIPK23 gene, displayed a strongerchlorosis, reduced chlorophyll- and Fe content, altered root structure as wellas reduced FRO2 activity than WT under Fe deficient conditions. This led to theconclusion, that kinases from the CIPK -family are involved in Fe accumulationand uptake, by positively mediating FRO2 activity (Tian et al.
, 2016). CBL1and CBL9 were identified to act as respective Ca2+ sensors upstreamof CIPK23 in this process (Xu et al., 2006; Tian et al., 2016).
Under low Fe, the plasma membrane ATPase AHA2 reducesthe soil pH and facilitates the solubilization of Fe3+ from soilparticles (Santi and Schmidt, 2009). Hence, differential regulation ofAHA2 via protein phosphorylation indirectly affects Fe accumulation and uptake.AHA2 interaction with 14-3-3 proteins has an activating effect on the protonpump (Jahn et al., 1997; Oecking et al., 1997; Baunsgaardet al., 1998; Fullone et al., 1998).
Phosphorylation of C-terminal threonine, Thr947, located within a Tyr-Thr-Valrecognition site for 14-3-3 proteins, facilitates the interaction of bothproteins and activates AHA2 (Fuglsang et al., 1999; Kinoshita and Shimazaki, 1999;Svennelid et al., 1999; Maudoux et al., 2000). Inaddition, the binding of the peptide hormone plantpeptide containing sulfated tyrosine 1 (PSY1) to the respective LEUCINE-RICHREPEAT RECEPTOR KINASE (LRR-RK) PSY1R, has a positive effect on AHA2 activity.The activated PSY1R interacts with and phosphorylates AHA2 at Thr881, assistingprotein extrusion (Fuglsang et al., 2014). Pump activation by Thr881phosphorylation appears to be independent of subsequent 14-3-3 binding (Niittyla et al.
, 2007). Thephosphorylation of C-terminal Ser931 by the CBL2 – CIPK11 module however causesan inhibition of AHA2 activity by prohibiting the interaction with 14-3-3proteins. cikp11 loss-of-functionmutants display an increase in tolerance towards high external pH (Fuglsang et al., 2007). Flagellin-mediatedphosphorylation of Ser899 also inhibits AHA2 action (Nuhse et al.
, 2007). In addition, peptide RAPIDALKALINIZATION FACTOR (RALF)-triggered activation of the cell surface receptorFERONIA also prompts Ser899 phosphorylation (Haruta et al., 2014).
The described phosphorylation of AHA2 is conveyed bydifferent overlapping environmental and biochemical stimuli, which mediate thesignal integration into downstream actions, either activating or inactivatingthe enzyme (Haruta et al., 2015). These examples show, which potentialpost-translational modifications have on the regulation of signaling pathways. Hence,it is of interest to expand this knowledge, putting a focus onphosphorylation-based protein regulation, since it is probable that moreproteins involved in Fe deficiency signaling might undergo post-translationalcontrol.