The question remains unanswered
to date, which kinases might be involved in phosphorylation of the above
identified amino acids. Several kinase families are connected to Fe-deficiency
responses such as MITOGEN-ACTIVATED
PROTEIN KINASES (MAPKs), targeting serine and threonine residues. MAPK3 and MAPK6 gene expression is upregulated under Fe deficiency (Ye et al., 2015). The important role of both kinases is displayed in mapk3 and mapk6 loss-of function mutants which show decreased gene expression
level of FRO2 and IRT1 (Ye et al., 2015). Both kinases are involved in the expression of ACC SYNTHASE (ACS) genes (Li et al., 2012; Li et
al., 2017). They additionally positively influence Fe acquisition through
phosphorylation-based stabilization of ACS proteins, needed for ethylene
production (Liu and Zhang, 2004;
Joo et al., 2008; Han et al., 2010). Besides, EIN3 was found to be phosphorylated and hence stabilized by
MAPK3 and MAPK6 (Yoo et al., 2008). Consequently, ethylene can promote FIT stability and
positively influence the Fe deficiency response (Lingam et al., 2011). Recently, the involvement of serine/threonine kinases
CALCIUM-DEPENDENT PROTEIN KINASES (CPKs) CPK5 and CPK6 in ACS expression has been shown (Li et al., 2017). Another subfamily of Ca2+-dependent
protein kinases, CIPKs, is connected to Fe stress signaling. A recent
microarray study revealed two robustly upregulated CIPKs in 6 day old WT seedlings,
grown under Fe deficiency, which are CIPK11 and CIPK7 (Mai et al., 2016). Recently, we could elucidate the positive effects of
CIPK11 on FIT activity (Gratz et al., submitted), while the involvement of
CIPK7 has still to be revealed.

Potential kinases, targeting
tyrosine residues might be from the Raf-like subfamilies of MAPKKKs (Jouannic et al., 1999;
Ichimura et al., 2002). The phosphorylation motif spanning FIT Tyr238 is a
kinase substrate motif of human JANUS KINASE 2 (JAK2) (Argetsinger et al.,
2004). The closest plant homologue is the MAPKKK Raf10, which is expressed in
roots, responsive to abiotic stresses and is classified as plant tyrosine
kinase (Jouannic et al., 1999;
Ichimura et al., 2002; Rudrabhatla et al., 2006; Lee et al., 2015).

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Brassinosteroids (BRs) are
negative regulators of the Fe uptake by reducing the expression of CsFRO1 and CsIRT1 (Wang et al., 2012) and were also shown to interfere with the Fe homeostasis
in Strategy II plants (Wang et al., 2015). Hence, it is possible, that members of the BR
signaling pathway are involved in FIT tyrosine phosphorylation, due to the
fact, that the kinases BRASSINOSTEROID INSENSITIVE 1 (BRI1) and BRASSINOSTEROID
INSENSITIVE 1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) are dual specificity kinases (Oh et al., 2009; Kim
and Wang, 2010; Jaillais et al., 2011). Hence, it will be the focus of future studies to reveal which kinases control
the dual regulation of FIT activity.