The Fe stress signaling. A recent microarray study

The question remains unansweredto date, which kinases might be involved in phosphorylation of the aboveidentified amino acids.

Several kinase families are connected to Fe-deficiencyresponses such as MITOGEN-ACTIVATEDPROTEIN 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 expressionlevel 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 etal.

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, 2017). They additionally positively influence Fe acquisition throughphosphorylation-based stabilization of ACS proteins, needed for ethyleneproduction (Liu and Zhang, 2004;Joo et al., 2008; Han et al., 2010). Besides, EIN3 was found to be phosphorylated and hence stabilized byMAPK3 and MAPK6 (Yoo et al., 2008).

Consequently, ethylene can promote FIT stability andpositively influence the Fe deficiency response (Lingam et al., 2011). Recently, the involvement of serine/threonine kinasesCALCIUM-DEPENDENT PROTEIN KINASES (CPKs) CPK5 and CPK6 in ACS expression has been shown (Li et al., 2017). Another subfamily of Ca2+-dependentprotein kinases, CIPKs, is connected to Fe stress signaling.

A recentmicroarray 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 ofCIPK11 on FIT activity (Gratz et al., submitted), while the involvement ofCIPK7 has still to be revealed.Potential kinases, targetingtyrosine 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 akinase substrate motif of human JANUS KINASE 2 (JAK2) (Argetsinger et al.,2004). The closest plant homologue is the MAPKKK Raf10, which is expressed inroots, responsive to abiotic stresses and is classified as plant tyrosinekinase (Jouannic et al., 1999;Ichimura et al., 2002; Rudrabhatla et al.

, 2006; Lee et al., 2015). Brassinosteroids (BRs) arenegative 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 homeostasisin Strategy II plants (Wang et al., 2015). Hence, it is possible, that members of the BRsignaling pathway are involved in FIT tyrosine phosphorylation, due to thefact, that the kinases BRASSINOSTEROID INSENSITIVE 1 (BRI1) and BRASSINOSTEROIDINSENSITIVE 1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) are dual specificity kinases (Oh et al., 2009; Kimand Wang, 2010; Jaillais et al.

, 2011). Hence, it will be the focus of future studies to reveal which kinases controlthe dual regulation of FIT activity.