In Arabidopsisthaliana, genes involved in the Fe deficiency response can be grouped into sub-clusters.Genes involved in Fe uptake and transport, that are directly or indirectlydependent on the major regulator of Strategy I, FIT, are grouped into the so called”FIT target network” and contains root-expressed genes like IRT1 (Colangelo and Guerinot, 2004; Ivanov et al., 2012;Mai et al., 2016). Anadditional cluster contains FIT-independent genes, such as PYE and its direct targets as well as BHLH039 and BHLH101. As associatedgenes function mainly in Fe homeostasis, this sub-cluster can be referred to as”Fe homeostasis network”, (Ivanov et al.
, 2012). 1.1.1 Regulationof Fe Deficiency Response by the FIT Target NetworkThe Fe regulated, root specific bHLH transcriptionfactor FER is responsible for the transcriptional regulation of Fe uptake genesand was first identified in Solanumlycopersicum (Ling et al., 2002; Brumbarova and Bauer, 2005).
The FIT gene encodes the respective FER homologin Arabidopsis (Bauer et al., 2004; Colangelo and Guerinot, 2004;Jakoby et al., 2004; Yuan et al., 2005). FIT promotor activity was detected inepidermal cells within the differentiationzone and elongation zone of the main root, in lateral roots, as well asin the central cylinder. fit1-1 loss-of-functioncauses lethality if not supplemented with Fe. (Colangelo and Guerinot, 2004).
FITregulates the expression of 72 out of 179 Fe-responsive genes (Colangelo and Guerinot, 2004). Interestingly, recently 13 novel geneswere identified, being strictly Fe-dependent and FIT-regulated (Mai et al., 2016). Within these 85 genes, genes involved in Fe uptake,like IRT1, ABCG37 or F6’H1, but inFe homeostasis, e.g. naturalresistance-associated macrophage protein1 (NRAMP1), NICOTIANAMINESYNTHASE1 (NAS1) or ZRT/IRT-LIKE PROTEIN8 (ZIP8)are present (Colangelo andGuerinot, 2004; Mai et al.