A accumulates in all root cell types.

A second, FIT-independent gene network (Figure 3)
controls various aspects within the Fe deficiency response, putting a focus on
the subgroup IVb bHLH protein PYE and its homologs (Heim et al., 2003). PYE
was found in a T-DNA screen for transcription factors, being impaired in Fe
stress tolerance. PYE is mainly
upregulated in the pericycle upon Fe deficiency, but PYE protein accumulates in
all root cell types. pye-1 knock-out mutants
display altered formation of root hairs, deformation of lateral roots cells,
severe leaf chlorosis and reduced root growth, chlorophyll- and Fe content under
Fe limited conditions. Hence, PYE is suggested to positively regulate Fe
deficiency responses in plants (Long et al., 2010). However, it negatively regulates
transcription of Fe homeostasis genes such as FERRIC REDUCTASE OXIDASE3 (FRO3),
NAS4 and ZINC-INDUCED FACILITATOR1 (ZIF1)
by binding to their promotors under Fe deficiency (Long et al., 2010). The protein interacts with all
four homologues from subgroup IVc bHLH034, bHLH104, bHLH105 (ILR3) and bHLH115,
collective called PYE-likes (PYELs) (Heim et al., 2003; Long et al., 2010; Selote et al.,
2015; Zhang et al., 2015). All PYELs
can form homo- and hetero-dimers (Heim et al., 2003; Selote et al., 2015; Zhang et al.,
2015; Li et al., 2016; Liang et al., 2017). Overexpression
of BHLH101 partially rescues the bhlh034 bhlh104 double mutant phenotype,
suggesting that PYELs are upstream regulators of subgroup Ib BHLHs (Li et al., 2016). Indeed, they bind to the promoters
of PYE and BHLH038/039/100/101, however, the binding of bHLH034 to PYEpro is only anticipated (Zhang et al., 2015; Li et al., 2016; Liang et al.,
2017). The
generation of PYELs knock-out mutants demonstrate reduced tolerance to Fe
deficiency, whereat PYELs overexpression lines show increased accumulation of Fe
and improved fitness. Hence, PYELs positively mediate Fe deficiency responses (Zhang et al., 2015; Li et al., 2016; Liang et al.,
2017). The
decreased Fe stress tolerance phenotype of bhlh115-1
knock-out mutant could be rescued by the overexpression of any PYEL, suggesting
functional redundancy. Although, it cannot be ruled out that PYELs might have additive,
tissue specific functions, since they only in part display the same expression
pattern (Liang et al., 2017).