The transcription of genes involved in
iron acquisition in plants is induced under
iron deficiency, but our understanding of
iron sensors and signals remains limited.
Iron Deficiency-responsive
Element-binding Factor 1 (IDEF1) and
Hemerythrin motif-containing Really Interesting New Gene- and Zinc-finger
proteins (HRZs)/BRUTUS (BTS) have recently emerged as candidate
iron sensors because of their functions as potent regulators of
iron deficiency responses and their
iron-binding properties. IDEF1 is a central transcriptional regulator of graminaceous genes involved in
iron uptake and utilization, predominantly during the early stages of
iron deficiency. HRZs/BTS are E3
ubiquitin ligases and negative regulators of
iron deficiency responses in both graminaceous and non-graminaceous plants. Rice OsHRZ1 and OsHRZ2 are also potent regulators of
iron accumulation. Characterizing these putative
iron sensors also provides clues to understanding the nature of
iron signals, which may involve ionized
iron itself, other metals,
oxygen, redox status,
heme and
iron-
sulfur clusters, in addition to metabolites affected by
iron deficiency. Systemic
iron responses may also be regulated by phloem-mobile
iron and its
chelators such as
nicotianamine.
Iron sensors and signals will be identified by demonstration of signal transmission by IDEF1, HRZs/BTS, or unknown factors.