FIT-Binding Proteins and Their Functions in the Regulation of Fe Homeostasis

Abstract

Iron, as an essential micronutrient, is required by all living organisms. In plants, the deficiency and excess of iron will impair their growth and development. For maintaining a proper intracellular iron concentration, plants evolved different regulation mechanisms to tightly control iron uptake, translocation and storage. FIT, a bHLH transcription factor, is the master regulator of the iron deficiency responses and homeostasis in Arabidopsis. It interacts with different proteins, functioning in controlling the expression of various genes involved in iron uptake and homeostasis. In this review, we summarize the recent progress in the studies of FIT and FIT-binding proteins, and give an overview of FIT-regulated network in iron deficiency response and homeostasis.

Keywords: Arabidopsis; FIT; FIT-binding protein; iron; plant nutrition; transcriptional regulation.

FIGURE 1

Outline of the regulation networks and functions of FIT-binding proteins in iron deficiency responses and homeostasis in root epidermis cells (upper part) and stele cells (lower part). In root epidermal cells, Ib bHLHs, MED16, MED25, EIN3/EIL1, and CIPK11 as positive modulators interacts with the key regulator FIT, functioning in activation of the expression of iron acquisition genes such as FRO2 and IRT1 under iron limitation condition, whereas DELLA, ZAT12, and IVa bHLHs as negative modulators compete with Ib bHLHs to bind FIT for negative regulation of FIT activity for avoidance of excessive iron uptake. In stele cells, FBP as a negative regulator sequestered FIT to obstruct the heterodimer formation of FIT with Ib bHLH protein to downregulate the expression of NAS1, NAS2, and NAS4 for balancing Fe and Zn homeostasis in Arabidopsis under low iron or high Zn stress condition. The rectangles represent transcripts of the corresponding genes, and the ovals mean proteins indicated. The blue and orange color depicted positive and negative regulators, respectively. The solid and dotted lines separately indicate the known and unknown regulation processes.