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. 2021 Jan 15;72(6):2045–2055. doi: 10.1093/jxb/erab003

Fig. 3.

Fig. 3.

bHLH-dependent transcriptional regulation of Fe deficiency in Arabidopsis thaliana. During Fe deficiency, URI (bHLH121) is phosphorylated and interacts with subgroup IVc bHLH transcription factors, bHLH34, bHLH104, bHLH115, and bHLH105. These heterodimers transcriptionally regulate expression of genes including subgroup Ib bHLH transcription factors, BTS, BTSL, and PYE (indicated by blue arrows). Heterodimerization of FIT with subgroup Ib bHLHs induces expression of Fe uptake genes IRT1 and FRO2. PYE is a known transcriptional repressor that inhibits expression of genes involved in Fe transport and sequestration. BTS and BTSL are also negative regulators of Fe deficiency whose expression is induced by Fe deficiency. During Fe-sufficient conditions, BTS and BTSL bind to available Fe and interact with members of bHLH subgroups IVc and Ib, and URI and FIT transcription factors, and cause their proteasomal degradation (indicated by red arrows). Fe-dependent turnover by BTS and BTSL is crucial for tight regulation of Fe deficiency response to prevent toxic effects of excess Fe. Abbreviations used are –Fe, Fe deficiency; +Fe, Fe sufficiency; P, phosphorylation; Fe, iron metal. Rounded rectangles and circle, proteins; rectangles, genes; solid black arrow, transcription and translation; blue arrows, transcriptional regulation; red arrows; proteasomal degradation; purple cylinder, BTS and BTSL; red X, transcriptional repression.