Fitting into the Harsh Reality: Regulation of Iron-deficiency Responses in Dicotyledonous Plants  Ivanov Rumen , Brumbarova Tzvetina , Bauer Petra   Molecular Plant  Volume 5, Issue 1, Pages 27-42 (January 2012) DOI: 10.1093/mp/ssr065 Copyright © 2012 The Authors. All rights reserved. Terms and Conditions

Figure 1 Reduction-Based Strategy for Iron Uptake in the Model Plant Arabidopsis thaliana. (A) Iron is first solubilized by rhizosphere acidification through the action of the H+-ATPase AHA2, and is then reduced from ferric (Fe3+) to ferrous (Fe2+) iron by the reductase FRO2. Bivalent iron is then imported into the root cell by the metal transporter IRT1. The activity of this uptake system is dependent on the action of the transcription factor FIT. Under iron deficiency, up-regulation of the iron-responsive genes is achieved through a complex including FIT and at least one of the two bHLH proteins (bHLH038 and bHLH039), and presumably also bHLH100 and bHLH101. The FIT gene is induced by this system and thus it undergoes a feed-forward regulation, where the gene product positively regulates the source gene. Post-transcriptional regulation of protein abundance is not depicted here. Induction of FRO2 and IRT1 activity is co-regulated in response to iron deficiency, while that of AHA2 seems to be regulated in an independent manner (yellow arrows). (B) Expression of the AHA2 gene is dependent on FIT. Expression was analyzed by quantitative RT–PCR in wild-type Arabidopsis (Col-0 ecotype), a FIT overexpressing line (FIT Ox), and the fit-3 mutant line under iron-sufficient (50 μM Fe) or iron-deficient (0 μM Fe) conditions. AHA2 gene was induced by iron deficiency in both Col-0 and the FIT Ox line but not in the fit-3 mutant. Therefore, FIT is necessary but not sufficient for AHA2 expression. Expression of FIT and IRT1 was monitored as control. t-tests showed all inductions to be statistically significant (p-value < 0.005). Molecular Plant 2012 5, 27-42DOI: (10.1093/mp/ssr065) Copyright © 2012 The Authors. All rights reserved. Terms and Conditions

Figure 2 Hypothetical Model of the Regulation of Iron Mobilization by PYE. PYE and BTS have opposite effects on plant mobilization judged by the phenotypes of the corresponding mutants. Both proteins were shown to interact with ILR3 and bHLH115. It was suggested that the PYE–ILR3/bHLH115 interaction might negatively regulate the known PYE target genes ZIF1, FRO3, and NAS4. On the other hand, it can be speculated that the BTS–ILR3/bHLH115 interaction might have the opposite effect. Additionally, PYE represses BTS expression, thus probably influencing BTS protein abundance. Thus, a fine balance in the regulation of iron mobilization is achieved. This model is based on the assumption that these protein–protein interactions occur in plant cells and that the BTS–ILR3/bHLH115 complex influences directly or indirectly the three target genes. Molecular Plant 2012 5, 27-42DOI: (10.1093/mp/ssr065) Copyright © 2012 The Authors. All rights reserved. Terms and Conditions

Figure 3 Iron Supply-Dependent Co-Expression Networks in Arabidopsis Roots. Genes stably up-regulated upon iron deficiency presented in Table 1 (labeled here in yellow) were used as a basis for the generation of the co-expression network. The cluster containing the PYE–BTS regulon is surrounded by a dashed red line. Networks were generated by the ATTED-II NetworkDrawer tool and the data were further processed using the Cytoscape Software. Molecular Plant 2012 5, 27-42DOI: (10.1093/mp/ssr065) Copyright © 2012 The Authors. All rights reserved. Terms and Conditions

Figure 4 Iron Supply-Dependent Co-Expression Networks in Arabidopsis Leaves. Genes stably up-regulated upon iron-sufficient (A) or deficient conditions (B–D) presented in Table 1 (labeled here in yellow) were used as a basis for the generation of the co-expression networks. In (D), the cluster containing the PYE–BTS regulon is surrounded by a dashed red line and that enriched in cell cycle and DNA repair genes by a dashed blue line. Networks were generated by the ATTED-II NetworkDrawer tool and the data were further processed using the Cytoscape Software. Molecular Plant 2012 5, 27-42DOI: (10.1093/mp/ssr065) Copyright © 2012 The Authors. All rights reserved. Terms and Conditions