Small-Molecules Selectively Modulate Iron-Deficiency Signaling Networks in Arabidopsis

Sakthivel Kailasam,Kuo-Chen Yeh,Wei-Fu Chien

doi:10.3389/fpls.2019.00008

Abstract

Plant growth requires optimal levels of iron (Fe). Fe is used for energy production, numerous enzymatic processes, and is indispensable for cellular metabolism. Recent studies have established the mechanism involved in Fe uptake and transport. However, our knowledge of Fe sensing and signaling is limited. Dissecting Fe signaling may be useful for crop improvement by Fe fortification. Here, we report two small-molecules, R3 and R6 [where R denotes repressor of IRON-REGULATED TRANSPORTER 1 (IRT1)], identified through a chemical screening, whose use blocked activation of the Fe-deficiency response in Arabidopsis thaliana. Physiological analysis of plants treated with R3 and R6 showed that these small molecules drastically attenuated the plant response to Fe starvation. Small-molecule treatment caused severe chlorosis and strongly reduced chlorophyll levels in plants. Fe content in shoots was decreased considerably by small-molecule treatments especially in Fe deficiency. Small-molecule treatments attenuated the Fe-deficiency-induced expression of the Fe uptake gene IRT1. Analysis of FER-LIKE IRON-DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT) and subgroup Ib basic helix-loop-helix (bHLH) gene (bHLH38/39/100/101) expression showed that R3 affects the FIT-network, whereas R6 affects both the FIT and Ib bHLH networks. An assessment of the effects of the structural analogs of R3 and R6 on the induction of Fe-dependent chlorosis revealed the functional motif of the investigated chemicals. Our findings suggest that small-molecules selectively modulate the distinct signaling routes that operate in response to Fe-deficiency. R3 and R6 likely interrupt the activity of key upstream signaling regulators whose activities are required for the activation of the Fe-starvation transcriptional cascade in Arabidopsis roots.

Highlights

Many cellular functions occurring during plant growth and development depend on iron (Fe) availability; plants regulate Fe homeostasis by tightly controlling its uptake and allocation
Our finding clearly reveals that these small-molecules modulate Fe-deficiency by targeting specific signaling branches to central transcription factors, further suggesting that multiple routes are used for transferring the Fe-deficiency born signals to the central transcription factors in roots
These results suggest that R3 and R6 may modulate endogenous IRON-REGULATED TRANSPORTER 1 (IRT1) expression

Summary

Introduction

Many cellular functions occurring during plant growth and development depend on iron (Fe) availability; plants regulate Fe homeostasis by tightly controlling its uptake and allocation. For Fe uptake, large amounts of coumarins, facilitated by PLEIOTROPIC DRUG RESISTANCE 9 (PDR9) (Fourcroy et al, 2014; Clemens and Weber, 2016) and protons, mediated by H+-ATPASE 2 (AHA2) (Santi and Schmidt, 2009) are pumped into the rhizosphere. These processes help to solubilize and mobilize the insoluble ferric Fe (Fe3+) in the rhizosphere (Chen et al, 2017; Jeong et al, 2017). The IRON-REGULATED TRANSPORTER 1 (IRT1), a plasma membrane localized divalent cation transporter, imports ferrous Fe from the extracellular space (Connolly et al, 2002; Vert et al, 2002)

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