Characterization of the Nicotianamine Exporter ENA1 in Rice

Tomoko Nozoye,Naoko K Nishizawa,Nicolaus Von Wirén,Tetsuya Higashiyama,Yoshikatsu Sato,Hiromi Nakanishi

doi:10.3389/fpls.2019.00502

Abstract

Under iron (Fe) deficiency, graminaceous plants produce and secrete Fe-chelating phytosiderophores of the mugineic acid (MA) family into the rhizosphere to solubilize and mediate uptake of sparingly soluble Fe in the soil. MAs and their biosynthetic intermediate, nicotianamine (NA), are also important for the translocation of divalent metals such as Fe and zinc (Zn) throughout the plant body. In this study, the physiological role of the efflux transporter EFFLUX TRANSPORTER OF NA (ENA1), which exports NA out of cells, was analyzed in rice. Promoter-GUS analysis showed that ENA1 was mainly expressed in roots, and strongly upregulated under Fe-deficient conditions. In epidermal onion cells and rice roots, green fluorescent protein-tagged ENA1 localized mainly to the plasma membrane, while a part of the fluorescence was observed in vesicular structures in the cytoplasm. In the younger stage after germination, ENA1-overexpressing rice plants exhibited truncated roots with many root hairs compared to wild-type plants, while these phenotype were not observed in high Zn-containing medium. In Arabidopsis, which use a different strategy for Fe uptake from rice, ENA1 overexpression did not show any apparent phenotypes. Oligo DNA microarray analysis in rice showed that ENA1 knockout affects the response to stress, especially in root plastids. These results suggest that ENA1 might be recycling between the plasma membrane and cellular compartments by vesicular transport, playing an important role in the transport of NA, which is important for the physiological response to Fe deficiency.

Highlights

Iron (Fe) is an essential element for all living organisms
Since the discovery of mugineic acid (MA), we have explored strategies to acquire insoluble Fe from the soil, and the molecular machinery involved in Fe acquisition in graminaceous plants has been largely identified (Kobayashi et al, 2018)
It has been suggested that the transporters for MAs and NA are involved in the regulation of metal flows in planta

Summary

Introduction

Iron (Fe) is an essential element for all living organisms. Fe plays a key role in electron transfer in both photosynthetic and respiratory reactions in chloroplasts and mitochondria. Fe deficiency leads to leaf chlorosis and decreased plant yield. Excess Fe is toxic, because it catalyzes the generation of free radicals. To acquire Fe from the rhizosphere, plants have evolved two strategies (Marschner et al, 1986). In strategy-I plants, Fe is mobilized via coumarin-type siderophores (Schmid et al, 2014; Rajniak et al, 2018; Tsai et al, 2018) and Fe(III) reduction prior to uptake in the form of ferrous Fe (Eide et al, 1996; Robinson et al, 1999; Vert et al, 2002). In strategy-II plants, Fe is mobilized via chelation through mugineic acid

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Results

Conclusion