Supraoptimal Iron Nutrition of Brassica napus Plants Suppresses the Iron Uptake of Chloroplasts by Down-Regulating Chloroplast Ferric Chelate Reductase.

Máté Sági-Kazár,Anna Koncz,Ádám Solti,Sophie Z Farkas,Tamás Visnovitz,Ferenc Fodor,Helga Zelenyánszki,Balázs Gyuris,Barbara Bánkúti,Béla Kovács,Barnabás Cseh,Brigitta Müller,Kálmán Szenthe,Brigitta Tóth,Ferenc Bánáti,Edit I Buzás

doi:10.3389/fpls.2021.658987

Abstract

Iron (Fe) is an essential micronutrient for plants. Due to the requirement for Fe of the photosynthetic apparatus, the majority of shoot Fe content is localised in the chloroplasts of mesophyll cells. The reduction-based mechanism has prime importance in the Fe uptake of chloroplasts operated by Ferric Reductase Oxidase 7 (FRO7) in the inner chloroplast envelope membrane. Orthologue of Arabidopsis thaliana FRO7 was identified in the Brassica napus genome. GFP-tagged construct of BnFRO7 showed integration to the chloroplast. The time-scale expression pattern of BnFRO7 was studied under three different conditions: deficient, optimal, and supraoptimal Fe nutrition in both leaves developed before and during the treatments. Although Fe deficiency has not increased BnFRO7 expression, the slight overload in the Fe nutrition of the plants induced significant alterations in both the pattern and extent of its expression leading to the transcript level suppression. The Fe uptake of isolated chloroplasts decreased under both Fe deficiency and supraoptimal Fe nutrition. Since the enzymatic characteristics of the ferric chelate reductase (FCR) activity of purified chloroplast inner envelope membranes showed a significant loss for the substrate affinity with an unchanged saturation rate, protein level regulation mechanisms are suggested to be also involved in the suppression of the reduction-based Fe uptake of chloroplasts together with the saturation of the requirement for Fe.

Highlights

Iron (Fe)-dependent redox reactions occur in a wide range of plant metabolic processes (Connorton et al, 2017)
Regarding the 6th leaves, the effect of dFe treatment was the most pronounced in the accumulation of S and Mo, and slight accumulation was found for Ca and Mg at the very end of the time of treatment
We identified B. napus orthologue of AtFRO7 based on sequence similarities, the existence of chloroplast targeting peptide and the incorporation of BnFRO7-GFP fusion construct to the chloroplast envelope

Summary

Introduction

Iron (Fe)-dependent redox reactions occur in a wide range of plant metabolic processes (Connorton et al, 2017). 80– 90% of cellular Fe is found in chloroplasts (Hantzis et al, 2018), localised mainly in photosystem I and under the excess of Fe or if the photosynthetic apparatus decomposes, in ferritin (Chen et al, 2019). For the operation of a single photosynthetic electron transport chain, a total of 22 Fe atoms are needed in heme groups of photosystem I and cytochrome b6/f, in Fe–S clusters of PSI, cytochrome b6/f and ferredoxin and as non-heme ferrocofactors in photosystem II [for review, see Schmidt et al (2020)]. Incorporation of Fe into Fe–S clusters has a priority that is coupled to the Fe acquisition of chloroplasts (Solti et al, 2012, 2016). Fe deficiency causes chlorosis on the leaves and a significant decrease in biomass production (Hantzis et al, 2018)

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