The developmental and iron nutritional pattern of PIC1 and NiCo does not support their interdependent and exclusive collaboration in chloroplast iron transport in Brassica napus

Characterization of genes involved in micronutrients and toxic metals detoxification in Brassica napus by genome-wide cDNA library screening

Stresses caused by deficiency/excess of mineral nutrients or of pollution of toxic metals have already become a primary factor in limiting crop production worldwide. Genes involved in minerals and toxic metals accumulation/tolerance could be potential candidates for improving crop plants with enhanced nutritional efficiency and environmental adaptability. In this study, we first generated a high-quality yeast expression cDNA library of Brassica napus (Westar), and 46 genes mediating excess micronutrients and toxic metals detoxification were screened using the yeast genetic complementation system, including 11, 5, 6, 14, 6, and 5 genes involved in cadmium (Cd), zinc (Zn), iron (Fe), manganese (Mn), boron (B), and copper (Cu) tolerance, respectively. Characterization of genes mediating excess ions stress resistance in this study is beneficial for us to further understand ions homeostasis in B. napus.

Iron transport mechanisms and their evolution focusing on chloroplasts

Iron (Fe) is an essential element for photosynthetic organisms, required for several vital biological functions. Photosynthesis, which takes place in the chloroplasts of higher plants, is the major Fe consumer. Although the components of the root Fe uptake system in dicotyledonous and monocotyledonous plants have been extensively studied, the Fe transport mechanisms of chloroplasts in these two groups of plants have received little attention. This review focuses on the comparative analysis of Fe transport processes in the evolutionary ancestors of chloroplasts (cyanobacteria) with the processes in embryophytes and green algae (Viridiplantae). The aim is to summarize how chloroplasts are integrated into cellular Fe homeostasis and how Fe transporters and Fe transport mechanisms have been modified by evolution.

Iron uptake of etioplasts is independent from photosynthesis but applies the reduction-based strategy

Introduction

Iron (Fe) is one of themost important cofactors in the photosynthetic apparatus, and its uptake by chloroplasts has also been associated with the operation of the photosynthetic electron transport chain during reduction-based plastidial Fe uptake. Therefore, plastidial Fe uptake was considered not to be operational in the absence of the photosynthetic activity. Nevertheless, Fe is also required for enzymatic functions unrelated to photosynthesis, highlighting the importance of Fe acquisition by non-photosynthetic plastids. Yet, it remains unclear how these plastids acquire Fe in the absence of photosynthetic function. Furthermore, plastids of etiolated tissues should already possess the ability to acquire Fe, since the biosynthesis of thylakoid membrane complexes requires a massive amount of readily available Fe. Thus, we aimed to investigate whether the reduction-based plastidial Fe uptake solely relies on the functioning photosynthetic apparatus.

Methods

In our combined structure, iron content and transcript amount analysis studies, we used Savoy cabbage plant as a model, which develops natural etiolation in the inner leaves of the heads due to the shading of the outer leaf layers.

Results

Foliar and plastidial Fe content of Savoy cabbage leaves decreased towards the inner leaf layers. The leaves of the innermost leaf layers proved to be etiolated, containing etioplasts that lacked the photosynthetic machinery and thus were photosynthetically inactive. However, we discovered that these etioplasts contained, and were able to take up, Fe. Although the relative transcript abundance of genes associated with plastidial Fe uptake and homeostasis decreased towards the inner leaf layers, both ferric chelate reductase FRO7 transcripts and activity were detected in the innermost leaf layer. Additionally, a significant NADP(H) pool and NAD(P)H dehydrogenase activity was detected in the etioplasts of the innermost leaf layer, indicating the presence of the reducing capacity that likely supports the reduction-based Fe uptake of etioplasts.

Discussion

Based on these findings, the reduction-based plastidial Fe acquisition should not be considered exclusively dependent on the photosynthetic functions.

Iron in leaves: chemical forms, signalling, and in-cell distribution

Iron (Fe) is an essential transition metal. Based on its redox-active nature under biological conditions, various Fe compounds serve as cofactors in redox enzymes. In plants, the photosynthetic machinery has the highest demand for Fe. In consequence, the delivery and incorporation of Fe into cofactors of the photosynthetic apparatus is the focus of Fe metabolism in leaves. Disturbance of foliar Fe homeostasis leads to impaired biosynthesis of chlorophylls and composition of the photosynthetic machinery. Nevertheless, mitochondrial function also has a significant demand for Fe. The proper incorporation of Fe into proteins and cofactors as well as a balanced intracellular Fe status in leaf cells require the ability to sense Fe, but may also rely on indirect signals that report on the physiological processes connected to Fe homeostasis. Although multiple pieces of information have been gained on Fe signalling in roots, the regulation of Fe status in leaves has not yet been clarified in detail. In this review, we give an overview on current knowledge of foliar Fe homeostasis, from the chemical forms to the allocation and sensing of Fe in leaves.

New aspects of the photodegradation of iron(III) citrate: spectroscopic studies and plant-related factors

Iron (Fe) is an essential cofactor for all livings. Although Fe membrane transport mechanisms often utilize Fe^II, uncoordinated or deliberated ferrous ions can initiate Fenton reactions. Fe^III citrate complexes are among the most important complexed forms of Fe^III especially in plants that, indeed, can undergo photoreduction. Since leaves as photosynthetic organs of higher plants are generally exposed to illumination in daytime, photoreaction of ferric species may have biological relevance in iron metabolism, the relevance of which is poorly understood. In present work Fe^III citrate transformation during the photodegradation in solution and after foliar application on leaves was studied by Mössbauer analysis directly. To obtain irradiation time dependence of the speciation of iron in solutions, four model solutions of different pH values (1.5, 3.3, 5.5, and 7.0) with Fe to citrate molar ratio 1:1.1 were exposed to light. Highly acidic conditions led to a complete reduction of Fe together with the formation of Fe^II citrate and hexaaqua complexes in equal concentration. At higher pH, the only product of the photodegradation was Fe^II citrate, which was later reoxidized and polymerized, resulting in the formation of polynuclear stable ferric compound. To test biological relevance, leaves of cabbage were treated with Fe^III citrate solution. X-ray fluorescence imaging indicated the accumulation of Fe in the treated leaf parts. Mössbauer analysis revealed the presence of several ferric species incorporated into the biological structure. The Fe speciation observed should be considered in biological systems where Fe^III citrate has a ubiquitous role in Fe acquisition and homeostasis.

Ferroportin 3 is a dual-targeted mitochondrial/chloroplast iron exporter necessary for iron homeostasis in Arabidopsis

Mitochondria and chloroplasts are organelles with high iron demand that are particularly susceptible to iron-induced oxidative stress. Despite the necessity of strict iron regulation in these organelles, much remains unknown about mitochondrial and chloroplast iron transport in plants. Here, we propose that Arabidopsis ferroportin 3 (FPN3) is an iron exporter that is dual-targeted to mitochondria and chloroplasts. FPN3 is expressed in shoots, regardless of iron conditions, but its transcripts accumulate under iron deficiency in roots. fpn3 mutants cannot grow as well as the wild type under iron-deficient conditions and their shoot iron levels are lower compared with the wild type. Analyses of iron homeostasis gene expression in fpn3 mutants and inductively coupled plasma mass spectrometry (ICP-MS) measurements show that iron levels in the mitochondria and chloroplasts are increased relative to the wild type, consistent with the proposed role of FPN3 as a mitochondrial/plastid iron exporter. In iron-deficient fpn3 mutants, abnormal mitochondrial ultrastructure was observed, whereas chloroplast ultrastructure was not affected, implying that FPN3 plays a critical role in the mitochondria. Overall, our study suggests that FPN3 is essential for optimal iron homeostasis.

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

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.

IRONing out stress problems in crops: a homeostatic perspective

Iron (Fe) is essential for plant growth and therefore plays a key role in influencing crop productivity worldwide. Apart from its central role in chlorophyll biosynthesis and oxidative phosphorylation (electron transfer), it is an important constituent of many enzymes involved in primary metabolism. Fe has different accessibilities to the roots in the rhizosphere depending upon whether it is ferrous (soluble) or ferric (insoluble) oxidation stages, which in turn, determine two kinds of Fe uptake strategies employed by the plants. The reduction strategy is exclusively found in non-graminaceous plants wherein the ferrous Fe²⁺ is absorbed and translocated from the soil through specialized transporters. In contrast, the chelation strategy (widespread in graminaceous plants) relies on the formation of Fe (III)-chelate complex as the necessary requirement of Fe uptake. Once inside the cell, Fe is translocated, compartmentalized and stored through a common set of physiological processes involving many transporters and enzymes whose functions are controlled by underlying genetic components, so that a fine balance of Fe homeostasis is maintained. Recently, molecular and mechanistic aspects of the process involving the role of transcription factors, signaling components, and cis-acting elements have been obtained, which has enabled a much better understanding of its ecophysiology. This mini-review summarizes recent developments in our understanding of Fe transport in higher plants with particular emphasis on crops in the context of major agronomically important abiotic stresses. It also highlights outstanding questions on the regulation of Fe homeostasis and lists potentially useful genes/regulatory pathways that may be useful for subsequent crop improvement under the stresses discussed through either conventional or transgenic approaches.

A rice chloroplast-localized ABC transporter ARG1 modulates cobalt and nickel homeostasis and contributes to photosynthetic capacity

Transport and homeostasis of transition metals in chloroplasts, which are accurately regulated to ensure supply and to prevent toxicity induced by these metals, are thus crucial for chloroplast function and photosynthetic performance. However, the mechanisms that maintain the balance of transition metals in chloroplasts remain largely unknown. We have characterized an albino-revertible green 1 (arg1) rice mutant. ARG1 encodes an evolutionarily conserved protein belonging to the ATP-binding cassette (ABC) transporter family. Protoplast transfection and immunogold-labelling assays showed that ARG1 is localized in the envelopes and thylakoid membranes of chloroplasts. Measurements of metal contents, metal transport, physiological and transcriptome changes revealed that ARG1 modulates cobalt (Co) and nickel (Ni) transport and homeostasis in chloroplasts to prevent excessive Co and Ni from competing with essential metal cofactors in chlorophyll and metal-binding proteins acting in photosynthesis. Natural allelic variation in ARG1 between indica and temperate japonica subspecies of rice is coupled with their different capabilities for Co transport and Co content within chloroplasts. This variation underpins the different photosynthetic capabilities in these subspecies. Our findings link the function of the ARG1 transporter to photosynthesis, and potentially facilitate breeding of rice cultivars with improved Co homeostasis and consequently improved photosynthetic performance.