The MAR1 transporter is an opportunistic entry point for antibiotics

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.

Research progress on the biosynthesis and delivery of iron-sulfur clusters in the plastid

Iron-sulfur (Fe-S) clusters are ancient protein cofactors ubiquitously exist in organisms. They are involved in many important life processes. Plastids are semi-autonomous organelles with a double membrane and it is believed to originate from a cyanobacterial endosymbiont. By learning form the research in cyanobacteria, a Fe-S cluster biosynthesis and delivery pathway has been proposed and partly demonstrated in plastids, including iron uptake, sulfur mobilization, Fe-S cluster assembly and delivery. Fe-S clusters are essential for the downstream Fe-S proteins to perform their normal biological functions. Because of the importance of Fe-S proteins in plastid, researchers have made a lot of research progress on this pathway in recent years. This review summarizes the detail research progress made in recent years. In addition, the scientific problems remained in this pathway are also discussed.

A dynamic compartment model for xylem loading and long-distance transport of iron explains the effect of kanamycin on metal uptake in Arabidopsis

Arabidopsis plants exposed to the antibiotic kanamycin (Kan) display altered metal homeostasis. Further, mutation of the WBC19 gene leads to increased sensitivity to kanamycin and changes in iron (Fe) and zinc (Zn) uptake. Here we propose a model that explain this surprising relationship between metal uptake and exposure to Kan. We first use knowledge about the metal uptake phenomenon to devise a transport and interaction diagram on which we base the construction of a dynamic compartment model. The model has three pathways for loading Fe and its chelators into the xylem. One pathway, involving an unknown transporter, loads Fe as a chelate with citrate (Ci) into the xylem. This transport step can be significantly inhibited by Kan. In parallel, FRD3 transports Ci into the xylem where it can chelate with free Fe. A third critical pathway involves WBC19, which transports metal-nicotianamine (NA), mainly as Fe-NA chelate, and possibly NA itself. To permit quantitative exploration and analysis, we use experimental time series data to parameterize this explanatory and predictive model. Its numerical analysis allows us to predict responses by a double mutant and explain the observed differences between data from wildtype, mutants and Kan inhibition experiments. Importantly, the model provides novel insights into metal homeostasis by permitting the reverse-engineering of mechanistic strategies with which the plant counteracts the effects of mutations and of the inhibition of iron transport by kanamycin.

Golgi-Localized OsFPN1 is Involved in Co and Ni Transport and Their Detoxification in Rice

Cobalt (Co) and nickel (Ni) are beneficial and essential elements for plants, respectively, with the latter required for urease activity, which hydrolyzes urea into ammonium in plants. However, excess Co and Ni are toxic to plants and their transport mechanisms in rice are unclear. Here, we analyzed an ethyl methanesulfonate (EMS)-mutagenized rice mutant, 1187_n, with increased Co and Ni contents in its brown rice and shoots. 1187_n has a mutation in OsFPN1, which was correlated with a high Co and Ni phenotype in F₂ crosses between the parental line and mutant. In addition, CRISPR/Cas9 mutants exhibited a phenotype similar to that of 1187_n, demonstrating that OsFPN1 is the causal gene of the mutant. In addition to the high Co and Ni in brown rice and shoots, the mutant also exhibited high Co and Ni concentrations in the xylem sap, but low concentrations in the roots, suggesting that OsFPN1 is involved in the root-to-shoot translocation of Co and Ni. The growth of 1187_n and CRISPR/Cas9 lines were suppressed under high Co and Ni condition, indicating OsFPN1 is required for the normal growth under high Co and Ni. An OsFPN1-green fluorescent protein (GFP) fusion protein was localized to the Golgi apparatus. Yeast carrying GFP-OsFPN1 increased sensitivity to high Co contents and decreased Co and Ni accumulation. These results suggest that OsFPN1 can transport Co and Ni and is vital detoxification in rice.

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.

Knockout of Tobacco Homologs of Arabidopsis Multi-Antibiotic Resistance 1 Gene Confers a Limited Resistance to Aminoglycoside Antibiotics

To explore a possible recessive selective marker for future DNA-free genome editing by direct delivery of a CRISPR/Cas9-single guide RNA (sgRNA) ribonucleoprotein complex, we knocked out homologs of the Arabidopsis Multi-Antibiotic Resistance 1 (MAR1)/RTS3 gene, mutations of which confer aminoglycoside resistance, in tobacco plants by an efficient Agrobacterium-mediated gene transfer. A Cas9 gene was introduced into Nicotiana tabacum and Nicotiana sylvestris together with an sgRNA gene for one of three different target sequences designed to perfectly match sequences in both S- and T-genome copies of N. tabacum MAR1 homologs (NtMAR1hs). All three sgRNAs directed the introduction of InDels into NtMAR1hs, as demonstrated by CAPS and amplicon sequencing analyses, albeit with varying efficiency. Leaves of regenerated transformant shoots were evaluated for aminoglycoside resistance on shoot-induction media containing different aminoglycoside antibiotics. All transformants tested were as sensitive to those antibiotics as non-transformed control plants, regardless of the mutation rates in NtMAR1hs. The NtMAR1hs-knockout seedlings of the T1 generation showed limited aminoglycoside resistance but failed to form shoots when cultured on shoot-induction media containing kanamycin. The results suggest that, like Arabidopsis MAR1, NtMAR1hs have a role in plants' sensitivity to aminoglycoside antibiotics, and that tobacco has some additional functional homologs.

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.

Veterinary antibiotics and plant physiology: An overview

Antibiotics are considered one of the greatest advances of medicine and, in addition to their use in treating a wide spectrum of illnesses, they have been widely employed to promote animal growth. As many of those pharmaceuticals are only partially absorbed by the digestive system, a considerable fraction is excreted in its original active form or only partially metabolized. Therefore, the use of animal excrement in agriculture represents one of the principal routes of insertion of antibiotics into the environment. Within that context, plants, principally those of agricultural interest, will be exposed to those compounds when present in the soil or when irrigated with contaminated water. Although not yet fully understood, there are reports of phytotoxic effects of antibiotics that can diminish agricultural production. This review is designed to provide a general and integrative overview of physiological alterations observed in plants caused by environmental exposures to veterinary-use antibiotics. This text principally focuses on the processes involved in antibody absorption and accumulation, and their effects on the primary (photosynthesis, respiration, nitrogen assimilation) and oxidative metabolisms of plants. We also bring attention to germinative and plant establishment processes under conditions of antibiotic contamination. The different effects of different antibiotics on plant physiology are listed here to provide a better understanding of their phytotoxicities.

Roles of subcellular metal homeostasis in crop improvement

Improvement of crop production in response to rapidly changing environmental conditions is a serious challenge facing plant breeders and biotechnologists. Iron (Fe), zinc (Zn), manganese (Mn), and copper (Cu) are essential micronutrients for plant growth and reproduction. These minerals are critical to several cellular processes including metabolism, photosynthesis, and cellular respiration. Regulating the uptake and distribution of these minerals could significantly improve plant growth and development, ultimately leading to increased crop production. Plant growth is limited by mineral deficiency, but on the other hand, excess Fe, Mn, Cu, and Zn can be toxic to plants; therefore, their uptake and distribution must be strictly regulated. Moreover, the distribution of these metals among subcellular organelles is extremely important for maintaining optimal cellular metabolism. Understanding the mechanisms controlling subcellular metal distribution and availability would enable development of crop plants that are better adapted to challenging and rapidly changing environmental conditions. Here, we describe advances in understanding of subcellular metal homeostasis, with a particular emphasis on cellular Fe homeostasis in Arabidopsis and rice, and discuss strategies for regulating cellular metabolism to improve plant production.

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

The accumulation of NiCo following the termination of the accumulation of iron in chloroplast suggests that NiCo is not solely involved in iron uptake processes of chloroplasts. Chloroplast iron (Fe) uptake is thought to be operated by a complex containing permease in chloroplast 1 (PIC1) and nickel-cobalt transporter (NiCo) proteins, whereas the role of other Fe homeostasis-related transporters such as multiple antibiotic resistance protein 1 (MAR1) is less characterized. Although pieces of information exist on the regulation of chloroplast Fe uptake, including the effect of plant Fe homeostasis, the whole system has not been revealed in detail yet. Thus, we aimed to follow leaf development-scale changes in the chloroplast Fe uptake components PIC1, NiCo and MAR1 under deficient, optimal and supraoptimal Fe nutrition using Brassica napus as model. Fe deficiency decreased both the photosynthetic activity and the Fe content of plastids. Supraoptimal Fe nutrition caused neither Fe accumulation in chloroplasts nor any toxic effects, thus only fully saturated the need for Fe in the leaves. In parallel with the increasing Fe supply of plants and ageing of the leaves, the expression of BnPIC1 was tendentiously repressed. Though transcript and protein amount of BnNiCo tendentiously increased during leaf development, it was even markedly upregulated in ageing leaves. The relative transcript amount of BnMAR1 increased mainly in ageing leaves facing Fe deficiency. Taken together chloroplast physiology, Fe content and transcript amount data, the exclusive participation of NiCo in the chloroplast Fe uptake is not supported. Saturation of the Fe requirement of chloroplasts seems to be linked to the delay of decomposing the photosynthetic apparatus and keeping chloroplast Fe homeostasis in a rather constant status together with a supressed Fe uptake machinery.

Characterization of the Soybean GmIREG Family Genes and the Function of GmIREG3 in Conferring Tolerance to Aluminum Stress

The IREG (IRON REGULATED/ferroportin) family of genes plays vital roles in regulating the homeostasis of iron and conferring metal stress. This study aims to identify soybean IREG family genes and characterize the function of GmIREG3 in conferring tolerance to aluminum stress. Bioinformatics and expression analyses were conducted to identify six soybean IREG family genes. One GmIREG, whose expression was significantly enhanced by aluminum stress, GmIREG3, was studied in more detail to determine its possible role in conferring tolerance to such stress. In total, six potential IREG-encoding genes with the domain of Ferroportin1 (PF06963) were characterized in the soybean genome. Analysis of the GmIREG3 root tissue expression patterns, subcellular localizations, and root relative elongation and aluminum content of transgenic Arabidopsis overexpressing GmIREG3 demonstrated that GmIREG3 is a tonoplast localization protein that increases transgenic Arabidopsis aluminum resistance but does not alter tolerance to Co and Ni. The systematic analysis of the GmIREG gene family reported herein provides valuable information for further studies on the biological roles of GmIREGs in conferring tolerance to metal stress. GmIREG3 contributes to aluminum resistance and plays a role similar to that of FeIREG3. The functions of other GmIREG genes need to be further clarified in terms of whether they confer tolerance to metal stress or other biological functions.

Essential and Detrimental - an Update on Intracellular Iron Trafficking and Homeostasis

Chloroplasts, mitochondria and vacuoles represent characteristic organelles of the plant cell, with a predominant function in cellular metabolism. Chloroplasts are the site of photosynthesis and therefore basic and essential for photoautotrophic growth of plants. Mitochondria produce energy during respiration and vacuoles act as internal waste and storage compartments. Moreover, chloroplasts and mitochondria are sites for the biosynthesis of various compounds of primary and secondary metabolism. For photosynthesis and energy generation, the internal membranes of chloroplasts and mitochondria are equipped with electron transport chains. To perform proper electron transfer and several biosynthetic functions, both organelles contain transition metals and here iron is by far the most abundant. Although iron is thus essential for plant growth and development, it becomes toxic when present in excess and/or in its free, ionic form. The harmful effect of the latter is caused by the generation of oxidative stress. As a consequence, iron transport and homeostasis have to be tightly controlled during plant growth and development. In addition to the corresponding transport and homeostasis proteins, the vacuole plays an important role as an intracellular iron storage and release compartment at certain developmental stages. In this review, we will summarize current knowledge on iron transport and homeostasis in chloroplasts, mitochondria and vacuoles. In addition, we aim to integrate the physiological impact of intracellular iron homeostasis on cellular and developmental processes.

Chloroplasts preferentially take up ferric-citrate over iron-nicotianamine complexes in Brassica napus

Fe uptake machinery of chloroplasts prefers to utilise Fe(III)-citrate over Fe-nicotianamine complexes. Iron uptake in chloroplasts is a process of prime importance. Although a few members of their iron transport machinery were identified, the substrate preference of the system is still unknown. Intact chloroplasts of oilseed rape (Brassica napus) were purified and subjected to iron uptake studies using natural and artificial iron complexes. Fe-nicotianamine (NA) complexes were characterised by 5 K, 5 T Mössbauer spectrometry. Expression of components of the chloroplast Fe uptake machinery was also studied. Fe(III)-NA contained a minor paramagnetic Fe(II) component (ca. 9%), a paramagnetic Fe(III) component exhibiting dimeric or oligomeric structure (ca. 20%), and a Fe(III) complex, likely being a monomeric structure, which undergoes slow electronic relaxation at 5 K (ca. 61%). Fe(II)-NA contained more than one similar chemical Fe(II) environment with no sign of Fe(III) components. Chloroplasts preferred Fe(III)-citrate compared to Fe(III)-NA and Fe(II)-NA, but also to Fe(III)-EDTA and Fe(III)-o,o'EDDHA, and the K_m value was lower for Fe(III)-citrate than for the Fe-NA complexes. Only the uptake of Fe(III)-citrate was light-dependent. Regarding the components of the chloroplast Fe uptake system, only genes of the reduction-based Fe uptake system showed high expression. Chloroplasts more effectively utilize Fe(III)-citrate, but hardly Fe-NA complexes in Fe uptake.

Chloroplast Iron Transport Proteins - Function and Impact on Plant Physiology

Chloroplasts originated about three billion years ago by endosymbiosis of an ancestor of today's cyanobacteria with a mitochondria-containing host cell. During evolution chloroplasts of higher plants established as the site for photosynthesis and thus became the basis for all life dependent on oxygen and carbohydrate supply. To fulfill this task, plastid organelles are loaded with the transition metals iron, copper, and manganese, which due to their redox properties are essential for photosynthetic electron transport. In consequence, chloroplasts for example represent the iron-richest system in plant cells. However, improvement of oxygenic photosynthesis in turn required adaptation of metal transport and homeostasis since metal-catalyzed generation of reactive oxygen species (ROS) causes oxidative damage. This is most acute in chloroplasts, where radicals and transition metals are side by side and ROS-production is a usual feature of photosynthetic electron transport. Thus, on the one hand when bound by proteins, chloroplast-intrinsic metals are a prerequisite for photoautotrophic life, but on the other hand become toxic when present in their highly reactive, radical generating, free ionic forms. In consequence, transport, storage and cofactor-assembly of metal ions in plastids have to be tightly controlled and are crucial throughout plant growth and development. In the recent years, proteins for iron transport have been isolated from chloroplast envelope membranes. Here, we discuss their putative functions and impact on cellular metal homeostasis as well as photosynthetic performance and plant metabolism. We further consider the potential of proteomic analyses to identify new players in the field.

Chloroplast Iron Transport Proteins - Function and Impact on Plant Physiology

Chloroplasts originated about three billion years ago by endosymbiosis of an ancestor of today's cyanobacteria with a mitochondria-containing host cell. During evolution chloroplasts of higher plants established as the site for photosynthesis and thus became the basis for all life dependent on oxygen and carbohydrate supply. To fulfill this task, plastid organelles are loaded with the transition metals iron, copper, and manganese, which due to their redox properties are essential for photosynthetic electron transport. In consequence, chloroplasts for example represent the iron-richest system in plant cells. However, improvement of oxygenic photosynthesis in turn required adaptation of metal transport and homeostasis since metal-catalyzed generation of reactive oxygen species (ROS) causes oxidative damage. This is most acute in chloroplasts, where radicals and transition metals are side by side and ROS-production is a usual feature of photosynthetic electron transport. Thus, on the one hand when bound by proteins, chloroplast-intrinsic metals are a prerequisite for photoautotrophic life, but on the other hand become toxic when present in their highly reactive, radical generating, free ionic forms. In consequence, transport, storage and cofactor-assembly of metal ions in plastids have to be tightly controlled and are crucial throughout plant growth and development. In the recent years, proteins for iron transport have been isolated from chloroplast envelope membranes. Here, we discuss their putative functions and impact on cellular metal homeostasis as well as photosynthetic performance and plant metabolism. We further consider the potential of proteomic analyses to identify new players in the field.

Targeting and biogenesis of transporters and channels in chloroplast envelope membranes: Unsolved questions

Chloroplasts produce carbohydrates, hormones, vitamins, amino acids, pigments, nucleotides, ATP, and secondary metabolites. Channels and transporters are required for the movement of molecules across the two chloroplast envelope membranes. These transporters and channel proteins are grouped into two different types, including β-barrel proteins and transmembrane-domain (TMD) containing proteins. Most β-barrel proteins are localized at the outer chloroplast membrane, and TMD-containing proteins are localized at the inner chloroplast membrane. Many of these transporters and channels are encoded by nuclear genes; therefore, they have to be imported into chloroplasts after translation on cytosolic ribosomes. These proteins should have specific targeting signals for their final destination in the chloroplast membrane and for assembly into specific complexes. In this review, we summarize recent progress in the identification, functional characterization, and biogenesis of transporters and channels at the chloroplast envelope membranes, and discuss outstanding questions regarding transporter and channel protein biogenesis.

RNA-seq analysis of the effect of kanamycin and the ABC transporter AtWBC19 on Arabidopsis thaliana seedlings reveals changes in metal content

Plants are exposed to antibiotics produced by soil microorganisms, but little is known about their responses at the transcriptional level. Likewise, few endogenous mechanisms of antibiotic resistance have been reported. The Arabidopsis thaliana ATP Binding Cassette (ABC) transporter AtWBC19 (ABCG19) is known to confer kanamycin resistance, but the exact mechanism of resistance is not well understood. Here we examined the transcriptomes of control seedlings and wbc19 mutant seedlings using RNA-seq analysis. Exposure to kanamycin indicated changes in the organization of the photosynthetic apparatus, metabolic fluxes and metal uptake. Elemental analysis showed a 60% and 80% reduction of iron uptake in control and wbc19 mutant seedlings respectively, upon exposure to kanamycin. The drop in iron content was accompanied by the upregulation of the gene encoding for FERRIC REDUCTION OXIDASE 6 (FRO6) in mutant seedlings but not by the differential expression of other transport genes known to be induced by iron deficiency. In addition, wbc19 mutants displayed a distinct expression profile in the absence of kanamycin. Most notably the expression of several zinc ion binding proteins, including ZINC TRANSPORTER 1 PRECURSOR (ZIP1) was increased, suggesting abnormal zinc uptake. Elemental analysis confirmed a 50% decrease of zinc content in wbc19 mutants. Thus, the antibiotic resistance gene WBC19 appears to also have a role in zinc uptake.

Biosolid-borne tetracyclines and sulfonamides in plants

Tetracyclines and sulfonamides used in human and animal medicine are released to terrestrial ecosystems from wastewater treatment plants or by direct manure application. The interactions between plants and these antibiotics are numerous and complex, including uptake and accumulation, phytometabolism, toxicity responses, and degradation in the rhizosphere. Uptake and accumulation of antibiotics have been studied in plants such as wheat, maize, potato, vegetables, and ornamentals. Once accumulated in plant tissue, organic contaminants can be metabolized through a sequential process of transformation, conjugation through glycosylation and glutathione pathways, and ultimately sequestration into plant tissue. While studies have yet to fully elucidate the phytometabolism of tetracyclines and sulfonamides, an in-depth review of plant and mammalian studies suggest multiple potential transformation and conjugation pathways for tetracyclines and sulfonamides. The presence of contaminants in the vicinity or within the plants can elicit stress responses and defense mechanisms that can help tolerate the negative effects of contaminants. Antibiotics can change microbial communities and enzyme activity in the rhizosphere, potentially inducing microbial antibiotic resistance. On the other hand, the interaction of microbes and root exudates on pharmaceuticals in the rhizosphere can result in degradation of the parent molecule to less toxic compounds. To fully characterize the environmental impacts of increased antibiotic use in human medicine and animal production, further research is essential to understand the effects of different antibiotics on plant physiology and productivity, uptake, translocation, and phytometabolism of antibiotics, and the role of antibiotics in the rhizosphere.

Uptake and incorporation of iron in sugar beet chloroplasts

Chloroplasts contain 80-90% of iron taken up by plant cells. Though some iron transport-related envelope proteins were identified recently, the mechanism of iron uptake into chloroplasts remained unresolved. To shed more light on the process of chloroplast iron uptake, trials were performed with isolated intact chloroplasts of sugar beet (Beta vulgaris). Iron uptake was followed by measuring the iron content of chloroplasts in the form of ferrous-bathophenantroline-disulphonate complex after solubilising the chloroplasts in reducing environment. Ferric citrate was preferred to ferrous citrate as substrate for chloroplasts. Strong dependency of ferric citrate uptake on photosynthetic electron transport activity suggests that ferric chelate reductase uses NADPH, and is localised in the inner envelope membrane. The K(m) for iron uptake from ferric-citrate pool was 14.65 ± 3.13 μM Fe((III))-citrate. The relatively fast incorporation of (57)Fe isotope into Fe-S clusters/heme, detected by Mössbauer spectroscopy, showed the efficiency of the biosynthetic machinery of these cofactors in isolated chloroplasts. The negative correlation between the chloroplast iron concentration and the rate of iron uptake refers to a strong feedback regulation of the uptake.

Towards a knowledge-based correction of iron chlorosis

Iron (Fe) deficiency-induced chlorosis is a major nutritional disorder in crops growing in calcareous soils. Iron deficiency in fruit tree crops causes chlorosis, decreases in vegetative growth and marked fruit yield and quality losses. Therefore, Fe fertilizers, either applied to the soil or delivered to the foliage, are used every year to control Fe deficiency in these crops. On the other hand, a substantial body of knowledge is available on the fundamentals of Fe uptake, long and short distance Fe transport and subcellular Fe allocation in plants. Most of this basic knowledge, however, applies only to Fe deficiency, with studies involving Fe fertilization (i.e., with Fe-deficient plants resupplied with Fe) being still scarce. This paper reviews recent developments in Fe-fertilizer research and the state-of-the-art of the knowledge on Fe acquisition, transport and utilization in plants. Also, the effects of Fe-fertilization on the plant responses to Fe deficiency are reviewed. Agronomical Fe-fertilization practices should benefit from the basic knowledge on plant Fe homeostasis already available; this should be considered as a long-term goal that can optimize fertilizer inputs, reduce grower's costs and minimize the environmental impact of fertilization.

The chloroplast permease PIC1 regulates plant growth and development by directing homeostasis and transport of iron

The membrane-spanning protein PIC1 (for permease in chloroplasts 1) in Arabidopsis (Arabidopsis thaliana) was previously described to mediate iron transport across the inner envelope membrane of chloroplasts. The albino phenotype of pic1 knockout mutants was reminiscent of iron-deficiency symptoms and characterized by severely impaired plastid development and plant growth. In addition, plants lacking PIC1 showed a striking increase in chloroplast ferritin clusters, which function in protection from oxidative stress by sequestering highly reactive free iron in their spherical protein shell. In contrast, PIC1-overexpressing lines (PIC1ox) in this study rather resembled ferritin loss-of-function plants. PIC1ox plants suffered from oxidative stress and leaf chlorosis, most likely originating from iron overload in chloroplasts. Later during growth, plants were characterized by reduced biomass as well as severely defective flower and seed development. As a result of PIC1 protein increase in the inner envelope membrane of plastids, flower tissue showed elevated levels of iron, while the content of other transition metals (copper, zinc, manganese) remained unchanged. Seeds, however, specifically revealed iron deficiency, suggesting that PIC1 overexpression sequestered iron in flower plastids, thereby becoming unavailable for seed iron loading. In addition, expression of genes associated with metal transport and homeostasis as well as photosynthesis was deregulated in PIC1ox plants. Thus, PIC1 function in plastid iron transport is closely linked to ferritin and plastid iron homeostasis. In consequence, PIC1 is crucial for balancing plant iron metabolism in general, thereby regulating plant growth and in particular fruit development.

The chloroplast permease PIC1 regulates plant growth and development by directing homeostasis and transport of iron

The membrane-spanning protein PIC1 (for permease in chloroplasts 1) in Arabidopsis (Arabidopsis thaliana) was previously described to mediate iron transport across the inner envelope membrane of chloroplasts. The albino phenotype of pic1 knockout mutants was reminiscent of iron-deficiency symptoms and characterized by severely impaired plastid development and plant growth. In addition, plants lacking PIC1 showed a striking increase in chloroplast ferritin clusters, which function in protection from oxidative stress by sequestering highly reactive free iron in their spherical protein shell. In contrast, PIC1-overexpressing lines (PIC1ox) in this study rather resembled ferritin loss-of-function plants. PIC1ox plants suffered from oxidative stress and leaf chlorosis, most likely originating from iron overload in chloroplasts. Later during growth, plants were characterized by reduced biomass as well as severely defective flower and seed development. As a result of PIC1 protein increase in the inner envelope membrane of plastids, flower tissue showed elevated levels of iron, while the content of other transition metals (copper, zinc, manganese) remained unchanged. Seeds, however, specifically revealed iron deficiency, suggesting that PIC1 overexpression sequestered iron in flower plastids, thereby becoming unavailable for seed iron loading. In addition, expression of genes associated with metal transport and homeostasis as well as photosynthesis was deregulated in PIC1ox plants. Thus, PIC1 function in plastid iron transport is closely linked to ferritin and plastid iron homeostasis. In consequence, PIC1 is crucial for balancing plant iron metabolism in general, thereby regulating plant growth and in particular fruit development.

Exploring multiple drug and herbicide resistance in plants--spotlight on transporter proteins

Multiple drug resistance (MDR) has been extensively studied in bacteria, yeast, and mammalian cells due to the great clinical significance of this problem. MDR is not well studied in plant systems, although plant genomes contain large numbers of genes encoding putative MDR transporters (MDRTs). Biochemical pathways in the chloroplast are the targets of many herbicides and antibiotics, yet very little data is available regarding mechanisms of drug transport across the chloroplast membrane. MDRTs typically have broad substrate specificities, and may transport essential compounds and metabolites in addition to toxins. Indeed, plant transporters belonging to MDR families have also been implicated in the transport of a wide variety of compounds including auxins, flavonoids, glutathione conjugates, metal chelators, herbicides and antibiotics, although definitive evidence that a single transporter is capable of moving both toxins and metabolites has not yet been provided. Current understanding of plant MDR can be expanded via the characterization of candidate genes, especially MDRTs predicted to localize to the chloroplast, and also via traditional forward genetic approaches. Novel plant MDRTs have the potential to become endogenous selectable markers, aid in phytoremediation strategies, and help us to understand how plants have evolved to cope with toxins in their environment.