A Novel Prokaryote-Type ECF/ABC Transporter Module in Chloroplast Metal Homeostasis

A review of phytoremediation of environmental lead (pb) contamination

An estimated one billion people globally are exposed to hazardous levels of lead (Pb), resulting in intellectual disabilities for over 600,000 children each year. This critical issue aligns with the expanding worldwide population and the demand for food security, emphasizing the urgency of effectively addressing heavy metal pollution especially from Pb for sustainable development. Phytoremediation, a highly favoured approach in conjunction with conventional physical, chemical, and microbial methods, is a promising approach to mitigating soil and environmental contamination. In this review, we delve into a range of soil pollution mitigation strategies, with focus on the mechanisms that underpin the phytoremediation of environmental Pb. This detailed exploration sheds light on the efficacy and complexities of utilizing plants for the detoxification and removal of lead from contaminated environments. It also examines strategies to enhance phytoremediation by incorporating microbiology, composting, nanotechnology, and foliar spraying. The potential remediation strategies largely depend on the investigation and incorporation of environmentally friendly catalysts, as well as the utilization of innovative methods such as genetic engineering to improve phytoremediation processes. Studies have also shown that biochar has the capability to lower heavy metal concentrations in plant branches by over 50%, without affecting the pH of the soil. Specifically, magnetic biochar (MBC) has been shown to decrease lead levels in plants by up to 42%. Employing these methods showcases an effective strategy to enhance the efficacy of remediation techniques and fosters sustainable solutions to the pervasive issue of Pb pollution, thereby contributing to sustainable development efforts globally.

A transcriptome-wide identification of ATP-binding cassette (ABC) transporters revealed participation of ABCB subfamily in abiotic stress management of Glycyrrhiza glabra L

Transcriptome-wide survey divulged a total of 181 ABC transporters in G. glabra which were phylogenetically classified into six subfamilies. Protein-Protein interactions revealed nine putative GgABCBs (-B6, -B14, -B15, -B25, -B26, -B31, -B40, -B42 &-B44) corresponding to five AtABCs orthologs (-B1, -B4, -B11, -B19, &-B21). Significant transcript accumulation of ABCB6 (31.8 folds), -B14 (147.5 folds), -B15 (17 folds), -B25 (19.7 folds), -B26 (18.31 folds), -B31 (61.89 folds), -B40 (1273 folds) and -B42 (51 folds) was observed under the influence of auxin. Auxin transport-specific inhibitor, N-1-naphthylphthalamic acid, showed its effectiveness only at higher (10 µM) concentration where it down regulated the expression of ABCBs, PINs (PIN FORMED) and TWD1 (TWISTED DWARF 1) genes in shoot tissues, while their expression was seen to enhance in the root tissues. Further, qRT-PCR analysis under various growth conditions (in-vitro, field and growth chamber), and subjected to abiotic stresses revealed differential expression implicating role of ABCBs in stress management. Seven of the nine genes were shown to be involved in the stress physiology of the plant. GgABCB6, 15, 25 and ABCB31 were induced in multiple stresses, while GgABCB26, 40 & 42 were exclusively triggered under drought stress. No study pertaining to the ABC transporters from G. glabra is available till date. The present investigation will give an insight to auxin transportation which has been found to be associated with plant growth architecture; the knowledge will help to understand the association between auxin transportation and plant responses under the influence of various conditions.

Types of Membrane Transporters and the Mechanisms of Interaction between Them and Reactive Oxygen Species in Plants

Membrane transporters are proteins that mediate the entry and exit of substances through the plasma membrane and organellar membranes and are capable of recognizing and binding to specific substances, thereby facilitating substance transport. Membrane transporters are divided into different types, e.g., ion transporters, sugar transporters, amino acid transporters, and aquaporins, based on the substances they transport. These membrane transporters inhibit reactive oxygen species (ROS) generation through ion regulation, sugar and amino acid transport, hormone induction, and other mechanisms. They can also promote enzymatic and nonenzymatic reactions in plants, activate antioxidant enzyme activity, and promote ROS scavenging. Moreover, membrane transporters can transport plant growth regulators, solute proteins, redox potential regulators, and other substances involved in ROS metabolism through corresponding metabolic pathways, ultimately achieving ROS homeostasis in plants. In turn, ROS, as signaling molecules, can affect the activity of membrane transporters under abiotic stress through collaboration with ions and involvement in hormone metabolic pathways. The research described in this review provides a theoretical basis for improving plant stress resistance, promoting plant growth and development, and breeding high-quality plant varieties.

Structure, function, and inhibition of catalytically asymmetric ABC transporters: Lessons from the PDR subfamily

ATP-binding cassette (ABC) superfamily comprises a large group of ubiquitous transmembrane proteins that play a crucial role in transporting a diverse spectrum of substrates across cellular membranes. They participate in a wide array of physiological and pathological processes including nutrient uptake, antigen presentation, toxin elimination, and drug resistance in cancer and microbial cells. ABC transporters couple ATP binding and hydrolysis to undergo conformational changes allowing substrate translocation. Within this superfamily, a set of ABC transporters has lost the capacity to hydrolyze ATP at one of their nucleotide-binding sites (NBS), called the non-catalytic NBS, whose importance became evident with extensive biochemistry carried out on yeast pleiotropic drug resistance (PDR) transporters. Recent single-particle cryogenic electron microscopy (cryo-EM) advances have further catapulted our understanding of the architecture of these pumps. We provide here a comprehensive overview of the structural and functional aspects of catalytically asymmetric ABC pumps with an emphasis on the PDR subfamily. Furthermore, given the increasing evidence of efflux-mediated antifungal resistance in clinical settings, we also discuss potential grounds to explore PDR transporters as therapeutic targets.

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.

Black sheep, dark horses, and colorful dogs: a review on the current state of the Gene Ontology with respect to iron homeostasis in Arabidopsis thaliana

Cellular homeostasis of the micronutrient iron is highly regulated in plants and responsive to nutrition, stress, and developmental signals. Genes for iron management encode metal and other transporters, enzymes synthesizing chelators and reducing substances, transcription factors, and several types of regulators. In transcriptome or proteome datasets, such iron homeostasis-related genes are frequently found to be differentially regulated. A common method to detect whether a specific cellular pathway is affected in the transcriptome data set is to perform Gene Ontology (GO) enrichment analysis. Hence, the GO database is a widely used resource for annotating genes and identifying enriched biological pathways in Arabidopsis thaliana. However, iron homeostasis-related GO terms do not consistently reflect gene associations and levels of evidence in iron homeostasis. Some genes in the existing iron homeostasis GO terms lack direct evidence of involvement in iron homeostasis. In other aspects, the existing GO terms for iron homeostasis are incomplete and do not reflect the known biological functions associated with iron homeostasis. This can lead to potential errors in the automatic annotation and interpretation of GO term enrichment analyses. We suggest that applicable evidence codes be used to add missing genes and their respective ortholog/paralog groups to make the iron homeostasis-related GO terms more complete and reliable. There is a high likelihood of finding new iron homeostasis-relevant members in gene groups and families like the ZIP, ZIF, ZIFL, MTP, OPT, MATE, ABCG, PDR, HMA, and HMP. Hence, we compiled comprehensive lists of genes involved in iron homeostasis that can be used for custom enrichment analysis in transcriptomic or proteomic studies, including genes with direct experimental evidence, those regulated by central transcription factors, and missing members of small gene families or ortholog/paralog groups. As we provide gene annotation and literature alongside, the gene lists can serve multiple computational approaches. In summary, these gene lists provide a valuable resource for researchers studying iron homeostasis in A. thaliana, while they also emphasize the importance of improving the accuracy and comprehensiveness of the Gene Ontology.

Deciphering the functional roles of transporter proteins in subcellular metal transportation of plants

MAIN CONCLUSION

The role of transporters in subcellular metal transport is of great significance for plants in coping with heavy metal stress and maintaining their proper growth and development. Heavy metal toxicity is a serious long-term threat to plant growth and agricultural production, becoming a global environmental concern. Excessive heavy metal accumulation not only damages the biochemical and physiological functions of plants but also causes chronic health hazard to human beings through the food chain. To deal with heavy metal stress, plants have evolved a series of elaborate mechanisms, especially a variety of spatially distributed transporters, to strictly regulate heavy metal uptake and distribution. Deciphering the subcellular role of transporter proteins in controlling metal absorption, transport and separation is of great significance for understanding how plants cope with heavy metal stress and improving their adaptability to environmental changes. Hence, we herein introduce the detrimental effects of excessive common essential and non-essential heavy metals on plant growth, and describe the structural and functional characteristics of transporter family members, with a particular emphasis on their roles in maintaining heavy metal homeostasis in various organelles. Besides, we discuss the potential of controlling transporter gene expression by transgenic approaches in response to heavy metal stress. This review will be valuable to researchers and breeders for enhancing plant tolerance to heavy metal contamination.

Response and Tolerance of Macleaya cordata to Excess Zinc Based on Transcriptome and Proteome Patterns

Macleaya cordata is a dominant plant of mine tailings and a zinc (Zn) accumulator with high Zn tolerance. In this study, M. cordata seedlings cultured in Hoagland solution were treated with 200 μmol·L^-1 of Zn for 1 day or 7 days, and then, their leaves were taken for a comparative analysis of the transcriptomes and proteomes between the leaves of the control and Zn treatments. Differentially expressed genes included those that were iron (Fe)-deficiency-induced, such as vacuolar iron transporter VIT, ABC transporter ABCI17 and ferric reduction oxidase FRO. Those genes were significantly upregulated by Zn and could be responsible for Zn transport in the leaves of M. cordata. Differentially expressed proteins, such as chlorophyll a/b-binding proteins, ATP-dependent protease, and vacuolar-type ATPase located on the tonoplast, were significantly upregulated by Zn and, thus, could be important in chlorophyll biosynthesis and cytoplasm pH stabilization. Moreover, the changes in Zn accumulation, the production of hydrogen peroxide, and the numbers of mesophyll cells in the leaves of M. cordata were consistent with the expression of the genes and proteins. Thus, the proteins involved in the homeostasis of Zn and Fe are hypothesized to be the keys to the tolerance and accumulation of Zn in M. cordata. Such mechanisms in M. cordata can suggest novel approaches to genetically engineering and biofortifying crops.

Physiological and molecular response and tolerance of Macleaya cordata to lead toxicity

BACKGROUND

Macleaya cordata is a traditional medicinal herb, and it has high tolerance and accumulation ability to heavy metals, which make it a good candidate species for studying phytoremediation. The objectives of this study were to investigate response and tolerance of M. cordata to lead (Pb) toxicity based on comparative analysis of transcriptome and proteome.

RESULTS

In this study, the seedlings of M. cordata cultured in Hoagland solution were treated with 100 µmol·L^- 1 Pb for 1 day (Pb 1d) or 7 days (Pb 7d), subsequently leaves of M. cordata were taken for the determination of Pb accumulation and hydrogen peroxide production (H₂O₂), meanwhile a total number of 223 significantly differentially expressed genes (DEGs) and 296 differentially expressed proteins (DEPs) were screened between control and Pb treatments. The results showed leaves of M. cordata had a special mechanism to maintain Pb at an appropriate level. Firstly, some DEGs were iron (Fe) deficiency-induced transporters, for example, genes of vacuolar iron transporter and three ABC transporter I family numbers were upregulated by Pb, which can maintain Fe homeostasis in cytoplasm or chloroplast. In addition, five genes of calcium (Ca²⁺) binding proteins were downregulated in Pb 1d, which may regulate cytoplasmic Ca²⁺ concentration and H₂O₂ signaling pathway. On the other hand, the cysteine synthase upregulated, glutathione S-transferase downregulated and glutathione reductase downregulated in Pb 7d can cause reduced glutathione accumulation and decrease Pb detoxification in leaves. Furthermore, DEPs of eight chlorophyll a/b binding proteins, five ATPases and eight ribosomal proteins can play a pivotal role on chloroplast turnover and ATP metabolism.

CONCLUSIONS

Our results suggest that the proteins involved in Fe homeostasis and chloroplast turnover in mesophyll cells may play key roles in tolerance of M. cordata to Pb. This study offers some novel insights into Pb tolerance mechanism of plants, and the potential valuable for environmental remediation of this important medicinal plant.

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.

Identification and characterization of ABC proteins in an important rice insect pest, Cnaphalocrocis medinalis unveil their response to Cry1C toxin

Rice leaffolder (Cnaphalocrocis medinalis) is an important insect pest in paddy fields. Due to their essential role in the physiology and insecticidal resistance, ATP-binding cassette (ABC) proteins were studied in many insects. In this study, we identified the ABC proteins in C. medinalis through genomic data and analyzed their molecular characteristics. A total of 37 sequences with nucleotide-binding domain (NBD) were identified as ABC proteins and belonged to eight families (ABCA-ABCH). Four structure styles of ABC proteins were found in C. medinalis, including full structure, half structure, single structure, and ABC2 structure. In addition to these structures, TMD-NBD-TMD, NBD-TMD-NBD, and NBD-TMD-NBD-NBD were found in C. medinalis ABC proteins. Docking studies suggested that in addition to the soluble ABC proteins, other ABC proteins including ABCC4, ABCH1, ABCG3, ABCB5, ABCG1, ABCC7, ABCB3, ABCA3, and ABCC5 binding with Cry1C had higher weighted scores. The upregulation of ABCB1 and downregulation of ABCB3, ABCC1, ABCC7, ABCG1, ABCG3, and ABCG6 were associated with the C. medinalis response to Cry1C toxin. Collectively, these results help elucidate the molecular characteristics of C. medinalis ABC proteins, pave the way for further functional studies of C. medinalis ABC proteins, including their interaction with Cry1C toxin, and provide potential insecticide targets.

Commonalities and specialties in photosynthetic functions of PROTON GRADIENT REGULATION5 variants in Arabidopsis

The PROTON GRADIENT REGULATION5 (PGR5) protein is required for trans-thylakoid proton gradient formation and acclimation to fluctuating light (FL). PGR5 functionally interacts with two other thylakoid proteins, PGR5-like 1 (PGRL1) and 2 (PGRL2); however, the molecular details of these interactions are largely unknown. In the Arabidopsis (Arabidopsis thaliana) pgr5-1 mutant, the PGR5G130S protein accumulates in only small amounts. In this work, we generated a knockout allele of PGR5 (pgr5-Cas) using CRISPR-Cas9 technology. Like pgr5-1, pgr5-Cas is seedling-lethal under FL, but photosynthesis and particularly cyclic electron flow, as well as chlorophyll content, are less severely affected in both pgr5-Cas and pgrl1ab (which lacks PGRL1 and PGR5) than in pgr5-1. These differences are associated with changes in the levels of 260 proteins, including components of the Calvin-Benson cycle, photosystems II and I, and the NDH complex, in pgr5-1 relative to the wild type (WT), pgr5-Cas, and pgrl1ab. Some of the differences between pgr5-1 and the other mutant lines could be tentatively assigned to second-site mutations in the pgr5-1 line, identified by whole-genome sequencing. However, others, particularly the more pronounced photosynthetic defects and PGRL1 depletion (compared to pgr5-Cas), are clearly due to specific negative effects of the amino-acid substitution in PGR5G130S, as demonstrated by complementation analysis. Moreover, pgr5-1 and pgr5-Cas plants are less tolerant to long-term exposure to high light than pgrl1ab plants. These results imply that, in addition to the previously reported necessity of PGRL1 for optimal PGR5 function, PGR5 is required alongside PGRL1 to avoid harmful effects on plant performance.

Wheat-Thinopyrum Substitution Lines Imprint Compensation Both From Recipients and Donors

Even frequently used in wheat breeding, we still have an insufficient understanding of the biology of the products via distant hybridization. In this study, a transcriptomic analysis was performed for six Triticum aestivum-Thinopyrum elongatum substitution lines in comparison with the host plants. All the six disomic substitution lines showed much stronger "transcriptomic-shock" occurred on alien genomes with 57.43-69.22% genes changed expression level but less on the recipient genome (2.19-8.97%). Genome-wide suppression of alien genes along chromosomes was observed with a high proportion of downregulated genes (39.69-48.21%). Oppositely, the wheat recipient showed genome-wide compensation with more upregulated genes, occurring on all chromosomes but not limited to the homeologous groups. Moreover, strong co-upregulation of the orthologs between wheat and Thinopyrum sub-genomes was enriched in photosynthesis with predicted chloroplastic localization, which indicates that the compensation happened not only on wheat host genomes but also on alien genomes.

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.

The Tiny Companion Matters: The Important Role of Protons in Active Transports in Plants

In plants, the translocation of molecules, such as ions, metabolites, and hormones, between different subcellular compartments or different cells is achieved by transmembrane transporters, which play important roles in growth, development, and adaptation to the environment. To facilitate transport in a specific direction, active transporters that can translocate their substrates against the concentration gradient are needed. Examples of major active transporters in plants include ATP-binding cassette (ABC) transporters, multidrug and toxic compound extrusion (MATE) transporters, monosaccharide transporters (MSTs), sucrose transporters (SUTs), and amino acid transporters. Transport via ABC transporters is driven by ATP. The electrochemical gradient across the membrane energizes these secondary transporters. The pH in each cell and subcellular compartment is tightly regulated and yet highly dynamic, especially when under stress. Here, the effects of cellular and subcellular pH on the activities of ABC transporters, MATE transporters, MSTs, SUTs, and amino acid transporters will be discussed to enhance our understanding of their mechanics. The relation of the altered transporter activities to various biological processes of plants will also be addressed. Although most molecular transport research has focused on the substrate, the role of protons, the tiny counterparts of the substrate, should also not be ignored.

ATP-binding cassette transporters in nonmodel plants

Knowledge about plant ATP-binding cassette (ABC) proteins is of great value for sustainable agriculture, economic yield, and the generation of high-quality products, especially under unfavorable growth conditions. We have learned much about ABC proteins in model organisms, notably Arabidopsis thaliana; however, the importance of research dedicated to these transporters extends far beyond Arabidopsis biology. Recent progress in genomic and transcriptomic approaches for nonmodel and noncanonical model plants allows us to look at ABC transporters from a wider perspective and consider chemodiversity and functionally driven adaptation as distinctive mechanisms during their evolution. Here, by considering several representatives from agriculturally important families and recent progress in functional characterization of nonArabidopsis ABC proteins, we aim to bring attention to understanding the evolutionary background, distribution among lineages and possible mechanisms underlying the adaptation of this versatile transport system for plant needs. Increasing the knowledge of ABC proteins in nonmodel plants will facilitate breeding and development of new varieties based on, for example, genetic variations of endogenous genes and/or genome editing, representing an alternative to transgenic approaches.

Metal tolerance in plants: Molecular and physicochemical interface determines the "not so heavy effect" of heavy metals

An increase in technological interventions and ruthless urbanization in the name of development has deteriorated our environment over time and caused the buildup of heavy metals (HMs) in the soil and water resources. These heavy metals are gaining increased access into our food chain through the plant and/or animal-based products, to adversely impact human health. The issue of how to restrict the entry of HMs or modulate their response in event of their ingress into the plant system is worrisome. The current knowledge on the interactive-regulatory role and contribution of different physical, biophysical, biochemical, physiological, and molecular factors that determine the heavy metal availability-uptake-partitioning dynamics in the soil-plant-environment needs to be updated. The present review critically analyses the interactive overlaps between different adaptation and tolerance strategies that may be causally related to their cellular localization, conjugation and homeostasis, a relative affinity for the transporters, rhizosphere modifications, activation of efflux pumps and vacuolar sequestration that singly or collectively determine a plant's response to HM stress. Recently postulated role of gaseous pollutants such as SO₂ and other secondary metabolites in heavy metal tolerance, which may be regulated at the whole plant and/or tissue/cell is discussed to delineate and work towards a "not so heavy" response of plants to heavy metals present in the contaminated soils.

2021 update on ATP-binding cassette (ABC) transporters: how they meet the needs of plants

Recent developments in the field of ABC proteins including newly identified functions and regulatory mechanisms expand the understanding of how they function in the development and physiology of plants.

Metal transporters in organelles and their roles in heavy metal transportation and sequestration mechanisms in plants

Heavy metal toxicity is one of the major concerns for agriculture and health. Accumulation of toxic heavy metals at high concentrations in edible parts of crop plants is the primary cause of disease in humans and cattle. A dramatic increase in industrialization, urbanization, and other high anthropogenic activities has led to the accumulation of heavy metals in agricultural soil, which has consequently disrupted soil conditions and affected crop yield. By now, plants have developed several mechanisms to cope with heavy metal stress. However, not all plants are equally effective in dealing with the toxicity of high heavy metal concentrations. Plants have modified their anatomy, morphophysiology, and molecular networks to survive under changing environmental conditions. Heavy metal sequestration is one of the essential processes evolved by some plants to deal with heavy metals' toxic concentration. Some plants even have the ability to accumulate metals in high quantities in the shoots/organelles without toxic effects. For intercellular and interorganeller metal transport, plants harbor spatially distributed various transporters which mainly help in uptake, translocation, and redistribution of metals. This review discusses different heavy metal transporters in different organelles and their roles in metal sequestration and redistribution to help plants cope with heavy metal stress. A good understanding of the processes at stake helps in developing more tolerant crops without affecting their productivity.

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.

Protuberances are organized distinct regions of long-term callus: histological and transcriptomic analyses in kiwifruit

KEY MESSAGE

Macroscopic, ultrastructural, and molecular features-like a ball shape, the presence of starch granules, and the up-regulation of genes involved in carbohydrate metabolism and secondary metabolite biosynthesis-distinguish PT regions within a callus. The modification of the mass of pluripotent cells into de novo shoot bud regeneration is highly relevant to developmental biology and for agriculture and biotechnology. This study deals with protuberances (PT), structures that appear during the organogenic long-term culturing of callus (OC) in kiwifruit. These ball-shaped regions of callus might be considered the first morphological sign of the subsequent shoot bud development. Sections of PT show the regular arrangement of some cells, especially on the surface, in contrast to the regions of OC beyond the PT. The cells of OC possess chloroplasts; however, starch granules were observed only in PTs' plastids. Transcriptomic data revealed unique gene expression for each kind of sample: OC, PT, and PT with visible shoot buds (PT-SH). Higher expression of the gene involved in lipid (glycerol-3-phosphate acyltransferase 5 [GPAT5]), carbohydrate (granule-bound starch synthase 1 [GBSS1]), and secondary metabolite (beta-glucosidase 45 [BGL45]) pathways were detected in PT and could be proposed as the markers of these structures. The up-regulation of the regulatory associated protein of TOR (RAPTOR1) was found in PT-SH. The highest expression of the actinidain gene in leaves from two-year-old regenerated plants suggests that the synthesis of this protein takes place in fully developed organs. The findings indicate that PT and PT-SH are specific structures within OC but have more features in common with callus tissue than with organs.

Role of ATP-binding cassette transporters in maintaining plant homeostasis under abiotic and biotic stresses

The ATP-binding cassette (ABC) transporters belong to a large protein family predominantly present in diverse species. ABC transporters are driven by ATP hydrolysis and can act as exporters as well as importers. These proteins are localized in the membranes of chloroplasts, mitochondria, peroxisomes and vacuoles. ABC proteins are involved in regulating diverse biological processes in plants, such as growth, development, uptake of nutrients, tolerance to biotic and abiotic stresses, tolerance to metal toxicity, stomatal closure, shape and size of grains, protection of pollens, transport of phytohormones, etc. In mitochondria and chloroplast, the iron metabolism and its transport across the membrane are mediated by ABC transporters. Tonoplast-localized ABC transporters are involved in internal detoxification of metal ion; thus protecting against the DNA impairment and maintaining cell growth. ABC transporters are involved in the transport of secondary metabolites inside the cells. Microorganisms also engage a large number of ABC transporters to import and expel substrates decisive for their pathogenesis. ABC transporters also suppress the seed embryonic growth until favorable conditions come. This review aims at giving insights on ABC transporters, their evolution, structure, functions and roles in different biological processes for helping the terrestrial plants to survive under adverse environmental conditions. These specialized plant membrane transporters ensure a sustainable economic yield and high-quality products, especially under unfavorable conditions of growth. These transporters can be suitably manipulated to develop 'Plants for the Future'.

ATP binding cassette importers in eukaryotic organisms

ATP-binding cassette (ABC) transporters are ubiquitous across all realms of life. Dogma suggests that bacterial ABC transporters include both importers and exporters, whilst eukaryotic members of this family are solely exporters, implying that ABC import function was lost during evolution. This view is being challenged, for example energy-coupling factor (ECF)-type ABC importers appear to fulfil important roles in both algae and plants where they form the ABCI sub-family. Herein we discuss whether bacterial Type I and Type II ABC importers also made the transition into extant eukaryotes. Various studies suggest that Type I importers exist in algae and the liverwort family of primitive non-vascular plants, but not in higher plants. The existence of eukaryotic Type II importers is also supported: a transmembrane protein homologous to vitamin B12 import system transmembrane protein (BtuC), hemin transport system transmembrane protein (HmuU) and high-affinity zinc uptake system membrane protein (ZnuB) is present in the Cyanophora paradoxa genome. This protein has homologs within the genomes of red algae. Furthermore, its candidate nucleotide-binding domain (NBD) shows closest similarity to other bacterial Type II importer NBDs such as BtuD. Functional studies suggest that Type I importers have roles in maintaining sulphate levels in the chloroplast, whilst Type II importers probably act as importers of Mn²⁺ or Zn²⁺ , as inferred by comparisons with bacterial homologs. Possible explanations for the presence of these transporters in simple plants, but not in other eukaryotic organisms, are considered. In order to utilise the existing nomenclature for eukaryotic ABC proteins, we propose that eukaryotic Type I and II importers be classified as ABCJ and ABCK transporters, respectively.

Evolutionary history of ATP-binding cassette proteins

ATP-binding cassette (ABC) proteins are found in every sequenced genome and evolved deep in the phylogenetic tree of life. ABC proteins form one of the largest homologous protein families, with most being involved in substrate transport across biological membranes, and a few cytoplasmic members regulating in essential processes like translation. The predominant ABC protein classification scheme is derived from human members, but the increasing number of fully sequenced genomes permits to reevaluate this paradigm in the light of the evolutionary history the ABC-protein superfamily. As we study the diversity of substrates, mechanisms, and physiological roles of ABC proteins, knowledge of the evolutionary relationships highlights similarities and differences that can be attributed to specific branches in protein divergence. While alignments and trees built on natural sequence variation account for the evolutionary divergence of ABC proteins, high-throughput experiments and next-generation sequencing creating experimental sequence variation are instrumental in identifying functional constraints. The combination of natural and experimentally produced sequence variation allows a broader and more rational study of the function and physiological roles of ABC proteins.

The OsABCI7 Transporter Interacts with OsHCF222 to Stabilize the Thylakoid Membrane in Rice

The thylakoid membrane is a highly complex membrane system in plants and plays crucial roles in the biogenesis of the photosynthetic apparatus and plant development. However, the genetic factors involved in chloroplast development and its relationship with intracellular metabolites are largely unknown. Here, a rice (Oryza sativa) chlorotic and necrotic leaf1 (cnl1) mutant was identified and map-based cloning revealed that a single base substitution followed by a 6-bp deletion in the ATP-binding cassette transporter I family member7 (OsABCI7) resulted in chlorotic and necrotic leaves with thylakoid membrane degradation, chlorophyll breakdown, photosynthesis impairment, and cell death in cnl1 Furthermore, the expression of OsABCI7 was inducible under lower temperatures, which severely affected cnl1 chloroplast development, and etiolated cnl1 seedlings were unable to recover to a normal green state under light conditions. Functional complementation and overexpression showed that OsABCI7 could rescue the cnl1 chlorotic and necrotic phenotype. OsABCI7 interacted with HIGH CHLOROPHYLL FLUORESCENCE222 (OsHCF222) to regulate cellular reactive oxygen species (ROS) homeostasis for thylakoid membrane stability. OsABCI7 localized to thylakoid membranes, while OsHCF222 targeted to endoplasmic reticulum and chloroplasts. Exogenous application of ascorbic acid eased the yellowish leaf phenotype by increasing chlorophyll content and alleviating ROS stress in cnl1 Unlike cnl1, the CRISPR/Cas9-mediated OsHCF222 knockout lines showed chlorotic leaves but were seedling lethal. Our results provide insight into the functions of ABC transporters in rice, especially within the relationship between ROS homeostasis and stability of thylakoid membranes.

Chloroplast Transition Metal Regulation for Efficient Photosynthesis

Plants require sunlight, water, CO₂, and essential nutrients to drive photosynthesis and fulfill their life cycle. The photosynthetic apparatus resides in chloroplasts and fundamentally relies on transition metals as catalysts and cofactors. Accordingly, chloroplasts are particularly rich in iron (Fe), manganese (Mn), and copper (Cu). Owing to their redox properties, those metals need to be carefully balanced within the cell. However, the regulation of transition metal homeostasis in chloroplasts is poorly understood. With the availability of the arabidopsis genome information and membrane protein databases, a wider catalogue for searching chloroplast metal transporters has considerably advanced the study of transition metal regulation. This review provides an updated overview of the chloroplast transition metal requirements and the transporters involved for efficient photosynthesis in higher plants.

Regulation of Iron Homeostasis and Use in Chloroplasts

Iron (Fe) is essential for life because of its role in protein cofactors. Photosynthesis, in particular photosynthetic electron transport, has a very high demand for Fe cofactors. Fe is commonly limiting in the environment, and therefore photosynthetic organisms must acclimate to Fe availability and avoid stress associated with Fe deficiency. In plants, adjustment of metabolism, of Fe utilization, and gene expression, is especially important in the chloroplasts during Fe limitation. In this review, we discuss Fe use, Fe transport, and mechanisms of acclimation to Fe limitation in photosynthetic lineages with a focus on the photosynthetic electron transport chain. We compare Fe homeostasis in Cyanobacteria, the evolutionary ancestors of chloroplasts, with Fe homeostasis in green algae and in land plants in order to provide a deeper understanding of how chloroplasts and photosynthesis may cope with Fe limitation.