Structural element responsible for the Fe(III)-phytosiderophore specific transport by HvYS1 transporter in barley

The multifaceted role of YSL proteins: Iron transport and emerging functions in plant metal homeostasis

Understanding metal transport in plants has always been critical. Several gene families have been identified in the last two decades that have aided in the understanding of channelized metal transport, including their uptake, distribution, and storage in plants. Identifying Yellow Stripe-like (YSL) genes has contributed to an improved understanding of metal homeostasis in plants, especially monocots. Several studies have demonstrated that these genes play a role in transporting metals complexed with phytosiderophores (PS) and/or nicotianamine (NA). In the current review, we have discussed and opinionated the signalling role of YSL protein in maintaining inter and intracellular metal homeostasis in plants. Although the genes are known to have a broader range of metal substrate specificity, these are primary iron (Fe) transporters, and a detailed Fe transport in plants is discussed. Furthermore, based on recent findings, alternative functions of these genes are also discussed. Overall, we provide a broader overview of YSL protein in modulating the Fe mobilization and provides evidence of the expanding functions in plants.

SbYS1 and SbWRKY72 regulate Cd tolerance and accumulation in sweet sorghum

MAIN CONCLUSION

SbYS1 and its upstream transcription factor SbWRKY72 were involved in Cd tolerance and accumulation and are valuable for developing sweet sorghum germplasm with high-Cd tolerance or accumulation ability through genetic manipulation. Cadmium (Cd) is highly toxic and can severely affect human health. Sweet sorghum, as an energy crop, shows great potential in extracting cadmium from Cd-contaminated soils. However, its molecular mechanisms of Cd-tolerance and -accumulation remain largely unknown. Here, we isolated a YSL family gene SbYS1 from the sweet sorghum genotype with high Cd accumulation ability and the expression of SbYS1 in roots was induced by cadmium. GUS staining experiment exhibited that SbYS1 was expressed in the epidermis and parenchyma tissues of roots. Further subcellular localization analysis suggested that SbYS1 was localized in the endoplasmic reticulum and plasma membrane. Yeast transformed with SbYS1 exhibited a sensitive phenotype compared to the control when exposed to Cd-NA (chelates of cadmium and nicotianamine), indicating that SbYS1 may absorb cadmium in the form of Cd-NA. Arabidopsis overexpressing SbYS1 had a longer root length and accumulated less Cd in roots and shoots. SbWRKY72 bound to the promoter of SbYS1 and negatively regulated the expression of SbYS1. Transgenic Arabidopsis of SbWRKY72 showed higher sensitivity to cadmium and increased cadmium accumulation in roots. Our results provide references for improving the phytoremediation efficiency of sweet sorghum by genetic manipulation in the future.

Identification and characterization of yellow stripe-like genes in maize suggest their roles in the uptake and transport of zinc and iron

BACKGROUND

Yellow Stripe-Like (YSL) proteins are involved in the uptake and transport of metal ions. They play important roles in maintaining the zinc and iron homeostasis in Arabidopsis, rice (Oryza sativa), and barley (Hordeum vulgare). However, proteins in this family have not been fully identified and comprehensively analyzed in maize (Zea mays L.).

RESULTS

In this study, we identified 19 ZmYSLs in the maize genome and analyzed their structural features. The results of a phylogenetic analysis showed that ZmYSLs are homologous to YSLs of Arabidopsis and rice, and these proteins are divided into four independent branches. Although their exons and introns have structural differences, the motif structure is relatively conserved. Analysis of the cis-regulatory elements in the promoters indicated that ZmYSLs might play a role in response to hypoxia and light. The results of RNA sequencing and quantitative real-time PCR analysis revealed that ZmYSLs are expressed in various tissues and respond differently to zinc and iron deficiency. The subcellular localization of ZmYSLs in the protoplast of maize mesophyll cells showed that they may function in the membrane system.

CONCLUSIONS

This study provided important information for the further functional analysis of ZmYSL, especially in the spatio-temporal expression and adaptation to nutrient deficiency stress. Our findings provided important genes resources for the maize biofortification.

Nicotianamine: A Key Player in Metal Homeostasis and Hyperaccumulation in Plants

Nicotianamine (NA) is a low-molecular-weight N-containing metal-binding ligand, whose accumulation in plant organs changes under metal deficiency or excess. Although NA biosynthesis can be induced in vivo by various metals, this non-proteinogenic amino acid is mainly involved in the detoxification and transport of iron, zinc, nickel, copper and manganese. This review summarizes the current knowledge on NA biosynthesis and its regulation, considers the mechanisms of NA secretion by plant roots, as well as the mechanisms of intracellular transport of NA and its complexes with metals, and its role in radial and long-distance metal transport. Its role in metal tolerance is also discussed. The NA contents in excluders, storing metals primarily in roots, and in hyperaccumulators, accumulating metals mainly in shoots, are compared. The available data suggest that NA plays an important role in maintaining metal homeostasis and hyperaccumulation mechanisms. The study of metal-binding compounds is of interdisciplinary significance, not only regarding their effects on metal toxicity in plants, but also in connection with the development of biofortification approaches to increase the metal contents, primarily of iron and zinc, in agricultural plants, since the deficiency of these elements in food crops seriously affects human health.

Mapping of the Quantitative Trait Loci and Candidate Genes Associated With Iron Efficiency in Maize

Iron (Fe) is a mineral micronutrient for plants, and Fe deficiency is a major abiotic stress in crop production because of its low solubility under aerobic and alkaline conditions. In this study, 18 maize inbred lines were used to preliminarily illustrate the physiological mechanism underlying Fe deficiency tolerance. Then biparental linkage analysis was performed to identify the quantitative trait loci (QTLs) and candidate genes associated with Fe deficiency tolerance using the recombinant inbred line (RIL) population derived from the most Fe-efficient (Ye478) and Fe-inefficient (Wu312) inbred lines. A total of 24 QTLs was identified under different Fe nutritional status in the Ye478 × Wu312 RIL population, explaining 6.1-26.6% of phenotypic variation, and ten candidate genes were identified. Plants have evolved two distinct mechanisms to solubilize and transport Fe to acclimate to Fe deficiency, including reduction-based strategy (strategy I) and chelation-based strategy (strategy II), and maize uses strategy II. However, not only genes involved in Fe homeostasis verified in strategy II plants (strategy II genes), which included ZmYS1, ZmYS3, and ZmTOM2, but also several genes associated with Fe homeostasis in strategy I plants (strategy I genes) were identified, including ZmFIT, ZmPYE, ZmILR3, ZmBTS, and ZmEIN2. Furthermore, strategy II gene ZmYS1 and strategy I gene ZmBTS were significantly upregulated in the Fe-deficient roots and shoots of maize inbred lines, and responded to Fe deficiency more in shoots than in roots. Under Fe deficiency, greater upregulations of ZmYS1 and ZmBTS were observed in Fe-efficient parent Ye478, not in Fe-inefficient parent Wu312. Beyond that, ZmEIN2 and ZmILR3, were found to be Fe deficiency-inducible in the shoots. These findings indicate that these candidate genes may be associated with Fe deficiency tolerance in maize. This study demonstrates the use of natural variation to identify important Fe deficiency-regulated genes and provides further insights for understanding the response to Fe deficiency stress in maize.

Low-molecular-weight ligands in plants: role in metal homeostasis and hyperaccumulation

Mineral nutrition is one of the key factors determining plant productivity. In plants, metal homeostasis is achieved through the functioning of a complex system governing metal uptake, translocation, distribution, and sequestration, leading to the maintenance of a regulated delivery of micronutrients to metal-requiring processes as well as detoxification of excess or non-essential metals. Low-molecular-weight ligands, such as nicotianamine, histidine, phytochelatins, phytosiderophores, and organic acids, play an important role in metal transport and detoxification in plants. Nicotianamine and histidine are also involved in metal hyperaccumulation, which determines the ability of some plant species to accumulate a large amount of metals in their shoots. In this review we extensively summarize and discuss the current knowledge of the main pathways for the biosynthesis of these ligands, their involvement in metal uptake, radial and long-distance transport, as well as metal influx, isolation and sequestration in plant tissues and cell compartments. It is analyzed how diverse endogenous ligand levels in plants can determine their different tolerance to metal toxic effects. This review focuses on recent advances in understanding the physiological role of these compounds in metal homeostasis, which is an essential task of modern ionomics and plant physiology. It is of key importance in studying the influence of metal deficiency or excess on various physiological processes, which is a prerequisite to the improvement of micronutrient uptake efficiency and crop productivity and to the development of a variety of applications in phytoremediation, phytomining, biofortification, and nutritional crop safety.

Methods for metal chelation in plant homeostasis: Review

Metal uptake, transport and storage in plants depend on specialized ligands with closely related functions. Individual studies differing by species, nutrient availability, tissue type, etc. are not comprehensive enough to understand plant metal homeostasis in its entirety. A thorough review is required that distinguishes the role of ligands directly involved in chelation from the myriad of plant responses to general stress. Distinguishing between the functions of metal chelating compounds is the primary focus of this review; reactive oxygen species mediation and other aspects of metal homeostasis are also discussed. High molecular weight ligands (polysaccharides, phytochelatin, metallothionein), low molecular weight ligands (nicotianamine, histidine, secondary metabolites) and select studies which demonstrate the complex nature of plant metal homeostasis are explored.

Delineating the future of iron biofortification studies in rice: challenges and future perspectives

Iron (Fe) deficiency in humans is a widespread problem worldwide. Fe biofortification of rice (Oryza sativa) is a promising approach to address human Fe deficiency. Since its conceptualization, various biofortification strategies have been developed, some of which have resulted in significant increases in grain Fe concentration. However, there are still many aspects that have not yet been addressed in the studies to date. In this review, we first overview the important rice Fe biofortification strategies reported to date and the complications associated with them. Next, we highlight the key outstanding questions and hypotheses related to rice Fe biofortification. Finally, we make suggestions for the direction of future rice biofortification studies.

Analysis of Yellow Striped Mutants of Zea mays Reveals Novel Loci Contributing to Iron Deficiency Chlorosis

The micronutrient iron (Fe) is essential for photosynthesis, respiration, and many other processes, but it is only sparingly soluble in aqueous solution, making adequate acquisition by plants a serious challenge. Fe is a limiting factor for plant growth on approximately 30% of the world's arable lands. Moreover, Fe deficiency in humans is a global health issue, affecting 1.62 billion people, or about 25% of the world's population. It is imperative that we gain a better understanding of the mechanisms that plants use to regulate iron homeostasis, since these will be important targets for future biofortification and crop improvement strategies. Grasses and non-grasses have evolved independent mechanisms for primary iron uptake from the soil. The grasses, which include most of the world's staple grains, have evolved a distinct 'chelation' mechanism to acquire iron from the soil. Strong iron chelators called phytosiderophores (PSs) are synthesized by grasses and secreted into the rhizosphere where they bind and solubilize Fe(III). The Fe(III)-PS complex is then taken up into root cells via transporters specific for the Fe(III)-PS complex. In this study, 31 novel, uncharacterized striped maize mutants available through the Maize Genetics Cooperation Stock Center (MGCSC) were analyzed to determine whether their mutant phenotypes are caused by decreased iron. Many of these proved to be either pale yellow or white striped mutants. Complementation tests were performed by crossing the MGCSC mutants to ys1 and ys3 reference mutants. This allowed assignment of 10 ys1 alleles and 4 ys3 alleles among the novel mutants. In addition, four ys∗ mutant lines were identified that are not allelic to either ys1 or ys3. Three of these were characterized as being non-allelic to each other and as having low iron in leaves. These represent new genes involved in iron acquisition by maize, and future cloning of these genes may reveal novel aspects of the grass iron acquisition mechanism.

Annotation and characterization of Cd-responsive metal transporter genes in rapeseed (Brassica napus)

In higher plants, heavy metal transporters are responsible for metal uptake, translocation and homeostasis. These metals include essential metals such as zinc (Zn) or manganese (Mn) and non-essential metals like cadmium (Cd) or lead (Pb). Although a few heavy metal transporters have been well identified in model plants (e.g. Arabidopsis and rice), little is known about their functionality in rapeseed (Brassica napus). B. napus is an important oil crop ranking the third largest sources of vegetable oil over the world. Importantly, B. napus has long been considered as a desirable candidate for phytoremediation owning to its massive dry weight productivity and moderate to high Cd accumulation. In this study, 270 metal transporter genes (MTGs) from B. napus genome were identified and annotated using bioinformatics and high-throughput sequencing. Most of the MTGs (74.8%, 202/270) were validated by RNA-sequencing (RNA-seq) the seedling libraries. Based on the sequence identity, nine superfamilies including YSL, OPT, NRAMP, COPT, ZIP, CDF/MTP, HMA, MRP and PDR have been classified. RNA-sequencing profiled 202 non-redundant MTGs from B. napus seedlings, of which, 108 MTGs were differentially expressed and 62 genes were significantly induced under Cd stress. These differentially expressed genes (DEGs) are dispersed in the rapeseed genome. Some of the genes were well confirmed by qRT-PCR. Analysis of the genomic distribution of MTGs on B. napus chromosomes revealed that their evolutional expansion was probably through localized allele duplications.

Isolation and characterization of a novel cadmium-regulated Yellow Stripe-Like transporter (SnYSL3) in Solanum nigrum

SnYSL3 encodes a plasma-localized transporter delivering various metal-nicotianamine complexes. The expression of SnYSL3 is up-regulated by excess Cd, suggesting an important role for SnYSL3 in response to Cd stress. The Yellow Stripe-Like (YSL) transporters have been proposed to participate in metal uptake and long-range transport in model plants. In this study, we isolated and characterized a novel member of the YSL gene family, SnYSL3, from the cadmium hyperaccumulator Solanum nigrum. SnYSL3 was constitutively expressed and encodes a plasma membrane-localized protein. In situ RNA hybridization localized the SnYSL3 transcripts predominantly in vascular tissues and epidermal cells of the roots and stems, while in leaves, the mRNA levels were high in the vasculature. The SnYSL3 expression level was up-regulated by excess Cd, excess Fe and Cu deficiency. Heterologous expression of SnYSL3 in yeast revealed that SnYSL3 transports nicotianamine complexes containing Fe(II), Cu, Zn and Cd. SnYSL3 overexpression in Arabidopsis thaliana decreased Fe and Mn concentrations in the roots and increased the root-to-shoot translocation ratios of Fe and Mn. Under Cd exposure, the transgenic plants showed increased translocation ratios of Fe and Cd, but no difference was observed in Mn translocation from roots to shoots between the transgenic and wild-type lines. Although the accurate function of SnYSL3 remains to be confirmed, these results suggest that SnYSL3 is a transporter delivering a broad range of metal-nicotianamine complexes and is potentially important for the response to heavy metal stress, especially due to Cd and Fe.

The helical propensity of the extracellular loop is responsible for the substrate specificity of Fe(III)-phytosiderophore transporters

Hordeum vulgare L. yellow stripe 1 (HvYS1) is a selective transporter of Fe(III)-phytosiderophores in barley that is responsible for iron acquisition from the soil. In contrast, maize Zea mays, yellow stripe 1 (ZmYS1) possesses broad substrate specificity. In this study, a quantitative evaluation of the transport activities of HvYS1 and ZmYS1 chimera proteins revealed that the seventh extracellular membrane loop is essential for substrate specificity. The loop peptides of both transporters were prepared and analysed by circular dichroism and NMR. The spectra revealed a higher propensity for α-helical conformation of the HvYS1 loop peptide and a largely disordered structure for that of ZmYS1. These structural differences are potentially responsible for the substrate specificities of the transporters.

DNA-protein cross-links involved in growth inhibition of rice seedlings exposed to Ga

Hydroponic experiments were conducted with rice seedlings (Oryza sativa L. cv. XZX45) exposed to gallium nitrate (Ga(3+)) to investigate the accumulation of Ga in plant tissues and phytotoxic responses. Results showed that phyto-transport of Ga was apparent, and roots were the dominant site for Ga accumulation. The total accumulation rates of Ga responded biphasically to Ga treatments by showing increases at low (1.06-8.52 mg Ga/L) and constants at high (8.52-15.63 mg Ga/L) concentrations, suggesting that accumulation kinetics of Ga followed a typical saturation curve. Higher amount of Ga accumulation in plant tissues led to significant inhibition in relative growth rate and water use efficiency in a dose-dependent manner. DNA-protein cross-links (DPCs) analysis revealed that overaccumulation of Ga in plant tissues positively stimulated formation of DPCs in roots. Likewise, the measure of root cell viability evaluated by Evan blue uptake showed a similar trend. These results suggested that Ga can be absorbed, transported, and accumulated in plant materials of rice seedlings. Overaccumulation of Ga in plant tissues provoked the formation of DPCs in roots, which resulted in cell death and growth inhibition of rice seedlings.

A domain-based approach for analyzing the function of aluminum-activated malate transporters from wheat (Triticum aestivum) and Arabidopsis thaliana in Xenopus oocytes

Wheat and Arabidopsis plants respond to aluminum (Al) ions by releasing malate from their root apices via Al-activated malate transporter. Malate anions bind with the toxic Al ions and contribute to the Al tolerance of these species. The genes encoding the transporters in wheat and Arabidopsis, TaALMT1 and AtALMT1, respectively, were expressed in Xenopus laevis oocytes and characterized electrophysiologically using the two-electrode voltage clamp system. The Al-activated currents generated by malate efflux were detected for TaALMT1 but not for AtALMT1. Chimeric proteins were generated by swapping the N- and C-terminal halves of TaALMT1 and AtALMT1 (Ta::At and At::Ta). When these chimeras were characterized in oocytes, Al-activated malate efflux was detected for the Ta::At chimera but not for At::Ta, suggesting that the N-terminal half of TaALMT1 is necessary for function in oocytes. An additional chimera, Ta(48)::At, generated by swapping 17 residues from the N-terminus of AtALMT1 with the equivalent 48 residues from TaALMT1, was sufficient to support transport activity. This 48 residue region includes a helical region with a putative transmembrane domain which is absent in AtALMT1. The deletion of this domain from Ta(48)::At led to the complete loss of transport activity. Furthermore, truncations and a deletion at the C-terminal end of TaALMT1 indicated that a putative helical structure in this region was also required for transport function. This study provides insights into the structure-function relationships of Al-activated ALMT proteins by identifying specific domains on the N- and C-termini of TaALMT1 that are critical for basal transport function and Al responsiveness in oocytes.

Early senescence of the oldest leaves of Fe-deficient barley plants may contribute to phytosiderophore release from the roots

Barley (Hordeum vulgare), which tolerates iron (Fe) deficiency, secretes a large amount of phytosiderophores from its roots. However, how barley is able to allocate resources for phytosiderophore synthesis when the carbon assimilation rate is reduced by Fe deficiency is unknown. We previously suggested that the acceleration of senescence in older leaves triggered by Fe deficiency may allow the recycling of assimilates to contribute to phytosiderophore synthesis. In this work, we show the relationship between an increase in the C/N ratio in older leaves and Fe-deficiency tolerance among three barley cultivars. The increase in the C/N ratio suggests an enhanced capacity for the retranslocation of carbohydrates or amino acids from older leaves to the sink organs. An increase in the sucrose concentration in Fe-deficient barley also suggests active redistribution of assimilates. This metabolic modulation may be supported by accelerated senescence of older leaves, as Fe deficiency increased the expression of senescence-associated genes. The older leaves of Fe-deficient barley maintained CO2 assimilation under Fe deficiency. Barley that had been Fe-deficient for 3 days preferentially allocated newly assimilated (13) C to the roots and nutrient solution. Interestingly, the oldest leaf of Fe-deficient barley released more (13) C into the nutrient solution than the second oldest leaf. Thus, the balance between anabolism and catabolism in older leaves, supported by highly regulated senescence, plays a key role in metabolic adaptation in Fe-deficient barley.

Alternative functions of Arabidopsis Yellow Stripe-Like3: from metal translocation to pathogen defense

Yellow stripe-like1 (YSL1) and YSL3 are involved in iron (Fe) and copper (Cu) translocation. Previously, we reported that upregulation of YSL1 and YSL3 under excess Cu caused high accumulation of Cu in the siz1 mutant, impaired in small ubiquitin-like modifier (SUMO) E3 ligase. Interestingly, the siz1 mutant contains high levels of salicylic acid (SA), involved in plant defense against biotrophic pathogens. In this study, we found that YSL1 and YSL3 were upregulated by SA. SA-regulated YSL3 but not YSL1 depended on nonexpressor of PR1 (NPR1). Susceptibility to the pathogen Pseudomonas syringe pv. tomato (Pst) DC3000 was greater for ysl3 than the wild type. Also, during Pst DC3000 infection, YSL3 was positively regulated by SA signaling through NPR1 and the upregulation was enhanced in the coi1 mutant that defective in the jasmonic acid (JA) receptor, coronatine insensitive1. This line of evidence indicates that the regulation of YSL3 is downstream of SA signaling and interplays with JA signaling for involvement in pathogen-induced defense. We provide new insights into the biological function of the metal transporter YSL3 in plant pathogen defense.

Arabidopsis thaliana Yellow Stripe1-Like4 and Yellow Stripe1-Like6 localize to internal cellular membranes and are involved in metal ion homeostasis

Several members of the Yellow Stripe1-Like (YSL) family of transporter proteins are able to transport metal-nicotianamine (NA) complexes. Substantial progress has been made in understanding the roles of the Arabidopsis YSLs that are most closely related to the founding member of the family, ZmYS1 (e.g., AtYSL1, AtYSL2 and AtYSL3), but there is little information concerning members of the other two well-conserved YSL clades. Here, we provide evidence that AtYSL4 and AtYSL6, which are the only genes in Arabidopsis belong to YSL Group II, are localized to vacuole membranes and to internal membranes resembling endoplasmic reticulum. Both single and double mutants for YSL4 and YSL6 were rigorously analyzed, and have surprisingly mild phenotypes, in spite of the strong and wide-ranging expression of YSL6. However, in the presence of toxic levels of Mn and Ni, plants with mutations in YSL4 and YSL6 and plants overexpressing GFP-tagged YSL6 showed growth defects, indicating a role for these transporters in heavy metal stress responses.

TcOPT3, a member of oligopeptide transporters from the hyperaccumulator Thlaspi caerulescens, is a novel Fe/Zn/Cd/Cu transporter

BACKGROUND

Thlaspi caerulescens is a natural selected heavy metal hyperaccumulator that can not only tolerate but also accumulate extremely high levels of heavy metals in the shoots. Thus, to identify the transportors involved in metal long-distance transportation is very important for understanding the mechanism of heavy metal accumulation in this hyperaccumulator.

METHODOLOGY/PRINCIPAL FINDINGS

We cloned and characterized a novel gene TcOPT3 of OPT family from T. caerulescens. TcOPT3 was pronouncedly expressed in aerial parts, including stem and leaf. Moreover, in situ hybridization analyses showed that TcOPT3 expressed in the plant vascular systems, especially in the pericycle cells that may be involved in the long-distance transportation. The expression of TcOPT3 was highly induced by iron (Fe) and zinc (Zn) deficiency, especially in the stem and leaf. Sub-cellular localization showed that TcOPT3 was a plasma membrane-localized protein. Furthermore, heterogonous expression of TcOPT3 by mutant yeast (Saccharomyces cerevisiae) complementation experiments demonstrated that TcOPT3 could transport Fe(2+) and Zn(2+). Moreover, expression of TcOPT3 in yeast increased metal (Fe, Zn, Cu and Cd) accumulation and resulted in an increased sensitivity to cadmium (Cd) and copper (Cu).

CONCLUSIONS

Our data demonstrated that TcOPT3 might encode an Fe/Zn/Cd/Cu influx transporter with broad-substrate. This is the first report showing that TcOPT3 may be involved in metal long-distance transportation and contribute to the heavy metal hyperaccumulation.

The zinc homeostasis network of land plants

The use of the essential element zinc (Zn) in the biochemistry of land plants is widespread, and thus comparable to that in other eukaryotes. Plants have evolved the ability to adjust to vast fluctuations in external Zn supply, and they can store considerable amounts of Zn inside cell vacuoles. Moreover, among plants there is overwhelming, but yet little explored, natural genetic diversity that phenotypically affects Zn homeostasis. This results in the ability of specific races or species to thrive in different soils ranging from extremely Zn-deficient to highly Zn-polluted. Zn homeostasis is maintained by a tightly regulated network of low-molecular-weight ligands, membrane transport and Zn-binding proteins, as well as regulators. Here we review Zn homeostasis of land plants largely based on the model plant Arabidopsis thaliana, for which our molecular understanding is most developed at present. There is some evidence for substantial conservation of Zn homeostasis networks among land pants, and this review can serve as a reference for future comparisons. Major progress has recently been made in our understanding of the regulation of transcriptional Zn deficiency responses and the role of the low-molecular-weight chelator nicotianamine in plant Zn homeostasis. Moreover, we have begun to understand how iron (Fe) and Zn homeostasis interact as a consequence of the chemical similarity between their divalent cations and the lack of specificity of the major root iron uptake transporter IRT1. The molecular analysis of Zn-hyperaccumulating plants reveals how metal homeostasis networks can be effectively modified. These insights are important for sustainable bio-fortification approaches. This article is part of a Special Issue entitled: Cell Biology of Metals.

Response of barley plants to Fe deficiency and Cd contamination as affected by S starvation

Both Fe deficiency and Cd exposure induce rapid changes in the S nutritional requirement of plants. The aim of this work was to characterize the strategies adopted by plants to cope with both Fe deficiency (release of phytosiderophores) and Cd contamination [production of glutathione (GSH) and phytochelatins] when grown under conditions of limited S supply. Experiments were performed in hydroponics, using barley plants grown under S sufficiency (1.2 mM sulphate) and S deficiency (0 mM sulphate), with or without Fe(III)-EDTA at 0.08 mM for 11 d and subsequently exposed to 0.05 mM Cd for 24 h or 72 h. In S-sufficient plants, Fe deficiency enhanced both root and shoot Cd concentrations and increased GSH and phytochelatin levels. In S-deficient plants, Fe starvation caused a slight increase in Cd concentration, but this change was accompanied neither by an increase in GSH nor by an accumulation of phytochelatins. Release of phytosiderophores, only detectable in Fe-deficient plants, was strongly decreased by S deficiency and further reduced after Cd treatment. In roots Cd exposure increased the expression of the high affinity sulphate transporter gene (HvST1) regardless of the S supply, and the expression of the Fe deficiency-responsive genes, HvYS1 and HvIDS2, irrespective of Fe supply. In conclusion, adequate S availability is necessary to cope with Fe deficiency and Cd toxicity in barley plants. Moreover, it appears that in Fe-deficient plants grown in the presence of Cd with limited S supply, sulphur may be preferentially employed in the pathway for biosynthesis of phytosiderophores, rather than for phytochelatin production.

Isolation and expression analysis of partial sequences of heavy metal transporters from Brassica juncea by coupling high throughput cloning with a molecular fingerprinting technique

Heavy metal transporters play a key role in regulating metal accumulation and transport in plants. These are important candidate genes to study in metal tolerant and accumulator plants for their potential use in environmental clean up. We coupled a degenerate primer-based RT-PCR approach with a molecular fingerprinting technique based on amplified rDNA restriction analysis (ARDRA) to identify novel ESTs corresponding to heavy metal transporters from metal accumulator Brassica juncea. We utilized this technique to clone several family members of natural resistance-associated macrophage proteins (NRAMP) and yellow stripe-like proteins (YSL) in a high throughput manner to distinguish between closely related isoforms and/or allelic variants from the allopolyploid B. juncea. Partial clones of 23 Brassica juncea NRAMPs and 27 YSLs were obtained with similarity to known Arabidopsis thaliana and Noccaea (Thlaspi) caerulescens NRAMP and YSL genes. The cloned transporters showed Brassica-specific changes in domains, which can have important functional consequences. Semi-quantitative RT-PCR-based expression analysis of chosen members indicated that even closely related isoforms/allelic variants of BjNRAMP and BjYSL have distinct tissue-specific and metal-dependent expressions which might be essential for adaptive fitness and heavy metal tolerance. Consistent to this, BjYSL6.1 and BjYSL5.8 were found to show elevated expressions specifically in cadmium-treated shoots and lead-treated roots of B. juncea, respectively.

Transporters contributing to iron trafficking in plants

This review will discuss recent progress in understanding the many roles of transporters in the whole-plant physiological processes that maintain iron (Fe) homeostasis. These processes include uptake from the soil via roots, control of transport from roots to above-ground parts of the plant, unloading of Fe from the xylem in above-ground parts, loading of Fe into mitochondria and plastids, transport of Fe to reproductive parts of the plant, and Fe mobilization during seed germination. In addition, we will discuss the mechanisms that plants use to cope with an apparently unintended consequence of Fe acquisition: the uptake of toxic heavy metals via Fe transporters. Rapid progress has been made in understanding the transport processes involved in each of these areas in the last 5 years and this review will focus on this recent progress. We will also highlight the key questions regarding transport steps that remain to be elucidated.

OsYSL18 is a rice iron(III)-deoxymugineic acid transporter specifically expressed in reproductive organs and phloem of lamina joints

Iron uptake and translocation in plants are important processes for both plant and human nutrition, whereas relatively little is known about the molecular mechanisms of iron transport within the plant body. Several reports have shown that yellow stripe 1 (YS1) and YS1-like (YSL) transporters mediate metal-phytosiderophore uptake and/or metal-nicotianamine translocation. Among the 18 YSL genes in rice (OsYSLs), OsYSL18 is predicted to encode a polypeptide of 679 amino acids containing 13 putative transmembrane domains. An OsYSL18-green fluorescent protein (GFP) fusion was localized to the plasma membrane when transiently expressed in onion epidermal cells. Electrophysiological measurements using Xenopus laevis oocytes showed that OsYSL18 transports iron(III)-deoxymugineic acid, but not iron(II)-nicotianamine, zinc(II)-deoxymugineic acid, or zinc(II)-nicotianamine. Reverse transcriptase PCR analysis revealed more OsYSL18 transcripts in flowers than in shoots or roots. OsYSL18 promoter-beta-glucuronidase (GUS) analysis revealed that OsYSL18 was expressed in reproductive organs including the pollen tube. In vegetative organs, OsYSL18 was specifically expressed in lamina joints, the inner cortex of crown roots, and phloem parenchyma and companion cells at the basal part of every leaf sheath. These results suggest that OsYSL18 is an iron-phytosiderophore transporter involved in the translocation of iron in reproductive organs and phloem in joints.

Metal movement within the plant: contribution of nicotianamine and yellow stripe 1-like transporters

Background Since the identification of the genes controlling the root acquisition of iron (Fe), the control of inter- and intracellular distribution has become an important challenge in understanding metal homeostasis. The identification of the yellow stripe-like (YSL) transporter family has paved the way to decipher the mechanisms of long-distance transport of Fe. Scope Once in the plant, Fe will systematically react with organic ligands whose identity is poorly known so far. Among potential ligands, nicotianamine has been identified as an important molecule for the circulation and delivery of metals since it participates in the loading of copper (Cu) and nickel in xylem and prevents Fe precipitation in leaves. Nicotianamine is a precursor of phytosiderophores, which are high-affinity Fe ligands exclusively synthesized by Poaceae species and excreted by roots for the chelation and acquisition of Fe. Maize YS1 is the founding member of a family of membrane transporters called YS1-like (YSL), which functions in root Fe-phytosiderophore uptake from the soil. Next to this well-known Fe acquisition role, most of the other YSL family members are likely to function in plant-wide distribution of metals since (a) they are produced in vascular tissues throughout the plant and (b) they are found in non-Poaceae species that do not synthesize phytosiderophores. The hypothesized activity as Fe-nicotianamine transporters of several YSL members has been demonstrated experimentally by heterologous expression in yeast or by electrophysiology in Xenopus oocytes but, despite numerous attempts, proof of the arabidopsis YSL substrate specificity is still lacking. Reverse genetics, however, has revealed a role for AtYSL members in the remobilization of Cu and zinc from senescing leaves, in the formation of pollen and in the Fe, zinc and Cu loading of seeds. Conclusions Preliminary data on the YSL family of transporters clearly argues in favour of its role in the long-distance transport of metals through and between vascular tissues to eventually support gametogenesis and embryo development.

Specific transporter for iron(III): Phytosiderophore complex involved in iron uptake by barley roots

Iron (Fe) is an essential element for plant growth. Gramineous plants have generally developed a distinct strategy to efficiently acquire insoluble Fe, which is characterized by the synthesis and secretion of an Fe-chelating substance, phytosiderophore (PS) such as mugineic acid (MA), and by a specific uptake system for Fe(III)-PS complexes. In a previous study, we identified a gene specifically encoding an Fe(III)-PS transporter (HvYS1) in barley. This gene as well as the encoded protein is specifically expressed in the epidermal cells of the roots, and gene expression is greatly enhanced under Fe-deficient conditions. The localization and substrate specificity of HvYS1 indicate that it is a specific transporter in barley roots. In contrast, ZmYS1, which has been reported as an Fe-PS transporter from maize, possesses broad substrate specificity despite a high homology with HvYS1. By assessing the transport activity of a series of HvYS1-ZmYS1 chimeras, we revealed that the outer membrane loop between the 6th and 7th transmembrane regions is essential for the substrate specificity. We also achieved an efficient short-step synthesis of MA and 2'-deoxymugineic acid (DMA). Our new synthetic method enabled us to use them in a large quantity for biological studies.