Metal response of transgenic tomato plants expressing P(1B) -ATPase

Heavy metal transporters: Functional mechanisms, regulation, and application in phytoremediation

Heavy metal pollution in soil is a global problem with serious impacts on human health and ecological security. Phytoextraction in phytoremediation, in which plants uptake and transport heavy metals (HMs) to the tissues of aerial parts, is the most environmentally friendly method to reduce the total amount of HMs in soil and has wide application prospects. However, the molecular mechanism of phytoextraction is still under investigation. The uptake, translocation, and retention of HMs in plants are mainly mediated by a variety of transporter proteins. A better understanding of the accumulation strategy of HMs via transporters in plants is a prerequisite for the improvement of phytoextraction. In this review, the biochemical structure and functions of HM transporter families in plants are systematically summarized, with emphasis on their roles in phytoremediation. The accumulation mechanism and regulatory pathways related to hormones, regulators, and reactive oxygen species (ROS) of HMs concerning these transporters are described in detail. Scientific efforts and practices for phytoremediation carried out in recent years suggest that creation of hyperaccumulators by transgenic or gene editing techniques targeted to these transporters and their regulators is the ultimate powerful path for the phytoremediation of HM contaminated soils.

Zinc uptake and HMA4 activity are required for micro- and macroelement balance in tobacco (Nicotiana tabacum)

The pleiotropic effects of zinc deficiency on ion homeostasis have already been described in several plants. Tobacco (Nicotiana tabacum) heavy metal ATPases HMA4.1 and HMA4.2 are involved in zinc and cadmium root-to-shoot translocation. In previous research, we have shown that N. tabacum HMA4 RNAi plants and HMA4 double-nonsense mutants exhibit strongly reduced zinc and cadmium levels in leaves as well as stunted growth. In this study, the ionome and transcriptome of these lines were investigated to better characterize the effect of reduced zinc levels and to understand the impaired growth phenotype. We found that, under standard greenhouse fertilization rates, these lines accumulated up to 4- to 6-fold more phosphorus, iron, manganese, and copper than their respective controls. Under field conditions, HMA4 double-mutant plants also exhibited similar accumulation phenotypes, albeit to a lower extent. In both HMA4 RNAi plants and HMA4 mutants, transcription analysis showed a local zinc-deficiency response in leaves as well as an FIT1-mediated iron-deficiency response in roots, likely contributing to iron and manganese uptake at the root level. A phosphate-starvation response involving HHO2 was also observed in HMA4-impaired plant leaves. The high level of phosphorus observed in HMA4-impaired plants is correlated with leaf swelling and necrosis. The upregulation of aquaporin genes is in line with cellular water influx and the observed leaf swelling phenotype. These results highlight the involvement of HMA4 in zinc homeostasis and related regulatory processes that balance the micro- and macroelements in above-ground organs.

Suppression of NtZIP4A/B Changes Zn and Cd Root-to-Shoot Translocation in a Zn/Cd Status-Dependent Manner

In tobacco, the efficiency of Zn translocation to shoots depends on Zn/Cd status. Previous studies pointed to the specific contribution of root parts in the regulation of this process, as well as the role of NtZIP4A/B (from the ZIP family; Zrt Irt-like Proteins). Here, to verify this hypothesis, NtZIP4A/B RNAi lines were generated. Then, in plants exposed to combinations of Zn and Cd concentrations in the medium, the consequences of NtZIP4A/B suppression for the translocation of both metals were determined. Furthermore, the apical, middle, and basal root parts were examined for accumulation of both metals, for Zn localization (using Zinpyr-1), and for modifications of the expression pattern of ZIP genes. Our results confirmed the role of NtZIP4A/B in the control of Zn/Cd-status-dependent transfer of both metals to shoots. Furthermore, they indicated that the middle and basal root parts contributed to the regulation of this process by acting as a reservoir for excess Zn and Cd. Expression studies identified several candidate ZIP genes that interact with NtZIP4A/B in the root in regulating Zn and Cd translocation to the shoot, primarily NtZIP1-like in the basal root part and NtZIP2 in the middle one.

Zn/Cd status-dependent accumulation of Zn and Cd in root parts in tobacco is accompanied by specific expression of ZIP genes

BACKGROUND

Root-to-shoot translocation of zinc (Zn) and cadmium (Cd) depends on the concentrations of both metals in the medium. A previous study on tobacco (Nicotiana tabacum) pointed to the contribution of NtZIP1, NtZIP2, NtZIP4 and NtIRT1-like in the regulation of this phenomenon. To learn more, Zn and Cd accumulation, root/shoot distribution and the expression of ZIP genes were investigated in the apical, middle and basal root parts.

RESULTS

We show that Zn/Cd status-dependent root-shoot distribution of both metals was related to distinct metal accumulation in root parts. At low Zn and Cd in the medium, the apical part contained the highest metal level; at higher concentrations, the middle and basal parts were the major sink for excess metal. The above were accompanied by root part-specific expression pattern modifications of ZIPs (NtZIP1-like, NtZIP2, NtZIP4A/B, NtZIP5A/B, NtZIP5-like, NtZIP8, NtZIP11, NtIRT1, and NtIRT1-like) that fell into four categories with respect to the root part. Furthermore, for lower Zn/Cd concentrations changes were noted for NtZIP5A/B and NtZIP5-like only, but at higher Zn and Cd levels for NtZIP1-like, NtZIP5-like, NtZIP8, NtZIP11, NtIRT1, and NtIRT1-like. NtZIP1, here renamed to NtZIP5B, was cloned and characterized. We found that it was a zinc deficiency-inducible transporter involved in zinc and cadmium uptake from the soil solution primarily by the middle root part.

CONCLUSIONS

We conclude that regulation of the longitudinal distribution of Zn and Cd is highly specific, and that the apical, middle and basal root parts play distinct roles in Zn/Cd status-dependent control of metal translocation efficiency to shoots, including the stimulation of Zn translocation to shoots in the presence of Cd. These results provide new insight into the root part-specific unique role of NtZIP5B and other ZIP genes in the longitudinal distribution of zinc and cadmium and their contribution to the regulation of root-to-shoot translocation.

Study of Zn accumulation and tolerance of HMA4 TILLING mutants of Brassica rapa grown under Zn deficiency and Zn toxicity

Nowadays, Zinc (Zn) deficiency is the most widespread micronutrient deficiency but simultaneously Zn toxicity is produced due to environmental pollution. A potential method to alleviate Zn deficiency and to reduce Zn concentration in soils is through the generation of plants with enhanced capacity for Zn accumulation and higher tolerance. This could be achieved through the modification of HMA4 transporter. BraA.hma4a-3 is a TILLING mutant plant that presents one modification in HMA4 transporter. Thus, in this study we analyzed the potential of BraA.hma4a-3 for Zn accumulation and Zn deficiency and toxicity tolerance. BraA.hma4a-3 and parental R-o-18 plants were grown with different Zn doses: 1 μM ZnSO₄ (Control), 0.01 μM ZnSO₄ (Zn deficiency) and 100 μM ZnSO₄ (Zn toxicity). Parameters of biomass, Zn concentration, photosynthesis, oxidative stress, N metabolism and amino acids (AAs) were measured. BraA.hma4a-3 did not affect plant biomass but did increase Zn accumulation in leaves under an adequate Zn supply and Fe under control and Zn deficiency doses. Regarding stress tolerance parameters and N metabolism, BraA.hma4a did not produce alterations under control conditions. In addition, under Zn toxicity, parameters suggest a greater tolerance. Briefly, the obtained results point to BraA.hma4a-3 as a useful mutant to increase Zn accumulation.

Functional Analysis of NtZIP4B and Zn Status-Dependent Expression Pattern of Tobacco ZIP Genes

Tobacco is frequently considered as a plant useful for phytoremediation of metal-contaminated soil, despite the mechanisms for regulation of uptake and accumulation being largely unknown. Here we cloned and characterized a new tobacco Zn and Cd transporter NtZIP4B from the ZIP family (ZRT-IRT-Like proteins). It complemented the Zn-uptake defective yeast mutant zrt1zrt2, and rendered the wild type DY1457 yeast more sensitive to Cd. Bioinformatic analysis and transient expression of the NtZIP4B-GFP fusion protein in tobacco leaves indicated its localization to the plasma membrane. Real-time q-PCR based analysis showed that it is expressed in all vegetative organs with the highest level in leaves. The Zn status determined transcript abundance; NtZIP4B was upregulated by Zn-deficiency and downregulated by Zn excess. At the tissue level, in roots NtZIP4B is expressed in the vasculature of the middle part of the roots and in surrounding tissues including the root epidermis; in leaves primarily in the vasculature. Bioinformatic analysis identified two copies of ZIP4 in tobacco, NtZIP4A and NtZIP4B with 97.57% homology at the amino acid level, with the same expression pattern for both, indicating a high degree of functional redundancy. Moreover, the present study provides new insights into the coordinated function of NtZIP1, NtZIP2, NtZIP4, NtZIP5, NtZIP8, NtIRT1, and NtIRT1-like in response to low-to-high Zn status. Leaves were the major site of NtZIP4, NtZIP5, and NtZIP8 expression, and roots for NtZIP1, NtZIP2, NtIRT1, and NtIRT1-like. Contrasting expression level in the apical and basal root parts indicates distinct roles in root-specific processes likely contributing to the regulation of Zn root-to-shoot translocation. In summary, new insight into the role of ZIP genes in Zn homeostasis pointing to their overlapping and complementary functions, offers opportunities for strategies to modify Zn and Cd root/shoot partition in tobacco.

NO-Dependent programmed cell death is involved in the formation of Zn-related lesions in tobacco leaves

A recent study indicated that the development of lesions on the leaf blades of tobacco exposed to zinc (Zn) excess can be considered a manifestation of a Zn-tolerance strategy at the organ level. Here, we investigated whether cell death leading to the formation of localized lesions is destructive in character (necrosis type) or results from programmed self-induced cell death (PCD). Selected parameters, including PCD markers, were determined in the leaves from tobacco plants grown in the presence of 200 μM Zn and compared with control conditions. TUNEL assay results showing internucleosomal DNA fragmentation in the nuclei of the cells from Zn-exposed leaves, together with an enhanced expression of three PCD marker genes (NtBI-1, Ntrboh, and NtSIPK), indicated the involvement of PCD in the formation of Zn-related lesions. It is known that NO is a key factor in the execution of PCD. Interestingly, upon exposure to high Zn, in situ localization of NO (visualized using DAF-2DA fluorescence) was restricted to groups of mesophyll cells, and was correlated with the pattern of Zn localization (determined using the fluorophore Zinpyr-1), similarly limited primarily to groups of "Zn accumulating cells". Furthermore, inhibition of the formation of lesions in the presence of l-NAME (an NO synthase inhibitor) was accompanied by the delayed appearance of Zn and by NO localization limited to these groups of cells. Altogether, we provide the first demonstration that Zn-related lesions in leaves develop from groups of mesophyll cells in which accumulation of high concentrations of Zn contributes to enhancement of the NO level and to initiation of PCD processes.

The ratio of Zn to Cd supply as a determinant of metal-homeostasis gene expression in tobacco and its modulation by overexpressing the metal exporter AtHMA4

This study links changes in the tobacco endogenous metal-homeostasis network caused by transgene expression with engineering of novel features. It also provides insight into the concentration-dependent mutual interactions between Zn and Cd, leading to differences in the metal partitioning between wild-type and transgenic plants. In tobacco, expression of the export protein AtHMA4 modified Zn/Cd root/shoot distribution, but the pattern depended on their concentrations in the medium. To address this phenomenon, the expression of genes identified by suppression subtractive hybridization and the Zn/Cd accumulation pattern were examined upon exposure to six variants of low/high Zn and Cd concentrations. Five tobacco metal-homeostasis genes were identified: NtZIP2, NtZIP4, NtIRT1-like, NtNAS, and NtVTL. In the wild type, their expression depended on combinations of low/high Zn and Cd concentrations; co-ordinated responses of NtZIP1, NtZIP2, and NtVTL were shown in medium containing 4 µM Cd, and at 0.5 µM versus 10 µM Zn. In transgenics, qualitative changes detected for NtZIP1, NtZIP4, NtIRT1-like, and NtVTL are considered crucial for modification of Zn/Cd supply-dependent Zn/Cd root/shoot distribution. Notwithstanding, NtVTL was the most responsive gene in wild-type and transgenic plants under all concentrations of Zn and Cd tested; thus it is a candidate gene for the regulation of metal cross-homeostasis processes involved in engineering new metal-related traits.

Engineering high Zn in tomato shoots through expression of AtHMA4 involves tissue-specific modification of endogenous genes

Background

To increase the Zn level in shoots, AtHMA4 was ectopically expressed in tomato under the constitutive CaMV 35S promoter. However, the Zn concentration in the shoots of transgenic plants failed to increase at all tested Zn levels in the medium. Modification of Zn root/shoot distribution in tomato expressing 35S::AtHMA4 depended on the concentration of Zn in the medium, thus indicating involvement of unknown endogenous metal-homeostasis mechanisms. To determine these mechanisms, those metal-homeostasis genes that were expressed differently in transgenic and wild-type plants were identified by microarray and RT-qPCR analysis using laser-assisted microdissected RNA isolated from two root sectors: (epidermis + cortex and stele), and leaf sectors (upper epidermis + palisade parenchyma and lower epidermis + spongy parenchyma).

Results

Zn-supply-dependent modification of Zn root/shoot distribution in AtHMA4-tomato (increase at 5 μM Zn, no change at 0.5 μM Zn) involved tissue-specific, distinct from that in the wild type, expression of tomato endogenous genes. First, it is suggested that an ethylene-dependent pathway underlies the detected changes in Zn root/shoot partitioning, as it was induced in transgenic plants in a distinct way depending on Zn exposure. Upon exposure to 5 or 0.5 μM Zn, in the epidermis + cortex of the transgenics’ roots the expression of the Strategy I Fe-uptake system (ethylene-dependent LeIRT1 and LeFER) was respectively lower or higher than in the wild type and was accompanied by respectively lower or higher expression of the identified ethylene genes (LeNR, LeACO4, LeACO5) and of LeChln. Second, the contribution of LeNRAMP2 expression in the stele is shown to be distinct for wild-type and transgenic plants at both Zn exposures. Ethylene was also suggested as an important factor in a pathway induced in the leaves of transgenic plants by high Zn in the apoplast, which results in the initiation of loading of the excess Zn into the mesophyll of “Zn accumulating cells”.

Conclusions

In transgenic tomato plants, the export activity of ectopically expressed AtHMA4 changes the cellular Zn status, which induces coordinated tissue-specific responses of endogenous ethylene-related genes and metal transporters. These changes constitute an important mechanism involved in the generation of the metal-related phenotype of transgenic tomato expressing AtHMA4.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2990-x) contains supplementary material, which is available to authorized users.

Determination the Usefulness of AhHMA4p1::AhHMA4 Expression in Biofortification Strategies

AhHMA4 from Arabidopsis thaliana encodes Zn/Cd export protein that controls Zn/Cd translocation to shoots. The focus of this manuscript is the evaluation of AhHMA4 expression in tomato for mineral biofortification (more Zn and less Cd in shoots and fruits). Hydroponic and soil-based experiments were performed. Transgenic and wild-type plants were grown on two dilution levels of Knop's medium (1/10, 1/2) with or without Cd, to determine if mineral composition affects the pattern of root/shoot partitioning of both metals due to AhHMA4 expression. Facilitation of Zn translocation to shoots of 19-day-old transgenic tomato was noted only when plants were grown in the more diluted medium. Moreover, the expression pattern of Zn-Cd-Fe cross-homeostasis genes (LeIRT1, LeChln, LeNRAMP1) was changed in transgenics in a medium composition-dependent fashion. In plants grown in soil (with/without Cd) up to maturity, expression of AhHMA4 resulted in more efficient translocation of Zn to shoots and restriction of Cd. These results indicate the usefulness of AhHMA4 expression to improve the growth of tomato on low-Zn soil, also contaminated with Cd.

Functional analysis of the three HMA4 copies of the metal hyperaccumulator Arabidopsis halleri

In Arabidopsis halleri, the HMA4 gene has an essential function in Zn/Cd hypertolerance and hyperaccumulation by mediating root-to-shoot translocation of metals. Constitutive high expression of AhHMA4 results from a tandem triplication and cis-activation of the promoter of all three copies. The three AhHMA4 copies possess divergent promoter sequences, but highly conserved coding sequences, and display identical expression profiles in the root and shoot vascular system. Here, an AhHMA4::GFP fusion was expressed under the control of each of the three A. halleri HMA4 promoters in a hma2hma4 double mutant of A. thaliana to individually examine the function of each AhHMA4 copy. The protein showed non-polar localization at the plasma membrane of the root pericycle cells of both A. thaliana and A. halleri. The expression of each AhHMA4::GFP copy complemented the severe Zn-deficiency phenotype of the hma2hma4 mutant by restoring root-to-shoot translocation of Zn. However, each copy had a different impact on metal homeostasis in the A. thaliana genetic background: AhHMA4 copies 2 and 3 were more highly expressed and provided higher Zn tolerance in roots and accumulation in shoots than copy 1, and AhHMA4 copy 3 also increased Cd tolerance in roots. These data suggest a certain extent of functional differentiation among the three A. halleri HMA4 copies, stemming from differences in expression levels rather than in expression profile. HMA4 is a key node of the Zn homeostasis network and small changes in expression level can have a major impact on Zn allocation to root or shoot tissues.

Got to hide your Zn away: Molecular control of Zn accumulation and biotechnological applications

Zinc (Zn) is an essential micronutrient for all organisms, with key catalytic and structural functions. Zn deficiency in plants, common in alkaline soils, results in growth arrest and sterility. On the other hand, Zn can become toxic at elevated concentrations. Several studies revealed molecules involved with metal acquisition in roots, distribution within the plant and translocation to seeds. Transmembrane Zn transport proteins and Zn chelators are involved in avoiding its toxic effects. Plant species with the capacity to hyperaccumulate and hypertolerate Zn have been characterized. Plants that accumulate and tolerate high amounts of Zn and produce abundant biomass may be useful for phytoremediation, allowing cleaning of metal-contaminated soils. The study of Zn hyperaccumulators may provide indications of genes and processes useful for biofortification, for developing crops with high amounts of nutrients in edible tissues. Future research needs to focus on functional characterization of Zn transporters in planta, elucidation of Zn uptake and sensing mechanisms, and on understanding the cross-talk between Zn homeostasis and other physiological processes. For this, new research should use multidisciplinary approaches, combining traditional and emerging techniques, such as genome-encoded metal sensors and multi-element imaging, quantification and speciation using synchrotron-based methods.

In vitro regeneration of eight cultivars of Brassica oleracea var. capitata

Eight cultivars of Brassica oleracea var. capitata and two types of explant (hypocotyl and cotyledon) were tested for their potential to regenerate under in vitro conditions. Hypocotyl and cotyledon explants from 10-d-old seedlings were subcultured onto different callus induction media based on Murashige and Skoog (MS) basal medium supplemented with 1% sucrose and different concentrations and combinations of plant growth regulators. Hypocotyl explants were found to be more suitable for callus induction and organogenesis than cotyledon explants for all cultivars tested. In terms of regeneration, the cv. 'Amager' was significantly more responsive than the other cultivars tested and produced the highest number of shoots/buds per explant. Moreover, among five types of media tested, MS + 8.88 μM 6-benzyloaminopurine (BAP) + 0.53 μM α-naphthylacetic acid (NAA) was most effective for shoot regeneration. Rooting was achieved within 10-15 d on all the rooting media, but MS medium containing 5.37 μM NAA produced the maximum number of strong and healthy roots. Plantlets (95%) were subsequently established in the greenhouse, and no phenotypic variations were observed among regenerated plants. This plant regeneration protocol could be suitable for a wide range of cabbage cultivars.

OsACA6, a P-type 2B Ca(2+) ATPase functions in cadmium stress tolerance in tobacco by reducing the oxidative stress load

MAIN CONCLUSION

The present study demonstrates the first direct evidence of the novel role of OsACA6 in providing Cd (2+) stress tolerance in transgenic tobacco by maintaining cellular ion homeostasis and modulating ROS-scavenging pathway. Cadmium, a non-essential toxic heavy metal, interferes with the plant growth and development. It reaches the leaves through xylem and may become part of the food chain, thus causing detrimental effects to human health. Therefore, there is an urgent need to develop strategies for engineering plants for Cd(2+) tolerance and less accumulation. The members of P-type ATPases family transport metal ions including Cd(2+), and thus play important role an ion homeostasis. The present study elucidates the role of P-type 2B Ca(2+) ATPase (OsACA6) in Cd(2+) stress tolerance. The transcript levels of OsACA6 were up-regulated upon Cd(2+), Zn(2+) and Mn(2+) exposure. Transgenic tobacco expressing OsACA6 showed tolerance towards Cd(2+) stress as demonstrated by several physiological indices including root length, biomass, chlorophyll, malondialdehyde and hydrogen peroxide content. The roots of the transgenic lines accumulated more Cd(2+) as compared to shoot. Further, confocal laser scanning microscopy showed that Cd(2+) exposure altered Ca(2+) uptake in OsACA6 transgenic plants. OsACA6 expression in tobacco also protected the transgenic plants from oxidative stress by enhancing the activity of enzymatic (SOD, CAT, APX, GR) and non-enzymatic (GSH and AsA) antioxidant machinery. Transgenic lines also tolerated Zn(2+) and Mn(2+) stress; however, tolerance for these ions was not as significant as observed for Cd(2+) exposure. Thus, overexpression of OsACA6 confers Cd(2+) stress tolerance in transgenic lines by maintaining cellular ion homeostasis and modulating reactive oxygen species (ROS)-scavenging pathway. The results of the present study will help to develop strategies for engineering Cd(2+) stress tolerance in economically important crop plants.

Approach to engineer tomato by expression of AtHMA4 to enhance Zn in the aerial parts

The aim of this work was to assess the potential for using AtHMA4 to engineer enhanced efficiency of Zn translocation to shoots, and to increase the Zn concentration in aerial tissues of tomato. AtHMA4, a P1B-ATPase, encodes a Zn export protein known to be involved in the control of Zn root-to-shoot translocation. In this work, 35S::AtHMA4 was expressed in tomato (Lycopersicon esculentum var. Beta). Wild-type and transgenic plants were tested for Zn and Cd tolerance; Zn, Fe and Cd accumulation patterns, and for the expression of endogenous Zn/Fe-homeostasis genes. At 10μM Zn exposure, a higher Zn concentration was observed in leaves of AtHMA4-expressing lines compared to wild-type, which is promising in terms of Zn biofortification. AtHMA4 also transports Cd and at 0.25μM Cd the transgenic plants showed similar levels of this element in leaves to wild-type but lower levels in roots, therefore indicating a reduction of Cd uptake due to AtHMA4 expression. Expression of this transgene AtHMA4 also resulted in distinct changes in Fe accumulation in Zn-exposed plants, and Fe/Zn-accumulation in Cd-exposed plants, even though Fe is not a substrate for AtHMA4. Analysis of the transcript abundance of key Zn/Fe-homeostasis genes showed that the pattern was distinct for transgenic and wild-type plants. The reduction of Fe accumulation observed in AtHMA4-transformants was accompanied by up-regulation of Fe-deficiency marker genes (LeFER, LeFRO1, LeIRT1), whereas down-regulation was detected in plants with the status of Fe-sufficiency. Furthermore, results strongly suggest the importance of the up-regulation of LeCHLN in the roots of AtHMA4-expressing plants for efficient translocation of Zn to the shoots. Thus, the modifications of Zn/Fe/Cd translocation to aerial plant parts due to AtHMA4 expression are closely related to the alteration of the endogenous Zn-Fe-Cd cross-homeostasis network of tomato.

Histidine-mediated xylem loading of zinc is a species-wide character in Noccaea caerulescens

Histidine plays a crucial role in nickel (Ni) translocation in Ni-hyperaccumulating plants. Here, we investigated its role in zinc (Zn) translocation in four accessions of the Zn hyperaccumulator, Noccaea caerulescens, using the related non-hyperaccumulator, Thlaspi arvense, as a reference. We compared the effects of exogenous histidine supply on Zn xylem loading, and of Zn-histidine complex formation on Zn uptake in energized tonoplast vesicles. The Zn distribution patterns over root tissues were also compared. Exogenous histidine supply enhanced Zn xylem loading in all the N. caerulescens accessions, but decreased it in T. arvense. Zn distribution patterns over root tissues were similar, apart from the accumulation in cortical and endodermal cells, which was much lower in N. caerulescens than in T. arvense. Zn uptake in energized tonoplast vesicles was inhibited significantly in N. caerulescens, but not affected significantly in T. arvense, when Zn was supplied in combination with histidine in a 1:2 molar ratio. Histidine-mediated Zn xylem loading seems to be a species-wide character in N. caerulescens. It may well have evolved as a component trait of the hyperaccumulation machinery for Zn, rather than for Ni.

Metal species involved in long distance metal transport in plants

The mechanisms plants use to transport metals from roots to shoots are not completely understood. It has long been proposed that organic molecules participate in metal translocation within the plant. However, until recently the identity of the complexes involved in the long-distance transport of metals could only be inferred by using indirect methods, such as analyzing separately the concentrations of metals and putative ligands and then using in silico chemical speciation software to predict metal species. Molecular biology approaches also have provided a breadth of information about putative metal ligands and metal complexes occurring in plant fluids. The new advances in analytical techniques based on mass spectrometry and the increased use of synchrotron X-ray spectroscopy have allowed for the identification of some metal-ligand species in plant fluids such as the xylem and phloem saps. Also, some proteins present in plant fluids can bind metals and a few studies have explored this possibility. This study reviews the analytical challenges researchers have to face to understand long-distance metal transport in plants as well as the recent advances in the identification of the ligand and metal-ligand complexes in plant fluids.

Phenotypic and molecular consequences of overexpression of metal-homeostasis genes

Metal hyperaccumulating plants are able to store very large amounts of metals in their shoots. There are a number of reasons why it is important to be able to introduce metal hyperaccumulation traits into non-accumulating species (e.g., phytoremediation or biofortification in minerals) and to engineer a desired level of accumulation and distribution of metals. Metal homeostasis genes have therefore been used for these purposes. Engineered accumulation levels, however, have often been far from expected, and transgenic plants frequently display phenotypic features not related to the physiological function of the introduced gene. In this review, we focus on an aspect often neglected in research on plants expressing metal homeostasis genes: the specific regulation of endogenous metal homeostasis genes of the host plant in response to the transgene-induced imbalance of the metal status. These modifications constitute one of the major mechanisms involved in the generation of the plant's phenotype, including unexpected characteristics. Interestingly, activation of so-called "metal cross-homeostasis" has emerged as a factor of primary importance.

Modulation of Zn/Cd P1B2-ATPase activities in Arabidopsis impacts differently on Zn and Cd contents in shoots and seeds

Changes in the expression levels of P_1B2-ATPases in Arabidopsis impacts Zn and Cd contents in shoots and seeds.

Managing heavy metal toxicity stress in plants: biological and biotechnological tools

The maintenance of ion homeostasis in plant cells is a fundamental physiological requirement for sustainable plant growth, development and production. Plants exposed to high concentrations of heavy metals must respond in order to avoid the deleterious effects of heavy metal toxicity at the structural, physiological and molecular levels. Plant strategies for coping with heavy metal toxicity are genotype-specific and, at least to some extent, modulated by environmental conditions. There is considerable interest in the mechanisms underpinning plant metal tolerance, a complex process that enables plants to survive metal ion stress and adapt to maintain growth and development without exhibiting symptoms of toxicity. This review briefly summarizes some recent cell biological, molecular and proteomic findings concerning the responses of plant roots to heavy metal ions in the rhizosphere, metal ion-induced reactions at the cell wall-plasma membrane interface, and various aspects of heavy metal ion uptake and transport in plants via membrane transporters. The molecular and genetic approaches that are discussed are analyzed in the context of their potential practical applications in biotechnological approaches for engineering increased heavy metal tolerance in crops and other useful plants.

Expression of HMA4 cDNAs of the zinc hyperaccumulator Noccaea caerulescens from endogenous NcHMA4 promoters does not complement the zinc-deficiency phenotype of the Arabidopsis thaliana hma2hma4 double mutant

Noccaea caerulescens (Nc) exhibits a very high constitutive expression of the heavy metal transporting ATPase, HMA4, as compared to the non-hyperaccumulator Arabidopsis thaliana (At), due to copy number expansion and altered cis-regulation. We screened a BAC library for HMA4 and found that HMA4 is triplicated in the genome of a N. caerulescens accession from a former Zn mine near La Calamine (LC), Belgium. We amplified multiple HMA4 promoter sequences from three calamine N. caerulescens accessions, and expressed AtHMA4 and different NcHMA4 cDNAs under At and Nc HMA4 promoters in the A. thaliana (Col) hma2hma4 double mutant. Transgenic lines expressing HMA4 under the At promoter were always fully complemented for root-to-shoot Zn translocation and developed normally at a 2-μM Zn supply, whereas the lines expressing HMA4 under Nc promoters usually showed only slightly enhanced root to shoot Zn translocation rates in comparison with the double mutant, probably owing to ectopic expression in the roots, respectively. When expression of the Zn deficiency responsive marker gene ZIP4 was tested, the transgenic lines expressing AtHMA4 under an NcHMA4-1-LC promoter showed on average a 7-fold higher expression in the leaves, in comparison with the double hma2hma4 mutant, showing that this construct aggravated, rather than alleviated the severity of foliar Zn deficiency in the mutant, possible owing to expression in the leaf mesophyll.

Expression of HvHMA2 in tobacco modifies Zn-Fe-Cd homeostasis

HvHMA2 is a plasma membrane P1B-ATPase from barley that functions in Zn/Cd root-to-shoot transport. To assess the usefulness of HvHMA2 for modifying the metal content in aerial plant parts, it was expressed in tobacco under the CaMV35S promoter. Transformation with HvHMA2 did not produce one unique pattern of Zn and Cd accumulation; instead it depended on external metal supply. Thus Zn and Cd root-to-shoot translocation was facilitated, but not at all applied Zn/Cd concentrations. Metal uptake was restricted in HvHMA2-transformed plants and the level in the shoot was not enhanced. It was shown that HvHMA2 localizes to the plasma membrane of tobacco cells, and overloads the apoplast with Zn, which could explain the overall decrease in metal uptake observed. Despite the lower levels in the shoot, HvHMA2 transformants showed increased Zn sensitivity. Moreover, introduction of HvHMA2 into tobacco interfered with Fe metabolism and Fe accumulation was modified in HvHMA2-transformants in a Zn- and Cd-concentration dependent manner. The results indicate that ectopic expression of the export protein HvHMA2 in tobacco interferes with tobacco metal Zn-Cd-Fe cross-homeostasis, inducing internal mechanisms regulating metal uptake and tolerance.

Development of Zn-related necrosis in tobacco is enhanced by expressing AtHMA4 and depends on the apoplastic Zn levels

AtHMA4 was previously shown to contribute to the control of Zn root-to-shoot translocation and tolerance to high Zn. However, heterologous expression of 35S::AtHMA4 in tobacco (Nicotiana tabacum cv. Xanthi) results in enhanced Zn sensitivity. This study provides a better understanding of the development of this Zn-sensitive phenotype and demonstrates that substantial modifications of Zn homeostasis occur due to AtHMA4 expression. We show that ectopically expressing AtHMA4 in tobacco results in overloading the root and leaf apoplast with Zn. The tissue and cellular distribution of Zn, monitored using Zinpyr-1, was altered in the AtHMA4-expressing plants compared with wild type. Increased loading of the leaf apoplast with Zn in AtHMA4 transformants induced necrosis; this appeared at lower levels of Zn supply in the transgenics compared with wild type. This study suggests that Zn concentration may be sensed in the apoplast of leaves, and if concentrations are above a certain threshold then particular groups of cells accumulate Zn and necrosis is initiated. Therefore, this could be considered as a mechanism for protecting the other parts of the photosynthetically active leaf from Zn toxicity.

Cadmium accumulation in tomato cultivars and its effect on expression of metal transport-related genes

Cadmium (Cd) content was quantified in the shoot of six tomato cultivars and the Cd effect on the expression of LeNRAMP3, LeFER, LeIRT1 and LeNRAMP1 was evaluated. The six tomato cultivars accumulated high Cd concentrations and were able to transport Cd to the fruits. Among the evaluated genes, the Cd-induced level of LeFER expression appeared to provide an evidence regarding the capacity of foliar Cd accumulation in tomato. The results indicate that tomato represents a possible pathway for Cd entry into the food chain and an attractive model organism for the elucidation of the mechanisms involved in Cd accumulation.

Recent developments in plant zinc homeostasis and the path toward improved biofortification and phytoremediation programs

Zinc (Zn) is an essential micronutrient for all living organisms. Plants serve as a major entry point for this element into the food chain. Zn deficiency has become a widespread nutritional condition, which mirror the inadequate Zn reserves in significant proportion of the earth's arable land. A recent assessment by the World Health Organization revealed that one third of the world's population is at risk of Zn deficiency. To counter this alarming situation, substantial efforts have been made to increase Zn content and availability in staple crops and grains. Nevertheless, the absence of fundamental information has held back progress in this field. Developing a better understanding of how Zn homeostasis is regulated in plants, such as Zn transporters at loading bottlenecks, is of primary interest to biofortification and phytoremediation programs. Many reviews have been published on this subject, and here we briefly summarize the regulation of one limiting step in Zn distribution within plants - the loading of Zn into root xylem.

Comparative transcriptome analysis of cadmium responses in Solanum nigrum and Solanum torvum

• Solanum nigrum is a cadmium (Cd) accumulator, whereas Solanum torvum is a low Cd-accumulating plant. The molecular mechanisms that are responsible for differential cadmium (Cd) accumulation in the two Solanum species are poorly understood. • Here, grafting experiments confirmed that increased Cd loading into the root xylem was responsible for the differential Cd accumulation in the two Solanum species. An iron (Fe) supply assay indicated that low Fe accumulation in S. torvum leaves is related to its Cd sensitivity. • Transcriptome analyses revealed higher expression of the genes that encode several metal transporters as well as antioxidant-related genes, and several organic and amino acid biosynthesis/metabolism-related genes in Cd-treated S. nigrum. Our data also indicated that the different responsive mechanisms of the transporter genes to Fe deficiency might be responsible for differential uptake and redistribution of metals in the two Solanum species • These results form a basis upon which to further explore the molecular mechanisms of Cd accumulation and tolerance, and provide an insight into novel strategies that can be used for phytoremediation and food safety.