Zn Homeostasis In Plants Research Articles

Zinc toxicity in plants: a review.

This review highlights the most recent updated information available about Zn phytotoxicity at physiological, biochemical and molecular levels, uptake mechanisms as well as excess Zn homeostasis in plants. Zinc (Zn) is a natural component of soil in terrestrial environments and is a vital element for plant growth, as it performs imperative functions in numerous metabolic pathways. However, potentially noxious levels of Zn in soils can result in various alterations in plants like reduced growth, photosynthetic and respiratory rate, imbalanced mineral nutrition and enhanced generation of reactive oxygen species. Zn enters into soils through various sources, such as weathering of rocks, forest fires, volcanoes, mining and smelting activities, manure, sewage sludge and phosphatic fertilizers. The rising alarm in environmental facet, as well as, the narrow gap between Zn essentiality and toxicity in plants has drawn the attention of the scientific community to its effects on plants and crucial role in agricultural sustainability. Hence, this review focuses on the most recent updates about various physiological and biochemical functions perturbed by high levels of Zn, its mechanisms of uptake and transport as well as molecular aspects of surplus Zn homeostasis in plants. Moreover, this review attempts to understand the mechanisms of Zn toxicity in plants and to present novel perspectives intended to drive future investigations on the topic. The findings will further throw light on various mechanisms adopted by plants to cope with Zn stress which will be of great significance to breeders for enhancing tolerance to Zn contamination.

Arabidopsis thaliana zinc accumulation in leaf trichomes is correlated with zinc concentration in leaves

Zinc (Zn) is a key micronutrient for plants and animals, and understanding Zn homeostasis in plants can improve both agriculture and human health. While root Zn transporters in plant model species have been characterized in detail, comparatively little is known about shoot processes controlling Zn concentrations and spatial distribution. Previous work showed that Zn hyperaccumulator species such as Arabidopsis halleri accumulate Zn and other metals in leaf trichomes. To date there is no systematic study regarding Zn accumulation in the trichomes of the non-accumulating, genetic model species A. thaliana. Here, we used Synchrotron X-Ray Fluorescence mapping to show that Zn accumulates at the base of trichomes of A. thaliana. Using transgenic and natural accessions of A thaliana that vary in bulk leaf Zn concentration, we demonstrate that higher leaf Zn increases total Zn found at the base of trichome cells. Our data indicates that Zn accumulation in trichomes is a function of the Zn status of the plant, and provides the basis for future studies on a genetically tractable plant species to understand the molecular steps involved in Zn spatial distribution in leaves.

Computational characterization and expression profile of MTP1 gene associated with zinc homeostasis across dicot plant species

This study characterizes different MTP1 (metal tolerance protein)/ZAT (zinc transporter) homologs involved in zinc (Zn) homeostasis in plants. BLAST analysis of AtMTP1 protein against 15 plant species showed 21 MTP1 homologs. These MTP1 protein homologs generally contain ~400 residues, six transmembrane helices and cation transmembrane transporter activity (GO:0008324), which can be utilized in predicting Zn uptake and tolerance mechanisms. These MTP1 genes having 1 exon are located on chromosomes 2, 7, and 14. Motifs contain conserved sequences of 41–50 residues belonging to the cation efflux family, which may help target binding sites and transcription factors. Further, AtMTP1 shows close similarities with Glycine max and Medicago trunculata. Interactome analysis suggests the association of AtMTP1 with cadmium/zinc-transporting ATPase and ZIP metal ion transporter. The AtMTP1 network is predominantly connected to cadmium/zinc-transporting ATPase (heavy metal ATPase 2, HMA2; heavy metal ATPase 3, HMA3; heavy metal ATPase 4, HMA4), cation efflux protein (MTP11), and metal tolerance protein C3 (AT4G58060). The Genevestigator predicts the high expression potential of AtMTP1 in the apical root during senescence, seedling, and bolting stages in an association with 11 co-expressed genes, mainly linked to estradiol toxicity and heat stress. Besides, AtMTP1 protein homologs possess conserved N-glyco motifs and physicochemical properties. The similarity and interactions of AtMTP1 gene with other genes suggest that Zn homeostasis in plants is associated with the regulation of different genes. These findings may advance our understanding to further develop plants capable of maintaining Zn homeostasis under adverse conditions.

Physiological responses of rice (Oryza sativa L.) oszip7 loss-of-function plants exposed to varying Zn concentrations.

Rice is a daily staple for half of the world's population. However, rice grains are poor in micronutrients such as Fe and Zn, the two most commonly deficient minerals in the human diet. In plants, Fe and Zn must be absorbed from the soil, distributed and stored, so that their concentrations are maintained at sufficient but non-toxic levels. The understanding of mechanisms of Fe and Zn homeostasis in plants has the potential to benefit agriculture, improving the use of micronutrients by plants, as well as to indicate approaches that aim at biofortification of the grains. ZIP transporters are commonly associated with Zn uptake, but there are few reports about their physiological relevance in planta. Here we describe a Tos17 loss-of-function line for the Zn plasma membrane transporter OsZIP7 (oszip7). We showed that the absence of functional OsZIP7 leads to deregulated Zn partitioning, increasing Zn accumulation in roots but decreasing in shoots and seeds. We also demonstrated that, upon Zn deficiency, oszip7 plants slightly increase their photosynthetic performance, suggesting that these plants might be primed for Zn deficiency which makes them more tolerant. On the other hand, we found that Zn excess is more deleterious to oszip7 plants compared to wild type, which may be linked to secondary effects in concentrations of other elements such as Fe. Our data suggest that OsZIP7 is important for Zn homeostasis under physiological Zn concentrations, and that Fe homeostasis might be affected due to loss of function of OsZIP7.

Cellular Fractionation and Nanoscopic X-Ray Fluorescence Imaging Analyses Reveal Changes of Zinc Distribution in Leaf Cells of Iron-Deficient Plants.

Multilevel interactions among nutrients occur in the soil-plant system. Among them, Fe and Zn homeostasis in plants are of great relevance because of their importance for plant and human nutrition. However, the mechanisms underlying the interplay between Fe and Zn in plants are still poorly understood. In order to elucidate how Zn interacts with Fe homeostasis, it is crucial to assess Zn distribution either in the plant tissues or within the cells. In this study, we investigated the subcellular Zn distribution in Fe-deficient leaf cells of cucumber plants by using two different approaches: cellular fractionation coupled with inductively coupled plasma mass spectrometry (ICP/MS) and nanoscopic synchrotron X-ray fluorescence imaging. Fe-deficient leaves showed a strong accumulation of Zn as well as a strong alteration of the organelles’ ultrastructure at the cellular level. The cellular fractionation-ICP/MS approach revealed that Zn accumulates in both chloroplasts and mitochondria of Fe deficient leaves. Nano-XRF imaging revealed Zn accumulation in chloroplast and mitochondrial compartments, with a higher concentration in chloroplasts. Such results show that (i) both approaches are suitable to investigate Zn distribution at the subcellular level and (ii) cellular Fe and Zn interactions take place mainly in the organelles, especially in the chloroplasts.

A FIT-binding protein is involved in modulating iron and zinc homeostasis in Arabidopsis.

Fe and Zn are essential micronutrients for plant growth, and the interrelationship regarding their homeostasis is very complicated. In this study, we identified a FIT-binding protein (FBP) using the yeast two-hybrid system. The C-terminus of FBP binds to the bHLH domain of FIT, abolishing the DNA-binding capacity of FIT. Knockout of FBP results in an enhanced expression of NAS genes and a higher nicotianamine content, and the fbp mutant exhibits tolerance to excessive Zn. Physiological analyses reveal that the mutant fbp retains a larger amount of Zn in roots and transfers a greater proportion of Fe to shoots than that in wild type under Zn-excessive stress. As FBP is expressed in the root stele, the negative regulation caused by sequestration of FIT is restricted to this tissue, whereas other FIT-regulated genes, such as IRT1 and FRO2, which mainly expressed in root epidermis, do not show transcriptional upregulation in the fbp mutant. As an antagonistic partner, FBP offers a new approach to spatially fine-tune the expression of genes controlled by FIT. In conclusion, our findings provide a new insight to understand the interrelationship of Fe and Zn homeostasis in plants.

Identification of a novel pathway involving a GATA transcription factor in yeast and possibly in plant Zn uptake and homeostasis

To gain a better understanding of the regulation of Zn homeostasis in plants and the degree of conservation of Zn homeostasis between plants and yeast, a cDNA library from the Zn/Cd hyperaccumulating plant species, Noccaea caerulescens, was screened for its ability to restore growth under Zn limiting conditions in the yeast mutant zap1Δ. ZAP1 is a transcription factor that activates the Zn dependent transcription of yeast genes involved in Zn uptake, including ZRT1, the yeast high affinity Zn transporter. From this screen two members of the E2F family of transcription factors were found to activate ZRT1 expression in a Zn independent manner. The activation of ZRT1 by the plant E2F proteins involves E2F-mediated activation of a yeast GATA transcription factor which in turn activates ZRT1 expression. A ZRT1 promoter region necessary for activation by E2F and GATA proteins is upstream of two zinc responsive elements previously shown to bind ZAP1 in ZRT1. This activation may not involve direct binding of E2F to the ZRT1 promoter. The expression of E2F genes in yeast does not replace function of ZAP1; instead it appears to activate a novel GATA regulatory pathway involved in Zn uptake and homeostasis that is not Zn responsive.

Nicotianamine is a major player in plant Zn homeostasis

Nicotianamine (NA) is among the most studied plant metal chelators. A large body of evidence supports its crucial role for Fe distribution in plants and as a precursor of phytosiderophore synthesis in grasses. NA forms stable complexes in vitro not only with Fe(II) and Fe(III) but also with various other divalent metal cations including Zn(II). Early observations indicated a possible contribution of NA to Zn trafficking in plants. Numerous studies on transgenic monocot and dicot plants with modulated NA levels have since then reported Zn accumulation phenotypes. NAS genes were shown to represent promising targets for biofortification efforts. For instance, NA was found to bind Zn in rice grains in a form bioavailable for humans. Recently, additional strong support for the existence of Zn-NA complexes in planta has been obtained in rice, Arabidopsis thaliana and the Zn hyperaccumulating plant A. halleri. We review the evidence for a role of NA in the intercellular and long-distance transport of Zn in plants and discuss open questions.

Expression of a rice Zn transporter, OsZIP1, increases Zn concentration in tobacco and finger millet transgenic plants

Zinc (Zn) deficiency is very widespread both from the plant and human nutrition perspective. One of the approaches to improve Zn in crop plants is by over-expression of Zn transporters such that plants can uptake and accumulate additional Zn added to soil. The ZIP family transporters generally contribute to Zn homeostasis in plants by regulating Zn transport into the cell. We over-expressed OsZIP1 in finger millet and model plant tobacco under the control of constitutive (35S) and endosperm-specific (Bx17) promoters to study the improvement in Zn accumulation. The transcript analysis revealed the induction of ZIP1 in leaf and root tissue under Zn deprivation in finger millet cultivars. Ectopic expression of OsZIP1 in tobacco under CaMV35S (n35S) and Bx17 (nBx17) improved seed Zn concentration compared to untransformed wild-type plants. In addition, we successfully developed finger millet transgenic plants, f35S and fBx17, expressing the OsZIP1 under 35S (f35S) and Bx17 (fBx17) promoter, respectively. The transgenic plants accumulated significantly higher Zn in seeds compared to untransformed wild-type finger millet plants. Plants expressing the gene under Bx17 promoter accumulated more Zn in seed than 35S plants. Apart from Zn, finger millet transgenic plants also showed increased Mn content in seeds. Our results show the involvement of OsZIP1 in improving Zn concentration in tobacco and finger millet. Using endosperm-specific promoter, it is possible to improve the seed Zn concentration in the edible part of cereals.

Nicotianamine forms complexes with Zn(ii)in vivo

The non-proteinogenic amino acid nicotianamine (NA) is a major player in plant metal homeostasis. It is known to form complexes with different transition metals in vitro. Available evidence associates NA with translocation of Fe, and possibly other micronutrients, to and between different plant cells and tissues. To date, however, it is still extremely challenging to detect metal-ligand complexes in vivo because tissue disruption immediately changes the chemical environment and thereby the availability of binding partners. In order to overcome this limitation we used various Schizosaccharomyces pombe strains expressing a plant NAS gene to study formation of metal-NA complexes in vivo. Tolerance, accumulation and competition data clearly indicated formation of Zn(ii)-NA but not of Cu(ii)-NA complexes. Zn(ii)-NA was then identified by X-ray absorption spectroscopy (XAS). About half of the cellular Zn was found to be bound by NA in NAS-expressing cells while no NA-like ligands were detected by XAS in control cells not expressing NAS. Given the high conservation of eukaryotic metal homeostasis components, these results strongly suggest the possible existence of Zn(ii)-NA complexes also in planta. Reported observations implicating NA in plant Zn homeostasis would then indeed be attributable to direct interaction of Zn(ii) with NA rather than only indirectly to perturbations in Fe metabolism. Re-evaluation of extended X-ray absorption fine structure (EXAFS) spectra for the Zn hyperaccumulator Thlaspi caerulescens showed that NA is as expected not a major storage ligand for Zn. Instead it is hypothesized to be involved in efficient translocation of Zn to above-ground tissues in hyperaccumulators.

Facing the challenges of Cu, Fe and Zn homeostasis in plants

Plants have recently moved into the spotlight owing to the growing realization that the world needs solutions to energy and food production that are sustainable and environmentally sound. Iron, copper and zinc are essential for plant growth and development, yet the same properties that make these transition metals indispensable can also make them deadly in excess. Iron and copper are most often used for their redox properties, whereas zinc is primarily used for its ability to act as a Lewis acid. Here we review recent advances in the field of metal homeostasis and integrate the findings on uptake and transport of these three metals.

The use of the zinc‐fluorophore, Zinpyr‐1, in the study of zinc homeostasis in Arabidopsis roots

* The usefulness of the zinc (Zn)-fluorophore, Zinpyr-1, to examine the localization of Zn in the roots of Arabidopsis has been investigated. * In wild-type roots Zinpyr-1 fluorescence was predominantly in the xylem. The fluorescence signal was abolished by the application of the Zn-chelator, N,N,N',N-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), and was increased by increasing exogenous Zn in the medium, indicating that fluorescence reflected relative Zn concentrations. * In the hma2, hma4 double mutant, which is deficient in root to shoot Zn translocation, Zinpyr-1 fluorescence was low in the xylem and high in the adjacent pericycle cells in which HMA2 and HMA4 are specifically expressed in a wild type. Zinpyr-1 fluorescence was also increased in the endodermis. * These results show that Zinpyr-1 can be used to examine the effects of mutations in Zn transporters on the localization of Zn in Arabidopsis roots and should be a useful addition to the tools available for studying Zn homeostasis in plants.

Comparative microarray analysis of Arabidopsis thaliana and Arabidopsis halleri roots identifies nicotianamine synthase, a ZIP transporter and other genes as potential metal hyperaccumulation factors.

The hyperaccumulation of zinc (Zn) and cadmium (Cd) is a constitutive property of the metallophyte Arabidopsis halleri. We therefore used Arabidopsis GeneChips to identify genes more active in roots of A. halleri as compared to A. thaliana under control conditions. The two genes showing highest expression in A. halleri roots relative to A. thaliana roots out of more than 8000 genes present on the chip encode a nicotianamine (NA) synthase and a putative Zn2+ uptake system. The significantly higher activity of these and other genes involved in metal homeostasis under various growth conditions was confirmed by Northern and RT-PCR analyses. A. halleri roots also show higher NA synthase protein levels. Furthermore, we developed a capillary liquid chromatography electrospray ionization quadrupole time-of-flight mass spectrometry (CapLC-ESI-QTOF-MS)-based NA analysis procedure and consistently found higher NA levels in roots of A. halleri. Expression of a NA synthase in Zn2+-hypersensitive Schizosaccharomyces pombe cells demonstrated that formation of NA can confer Zn2+ tolerance. Taken together, these observations implicate NA in plant Zn homeostasis and NA synthase in the hyperaccumulation of Zn by A. halleri. Furthermore, the results show that comparative microarray analysis of closely related species can be a valuable tool for the elucidation of phenotypic differences between such species.