Phytochelatin Synthase has Contrasting Effects on Cadmium and Arsenic Accumulation in Rice Grains

Producing cadmium-free Indica rice by overexpressing OsHMA3

BACKGROUND

Considerable proportions of rice grains produced in some areas in southern China contain high concentrations of cadmium (Cd), leading to unsafe levels of dietary Cd intake. Cultivars of Indica rice, widely grown in southern China, are particularly prone to high Cd accumulation in the grain. Effective methods are needed to decrease Cd accumulation in Indica rice.

METHODS

OsHMA3, encoding a tonoplast Cd transporter, was overexpressed in an elite Indica rice cultivar (Zhongjiazao 17) driven by CaMV 35S promoter. The effects on Cd translocation, accumulation and tolerance, as well as on the agronomic traits and micronutrient concentrations were evaluated.

RESULTS

OsHMA3 overexpression markedly decreased Cd translocation from roots to shoots and increased Cd tolerance. OsHMA3 overexpression decreased Cd concentrations in brown rice by 94-98%, to levels just above the detection limit, when rice plants were grown in two Cd-contaminated paddy soils. OsHMA3 overexpression generally had no significant effect on grain yield and the concentrations of the essential micronutrients including zinc, iron, copper and manganese in field trials.

CONCLUSION

Overexpression of OsHMA3 is a highly effective method to reduce Cd accumulation in Indica rice, producing rice grains that were almost Cd free with little effect on grain yield or essential micronutrient concentrations.

Ectopic expression of a bacterial mercury transporter MerC in root epidermis for efficient mercury accumulation in shoots of Arabidopsis plants

For mercury phytoextraction, we previously demonstrated in Arabidopsis thaliana that a constitutive and ubiquitous promoter-driven expression of a bacterial mercury transporter MerC fused with SYP121, a plant SNARE for plasma membrane protein trafficking increases plant mercury accumulation. To advance regulation of ectopic expression of the bacterial transporter in the plant system, the present study examined whether merC-SYP121 expression driven by a root epidermis specific promoter (pEpi) is sufficient to enhance mercury accumulation in plant tissues. We generated five independent transgenic Arabidopsis plant lines (hereafter pEpi lines) expressing a transgene encoding MerC-SYP121 N-terminally tagged with a fluorescent protein mTRQ2 under the control of pEpi, a root epidermal promoter. Confocal microscopy analysis of the pEpi lines showed that mTRQ2-MerC-SYP121 was preferentially expressed in lateral root cap in the root meristematic zone and epidermal cells in the elongation zone of the roots. Mercury accumulation in shoots of the pEpi lines exposed to inorganic mercury was overall higher than the wild-type and comparable to the over-expressing line. The results suggest that cell-type specific expression of the bacterial transporter MerC in plant roots sufficiently enhances mercury accumulation in shoots, which could be a useful phenotype for improving efficiency of mercury phytoremediation.

Recent advances in arsenic metabolism in plants: current status, challenges and highlighted biotechnological intervention to reduce grain arsenic in rice

Arsenic (As), classified as a “metalloid” element, is well known for its carcinogenicity and other toxic effects to humans.

In silico prediction, phylogenetic and bioinformatic analysis of SoPCS gene, survey of its protein characterization and gene expression in response to cadmium in Saccharum officinarum

Phytochelatin synthase isolated from microorganisms, yeasts, algae and plant, serve a fundamental role in reducing heavy metals. In this research the in silico PCS gene structure (SoPCS) of sugarcane, its secondary and 3D protein structure, physicochemical properties, cell localization and phylogenetic tree were predicted utilizing bioinformatics tools. SoPCS expression in the leaves and roots of sugarcane in tissue culture treated with cadmium was also studied utilizing real time PCR. The predicted SoPCS gene contains 1524 nucleotides, a protein encoded with 508 amino acids of which the molecular weight is 55953.3 Da, 6 exons and 5 introns. The subcellular position of the enzyme is mitochondrion or cytoplasmic. Two domains belonging to the phytochelatin synthase family with similar features was found in Pfam having more than 97% similarity with the predicted SoPCS protein. Phylogeny analyses of plant species were well isolated from other organisms. Ten disulfide-bonded cysteines were excluded from the structure of SoPCS. The predicted 3D structure of SoPCS showed that it is able to bind to L-gamma-glutamylcysteine as substrate. The binding site sequence of PCS included amino acids 52(Q),55(P),56(A),57(F), 58(C),103(G),104(I),151(S),163(G),165(F),206(D), 213(R). The common amino acid with conserved sequence in the binding site of the plant was 103Gly. Gene expression indicated that SoPCS has an important role in the response of sugarcane to cadmium with potential use in genetic engineering to remove metal contaminants in the environment. This is the first characterization of a PCS from sugarcane.

Prospects of genetic engineering utilizing potential genes for regulating arsenic accumulation in plants

The rapid pace of industrial, agricultural and anthropogenic activities in the 20^th century has resulted in contamination of heavy metals across the globe. Arsenic (As) is a ubiquitous, naturally occurring toxic metalloid, contaminating the soil and water and affecting human health in several countries. Several physicochemical methods exist for the cleanup of As contamination but these are expensive and disastrous to microbes and soil. Plant based remediation approaches are low cost and environmentally safe. Hence, extensive biochemical, molecular and genetic experiments have been conducted to understand plants' responses to As stress and have led to the identification of potential genes. The available knowledge needs to be utilized to either reduce As accumulation in crop plants (rice) or to enhance As levels in shoots of hyperaccumulators (Pteris vittata). Gene manipulation using biotechnological tools can be an effective approach to exploit the potential genes (plasmamembrane and vacuolar transporters, glutathione and phytochelatin biosynthetic enzymes, etc.) playing pivotal roles in uptake, translocation, transformation, complexation, and compartmentalization of As in plants. The transgenic plants with increased tolerance to As and altered (increased/decreased) As accumulation have been developed. The need, however, exists to design plants with altered expression of two or more genes for harmonizing various events (like arsenate reduction, arsenite complexation, sequestration and translocation) so as to achieve desirable reduction (crop plants) or increase (phytoremediator plants) in As content. This review sheds light on transgenic approaches adopted to modulate As levels in plants and proposes future directions to achieve desirable results.

Sulfate supply enhances cadmium tolerance in Vicia faba L. plants

Sulfur deficiency and cadmium (Cd) pollution are two ubiquitous constraints affecting plant growth in agricultural soils. However, facing the situation of sulfur deficiency, whether surplus sulfate supply can affect Cd toxicity in plants is still unclear. Therefore, in the present study, experiments with deficient, sufficient, and excess sulfate levels under Cd stress were conducted in faba bean plants hydroponically. We found that sulfate supply significantly increased biomass of Cd-stressed plants when compared with deficient sulfate treatment. Intriguingly, sulfate application also increased Cd concentrations in leaves. Based on increased Cd concentrations without retarding plant growth, we conclude that sulfate supply enhances Cd tolerance in faba bean plants. Sulfate application increased CdSO₄⁰ proportion in the growth medium which is partially related to the increase of Cd in plants because the diffusion of CdSO₄⁰ is faster than Cd²⁺ in plants. Further study on Cd localization showed that this heavy metal was prone to accumulate in the epidermis of leaves as affected by sulfate which might contribute to enhancement of Cd tolerance. Oxidative stress induced by Cd toxicity was alleviated by surplus sulfate supply compared with deficient sulfate. Although capacities of total antioxidants were increased by sulfate in Cd-stressed plants, phenolic compounds as one kind of important antioxidants were unchanged, suggesting that sulfate has no effect on phenolic compounds for scavenging ROS under Cd stress. Taken together, sulfate accelerates Cd accumulation in the epidermis of leaves in faba bean giving rise to higher Cd tolerance.

OsWRKY28 Regulates Phosphate and Arsenate Accumulation, Root System Architecture and Fertility in Rice

WRKYs are transcriptional factors involved in stress tolerance and development of plants. In the present study, we characterized OsWRKY28, a group IIa WRKY gene, in rice, because its expression was found to be upregulated by arsenate exposure in previous transcriptomic studies. Subcellular localization using YFP-OsWRKY28 fusion protein showed that the protein was localized in the nuclei. Transgenic rice plants expressing pOsWRKY28::GUS suggested that the gene was expressed in various tissues in the whole plant, with a strong expression in the root tips, lateral roots and reproductive organs. The expression of OsWRKY28 was markedly induced by arsenate and other oxidative stresses. In a hydroponic experiment, loss-of-function mutation in OsWRKY28 resulted in lower accumulation of arsenate and phosphate concentration in the shoots. The mutants showed altered root system architecture, with fewer lateral roots and shorter total root length than wild-type plants. In a soil pot experiment, the mutants produced lower grain yield than wild-type because of reduced fertility and smaller effective tiller numbers. Transcriptomic profiling using RNA-seq showed altered expression in the mutant of genes involved in the biosynthesis of phytohormones, especially jasmonic acid (JA). Exogenous JA treatments mimicked the phenotypes of the oswrky28 mutants with inhibited root elongation and decreased arsenate/phosphate translocation. Our results suggested that OsWRKY28 affected arsenate/phosphate accumulation, root development at the seedling stage and fertility at the reproductive stage possibly by influencing homeostasis of JA or other phytohormones.

Sulfate facilitates cadmium accumulation in leaves of Vicia faba L. at flowering stage

Cadmium (Cd) is a toxic element, and is prevalent all over the world because of industrialization, mining, sewage sludge, or pesticide supply. Sulfur deficiency is also a frequent problem faced in agriculture. To date, information relating to effects of sulfate on Cd toxicity is still limited. To elucidate how sulfate affects Cd accumulation in faba bean, subcellular accumulation of Cd in leaves consisting of apoplastic washing fluid, symplastic fluid and the cell wall under deficient, sufficient and excess sulfate treatments were investigated in the present study. By using stable isotope of Cd (¹⁰⁶Cd), we also traced Cd accumulation in young leaves at flowering stage from early and newly uptake of Cd in the same plants as affected by sulfate. We found that excess sulfate supply significantly increased newly uptake of Cd without affecting early uptake of Cd when compared with sufficient sulfate treatment, which resulted in enhanced total Cd in leaves by excess sulfate. Since newly uptake of Cd in leaves was from root uptake directly, we conclude that excess sulfate supply enhanced Cd originated from root uptake directly rather than re-translocation from old leaves, which is related to increased Cd accumulation in young leaves of faba bean. Subcellular analysis showed that the enhanced Cd by excess sulfate addition was a consequence of enhanced Cd in cell walls, while Cd accumulation in the apoplastic washing fluid and symplastic fluid were unchanged. The increased Cd by excess sulfate supply might be related to increased proportion of Cd speciation CdSO₄⁰ in the growth medium because of faster diffusion of CdSO₄⁰ than Cd²⁺. To test whether macronutrients, micronutrients, sulfate and non-protein thiol involved in cell wall-Cd accumulation as affected by sulfate, correlations of subcellular Cd with subcellular macronutrients, micronutrients, sulfate, and non-protein thiol were analyzed. We found that cell wall-Cd was negatively correlated with K and Ca concentrations, whereas cell wall-Cd was positively correlated with Cu and symplastic non-protein thiol concentrations. However, when compared with sufficient sulfate, excess sulfate decreased K concentration and increased symplastic non-protein thiol concentration without changing Ca and Cu concentrations. Based on those results, reduction of K concentration and enhancement of symplastic non-protein thiol concentration by sulfate supply might be a reason for increase of cell wall-Cd concentration. Taken together, increased Cd in cell walls of leaves by sulfate supply contributes to enhance Cd accumulation.

Vacuolar Transporters for Cadmium and Arsenic in Plants and their Applications in Phytoremediation and Crop Development

Soil contamination by heavy metals and metalloids such as cadmium (Cd) and arsenic (As) poses a major threat to the environment and to human health. Vacuolar sequestration is one of the main mechanisms by which plants control toxic materials including Cd and As. Understanding the mechanisms of heavy metal tolerance and accumulation can be useful for both phytoremediation and safe crop development. In this review, we summarize recent advances in deciphering the molecular mechanisms underlying vacuolar sequestration of Cd and As, and discuss potential biotechnological applications of this knowledge and efforts towards attaining these goals.

Identification of C-terminal Regions in Arabidopsis thaliana Phytochelatin Synthase 1 Specifically Involved in Activation by Arsenite

Phytochelatins (PCs) are major chelators of toxic elements including inorganic arsenic (As) in plant cells. Their synthesis confers tolerance and influences within-plant mobility. Previous studies had shown that various metal/metalloid ions differentially activate PC synthesis. Here we identified C-terminal parts involved in arsenite- [As(III)] dependent activation of AtPCS1, the primary Arabidopsis PC synthase. The T-DNA insertion in the AtPCS1 mutant cad1-6 causes a truncation in the C-terminal regulatory domain that differentially affects activation by cadmium (Cd) and zinc (Zn). Comparisons of cad1-6 with the AtPCS1 null mutant cad1-3 and the double mutant of tonoplast PC transporters abcc1/2 revealed As(III) hypersensitivity of cad1-6 equal to that of cad1-3. Both cad1-6 and cad1-3 showed increased As distribution to shoots compared with Col-0, whereas Zn accumulation in shoots was equally lower in cad1-6 and cad1-3. Supporting these phenotypes of cad1-6, PC accumulation in the As(III)-exposed plants were at trace level in both cad1-6 and cad1-3, suggesting that the truncated AtPCS1 of cad1-6 is defective in PCS activity in response to As(III). Analysis of a C-terminal deletion series of AtPCS1 using the PCS-deficient mutant of fission yeast suggested important regions within the C-terminal domain for As(III)-dependent PC synthesis, which were different from the regions previously suggested for Cd- or Zn-dependent activation. Interestingly, we identified a truncated variant more strongly activated than the wild-type protein. This variant could potentially be used as a tool to better restrict As mobility in plants.

Rice phytochelatin synthases OsPCS1 and OsPCS2 make different contributions to cadmium and arsenic tolerance

Cadmium (Cd) and arsenic (As) pollution in paddy soil and their accumulation in rice (Oryza sativa) pose serious threats to human health. Rice internally detoxifies these toxic metal and metalloid to some extent, resulting in their accumulation within the edible parts. However, the mechanisms of Cd and As detoxification in rice have been poorly elucidated. Plants synthesize thiol-rich metal-chelating peptides, termed phytochelatins (PCs). We characterized rice PC synthase (PCS) and investigated its contribution to Cd and As tolerance in rice. We identified two PCS homolog genes, OsPCS1 and OsPCS2, in the rice genome. The expression of OsPCS1 was upregulated by As(III) stress in the roots but that of OsPCS2 was not significantly affected. The expression level of OsPCS2 was higher than that of OsPCS1 in the shoots and roots. Recombinant OsPCS1 and OsPCS2 proteins differed in their metal activation. OsPCS1 was more strongly activated by As(III) than by Cd; however, OsPCS2 was more strongly activated by Cd than by As(III). Genetically engineered plants having their OsPCS2 expression silenced via RNA interference (OsPCS2 RNAi) contained less PCs and more glutathione (GSH), a substrate of PC synthesis, than wild-type plants, although there was no significant difference in OsPCS1 RNAi plants. OsPCS2 RNAi plants were sensitive to As(III) stress, but Cd tolerance was little affected. On the other hand, treatment with buthionine sulfoximine, an inhibitor of GSH biosynthesis, significantly decreased Cd and As tolerance of rice seedlings. These findings indicate that OsPCS2 is a major isozyme controlling PC synthesis, and that PCs are important for As tolerance in rice. However, PC synthesis may make a smaller contribution to Cd tolerance in rice, and GSH plays crucial roles, not only as a substrate of PC synthesis.